Nonmetallic Materials and Composites at Low Temperatures

Frontmatter

Thermal Expansion of Non-Metals

Abstract

The thermal expansion of a solid is determined by two factors, namely the internal stress generated by temperature change and the elastic stiffness. In many non-metallic crystals both these factors show considerable anisotropy which leads to expansion coefficients and associated Grüneisen parameters varying greatly with temperature and from one material to another. Experimental data extending down to 2 K are available for many materials of the common crystal structures such as rock-salt, fluorite, zincblende, rutile, caesium chloride, rare gas solids (f.c.c). Data are scarce for most anisotropic crystals, mixed systems and composites. A brief review is given of the available experimental data for non-metallic crystals and some glasses and polymers.

Guy K. White

The Low Temperature Thermal Conductivity of Rubber

Abstract

The low temperature thermal conductivity, κ, of nitrile and natural rubber has been measured, together with the influences on κ of stretching and ‘filling’ with Carbon Black. These new results, as well as those presented previously, are used in an attempt to establish a qualitative relationship between the observed properties and polymer structure.

J. J. Freeman, D. Greig

The Thermal Conductivity of Polymers Below 1K

Abstract

It has been well-established for a number of years that the thermal conductivity, κ, of all amorphous materials — including amorphous polymers — exhibits “universal” behaviour below 1K, falling as T² with magnitudes that are very similar between one material and the next. For semicrystalline polymers, on the other hand, the behaviour is quite different. At 1K the values of κ are an order of magnitude lower than in amorphous materials, with a temperature dependence in which κ is proportional to T. In this report we summarise recent measurements of κ below 1K, with emphasis on the effects of (i) changes in the degree of crystallinity and (ii) extrusion. Very generally κ tends to the universal T² dependence at very low temperatures (below 100mK), and when either the crystallinity is small or the extrusion ratio is large. We shall also report on measurements on a single crystal polymer, polydiacetylene, in which, as expected for a crystalline solid, the temperature dependence of κ approaches T³.

D. Greig, M. Sahota

Mechanical Properties of Three Candidate Organic Insulator Materials for Fusion Reactors

Abstract

Insulators in superconducting magnets for fusion reactors will be exposed to radiation doses of 10 to 100 Mgy with more than half the dose coming from fast neutrons. One polyimide- and two epoxy-based laminates* were neutron irradiated to total doses of 2.6 and 4.1 × 10²¹ n/m², E>0.1 MeV at 4.2 K in the Intense Pulsed Neutron Source at Argonne National Laboratory. Flexural properties were determined at 75 K and compared to results from previous studies of gamma irradiation effects at Oak Ridge National Laboratory.

D. S. Tucker, G. F. Hurley, J. C. Kennedy

A New Way to Calculate the Fracture Toughness of Mouldings on Impact Load in Comparison with Linear Fracture Mechanics

Abstract

The tests for determining fracture data which are described in literature often deal with the methods of linear-elastic fracture mechanics (LEFM). The critical stress intensity factor K_IC can only be determined with unusually large material thicknesses and, in addition to that, is strongly dependent on temperature and time. Therefore, great problems are involved in transferring the LEFM-data to thin-wall mouldings. Even more difficulties are encountered in the case of multilayer fiber-reinforced thermoset mouldings, where the LEFM only yields efficient results if cracks propagate between the layers in the resin.

In recent years, the Institut für Kunststoffverarbeitung (IKV) have determined material characteristics in special tensile impact tests. These data — the minimum fracture strain and the minimum volume-specific work to fracture — represent real minima, which are not exceeded even in cases of fastest loading and lowest temperatures. Thus, they are suited for a safe dimensioning against impact. It was proven that the data for mouldings exposed to bending impact can also be safely calculated. Among others a method proposed by Oberbach is used which allows for geometrical influences (direction and location of impact) with the aid of a factor that can be determined during static load or by a FEM-calculation. It is expected that this new way of calculating can also be applied to more complicated mouldings. Relevant tests are being prepared.

Hartmut Rest, Georg Menges

Craze Formation and Cryogenic Toughness

Abstract

Crazing is the predominant plastic deformation mechanism in thermoplastics preceeding brittle fracture. The energies needed to form and to stretch crazes up to their break also mainly determine the fracture energy. Using the fracture mechanics approach together with the Dugdale type of plastic zone the single craze at a crack tip can be well characterized as may be seen from experimental investigations using the microscopic optical interference technique. At low temperatures the good deformation behaviour of PC is associated with the crazing mechanism.

W. Döll, L. Könczöl, M. G. Schinker

A Simple Device for Short Time Cryogenic Tests and its Application in High Velocity Tensile Measurements

Abstract

One undesired property of polymers is their brittleness at low temperatures. Only some linear polymers, e.g. PC, PSU or PE show some ductility even at 4.2 K. On the basis of previous experience it is expected that brittleness increases with increasing deformation rate. Investigations¹ of the strain rate dependence on some selected polymers at 77 K showed that only up to a certain specific strain rate the fracture strain ε_F decreases. At higher strain rates the fracture stress and strain increase remarkably. An explanation of this behavior is given.

K. Pöhlmann, A. Heneka

Apparatus Permitting Rapid Consecutive Flexural Testing at 4.2 K

Abstract

Testing materials at low temperatures is a time consuming and expensive procedure. For this reason, it is understandable that many research programs are limited in scope, both in the number of replicates, and in the number of materials that are examined in detail.

D. Evans, R. Luckock, J. T. Morgan

Radiation Effects on Polybutyleneterephthalate Resins at Low Temperature

Abstract

The mechanical properties of irradiated polybutyleneterephthalate (PBT) resins were measured by means of flexural tests. The specimens irradiated at ambient temperature showed no appreciable change both in flexural strength and in flexural modulus. When the specimens were irradiated at 20 K and warmed up to 77 K after irradiation, only a slight mechanical change occurred in mica flake reinforced PBT, whereas remarkable reduction of flexural strength was observed in glass fiber reinforced PBT. Observation on fracture surface in irradiated glass fiber reinforced PBT with scanning electron microscope revealed a marked debonding in the matrix-filler interface.

Hitoshi Yamaoka, Kiyomi Miyata, Yoshio Nakayama, Hiroyuki Yoshida

Low Temperature Properties Probed by Selective Laser-Excitation

Abstract

A sensitive probe for the analysis of low temperature properties of nonmetallic materials is provided by persistent spectral hole burning. The microscopic probe is based on narrow holes in the spectra of optical centers in solids, e.g. dye molecules in polymers or dyes adsorbed at crystal surfaces. The investigations concern thermal, electrical and mechanical properties of amorphous and crystalline materials. In the case of thermal properties the method is combined with heat-pulse technique and provides information about thermal conductivity and phonon processes in the double-well potentials of amorphous solids. Electric field effects are also studied, demonstrating applications in optical data storage.

U. Bogner

Optical Investigation of Pressure-Induced Matrix Changes in Polymers under Low Temeprature Conditions

Abstract

Photochemical hole burning (PHB) is a laser spectroscopy method yielding high optical resolution at temperatures below 30 K. The method utilizes dye molecules embedded in polymer hosts to detect minute matrix changes resulting in optical changes of linewidths and line positions of as little as 10⁻³ Å. Using the dye molecule phthalocyanine in the polymer matrices polyethylene (PE) and polymethylmethacrylate (PMMA), it could be shown, that pressure changes of as little as 100 hPa lead to appreciable broadening of the line profile. At higher pressures up to 3 ·10⁴ hPa we observed irreversible line broadening phenomena, which we attribute to irreversible site changes of the host-guest system. Temperature-induced line broadening will be compared to the pressure data.

W. Richter, G. Schulte, D. Haarer

Low Frequency Measurements on Polymethylmethacrylate

Abstract

Variations of the sound velocity and the internal friction have been measured in commercial polymethylmethacrylate from 0.3 K to 290 K at frequencies below 1 kHz using a vibrating reed technique. Below 6 K the polymer shows the acoustic properties known from inorganic glasses: a temperature independent plateau in the absorption and a logarithmic variation of the sound velocity with temperature. The results at low temperatures are discussed within the framework of the Tunneling Model of glasses.

N. Geis, G. Kasper, S. Hunklinger

Very Low Frequency Loss Measurements in Glasses

Abstract

At low temperatures the thermal, elastic and dielectric properties of glasses are known to be determined by low-energy excitations which seem to be intrinsic to the glassy state /1/. Recently measurements of the time dependence of the specific heat /2,3,4/ have shown that some of these excitations relax very slowly into thermal equilibrium. Such a behaviour, especially a logarithmic time dependence of the specific heat, is indeed predicted by the tunneling model /5,6/ which until now was most successful in explaining these low-temperature anomalies. A quantitative analysis of these time-dependent thermal measurements, however, is very difficult and does not allow to unambiguously confirm the predictions of the tunneling model.

Manfred von Schickfus, Herbert Tietje, Eberhard Gmelin

Mechanical and Dielectric Losses of Polymers in the Temperature Range of 5K to 293K

Abstract

Technical applications and scientific research are main reasons for studying losses of polymers at low temperatures.

G. Hartwig, G. Schwarz

Mechanical Properties of Unidirectionally Reinforced Materials

Abstract

Mechanical properties of unidirectionally reinforced composite materials with the systematic variation of reinforcement and matrix have been studied aiming at standardization of materials, testing methods and developing advanced materials. Mechanical properties on identical materials were measured at three different laboratories independently and the results were compared each other.

The variation of mechanical properties originated from reinforcement, epoxy matrix and the deviation of data caused by different experimental method is discussed. The possibility of standardization and developement of advanced materials are also discussed.

Toichi Okada, Shigehiro Nishijima, Hitoshi Yamaoka, Kiyomi Miyata, Yasuhiro Tsuchida, Koichi Mizobuchi, Yasuo Kuraoka, Shingo Namba

Dynamic Young’s Modulus and Internal Friction in Composite Materials

Abstract

The strain amplitude dependence of Dynamic Young’s modulus and internal friction have been studied on variously fatigued glass cloth reinforced epoxies and was compared with laser transmittance. The temperature dependence was also measured in temperature range from 100 to 380 K.

The strain amplitude dependence in Young’s modulus and internal friction come to notable as the fatigue progresses. The close correlation was revealed between laser transmittance and strain amplitude dependence. The degree of interface fracture could be estimated by strain amplitude dependence. The different temperature dependence of internal friction were found in different FRP of which Young’s modulus are just same at room temperature, because the temperature dependence of internal friction mainly reflects the behavior of matrix in FRP.

Shigehiro Nishijima, Kenichi Matsushita, Toichi Okada, Taira Okamoto, Takeshi Hagihara

Low Temperature Ductile Matrices for Advanced Fiber Composites

Abstract

Fiber composites are attractive alternatives to metals because of their high specific strength or stiffness or their excellent fatigue behavior. They are a necessary supplement to metals because of their low electrical and thermal conductivities, the latter being related to strength or stiffness¹. Their disadvantage arises from the weak polymeric matrix and results in a low interlaminar shear strength and a low transverse strength. At low temperatures the majority of currently used matrices are brittle and they do not permit relaxation of residual stresses and stress concentrations to occur. In the course of cooling the different thermal contractions of fiber and matrix give rise to thermal residual stresses and strains which influence most of the mechanical properties.²

G. Hartwig

Fabrication of Carbon Fibre Reinforced Thermoplastics Polycarbonate and Polysulfone

Abstract

In recent years the market for advanced composite preparation was oversupplied by thermoplastics as a new group of matrix systems.^1–4 Within these polymers polycarbonate PC and polysulfone PSU seem to be most suitable for carbon fibre reinforced thermoplastics at cryogenic temperature application because of their highest tensile strains up to 4.2 K.⁵ In this temperature range sufficient adhesion between carbon fibre and thermoplastic matrix is only achieved if the contamination of the active carbon fibre surface by water adsorption is hindered. A preparation method is presented that offers the possibility to form composites with thermoplastics PC and PSU by prepreg technique. Controlling quality parameters like pressure and press temperature are discussed with respect to the properties of the carbon fibre reinforced thermoplastic composites.

E. Fitzer, G. Hannes, H. Jäger

Interlaminar Shear Strength of Carbon-Fibre Reinforced Thermoplastics Polycarbonate and Polysulfone

Abstract

The interlaminar shear strength (ILSS) of carbon-fibre reinforced polymers (CFRP) at room temperature (RT) depends on:

a)

properties of the reinforcing fibre

 

b)

properties of the pure matrix system

 

c)

interaction between fibre surface and matrix, caused by active surface groups

At cryogenic temperatures two effects are superposed:

a)

increasing matrix strength

 

b)

“shrink-on” of the matrix on the fibre because of the strong matrix contraction by cooling

 

 

Both effects can cause improved mechanical composite properties. Negative side-effect of this shrinkage is the built-up of enormous thermal residual stresses at the fibre-matrix interface and therfore a more brittle fracture behaviour. Matrix systems having some low temperature ductility and high cryogenic fracture strain are necessary to overcome these problems.

Thermoplastic materials like polycarbonate (PC), polysulfone (PSU) and polyethersulfone (PES) seem to be promising candidate matrix materials.

The ILSS values, measured by short beam test on PC and PSU reinforced with HM- and HT-fibres at RT, 77K and 4.2 K are presented and discussed. A comparison is given of the different fracture behaviour of carbon fibre reinforced epoxies, polycarbonate and polysulfone.

G. Hartwig, H. Jäger, S. Knaak

Acoustic Emission and Interlaminar Shear Strength

Abstract

Acoustic emission from glass cloth reinforced epoxy during interlaminar shear test has been studied in order to clarify the interlaminar properties. At liquid nitrogen temperature the serration was found on load-displacement curves which corresponds with step wise AE increase. At room temperature the macroscopic yielding exist on load-displacement curve which brought the sudden increase of AE energy rate. The detection of interlaminar failure and the estimation of its degree has become possible using AE technique.

Shigehiro Nishijima, Toichi Okada, Shingo Namba

Mechanical Properties of Low Thermal Contraction GFRP

Abstract

Liquid helium duct spacers in large superconducting magnets must have high compressive strength to withstand large electromagnetic forces. They must also show a low thermal contraction, in the direction perpendicular to the laminations, to maintain windings stability at liquid helium temperature. To attain higher glass content or higher density, we adopted glass roving cloth with mineral fillers and unidirectional woven glass cloths. E-,S-, and R-glass cloths were examined as reinforcements and epoxy and bismaleimide triazine, as matrix resins.

The thermal contraction of GFRP with unidirectional woven cloths (cross ply) was nearly equal to that of stainless steel; compressive strength at cryogenic temperature was nearly 1200–1300 MPa. GFRP with glass roving cloths and fillers also showed low thermal contraction, however, their compressive strength was nearly 800–900 MPa.

K. Fukushi, M. Nagai, Y. Kamata, K. Kadotani

Adhesive Properties at Low Temperature of Epoxy Resin Pre-Impregnated Tape

Abstract

The work reported here was carried out as part of an investigation into the feasibility of building a large superconducting solenoid (approx. 5m diameter) using epoxide resin pre-impregnated glass fabric as coil insulant and bonding material.

D. Evans, J. T. Morgan

MDF Cements for Low Temperature Applications

Abstract

A pilot study to investigate some low temperature properties of macro-defect free cements has been carried out. That based on Ordinary Portland Cement was initially unsuitable for use at cryogenic temperatures due to the development of thermal cracks on cooling. However, the addition of a latex enables strength to be maintained down to 4.2K. That made from High Alumina Cement is also found to be suitable for use down to 4.2K. Both types are non-porous, no leaks being detectable through a 2mm thick sheet when using a helium leak detector. Thermal contraction data has also been obtained.

Possible cryogenic applications include components for superconducting magnets, vacuum equipment, and containers for refrigerants.

B. A. Hands, A. J. Smith, G. W. Groves, D. D. Double

Present and Future Application of Nonmetallic Materials in Cryogenic Technology: Summary of the Panel Discussion

Abstract

This paper was prepared in order to facilitate the course of a panel discussion on the above topic. For simplicity the discussion is restricted to polymers — pure polymers, powder filled polymers and fibre reinforced polymers. Because of their outstanding properties, fibre reinforced polymers especially play an important role in cryogenic technology. In future this importance will further increase in accordance with the progress of knowledge in this field and the growing application of cryogenic technologies. Special attention is directed to the application of superconductivity whereas other application, like space research and cryogenic wind tunnels are not considered.

G. Bogner

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