Showing papers in "Fatigue & Fracture of Engineering Materials & Structures in 2006"

Extrapolation of load histories and spectra

Abstract: In fatigue life assessments, both the material properties and the load characteristics are essential parameters. The life of a component can be experimentally found by performing fatigue tests. In order to get reliable predictions of the life in service, the tests should be performed using variable amplitude loadings that are representative for the service loads. This paper concentrates on the problem of extrapolating a measured load history to a longer time period, for example to a full design life. Using statistical extreme value theory, a new method for extrapolation in the time domain is presented. The extrapolated load spectrum, obtained from the extrapolated time signal, is compared to the result using a method for extrapolating the rainflow matrix.

Prediction of corrosion fatigue lives of aluminium alloy on the basis of corrosion pit growth law

Abstract: Tension-compressionandrotating-bendingfatiguetestswerecarriedoutusingaluminiumalloy 2024-T3, in 3% NaCl solution. The corrosion pit growth characteristics, and alsothe fatigue crack initiation and propagation behaviour were investigated in detail. Theresults obtained are summarized as follows: (i) Most of corrosion fatigue life (60–80%)is occupied with a period of corrosion pit growth at low-stress amplitude. The corrosionpit growth law can be expressed as functions of stress amplitude σ a and an elapsed time t . (ii) The critical stress intensity factor for crack initiation from the corrosion pit wasdetermined as 0.25 MPa√m. This value is the same as the threshold stress intensity factorrange for crack propagation. (iii) Corrosion fatigue life can be estimated on the basis ofcorrosion pit growth law and crack propagation law. The estimated fatigue lives agree wellwith the experimental data. Keywords corrosion fatigue; corrosion pit; stress amplitude; stress frequency.INTRODUCTIONAluminium alloys have been widely used as structuralmembersoftransportmachines,suchasshipsandaircraftsfor the purpose of reducing their weight. Such structuresare usually exposed to the natural environment during theoperation.It is well known that metal fatigue life is lowered sub-stantially by the action of a corrosive environment whencompared with that in an inert atmosphere. This is be-cause a corrosion pit can generate and grow on the mate-rial surface, followed by crack initiation and propagationin corrosive environments, even at very low-stress ranges.Therefore, for quantitative analysis of the corrosion fa-tigue life, it is very important to clarify the corrosion pitgrowth characteristics and the crack initiation behaviourfrom the corrosion pit. Until now, many researches oncorrosion pit growth characteristics, and crack initiationand propagation behaviour have been made.Wan

Characterisation of equivalent initial flaw sizes in 7050 aluminium alloy

Abstract: This paper summarises a collation of crack growth related data from a significant number of fatigue tests on commercial quality 7050-series aluminium alloy tested under various F/A-18 aircraft spectra. The data presented consist of quantitative fractography measurements of the fracture surfaces including effective initiating defect size and type (mechanical, environmental or chemical). Three different surface conditions were considered: chemically etched, glass bead peened and machined. The purpose of providing these data was to facilitate analyses on the parameters governing the propagation of fatigue cracks in the 7050-series aluminium alloy. Here an investigation to determine whether surface finish or applied stress and spectra have any bearing on the initial defect size or the damage type is summarised. Based on this investigation, it appears that the applied stress and spectra have no correlation to the size of the equivalent initiating flaw; however, the surface finish appears to influence the various crack-initiating mechanisms and to govern the formation of the damage type.

A meso‐level approach to the 3D numerical analysis of cracking and fracture of concrete materials

Abstract: A meso-mechanical model for the numerical analysis of concrete specimens in 3 D has been recently proposed. In this approach, concrete is represented as a composite material with the larger aggregates embedded in a mortar-plus-aggregates matrix. Both continuum-type components are considered linear elastic, while the possibilities of failure are provided with the systematic use of zero-thickness interface elements equipped with a cohesive fracture constitutive law. These elements are inserted along all potential crack planes in the mesh a priori of the analysis. In this paper, the basic features of the model are summarized, and then results of calculations are presented, which include uniaxial tension and compression loading of 14-aggregate cubical specimen along X, Y and Z axes. The results confirm the consistency of the approach with physical phenomena and well-known features of concrete behaviour, and show low scatter when different loading directions are considered. Those cases can also be considered as different specimens subjected to the same type of loading.

The CAFE model of fracture—application to a TMCR steel

Abstract: CAFE, a combination of Cellular Automata (CA) to describe the material microstructural properties, and Finite Elements to represent the macroscopic strain, allows the evolving states of microstructural change to be analysed in a deforming body. The paper explores the potential of the technique for the assessment of the ductile-to-brittle transition behaviour of ferritic steels. The most important features are due to the microstructural control of cleavage. The model was applied to the Charpy test on a TMCR (Thermo-Mechanically Controlled Rolled) steel. The results include simulations of the full Charpy energy transition curve, the varying amount of scatter in the data, and, in distinction to most other models in the literature, a full description of the state of the fracture surface on each specimen. The results agree reasonably well with the experimental data. The model follows the development of both ductile and cleavage micro-mechanisms throughout the deformation of the structure or component. As a consequence, it allows cleavage to develop as a progressive point-to-point process. The model can, and does, predict the arrest of cleavage cracks by ductile processes. It can be easily generalized to include more sophisticated aspects of a material microstructure.

Water pipeline failure due to water hammer effects

Abstract: A numerical model has been established in order to simulate the propagation of pressure waves in water networks. The present model formulation is based on a system of partial hyperbolic differential equations. This system has been solved via the characteristics method. The current model provides the necessary data and the necessary damping of water hammer waves, taking into account the structure of the pipe network and the pressure loss. The numerical algorithm estimates the maximum pressure values resulting from the water hammer when closing valves in the network and consequently, the maximum stresses in the pipes have been calculated. In the case of simultaneous closing of several valves, the over pressure can exceed the admissible pressure. In this case, the severity of a defect such as a corrosion crater (pit) has been estimated by computing a safety factor for the stress distribution at the defect tip. This allows the applied notch stress intensity factor to be obtained. To investigate the defect geometry effects, semi-spherical and semi-elliptical defects are deemed to exist in up to one-half of the thickness of the pipe wall. The outcomes have been introduced into the structural integrity assessment procedure (SINTAP) failure diagram assessment (FAD) in order to obtain the safety factor value. Conventionally, it is considered that a failure hazard exists if this safety factor is less than two.

Fatigue crack growth under variable amplitude loading Part II: analytical and numerical investigations

Abstract: In part I, the effects of variable amplitude loadings on the fatigue crack growths were illustrated by means of experimental results. Within the scope of part II, systematic analytical and numerical investigations are presented. Using different analytical concepts it can be shown that the lifetime depends both on the concept used and on the loading sequence. Also, the influence of the parameters that must be fitted by experimental data for all analytical prediction models has been investigated. By means of detailed elastic–plastic finite element simulations it becomes obvious that not only the crack opening caused by large plastic deformations subsequent to overloads and block loadings, but also the stress field in the ligament is an indicator for the retardation effect. If the σy-stresses both at maximum and minimum loading are identical with the σy-stress distribution of an appropriate constant amplitude (CA) loading, one can assume that the interaction effect is annihilated.

An embedded cohesive crack model for finite element analysis of mixed mode fracture of concrete

Abstract: An embedded cohesive crack model is proposed for the analysis of the mixed mode fracture of concrete in the framework of the Finite Element Method. Different models, based on the strong discontinuity approach, have been proposed in the last decade to simulate the fracture of concrete and other quasi-brittle materials. This paper presents a simple embedded crack model based on the cohesive crack approach. The predominant local mode I crack growth of the cohesive materials is utilized and the cohesive softening curve (stress vs. crack opening) is implemented by means of a central force traction vector. The model only requires the elastic constants and the mode I softening curve. The need for a tracking algorithm is avoided using a consistent procedure for the selection of the separated nodes. Numerical simulations of well-known experiments are presented to show the ability of the proposed model to simulate the mixed mode fracture of concrete.

Uniaxial ratchetting in steels with different cyclic softening/hardening behaviours

Abstract: The cyclic deformation of three structural steels, SS316L stainless steel, 40Cr3MoV bainitic steel and 2 5CDV4.11 steel, were studied experimentally by uniaxial cyclic straining or stressing tests at room temperature. The cyclic softening/hardening behaviours of the steels were discussed first by cyclic straining tests; and then the effects of cyclic softening/hardening behaviours on the uniaxial ratchetting of the materials were investigated by asymmetrical cyclic stressing tests. It is concluded from the experimental results that the ratchetting greatly depends on the cyclic softening/hardening behaviours of the materials, as well as the loading history. Different ratchetting and failure behaviours are observed for the prescribed steels. It is also stated that the proposed unified visco-plastic constitutive model can provide a fairly reasonable simulation of the uniaxial ratchetting of SS316L stainless steel and 25CDV4.11 steel; but cannot simulate the ratchetting of 40Cr3MoV bainitic steel since the dependence of cyclic softening behaviours on the applied inelastic strain amplitude cannot be reasonably described in the discussed constitutive model. Some significant conclusions are obtained, which are useful to construct constitutive model to describe the ratchetting of the materials with different cyclic softening/hardening behaviours.

Effects of hydrogen charge on microscopic fatigue behaviour of annealed carbon steels

Abstract: The effects of hydrogen charge on cyclic stress-strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS-S45C) were studied. The total strain range of the stress-strain hysteresis loop in the hydrogen-charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen-charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen-charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen - slip band interaction may play an important role in the hydrogen- influenced metal fatigue.

Cold Drawn Steel Wires-Processing, Residual Stresses and Ductility-Part I: Metallography and Finite Element Analyses

Abstract: Cold drawing steel wires lead to an increase of their mechanical strength and to a drop of their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by plastic deformation, but the mechanisms controlling their elongation to failure have received relatively little attention. It is usually found that heavily deformed materials show a tendency to plastic strain localization and necking. However, in this paper it is shown that, though the steel wires are plastically deformed up to strain levels as high as 3.5, a significant capability of plastic deformation is preserved in as-drawn wires. This apparent contradiction is resolved by the existence of residual stresses inside the wire. Finite element analyses have been conducted in order to show that residual stresses, inherited from the drawing process, are sufficient to produce a significant hardening effect during a post-drawing tensile test, without introducing any hardening in the local material behaviour. The main conclusion of this paper is that once the material has lost its hardening capabilities, residual stresses, inherited from the process, control the elongation of cold drawn wires. The finite element method allowed also the determination of the residual stress field that would lead to the best agreement between the simulated and the experimental stress strain curve of as-drawn wires.

A computational lifetime prediction of a thermal shock experiment. Part I: thermomechanical modelling and lifetime prediction

Abstract: The SPLASH experiment has been designed in 1985 by the CEA to simulate thermal fatigue due to short cooling shocks on steel specimens and is similar to the device reported by Marsh in Ref. (1). The purpose of this paper is to discuss the mechanical and the fatigue analysis of the experiment using results from FEM computations. The lifetime predictions are obtained using a modified dissipated energy with a maximal pressure term and agree with the experimental observations. The numerical analysis of the mechanical state shows an important evolution of the triaxiality ratio during the loading cycle. Further comparisons and discussions of the fatigue criteria are provided in the second part of the paper (Part II) 2 .

Prediction of rubber fatigue life under multiaxial loading

Abstract: The process of fatigue failure of rubbers is generally described by two phases: crack initiation and crack propagation. This study concerns the crack initiation in such materials submitted to a cyclical loading. Concerning this aspect, either criteria based upon maximum stretch or strain energy density have been developed in the literature [1, 2, 3]. More recently, a parameter predicting the onset of primary crack and its probable orientation has been introduced by Mars [4, 5]. This criterion is based on the so-called “cracking strain energy density (CSED)” Wc and postulate that crack initition will occur in the plane in which the value of Wc is a maximum. The cracking strain energy density parameter represents only the portion of strain energy density available to initiate a crack in a given plane. It is defined incrementally as the dot product of traction vector with \( \vec \sigma \) the strain increment vector \( d\vec \varepsilon \) on this material plane (figure 1): $$ dW_c = \vec \sigma .d\vec \varepsilon = (\vec r^T .\mathop \sigma \limits^ \approx ).(d\mathop \varepsilon \limits^ \approx .\vec r) $$ (1)

Fatigue crack growth under variable amplitude loading Part I: experimental investigations

Abstract: During use, a component or a structure is exposed to variable amplitude loading, which influences the lifetime. Within the scope of this work, systematic investigations of different loading situations are carried out by means of experimental studies (part I) as well as analytical and numerical studies (part II). The experimental investigations show that overloads lead to retardation effects, which are influenced by several factors, e.g. the overload ratio, baseline-level loading, number of overloads or the fraction of mixed mode. In a high-low-high block loading, both retarded and accelerated crack growth can be obtained, which is also influenced, e.g. by the block loading ratio and the length of the block. Moreover, experimental studies have been performed with load spectra, like FELIX/28, CARLOS vertical and WISPER. They have been applied in original form as well as in counted and reconstructed sequences.

Elastic¿plastic fatigue crack growth in 18G2A steel under proportional bending with torsion loading

Abstract: The paper presents the results of fatigue crack growth on low-alloy 18G2A steel under proportional bending with torsion loading. Specimens with square sections and a stress concentration in the form of external one-sided sharp notch were used. The tests were performed under the stress ratios R = -1, -0.5 and 0. The test results were described by the ΔJ-integral range and compared with the ΔK stress intensity factor range. It has been found that there is a good agreement between the test results and the model of crack growth rate, which includes the ΔJ-integral range.

Damage analysis for structural integrity and durability of composite materials

Abstract: Composite structures for mechanical and aerospace applications are designed to retain structural integrity and remain durable for the intended service life. Since the early 1970s important advances have been made in characterizing and modelling the underlying mechanical behaviour and developing tools and methodologies for predicting fracture and fatigue of composite materials. This paper presents a review of the concepts and analyses related to this area, and illustrates these by a few examples. The topics discussed are composite material strength in tension, compression and shear, damage and its progression in monotonic and cyclic loading, fatigue life prediction and damage induced changes in visco-elastic response.

The influence of porosity on the fatigue strength of high-pressure die cast aluminium

Abstract: Aluminium is a lightweight material with high strength and good corrosion resistance among other beneficial properties. Thanks to these properties, aluminium is more extensively used in the vehicle ...

Generalized stress intensity factors due to steady and transient thermal loads with applications to welded joints

Abstract: This paper deals with the elastic and plastic stress fields induced by thermal loads in the vicinity of sharp V-notch tips in plates. Under the hypothesis of steady-state heat transfer and plane-strain conditions, the thermal and mechanical problem requires the numerical solution of an ordinary differential equation (ODE) system, obtained by extending the ‘stress function approach’. The intensity of the stress distributions ahead of V-notch tips can be expressed in terms of thermal notch stress intensity factors (thermal NSIFs), as for external loads. The problem becomes much more demanding in the presence of transient thermal loads. The residual asymptotic stress distribution arising from the solidification of a fusion zone during an arc welding process is obtained by considering different boundary conditions. An aluminium butt-welded joint is analysed after having modelled the weld toe region as a sharp V-notch. A finite element (FE) simulation of the welding process is carried out by means of SYSWELD code (version 2004.1) modelling the arc welding torch by means of Goldak's source. Near the weld toe, the intensity of the residual stress field is given in terms of elastic or elastic—plastic generalized NSIFs.

Fracture behaviour of an Al2O3-ZrO2 multi-layered ceramic with residual stresses due to phase transformations

Abstract: The fracture behaviour of a ceramic multi-layer designed with thin internal compressive layers and obtained by slip casting is studied. It consists of nine alternated Al 2 O 3 -5vol%tZrO 2 and Al 2 O 3 -30vol%mZrO 2 layers of530 μm and 100 μm thickness, respectively. Mechanical characterization includes evaluation of Vickers Hardness, Young's modulus and fracture strength under four-point bending. In addition, the residual stress magnitude and distribution in the laminate is determined both analytically, from calculations using the differential strain between layers and the elastic properties, and experimentally, using indentation techniques. The experimental findings in terms of mechanical strength and fractography show a subcritical growth of the natural flaws in the laminate before catastrophic failure occurs, owing to the relevant role of the thin Al 2 O 3 -30vol%mZrO 2 layers with compressive stresses inherent to the zirconia phase transformation. These layers are also responsible for the increase in toughness to levels of at least three times that of the reference Al 2 O 3 -5vol%tZrO 2 monolith.

Comparative study on biaxial low‐cycle fatigue behaviour of three structural steels

Abstract: In this study the uniaxial/biaxial low-cycle fatigue behaviour of three structural steels (Ck45 normalized steel, 42CrMo4 quenched and tempered steel and AISI 303 stainless steel) are studied, evaluated and compared. Two parameters are considered for estimating non-proportional fatigue lives: the coefficient of additional hardening and the factor of non-proportionality. A series of tests of uniaxial/biaxial low-cycle fatigue composed of tension/compression with cyclic torsion were carried out on a biaxial servo-hydraulic testing machine. Several loading paths were carried out, including proportional and non-proportional ones, in order to verify the additional hardening caused by different loading paths. The experiments showed that the three materials studied have very different additional hardening behaviour. Generally, the transient process from the initial loading cycle to stabilized loading cycle occurs in a few cycles. The stabilized cyclic stress/strain parameters are controlling parameters for fatigue damage. A factor of non-proportionality of the loading paths is evaluated based on the Minimum Circumscribed Ellipse approach. It is shown that the microstructure has a great influence on the additional hardening and the hardening effect is dependent on the loading path and also the intensity of the loading.

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

Abstract: The creep behaviour of directionally solidified (DS) Ni-base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.

Thermographic analysis in ultrasonic fatigue tests

Abstract: r Three kinds of alloys used in the automotive industry for structures and engine components are subjected to very high cycle fatigue damage during working service. In the study, fatigue damage progression of the three kinds of alloys was investigated in the very high cycles fatigue regime by means of thermographic analysis. The specimens have been tested at ambient temperature without cooling air using a piezoelectric fatigue system (20 kHz) with a stress ratio of R = -1 to determine the effects of increased specimen temperature caused by internal damping due to cycling at a very high frequency. The temperature evolution of specimens for different alloys at various load levels have been recorded. Results indicated that the temperature evolution of the fatigue specimen depends on the material, testing frequency and loading level; the temperature increased just at the beginning of the test, which corresponds to the thermal dissipation in the specimen. In the process of small crack propagation, the temperature in a local plastic zone increased sharply, evidently irreversible local plastic deformation can result in a local temperature increase. The basis of microstructural characterization and fracture surface analysis by scanning electron microscopy, it was found that the temperature field in an ultrasonic fatigue specimen corresponds to the fatigue damage process.

Analytical and experimental studies on fatigue crack path under complex multi‐axial loading

Abstract: In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Fractographical analyses of the failure surface and the crack orientation are helpful for identifying the effects of the non-proportional multi-axial loading. There are many factors that influence fatigue crack paths. This paper studies the effects of multi-axial loading path on the crack path. Two kinds of materials were studied and compared in this paper: AISI 303 stainless steel and 42CrMo4 steel. Experiments were conducted in a biaxial testing machine INSTRON 8800. Six different biaxial loading paths were selected and applied in the tests to observe the effects of multi-axial loading paths on the additional hardening, fatigue life and the crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It was shown that the two materials studied had different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were made using the critical plane approaches such as the Brown-Miller, the Findley, the Wang-Brown, the Fatemi-Socie, the Smith-Watson-Topper and the Liu's criteria. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the critical plane models give satisfactory predictions for the orientations of early crack growth of the 42CrMo4 steel, but less accurate predictions were obtained for the AISI 303 stainless steel. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multi-axial microstructure characteristics.

A computational lifetime prediction of a thermal shock experiment. Part II: discussion on difference fatigue criteria

Abstract: The SPLASH experiment has been designed in 1985 by the CEA to simulate thermal fatigue due to cooling shocks on steel specimens and is similar to the device reported by Marsh in Ref. [1]. The purpose of this paper is to discuss the application of different fatigue criteria in this case. The fatigue criteria: dissipated energy, Manson Coffin, Park and Nelson, dissipated energy with a pressure term, are determined for the experiment using results from FEM computations presented in the first part of the paper (Part I) 2 and compared with results from uniaxial and multiaxial experiments from literature. The work emphasizes the evolution of the triaxiality ratio during the loading cycle.

T-stress solutions for two-dimensional crack problems in anisotropic elasticity using the boundary element method

Abstract: The importance of a two-parameter approach in the fracture mechanics analysis of many cracked components is increasingly being recognized in engineering industry. In addition to the stress intensity factor, the T stress is the second parameter considered in fracture assessments. In this paper, the path-independent mutual M-integral method to evaluate the T stress is extended to treat plane, generally anisotropic cracked bodies. It is implemented into the boundary element method for two-dimensional elasticity. Examples are presented to demonstrate the veracity of the formulations developed and its applicability. The numerical solutions obtained show that material anisotropy can have a significant effect on the T stress for a given cracked geometry.

Crack growth predictions in a complex helicopter component under spectrum loading

Abstract: Fatigue crack growth predictions have been made on a helicopter round-robin crack configuration. The crack configuration was a small corner defect at the edge of a large central hole in a flanged plate made of 7010 aluminium alloy and the component was subjected to a simulated helicopter spectrum loading. The crack growth rate data and the stress-intensity factor (K) solution for the crack configuration were provided in the round-robin. The FASTRAN life-prediction code was used to predict fatigue crack growth under various load histories on the aluminium alloy, such as Rotorix and Asterix, on both compact tension C(T) specimens and the complex crack configuration. A BEASY three-dimensional stress-intensity factor solution for the round-robin problem was also provided for this paper and is compared with the original K solution. Comparisons are made between measured and predicted fatigue crack growth lives for both crack configurations. The predicted lives for the C(T) specimens were 15–30% longer than the measured lives; and crack growth in the round-robin configuration agreed very well in the early stages of crack growth, but the life was 30% short of the test results at the final crack length.

Cleavage fracture of RPV steel following warm pre‐stressing: micromechanical analysis and interpretation through a new model

Abstract: In this paper, the warm pre-stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhance- ment of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and −150 ◦ C are considered: Load-Unload-Cool-Fracture (LUCF), Load-Cool-Fracture (LCF) and Load-Cool with Increasing K-Fracture (LCIKF). All experiments complied with the conservative prin- ciple, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre-load (K wps = 40 MPa √ m), and that a minimum critical slope (−� K/� T) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and −150 ◦ C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) speci- mens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results.

Some comments on surface cracks in rails

Abstract: The present work considers the problem of whether a shallow angle surface crack on a rail may propagate either parallel or perpendicular to the surface. After referring to a classification of crack-like defects, a distinct surface crack with a typical length of 1 mm and more, inclined to the surface by an angle of approx. 30°, is introduced. Then various strain and stress states are discussed (eigenstress, residual and load stress state) which may control the crack growth. Stress intensity factor equations are provided for the internal and load stress states. In particular, the deformation process due to a single pass (overrolling) by a wheel is investigated. Recommendations are presented on how to relate the stress intensity factor ranges to the actual deformation process. For a typical loading condition, the positive part of the mode I stress intensity factor range is near to the effective threshold value of a rail steel. The influence of the material and its strength on the crack propagation behaviour is discussed qualitatively.

Validation of constraint‐based methodology in structural integrity of ferritic steels for nuclear reactor pressure vessels

Abstract: VOCALIST (validation of constraint-based methodology in structural integrity) was a shared cost action project co-financed by DG Research of the European Commission under the Fifth Framework of the European Atomic Energy Community (EURATOM). The motivation for VOCALIST was based on the understanding that the pattern of crack-tip stresses and strains causing plastic flow and fracture in components is different to that in test specimens. This gives rise to the so-called constraint effect. Crack-tip constraint in components is generally lower than in test specimens. Effective toughness is correspondingly higher. The fracture toughness measured on test specimens is thus likely to underestimate that exhibited by cracks in components. The purpose of VOCALIST was to develop validated models of the constraint effect and associated best practice advice, with the objective of aiding improvements in defect assessment methodology for predicting safety margins and making component lifetime management decisions. The main focus in VOCALIST was an assessment of constraint effects on the cleavage fracture toughness of ferritic steels used in the fabrication of nuclear reactor pressure vessels, because of relevance to the development of improved safety assessments for plant under postulated accident conditions. This paper provides a detailed summary of the main results and conclusions from VOCALIST and points out their contribution to advances in constraint-based methodology for structural integrity assessment. In particular, the output from VOCALIST has improved confidence in the use of K J − T stress and K J − Q approaches to assessments of cleavage fracture where the effects of in-plane constraint are dominant. Cleavage fracture models based on the Weibull stress, σ W, have been shown to be reliable, although current best practice advice suggests that σ W should be computed in terms of hydrostatic stress (as distinct from maximum principal stress) for problems involving out-of-plane loading. Correspondingly, the results suggest that the hydrostatic parameter, QH, is the appropriate one with which to characterize crack-tip constraint in analysing such problems. The materials characterization test results generated as part of VOCALIST have provided added confidence in the use of sub-size specimens to deter- mine the Master Curve reference temperature, T 0, for as-received and degraded ferritic RPV materials. The usefulness of correlating the Master Curve reference temperature, T 0, with the constraint parameter, Q, has been demonstrated; however, the trend curves

A weight function-critical plane approach for low-cycle fatigue under variable amplitude multiaxial loading

Abstract: Low-cycle fatigue data of type 304 stainless steel obtained under axial-torsional loading of variable amplitudes are analyzed using four multiaxial fatigue parameters: SWT, KBM, FS and LKN. Rainflow cycle counting and Morrow's plastic work interaction rule are used to calculate fatigue damage. The performance of a fatigue model is dependent on the fatigue parameter, the critical plane and the damage accumulation rule employed in the model. The conservatism and non-conservatism of predicted lives are examined for some combinations of these variables. A new critical plane called the weight function-critical plane is introduced for variable amplitude loading. This approach is found to improve the KBM-based life predictions.