The engineering rockmass is often prone to compression, therefore how to establish a fracture criterion to reflect the rockmass compression failure is vital. Take the rockmass with one oblique crack for instance, the mechanical behavior of the oblique crack face closure and friction sliding under uniaxial compression is firstly analyzed, and then the Kolossoff-Muskhelishvilli stress function of the rockmass under uniaxial compression is established based on the complex stress functions. Next, the calculation methods of the stress intensity factor (SIF) K and three T-stress components at or near the crack tip considering three kinds of crack parameters (e.g. geometry parameter, friction strength parameter and deformation parameter) are obtained. Thirdly, the improved maximum tangential stress (MTS) criterion is obtained considering T-stress and three kinds of crack parameters. Finally, the relevant experiments done by other researchers are adopted to verify the improved MTS criterion. Meanwhile, the effect of γ, the crack normal stiffness kn and shear stiffness ks, and the crack friction coefficient f on the wing-crack initiation angle are discussed with the parametric sensitivity analysis.

Infrared Thermography has been under review in the last thirty years due to its versatility and potential in the detection of the thermal signature associated with intrinsic energy phenomena due to dissipative processes, specifically those relying on mechanical fatigue. Nowadays, it is a well-established technique that can support mechanical and structural engineers to implement a damage-tolerant design, assess the residual life, and finally characterize the fatigue behavior of materials. The aim of this work is to review all thermography-based approaches and procedures for fatigue limit estimation by rapid tests, draw considerations on the role of thermal methods in mechanical design and machine construction, and discuss the pros and cons of each method and open points. This review is intended on one hand, as a warning/guideline to direct new research toward issues that have not been fully resolved or understood yet and on the other hand, to sum up, what has already been done in the field.

Due to the additional local stress concentration caused by undercut and misaligned defects, welded joints own less fatigue strength. This work uses probabilistic technique and fracture mechanics theory to quantitatively investigate the impacts of undercuts and misalignments on fatigue performance and reliability of Load-carrying Cruciform Welded Joint (LCWJ). Firstly, the geometrical characteristics summary of the size and type of undercuts and misalignments are provided from researches and experiments. Subsequently. Evaluations of the stress levels are combined with the nominal loadings in LCWJs, probabilistic distributions of material fracture properties, and various configurations of geometries and flaws. Meanwhile, the estimations of fatigue strengths are conducted by the probabilistic reliability theory by taking into account the distributions of the actual fatigue data. The findings reveal a clear disparity between the base metal and weldment test results. Different reliability levels for various defect types and sizes and in LCWJs are caused by the tolerance limits.

Four different types of specimens were prepared from the center and the periphery of two large IN706 forged discs of commercial scale, and low cycle fatigue (LCF) tests were conducted at 650 oC. The IN706 specimens with lower LCF lives showed relatively large fraction of cleavage-like fracture along linearly aligned η (Ni 3Ti) precipitates in the area of crack propagation. The amount of η precipitates for those specimens with greater LCF lives was negligible, and the fracture mode of crack propagation was dominantly intergranular. Crack initiation was mainly by persistent slip band (PSB) cracking at the surface, and no notable difference was found for each specimen. The correlation between tensile properties, grain size and LCF lives of IN706 at 650 oC appeared to be not as significant as expected.

In order to raise the hardness and strength of the surface layer of mechanical components and induce favourable residual compressive stresses, case-hardening procedures have become established in the heat treatment of steel. In this work, a calculation concept for the fatigue strength of components that have been case-hardened through carburizing heat-treatment is being developed. The residual stress and the load stresses in complex-shaped, carburized materials are determined using a finite element (FE) model. The fatigue limit of the components is derived using probabilistic methods and taking into account hardness gradients, residual stresses, and non-metallic inclusions. The model is validated with available axial bending fatigue test data and then used to predict the rotating bending fatigue limit of samples with various geometries and heat-treatment conditions. This work demonstrates the capability of combining probabilistic and FE-based modelling to represent complex interactions between variables that affect the fatigue of heat-treated components, such as steel cleanliness, notch shape, case-hardening depth, or loading conditions.

The fatigue analysis of structural components is a relevant research topic in both scientific and industrial communities. Despite major advances in understanding, fatigue damage remains a significant issue for both metallic and non-metallic components, sometimes leading to unexpected failures of in-service parts. Among the different assessment methodologies, critical plane methods have gained significance as they enable identification of a component’s critical location and direction of early crack propagation. However, the standard plane scanning method for calculating critical plane factors is computationally intensive and, for that, it is only applied when the component critical regions are already known. When critical areas are not easily identifiable due to complex geometries, loads or constraints, a more efficient method for evaluating critical plane factors would be required. This work presents a closed form solution for efficiently evaluating the Fatemi-Socie critical plane factor, in case of linear-elastic material behaviour and proportional loading conditions, based on tensor invariants and coordinates transformation laws. The proposed algorithm was tested on different test cases (i.e. hourglass, notched and welded joint geometries) under different loading conditions (i.e. tensile, bending and torsion) and showed a significant reduction in computation time compared to the standard plane scanning method.

This study aims to revealing the relationship between retained austenite (RA) morphology, deformation-induced transformation (DIPT) behavior and surface wear behavior. The hardening and fatigue behavior during wear of continuous film-like RA (CFRA), blocky RA (BRA), discontinuous film-like RA (DFRA) and point or flocculent RA (PRA) achieved through different tempering process was investigated using quasi-in-situ fatigue test and impact abrasive wear test. RA morphology exhibited a significant influence on hardening and fatigue behavior during wear. CFRA exhibited the best wear properties, owing to its excellent fatigue performance and hardening capacity. The BRA displayed relative high wear performance, with adequate hardening capacity but weak fatigue performance. DFRA demonstrated comparable wear resistance to BRA due to better fatigue performance. PRA exhibited deficient wear performance due to insufficient fatigue properties and low work hardening ability.

This study explores the effect of adding 3wt.% Y to pure magnesium (Mg) on its mechanical behavior under cyclic torsional loadings at room temperature. The research examines deformation and cracking modes in both pure Mg and Mg-3Y samples. Deformation modes are monitored using quasi-in-situ EBSD observations coupled with slip trace analysis. The findings reveal that basal slip dominates the cyclic deformation throughout the fatigue life of the pure Mg sample, while both basal and pyramidal slip dominate the cyclic deformation in the Mg-3Y sample. Intergranular cracking is the primary cracking mode for both samples under cyclic torsional loadings. Basal and pyramidal slip PSB cracking serves as a primary transgranular cracking mode in the pure Mg and Mg-3Y samples, respectively. The study also investigates the underlying mechanism governing the activity of various deformation modes, cracking modes, and mechanical behavior.

The effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviours of a directionally superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than the specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young’s modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

This study is focused on the anisotropic fatigue crack growth (FCG) behaviour of an aluminium AA7050-T7451 plate. L-T and S-T orientations were studied in M(T) samples with W=50 mm, in mode I loading, with R-ratio of +0.05. A numerical approach was used, assuming that crack tip plastic strain is the crack driving force. A purely kinematic elastic-plastic model was calibrated using experimental data from low cycle fatigue tests of smooth specimens in L and S orientations. The predicted FCG rates agree well with experimental trends in the Paris’ regime, suggesting that cyclic plastic deformation is the main damage mechanism. The numerical model was used to estimate the stress ratio effect for both orientations, which was found to be linked with crack closure variations. However, the closure free predicted trends for both microstructural orientations at R=0.05 are not overlapped, suggesting an effect of microstructure not linked to crack closure.

Maintaining aircraft airworthiness to ensure the fleet's safe operation and maintain its readiness is critically dependent on accurate modelling and reliable predictions of fatigue crack growth. In this process a knowledge of the representative initial discontinuity sizes that cause fatigue crack nucleation and early growth in aircraft is essential. Here the effective pre-crack size of aluminium alloy 2024, from samples of aircraft production material and tested under aircraft spectra, are considered.

The influence of preloading on the residual fatigue life of nickel-based superalloys under elevated temperatures was investigated experimentally. A powder metallurgy nickel-based superalloy FGH96 was preloaded with different number of cycles, and the residual fatigue life was tested under subsequent high-amplitude loads. The test results show that the fatigue life of the material was increased when the preloading cycle number was within a specific range. At the same time, the fatigue life of virgin specimens under high-amplitude loads shows little scatter, which provides the possibility to consider the strengthening effect of low-amplitude loads. A novel damage accumulation model was proposed to incorporate the strengthening effect of low-amplitude loads into the life prediction framework. The proposed model provides better life prediction than some existing models. Finally, the proposed model was validated using experimental data for various materials under the low-high loading sequence.

The influence of pre-strain on the tensile and fatigue properties of a dual phase DP600 was studied. The material was pre-strained by uni-axial tension in rolling and transverse direction. Thereafter, specimens were cut from the deformed plates in parallel or orthogonal to pre-strain direction. It was found that pre-strain increases yield and tensile strength. Results suggested that strain path change primarily affects the elastic-plastic transition during early stage of reloading. Pre-strained specimens showed an increase in high cycle regimes as a consequence of yield strength increment, irrespective of imposed pre-straining direction. A modified stress life equation which accounts for pre-strain was proposed and showed good agreement with experimental data. Bake hardening enhanced both tensile and high cycle fatigue resistance. Walker equation was successfully fitted to account tensile mean stress. In low cycle fatigue, negligible influence of pre-strain was observed due to cyclic softening and residual stress relaxation.

This article aims to analyze the influence of high cycle fatigue damage on the mechanical properties and low cycle fatigue performance of Q690D. Monotonic tensile and cyclic loading tests were performed on Q690D specimens with different degrees of high cycle fatigue damage. Degradation models were established to describe the declining trending of mechanical properties with the increase of pre-fatigue damage. Manson-Coffin models for Q690D steels of different pre-damage levels were established. Besides, a comparison was presented between Q690D and Q355B. The research work in this article provides a fundamental reference for the appropriate assessment of the mechanical performance of Q690D high-strength steel structures after long-term alternating loading.

The effects of various heat treatments on the microstructure and mechanical properties of laser beam powder bed fused AlSi10Mg were investigated. Specimens were solutionized at three different temperatures of 425, 475 and 525 °C followed by natural aging (T4) prior to microstructural and mechanical characterization. In addition, the effect of aging was studied by artificially aging (i.e., T7) some of the solutionized specimens at 165 °C. Solutionizing at all temperatures was observed to fully dissolve the additive manufacturing (AM) induced dendritic microstructure, leaving bulky Si and needle-shaped β-AlFeSi precipitates in the grain interiors and boundaries. Tensile results revealed that T4 specimens exhibited more ductility, while T7 specimens showed substantially higher strengths with slightly reduced ductility. Interestingly, no significant effect of heat treatment on strain-life fatigue behavior was observed. Fractography found the Si-particles to be responsible for tensile fracture, while AM volumetric defects were the main initiators of fatigue cracks.

High-temperature damage characteristics related to the deformation behavior of Sanicro25 alloy were investigated under cyclic loading. The results show that the high mean stress can lead to creep-like deformation behavior. The increase of mean stress increased the strain rate and weakened the life. Comparatively, the increase in stress amplitude only accelerated the third-regime strain rate, leading to a decrease in life and fracture strain. Under different stress amplitudes, microcrack initiation at Z phase interfaces can be certified by observing micropore and high geometrically necessary dislocation (GND) density around Z phase. Subsequently, the increase in stress amplitude can significantly affect crack propagation. Under constant loading ( σ a =0MPa ), the recrystallization of the transgranular crack tip at the grain boundary can change the tendency of crack propagation, making crack propagation along the grain boundaries. Conversely, transgranular cracks can continue to propagate, assisted by intragranular dislocation accumulation at higher stress amplitude ( σ a =130MPa ).

A new algorithm optimization-based hybrid neural network model is proposed in the present study for the multiaxial fatigue life prediction of various metallic materials. Firstly, a convolutional neural network (CNN) is applied to extract the in-depth features from the loading sequence comprised of the critical fatigue loading conditions. Meanwhile, the multiaxial historical loading information with time-series features is retained. Then, a long short-term memory (LSTM) network is adopted to capture the time-series features and in-depth features of the CNN output. Finally, a full connection layer is used to achieve dimensional transformation, which makes the fatigue life predictable. Herein, the hyperparameters of the LSTM network are automatically determined using the slime mould algorithm (SMA). The test results demonstrate that the proposed model has pleasant prediction performance and extrapolation capability, and it is suitable for the life prediction of various metallic materials under uniaxial, proportional multiaxial, non-proportional multiaxial loading conditions.

Fatigue results from the occurrence of several damage mechanisms and their interactions. The cyclic plastic strain and damage accumulation at the crack tip are widely pointed as the main agents behind FCG. In this work, the authors propose the prediction of FCG through a node release numerical model that offers several possibilities regarding the modelling of the mechanisms behind fatigue. A hybrid propagation method is presented where both cumulative plastic strain and porous damage represent parallel propagation criteria. Accordingly, the node is released once either a critical plastic strain or a critical porosity, at the crack tip, is reached. The Gurson-Tvergaard-Needleman (GTN) damage model is employed to predict porous damage evolution through the processes of nucleation and growth of microvoids. The model is validated through comparison with experimental data. Finally, the interactions between plastic strain, porous damage, crack closure and stress triaxiality are accessed.