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Über dieses Buch

The first International Conference on Ageing Studies and Lifetime Extension of Materials was held on th July 12-14 , 1999 at St. Catherine's College, Oxford, United Kingdom. Over 230 delegates attended during the three days and heard nearly ninety papers, together with over thirty poster presentations. Sixteen of these papers were keynotes from invited speakers eminent in their field of research. The proceedings were organised into six separate sessions: observation and understanding of real-time and accelerated ageing; experimental techniques; modelling and theoretical studies; lifetime prediction and validation; lifetime extension; and material design for ageing. In doing this, it was hoped to cover most issues of scientific concern inthefield ofmaterials ageing. One important aspect was that the conference did not concentrateon any particular group or type ofmaterial; rather the aim was to attract contributions from workers engaged in ageing studies with as wide a range of materials as possible. In this way, it was hoped that delegates could interactwith and learnfrom those whom they perhapswould not normally come across and that metallurgists could learn from polymer scientists, ceramicists could talk to modellers, and so on, in this important field. A read through the diverse papers contained within these proceedings will confirm that this aim was happily satisfied. Why hold such a meeting? In the modem world, engineered systems are expected to last longer.

Inhaltsverzeichnis

Frontmatter

Plenary Session

Frontmatter

High Pressure Gas Pipelines - Their Ageing and Propensity Towards Intergranular Stress Corrosion Cracking Failure

Natural gas is usually transmitted from its source to regions of consumption in steel pipelines operating at pressures typically in the region of 7000 kPa. The lines are buried under 1 m or more of soil and, to counteract their tendency to corrode in such an environment, they have coatings applied over the outer surface and are subjected to cathodic protection. While those approaches to preventing corrosion are usually effective they are not invariably so, because coatings deteriorate with time, developing defects that allow the ingress of ground waters to the steel. Disbonding of the coating from the steel extends the area over which water has access to the pipe and cathodic current may not penetrate sufficiently into the crevice between pipe and coating to provide protection. The consequence is usually that pits develop and if they penetrate the pipe wall a gas leak will occur. However, in some situations stress corrosion cracks may be generated, sometimes without penetrating the wall, and if they reach an adequate size the fracture toughness of the steel will be exceeded, obviously with much more dire consequences than those for a simple leak. Ground waters are usually dilute solutions containing HCO3-, C1- and SO42-, and often free CO2, with balancing cations of Na, K, Ca and Mg, and, while capable of promoting pitting, such solutions do not promote intergranular stress corrosion cracking. The latter requires the generation of a relatively concentrated bicarbonate solution, as considered later.

Redvers N. Parkins

Ageing Studies of Nuclear Waste Forms

One of the unique and scientifically most difficult aspects of nuclear waste management is the extrapolation of short-term laboratory data (hours to years) to the long time periods (103 to 105 years) required by the performance objectives set in regulations. It will be necessary to establish the extent to which such extrapolations are useful or valid.

Rodney C. Ewing

Plutonium Aging: From Mystery to Enigma

The unusual behavior of plutonium in the solid state is a result of plutonium sitting at a transition in the actinide series between 5f electrons participating in bonding vs. being localized or inert in the core of the atom. The metallurgical consequences of this peculiar electronic structure are examined. We then review the consequences of aging on the properties of plutonium and some of its alloys. The principal mechanisms of aging are through surface corrosion (especially with oxygen and hydrogen) and through self-irradiation damage.

Siegfried S. Hecker, Joseph C. Martz

Observation and Understanding of Real-Time and Accelerated Ageing

Frontmatter

An Ageing Study of Air Activated Acrylic Adhesives using Dynamic Mechanical Thermal Analysis

Air activated acrylic adhesives may be classed as one-part, rapid room-temperature curing adhesives which cure by redox initiated polymerisation of (meth)acrylate monomers. Their cure process is triggered by ambient atmosphere (oxygen) and results in cured adhesive properties similar to conventional two-part (modified) acrylic adhesives. Following the relatively recent development of these novel adhesives, there was an obvious need for a test which would characterise their curing behaviour. The development of the test method involved consideration of the industrial application of the adhesive as well as its novel cure chemistry. Dynamic Mechanical Thermal Analysis (DMTA) was selected as the most suitable technique while torsional oscillation of the adhesive in the parallel plate mode was determined as the optimum experimental arrangement for the desired test conditions. The developed test method was subsequently used in the assessment of the effects of ageing (if any) on these adhesives. The main objective of this ageing study was to investigate if the rate of cure of air activated acrylic adhesives was retarded on storage at room temperature over a period of one year. Tubes of adhesive were tested initially at ‘zero time’, then at the selected time intervals of one month, three, six and nine months, while the final set of measurements were performed on tubes that had been stored for twelve months. The findings of this investigation are reported and analysed using the statistical tool of ANOVA. As a result of this work, some conclusions are drawn regarding the effects of ageing on the cure characteristics of these novel adhesives.

E. Lyons, J. J. Leahy

Ageing of Polyethylene and Conducting Polymer Blends

Thermal oxidation of the Undoped Conducting Polymer

The thermal oxidation of pure poly(3-butylthiophene) (P3BT) films has been studied at 80°C in air. Infrared spectroscopy (FTIR) was used for the identification of the key products. The major carbonyl group is ketone in the a-carbon position of the butyl side chain. The thermal oxidation of blended polyethylene(PE) and poly(3-butylthiophene) films at 80°C, in air or nitrogen, has shown that P3BT is preferentially oxidised, according to the same mechanisms.

Philippe Mazabraud, Patrick Hourquebie, Hakim Janah

Real Time Ageing of Polymeric Components in Closed Systems

Results from real time ageing of polymeric components in closed systems are discussed and compared with data from accelerated ageing studies. Polymers considered have included polysiloxanes, polychloroprenes, polyurethanes, polyimides and epoxies. The polymeric materials have been exposed to oxidative and reducing atmospheres and radiative, thermal and mechanical stresses. Test conditions and the results of changes in physical and chemical properties are described and related to the chemistry of the materials. Valuable data can be obtained from accelerated ageing studies but real time ageing data are required to support and confirm specific trends in ageing phenomenology.

G. F. Hayes

Accelerated Compression Set at Elevated Temperature in Rigid Polymer Foams

We have extended our study of the recovery behaviour of rigid polymer foams following long periods of compression (Miller Tate and Talal, 1999) to include samples constrained at temperatures above ambient. Samples of polyurethane, polyethylene and polyimide, which represented highly crosslinked, lightly crosslinked and uncrosslinked foams respectively, were maintained under a range of compressive strains for periods ranging from three days to one month and their dimensional recovery followed at ambient temperature after release. All materials were of similar density (~100 kg m-3).The different materials showed markedly different behaviour, suggesting that the dominant effect of elevated temperature testing is to induce minor phase changes in the polymer, thus incrementally reducing the mechanical resilience of the polymer chain to applied stress and yielding results which do not accord with an Arrhenius (or other simple) model.

Philip C. Miller Tate, Sina Talal, Christopher J Page, R Keith Scarrow

Kinetics and Mechanisms of the Thermal Degradation of Nylon 6

a pyrolysis-gas-chromatography study

The kinetics of the thermal degradation of Nylon 6 have been studied over the temperature range 350 to 500 °C using pyrolysis-gas-chromatography (py-g.c). Samples of Nylon 6 in the microgramme range were deposited as thin films on to a pyrolysis filament. The latter can reach any chosen degradation temperature chosen up to 800 °C in ca. 20 milliseconds and maintain this temperature to ± 0.5 °C. It was found that, under these conditions, monomer (caprolactam) was almost exclusively evolved as the pyrolysis product. Kinetic data gained from a sequential pyrolysis method revealed that the monomer is evolved by two independent parallel processes, the faster occurring in only a minor fraction of the sample. The specific rate constant determined for the faster process was found to be two orders of magnitude greater than that of the slower process at 350 °C, decreasing to one order of magnitude higher at 500 °C. The slower process was found to have both a higher activation energy (170 ± 20 kJ mol-1) and a higher A factor (2.2 ± 0.3 × 1010 s-1) than the faster process (100 ± 20 kJ mol-1 and 2.8 ± 0.7 x 106 s-1, respectively). This implies that the rate of the slower process approaches mat of the faster process as the degradation temperature increases. It is proposed that the slow process involves the monomer forming within the length of the polymer chain by a ‘middle biting, inchworm’ process, whereas the faster process is associated with backbiting from the ends of the chain, in only a restricted number of polymer molecules with labile end groups (NH2 groups are suggested as possibilities). This leads to the conclusion that the stability of Nylon 6 might be improved by blocking these active end groups.

R. S. Lehrle, I. W. Parsons, M. Rollinson

Ageing & Characterisation of Polysiloxane Rubbers

Cured RTV 5370 siloxane rubbers have been thermally aged in an inert gas atmosphere in sealed containers at temperatures up to 80°C. In a separate experiment, samples of rubber were aged in the presence of water (‘hydrolysed’) at 190°C for 48 hours both in a sealed inert gas atmosphere and also open to the air. Techniques used to probe the samples included mechanical testing, IRHD rubber hardness, thermogravimetry (TGA), Differential Scanning Calorimetry (DSC), ATR infrared spectroscopy, toluene extractable matter and GC-MS of the toluene extractable fraction. Overall, the results suggest a common degradation mechanism for all the samples aged in a closed system (including the ‘hydrolysed’ material). This ageing mechanism is possibly acid accelerated hydrolysis (via hydronium ions) of the siloxane network as indicated by the presence of 2-ethylhexanoic acid.

M. Patel, A. R. Skinner

Geometry Effects During Radiation Ageing of Elastomeric Seal Materials

Lifetime prediction models for elastomeric seals have been developed and validated for a range of elastomeric seal materials. At AEA Technology, a predictive model named SEALS has been developed using well-established time-temperature superposition methods (Burnay 1991) and the model has been validated using a long-term seals ageing programme. Recent work, however, has indicated that differences in compression set values can be obtained on O-rings of the same material but of different sizes, aged under the same environmental conditions. In nitrile O-rings which were exposed to thermal ageing only at 90°C, higher compression set values were observed in O-rings with the smallest cord diameter. This type of geometry effect tended to be smaller in O-rings which were radiation aged at ambient temperature than that seen in thermal ageing.

S. G. Burnay, J. Dawson

Characterisation of the Moisture Absorption and Thermal Ageing Behaviour of Polymeric Composite Systems Using Raman Spectroscopy

Advanced polymeric materials and their respective composites are fast becoming one of the world’s most frequently used engineering materials. They find application in the manufacture of e.g. boat hulls, high performance motor vehicles, aircraft components and sports goods. Their high specific strength and specific stiffness give them the edge in applications where weight savings are critical, but their long-term durability is often questioned. These materials are susceptible to environmental conditions such as temperature and humidity. There is also a lack of relevant data, due to the long time-scales required for testing. In this study, the Raman technique has been used to monitor the degradation of two composite systems, namely: a rubber toughened vinylester material used in the marine industry and a high temperature bismaleimide/carbon fibre aerospace composite. Preliminary Raman studies show that the toughening rubber particles dispersed in the cured vinylester resin are leached out during hygrothermal ageing. The weight gain during ageing suggests that this leaching process occurs concurrently with the absorption of water molecules. An increase in the degree of cross-linking is observed when bismaleimide/carbon fibre composite is aged at high temperature. This cross-linking tendency decreases with increasing depth within the carbon fibre bundle.

V. Otieno-Alego, D. Creagh, B. Jar, B. Fox, A. Lowe

Ageing Processes of Nitrocellulose in Plastic Bonded Explosives

Nitrocellulose (NC) is present as a binder in a range of plastic bonded explosives (PBXs) and its degradation with time is of ongoing interest to the explosives industry. The decomposition kinetics of NC have been investigated by ageing nitrocellulose-containing PBXs at elevated temperatures and monitoring the reduction in NC molecular weight using gel-permeation chromatography (GPC). The kinetics can be described by a first order, random chain scission model and exhibit linear Arrhenius-type behaviour over the temperature range studied, suggesting a single degradation mechanism applies at temperatures below 70 °C. Our current understanding of this degradation mechanism is discussed. Results are presented which show the correlation between NC molecular weight and the mechanical properties of a PBX and the concept of ‘instant ageing’ is introduced whereby explosives are formulated using a low molecular weight nitrocellulose to mimic the probable composition of a PBX had it aged naturally. It is shown that the molecular characteristics of NC vary according to whether the cellulose used in its manufacture is derived from wood pulp or from cotton. This appears to have no significant effect upon the bulk properties of the NC-containing plastic bonded explosives under consideration here.

G. R. Kennedy, P. R. Deacon, N. J. Herbert, A. L. Lewis, T. E. Lilly, A. F. Macdonald, G. Miles, M. K. Till

Ageing Effects on the Mechanical Properties of a Polymer Bonded Explosive

Polymer bonded explosives (PBXs) are highly filled composites typically comprising 90–95% of crystalline secondary-explosive filler and a polymer binder. They are designed for safety as well as performance and have (compared with cast explosives) lower sensitivity, high extensibility and long in service life. Ageing effects can have critical implications on long-term safety and performance and so an understanding of ageing effects is an important component of any “stockpile stewardship” programme. The explosive filler changes little with time, but the nature of the polymer binder can change significantly during the service lifetime of a charge. In this paper, we study the effects of ageing on several variants of a nitrocellulose (NC)-containing PBX. The PBXs studied included NC from different biological sources. Two main experimental techniques, Digital Image Cross-Correlation (DICC) and Environmental Scanning Electron Microscopy (ESEM) have been used to study the effects of ageing on these compositions. We describe the techniques and present some preliminary results below.

H. T. Goldrein, P. J. Rae, S. J. P. Palmer, A. L. Lewis

Accelerated Ageing of Pyrotechnics: 16mm Signal Cartridge - White

The 16mm Signal Cartridge White is used by all three of the UK services. The explosive components are a rimfire cap, a propelling charge of sulphur free gunpowder (SFG), a priming composition based on silicon, potassium nitrate and sulphurless mealed gunpowder, and the main pyrotechnic composition which contains magnesium, barium and strontium nitrates, a colour enhancer and a binder.Cartridges from two different lots were subjected to diurnal temperature cycling for periods of up to 50 weeks. At intervals of 10 weeks, six cartridges were removed from the ageing trial and subjected to critical examination.Quantitative chemical analysis of the main pyrotechnic composition was undertaken, free and total magnesium determinations were made and the heat of reaction was measured using an adiabatic bomb calorimeter.The SFG was subjected to microscopic examination to assess possible caking and/or leaching of the potassium nitrate. The temperature of ignition of the three pyrotechnic compositions was measured by differential thermal analysis.Additionally, the cap was subjected to fire/no fire testing.The performance of the cartridges was determined by proof fire testing which included measurement of the burning time and the heights attained on firing. Other stores enabled the burning time and light output of the main composition to be determined.The results of these tests are reported and discussed in detail, and recommendations for the safe in-service life of this signal cartridge are made.

G. Manton, T. T. Griffiths

Ageing Study of Polyetheretherketone Exposed to Water and Brine by Thermal Analysis and Dielectric Spectroscopy

Changes in the physical properties of semi-crystalline polyetheretherketone (PEEK) were monitored as a function of time of exposure using gravimetric measurement, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC) and dielectric spectroscopy. It is observed that plasticisation of a crystallite distribution generated in the formation of the sample occurred during the water uptake process. The water and brine sorption behaviour follows a Fickian Case I process with a maximum solubility of 0.43 w/w % at 75°C. The observed sorption behaviour was independent of the salinity of the water to which the polymer was exposed. Low frequency dielectric spectroscopy exhibits a very close correlation with the gravimetric measurements and indicates that this method provides a useful non-destructive evaluation technique for the assessment of water content in filler free thermoplastics.

Eric Boinard, Richard A. Pethrick, Colin J. MacFarlane

The Effects of Gamma Radiation Sterilisation on Ultra High Molecular Weight Polyethylene

Ultra high molecular weight polyethylene (UHMWPE) is effectively a high density polyethylene with a molecular weight in excess of 3 × 106 g/mol. Whilst it is chemically identical to HDPE, the large chain length of UHMWPE confers on it the greatest abrasion resistance of any similar thermoplastic, with other favourable properties such as low coefficient of friction and high impact resistance. As UHMWPE also has good biocompatibility, it is commonly used as a prosthetic material, particularly as bearing surfaces in hip and knee replacements, Figure 1.

J. Dawson

Ageing Characteristics of Gamma Irradiated Ultra High Molecular Weight Polyethylene

Ultra high molecular weight polyethylene, UHMWPE, is used as an articulating surface in prosthetic surgery and is sterilised by γ-irradiation. This causes both immediate and time dependent chain scission processes to occur, releasing material for further crystal growth, leading to an increase in stiffness and predisposing the surface to wear.13C NMR was used to characterise the aliphatic region of UHMWPE on the basis of differences in chemical shift and relaxation behaviour. The effects of combined γ-irradiation and ageing in air on these three regions was examined and quantified. CPMAS spectra of the unirradiated material show three semi- resolved peaks which were assigned to amorphous, crystalline and interfacial regions. The spectra for material irradiated at 50Mrad were similar, with a notable reduction in the intensity of the amorphous peak.Longitudinally relaxed spectra of the samples show that the T1 values of the crystalline and interfacial regions are similar and longer than the amorphous region, whereas the T2 values of the crystalline- interfacial components were observed to be significantly shorter than the amorphous peak in the transverse relaxed spectra.Analysis of the spectra was undertaken in terms of Lorentzian curves by non-linear least squares fitting, allowing integration and quantitative measurement of changes in each of the three components after irradiation.

P. O’Neill, C. Birkinshaw, J. J. Leahy

Investigation into Accelerated Ageing of Glass Reinforced Plastic (GRP) for use in Naval Vessels

GRP’s have been widely used in the marine environment for the hulls of mine sweeper vessels over the last 20 years. Lifetime predictions for GRP vessels have been difficult, due to the lack of experimental data on ageing of GRP compositions. Studies into the long term ageing effect of immersion of GRP’s in seawater are therefore required in order to identify degradation pathways and to help predict useful lifetimes of GRP fabricated hulls.This study describes the development of an immersion regime for GRP resin systems to simulate ageing. Ambient, 30°C, 40°C and 60°C seawater baths were utilised over the course of one year and the effect of this artificial ageing was measured, at regular intervals, using chemical and mechanical test methods. The samples examined were a twenty-two year old unsaturated isophthalic polyester system, a new unsaturated isophthalic polyester and a vinyl ester system. All the resins used are approved systems for use in naval composite structures.Good correlation between the mechanical properties and the chemical changes has been found. Reaction mechanisms and theoretical degradation pathways for the GRP systems are currently being examined, together with supplementary analyses of water absorption and chemical leaching effects.High temperature accelerated ageing appears to be unsuitable for lifetime prediction of the GRP systems examined. Ongoing work will attempt to correlate material degradation under accelerated ageing conditions with that experienced in-service to enable lifetime predictions to be made for GRP hulled vessels.

C. Andrews, G. Kirby, D. Breach, T. Taylor, J. Bradley

Electronic Phases and Ageing of Plutonium

The action of pressure can change, besides the crystalline structure, the material electronic structure. This phenomenon can create conditions for radioactive material structural and compositional instability during storage. The paper presents results of processing of P. Bridgman’s experimental data on static compression of uranium and plutonium metal samples which testify for the existence of two various uranium electronic phases and three plutonium phases up to a pressure P=10 GPa. The electronic phases of solids were originally discovered when pronounced changes in material properties (for example, compressibility), without any change in the material crystalline structure, were detected in experiments under pressure.

B. A. Nadykto, O. B. Nadykto

Low Temperature Equilibrium Ageing Under Self-Irradiation in Binary Alloys of Pu with Elements of Group III -B

The questions of low temperature phase equilibrium achievement in alloys of Pu with group III-B elements were considered in connection with the ageing of alloys as a process going in the direction of thermodynamical equilibrium state achievement and against the background of self-irradiation by alphadecay Pu products.

L. F. Timofeeva

A Survey of Plutonium Characteristics as a Function of Age: The ARIES Sampling Program

A survey of a wide variety of plutonium samples from a large number of retired nuclear weapons is providing a substantive statistical foundation for description of characteristics of plutonium as a function of age. This study will provide a statistically significant body of data, with which to compare changes observed with age in other units. A statistical description of changes expected with age will ameliorate the problem of insufficiently detailed manufacturing records and the absence of a baseline from which age- induced changes can be measured.The ARIES line (Advanced Recovery and Integrated Extraction System) is a system that disassembles old pits and converts the weapons-grade plutonium to oxide as feed material for commercially usable MOX fuel, thus providing an elegant means for reducing the nuclear stockpile. The large number and variety of units to be dismantled by the ARIES line provides the opportunity to sample several plutonium alloys with a broad range of ages.Material is examined by a variety of techniques, all but one of which are currently in use for plutonium sampling. The effect of annealing due to disturbance of the lattice by radioactive decay is examined by determining local concentrations of impurities, by examination of microstructure and by hardness. Phase changes may be effected by annealing or migration of impurities and nucleated by damage cascades arising from radioactive decay. Phase changes are sought using hardness measurements, metallography, and X-ray diffraction. Void swelling as well as other radiation damage effects are examined, first by precision density measurement and subsequently by other techniques as their magnitude warrants. Distribution and migration of helium and other products of radioactive decay are examined. Mechanical properties are measured as possible using microindentation techniques, as the samples are too small to accommodate standard testing.Statistical analysis, determination of the underlying mean behaviour and population variability provides a template for evaluation of subsequent samples. The problem of absent baseline data for some plutonium parts manufactured in the past will be addressed by application of statistical methods to this large database. This insufficient baseline can be addressed by collecting measurements on a sufficient number of units to improve the performance of extrapolation of data to zero time, for each of the several compositions and alloys. This extrapolation will provide a foundation for evaluation of ageing effects that may account for observations by more sophisticated methods. Statistical analysis may be used to assess proposed functions to model ageing behaviour.

R. N. Mulford, W. Brown, T. Huntley, J. Wendelberger

Nanospatial Distribution of Lead in Glasses and its Influence on Chemical Stability with Ageing

Lead-containing glasses are common materials used for many applications such as crystal glass (25 wt% PbO) and protective barriers against ionising radiation. When they are in contact with aqueous solutions, these glasses can be altered. The glass-water interaction involves a release of some elements, including lead, from the glass surface. As lead has a well-known ecotoxicity, its mobility during alteration processes is an important environmental problem (Krajewski and Ravaglioli, 1982; Ahmed and Youssof, 1997).

Didier Chaulet, Anne Bouquillon, Jean-Hugues Thomassin, Philippe Le Coustumer, Guillaume Devès, Patrick Baillif

Melt Corrosion of Refractories

Direct and indirect corrosion mechanisms of solids by liquids at high temperature and experimental techniques used to study them are discussed. These include accelerated static/dynamic corrosion tests,post mortem characterisation of corroded microstructures, thermodynamic calculations and phase equilibrium diagrams.

W. E. Lee, S. Zhang

Ageing Under Self-Irradiation: Influence on Positron Annihilation Characteristics in Plutonium-Hafnium Oxide Ceramics

Sintered samples of Pu oxides and their solid solution with Hf, Y and Ce have been studied by positron annihilation. The presence of several defect types has been defined. The influence of crystal lattice type on defect characteristics has been shown. It is also shown that the positron lifetime value in Pu oxide is 20 % more than in Hf oxide. The integral characteristics of the spectra (width of spectrum at half height) are changed during the process of self-irradiation; this is individual for every sample of ceramic.

L. F. Timofeeva, V. M. Filin, V. I. Bulkin

Field Monitoring of the Ageing of Composite High Voltage Insulators

Composite high voltage insulators have been introduced in power networks throughout the world to replace ceramic insulators. Particular advantages include low weight, vandal resistance and lower discharge and electrical interference. The disadvantage, and thus the challenge in management of large populations of composite insulators, is the possible loss of insulation properties on environmental ageing, leading to uncertainty as to the safe useful lifetime of the insulator. One approach which exploits the extensive understanding of the degradation mechanism of elastomers such as EPDM, is to assess the condition of the shed by identifying the oxidation products removed by a non-destructive surface sampling method, including a remote live-line sampling tool. A xylene impregnated swab is used to remove surface material from representative sheds. The swabs are analysed by FT-IR emission spectroscopy (IES) and examination of the carbonyl region of the emission spectrum allows relative amounts of the different oxidation products to be determined. Analysis of a large number of insulators from widely varying environments has shown that there is a variation in the extent of oxidation and product distribution which is consistent with corona discharge, ozone damage and extensive UV exposure. By using cluster analysis on the growing database of insulators which have been sampled, it is possible to determine the predominant mode of degradation in particular geographical locations as well as assessing which commercial insulator showed higher durability in the particular environment. The use of IES has been complemented by diffuse reflectance IR for the analysis of chalking of filler and SEM and XPS analysis on thin slivers of polymer to determine the extent of microcracking, depth of degraded layer and elemental composition. Cluster analysis has also been performed on these data to provide a total picture of the degradation of composite insulators and so guide the procurement, maintenance and replacement strategies of the power distribution authorities.

Graeme A. George, Gregory A. Cash, Heping Liu, Ben G. S. Goss, David Birtwhistle, Andrej Krivda

Experimental Techniques

Frontmatter

Advanced Analytical Techniques for Studying Polymer Oxidation

Chemiluminescence Combined with Scanning Calorimetry

The analytical challenges of polymer oxidation are reviewed and the need for both better methods of chemical analysis and better accelerated ageing tests is highlighted. Newer spectroscopic approaches, in particular the use of gas-phase derivatisation reactions are discussed. The application of chemiluminescence, particularly when coupled to scanning calorimetry, is shown to be a significant advance in oxidation induction time testing. Increased oxygen pressure and chemiluminescence imaging are both promising developments.

Norman C. Billingham, Vincent Dudler

Cable Condition Monitoring Using Indenter Measurements

There has been a considerable amount of work worldwide over the last 10 to 15 years on the development of methods for monitoring ageing degradation in polymeric cable materials (see for example EPRI workshop, 1993). Of the many methods assessed, only a few have so far shown practical promise for use in real plant. One of the most promising non-destructive methods is that based on indenter measurements (Carfagno et al., 1989). In this type of test, a small probe (usually with a truncated cone tip) is pushed into the surface of the material under controlled conditions and the load/displacement curve is measured. The slope of the curve is known as the indenter modulus (IM) and shows a good correlation with ageing degradation in a wide range of materials used in both cables and seals. The method can be applied to both thermal and radiation ageing of polymeric materials.

S. G. Burnay

Oxidation of Stressed Polymers as Studied by Chemiluminescence

The oxidation of polyamide 6 film and injection moulded specimens of Polypropylene (PP), high density polyethylene (HDPE) and polyamide 6 (PA6) subjected to constant load has been studied with chemiluminescence. Stress induced chemiluminescence (SCL) was used to study the oxidation in situ of specimens exposed to loads between 0 and 100 % of load at yield. No change in the course of oxidation was found due to load. Conventional chemiluminescence (CL) was used to calculate the total luminescence intensity (TLI) for specimens which had been pre-aged under stress. The results from these measurements, in accordance with FTIR measurements, showed no obvious sign of acceleration of the oxidation due to stress. Imaging chemiluminescence (ICL) was used to monitor the spatial distribution of oxidation over a stress profile in pre-aged specimens. The resulting images showed no sign of an increased oxidation with increased stress.

Karin Jacobson, Bengt Stenberg, Björn Terselius, Torbjörn Reitberger

Analytical Techniques & Modelling of the Lithium Hydride Ageing Reactions

Lithium hydride, the “lightest” inorganic salt possible, and the simplest neutral heteropolar diatomic molecule, has been the object of intense theoretical and spectroscopic study since the 1930s. Its unique nuclear chemistry and high hydrogen density allows it an unsurpassed place, in certain isotopic forms, in modern nuclear weapons. The basic chemistry is of great interest and the reaction with water has been extensively studied. However, much of the work reported to date has examined the reactions over relatively short timescales and been completely inconclusive in terms of the mechanisms/kinetics of lithium hydride chemistry and its long-term ageing. High vacuum mass spectroscopy leak detection studies of the very slow hydrogen evolution rates was/is the historical technique, but is not reported here. Research reported includes the use of modern analytical techniques such as solid state 6Li NMR, as well as an adapted thermal analysis technique specifically for atmospherically sensitive materials. Quantum mechanical modelling and molecular mechanical calculations of the basic hydride/water reactions are also reported. The research aims to expand our knowledge of the complex chemistry that occurs during the apparently simple long term hydrogen outgassing of bulk lithium hydride.

Duncan A. Broughton

Ultra-Accelerated Thermal Ageing Tests

Slow Thermal Ramp Methods in Predicting Life Expectancy

Isothermal testing of polymer ageing requires long-term exposure if realistic results are to be obtained, which means a time-consuming and hence expensive exercise, which may have to be repeated many times over many different conditions. Rates of ageing can be accelerated by increasing the temperature of tests or by using a more aggressive environment, but then results must be extrapolated to working conditions, assuming that the same mechanisms still apply. In this paper, we demonstrate that a slow thermal ramp technique, similar to thermo-gravimetric analysis, allows the qualitative and, in some cases, quantitative assessment of ageing to be made in a relatively short time, 15 to 20 days, compared to up to 2 years for isothermal tests. We will illustrate the method in relation to recent work on the ageing of Kraft electrical insulation paper in insulating oil and compare results from isothermal and thermal ramp experiments. In this case, Degree of Polymerisation (DP) was used as a key indicator of degradation.

Alan M. Emsley, Richard J. Heywood, Gary C. Stevens

Application of Tritium Tracer Techniques to Materials Ageing and Lifetime Extension Studies

With materials used in defence related areas it is important not only to ensure safe conditions during handling but also that the good performance characteristics of the materials are retained on storage. Furthermore, there will in some cases be the need to extend their effective lifetimes. In addition it will be necessary to know what products / pollutants are formed during use. These are demanding requirements and, to improve our understanding, the best approach is to study the mechanisms of the reactions involved. Invariably these are complex and can only be studied well away from the customary standard laboratory conditions, e.g. in water at 25°C. Consequently, a good approach is to build a knowledge data-base starting with model compounds. Ideally, the approach chosen should be widely applicable, with the same techniques being capable of use for different reactions. Furthermore, if extremely sensitive methods could be developed so that relatively slow rates can be accurately determined over short time intervals, that would be extremely useful. There would then be no need to use long and hazardous extrapolations in interpreting the experimental data.

Paul R. Deacon, Hemant J. Desai, Russell N. Garman, John R. Jones

Ageing and Lifetime of Electronic Components Studies By Means of a High-Resolution In-Situ Measurement Technique

Since the invention of the integrated circuit (IC) in 1959, there has been an enormous growth in performance and complexity of electronic systems by drastically reducing the basic building blocks and increasing the circuit density. Due to this ongoing miniaturization of microelectronics, new physical mechanisms can be triggered or known physical mechanisms can be enhanced inducing ageing and failure of the electronic material systems. The time evolution of these ageing mechanisms has to be understood in order to guarantee the lifetime of several years required for products such as personal computers, automobiles, home appliances etc. The study of failure mechanisms in electronic components requires a combination of advanced electrical, analytical and statistical tools.

K. Croes, J. V. Manca, W. De Ceuninck, L. De Schepper, L. Tielemans

Techniques for Monitoring Material Degradation

The paper covers the development of techniques in two main areas. The first is that of damage, strength and transmission losses of “window” materials in aerospace, aircraft and missile applications. In such applications, the “window” may be subjected to liquid impact (rain) or solid particle impact (dust, sand, ice). The “window” or dome may need to be transparent in the visible, the micro-wave or infra-red. Relevant impact velocities may be up to Mach 2 for aircraft and up to Mach 4 and beyond for missiles. The paper describes techniques which we have developed for producing very controlled and accurate impacts for rain, dust, sand and ice. The theory of liquid impact is used to show that water drop impacts can be simulated by jet impact provided that the liquid is coherent and has a smooth, curved front surface. A multiple impact jet apparatus (MIJA) for measuring the rain erosion response of materials is described. For dust and sand, we have designed an erosion rig of the gas flow type. The apparatus can handle particles from 20 μm up to millimetre size at velocities up to ca. 250 m s−1. For large solid particles, including ice, we use a range of gas guns. Advantages of our approach are that we can produce controlled and reproducible impacts and can provide data on threshold velocities for damage and strength and transmission losses. Data on a wide range of materials, including chemically vapour deposited (CVD) diamond, is presented. The second area concerns failure mechanisms and strength loss in polymer bonded explosives (PBXs). The advantages of using environmental scanning electron microscopy (ESEM) for high magnification deformation studies are illustrated and discussed. For quantitative measurement of strains to failure, a combination of optical microscopy and moiré interferometry has been developed. This method allows strain to be measured on the sub-micron level while at the same time recording the underlying microstructure. Such methods are of general interest to degradation mechanisms in composites.

J. E. Field

Application of the Pressurised Unsaturated Flow (PUF) Test for Accelerated Ageing of Waste Forms

The pressurised unsaturated flow (PUF) test method is an emerging technique for studying the long-term durability or weathering of waste forms and other geologic materials. We have used the technique to monitor the complex coupling between primary phase dissolution, secondary phase precipitation and unsaturated flow behaviour, including changes in the hydraulic properties of the materials. New experimental data with low-activity waste glasses has shown that the technique effectively accelerates the hydrolysis and ageing processes by as much as 50 times over conventional static tests run at the same temperature. Rapid, sustained increases in the corrosion rate of three low-activity waste glasses were detected with the PUF technique in only a few days that required months to observe in static experiments. The ability to quickly detect long-term performance problems with waste forms may make it possible to formulate second generation materials concurrently with meaningful long-term durability screening.

B. P. McGrail, P. F. Martin, C. W. Lindenmeier, H. T. Schaef

Making and Using Quantitative Measurements of Thermal Degradation

-Pyrolysis experiments and their wide range of applications

Experimental approaches: Thermal degradation studies of polymers present problems because at elevated temperatures the rates may be so high that the reaction times may be measured in seconds. This means that degradation temperatures of several hundred degrees must not only be attained in milliseconds, but then precisely maintained. Moreover, even if a pyrolysis filament can be made to have these characteristics, the polymer coated on it will not follow the temperature profile unless it is in the form of a very thin film and in practice this means sample sizes must be of the order of micrograms. Finally the products must be swept away, characterised and measured, with equipment of adequate sensitivity, before there has been any opportunity for secondary reactions. Considerable progress has been made in overcoming all of these difficulties by the pyrolysis-g.c. technique and reliable quantitative data can now be obtained on most polymeric systems. What data should be obtained & how is the information utilised? Data of different types may be measured with certain kinetic or mechanistic objectives in mind: (1) measurement of overall rate constants and activation energies; (2) verifying whether random scission is occurring; (3) studying chain depropagation, initiation and termination mechanisms; (4) verifying chain transfer mechanisms and (5) testing for secondary (consecutive) reactions. More complex examples and recent developments: (1) effects of blending on thermal stability; (2) studying the thermal stability of cross-linked samples; (3) thermal stability of low-MW oils; (4) predicting yields and structures of degradation products; (5) direct sample comparisons.

Roy S. Lehrle

Modelling Radiation Damage in Polyurethane Materials Based Upon Polyester Polyols and Methylenediphenyldiisocyanate Formulations

Bond strength measurements of test samples containing an aluminium/polyurethane interface, which was exposed to relatively low doses of simulated α radiation, indicate considerable degradation. Gas analysis of volatile evolution resulting from bombardment with simulated α irradiation indicates chain scissioning of the ester to be the dominant degradation mechanism. However, the extent of degradation indicated by volatile evolution appears inconsistent with the observed degradation in bond strength. The polyurethane system subjected to alpha radiation was exposed to gamma radiation, in conjunction with a number of comparable systems. Analysis of gamma irradiated samples indicates that two competing reactions result from irradiation: chain scissioning and crosslinking of residual double bonds present in the poly(ester) used to formulate the polyurethane. Such bonds are in finite supply and are eventually exhausted. Crosslinking occurs at low doses, but causes a large decrease in the hardness of the material. This is ascribed to a loss of adhesion between the polymer matrix and the silica filler particles incorporated into the formulation. The loss of adhesion occurs at low doses and it is postulated that a similar loss of adhesion occurred in the bond test samples exposed to low doses of simulated alpha radiation, causing the observed drop in bond strength.

J. J. Murphy, M. Patel, A. R. Skinner, P. F. Smith

Thermal Ageing of Polymers as Studied by 1H Broadband and 13C High Resolution Solid State NMR

Solid state NMR, based on both structural and dynamical approaches, was used to follow the curing process and thermal ageing of EPDM elastomer. The methods, including standard wideline and high resolution experiments, as well as two dimensional WISE (Wideline Separation experiment), were shown to be sensitive to chemical changes of a few tenths of a percent of the carbon content.

Pascal Palmas, Laurent Martel

Electron Spin Resonance Imaging (ESRI) of Degradation and Stabilization Processes in Polymers:

Poly(acrylonitrile-butadiene-styrene) (ABS) Polymers

Polymeric materials, exposed to heat, mechanical stress and ionizing or UV irradiation, undergo degradation in the presence of oxygen due to the formation of reactive intermediates such as free radicals R., RO. and ROO., and hydroperoxides ROOH.1–6 The degradation process can be accelerated by chromophores, free radicals and metallic residues from the polymerization reactions. Often the deleterious effects are not immediately detected, but develop over longer periods. The gradual changes in the polymer properties observed in many systems, including polyolefìns, and the ultimately grave results are due to trapped radicals that react slowly, to peroxy radicals that decompose in time with formation of reactive radicals and gas molecules, and to trapped gases that lead to local stresses and to cracking.2,5 While the timescale of these changes may vary, the final results are dramatic: degradation of the structure and collapse of the mechanical properties. The accelerated rate of ozone depletion in the stratosphere due to environmental factors is expected to raise the level of UV-B radiation (≈290–320 nm), thus adding severity to the problem of degradation and urgency to the need for solutions.

Mikhail V. Motyakin, John L. Gerlock, Shulamith Schlick

Chemometrics In The Study Of Polymer Degradation And Life Prediction By Spectroscopic Methods

The correlation of polymer structure with properties has been greatly assisted by the application of statistical methods — chemometrics — to spectroscopic and physical multi-variate data. Chemometrics is a multivariate methodology that allows the information from entire spectra to be used and can be considered as a maximal data compression technique. The extracted “factors” can then be regressed against property data and, given a characterised set of “training” samples, can then be used in a predictive mode. For example, the relationship between the near infrared (MR) spectra of polyolefins and their density and melt flow index (MFI) is already being used for on-line process analysis and control of powder and pellet grades.We will show that simple spectroscopic methods, such as Raman, infrared and near infrared techniques, when coupled with chemometrics, are capable of providing molecular weight and distribution information, and we will illustrate this with reference to poly(dimethylsiloxanes) (PDMS) and cellulosic materials. We will further show the application of the method to tracking structural and property changes in synthetic and natural polymers due to degradation.

M. J. Shenton, H. Herman, G. C. Stevens

Early Glass Alteration Monitored by Electrochemical Impedance Spectroscopy (EIS)

When glassy materials are in contact with aqueous solutions, alteration processes occur (Hench, 1975; Bunker, 1994). These phenomena manifest themselves by a modification of the glass surface and by a mass transfer of some elements across the glass/liquid interface. Experimental studies of alteration allow the estimation of the chemical resistance of glasses and the determination of the processes involved in their deterioration. So, these studies have many industrial applications (containers for aggressive solutions and elaboration of vitrified matrices for toxic waste storage.)

Didier Chaulet, Serguei Martemianov, Jean-Hugues Thomassin

Modelling and Theoretical Studies

Frontmatter

A Model of the Effects of Radiation on the Microstructure of Metals

Engineering-scale models of the effects of radiation on the mechanical properties of materials can be modelled by considering relevant physical processes at the microstructural level.The initial displacement of atoms by quanta of radiation and the subsequent formation of damage cascades are treated by direct simulation (molecular dynamics) and by a statistical model. The interaction between damage cascades may be significant at high dose rates; this is being investigated by simulation and is included in the macroscopic model.The mobility and interactions of vacancies and interstitial atoms (including helium from alpha decay) are estimated by simulation and by a hierarchy of simplified models. Sample calculations are shown of the growth of micro-voids and dislocation loops and the migration of helium in a face-centred cubic metal.The microstructure, evolving under the influence of radiation, can be related to macroscopic mechanical properties such as plasticity. Thermal and disordering effects such as recrystallisation and localised melt are introduced here.The methods developed are demonstrated by application to an idealised model of a-decay in plutonium.

D. C. Swift

Modelling Long-Term Precipitation Growth in Austenitic Steels for Boiler Applications

Conventional models of precipitation growth have been developed to describe short-term ageing effects and are thus limited in their applicability to long-term service environments. In this study, analytical equations describing the kinetics of precipitation growth have been abandoned in favour of an iterative technique, which assesses the relevant driving forces at each individual step in the calculation. The model has been applied to study intergranular precipitation of M23C6 carbides in AlSI-type 316 austenitic stainless steel, after ageing times of the order 100,000 hours. Model predictions are compared with one of the more conventional models.

Colin C. Goodwin, Roy G. Faulkner, Mike W. Spindler

Testing and Modelling for Prediction of Hydrogen Embrittlement

A critical evaluation of testing and modelling for prediction of hydrogen embrittlement has been undertaken. Exposure time, temperature excursions and mechanical test method are shown to be factors which affect the reliable experimental determination of threshold stress/stress intensity factor for cracking and crack growth rates. In relation to test time, the key issue is the distance of the site of cracking from the primary source of hydrogen atoms, which will be influenced by exposure conditions, test specimen configuration and activity of the alloy. In some systems very long tests may be inevitable to ensure steady-state hydrogen uptake has been attained. Excursions from high to low temperature can be important in service and have been shown to be of significance in laboratory testing, but further studies are required. Mechanical test methods based on dynamic straining will provide a more conservative estimate of the threshold stress/stress intensity factor for cracking. It is concluded that guidelines for hydrogen embrittlement testing are needed to complement existing test standards.Mechanistic modelling of hydrogen embrittlement threshold and crack growth kinetics is fundamentally challenging. Analytical models have been developed but with questionable assumptions and a limited range of application. The primary obstacle is the need to predict the time-dependent distribution of hydrogen atoms at the crack tip as a function of test and operational variables. Progress has been made but existing models have limitations. Nevertheless, if further progress is to be made it is in the development of more robust models of crack-tip hydrogen coupled with a criterion for failure.

Alan Turnbull

Model of Shock-Wave Material on Alpha Decay

During storage, plutonium containing materials experience changes caused by radioactive decay products. The natural crystal effect of radioactive radiation was detected long ago by geologists as coloured rings (“pleochroic haloes”) in mineral sections. The English geologist Joly (1907) attributed these haloes to action of uranium α -particle decay [1]. A group of α - particles of various energy corresponds with a system of visible rings. After long exposure to α - particles, a material transfers from the crystalline to the amorphous state. For natural apatites and rare-earth silicate apatites containing actinides, such a transition was studied in ref. [2]. The effect of α - decay on the Gd2Ti2O7 pyrochlor phase containing 244Cm and 240Pu was studied with X-ray diffraction and electron microscopy [3, 4]. At a dose of 3.1.1018 α - decay/g the material becomes completely amorphous, without any traces of the initial crystalline structure. For zircon amorphism sets in at a dose of 1019 α -decay/g [5].

B. A. Nadykto, O. B. Nadykto

Modelling Dimensional Change With Radiolytic Oxidation in AGR Moderator Graphite

Advanced Gas Cooled Reactors (AGRs) operated by British Energy Generation Ltd are graphite-moderated and cooled by CO2. The moderator cores are constructed from graphite bricks which must allow unhindered movement of the control rods and fuel stringers. During the life of a reactor, the moderator core bricks are subject to irradiation both by neutrons and by ionising radiations. Neutron irradiation induces dimensional change and changes in mechanical, physical and electrical properties of the graphite; whereas the ionising radiation (principally γ) brings about oxidation of the graphite by the CO2 coolant. Radiolytic oxidation results in weight loss from the graphite components and the development of internal porosity leading to a reduction in strength. A full understanding of radiolytic oxidation is difficult to achieve because it requires a good description of the size and shape of the open porosity and that of the porosity which may be opened to gas by oxidation. Information on the effects of radiolytic oxidation on graphite properties is currently limited to ~40% weight loss. However, higher local weight losses may develop in some peak-rated bricks towards the end of the anticipated core life.

Gareth B. Neighbour

Time Decay Modelling of UV-Induced Refractive Index Change of Silica Glasses for Optical Applications

We describe an approach for analysing the experimental physical data recorded in disordered media. It is currently used with success for refractive index Bragg gratings and is therefore the example of this paper. This approach allows us to model the time behaviour for life-time prediction and to give a method to extend the stability of the devices.

B. Poumellec

A Methodology for Examining System Aging Due to Interactions Between Chemically Incompatible Materials

We start with a stored and unused population of fielded engineered units that are composed of chemically incompatible materials. The units age primarily through heterogeneous chemical reactions between the materials resulting in possible degradation in performance. The engineered units are unused in storage, but may be called into actual service at any time. We sample several units from the population per year and perform a number of non-destructive evaluation (NDE) techniques, such as radiography, low-frequency vibration analysis and ultrasonic imaging on the selected units. From those units, some are selected for destructive testing (D-test) involving disassembly and testing of internal parts and components. Chemical analyses, mechanical properties measurements and other tests are performed. All of the above steps provide information that is used in the system simulation mathematical model. The system simulation model incorporates chemical reactions and gas-solid transport processes, along with changes in both the surface and bulk properties of the solids. Model results are used to suggest improvements in NDE analyses of the units and improvements in component and material analyses. Model results give trending indications of individual component and overall system changes over time, plus some understanding of the mechanisms involved which allow science-based predictions of the aged state of the units in future times. The NDE, D-test and model results can also be used to assess statistically the reliability and performance of the overall aging population of units.

J. P. Deininger, J. A. Tanski

Modelling, Predicting and Extending the Service Life of Structural Adhesive Joints

As a means of joining materials, adhesives have been used by mankind for many centuries. However, it is only in the last fifty years or so that the science and technology of adhesion and adhesives has really progressed significantly and the major advances that have been made may be traced from the middle of the 1940’s. The main reason for this is that the adhesives employed in nearly all the technically demanding applications are based upon synthetic polymers and such polymers have only been available for about the last fifty years. Synthetic polymers possess the balance of properties that enables them to adhere readily to other materials and to have an adequate strength so that they are capable of transmitting the applied loads from one substrate to the other.

Anthony J. Kinloch

Modelling of Stabilizer Reactions in Gun and Rocket Propellants

Most of the primary stabilizers for gun and rocket propellants such as DPA, 2-NO2-DPA, EC, Ak II and MNA form stabilizing consecutive products. It is of interest to include the consecutive products in the prediction of service time period in order to get reliable data. This is achievable with kinetic modelling and enables the extension of the service time period in a defined way and also the full use of the service time period capacity of propellants. Two main groups of models are discussed. The first group of models is applicable already with the data on the decrease of the primary stabilizer only. Model limits, mechanistic aspects of the main decomposition reactions of NC and constraints influencing the prediction of service time period are pointed out. The second group also includes the concentrations of the consecutive stabilizer products. The non-simplified model uses complete reaction mechanistic schemes; the simplified one applies the steady-state approximation for the concentrations of the autocatalytically effective products. It will be shown that this approximation is not allowed with these stabilizers. The non-simplified model produces the reactivities of the consecutive products, relative to the primary stabilizer, for their reactions with the autocatalytic products, directly for the propellant matrix situation. Therewith an effective stabilizer concentration expressed as effective primary stabilizer is obtained, which is used to predict the safe service time period. Examples of both types of modelling are discussed with a DPA-stabilized, singlebase, gun propellant. The extension of service time period is demonstrated.

Manfred A. Bohn

The Mechanical Characterization and Stress Relaxation of a Filled Silicone Foam

Understanding the time dependent behaviour of a compressed flexible foam

Stress relaxation behavior of polymeric materials under strain can only be understood by conducting experiments that are comparable in duration to the time regime of interest. Stress relaxation is the decay in stress over time that results when a viscoelastic material is deformed and held at constant strain. In polymers, the stress decays in part as a consequence of molecular motion of polymer chains but also as a result of chemical and physical bond changes within the strained material. A polymer is considered thermo-rheologically simple when a single mechanism controls the relaxation response (Schwarzl and Stavennan, 1952). Such a unique thermal dependency lends itself neatly to the thermal acceleration of aging, such as time-temperature superposition, that yields years’ worth of data in a few hours. However, many polymeric materials are thermo-rheologically complex. That is, several mechanisms contribute to the overall relaxation and each has its own thermal dependency. In these situations, one must understand the relative contributions of each mechanism, which are likely to change, in the time domain of interest. For example, the time scale for viscoelastic or physical relaxation may vary from less than one second to many years whereas chemical degradation typically occurs over hours to years. Therefore, the relaxation response of a polymeric material within a few seconds is likely to be dominated by physical processes, with very little contribution from chemical degradation. This is advantageous for characterizing the physical response of the material but can not be applied to long times when chemical changes with different thermal dependencies dominate.

J. E. Coons, T. S. Stephens, R. Mooday, A. L. Graham, W. P. Steckle

Modelling Aged Polymers with Finite Element Analysis

We have modelled the stress relaxation behaviour of silastic foams over a 5-year period. We have used relaxation data from tests at Los Alamos and other facilities. The model behaviour is consistent with data taken on such foams at the Allied Signal Plant in Kansas City, Missouri. Initial compression for these tests can be as high as 55%. Variability of the initial properties of the foams significantly influences the long term relaxation behaviour as these strains are high enough to be in the “lock-up” region of the compressive stress-strain curve.We are incorporating a Rusch-like ageing model for silastic foams. The model includes the influence of initial strain, temperature and time. It allows for the development of permanent strains. We are also analysing models of incompressible polymers and rigid foams. We will attempt to incorporate Rusch-like ageing models for these materials also.

Matthew W. Lewis, Charles A. Anderson

The Use of the Zhurkov Equation to Describe Polymer Ageing Processes

The use of the Zhurkov equation to predict the durability of polymers is discussed. It is expected that it will be obeyed when failure is controlled by a single, stress-biased, thermally activated process (e.g. a chain scission event). The mechanisms of failure are therefore considered and conditions identified for which the Zhurkov equation is likely to apply. Crack growth through chain scission at the crack tip is considered and the effect of the stress concentration is analysed. Estimates for the activation energy (42kJ/mol) and the activation volume (0.35nm3) are derived from data obtained with polystyrene samples.

J. R. White

Quantitative Kinetic Modelling of the Pyrolysis of Polyisobutylene

- characteristics of 55 volatile products predicted

Above 300 °C, high MW polyisobutylene degrades to more than 60 volatile products. This paper sets out to predict their molecular weights, precise structures and relative yields by quantitatively modelling the detailed pyrolysis mechanisms. A preliminary statistical analysis of the observed oligomeric species shows that random scission is the predominant mechanism producing oligomers and an initial model is set up on this basis. Monomer, dimer, and trimer are however being supplemented by depropagation plus intramolecular transfer (backbiting) mechanisms and the relative yields and structures of the products from these sources are then predicted. All predictions are then combined to obtain a complete model for the structures and the relative yields of all products from the pyrolysis. The model leads to the prediction of 55 pyrolysis products, compared with the 63 products detected experimentally. Also, the predicted yields are similar to those in the experimental chromatogram. Finally, the predicted weight fractions of all 55 products compare well with those observed experimentally. The procedure is useful in that it assesses the relative importance of possible degradation mechanisms and helps to elucidate which structural features contribute towards the thermal instability of the polymer. The approach may also be developed to predict the more complex thermal degradation patterns of oils, which are more difficult to study experimentally.

Roy S. Lehrle, Caroline S. Pattenden

Lifetime Prediction and Validation

Frontmatter

Reverse Temperature Effect During Radiation Ageing of XLPE Cable Insulation

The behaviour of polymer based cable insulation and jacket materials during radiation ageing has been studied extensively over the last 10 years and predictive models have been developed (IEC 1996) for many of the materials used in UK nuclear power stations. Most of these polymeric materials behave in a similar manner, with ageing being a function of the radiation dose rate, total dose and ageing temperature. In general, an increase in ageing temperature results in an increase in the rate of degradation, as would be expected from normal chemical kinetics. However, results from long term ageing tests on cable materials have shown that cross-linked polyethylene (XLPE) insulation materials can show a reverse temperature effect, with the material ageing more rapidly at 20°C than at higher temperatures. This is opposite to the expected behaviour where elevated temperature would normally accelerate degradation. This reverse temperature effect is believed to be dependent on the semi-crystalline nature of XLPE, with significant recrystallisation occurring during radiation ageing at elevated temperature, ‘repairing’ the degradation associated with the irradiation. At lower temperatures, molecular mobility is too low for significant recrystallisation to occur. Such effects are likely to be generic to semi-crystalline polymers.

S. G. Burnay, J. Dawson

Polypropylene Degradation and Durability Estimates Based on the Master Curve Concept

Material degradation and life prediction studies need to overcome numerous challenges. Among the most formidable is the long term, real time data generation. The typical experimental time span seldom goes beyond a year and various ways to accelerate the ageing process frequently encounter fundamental difficulties which reduce the value of the extrapolation or worse, totally negate it.

Lecon Woo, Michael Ling, Atul R. Khare, Y. Samuel Ding

Prediction of the Lifetime Integrity of a Nuclear Waste Container Material Based on Thermal Stability Studies

The kinetics of precipitation in C-22 alloy (UNS N06022) were estimated through isothermal aging experiments at temperatures ranging from 427°C to 800°C for times up to 40,000 h. The phases that formed, at least after the longer aging times, were identified by transmission electron microscopy (TEM). Aged samples were also examined using scanning electron microscopy (SEM) to determine when precipitation began on grain boundaries, completely covered grain boundaries, and began forming within the grains. This data was used to evaluate the kinetics of precipitation at lower temperatures using a simple exponential temperature dependence. The corrosion resistance of aged samples was investigated using the ASTM Specification G28B technique. The time to cause a significant increase in corrosion rate was also determined as a function of temperature and fitted an exponential temperature dependence. Preliminary results indicate that C-22 alloy base metal would remain a highly corrosion resistant, single-phase material for greater than 10,000 years at temperatures below approximately 300°C.

Tammy S. Edgecumbe Summers, Tien Shen, Raúl B. Rebak

Ageing Studies and Lifetime Extension of Polymer-Based Composite Rocket Motors Containing Ammonium Perchlorate Solids

Polymer based composite rocket motors containing ammonium perchlorate (AP) as the active ingredient are retrieved from the field at regular intervals for quality evaluation (QE) studies. During QE the motors are subjected to static firings. The ballistic performances of these aged motors are compared to the ballistic parameters originally obtained at zero time during lot acceptance tests (LAT). During the in-service time period, a number of factors can affect the ultimate ballistic performance of the rocket motor. Among the factors that can adversely affect the ultimate safe life of the rocket motor are moisture, temperature profiles, soluble impurities within the polymer matrix and motor vibration.In addition to the propellant, other components of the rocket motor such as o-rings can undergo chemical degradation thus shortening the life of the rocket motor.Several of the rocket motors from the manufactured lots that are undergoing LATs and QEs are dissected and the propellant characterized by scanning electron microscopy (SEM), energy dispersive X-ray microanalysis and mechanical properties tests. The analysis of the zero time and aged rocket motors are used to predict the service life of CTPB/AP rocket systems. These results will be discussed. Emphasis will be placed on the mechanisms leading to performance degradation.

S. M. Caulder, E. Hernandez

Battery Degradation and Ageing

Batteries are subject to degradation in storage due to a variety of chemical mechanisms, such as limited thermal stability of materials in storage, e.g. silver oxide in silver - zinc batteries, or corrosion of metal electrodes, e.g. lead in lead - acid batteries or lithium in lithium / thionyl chloride batteries. Battery performance can degrade during use, due to parasitic reactions, such as lithium metal / battery electrolyte reactions in hthium metal rechargeable batteries. Rates of degradation can be related to a number of factors, such as storage temperature or temperature variations. Battery standards require testing after various storage temperature regimes to detect this.The effect of degradation of performance can be estimated by real time storage measurements or by accelerated ageing at high temperatures. Other methods for estimation of degradation rates include thermal measurements (microcalorimetry). Causes of increased rates of battery degradation include inaccurate control of charging voltages, e.g. overcharging of lead - acid batteries will cause overheating and excessive loss of electrolyte through gassing. Maintenance of batteries is necessary to ensure good performance, e.g. complete discharge of nickel - cadmium batteries to avoid capacity loss due to the ‘memory effect’ or routine charging of lead - acid batteries to avoid capacity loss in storage due to sulphation (formation of unreactive lead sulphate in the battery plates).Batteries can be designed to avoid degradation in storage by use of reserve designs in which one component, usually the electrolyte, is omitted in manufacture and is then added just before use, e.g. in sea -water batteries, the magnesium anode would rapidly corrode so batteries are stored dry (without electrolyte) and the battery is activated by immersion in sea water.

A. G. Ritchie, B. Lakeman, P. Burr, P. Carter, P. N. Barnes, P. Bowles

Lessons Learned in Service Life Prediction

Service life of rocket motors has been of interest to the military. This is increasingly important today as there is a desire to extend the useable service of rocket motors beyond their initial design life. One important aspect of service life prediction is the ability to determine the age-life of a system from subscale testing. Subscale testing can include specimens from witness samples, motor dissections, or analog simulations. These attempts have met with moderate success. This presentation will discuss some of the lessons learned by the authors over the many years of tactical and strategic solid rocket motor aging and surveillance efforts. The discussion will be focused mainly on the solid rocket propellant and bondline interfaces of the rocket motor.

Edmund K. Liu, Richard K. McCamey

Development of Accelerated Life Testing Procedures for Solar Absorber Coatings

The oil crisis in the early seventies showed drastically the energy dependencies of countries with too few fossil energy resources. The exploration of alternatives yielded three options mainly: nuclear, renewable energy and energy saving techniques. Energy saving and renewables also have the advantage of sustainability and low environmental impact. The disadvantages of renewables are their fluctuations, requiring storage media, and their low power density (1 kW/m2 in maximum), hence their incompatibility with the traditional power grids. But their exploitation offers the possibility of a really sustainable energy supply in the future. This paper is dedicated to some aspects of solar-thermal energy conversion for low temperature heat used in domestic hot water, heating or cooling systems. Because of the low power density of solar irradiation, the energy has to be collected from an appropriate area by the so-called solar collector, shown in the schematic in figure 1. This device looks very simple at a first glance: The solar irradiation is absorbed and converted into heat by the absorber coating on top of a flat heat exchanger, where the heat is transferred to a fluid for transportation to thermal storage. The absorber is thermally isolated in order to reduce thermal losses. The glazing, acting as a convection barrier, is used for the isolation of the front side. Solar collectors in moderate climates need a high performance to become competitive with conventional heat sources.

M. Koehl, M. Heck, Bo Carlsson, K. Möller, U. Frei, S. Brunold

Methodologies for Service Life Predictions of Elastomeric Components in Oilfield Engineering

Oil and gas production is annually a multi-billion business worldwide, involving much hardware (oil rigs, drilling rigs and ships, well and wellhead equipment, plus numerous ancillaries). From a material viewpoint, the vast bulk used is metallic but, at the scale involved, the quantity of polymeric material employed is still significant - and the role of polymers in the harsh environments associated with gas/oil extraction is critical if acceptable service lives are to be achieved by the various components.

R. P. Campion

Lifetime of Polymer Pipes in Water Distribution Systems

This research was a contribution to an ongoing study by the Water Research Centre (WRc) and Anglian Water Services in their assessment of the life of pipes of different materials in water distribution networks. These networks have been constructed over a span of many years, employing improved pipe materials and jointing techniques as these became available. In the past, the pipe materials included, for example, cast and ductile iron, steel and reinforced cement. More recently, there has been a move to the use of polymer pipes, which have the advantage that they do not suffer from corrosion as metal pipes do and their inner surfaces remain clean. Most polymer pipes can be supplied in long lengths, which reduces the number of joints necessary and the pipes can be welded or a variety of mechanical jointing methods can be used. The lightweight and ease-of-handling of polymer pipes reduces the need for costly, heavy installation equipment. Also, the completed polymer pipe installation has a good hydraulic performance because the pipes have, and retain for a long life, smooth inner surfaces.

John P. Dear, Nick S. Mason

Lifetime Extension

Frontmatter

In-Service Degradation and Life Extension of Nuclear Reactor Vessels: Combining Experiments and Modelling

The reactor pressure vessel (RPV) is the primary containment in a commercial nuclear power plant and represents the first line of defense in the case of an accident. Thus, the structural integrity of the RPV, constructed of ferritic low-alloy steels, is of paramount importance to the overall safe operation of the plant. The RPV operates at relatively high pressure and temperature and is subjected to neutron irradiation. As part of an overall investigation of RPV aging, the Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory (ORNL), sponsored by the U.S. Nuclear Regulatory Commission, includes irradiation and testing of mechanical property and fracture toughness specimens, as well as a task to determine the microstructural basis for radiation-induced property changes in RPV steels. This modeling and microstructural characterization task aids in understanding and applying the results obtained from the experiments through the development of predictive models used in forecasting the aging behavior of RPVs. A series of experiments have been conducted over the past three decades to determine the effects of neutron irradiation on the fracture toughness and crack-arrest toughness of RPV steels with fracture toughness specimens up to 100 mm and 250 mm thick in the irradiated and unirradiated conditions, respectively. Furthermore, experiments have also been conducted to investigate the effects of thermal annealing on mitigation of the irradiation-induced embrittlement as well as on the effects of reirradiation. The modeling research focuses on the development of an improved description of primary damage formation in irradiated materials and the further development and use of predictive models of radiation-induced microstructural evolution and its impact on the mechanical behavior of RPV materials. Molecular dynamics cascade simulations have been extended to higher energies and temperatures and have revealed significant observations regarding defect survival, interstitial clustering dependencies and vacancy cluster formation. The microstructural characterization research focuses on the methods of atom probe-field ion microscopy (APFIM) and small-angle neutron scattering (SANS) to determine the matrix copper content and the chemical composition of radiation-induced precipitates. The new ORNL energy-compensated optical position-sensitive atom probe has been used for atom probe tomography (APT) studies of irradiated and thermally aged RPV welds. The results from SANS experiments are compared with those of the APFIM and APT investigations to provide detailed characterization of the features responsible for the observed changes in mechanical properties. Results from the modeling studies and the experiments are compared so that the models can be evaluated for modification and experiments can be planned with a view to providing the information needed for continued evolution of the models.

R. K. Nanstad, R. E. Stoller, M. K. Miller, M. A. Sokolov

The Treatment of the Brittle Fracture Boundary & the Statistical Analysis of Raw K1C Data From Tests on Irradiated Material from a Decommissioned Naval Reactor

The paper describes the future sampling and testing programme from a docked and de-fuelled nuclear submarine’s Reactor Pressure Vessel (RPV). The subsequent analysis of fracture toughness data is an important feature of this process and Bayesian statistical techniques for these analyses are presented and compared with an ASTM procedure.

P. R. Chard-Tuckey, J. E. Moorman

Service Extension Using Predictable Residual Stresses

The idea of bombarding the surface of a metal with small particles is nothing new; in the early part of the century the objective was to clean/prepare the surface of the material by “blasting”, but latterly the process of shot peening has shown that significantly improved material mechanical properties can be obtained. The two processes can seem similar, but are in fact very different, in terms of ultimate objective, control and application.

Jeremy P. Allen

Structural Optimisation With Damage Tolerance Constraints

This paper presents recent developments in the optimal design of structural components with fracture constraints. To minimise the computational effort it is suggested that an initial “near optimum” shape be used. Indeed, one approach could be to begin with the optimal shape for the non cracked geometry. This initial near optimum shape could then be used in conjunction with the alternating finite element method for multiple cracks, which builds on existing CAD based finite element models, and the resultant formulation then linked to available optimisation codes. This approach is illustrated by considering the problem of an optimum cut-out geometry for a rectangular plate subjected to a 4:1 and a 2:1 biaxial stress field. When considering the problem of shape optimisation with static fracture constraints, it was found that the optimal shape for the uncracked geometry was not the same as the optimal solution for the cracked problem. It was also found that for the optimal case, the stress intensity factors for same length cracks emanating at any arbitrary point around the hole were approximately constant along most of its circumference. This behaviour is intuitively expected for an optimised geometry where it all locations around the hole should be equally critical. The analysis also reveals that, as the geometry of the cut out or hole changes, the location of the crack that causes the maximum stress intensity factor can also change. This makes it necessary to consider flaws at a range of locations around the hole.

R. Jones, P. Chaperon, J. P. G. Sawyer

In Situ and Laboratory Studies of the Ageing of Protective Wax Coatings on Metal Surfaces of Museum Objects and Outdoor Statues

A variety of waxes have been used by conservators for the preservation of objects of cultural heritage significance in the custody of museums. These waxes are required to behave reversibly when applied to surfaces: they must be readily applied to surfaces, be easily removed, and be robust. Techniques used for the in situ and laboratory study of the degradation of waxy coatings were electrochemical impedance spectroscopy and Raman imaging microscopy. Ex situ techniques included electron microscopy (SEM & TEM) and atomic force microscopy. This paper reviews studies, in situ and ex situ, of changes in the integrity of a number of different wax coatings used by museum conservators. These include such commercial preparations as Besq 195, Cor-Trol 400, Dinitrol 4010 and their use in environments ranging from inside museums to external sites both inland and by the seaside.

V. Otieno-Alego, D. Creagh, D. Hallam, A. Viduka, G. Heath

Accelerated Ageing of Pyrotechnics: Evaluation of Pyrotechnic Degradation

The ageing of pyrotechnics along with degradation of compositions is a problem which has been recognised for many years; however, it is an area which has received little attention. In-service lives of many pyrotechnic stores and components are some of the shortest of all types of energetic materials: even when taking safety considerations into account, these are often at best conservative. The implications of this are that the general policy is to replace stores at given ages irrespective of their state of deterioration or to use them for training rather than operational use. With the ever-increasing demands to cut costs, these replacement policies are now questionable. The initial in-service lives of UK munitions are centred on Ordnance Board assessments, which are often based upon in-service proof testing and in-service surveillance programmes. Initial in-service lives of munitions are being extended following life extension trials, which include subjecting the stores to accelerated ageing using conditions such as diurnal temperature and humidity cycling, followed by mechanical stressing such as shock and vibration. Evaluation of the degree and levels of pyrotechnic composition degradation is usually based on proof fire testing of complete stores to ensure that performance specification requirements are attained. Although from this it may be possible to ascertain which performance parameter is the most likely to fail or terminate the in-service life of the pyrotechnic component, there is little or no correlation between chemical degradation or stability of the pyrotechnic compositions and proof performance results.This paper details the analytical techniques employed at DERA for evaluation of chemical changes in pyrotechnic compositions and procedures employed to determine performance characteristics of complete pyrotechnic stores and individual compositions/components, prior to and following accelerated ageing.

G. Manton

Material Design for Ageing

Frontmatter

Irradiation Induced Ageing of Optical Glasses and Glass Ceramics

An overview is given on how irradiation induced compaction is affecting the performance of optical precision components in space applications and in inicrolithography. Low expansion materials used as mirror substrates in space are compacted in a thin layer when exposed to space radiation, As a consequence, the mirrors bend; although theses effects are small they have to be taken into account in applications with very high performance requirements as e.g. in the SILEX-telescope.Similarly, fused silica used in deep-UV-microlithography compacts under the influence of excimer-laser irradiation. The compaction can be well described by a simple law.

Wolfgang Pannhorst

Improvements of Gamma-Induced Transmission Damage in Silica Fibers in the Deep UV-Region

UV-improved all-silica fibres with higher solarization-resistance against UV-light have been well known for three years; mainly the most inconvenient UV-absorption band around 215 nm has been reduced by passivating the defects. In addition, ionizing radiation such as gamma-radiation or X-rays can create similar defects, as shown in bulk samples of undoped silica having high-OH content; however two absorption bands around 215 nm and 230 nm with a similar defect structure are clearly observed. Using the same passivation mechanism, an improved UV-transmission below 250 nm is observed in fiber samples up to 5 m in length treated with Gamma-irradiation. During and after the damage, annealing processes with and without UV-light have been undertaken and studied for the first time. Because there is an acceptable transmission in the UV-region above 200 nm, the light of the commonly-used excimer-lasers and deuterium-lamps can be transmitted during Gamma-irradiation. Based on these results, the standard process of gamma-sterilisation using approx. 2 Mrad can be used for these fibres.

J. Assmus, K. F. Klein, G. Nelson, J. Clarkin, F. Gutermuth, J. Kiefer

Chemical Durability of Candidate Wasteforms for the Immobilisation of Chloride-Containing Intermediate Level Radioactive Wastes

Details of our work on the development and characterisation of candidates for the immobilisation of chloride-containing intermediate level radioactive wastes are presented and discussed. Particular attention is given to chemical durability, this being one of the main factors which will determine the lifetime behaviour of these systems. It is shown that a glass-encapsulated calcium phosphate ceramic shows particular promise as a candidate wasteform for these chloride-containing wastes.

I. W. Donald, B. L. Metcalfe, M. E. Brenchley, R. S. Greedharee

Interfacial Reactions and their Influence on the Lifetime Behaviour of Glass-Ceramic-to-Metal Seals and Coatings

An overview is given covering glass-ceramic-to-metal seal and coating systems that have been reported in the literature, including a number of systems studied at AWE. Factors influencing the lifetime behaviour of these systems are reviewed and discussed, with particular emphasis given to the effect that interfacial reactions can have on the resultant and longer term properties. Steps that can be taken to enhance desirable interfacial reactions and minimise those that are undesirable are highlighted, thus emphasising the concept of preventative ageing through careful control of the interfacial chemistry and microstructure. Systems covered include lithium zinc silicate and lithium aluminosilicate glass-ceramic compositions bonded to stainless steels, high alloy steels, and nickel-based superalloys. Alternative glass-ceramic compositions bonded to a variety of metals including copper, molybdenum, titanium and tantalum are also reviewed.

I. W. Donald

Mechanical Property Degradation Induced by Elevated Temperature Environmental Attack in MCrAlY Coatings of Gas Turbine Blades

This paper presents the influence of in-service environmental attack on the mechanical properties and microstructure/composition of plasma sprayed MCrAlY coatings over Ni based superalloy substrates in gas turbine (GT) blades using a small punch (SP) testing technique and scanning Auger microprobe analysis. SP tests on near surface CoNiCrAlY coatings demonstrated strong dependence of mechanical degradation on the elevated temperature environmental condition. In-service operation under the combined fuels of kerosene and liquefied natural gas (LNG) led to a two-fold increase in the ductile-brittle transition temperature over coatings observed mainly under LNG because of more extensive oxidation and grain boundary sulfidation. In CoCrAlY coatings of GT blades operated at higher temperatures than the CoNiCrAlY ones under LNG fuels, substantial oxidation/carbonization and nitridation occurred in near surface and interface regions of concave coatings, respectively, but not in convex coatings. Brittle cracks in the near surface and interface of concave coatings more easily initiated up to 950 °C than in the convex coatings. It was found that the oxidation/carbonization and nitridation in the concave CoCrAlY coatings produced a greater ductility loss than the oxidation/sulfidation in the CoNiCrAlY coatings.

J. Kameda, T. E. Bloomer, Y. Sugita, A. Ito, S. Sakurai

Vulcanized Rubber-Based Composite Materials with Temperature/Pressure Sensitivity

The mechanical properties of a vulcanized rubber based composite material obtained through an homogeneous blend of polynorbornene and EPDM, with a Tg of about 0 °C, were evaluated at temperatures between −0 °C to 20 °C. In compression tests the material exhibited elastomeric behaviour with a degree of deformation of 62% under a stress of 100 kgf/cm2 at room temperature and rigid behaviour with a degree of deformation of 1.83% at −10 °C. The Shore A hardness (Hs) was 57 at 20 °C and 87 at −5 °C. The elongation in the tensile test was 480% in the elastomeric state (20 °C) and was kept to 300% even in the rigid state (−5 °C). The temperature/pressure sensitive material undergoes change in its state depending upon temperature and pressure. Spikes made from the material and used for tires and shoes exhibited a satisfactory degree of traction on icy surfaces.

H. Yamada, M. Ueki, Y. Miyano, H. Yoshida

Backmatter

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