Search

In this study, a miniaturized cruciform-shaped sample geometry which allows reaching high plastic strain under equibiaxial loading with reduced thickness at the test section is presented. The thinning method results in excellent surface quality that can be used for electron backscatter diffraction (EBSD) and high-resolution digital image correlation (HRDIC) investigations. The new cruciform geometry is used to study the slip activity in metastable austenitic stainless steel 304 during uniaxial and equibiaxial deformation using HRDIC. The results are discussed with respect to the Schmid law and the effect of multiple slip activity on the nucleation of martensitic transformations. [ABSTRACT FROM AUTHOR]

Highlights • An approach for fatigue damage prediction under multiaxial fatigue was proposed. • The critical plane approach was introduced to predict the fatigue damage. • Multiaxial stress of deck and U-rib welds in different positions was analysed. • The rotation angle of the maximum principal stress was calculated. Abstract Assessment approach for multiaxial fatigue damage of deck and U-rib weld in steel bridge decks was studied. The orthotropic steel bridge deck was modelled to analyse the multiaxial stress state of the weld under wheel load. A multiaxial fatigue loading experiment was carried out to assess the fatigue strength of deck and U-rib weld based on equivalent stress and maximum principal stress. The critical plane approach which is suitable for deck-U rib welds was also established. The assessment results were then compared to those obtained by uniaxial fatigue approach. Reasonable multiaxial fatigue principles were proposed for deck and U-rib welds in different positions. The results show that multiaxial fatigue stress is observed in the deck-U rib weld. The shear stress may exceed normal stress and become the main reason for fatigue damage, thus a multiaxial fatigue approach is necessary for fatigue damage assessment. The nominal stress approach is suggested for fatigue assessment of deck and U-rib weld between diaphragms, which leads to conservative results and is more applicable to engineering practice. The critical plane approach is suggested for fatigue assessment of deck and U-rib weld on the diaphragm considering multiaxial fatigue effects and non-proportional stress in the weld. [ABSTRACT FROM AUTHOR]

Uniaxial damage modelling use constituent components of the stress/strain measured data which have inherent scatter. Models developed for this purpose have also attempted to address the issues related to failure mechanisms within a multiaxial stress state context. This paper presents the multiscale approach to constraint by using the relationship between Monkman-Grant (MG) uniform failure strain and local sub-grain stress state to predict creep damage and rupture under uniaxial/multiaxial conditions. In essence, a global geometric constraint and a time-dependent local sub-grain constraint is defined with the latter controlling the failure response of the geometry. The model compliments and is in agreement with the established NSW model is also used to predict the lower/upper bound of cracking rates in crack dominated geometries. The model is employed into a finite element (FE) to assess its capability to numerically predict the rupture of plain and multiaxial notched bar specimens based on appropriate void growth models. For verification, creep constitutive properties of long term data from uniaxial and multiaxial tests on Grade 92 martensitic steels from various databases, where available, are used to establish the procedure. Given the level of scatter in the data and fabrication and testing uncertainties that cannot be accounted for in such databases, it is shown that the model is sufficiently simple and robust to be developed for use in conservatively predicting very long term failure times. [ABSTRACT FROM AUTHOR]

Numerical and analytical methods for predicting uniaxial damage have largely depended on the constituent components of the stress/strain measured data which have inherent scatter. Models developed for this purpose have also attempted, with some degree of success, to address the fundamental issues of failure mechanisms within a multiaxial stress state context. This paper presents a new analytical/empirical/a postpriori unifying approach to predict creep damage and rupture under uniaxial/multiaxial and crack growth conditions by deriving a multiscale based constraint criterion. Essentially, the model links the global constraint due to geometry in a globally isotropic material with a microstructural constraint arising from creep diffusional processes occurring in a sub-grain locally anisotropic microstructure. Furthermore, it is shown that the model is consistent with the established NSW crack growth model (Nikbin et al., 1984, 1986; Tan et al., 2001) which is routinely used to determine the plane stress/strain bounds for cracking rates in fracture mechanics geometries and cracked components. The concept assumes that at very short times an initial upper shelf material tensile strength and global plasticity and power law creep control creep damage failure and sub grain multiaxial axial stress state dependent failure strain dominates the long term diffusion/dislocation controlled creep response. It is established that the material yield strength in the short term and a measure of creep failure strain at the creep secondary/tertiary transition region described at the limits by the Monkman-Grant failure strain (Monkman and Grant, 1963), are the important variables in both the uniaxial and multiaxial failure processes. For verification creep constitutive properties from long term data from uniaxial and multiaxial and crack growth tests on Grade P91/92 martensitic steels from various databases (EPRI, private communications; NIMS data base), are used to establish the procedure. [ABSTRACT FROM AUTHOR]

This study presents experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy subjected to both uniaxial and multiaxial variable amplitude flight loading spectra. Experimental crack growth behavior is compared to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. UniGrow is based on the idea that residual stress distributions surrounding the crack tip are responsible for causing load sequence effects in variable amplitude crack growth, while FASTRAN attributes these effects to varying degrees of plasticity induced closure in the crack wake. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce conservative growth life predictions despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This is attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were observed in constant amplitude crack growth tests to increase experimental crack growth rates. Additionally, by comparing experimental crack growth lives between tests performed using full and edited versions of the same loading history, it was found that a 94% reduction in loading history length resulted in differences in experimental crack growth life of less than a factor of 5. Since cracks in this study were initiated naturally, the effect of initial crack geometry assumptions on crack growth predictions was also investigated. [ABSTRACT FROM AUTHOR]

• Effects of peak strain amplitude and strain gradient on test fatigue life studied. • Localized strain using FEA is compared with measured strain. • Fatigue life predictions using equivalent strain/critical plane based models. • Fatigue life assessment considers information at hole tip/characteristic distance. Present work is aimed at performing fatigue tests on notched C-Mn steel tubes. Tests were conducted on different sizes of notches under remote axial strain-controlled conditions. Individual effects of peak equivalent strain amplitude and strain gradient are brought out on experimental fatigue life. The localized measured strains are compared with corresponding outcome of FE analyses. Fatigue life for notched specimens was predicted based on peak equivalent strain amplitude/existing critical plane model considering stress/strain information at peak/characteristic distance locations. The characteristic distance location with critical plane model and peak equivalent strain amplitude results in improved fatigue life predictions. [ABSTRACT FROM AUTHOR]

Weld start and end points are fatigue failure sensitive locations. Their fatigue behaviour especially in thin sheet structures under multiaxial load conditions is not sufficiently explored so far. Therefore, a research project was initiated to increase the knowledge concerning this topic, which is of special interest in the automotive industry. In the present study, fatigue tests on welded joints were conducted. In the numerical part of the study, notch stresses were calculated with an idealised weld end model. A numerical method which combines the geometrical and statistical size effect to an integrated approach was used, in order to consider the size effects. [ABSTRACT FROM AUTHOR]

This study concerns the multiaxial static and fatigue strength properties. Short-glass-fibre-reinforced phenolic-resin composites (SGP) molded by injection and compression processes were subjected to tensiontorsion combined static and fatigue tests at room temperature under various test conditions. Tension - torsion combined static strength well agreed with Tsai-Hill failure criteria without depending on processes. Relationships between the maximum principal stress, σp1, max, and the number of fracture cycles, Nf, were approximately linear in the whole range of up to 106 cycles. For a unified evaluation of multiaxial fatigue life for SGP, non-dimensional effective stress, σ*, defined by modifying Tsai-Hill failure criteria was applied. The slopes of σ*- Nf curves according to Baskin's law were almost identical to the injection (n = 26.3) and compression (n = 26.2). We finally confirmed that the multiaxial fatigue life of SFRP could be predicted by using σ* with a unique Wöhler curve without relying on molding processes. [ABSTRACT FROM AUTHOR]

Fatigue cracking of longerons manufactured from Al-alloy AVT-1 for helicopter in-service rotorblades was considered and crack growth period and equivalent of tensile stress for different blade sections were estimated. Complicated case of in-service blades multiaxial cyclically bending-rotating and tension can be considered based on introduced earlier master curve constructed for aluminum alloys in the simple case of uniaxial tension with stress R-ratio near to zero. Calculated equivalent tensile stress was compared for different blade sections and it was shown that in-service blades experienced not principle difference in this value in the crack growth direction by the investigated sections. It is not above the designed equivalent stress level. Crack growth period estimation in longerons based on fatigue striation spacing or meso-beach-marks measurements has shown that monitoring system introduced designer in longerons can be effectively used for in-time crack detecting independently on the failed section when can appeared because of various type of material faults or in-service damages. [ABSTRACT FROM AUTHOR]

The present study reports the effects of combined torsional and compressive cyclic loading on trabecular bone in order to mimic true physiological conditions and thereby provides improved data that represents clinical and real life conditions. However, only compressive behaviour is evaluated in most previous studies of bone mechanics. From the monotonic evaluation, it is observed that lower stress is needed for the onset of microcrack in the sample under torsional loading, compared to the stress needed in compression. Trabecular bone samples were subjected to a combination of torsion and compression fatigue at different stress levels during which they were compared to compressive axial fatigue. The stress levels were determined by considering the monotonic strength at 25–50% for both compressive and shear stresses. Significant decrease in fatigue lifetime is observed in between samples of pure compression fatigue and those with superpositioned torsional loading ( p <0.05). The reduction in fatigue lifetime became more evident at a high torsional stress level. In this case, the failure of the sample is said to be ‘torsional dominant’. Fatigue behaviour of bovine trabecular bone begins with plastic deformation, followed by strain accumulation and modulus reduction. As the strain rate increases, more energy dissipates and the sample finally failed. Further, the analysis of fractograph revealed something on the trabeculae by bending in sample with superpositioned torsional loading. In conclusion, torsional loading decreases the quality of the trabecular properties in terms of stiffness, life and structural integrity. It is hoped that results from this study will improve the understanding of the behaviour of trabecular bone under combined fatigue and help to develop future assessments of trabecular failure. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work is to model short crack growth under fretting fatigue loading conditions by considering a criterion based on linear elastic fracture mechanics quantities, which also accounts for the first non-singular terms of the asymptotic expansion, namely the T-stresses. The Modes I and II Stress Intensity Factors and the T-stresses were computed by the finite element method under plane strain hypothesis. To assess the model fretting fatigue tests were carried out using two cylindrical fretting pads, which were loaded against a flat dogbone tensile test piece, both made of a Ti–6Al–4V titanium alloy. The model was capable to correctly estimate short crack arrest and to find the threshold fretting conditions separating failure from infinite life (here defined by tests which reached one million cycles). An optimization technique was implemented to the numerical model so that it could also estimate crack path. [Copyright &y& Elsevier]

Highlights: [•] The multiaxial elastic–plastic stress–strain model for notched components is developed. [•] The model is used for elastic–plastic stress–strain analysis based linear-elastic FE results. [•] The model is validated against the experimental data of SAE 1070 steel notched bar. [•] The model can be employed to perform fatigue life estimation for notched components. [ABSTRACT FROM AUTHOR]

Abstract: Reliable design of industrial components against high cycle multiaxial fatigue requires a model capable of predicting both stress gradient and load type effects. Indeed, taking into account gradient effects is of prior importance for the applicability of fatigue models to real structures. In this paper, a fatigue life assessment method is proposed for proportional and non-proportional multiaxial variable amplitude loadings in the range – cycles. This method derives from the fatigue criterion initially proposed by Palin-Luc and Lasserre (1998) [2] and revisited by Banvillet et al. (2003) [16] for multiaxial constant amplitude loading. The new proposal consists of a complete reformulation and extension of the previously cited energy based fatigue strength criteria. It includes two major improvements of the existing criteria. The first one consists in a fatigue criterion for multiaxial variable amplitude loadings while only constant amplitude loadings were considered in the above cited works. The second one is an extension to an incremental fatigue life assessment method for proportional and non-proportional multiaxial variable amplitude loadings. No cycle counting technique is needed whatever the variable amplitude loadings type considered (uniaxial or multiaxial). The predictions of the method for constant and variable amplitude multiaxial loadings are compared with experimental results on specimens from literature and from new experiments on a ferrito-perlitic steel. The above mentioned method has been implemented as a post-processor of a finite element software. An application to a railway wheel is finally presented. [Copyright &y& Elsevier]

In this article, a computational modeling method of the multiaxial stress–strain notch analysis has been developed to compute elasto-plastic notch-tip stress–strain responses using linear elastic finite element results of notched components. Application and validation of the multiaxial stress–strain notch analysis model were presented by comparing computed results of the model to the experimental data of SAE 1070 steel notched shaft subjected to several nonproportional load paths. Based on the comparison between the experimental and computed strain histories, the elasto-plastic stress–strain model predicted notch strains with reasonable accuracy using linear elastic finite element stress histories. The elasto-plastic stress–strain notch analysis model provides an efficient and simple analysis method preferable to expensive experimental component tests and more complex and time-consuming incremental nonlinear finite element analysis. The elasto-plastic stress–strain model can thus be employed to perform fatigue life and fatigue damage estimates associated with the local material deformation. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this work is to develop a simple multiaxial version of a rainflow algorithm that allows the proper calculation of multiaxial fatigue damage induced by non-proportional load histories. One of the issues in such algorithm involves a complementary problem, how to properly quantify equivalent stress or strain ranges and mean components associated with each rainflow-counted cycle. A traditional way to estimate such ranges is to use enclosing surface methods, which search for convex enclosures like balls or prisms, of the entire history path in stress or strain diagrams. To treat these two intrinsically related problems, this work is divided into two parts. Part I deals with how to compute equivalent stress or strain ranges in multiaxial NP histories using enclosing surface methods. The available methods are first reviewed, and new enclosing surface models are proposed, based on Deperrois’ idea of longest chords. Then, these methods are compared using results from more than 3×106 Monte Carlo simulations of random and especially chosen path topologies in two to five-dimensional stress or strain diagrams. Moreover, a new simpler but powerful approach to evaluate equivalent stress and strain ranges in NP histories is presented, called the Moment Of Inertia (MOI) method. The MOI method is not based on enclosing surfaces, it assumes instead that the path contour in the stress or strain diagram is analogous to a homogeneous wire with a unit mass. The center of mass of such wire gives then the mean component of the path, while the moments of inertia of the wire can be used to obtain the equivalent stress or strain ranges. Experimental results for 15 different multiaxial histories prove the effectiveness of the MOI method to predict the associated fatigue lives, when compared to the existing enclosing surface methods. Part II of this paper presents a multiaxial rainflow counting algorithm that allows the MOI and enclosing surface methods to be generalized to non-periodic NP histories and to periodic NP histories formed by complex blocks with multiple cycles each. [Copyright &y& Elsevier]

This paper represents the author’s contribution to the second world-wide failure exercise using his failure mode concept modelling capability. The second world-wide failure exercise deals with the behaviour of isotropic material and unidirectional as well as multidirectional unidirectional laminae-composed laminates subjected to three-dimensional (triaxial) states of stress. Twelve challenging test cases were provided by the organisers and those covered stress–strain curves and failure envelopes under three-dimensional stress states. The application of the new failure mode concept model has extended the three-dimensional modelling by taking into account the effects of hydrostatic pressure and second glass temperature shift factor on the stress–strain curves and failure envelopes. The failure mode concept model was capable of successfully solving the majority of all the problems and a comparison between the predictions and test data is planned to be published in Part B of the second world-wide failure exercise. [ABSTRACT FROM AUTHOR]

Abstract: A new equivalent stress amplitude expression has been developed for the assessment of fatigue life in components under multiaxial loading. The expression was generated by incorporating non-linear/plastic stress–strain relation into a mechanical energy calculation, and then applying the calculation to the distortion energy theory for a cyclic loading case. Therefore, the new uniaxial equivalent stress expression determines an appropriate stress amplitude value for multiaxial cyclic loading. The purpose of the equivalent stress value is to determine multiaxial fatigue failure using an energy-based fatigue life prediction criterion. The governing understanding behind the criterion states that the physical damage quantity for failure is equal to the accumulated strain energy in a monotonic fracture, which is also equal to the accumulated strain energy during fatigue failure. Using the new equivalent stress amplitude expression and the energy-based life prediction method, a comparison is made between prediction results and multiaxial empirical data. The multiaxial data was acquired by a vibration-based biaxial bending fatigue test and a torsion fatigue test with an assumed axial misalignment. The results of the comparison provide encouragement regarding the capability of the newly developed equivalent stress amplitude expression for fatigue life prediction. [Copyright &y& Elsevier]

Abstract: An innovative procedure is presented here for determining the minimum low cycle fatigue (LCF) life of turbine disk bores under multiaxial states of stress. The procedure statistically combines multiaxial data from spin pit tests with the data from uniaxial LCF tests. The connecting link between the two data sets is a new probability density function which can be derived from the uniaxial data and subsequently applied to the probability of failure calculated for a set of mini disks. The overall objective was to establish which of the criteria, principal stress-based versus effective stress-based, is valid for life estimation of the turbine disk bores made of DP-718. The minidisks, designed to simulate the same biaxial stress ratio as in an actual turbine disk bore, were tested at 589K by cycling between 5000 and 52,000rpm in a spin pit. The testing was interrupted to check for fatigue crack initiation in the bore by means of a proprietary vibration monitor. Only one of the disks showed a valid crack initiation. The initiation life is defined as the number of cycles required to initiate a crack of 381μm (0.015in.) depth, and was derived experimentally through subsequent striation counting. The approach established principal stress range with its associated stress ratio as the proper lifing parameter for DP-718 mini disks. Fractographic observations of similarity in the crack initiation modes observed for the alloy support the methodology. [Copyright &y& Elsevier]

Abstract: When an anisotropic material is subject to creep conditions and a complex state of stress, an anisotropic creep damage behavior is observed. Previous research has focused on the anisotropic creep damage behavior of isotropic materials but few constitutive models have been developed for anisotropic creeping solids. This paper describes the development of a new anisotropic tertiary creep damage constitutive model for anisotropic materials. An advanced tensorial damage formulation is implemented which includes both material orientation relative to loading and the degree of creep damage anisotropy in the model. A variation of the Norton-power law for secondary creep is implemented which includes the Hill’s anisotropic analogy. Experiments are conducted on the directionally-solidified bucket material DS GTD-111. The constitutive model is implemented in a user programmable feature (UPF) in ANSYS FEA software. The ability of the constitutive model to regress to the Kachanov-Rabotnov isotropic tertiary creep damage model is demonstrated through comparison with uniaxial experiments. A parametric study of both material orientation and stress rotation are conducted. Results indicate that creep deformation is modeled accurately; however an improved damage evolution law may be necessary. [Copyright &y& Elsevier]

Abstract: Thin-walled tubular specimens were employed to study the cyclic deformation of extruded AZ61A magnesium alloy. Experiments were conducted under fully reversed strain-controlled tension–compression, torsion, and combined axial–torsion in ambient air. Mechanical twinning was found to significantly influence the inelastic deformation of the material. Cyclic hardening was observed at all the strain amplitudes under investigation. For tension–compression at strain amplitudes higher than 0.5%, the stress–strain hysteresis loop was asymmetric with a positive mean stress. This was associated with mechanical twinning in the compression phase and detwinning in the subsequent tension phase. Under cyclic torsion, the stress–strain hysteresis loops were symmetric although mechanical twinning was observed at high shear strain amplitudes. When the material was subjected to combined axial–torsion loading, the alternative occurrence of twinning and detwinning processes under axial stress resulted in asymmetric shear stress–shear strain hysteresis loops. Nonproportional hardening was not observed due to limited number of slip systems and the formation of mechanical twins. Microstructures after the stabilization of cyclic deformation were observed and the dominant mechanisms governing cyclic deformation were discussed. Existing cyclic plasticity models were discussed in light of their capabilities for modeling the observed cyclic deformation of the magnesium alloy. [Copyright &y& Elsevier]

• The torsional and coupled torsion-tension/compression behaviors of magnesium alloy solid rod are systematically investigated through experiments and the crystal plasticity model. • Torsional behavior and the resultant Swift effect depend strongly on the texture. • The axial stress would affect the twinning activities drastically in different loadings. • The difference between shears and torsion is attributed to the significant bulk stress caused by the strong interaction between the cylindrical elements. Extensive attention has been paid to magnesium (Mg) alloys considering their potential as lightweight structural materials. However, it is still challenging to process and manufacture Mg materials that carry high strength and good ductility. This issue mainly arises from a lack of understanding toward the anisotropic mechanical behavior of Mg alloys in response to large deformation in a multiaxial stress state, which is unavoidably in existence under various material processing routes. In this regard, we seek to understand the torsional and coupled torsion-tension/compression behaviors of a magnesium alloy subject to large-strain deformation. The elastic viscoplastic self-consistent (EVPSC) model, which incorporates the twinning-detwinning (TDT) scheme and takes a torsion-specific finite element (TFE) approach, was employed to illuminate the inhomogeneous and multiaxial features of the torsional deformation in the AZ31 Mg alloy subjected to free/fixed-end torsion and coupled torsion-tension/compression. Experimental validation was conducted by characterizing the mechanical responses of the torsional specimens under the loading paths of free-end torsion and coupled torsion-tension. Our model successfully captures the Swift effect along with the shear texture, which is hardly predicted by conventional constitutive models. In addition, our model reveals that twinning is nearly equally active under free-/fixed-end torsion, while twinning under coupled torsion-tension and torsion-compression is promoted and suppressed, respectively. The comparison between the simple shear and pure torsion by simulation demonstrates that the significant bulk stress existing within the torsional rod is ascribed to the strong interaction between the cylindrical elements in the EVPSC model. As a final thought, we believe that the TFE-EVPSC-TDT model not only highlights the inhomogeneous and multiaxial features underlying the torsional and coupled torsion-tension/compression behaviors of the Mg alloy solid rod, but more significantly, can be used as a numerical tool for designing/tuning gradient twinning structures that may lead to optimized properties of Mg alloys. [ABSTRACT FROM AUTHOR]

Abstract: A facility has been developed capable of high strain rate material characterization in multiaxial flexure. Using this novel equipment, a formulation of a glass filled nylon composite (Ultramid B3WG6 BGVW) has been tested in biaxial flexure at a variety of strain rates. For the material tested, the flexural modulus was shown to have no significant dependence on the rate of application of load. The measured modulus was found to be over a range of –. A FE model was created and found to have an excellent correlation over the entire range of strain rate. The capabilities and limitations of the experimental facility are discussed. [Copyright &y& Elsevier]

Given the previous complete-potential structure framework [see Int. J. Plasticity 10(3) (1994) 263], together with the notion of strain- and stress- partitioning in terms of separate contributions of several submechanisms (viscoelastic and viscoplastic) to the thermodynamic functions (stored energy and dissipation), see [Int. J. of Plasticity 17(10) (2001) 1305], a detailed viscoelastoplastic multimechanism characterization of a specific hardening functional form of the model is presented and discussed. TIMETAL 21S is the material of choice as a comprehensive test matrix, including creep, relaxation, constant strain-rate tension tests, etc. are available at various temperatures. Discussion of these correlations tests, together with comparisons to several other experimental results, are given to assess the performance and predictive capabilities of the present model particularly with regard to the notion of hardening saturation as well as the interaction of multiplicity of dissipative (reversible/irreversible) mechanisms. [Copyright &y& Elsevier]

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In practice, calculations of creep damage are further complicated by the presence of multiaxial states of stress. In the case of the time fraction approach, there are a number of models that can be used to predict the effect of state of stress on creep rupture strength. In particular, Huddleston developed a model from data on stainless steels. The R5 procedure uses a ductility exhaustion approach to calculate creep damage and includes a model for use under triaxial states of stress. The aim of this paper is to describe the development of this model, which is based on considerations of cavity nucleation and growth and was developed from multiaxial creep data on Type 304 and 316 steels. [ABSTRACT FROM AUTHOR]

Non-proportional multiaxial fatigue tests of tubular specimens were performed under purely alternating strain-controlled loading. Different loading paths with different phase shifts were applied. With increasing phase shift at the same equivalent load, the lifetime was found to increase. For lifetime prediction a model based on the Manson–Coffin law was developed. By including the hydrostatic loading part, it was possible to compare the results of the multiaxial fatigue tests with uniaxially ascertained results. To obtain more information about the microcrack behaviour under multiaxial non-proportional loading, sonic emission studies and fractographic analyses were performed. The results suggest a discontinuous microcrack propagation. Motivated by the good agreement between these observations and some microcrack propagation models known from literature, a simplified model was proposed for micro and short crack propagation. This model which is based on the J-integral range ΔJ yields a quite good agreement between the experimentally observed and the calculated lifetimes. [Copyright &y& Elsevier]

Simulations of the nucleation of dislocations, glide and annihilation ahead of a fatigue crack growing along a localized slip band (a ‘long’ Stage I crack or a Stage II crack with a K value close to the threshold) are performed for the case of push–pull or reversed torsion loadings, ignoring, in a first approach, the effect of grain boundaries. The crack growth rates are deduced from the dislocation flux at the crack tip. An influence of the normal stress on the friction between the crack flanks as well as on the condition for dislocation emission is introduced. A slower Stage I growth rate is then predicted for reversed torsion, consistent with experimental data. [ABSTRACT FROM AUTHOR]