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.

This paper presents and compares two combined experimental-numerical techniques for the investigation of fracture properties of additively manufactured polymer parts using digital image correlation (DIC) measurements. The first method uses only measured kinematic fields, and the second is based on Finite Element simulations driven by measured boundary conditions. A micro Single Edge Notched Tensile sample manufactured by fused filament fabrication of ABS is studied. It is shown that both methods locally extract J-integrals, and the crack tip is accurately located by the FE-based method. By comparing computed displacements to those measured via DIC it is possible to locally check the validity of the numerical model. The initiation and propagation stages are analyzed independently thanks to two different magnifications of acquired image series

Fatigue cracks initiated from holes in several zones and structural components of the RRJ-95 aircraft frames were investigated. Using the method of quantitative fractography the crack growth duration in the brackets of the in-service airframe and in the wing panels during full-scale bench tests was estimated the spacing of meso-beach-marks (MBM) and fatigue striations. The applied program of bench test consisted of blocks of variable loads that were equivalent to the wing loading in flight and reproducing schematized flight-cycle. It was shown that the duration of fatigue crack propagation in several structural components of the RRJ-95 aircraft frames was approximately the same as for the crack nucleation duration. The total lifetime is sufficiently long for cracks in the structural components to be detected and reliably monitored with a large operating time interval between adjacent inspections.

Fatigue experiments and numerical simulations based on the Linear Elastic Fracture Mechanics (LEFM) theory were conducted on the Even-Matched (EM) and Under-Matched (UM) 10CrNi3MoV Load-carrying Cruciform Welded Joints (LCWJs). The study firstly experimentally investigated the Fatigue Crack Growth Rate (FCGR) of base metal, EM, and UM weldments. The corresponding Paris parameters as essential input data are provided to assess the fatigue crack propagation behavior for weld toe and weld root failure of LCWJs. On the one hand, the Stress Intensity Factors (SIFs) at weld toe and weld root were calculated considering the effects of LCWJ specimen geometries, initial crack types, and sizes. The comparisons between simulated results and standards analytical solutions were executed, which exhibit good accordance. It proved that the fatigue fracture simulation procedure based on LEFM is appropriate for the fatigue assessment of LCWJs. Eventually, it conducted the parametric analysis by predicted S-N curves, which included in the weld length, initial crack shape, initial crack size, penetration length, and materials fracture parameter, to explore some safety assessment reference lines for both failure modes of LCWJ.

In this work, the effect of residual stress on mixed-mode crack propagation behavior in friction stir welded (FSW) 7075-T6 panel under biaxial loading was investigated. The cruciform sample was designed and manufactured by FSW. Residual stress profiles across the welded sample were measured by the X-ray diffraction technique. Crack propagation behaviors were simulated with five different biaxial loading ratios. Stress intensity factors (KΙ and KΙΙ) were evaluated by finite element method (FEM) and used to study the effects of residual stress on crack behaviors. It was observed that residual stress has a considerable effect on the mixed-mode crack growth. In most of the cases, the crack deflection is mainly affected by residual stress at the beginning of crack propagation. The variation of crack propagation path is strongly linked with the residual stress as well as the biaxial loading ratio. In addition, KΙ and KΙΙ are susceptible to residual stress under biaxial loading conditions. Residual stresses contribute to a higher proportion of KΙΙ compared to that of KΙ. KΙ and KΙΙ in the retreating side are more affected by the residual stress.

The influence of the hole surface integrity on the fatigue life of 2024-T351 aluminium drilled parts was investigated. Fatigue tests were conducted on open-hole specimens and a large campaign was carried out to characterize the hole surface integrity (roughness measurements, hardness measurements, metallographic observations and residual stress analysis). An innovative technique, the Hole Opening Comparative Technique, was set up in order to analyse the residual stress state of the parts. In this study, strain hardening of the hole subsurface seems to be the main factor influencing the fatigue behaviour, associated with a residual stress state of the part.

The paper deals with the problem of ultra-light and light aircraft structural health monitoring. The phenomenological basis and engineering decisions for methods to monitor accumulated fatigue damage and to register overstress of aircraft principal structural elements have been shown. The proposed fatigue indicator is lightweight and autonomous; the output information of the indicator reflects the inhering feature of the fatigue damage – appearance and growth of surface extrusions and intrusions. The overstress indicator simplicity and correctness of the information is provided by “fuse” principle of the design.

Abstract: In order to study the dynamic fracture properties of FRP strengthened concrete, the three-point bending experiment with different initial crack-depth ratios was carried out under dynamic loading. The effects of crack-depth ratios on the fracture parameters were analyzed. Combined with Pearson Correlation Coefficient formula, the correlation between crack-depth ratios and fracture parameters was quantitatively verified. The conclusions could be drawn that, there are three critical points in the damage process: the crack initiation point, first peak point and ultimate bearing capacity point. With the increase of crack-depth ratio, the crack initiation load decreases, but the first peak load and ultimate load increase first and then decrease. The stress intensity factors of three points also increase first and then decrease. The increment of critical crack-depth ratio and flexibility coefficient are related to crack-depth ratio, which could reflect the effect of crack-depth ratio on the deformation capacity of FRP strengthened concrete.

An experimental protocol has been set up in order to study the Low Cycle Fatigue (LCF) damage micromechanisms in a Lost Foam Casting (LFC) A319 alloy at room temperature. The microstructure of the studied material was characterized by using X-ray Computed Tomography (X-ray CT) prior to the LCF tests performed with surface in-situ observations, which allow crack initiation and propagation being tracked in real-time. The mechanical fields measured by Digital Image Correlation (DIC) method allowed establishing the relations between strain localizations, damage evolutions and microstructure while a developed etching method, which gives a natural texture to the surface, makes DIC feasible to an acceptable resolution without masking the microstructure. The results showed that crack initiation is ascribed to strain localizations induced by large pore and/or the propagation of a previously nucleated crack. Cracks propagate along hard inclusions but the orientation of hard inclusions has also an influence on crack path.

In this study, an approach utilizing a conjunction of the extended finite element method (XFEM) and the GTN micro-mechanical damage model is proposed for predicting the ductile crack propagation of a mill-annealed Ti-6Al-4V alloy. The cohesive model based XFEM approach is used to capture the continuous crack propagation process and the GTN model is applied to describe the constitutive behavior of the material. Simulations are conducted by using the standard finite element code ABAQUS following a Newton-Raphson algorithm solution with employing the user material subroutine of the GTN model. In comparison to the experimental results of the smooth, notched and cracked titanium specimens, this approach is shown to be an efficient method for simulating the ductile crack propagation process under different stress triaxialities.

Aiming at the structural safety problems caused by the coupling and randomness of factors such as the geometric characteristics, load status, service characteristics and failure mechanism of the in-service bridge structure in time and space, a real-time prediction method of fatigue life of bridge structure under digital twin is proposed. Based on the physical entity of the general bridge crane, the status information acquisition system is used to get the current service status information of the crane equipment, and the obtained current service status information is combined with inherent information and historical service information to construct a fuzzy information database, which is clearly quantified to form twin data; based on the characteristics of the equipment structure and the technology cycle process, the load, strength, defect and life analysis models are determined to form a multi-theoretical calculation model of the bridge structure through information transmission; using the informational data of the active service state, the multi-theoretical computational model is modified to obtain the main factors affecting the fatigue life, and fuzzy comprehensive evaluation theory is combined to determine the comprehensive influence factor; the Kriging proxy model is constructed through experimental design to obtain the response relationship between inherent and service information and the fatigue life of the bridge structure, and the whole life cycle process of the equipment is reflected by completing the real-time prediction of the fatigue life of the bridge structure in the virtual space. Taking the QD20/10t×43m×12m general bridge crane as an example, the feasibility and applicability of the proposed method are verified by comparison, which provides a strong theoretical basis for the safe service and timely scrapping of the crane.

The grain boundary strengthening effect (GBSE) was investigated for nickel-based superalloy bicrystals with various misorientations under different temperatures and loading directions. The results show that the GBSE is enhanced with the increase of misorientation and insensitive to the temperature. Besides, the loading direction relative to GB also has strong effect on GBSE. On the other hand, the GBSE is often accompanied by GB cracking. The trend of GB cracking is heightened with the increase of misorientation and temperature. However, the trend of GB cracking is reduced strongly when the loading direction transforms from parallel direction to vertical direction.

The inhomogeneity of friction stir weld (FSW) joint of a 7085 Al-based alloy containing trace Sc was investigated by comparing the hardening phase morphology, grain structure, mechanical properties and fatigue crack growth (FCG) behavior in based metal (BM), heat-affected zone (HAZ) and weld nugget zone (WNZ). The results showed that microstructural evolution during FSW was remarkably affected by Sc addition. The recrystallization and growth of grains in BM and HAZ were restrained by coherent Al3(Sc,Zr) particles. In HAZ, partially dissolution and slight growth of precipitates resulted in softening but contributed to enhance the FCG resistance according to shear mechanism. In WNZ, high frictional heating and intense plastic deformation induced recrystallization and supersaturated solid solution which decomposed to form new reprecipitates, including many new GPII zones, ′ phases and some coarse equilibrium phases attached to grown incoherent Al3(Sc,Zr). Owing to combined influences of fine random orientated grain structure and reprecipitates, WNZ possessed the best fatigue endurance.

A novel multi-axial energy-based approach is presented and used to demonstrate the influence of different finite element (FE) modelling techniques on the prediction of the fatigue life of a rubber composite with long oriented fibres. It is shown that the simplest modelling methods using 2D elements with rebar layers, layered 2D elements or layered 3D elements do not allow for a precise determination of the critical location and damage value. In contrast, modelling methods with 3D matrix and discrete reinforcement provide much better results. The predicted critical location corresponds to the measured one, although the predicted fatigue life still differs from the measured results. The most complex microscopic modelling method shows the best agreement between the predicted and measured fatigue life. Since microscopic modelling is not suitable for modelling larger products made of rubber fibre composite, it is also noted that modelling techniques with 3D matrix and discrete reinforcing elements can be used with the same accuracy if the fatigue life curve is obtained from measurements on the specimens made of composite material rather than the specimens made of the critical base material (rubber).

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.

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.

This paper presents a novel numerical model, based on the Finite Element (FE) method, for the simulation of a welding process aimed to make a two-passes V-groove butt joint. Specifically, a particular attention has been paid on the prediction of the residual stresses and distortions caused by the welding process. At this purpose, an elasto-plastic temperature dependent material model and the “element birth and death” technique, for the simulation of the weld filler supply over the time, have been considered within this paper. The main advancement with respect to the State of the Art herein proposed concerns the development of a modelling technique able to simulate the plates interaction during the welding operation when an only plate is modelled, taking advantage of the symmetry of the joint; this phenomenon is usually neglected in such type of prediction models because of their complexity. Problems arising in the development of this modelling technique have been widely described and solved herein: transient thermal field generated by the welding process introduces several deformations inside the plates, leading to their interaction, never faced in literature. Moreover, the heat amount is supplied to the finite elements as volumetric generation of the internal energy, allowing overcoming the time-consuming calibration phase needed to use the Goldak’s model, commonly adopted in literature. The proposed FE modelling technique has been established against an experimental test, with regard to the temperatures field and to the joint distortion. Predicted results showed a good agreement with experimental ones. Finally, the residual stresses distribution in the joint has been evaluated.

The effect of pneumatic shot peening (PSP) on the fatigue properties of K403 turbine blades has been investigated under the high and low cycle combined fatigue (CCF) load with two types of blades: untreated blades and PSP treated blades. It is found that there is a threshold vibration stress which should be 194MPa. The PSP has a positive effect on the CCF life of blades mainly due to the compressive residual stress resulting in the reduction of the number of crack sources and propagation rate when vibration stresses are below the threshold vibration stress. However, the PSP treatment has no or negative effect when vibration stresses are above the threshold value. The compressive residual stress is released along with the microstructure changes of K403. Meanwhile, the microstructure changes, reflected in the precipitation of the lamellar MC carbides and σ phases, can accelerate the process of crack initiation and propagation.