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Über dieses Buch

Structures that are essential for economy and security such as energy production, transportation and supply, water supply, buildings, are susceptible to failure, because of defects already present in the material, or created at fabrication, or appearing during service. Methods of assesment of the nocivity of these defects are needed, to predict the remaining service life and the eventual emergency of stopping service and repairing, if possible. To reach this objectives, this book presents the last methods derived from the classical linear, non-linear fracture mechanics concepts, including fatigue and notch fracture mechanics. Several examples of structures rehabilitations and repairing are given.

This book gathers the presentation made during the Advanced Research Workshop held in Portoroz (Slovenia) in October 2008, under the auspices of NATO Science for Peace and Security Programme. It is edited by Professor Guy Pluvinage from the University Paul Verlaine – Metz (France) and Professor Aleksandar Sedmak from the University of Belgrade, Faculty of Mechanical Engineering. Both have a long and rich experience in analysis of theoretical and practical cases in safety and reliability of structures. Other contributors are all known as experts in the areas of fatigue, facture and reliability of structures.

Inhaltsverzeichnis

Frontmatter

Experimental and Numerical Aspects of Structure Integrity

The paper presents an approach in solving the problem of fracture mechanics treatment of surface cracks, which are a typical source for the structural failure. Difficulties associated with predicting structural integrity is that the surface cracks are three dimensional, whereas fracture mechanics methods using characterization parameters as K

Ic

, J

Ic

, COD are derived from two-dimensional assumptions. The method proposed for the solution is based on consideration of the elasto-plastic stress—strain behavior independent on the collapse conditions. For the investigation and development of the method, systematic numerical and experimental investigations have been performed. The main goal of the verification part of the tests' program was to justify the proposed method and to demonstrate the transferability of data, i.e. use of measurements taken from simple test specimens for a prediction of failure in large and/or complex structural components. The agreement between the prediction and test results using proposed procedure is very good. It has been shown that the proposed method gives more accurate prediction of verification test results and is less conservative than the current methods. In all cases where unnecessary conservatism is undesirable this method is more advantageous.

Petar Agatonović

Fatigue Failure Risk Assessment in Load Carrying Components

A methodology is presented for integrating probabilistic fracture mechanics (PFM) with quantitative non-destructive inspection (NDI) for the purpose of failure risk assessment in load-carrying elements, mainly, under cycling loading. The definition of the failure risk in structural components is made in the context of the general approach of structural reliability with highlights on the sources of uncertainties and variability encountered in the analysis. The quality of NDI is accounted by the probability of detection (POD), as function of the flaw size. The main focus in the presentation is placed on the fatigue failure risk assessment in conjunction with the quality and timing of the envisaged NDI. The management of failure risk in aerospace technology is exemplified in the framework of established philosophies known as fail-safe and damage tolerance approach. Fail-safe or total life (TL) approach is outlined by the probabilistic assessment of the fatigue life of landing gears components under realistic loading spectra encountered by a combat aircraft. A parametric analysis of the interplay between the iso-probable fatigue life and deterministic safety factors, as applied to the mean life, is outlined. Damage-tolerance (DT) approach to structural safety, having at the core fracture mechanics technology, is presented in the framework of probabilistic paradigm with its inter-relation with variability and uncertainty associated with non-destructive inspection practice. On the base of Monte Carlo computer simulation, a rationale have been developed encompassing the fatigue crack growth (FCG) both under short- and long-crack regime, thus addressing the entire fatigue life. For this purpose, the concepts of “initial fictitious crack” size and “equivalent initial flaw” size are discussed for the purpose of the implementation in the FCG analysis. By computer simulation, it is exemplified the FCG in an aluminum alloy of class 2024 T, for a coupon with central hole. The scatter of the total fatigue life and the crack size at a given life in the simulation follows from the probabilistic input of material strength characteristics (ultimate tensile strength, yield point and fracture toughness) as well as from probabilistic FCG parameters entering a Paris—Klensil type relationship. Further, by fitting the simulated data into continuous statistical distributions enabled to model the key distributions involved in the fatigue failure risk assessment. Finally, the probability of failure, at a specific timing during the fatigue life, is estimated by Monte Carlo massive simulation of the final failure on the base of Failure Analysis Diagram (FAD) approach. It is demonstrated the benefit of applying a non-destructive inspection technique qualified by a specific probability of detection (POD) dependence on the crack size. Quantitatively, the decrease of failure risk is evinced in terms of failure probability. Computer re-sampling simulation known as “bootstrap” technique has been applied for constructing confidence intervals on POD vs. crack size in order to use in the risk analysis safe bounds of POD vs. crack size which, otherwise, are established, merely, on expert bases. It is also discussed the integration of quantitative NDI with probabilistic fracture mechanics, from the perspective of assessing a better timing and capability ranking of non-destructive inspection procedures.

Ing. Dragos D. Cioclov

Assessment of Localised Corrosion Damaging of Welded Dissimilar Pipes

The procedure of localised corrosion damaging assessment on internal surface of welded dissimilar pipes is developed. Method is based on numerical-analytic model of dissimilar welded joint as three-electrode electrochemical system with using of the fundamental electrochemical parameters of composing materials, which were received by standard potentiometric methods. The data on general characterisation and maximal depth of localised damaging on internal surface of pipes are presented and analysed with dependence of term exploitation and composition of operating environment.

Ihor Dmytrakh, Volodymyr Panasyuk

Procedures For Structure Integrity Assessment

Quite few analytical flow assessments methods as specific standard and guidelines there have been developed in recent years. Today, as one of a most comprehensive assessment procedure is SINTAP — Structural INTegrity Assessment Procedure. The SINTAP introduced the basic principles of R6 (rev. 4) and ETM. SINTAP procedure is possible to performed assessment for inhomogeneous configurations such as strength mis-matched weldments and an effect of residual stresses. Nevertheless, the SINTAP procedure take into account temperature transition region from ductile-to-brittle behavior of material. In the paper the SINTAP procedure was applied to the failure analysis of cracked component. By results of used example was shown that SINTAP procedure gives reliable conservative results where the safe factor decreasing by increasing the quality of input data.

Nenad Gubeljak, Jožef Predan

Experience in Non Destructive Testing of Process Equipment

The paper gives an overview of common non-destructive methods used in-service and during shut-down of the plant. Their choice is dependent on the aim of the testing, component of process equipment and material. Ultrasonic testing is used as the method for materials evaluation, too.

Jano Kurai, Miodrag Kirić

Crack—Interface Interaction in Composite Materials

The presence of cracks has a major impact on the reliability of advanced materials, like fiber or particle reinforced composites or laminated composites. This paper presents different aspects of the interaction between crack and interface: stress field and fracture parameters for a crack approaching the interface and the crack deflection versus penetration for a crack with the tip on the interface. Different material combinations and mixed mode loads were considered by using a bi-axial bi-material specimen. The stress filed and fracture parameters were obtained numerically using the finite element method.

Liviu Marsavina, Tomasz Sadowski

Measurement of the Resistance to Fracture Emanating from Scratches in Gas Pipes Using Non-Standard Curved Specimens

The experimental procedure for measurement of the notch fracture toughness of the API 5L X52 steel using a non-standard curved specimen with a notch simulating the expected scratch damage of gas pipelines has been developed.

The concept of the notch stress intensity factor based on the volumetric method has been employed and the corresponding notch fracture toughness

K

ρ,c

was obtained. In this case, the notch fracture toughness was calculated using the critical load determined by acoustic emission and the results of finite element analysis of the elastic-plastic stress distribution ahead of the notch tip. The notch fracture toughness

J

ρ,c

in terms of the J-integral has been estimated by means of the load separation method which allows measuring the η-factor. In this case, the original representation of the J-integral as total energy release rate and the tests records have been used. The proposed procedure allows avoiding calculation of the stress intensity factor for non-standard specimens to determine the notch fracture toughness

J

ρ,c

.

Development of the present methodology encourages replacing the conservative crack-like defects approach. The results on the notch fracture toughness of the API 5L X52 steel can be used in the modified failure assessment diagram for structural integrity assessment of gas pipelines damaged by scratches.

J. Capelle, J. Gilgert, Yu. G. Matvienko, G. Pluvinage

Safe and Reliable Design Methods for Metallic Components and Structures Design Methods

Tools available for structure design are various and are different according to the fact that the risk is associated with an eventual presence of defect and possibility of brittle fracture or a ductile failure. The following design tools are described: (1) allowable stress and safety factor, (2) linear fracture mechanics and safety factors, (3) crack driving force, (4) failure assessment diagram, (5) allowable strain, (6) critical gross strain, (7) strain based design.

G. Pluvinage

Development and Application of Crack Parameters

The development, definition and implementation of crack parameters (stress intensity factor, crack tip open displacement, J integral, final stretch zone) are presented. They are now used for structural integrity and residual life assessment and fracture analysis of cracked components in elastic and plastic range, what is illustrated by typical examples.

Aleksandar Sedmak, Ljubica Milović, Jasmina Lozanović

Welded Joints Behaviour in Service with Special Reference to Pressure Equipment

Quality and structural integrity of welded pressure equipment are defined by The Pressure Equipment Directive (97/23/EC). The problem is complex, due to welded joint imperfections, matching effect, heterogeneity of microstructure and material properties. Equipment can be fit for service with defects unacceptable by codes, what should be proved by fracture mechanics approach, especially for cracks in the heat-affected-zone.

Stojan Sedmak, Katarina Gerić, Zijah Burzić, Vencislav Grabulov, Radomir Jovičić

Security of Gas Pipelines

Security of the natural gas supply strongly depends on the integrity of the transportation pipelines. The statistical evidence shows that the most probable cause of the break occurrence at natural gas pipeline is the external third party interference. A potential damage to the surrounding objects and violation of the people lives during the pipeline accident depends on the mass flow rate of the natural gas leakage from the break. The paper presents an efficient method for the prediction of the natural gas leakage rates in case of pipeline accidents, as well as for the prediction of transient gas dynamic forces that are generated in case of an unsteady fluid flow and a fluid discharge from the pressurized volume to the surrounding. The method application is demonstrated on the test cases of natural gas outflow from a high pressure main transportation gas pipeline and a break occurrence at the distribution pipeline. Obtained data are a necessary input to the safe design of pipeline structures and supports.

Vladimir Stevanović

Safety, Reliability and Risk. Engineering and Economical Aspects

The risk-based approach (RBA) of the safety is able to create an equilibrium situation between the safety level and the investment value, i.e. RBA is only the tool for discussion between engineers and economists in safety issue. This will be illustrated through risk-based inspection (RBI) and reliability centred maintenance (RCM) in petrochemical industry.

L. Tóth, Lenkey Biro Gy

Assessing the Development of Fatigue Cracks: From Griffith Fundamentals to the Latest Applications in Fracture Mechanics

Engineering analysis of fatigue crack growth depend on different loading conditions, wherever those conditions have been studied, in laboratory or real-world practice. Such analyses can be done on basis of different parameters: the stress intensity factor range, ֵ

K

, introduced in linear elastic fracture mechanics; the

J

-integral range, ֵ

J

, employed in elastic-plastic material characterization; the square-root area parameter of Murakami, effective in the presence of small defects and non-metallic inclusions. An alternative presentation of fatigue data has been proposed that uses the crack growth rate against a newly introduced parameter, namely an energy fatigue-function ֵ

W

based at different conditions on different parameters, ֵ

K

or ֵ

J

or the square-root area parameter of Murakami. This alternative presentation shows fatigue data as forming an almost straight line, which may be termed the “natural fatigue tendency” of a material, and specified more precisely at a given stress range. Also the present study introduces a physical interpretation of the line presentation of fatigue data and some illustrations of the “natural fatigue tendency” for different materials under different conditions.

Donka Angelova

Evaluation of Service Security of Steel Structures

The verification of existing steel structures especially steel bridges is in present one of the main problems of the structural engineers. The majority of existing railway steel bridges that have been built at the turn of the last century are riveted structures. Today many of these structures have already achieved a considerably age; therefore the establishment of the remaining fatigue safety of these structures is one of the most important tasks of contemporary society. Many of these bridges are still in operation after damages, several phases of repair and strengthening. The problem of these structures is the assessment of the present safety for modern traffic loads and the remaining service life. Along with the classical method of damage accumulation, a new approach based on the fracture mechanics principles is proposed. The paper presents the Romanian Methodology in this field with some case studies.

Edward Petzek, Radu Băncila

Fatigue Design of Notched Components by a Multiscale Approach Based on Shakedown

This paper shows that fatigue limit of notched specimens under cyclic loading can be simply and accurately estimated by using elastic-plastic computations and averaging stress over a critical volume obtained by an optimisation process minimizing the dispersion between experiments and simulations. The fatigue limit criterion considered is the Dang Van one. Fatigue tests (tension-compression, bending and torsion) carried out by the Cetim, are used to calibrate the critical volume.

K. Dang Van, H. M. Maitournan, J. F. Flavenot

Measurements for Mechanical Reliability of Thin Films

This paper reviews techniques for measurement of basic mechanical properties of thin films. Emphasis is placed on the adaptations needed to prepare, handle, and characterize thin films, and on adaptations of fracture mechanics for adhesion strength. The paper also describes a recent development, the use of electrical current as a controlled means of applying thermo-mechanical stresses to electrical conductors to characterize their fatigue behavior.

David T. Read, Alex A. Volinsky

From Macro to Meso and Nano Material Failure. Quantized Cohesive Model for Fractal Cracks

A discretization procedure for the cohesive model of a fractal crack requires that all pertinent entities describing the influence of the cohesive stress that restrains opening of the crack, such as effective stress intensity factor, the modulus of cohesion, extent of the end zone and the opening displacement within the high-strain region adjacent to the crack tip are re-visited and replaced by certain averages over a finite length referred to as either “unit step growth” or “fracture quantum”. Thus, two novel aspects of the model enter the theory: (1) degree of fractality related to the roughness of the newly created surface, and (2) discrete nature of the propagating crack. Both variables are shown to increase the equilibrium length of the cohesive zone. At the point of incipient fracture this length becomes the characteristic material length parameter L

c

.

Novel properties of the present model provide a better insight and an effective tool to explain multiscale nature of fracture process and the associated transitions from nano- to micro- and macro-levels of material response to deformation and fracture. These multiscale features of any real material appear to be inherent defense mechanisms provided by nature.

As the degree of fractality increases, the characteristic material length is shown to rapidly grow to the levels around three orders of magnitude higher than those predicted for the classic case. Such effect is helpful in explaining an unusual size-sensitivity of fracture testing in materials with cementitious bonding such as concrete and certain types of ceramics, where fractal cracks are commonly observed.

In the limit of vanishing quantum fracture and/or reduced degree of fractality the quantized cohesive model of a fractal crack, as presented here, reduces to the well-known classic models of Dugdale—Barenblatt or to the LEFM or the QFM fracture theories.

Michael P. Wnuk*

Reparability of Damaged Fluid Transport Pipes

Most of energy fluids and also water are transported over very long distance by pipelines. There is a quite large spectrum of situations defined by: nature and pressure of the transported fluid, environmental and political situations, and economical constraints. If a pipeline is damaged and becomes unable to continue its service or presents some risks towards the local population, or is an economical challenge, the question of repairing appears. Therefore, the technical choices should be optimized with respect to the induced costs and to the social, economical and, perhaps, political costs. This paper will present various situations and the available repairing solutions, with respect to their costs in view to be a guide for the person in charge to make decision.

Philippe Jodin

Damage Control and Repair for Security of Buildings

Civil engineering objects, especially buildings, suffer the damage and failure under earthquake. Invented DC Damper System can increase the resistance and strengthen the construction, enabling tougher behaviour under seismic load. Experimental research and experience in repairing 350 damaged objects on four continents is the base for system development.

Dragoslav Šumarac, Zoran PetraŠkoviĆ

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