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Since the discovery of fatigue phenomena, scientific research has constantly sought to understand and anticipate the failure of materials due to fatigue to mitigate unforeseen accidents and malfunctions in various technical fields. Numerous studies using acoustic emission (AE) – a key method in non-destructive testing – have shown a correlation between acoustic activity and fatigue damage. However, these measurements suffer from the non-specific nature of AE signals, which may be due to various physical sources. To investigate further the mechanisms of AE emission associated with fatigue, we study the groups of acoustic signals generated by fatigue cracking in metals. These so-called acoustic multiplets are characterized by highly correlated waveforms, are repeatedly triggered over many successive loading cycles at nearby stress levels and originate from a single location. These acoustic signatures produced during the propagation of fatigue cracks in alloys are automatically detected by a dedicated algorithm, grouped into multiplets and analyzed to understand the physical mechanisms from which they originate. By synchronizing their detection with digital image correlation measurements of fracture mechanics quantities, the investigation of this acoustic emission phenomenon shows that two mechanisms are at the origin of the multiplets: repeated local friction over fracture surfaces, and incremental crack propagation in the Paris regime, probably due to the reactivation of crack tip plasticity at each cycle. These two multiplet types serve as acoustic signatures, distinctly indicating the existence and propagation of a fatigue crack. • Acoustic multiplets are easily detected groups of acoustic emission (AE) signals • AE and digital image correlation analysis investigate multiplets emission mechanisms • Multiplets are associated to crack surfaces contact or incremental crack propagation • One type of acoustic multiplets are emitted at constant stress intensity factor • Multiplets are signatures of the presence or propagation of a fatigue crack [ABSTRACT FROM AUTHOR]

The plasticity-induced crack closure phenomenon has been the subject of many previous theoretical and experimental studies. From these studies it was found that in the absence of applied loading, a fatigue crack is likely to be partially closed due to the plastic wake, which is generated behind the tip of a propagating crack. This partial, or residual, closure has many implications for theoretical modelling as well as practical applications. However, no theoretical models have been developed to evaluate what part of a propagating fatigue crack is closed in the free from stress state. Direct 3D FE simulations of the plasticity-induced crack closure are very challenging, time consuming, and are not feasible for a large number of fatigue cycles. Therefore, the current paper utilises a 2D strip-yield idealisation and the distributed dislocation technique to investigate the effects of the stress amplitude and R-ratio of fatigue loading on the residual crack closure. The outcomes of this work may assist in the evaluation of the adequacy and limitations of the current analytical and computational models describing the behaviour of structures containing fatigue cracks, as well as in the interpretation of data from nondestructive defect inspections. [Display omitted] • Evaluation of the closed part of a fatigue crack when free from applied stress. • Analytical strip-yield model solved using the distributed dislocation technique. • Parametric study for a range of parameters of constant amplitude fatigue loading. • Assessment of the adequacy of slit and breathing crack models. [ABSTRACT FROM AUTHOR]

In this paper, an experimental procedure to evaluate the elastic-plastic J-integral at the tip of a fatigue crack is presented. According to this new approach, the elastic component of the J-integral is derived from Thermoelastic Stress Analysis, while the plastic component of the J-integral is derived from the heat energy loss. An analytical link is proposed to apply this new experimental technique. Therefore, the elastic-plastic J-integral range was evaluated starting from infrared temperature maps measured in situ during crack propagation tests of AISI 304L stainless steel specimens. It was found that the range of the infrared thermography-based J-integral correlated well the crack growth data generated in small as well as large scale yielding conditions. Finally, the experimental values of the J-integral were successfully compared with the corresponding numerical values obtained from elastic-plastic finite element analyses.

The paper revisits the way transient trapping is introduced in the literature based on the Sofronis and McMeeking model (Sofronis and McMeeking, 1989) [1] of hydrogen transport. It is shown that the direct use of the improved formulation made by Krom et al. (1999) [2] for transient trapping may lead to non-physical results of hydrogen concentration in case of an insulated system. The use of McNabb and Foster trapping kinetic equation is more relevant, and its ability to model both trap creation and kinetic trapping is investigated on a Small Scale Yielding configuration for the sake of comparison with a reference case from the literature. A parametric study is conducted, exhibiting differences with literature, and emphasizes on the significant effect of trapping kinetics on the hydrogen distribution. • The hydrogen transport equation is rewritten with a transient trapping process. • The Krom et al. Small Scale Yielding configuration is considered. • The influence of the loading time on the hydrogen repartition is focused on. • The influence of the trapping kinetic on hydrogen repartition is investigated. [ABSTRACT FROM AUTHOR]

High-temperature micro-fracture tests of a industrial grade W–1%Ta alloy were performed from room temperature up to 700 °C, using chevron-notched micro-cantilevers. A gradual increase of conditional fracture toughness (K Qc) was measured with increasing temperature, and a microscale brittle-to-ductile transition temperature (BDTT) was found at ~600 °C. This is slightly higher than macroscopic three-point bending tests from the same material (~400 °C), and contradicts most literatures for pure tungsten that shows a significant lower micro-BDTT. The results suggested that the BDTT is independent of the specimen size, and the higher micro-BDTT is due to the tantalum in this alloy. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Comparison of crystal plasticity simulations using FEM and FFT implementations. • Assessed reliability of CP-FFT near discontinuities including crack tip and GBs. • Differences in micromechanical fields and at the slip system level are quantified. • Describes issues with Gibbs phenomenon, periodicity constraints, traction continuity. • Discusses computational efficiency of parallelized implementation of CP-FFT. Crystal plasticity simulations enable a physically-based description of the micromechanical fields associated with microstructural attributes in polycrystalline materials. Fast Fourier transforms implementation of crystal plasticity (CP-FFT) in periodic unit cells offers increased computational efficiency compared to traditional finite element methods (CP-FEM). Albeit, questions may arise concerning the accuracy of the CP-FFT approach given its underlying spectral framework, specifically in regards to: potential artifacts due to violating the traction-free conditions at internal free surface within the unit cell, Gibbs phenomenon near regions with sharp changes in local mechanical properties, and representation of free surfaces under the constraints imposed by the requirement of periodicity of the unit cell. This paper presents a custom massively parallelized implementation of CP-FFT. A direct comparison of simulation results for polycrystalline deformation near a crack in a Ti55531 alloy is presented between CP-FEM and CP-FFT. The micromechanical fields and slip system level values are quantified and are similar between the two methods. Up to 10% difference in the stress fields is reported at the crack tip and this value dissipates moving away from the crack tip after 5 voxels (7 µm). The results point towards the reliable use of CP-FFT simulations to model polycrystalline deformation with caveats based on the treatment of the discussed artifacts. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The effect of single overload on the fatigue crack growth in 2024-T3 and 7075-T6 Al alloys was analyzed. Fatigue tests under constant-amplitude loading with overload peak were carried out on V-notched specimens. Fractographic analysis was used as a principal approach to explain the crack growth retardation due to the overload. Scanning electron microscopy (SEM) analyses were conducted on the fractured surface of failed specimens to study the retardation effect. The obtained results show that the overload application generates a plastic zone in both aluminum alloys. The generated plastic zone is three times larger in the case of 2024-T3 compared to 7075-T6, and thus, a significant crack retardation was induced for 2024-T3. The retardation effect due to the overload for 2024-T3 and 7075-T6 lasted for about 10 mm and 1 mm, respectively, from the point of overload application. [ABSTRACT FROM AUTHOR]

Metallic materials dissipate thermal energy when subjected to fatigue. Some of them, due a favorable combination of thermo-physical material properties, exhibit a significant temperature rise, which can be easily measured in-situ by means of thermocouples or infrared cameras. The heat energy dissipated in a unit volume of material per cycle (the Q parameter) has proven to be effective as a fatigue damage index in case of AISI 304L plain and notched specimens. Originally conceived and applied as a point-related quantity, recently Q has been averaged at the tip of propagating fatigue cracks (the Q* parameter) in order to correlate crack growth data gathered from fracture mechanics tests. The use of Q* seems interesting because (i) it can be evaluated in-situ from infrared temperature maps and (ii) crack acceleration due to excessive plasticity is likely to be accounted for.

In this paper, an experimental procedure to evaluate the elastic-plastic J-integral at the tip of a fatigue crack is presented. According to this new approach, the elastic component of the J-integral is derived from Thermoelastic Stress Analysis, while the plastic component of the J-integral is derived from the heat energy loss. An analytical link is proposed to apply this new experimental technique. Therefore, the elastic-plastic J-integral range was evaluated starting from infrared temperature maps measured in situ during crack propagation tests of AISI 304L stainless steel specimens. It was found that the range of the infrared thermography-based J-integral correlated well the crack growth data generated in small as well as large scale yielding conditions. Finally, the experimental values of the J-integral were successfully compared with the corresponding numerical values obtained from elastic-plastic finite element analyses.

• An analytical relation temperature field vs elastic-plastic J has been formalised. • Elastic component of J was estimated from thermoelastic stress analysis. • Plastic component of J was estimated from heat dissipation. • The experimental and numerical the elastic-plastic J are in agreement. The elastic-plastic J-integral is adopted to correlate fatigue crack growth data of ductile metals. An analytical link is known to exist between the J-integral and the strain energy density averaged in a control volume embracing the crack tip. On the other hand, the strain energy fluctuation is the source of temperature variations close to a fatigue crack tip of a metal material; hence the possibility to measure the J-integral from infrared thermographic scanning at the crack tip is envisaged and it is the focus of this paper. It is proposed that the elastic component of the J-integral is derived from a thermoelastic stress analysis, while the plastic component of the J-integral is derived from the heat energy loss. An analytical expression is formalised to apply this novel approach. Therefore, the elastic-plastic J-integral range was evaluated starting from infrared temperature maps measured in situ during crack propagation tests of AISI 304L stainless steel specimens. The range of the infrared thermography-based J-integral correlated well the crack growth data generated in small as well as large scale yielding conditions. Finally, the experimental values of the J-integral were successfully compared with the corresponding numerical values obtained from elastic-plastic finite element analyses. [ABSTRACT FROM AUTHOR]

In this paper, an experimental procedure to evaluate the elasticplastic J-integral at the tip of a fatigue crack is presented. According to this new approach, the elastic component of the J-integral is derived from Thermoelastic Stress Analysis, while the plastic component of the J-integral is derived from the heat energy loss. An analytical link is proposed to apply this new experimental technique. Therefore, the elastic-plastic J-integral range was evaluated starting from infrared temperature maps measured in situ during crack propagation tests of AISI 304L stainless steel specimens. It was found that the range of the infrared thermography-based J-integral correlated well the crack growth data generated in small as well as large scale yielding conditions. Finally, the experimental values of the J-integral were successfully compared with the corresponding numerical values obtained from elasticplastic finite element analyses. [ABSTRACT FROM AUTHOR]

Highlights • Intergranular fracture occurs without clear modification in crack tip plasticity. • Transgranular fracture occurs with great reduction of crack tip plasticity. • Crack tip plasticity reduction is associated with fatigue crack growth enhancement. • Mechanism models of hydrogen-affected fatigue crack growth are proposed. • Frequency and hydrogen pressure dependencies can be explained by proposed models. Abstract Hydrogen-Affected Fatigue Crack Growth (HAFCG) in commercially pure iron has been characterized in terms of hydrogen gas pressure, loading frequency and stress intensity factor range Δ K. A higher hydrogen gas pressure decreases the critical value of Δ K triggering the HAFCG enhancement, and a lower loading frequency increases the HAFCG enhancement. Intergranular FCG in the non-accelerated regime is likely caused by the hydrogen-induced microvoid coalescence along the grain boundary, while a brittle cyclic cleavage fracture in the accelerated regime can be explained in terms of crack tip sharpening and hydrogen-enhanced decohesion process. [ABSTRACT FROM AUTHOR]

Abstract The aim of this paper is to elucidate the mechanisms controlling the brittle-to-ductile transition (BDT) in pre-deformed, textured, polycrystalline body-centred cubic (bcc) metals by the example of cold rolled tungsten (W). For this purpose, five sheets were rolled out from one and the same sintered ingot, by various levels, representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (this refers to 83.5%, 91.8%, 95.0%, 96.7%, and 98.3% in the technical notation). Toughness tests show that the BDT temperature decreases with increasing degree of deformation from 115 °C ± 15 °C (388 K ± 15 K) down to −65 °C ± 15 °C (208 K ± 15 K). This is an improvement of >600 K compared with a sintered ingot. In this paper we perform an in-depth analysis of the microstructure of the five sheets mentioned above. This analysis includes the assessment of (i) crystallographic texture, (ii) grain size and (iii) dislocation density. A comparison between microstructural features and experimental data confirms our working hypothesis which states that the BDT is controlled by the glide of screw dislocations and that the transition temperature decreases with decreasing spacing, λ , of dislocation sources along the crack front. Sources for dislocations may be the intersection points of grain boundaries with the crack front (BDT-temperature-grain-size-relation) or dislocation multiplication processes such as e.g., the expansion of open and closed loops (impact of dislocation density). Graphical abstract Unlabelled Image Highlights • Five tungsten sheets were rolled out from one and the same sintered ingot. • The evolution of texture, grain size and dislocation density is displayed. • A correlation was found between microstructural features and BDT temperatures. • The brittle-to-ductile transition is controlled by the glide of screw dislocations. • BDT temperature = f(spacing, λ , of dislocation sources along the crack front). [ABSTRACT FROM AUTHOR]

A new method for the analysis of plasticity-induced closure of finite cracks is developed based on the distributed dislocation technique. The method is applied to analyse closure and opening processes for an embedded fatigue crack propagating under constant amplitude cyclic loading. The obtained solution and presented results can serve as a benchmark for numerical procedures utilising the same yield-strip methodology. Comparison with past numerical studies reveals similar trends, though there are some differences in the crack tip opening values, particularly for low and negative R-ratios. The latter may indicate the need for a better approach to the discretisation of the contact, direct and reverse plasticity zones in the numerical procedures which utilise the same methodology. The developed method is general, and it can be adapted to evaluate the crack tip opening stress for other crack geometries and loading conditions. [Display omitted] • Analytical method for analysis of cracks with arbitrary plastic wake is developed. • Exact solutions for a crack growing under constant amplitude loading is presented. • The effects of R-ratio and maximum load amplitude on crack closure are investigated. • Solutions are compared with numerical studies using the same yield-strip methodology. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Crack acceleration occurs in AISI 304L at high ΔK, due to excessive plasticity. • Crack growth data are correlated by the elastic-plastic ΔJ evaluated by FEM. • Heat dissipation inside a properly defined structural volume at the crack tip was measured. • ΔJ was estimated also from heat dissipation measured experimentally. • An analytical relation between heat dissipation and ΔJ has been formalised. Abstract The heat energy exchanged in a unit volume of material per cycle, Q ¯ , has been previously proposed and adopted by the authors as a fatigue damage indicator, because it accounts for the response of the material to the external applied loads as a result of energy conversion from the intrinsic mechanical dissipation. Q ¯ can be determined by means of temperature measurements performed in situ during fatigue tests, at least for certain classes of metallic and composites materials. Originally conceived as a point-related physical quantity, recently a theoretical frame and an experimental procedure have been defined to average Q ¯ inside a volume surrounding the tip of a fatigue crack. The resulting averaged heat energy per cycle Q ¯ ∗ is an appropriate candidate to correlate crack growth data generated from fracture mechanics tests, especially at relatively high applied ΔK values, when the Linear Elastic Fracture Mechanics hypotheses are violated. The present paper is focussed on the use of Q ¯ ∗ as an elastic-plastic fracture mechanics parameter. First of all, the size of the structural volume, where Q ¯ is to be averaged to calculate Q ¯ ∗ , was defined with reference to an AISI 304L stainless steel. After that, the averaged heat energy Q ¯ ∗ was determined during crack propagation tests starting from the temperature maps measured in situ by means of an infrared camera. The use of Q ¯ ∗ resulted promising because it could take into account crack acceleration due to excessive plasticity. Finally, in the context of energy-based approaches to fatigue crack propagation problems, a link between the averaged heat energy Q ¯ ∗ and Rice's J-integral has been discussed and formalised. [ABSTRACT FROM AUTHOR]

Abstract: Many cutting processes, such as chopping, slicing, and carving, consist in 2 different stages: an initial stage of indentation, in which the cutting tool is pushed into the material under the action of an external force, and a second stage, where the target material undergoes a progressive separation. This second stage is characterised by the formation of a fracture surface followed by the cut propagation due to the increasing external force, until eventually a steady state might occur. The purpose of this paper is to analyse the cutting process by means of some concepts of fracture mechanics and discuss the occurrence of the steady state. A simple model is used to obtain an analytic expression of the stress intensity factor at the tip of the cut and investigate the evolution of the fracture process. It is found that the cut propagation depends on the wedge sharpness. The analytic results are compared with finite element analyses, where the effect of tip blunting due to plasticity is taken into account. The influence of the cutting tool geometry is also discussed. [ABSTRACT FROM AUTHOR]

This paper attempts to elucidate fundamental mechanisms of the brittle-to-ductile transition (BDT) in textured, pre-deformed, polycrystalline body-centred cubic metals. For this purpose, five tungsten sheets have been rolled out from one and the same sintered ingot representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (logarithmic notation). Electron backscatter diffraction measurements display the evolution of the microstructure with increasing degree of deformation. The mean grain size of the five sheets in the normal direction is 1.1, 0.59, 0.48, 0.37, and 0.3 μm. Toughness tests based on the K-concept have been performed and the brittle-to-ductile transition temperature of the five sheets has been determined. The crack system used was the L-T crack system. The results have been benchmarked against the BDT temperature of a sintered ingot. The results show a decrease of the BDT temperature with increasing degree of deformation starting from 600 °C (873 K) for the sintered ingot via 115 °C (388 K), 85 °C (358 K), 75 °C (348 K), 60 °C (333K) down to − 65 °C (208 K) for the most heavily deformed tungsten plate. The results are discussed against the background of (i) rolling induced lattice defects such as (i-a) vacancies, (i-b) dislocations (i-c) grain boundaries, (ii) crystallographic texture, (iii) the state of stress, (iv) impurities, (v) sinter pores, and (vi) the geometrical arrangement of areas with low and high fracture toughness. The authors postulate that the BDT is still controlled by the kink-pair nucleation process of a screw dislocation and that the BDT temperature scales with the spacing of dislocation sources (e.g. grain boundary ledges, low angle boundaries, debris loops) along the crack front. [ABSTRACT FROM AUTHOR]

Ti exhibits complex plastic deformation controlled by active dislocation and twinning systems. Understandings on dislocation cores and twin interfaces are currently not complete or quantitative, despite extensive experimental and simulation studies. Here, we determine the dislocation core, twin, and crack properties in both HCP and BCC Ti using a Deep Potential (DP), DFT and linear elastic fracture mechanics. We compute the core structures, critical resolved shear stresses and mobilities of the 〈 a 〉 , 〈 c + a 〉 , 〈 c 〉 dislocations in HCP and the 〈 111 〉 / 2 dislocations in BCC Ti. The 〈 a 〉 slip consists of slow core migration on pyramidal-I planes and fast migration on prism-planes, and is kinetically limited by cross-slips among them. This behaviour is consistent with "locking–unlocking" phenomena in TEM and is likely an intrinsic property. Large-scale DFT calculations provide a peek at the screw 〈 c + a 〉 core and glide behaviour, which is further quantified using DP-Ti. The screw 〈 c + a 〉 is unstable on pyramidal-II planes. The mixed 〈 c + a 〉 is nearly sessile on pyramidal-I planes, consistent with observations of long dislocations in this orientation. The edge and mixed 〈 c + a 〉 are unstable against a pyramidal-to-basal (PB) transition and become sessile at high temperatures, corroborate the difficulties in 〈 c 〉 -axis compression of Ti. Cracks on basal planes are predicted and shown to be intrinsically brittle, consistent with cleavage facets primarily on this plane in experiments. Twin boundary energies vary considerably but all twin boundary structures possess a mirror reflection symmetry. Finally, in BCC Ti, the 〈 111 〉 / 2 screw has a degenerate core with average glide on { 112 } planes; the 〈 111 〉 / 2 edge and mixed dislocations have non-dissociated cores on { 110 } planes. This work paints a self-consistent, complete picture on all dislocations in Ti, rationalises previous experimental observations on Ti plasticity and fracture, and points to future HRTEM examinations of unusual dislocations such as the mixed and PB transformed 〈 c + a 〉 cores. • Dislocation, twin and crack-tip properties are quantitatively determined in Ti. • 〈 a 〉 slip in HCP-Ti consists of slow and fast core migrations on two different planes. • The edge and mixed 〈 c + a 〉 dislocations become unstable and sessile at high temperatures. • The 〈 111 〉 / 2 screw in BCC-Ti has a degenerate core with average glide on { 112 } planes. • Sharp crack-tips on the basal plane are intrinsically brittle in HCP Ti. [ABSTRACT FROM AUTHOR]

Metallic materials dissipate thermal energy when subjected to fatigue. Some of them, due a favorable combination of thermo-physical material properties, exhibit a significant temperature rise, which can be easily measured in-situ by means of thermocouples or infrared cameras. The heat energy dissipated in a unit volume of material per cycle (the Q parameter) has proven to be effective as a fatigue damage index in case of AISI 304L plain and notched specimens. Originally conceived and applied as a point-related quantity, recently Q has been averaged at the tip of propagating fatigue cracks (the Q* parameter) in order to correlate crack growth data gathered from fracture mechanics tests. The use of Q* seems interesting because (i) it can be evaluated in-situ from infrared temperature maps and (ii) crack acceleration due to excessive plasticity is likely to be accounted for. [ABSTRACT FROM AUTHOR]

The paper investigates the crack tip plasticity of laser stake-welded T-joints when loaded in tension and bending. Irwin’s first-order plastic zone size is kept equal for the selected load levels, and finite element (FE) analyses are carried out to investigate the stress redistribution in the plastic zone varying the applied loads. The results allow us to explain why the fatigue slope m is different for bending and tension. Moreover, a new method is proposed that permits the number of cycles to failure under bending to be derived directly from the tension fatigue curve, employing an effective J-integral. The results prove that the method is able to give an estimation of the number of cycles to failure under bending with a very good approximation and acceptable errors. [ABSTRACT FROM AUTHOR]

Plastic deformation around a stationary mode I crack in an isotropic material is studied within the continuum theory of stress gradient plasticity (GP). This model, as a lower-order plasticity theory, has been quite successful in predicting the size-dependent plastic behavior in micro-torsion as well as micro-bending. However, its application to a problem involving a complex stress state reveals here that it has to be modified with a proper measure of stress gradient in the continuum context. To this end, a new measure of stress gradient is proposed and assessed appropriately. It is then employed to investigate crack tip stress fields within the GP theory. Analyses show that a higher stress level is predicted near the crack tip using the GP theory when compared with the classical plasticity predictions. The increase of stress level due to stress gradient effects is predicted around 35% which is enough for cleavage cracking. However, relatively higher stress levels were expected due to large stress gradients near the crack tip. Therefore, either a higher-order formulation or involving higher-order spatial gradients of stress is required to study fracture with the GP theory. [ABSTRACT FROM AUTHOR]

The in-plane classical dislocation-based linear elastic fracture mechanics analysis is extended to the case of strain gradient elasticity. Nonsingular stress and smooth-closure crack profiles are derived. As in the classical treatment, the crack is represented by a distribution of climb edge dislocations (for Mode I) or glide edge dislocations (for mode II). These distributions are determined through the solution of corresponding integral equations based on variationally consistent boundary conditions. An incompatible framework is used and the nonsingular full-field plastic distortion tensor components are calculated. Numerical results and related graphs are provided illustrating the nonsingular behaviour of the stress/strain components and the smooth cusp-like closure of the crack faces at the crack tip. The work provides an alternative approach to celebrated 'Barenblatt's treatment' of cracks, without the introduction of a cohesive zone and related to intermolecular forces ahead of the physical crack tip. It also supplements a recent paper by the authors in which the mode III crack, represented by an array of screw dislocations, was solved within the present gradient elasticity framework. [ABSTRACT FROM AUTHOR]

Molecular Dynamics (MD) simulations are often used for comprehending evolving deformation mechanisms in materials at the atomic scale and also for assessing continuum-scale material properties. A major limitation of conventional MD simulations is that very small MD time-scale ( ∼ fs ), restrict the achievable strain-rates to be much higher ( ∼ 10 7 or higher) than experimentally observed rates, needed for continuum scale modeling, e.g. using crystal plasticity finite element methods. A strain-boost hyperdynamics method based accelerated MD tool is adopted and developed to overcome these limitations. This method biases the atomic system to make it evolve at much faster time-scales and achieve strain-rates that are at least three order of magnitude smaller than the lowest strain-rate achievable in MD. The hyperdynamics algorithm is implemented in a parallel version of LAMMPS, and validated with conventional MD. It is then used to predict evolution of plastic state variables at lower strain-rates for a Nickel single crystal with an embedded atomistic crack. It is shown that at lower strain-rates, not only the evolution of plastic variables are different, but for some configurations there is a shift in the plastic deformation mechanism from twin dominated to dislocation dominated. [ABSTRACT FROM AUTHOR]

The influence of the thickness in mode I fracture tests is investigated with extensive 3D finite elements calculations. A realistic constitutive law for the bulk is used. Failure by crazing is described with a cohesive model. A transition from ductile tearing to fracture by crazing is observed with increasing thickness. Ductile tearing takes place for thin samples even if possible crazing is allowed. Crazing takes place for thick samples with a noticeable difference in the load level for craze initiation to that for crack propagation. The consequence on the appropriate geometry for the estimation of the minimum toughness is discussed. [ABSTRACT FROM AUTHOR]

Confined crack tip plasticity model is employed to predict time independent fatigue crack growth rate (FCGR) behavior of HAYNES® 282® alloy at temperatures 1200F and 1400F. Crack growth tests were done in lab air, vacuum and steam environments at load ratios R = K min /K max ranging from 0.05 to 0.5. Calibrated model predicts average cyclic crack growth rate behavior of the material reasonably well. Predictions do not capture the accelerated fatigue crack growth rates observed in the data at low load levels. Such effects are believed to be caused by environmentally driven factors, which are not expected to be predicted by plasticity based models. [ABSTRACT FROM AUTHOR]

This paper analyzes crack tip stress and strain fields at crack initiation under high strain rates. In the range of high strain rates, the mechanical behavior of a material is characterized by an increased in strain rate sensitivity. However, due to geometry and constraint of various test specimen, it is difficult to quantify strain rate on the at constant test speed. To quantify the strain rate, we simulate crack tip stress and strain fields using three-dimensional finite element model. Our analysis models are fracture toughness (CT) specimen and full scale circumferential cracked pipe component. Dynamic material properties used in the finite element analysis are based on Pipe Fracture Encyclopedia. Pipe Fracture Encyclopedia provides strain rate dependent tensile test, fracture toughness test and full-scale four point bending test. All experiments were tested at 288 °C and test specimens were extracted from A106 Gr, B carbon steel pipes. Three different strain rate tensile test (4 x 10 -4 /s, 3.4/s and 11.6/s) are fitted using Johnson-Cook model. Our results show that, applied strain rate and strain rate at characteristic length have similar values as in the CT specimen simulation result. Similar outcomes were shown in the pipe simulation result. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper investigates the effect of a variation in material properties on the crack tip opening displacement, a parameter often used in the prediction of fatigue and fracture. This situation is typical when a component is subjected to a relatively slow temperature fluctuation or the material properties undergo direct changes, such as due to a phase transformation. An analytical strip-yield model is developed using the small-scale yielding assumption and theory of complex potentials. Four cases of crack tip plasticity behaviour are identified for the different combinations of parameters controlling the variation in material properties. Results of calculations over a wide range of material properties are presented and show a significant effect on the crack tip opening displacement. Finite element simulations are conducted to verify the analytical findings. The implications of the outcomes in relation to several practical situations are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: Materials in rotating machinery are typically subjected to loading conditions that combine both low cycle fatigue (LCF) and high cycle vibratory fatigue (HCF). In operating conditions, the number of allowable LCF cycles may be limited, but under no circumstances, superimposed HCF vibratory cycles can be allowed to produce fatigue damage. This paper aims at proposing an approach to account for the effect of variable amplitude LCF cycles on the endurance domain relative to HCF cycles. For this purpose, a LEFM-based approach is proposed. The material is assumed to contain flaws, and their growth is assumed to stem from crack tip plasticity. As a consequence of these “classical” assumptions, the endurance domain of the material can alternatively be viewed as an elastic domain for the crack tip region. A generalized von Mises yield criterion is hence introduced to model that elastic domain. For this purpose, rather than considering the distortional elastic energy density (von Mises), the Westergaard’s stress field is used to calculate the distortional elastic energy within a distance δ from the crack tip. A non-local yield criterion for the crack tip region is obtained. The first non-singular terms (e.g., the T-stresses) are included in the Westergaard’s stress functions to make it possible to use it also for mechanically short cracks. Then to account for the effect of LCF cycles, the elastic domain is allowed to evolve when crack tip plasticity occurs. The center of the endurance domain is defined as an internal variable that stands for the internal stress field within the crack tip region due to constrained plastic deformation. As long as the loading path remains inside the elastic domain, no plasticity, and hence no crack growth, is expected. If the yield surface is reached, plastic strain occurs and the elastic domain is displaced. In this study, elastic–plastic finite element computations are used to determine whether the crack tip region behaves elastically or plastically. For this purpose, the velocity field within the crack tip region is partitioned into elastic and plastic parts, each part being approached as the product of an intensity factor and a spatial reference field. The plastic intensity factor is then used as a global measure of the plasticity rate within the crack tip region. In this paper, the approach was applied to a forged Ti–6Al–4V titanium alloy, used in compressor blades of gas turbine engines. [Copyright &y& Elsevier]

Abstract: A theoretical model is suggested which describes generation and growth of nanoscale cracks (nanocracks) near tips of blunt cracks in nanocrystalline solids. Within the model, stress concentration near pre-existent/growing blunt cracks induces grain boundary sliding which leads to formation of dislocations at triple junctions of grain boundaries. The superposition of the external stress concentrated near tips of blunt cracks and stresses created by these dislocations is capable of initiating generation and growth of nanocracks. The characteristics of the nanocrack generation and growth in nanocrystalline Al and α-Fe are calculated, and the effects of these processes on toughness/ductility of nanocrystalline solids are discussed. [Copyright &y& Elsevier]

In this study, the effects of plasticity-induced crack closure and overloads were investigated using thermoelastic stress analysis (TSA) in aluminum alloy 2024. The amplitude of the stress intensity factors were evaluated during crack growth by fitting a Muskhelishvili-type description of the crack tip stress fields to the TSA data using the multipoint overdeterministic method. A new method to directly measure the extent of the crack tip plastic zone is proposed based on the phase difference between the TSA and forcing signals. The presence of crack tip plasticity was clearly identified and correlated with changes in the stress intensity factor values obtained from the TSA data both during constant amplitude loading as well as single and multiple overloads. Immediately post-overload the plastic zone increased in size by up to 50% whereas the amplitude of the stress intensity factor and the crack growth rate decreased until the crack grew through the plastic region created by the overload when the pre-overload value of the crack growth was re-attained. The experimental data of mode I stress intensity factor, crack growth rate and radius of crack tip plastic zone obtained from thermoelastic data in the region affected by the overload were used to estimate the exponent in the Wheeler crack growth model which was found to be a function of percentage overload and the number of cycles of overload. [ABSTRACT FROM AUTHOR]

Abstract: C–Mn steels are prone to static and dynamic strain aging (SSA and DSA) that induce Lüders or Portevin–Le Chatelier strain localization phenomena. Tensile tests and fracture tests were carried out between 20°C and 350°C. Negative strain rate sensitivity (SRS) and discontinuous yielding were evidenced, in relation with a drop in fracture toughness. The Kubin–Estrin–McCormick (KEMC) constitutive law, well suited for this kind of material response, was identified so as to model the mechanical behavior of the ferritic steel from 20°C up to 350°C. It is shown that both static and dynamic strain aging can be modeled by introducing a monotonic temperature dependence of all the parameters except the strain hardening parameter. Extra-hardening due to dynamic strain aging is clearly evidenced. The influence of SSA and DSA on the plastic zone development at the crack tip of CT specimens is analyzed on the basis of a finite element study. [Copyright &y& Elsevier]

Abstract: Extremum/work principles for a rigid–plastic body have been discussed in classical theory of plasticity to be of immense significance. Unfortunately, till now, these extremum theorems have been used only as a crude method of obtaining the limit load of a rigid–plastic body, using successive approximations by upper and lower bound estimates. On the other hand slip-line fields (SLF) have been extensively used not only for evaluation of limit load but also for obtaining sufficiently accurate estimates of stresses in the plastic region as well as in the vicinity of crack tip. Till now, these two methods of plastic analyses, that is, the work principles and SLF have remained more or less independent apart from the fact that both are upper bounds as they use kinematically admissible velocity fields. Recently, a new load bounding technique, modified upper bound (MUB) Approach, was proposed by Khan and Ghosh [Khan, I.A., Ghosh, A.K., 2007. A modified upper bound approach to limit analysis for plane strain deeply cracked specimens. International Journal of Solids and Structures 44 (10), 3114–3135]. In this article, a rigorous mathematical basis of this load bounding technique is presented and it is demonstrated that the method is actually a new form of the general extremum/work principles. The equivalence of this new form of work principle, that is, MUB with the classical SLF analysis, for a rigid–plastic material in plane strain, has been discussed in detail. Since plastic deformation fields depend on specimen geometry and type of loading specific cases have been considered. Both cracked and uncracked configurations have been analysed to establish this equivalence in general. Various simplifications resulting from the use of this new load bounding technique over SLF method has been demonstrated. Several standard problems of plane strain analysed by SLF method and validated by experiments in past have been considered in this article. As a novel application of the proposed method, single-edge-cracked plate under combined bending and tensile load has been analysed. For this specimen SLF solutions are available only for bending with small tensile load (defined in Section 3.2.4) while classical upper bound solutions are valid for bending with large tensile load. In this work a completely analytical formulation for yield locus for the entire range of tensile and bending load has been obtained. Apart from accurate evaluation of limit load, detailed evaluation of crack tip stresses and hence constraint near the crack tip has been performed using this new form of work principles. [Copyright &y& Elsevier]

Abstract: The anisotropic nature of mixed modes I–II crack tip plastic core region and crack initiation is investigated in this study using an angled crack plate problem under various loading conditions. Hill’s anisotropic yield criterion along with singular elastic stress field at the crack tip is employed to obtain the non-dimensional variable-radius crack tip plastic core region. In addition, the R-criterion for crack initiation proposed by the authors for isotropic materials is also extended to include anisotropy. The effect of Hill’s anisotropic constants on the shape and size of the crack tip plastic core region and crack initiation angle is presented for both plane stress and plane strain conditions at the crack tip. The study shows a significant effect of anisotropy on the crack tip core region and crack initiation angle and calls for further development of anisotropic crack initiation theory. [Copyright &y& Elsevier]

Abstract: Analytical and numerical approaches are used to solve an axisymmetric crack problem with a refined Barenblatt–Dugdale approach. The analytical method utilizes potential theory in classical linear elasticity, where a suitable potential is selected for the treatment of the mixed boundary problem. The closed-form solution for the problem with constant pressure applied near the tip of a penny-shaped crack is studied to illustrate the methodology of the analysis and also to provide a fundamental solution for the numerical approach. Taking advantage of the superposition principle, an exact solution is derived to predict the extent of the plastic zone where a Tresca yield condition is imposed, which also provides a useful benchmark for the numerical study presented in the second part. For an axisymmetric crack, the numerical discretization is required only in the radial direction, which renders the programming work efficient. Through an iterative scheme, the numerical method is able to determine the size of the crack tip plasticity, which is governed by the nonlinear von Mises criterion. The relationships between the applied load and the length of the plastic zone are compared for three different yielding conditions. To cite this article: S. Chaiyat et al., C. R. Mecanique 336 (2008). [Copyright &y& Elsevier]

This paper describes a method for modelling cyclic crack tip plasticity effects based on the distributed dislocation technique (DDT). A strip-yield model is utilised to allow for the determination of the crack opening displacement, size of the plastic zones and in the case of a fatigue crack, the wake of plasticity. The DDT can be easily implemented for a wide range of cracked geometries with reliable control over the accuracy and convergence. Thickness effects can also be incorporated through a recently obtained solution for an edge dislocation in an infinite plate of finite thickness. Results for finite length cracks that have had limited growth, such that there is no plastic wake, are presented for a range of applied loads and R-ratios. Further results are provided for a steady-state fatigue crack in a plate of finite thickness. The present results are compared with analytical solutions and they show an excellent agreement. [ABSTRACT FROM AUTHOR]

Abstract: The plastic blunting process during stage II fatigue crack growth was studied in pure polycrystalline Ni to investigate effects of strain localization and inelastic behavior on the kinematics of crack advance. Correlations were obtained between strain fields ahead of a fatigue crack, crack advance per cycle and crack growth kinetics. Strain fields were quantified using a combination of in situ loading experiments, scanning electron microscopy and digital image correlation for 8<ΔK <20MPam1/2 and a fixed load ratio of 0.1. Results indicate that strain localized along a dominant deformation band, which was usually crystallographic and carried mostly pure shear for large loads and was of mixed character for lower loads. Instances of double deformation bands were observed, with bands acting either in a simultaneous or alternating fashion. It was found that the area integral of the opening strain for values larger than a given threshold, an “integrated” strain, had a power-law relationship with ΔK, with the exponent approximately equal to the Paris exponent (m). Therefore, the crack growth rate was proportional to the integrated strain. An analysis based on this correlation and the presence of dominant shear bands indicated that the integrated strain is related to the accumulated displacement in the band. This, in turn, is proportional to the product of the cyclic plastic zone radius and the average shear strain ahead of the tip, which represents a basic length scale for plastic blunting. Assumptions on the load dependence of these quantities, based on their observed spatial variation, allowed estimating , where n′ is the cyclic hardening exponent (0< n′<1). This gives 3< m <4, which accounts for about 50% of the observed values of m between 1.5 and 6 for a wide variety of metallic materials. [Copyright &y& Elsevier]

Abstract: Results are reported for pressure–shear plate impact experiments in which pre-cracked 4340 steel plates are subjected to Mode II loading. Experiments show the propagation of a shear band ahead of the initial crack. Finite element simulations are used to interpret the results. Normal and transverse velocity–time profiles measured at the rear surface of the target can be simulated reasonably well using even an elastic model for the material response. A propagating shear band is obtained when the material is modeled as having reduced shearing resistance described by a thermo-viscoplastic power law, and complete loss of shearing resistance when the shear strain reaches a critical value. However, the predicted speed of propagation of the tip of the shear band is substantially less than required to explain the lengths of the bands observed in the experiments. Adjustments of parameters of the power-law model have little effect on the overall length of the band. Possible reasons for differences between predicted and measured shear band speeds are examined. Further reduction in the shearing resistance in the shear band appears to be essential for the simulated bands to be as long as those observed in the experiments. [Copyright &y& Elsevier]

Abstract: The present paper reports the results of a study on fatigue crack growth in a system with an interface between two elastic–plastic solids of different yield strength. Fatigue crack growth analysis is conducted by the use of a cohesive zone model. Irreversibility of material separation processes is introduced through the load history dependent degradation of the cohesive strength. Fatigue crack growth is considered to occur along the direction perpendicular to the interface and along the interface. Crack growth rate acceleration, deceleration or arrest, as well as crack bifurcation at the interface are predicted in dependence of the plastic property mismatch of the two solids and the interface properties. The outcome of the simulations is in very good agreement with trends of published experimental data. [Copyright &y& Elsevier]