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.
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.
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.
The operational conditions in next generation power plants and other high temperature applications such as turbine blades in jet engines demand the component to perform under extreme conditions where metallic materials show time dependent deformation under cyclic loading conditions. Under creep-fatigue loading condition, the crack tip is exposed to both time dependent and independent plastic deformation. Conventional crack characterizing parameters such as (C t ) , avg has shown good correlation with dominant damage based crack velocity, (d a/d t ) avg . However, the true definition or prediction of crack driving forces under such scenario are vague due to limited theoretical validity of the conventional crack tip characterizing parameters, such as J or C t. In this work, the concept of configurational forces are applied for the first time to understand the creep-fatigue crack growth behaviour. The crack growth is simulated using node-release technique and the configurational forces are calculated using post processing the finite element results for calculation of dJ/dt.
In order to reduce the vibration fatigue test time of aeronautical engineering components made of aluminum-alloy, a random vibration fatigue acceleration model under narrow-band excitation is proposed in this paper. A three-parameter S- N curve is adopted to consider the effect of small stress response, while a scale factor α is introduced to consider the effect of stress distribution. The random vibration fatigue tests of 2024-T3 and 7075-T6 aluminum-alloy specimens with elliptical holes are performed, where the vibration fatigue lives of load spectra with the same bandwidth and different excitation acceleration levels are obtained. The test results show that the proposed model is in sound agreement with the test results.
The present paper is dedicated to the mechanical and fracture characterisation of a specific earthen material, that is, the shot-earth 772. Although such a material has been recently characterised from a microstructural, chemical and physical point of view, the knowledge of its mechanical and fracture properties (essential for extending its use in construction industry) is still lacking. Such characterisations are here performed both experimentally, through laboratory tests, and numerically, through a FE model. The experimental tests (i.e. flexural, compression and fracture tests) are carried out on shot-earth specimens according to Recommendations available for concrete and a method proposed by some of the present authors, named Modified Two-Parameter Model. The numerical analyses are performed by employing a micromechanical model (implemented in a non-linear 2D FE homemade code), which allows to simulate both flexural and fracture behaviour of the shot-earth examined. Finally, the obtained numerical results are compared with the experimental ones.
In order to accurately predict creep deformation and damage evolution of nickel-based superalloy GH4169, a novel damage constitutive model, which can be called TTC CDM-based model, was proposed based on TTC relations and continuum damage mechanics (CDM). The stress and temperature dependence of constants were all determined in the novel model, which overcame the weakness of the traditional CDM-based model and made the model have the satisfactory abilities of interpolation and extrapolation. Microstructural study has revealed that the creep fracture mode gradually converts from intergranular brittle fracture to transgranular ductile fracture as the stress decreases. And the critical conditions were identified. It was determined that the novel model accurately predicted the minimum creep rate, rupture time, creep deformation and damage evolution process of GH4169. Furthermore, the nonlinear creep damage accumulation effect was also revealed by the novel model, i.e. the total creep life of GH4169 will be reduced if high stress or high temperature condition was applied first, which was consistent with previous experimental results of variable creep load.
A new test specimen is proposed for investigation of mixed mode I/II/III fracture of materials. In the test specimen, by displacing the position of an inclined crack from the middle of the rectangular specimen, in addition to mode II loading, mixed mode I/III loading conditions are created under anti-symmetric four-point bending. To examine the applicability of the test set-up, the specimens made of PMMA (Polymethyl-methacrylate) with three crack angles and three different positions of crack with reference to the middle of the specimen are tested. The experimental fracture loads are compared with the theoretical predictions of the maximum principal stress fracture criterion. There is a satisfactory consistency between the test and theoretical results. Although the proposed test configuration has limitations in applying the mixed modes of I/III and II/III, it is an efficient test configuration easier and less expensive than other configurations utilizing complex crack geometry or complicated loading fixtures.
Staircase testing is a standard method for evaluating the fatigue strength of components. However, staircase testing assumes a normal distribution, while components can display bimodal behavior due to flaws in material, or issues during the manufacturing process. Three unique step loading data sets on different production crankshafts provide evidence that step loading reliably identifies material or manufacturing issues, that lower a component’s fatigue strength. Staircase testing has an 87% or greater chance of overestimating the component’s fatigue strength, which in turn overestimates the component’s expected reliability. For example, a component with a 99.9% reliability, based on staircase testing would only have a 74% reliability based on step loading. If a component contains an undetectable manufacturing defect, staircase testing has a 99% chance of overestimating the component’s fatigue strength. Step loading reliably improves the estimation of a component’s fatigue strength distribution while providing insights into a component’s defect tolerance.