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.
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%.