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%.