Neural network (NN) models have made a significant impact on fatigue-related engineering communities and are expected to increase rapidly soon due to the recent advancements in machine learning and artificial intelligence. A comprehensive review of fatigue modeling methods using NNs is lacking and will help to recognize past achievements and suggest future research directions. Thus, this paper presents a survey of 251 publications between 1990 and July 2021. The NN modeling in fatigue is classified into five applications: fatigue life prediction, fatigue crack, fatigue damage diagnosis, fatigue strength, and fatigue load. A wide range of NN architectures are employed in the literature and are summarized in this review. An overview of important considerations and current limitations for the application of NNs in fatigue is provided. Statistical analysis for the past and the current trend is provided with representative examples. Existing gaps and future research directions are also presented based on the reviewed articles.

This article describes how a step-stress accelerated life test (SSALT) can be designed for testing the fatigue life and reliability of structural components with a single failure mode. With simple numerical simulations of the crack’s propagation in the notched area of the structural part for different loading levels, the slope of the S-N curve for a structural component is initially estimated. Then, a very few fatigue-life experiments are carried out in the high-cycle domain to determine the intercept of the structure’s S-N curve. By considering the scatter from the material’s P-S-N curve, different SSALT designs for the structural component can be composed and checked for their expected acceleration factor. The procedure is experimentally validated for the case of a notched specimen and two different SSALT designs. From the results it can be concluded that the predicted durations of the SSALT experiments correlate well with the real experiments.

The fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 are analysed in the as-built and heat-treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as-built condition, fine sub-grains were found, while in the heat-treated state, the sub-grains were removed and the dislocation density recovered. SN-curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the √area-method.

Polymeric foams have good capacity of absorbing energy in compression, but are brittle in tension. Linear Elastic fracture Mechanics is successfully applied to assess the integrity of structures with polymeric foams. The fracture toughness represents an important parameter. The different approaches to estimate the fracture toughness of polymeric foams are reviewed, analytical and numerical micromechanical models and experimental investigations. Focus is given on the parameters influencing the fracture toughness of polymeric foams like specimen type, solid material, density, loading speed, size effect and temperature. Data on mixed mode loading and dynamic fracture toughness are also presented. The last part of the paper presents some results to increase the fractured toughness by reinforcing of polymeric foams.

Superaustenitic steel Sanicro 25 has been subjected to in-phase and out-of-phase thermomechanical fatigue cycles in the temperature range from 250°C to 700°C. Both constant strain rate cycling and cycling with 10 minutes dwell at peak temperature were applied. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were evaluated. The transmission electron microscopy was used to find modifications of the internal structure and precipitation of the nanoparticles. 10 min dwell at maximum temperature modified substantially the dislocation arrangement. Various nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X-ray spectroscopy in scanning transmission electron microscope. The damage mechanism operating under specific loading conditions was investigated on the surface as well as in the interior of the cycled specimens. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for crack nucleation and propagation. Effect of dwells on fatigue behaviour, modification of internal structure and damage mechanisms are analysed and discussed.

The fretting damage and wear debris on the dental implant-abutment interface (IAI) are unclear. In this study, fatigue cycle loading (FT) and chewing cycle loading (CW) test were applied to two implant systems, the fretting damage morphology and wear debris generation on the IAI were observed by a scanning electron microscope. The torque value of the central screw was measured by electronic torque tester. The fretting damage on the IAI was relatively slight and mainly plastic deformation in the FT group, which was more serious and mainly furrow wear in the CW group. Various forms of wear debris were generated. The removal torques were lower than its pre-tightening value in both groups, the decline and loss rate of the CW group was significantly higher. This study confirmed the critical roles of fretting damages and metal wear debris on the IAI in the implant-supported prosthesis.

Electron Beam Melting (EBM) is one of a few additive manufacturing technologies capable of making full-density functional metallic parts realized from raw materials in the form of powders. The ability of direct fabrications of metallic parts can accelerate product designs and developments in a wide range of metallic-part applications, especially for complex components, which are difficult to make by conventional manufacturing means. To capitalize on these benefits, it must be shown that the mechanical performances of parts produced by EBM can meet design requirements. In this research an intensive mechanical characterization aimed at determining static and fatigue performance of the alloy Ti6Al4V processed by EBM has been performed. The effect of both postprocessing treatments (HIP and surface finish) on the mechanical behavior was evaluated by mechanical testing, microstructural study, computed tomography analysis and fracture surface investigation.

The main aim of the current study is to evaluate the compressive quasi-static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via Laser Powder Bed Fusion (LPBF) process. The topologies herein considered are lattice based regular and irregular configurations of cubic, star and cross shaped unit cell along with trabecular based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3 – 20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending dominated behavior affecting more the topologies with vertical struts. The S – N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching dominated cubic and purely bending dominated cross based structures. Trabecular structures have shown desirable quasi-static and fatigue properties despite the random distribution of struts.

The paper discusses various partial solutions used for estimating fatigue life under variable amplitude multiaxial loading in the high-cycle fatigue domain. The concurring effects are treated, and their proposed solutions are commented upon. The major focus is on the categories of the phase shift effect and of cycle counting, and on the scope and quality of data, which support discussed theories. Results of own new experimental data set on specimens from S355 steel are provided. Fatigue life estimates for McDiarmid and Findley multiaxial methods and for two different methods of load path decomposition to cycles are shown to highlight some of the points open for discussion. It is concluded that the available experimental data are not sufficient to substantiate a clear decision to follow a definite algorithm.

Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti-6Al-4V were fabricated. The porosity for each group was measured and the results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The S-N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack-of-fusion defects in the cases of surface and internal crack initiation.

In last decades, many alleviation measures were proposed in order to improve the life of fretting fatigue affected components. The aim of such palliatives is that to counteract the high stress gradients that arise near the contact surface. In such a context, the shot peening treatment is worth noting. Therefore, in the present paper, the fatigue life of shot-peened aluminium and titanium alloy specimens, subject to fretting fatigue under partial slip regime, is assessed by means of the Carpinteri et al. criterion for fretting fatigue. Firstly, according to the superposition principle, the relaxed residual stresses (due to the shot peening treatment) are combined with the stress components due to fretting fatigue loading. Then fretting fatigue assessment is performed. In such a context, a novel theoretical law for the relaxed residual stress field is here proposed, the implementation of which shows very promising results in terms of fatigue life estimation of the shot-peened specimens examined.

A concept of initial cracking strength of concrete is elaborated in this study. A fracture model and associated methods for determining independent initial cracking strength and initial fracture toughness by using the three-point-bending (3-p-b) and wedge splitting (WS) concrete specimens are present. The initial cracking strength and initial fracture toughness can be simultaneously determined using a curve-fitting method from the proposed fracture model. All of the initial fracture curves can be obtained using the determined concrete materials. The initial loads of the 3-p-b and WS specimens can be predicted on the basis of the curves with ±15% ranges. Furthermore, analytical functions are used to obtain and determine the initial cracking strength and the initial fracture toughness of concretes directly. The determined values with ±15% ranges cover the most of initial loads of the 3-p-b and WS specimens.

The use of passive infrared thermography comprises great opportunities to improve understanding the fatigue damage process of crack-patched structures. Quasi-static and cyclic coupon tests are performed using metallic specimens with single-sided bonded patches and monitored with passive infrared thermography. Different test setups help to differentiate between metallic crack growth and adhesive damage on thermal images. Results show that metallic crack growth can be monitored from the patched side, also in combination with local delamination at the patch/metal interface. Thus, it is possible to analyse the overall degradation progress of the crack patched component under loading conditions and thereby to identify the driving damage mechanism of the particular repair configuration. Being able to understand the overall damage behaviour of crack patched components is essential to improve the ability of predicting its long-term behaviour.

Concepts for crack propagation as well as fatigue assessment under variable mechanical and thermal loadings are unestablished. For variable mechanical loadings, the damage parameter PJ is well known, for thermal loadings the damage parameter DTMF is established. Both parameters base on the effective cyclic J-Integral, still the definition is different. The damage parameters PJ considers the effective stress and strain ranges from the upper reversal point of each load cycle to the point of crack closure. Depending on the loading sequence the point of crack closure is treated as a history variable. In addition, with a crack length dependant fatigue limit, the most important sequence effects are considered. A new PJ-based concept is developed by considering additional sequence effects. In comparison to experimental results, the developed concept is able to reduce scattering in the range of constant amplitude loading tests.

The consideration of realistic load assumptions is important for the fatigue design of highly stressed nodular cast iron components for wind energy application. Especially in case of overloads causing elastic-plastic deformation, residual stresses may have a strong impact on fatigue life. In strain-controlled fatigue tests with constant and variable amplitudes, the influence of overloads on the lifetime was investigated. The overload was applied with the objective to create high tensile residual stresses. During fatigue testing the transient material behavior, cyclic hardening, cyclic relaxation of the residual stresses as well as quasi static creep effects, of the EN-GJS-400-18-LT was recorded and evaluated. To quantify the influence of the transient material behavior on the calculated lifetime, fatigue analyses are carried out with the strain-life approach, both with and without consideration of the transient material behavior. The results show that conservative damage sums are derived if the transient material behavior, especially the relaxation of tensile residual stresses, is neglected.

The present paper investigates the variability in the static and cyclic properties of two nominally identical supplies of the aeronautical Al grade 7075-T6. Samples were extracted from extruded bars of 15 mm and 60 mm diameter and with slightly different chemical composition. Noticeable differences were found in tensile strength, total elongation, low- and high-cycle fatigue strength, despite the nearly identical hardness value. The diverse mechanical behavior has been imputed to different extrusion ratio and therefore work hardening along with a more or less fine distribution of precipitates and dispersoids. The high-cycle fatigue strength was found to be in direct correlation with the monotonic yield strength and the size of the largest intermetallic precipitate. A simple equation based on Murakami sqrt(area) parameter is proposed to predict the fatigue endurance. Tensile tests and microstructural analyses are recommended instead of conventional hardness tests to have a tighter quality control on the mechanical properties of semifinished products.

Biaxial (axial and torsional loading) static tests were performed for the first time on EN-GJS-600–3 ductile cast iron tubular specimens obtained reproducing the solidification conditions typical of thick-walled castings. The experimental results were elaborated to determine the yield and fracture loci of the material, which exhibited significant deviations from those predicted by the Von Mises and Mohr-Coulomb criteria usually adopted for steels and grey cast iron, respectively. For this purpose, several alternative criteria proposed in the technical literature, some of them specifically devised for composite materials, have been calibrated and compared to account for the peculiar mechanical properties of this natural composite material.

Typically, the Crack Tip Opening Displacement (CTOD) is used only to quantify the crack closure phenomenon. However, more information about crack tip phenomena can be extracted from the CTOD curves, which can be used for a better understanding of fatigue crack growth. The main objective here is the development of a numerical tool for the automatic analysis of CTOD plots, which can be obtained either numerically using the Finite Element Method (FEM) or experimentally using Digital Image Correlation (DIC). The parameters extracted are the elastic and plastic CTOD in loading and unloading regimes, the corresponding load ranges, the crack opening and closure levels and the dissipated energy. This tool is expected to promote a fast and efficient analysis of DIC and FEM results, facilitating the implementation of CTOD analysis in the fatigue community.