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.
In order to provide a sufficient theoretical basis for the fatigue resistant design of the aircraft wheels, strain-controlled low-cycle fatigue (LCF) tests were carried out on specimens machined in the extrusion direction (ED) and transverse direction (TD) of die-forged 2014 aluminum alloy wheel. Although the TD specimens show lower tensile strength and yield strength, the fatigue test results reveal that the TD specimens show superior fatigue life compared with the ED specimens at total strain amplitudes of 0.5% ~ 0.8%. This is predominantly caused by the Al 12(MnSi) 2(FeCu) intermetallic particles near the surface layer lead to a relatively short crack initiation stage for the ED specimens. In contrast, TD specimens with finer and more uniform recrystallized grains have better resistance to fatigue crack initiation (FCI) and propagation (FCP).
The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = −1, 0.05, 0.3 and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2×10 5 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by σ w R = σ w R = - 1 ∙ 1 - R 2 0.63 and τ w R = τ w R = - 1 ∙ 1 - R 2 0.63 .
The synergistic combination of carbon nanotubes (CNTs) and ductile thermoplastic resin has shown large potential in the improvement of fracture resistance for the epoxy matrix composites using the interleaving toughening method in recent years. The hybrid structure of CNTs and thermoplastic resin in interlayers affects directly the interlaminar structure and the resultant crack propagation path of the interleaved composites. In this work, the CNTs and thermoplastic polyetherketone-cardo (PEK-C) were used to prepare the interlayer with different hybrid structures to interleave the carbon fiber reinforced epoxy composites and the influence of hybrid structure on the interlaminar structure and the fracture toughness was investigated. The results showed that PEK-C/CNT/PEK-C sandwich interlayer produced the best toughening effect in mode I interlaminar fracture toughness (GIC) and the GIC was 446.76 J/m 2, increased by 138.11% compared to blank composites, which benefited from the multilayered structure in the interlaminar region formed during curing process and the resultant tortuous crack propagation.
In order to make full use of the potential fatigue crack growth resistance provided by layered architectures, a validated crack path simulation algorithm for crack propagation through different elements of the layered architectures was established. The crack path approaching a material interface was predicted by using the maximum tangential strain (MTSN) criterion and the crack behaviour at the interface was simulated by a developed two-step method (a modified stress-and-energy-based cohesive zone method considering the change in direction of an interface penetrating crack). The crack path simulation by using this algorithm in layered example architectures indicates: 1) there are two criteria zones for the transition between crack deflection and penetration in terms of the relationship between interfacial strength and toughness; 2) the likelihood of a crack deflecting out of the interface will increase with the propagation of an interfacial crack and 3) the architecture difference which affects shielding or anti-shielding behaviour has a significant effect on crack deflection or penetration events.
The stress ratio ( R) effect, especially at elevated temperatures, is associated with the fatigue crack growth (FCG) prediction in aero-engine hot-end components under complex operation conditions. The FCG experiments with three R were conducted on Ni-based superalloy GH4169 at room temperature (RT), 550 °C, and 700 °C. The results indicate that the R-effect on the FCG of GH4196 is temperature-dependent. Therefore, efforts were made to identify the R-effect at various temperatures to describe the FCG. The concept of the crack-closure or the two-driving-force was examined to quantify the R-effect considering the temperature influence. Fractographic analyses on the fracture surface were performed to discuss the underlying mechanism responsible for the temperature influence. The study can contribute to the R-dependent FCG modeling at various temperatures.
The fatigue behaviour of continuous wire polymer composite (CWPC) fabricated by fused filament fabrication was investigated. Four compositions were examined: polylactic acid (PLA), PLA with copper wire (Cu), thermoplastic polyurethane (TPU), and TPU with Cu wire. Residual properties were measured after different sets of number of cycles (10 2, 10 4, 10 5). Residual strengths were 89.736% and 70.464% of the ultimate tensile strength of the original material after 10 5 cycles for PLA CWPC and TPU CWPC, respectively. A one-way analysis of variance (ANOVA) statistical tests showed insignificant changes in the residual strengths of PLA-based materials after an increasing number of cycles and significant changes for the TPU-based materials. CWPC electromechanical properties under fatigue test demonstrated reverse piezoresistivity behavior. A strain-controlled fatigue life analytical model was compared to the experimental results showing good agreement. This study demonstrates the applicability of FFF technique to print sensors with continuous integrated wire with tunable properties.
The maximum tangential stress (MTS) criterion is one of the most widely used criteria for predicting the direction of crack extension. The suitability of this criterion is examined under different loading conditions using extended finite element method (XFEM). Experimental and numerical results reported in the literature are considered to evaluate the validity and accuracy of the criterion. The results demonstrate that the MTS criterion evaluated by stress intensity factors (SIF) can accurately predict the direction of crack propagation in specimens under direct tensile loading. This criterion overestimates the angle of crack initiation in the specimen under indirect tensile loading, but underestimates the angle in the specimen subjected to three-point bending. It is concluded that the MTS criterion based on SIF could not accurately predict the direction of the crack initiation, which could, however, be determined properly based on the stress distribution around the crack tip obtained by XFEM numerical models.
The fracture behavior of ultra-high temperature ceramic matrix composites at high temperature has received increasing attention. However, few studies consider the effect of particle/crack interaction on the high temperature fracture strength. In this work, based on the energy storage capacity, energy balance method and fracture theory, the effect of particle/crack interaction is introduced into a temperature-dependent fracture strength model of monolithic ultra-high temperature ceramic matrix composites, which also considers effects of flaw size, grain size and residual thermal stress. Furthermore, by considering the influence of the laminated structure, a theoretical characterization model of the temperature-dependent fracture strength of laminated ceramic matrix composites is developed. The effect of particle/crack interaction is also included in this model. It should be noted that the predictions of the models agree well with the experimental data of both monolithic and laminated materials without using any fitting parameters. The effect of particle/crack interaction is found to have a significant weakening effect on the strength of materials at different temperatures. The theoretical models only need some simple basic material parameters to predict the fracture strength and mechanisms of ceramic matrix composites at high temperature, which have important practical significance for engineering applications.
The process of fatigue spalling in the rings of ball bearings at durability exceeding 10 8 cycles under in-service loading conditions is analyzed on the basis of fractography and the slices prepared in radial planes of rings. The cracks are shown to originate at subsurface from carbides inherent in the bearing steel or inclusions permissible by sizes for the material. Subsequently, the development of cracks perpendicular to the ring raceway surface takes place similarly as in the VHCF regime with the elliptical “fish-eye” formation. The subsequent crack growth was demonstrated step-by-step up to the ring material fragment separation. The total crack path by alternating stops of propagation and new crack nucleation under conditions of mixed-mode I+II+III mechanisms with the crack branching was discussed. In the final stage, the crack grows towards the ring raceway and either appears on the raceway surface or coalesce with a similar adjacent crack followed by fatigue spalling formation.