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.
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.
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.
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.
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 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.
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 work aims to investigate the anisotropic fracture and energy dissipation characteristics of marbles cored along an angle of 0°, 30°, 60° and 90° with respect to interbed planes, subjected to multi-level cyclic loading conditions. Rock fatigue deformation, strength, lifetime and dissipated energy first decreases and then increases with increasing interbed orientation, they get to the minimum for sample having 30° interbed orientation. Rock stiffness degradation is significant with the increase of cyclic level and the stiffness evolution is affected by interbed structure. The incremental rate of dissipated energy becomes faster with increase of cyclic loading level and it presents an abrupt increasing trend at the last cyclic loading level. A damage evolution model was first established based on the dissipated energy to describe the two-phase damage accumulation characteristics. It suggests that the proposed model fits well to the testing data and favorably represents the non-linear characteristics of damage accumulation.
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.
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.
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.
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.
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.
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.
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.
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.
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%.