A stress-based sectional critical plane model for multiaxial fatigue life prediction is proposed. The proposed model considers the effects of material properties and loading paths on the crack initiation and propagation behaviors. By introducing the ratio of maximum shear stress amplitude to maximum normal stress amplitude, it is divided into three sections in which the maximum normal stress plane, maximum damage plane and maximum shear stress amplitude plane are considered as the critical planes, respectively. To verify the accuracy and applicability of the proposed model, experimental data of 30CrMnSiA steel conducted by the authors and other test data of different materials from the existing literatures are utilized. For 30CrMnSiA steel, the prediction results of the proposed model demonstrate that 79.3% and 93.7% of the prediction results are within the ±2 times and ±3 times scatter band of fatigue life. For the experimental data from the existing literatures, more than 85% and 70% of the results predicted by the proposed model are within ±3 times scatter band of fatigue life for steel and aluminum alloy materials, respectively.

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.

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 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.

Unloading compliance (UC) method and normalization method (NM) are two of the most commonly used methods for determining the fracture toughness of materials. However, considerable differences often exist in the fracture toughness determined by these two methods, which solicits a new method to determine the fracture toughness accurately. In this paper, the compliance of crack length differences as measured by the crack length difference ratio Si is discovered, analysed and verified by experiments. Based on this compliance, a new accurate method, known as AJR, is developed and verified by test results. Factors that exhibit the advantages of the developed new AJR method are also investigated. It is found that the J-R curves determined by the new AJR method are more accurate than those determined by UC and NM. The new AJR method should be the first choice for steels with a small strain hardening ratio and low effective yield strength, and thicker CT specimens with shallower initial crack length. This is because the disagreement between UC and NM is unacceptably large. The developed new AJR method and the results presented in this paper can assist engineers and researchers to determine J-R curves and fracture toughness of steels more accurately and can contribute to the body of knowledge of fracture mechanics.

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 recently developed Edge Tracing (ET) method allows to estimate the radial deformation in axisymmetric tensile specimens via analysis of digital images recorded during the experiments. Images are processed to detect the sample’s contours and therefore, estimate the minimal cross–section diameter. This technique was mainly developed to characterize the elastic–plastic behavior well beyond the necking strain. The aim of this work is to extend the ET method to two case studies. Firstly, the post–necking behavior and failure of a low ductility Al–alloy are investigated. Low ductility alloys tend to fail brutally after reaching the maximum load. The major result is the capture of the sharp load drop which allowed to calibrate parameters of a GTN damage model. Secondly, the anisotropic elastic–plastic behavior of a “vintage” line pipe steel is characterized by a direct measurement of the Lankford coefficient. Assembled experimental data allowed to model the anisotropic plasticity in different loading directions.

Manufacturers have been promoting multi-material designs. So, the dissimilar material welding methods are being developed. We focused on heat welding technologies for friction stir spot welding of aluminum alloy and carbon fiber-reinforced plastic. This study investigated the effect of changes to jig constraint of joined members on the fatigue properties of joints. Also the fatigue life estimation was carried out by considering the singular stress at the welding joint interface. As a result, the fatigue strength of joint in a less constrained state is higher than that in a more constrained state. The singular stress intensity at the slit tip was uniformly predicted by the differences in welding parameters of joints.

The stiffness degradation represents one of the most interesting phenomena used for describing the fatigue behaviour of composites. In this regard, in literature, several works have been presented for modelling the fatigue life by studying the stiffness degradation. A critical aspect of modelling damage fatigue is represented by the difficulties in simulating the whole behaviour of material and then in describing the damage progression in all its stages. In addition, the validation of models requires the measurement of stiffness variations by means of experimental techniques. Above all for real components, the difficulties in defying proper models are accompanied by the difficulties in measuring stiffness degradation due to inapplicability of classic experimental techniques. In this work, the stiffness degradation of quasi-isotropic carbon-fibre-reinforced-polymer obtained by automated fiber placement, has been assessed by means of Thermoelastic Stress Analysis. The amplitude of temperature signal at the mechanical frequency (thermoelastic signal) was considered as an indicator of material degradation and compared to the data provided by an extensometer. The correlation between thermoelastic and mechanical data allowed to build a new experimental model for evaluating and predicting material stiffness degradation by just using thermoelastic data. The proposed approach seems to be very promising for stiffness degradation assessment of real and complex mechanical components subjected to actual loading conditions.

Even though friction stir welding (FSW) has been shown to produce high performing butt-joints, stress concentration at the weld edges in overlap FSW significantly reduces the performance of these joints. By combining FSW and adhesive bonding into a friction stir (FS) weld-bonding, joint mechanical performance is greatly improved. Quasi-static and fatigue strength of the proposed FS weld-bonding joints was assessed and benchmarked against overlap FSW and adhesive bonding. The characterization of the structural adhesive is also presented, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as well as mechanical characterization with curing temperature. A small process parameter study was made to select proper FSW parameters for AA6082-T6 overlap FSW and FS weld-bonded joints. The adhesive degradation temperature (357ºC) was found to be higher than reported temperatures in the adhesive during welding of FS weld-bonding joints. Higher curing temperatures were found to lead to increased strength while decreasing ductility of the adhesive. The addition of adhesive bonding to the overlap FSW to produce FS weld-bonding resulted in a significant increase in quasi-static and fatigue strength, achieving 79.9% of the fatigue strength of adhesive bonded joints at 106 cycles, while FSW had 41.6%.

The present investigation is concerned with high-cycle axial fatigue testing of a 2 mm AA6060-T6 HYB butt weld produced in the solid state using AA6082 filler metal addition. The results complement the three-point bend testing and the tensile testing done in two previous studies. In this study, optical microscope and scanning electron microscope examinations have been carried out to reveal the joint macro/microstructure and document possible surface and root defects deemed to affect fatigue life. In the as-welded condition, the HYB weld suffers from surface irregularities at the weld face and “kissing” bond formation in the root region. Despite of this, the subsequent testing shows that the fatigue properties exceed those reported for comparable AA6082-T6 gas metal arc butt welds and matching those reported for corresponding high-strength laser beam and friction stir weldments.

This paper presents a study on the effect of microstructure on the fatigue crack growth rate (FCGR) in advanced normalised-rolled (NR) and thermomechanical control process (TMCP) S355 steels in the Paris Region of the da/dN vs. ΔK log-log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Discussions were based mainly on the comparison between the crack path in the TMCP and NR steels. Fundamentally, three phenomena: crack-tip diversion, crack-front bifurcation and metal crumb formation were observed to influence the rate of fatigue crack growth (FCG). The prevalence of these phenomena appears to be a function of the nature of the material microstructure, environment and crack-tip loading conditions. The three factors appear to retard the crack growth by reducing or re-distributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. Increase in the FCGR was observed with decrease in the crack-tip diversion angle, branched-crack length and metal crumbs formed. In SW, the degree of the electrochemical dissolution of the microplastic zone (or crack-tip blunting) appears to be an additional factor influencing crack growth in ferrite-pearlite (α-P) steel. This study, generally tends to present microstructural features that strongly influenced FCGR in α-P steels in the Paris Region both in air and SW. This work is very important in the design of fatigue resistant steel.

Low-cycle fatigue testing of a lead-free solder (InnoLot) based on Sn-3.8Ag-0.7Cu (SAC387) with three simultaneous additions of bismuth, nickel and antimony was conducted using miniature-sized fatigue specimens at different temperatures and strain amplitudes. The experiments show a decline of the load capacity of the solder alloy with the number of loading cycles. The fatigue life of the solder is also decreased by the level of imposed temperature. The modified Coffin-Manson and Morrow models were used to analyze the behavior under fatigue and predict lifetime. The parameters in the two fatigue models which were determined by considering different temperatures and total strain amplitudes. Compared to other reference lead-free solders, the InnoLot solder shows much better fatigue strength. The better fatigue strength is found to result from the effect of BiNiSb elements. Also, lifetime predictions were made with both models for the solder alloy under different conditions.

Pipeline transmission is one of the most important ways of oil and gas transportation, and its safety is majorly threatened by fatigue fracture that generally occurs in heat-affected zone (HAZ) adjacent to the welded joint of the pipeline. Therefore, studying the fatigue properties of HAZ is important. In this work, the microstructure and mechanical properties in the HAZ of a X80 pipe were accurately simulated by thermal simulation. The fatigue life and crack growth rate of typical sub-HAZ were tested. Results showed that the fine grain HAZ had the lowest strength and fatigue life. By contrast, the coarse grain HAZ had the highest strength, but its fatigue life was lower than that of the intercritical HAZ. Furthermore, the microstructure of each sub-HAZ and its effect on fatigue properties were discussed in detail. The results will reveal the effects of microstructure and mechanical properties on the service safety of pipeline transportation.

Based on the physical phenomenon that the fatigue cracks initiate along specific slip plane, a slip plane damage based low cycle fatigue (LCF) lifetime model for the nickel- based single crystal superalloy is established. The predicted results indicate that the lifetime model can reflect the orientation effect. In addition, in order to characterize the dwell time dependence of the LCF lifetime, creep damage and compression-creep damage are introduced to the lifetime model. Finally, the lifetime predictions under LCF loading with tensile dwell time, compressive dwell time and LCF with tensile-compressive dwell time are conducted by employing the lifetime mode, respectively. The predicted lifetimes show a good agreement with the experimental data, which verifies the accuracy of the developed lifetime model in this paper.

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.

Droplet impingement of metallic surfaces at high impact velocities results, after some time, in erosion of the surface due to fatigue. By extending our previously published analytical model to enable the use of experimental fatigue data (S-N curves), here, for the first time, a wide range of experimental liquid droplet erosion incubation period test states for both ferrous (stainless steel AISI 316) and non-ferrous (aluminium 6061-T6) engineering metals have been investigated. To achieve this, the developed model includes additional surface hardening and a residual compressive stress state at the surface due to a water drop peening effect. As such, the interrelation of the physical and mechanical properties that follows from the model has been used to identify how changes in selected metal properties might enhance droplet impingement erosion incubation life. Model predictions for both metals, using fatigue data from S-N curves from different literature sources, showed for the droplet impact velocity range of 140 to 400 m/s an excellent agreement with results from a multi-regression equation as determined from an ASTM interlaboratory test program.

This study investigated the fretting fatigue behavior and mechanism of 35CrMoA steel of different contact stresses under diamond and square loading paths in the form of curved surface contact. The results show that multiple crack sources will initiate on the subsurface of the specimen under the combined effect of contact stress and cyclic stress. Under low contact stress, only one crack source dominates, causing the instantaneous fracture zone to be biased to the other side of the main crack source. Under high contact stress, the crack sources in both fretting zones play a dominant role, making the shape of the instantaneous fracture zone into a nearly circular shape with better symmetry; At the beginning of the fretting fatigue, cracks only propagate in the cross-section where they form. When they propagate to a certain depth, a component that propagates in the longitudinal direction will be generated.