Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.

Dissimilar welding used in the automotive area are possible joints with the GMAW process; however, its structural performance must be evaluated. The focus of this work is to study the microstructural–mechanical properties of dissimilar welding DPC340Y590T dual phase–JSC270C low carbon steels. Microhardness profile, tensile, and impact tests were used to evaluate the mechanical behavior, while optical and scanning electron microscopy were employed to evaluate the microstructural changes. The tensile strength was 540 and 275 MPa in dual phase and low carbon, respectively. Weld thermal cycles were obtained by means of K type thermocouples. The welding heat input generated martensite and grain growth in the dual phase heat affect zone, while grain growth and perlite phase increased in the low carbon heat affected zone. The variation in microhardness profile was produced by the presence of different phases, and the temperature at the end of dual phase heat affect zone was approximately 242 °C. During impact tests, the absorbed energies were 19.3, 50.7, and 50.2 J for low carbon, dual phase steel, and the welded dissimilar joint respectively. Finally, dissimilar welding subjected to tensile test failed in the low carbon steel (270 MPa), out of the heat affect zone, thus a good dissimilar joint between both steels was obtained.

A customized digital image correlation (DIC) system was implemented to monitor the strain produced in a cold-rolled AL-6XN stainless steel plate, 3.0 mm thick, subjected to quasi-static and cyclic loading tests. A comparison of the DIC strain measurements was made against those provided by conventional extensometers. Furthermore, the DIC system was used to monitor the fatigue crack initiation in low-cycle fatigue tests. The true stress–strain behavior for the AL-6XN material was properly captured by the DIC measurements. For low-cycle fatigue tests (strain control), the strain mapping generated by DIC allowed for identifying zones with higher strain than the nominal strain amplitude applied (${\epsilon }_a$) since the first stages of the fatigue life (FL). These zones become potential fatigue crack initiation sites.

The fatigue life curve was determined for the AL6XN stainless steel under strain-controlled Low Cycle Fatigue (LCF) tests. Additionally, a specific number of loading cycles were applied to new specimens made from the same AL6XN alloy batch to set an Accumulated Fatigue Damage (AFD) based on the Palmgren–Miner rule. The AFD was 0.25, 0.50 and 0.75; subsequently, these specimens were subjected to tensile tests. It was observed that all AFD specimens exhibited a yield strength increment with respect to the AL6XN material property, thus, it was similar to a strain-hardening mechanism. However, the stress–strain behavior and microstructure characterization showed a microvoid nucleation and growth mechanism that competed against the strain-hardening one. The fracture in the 0.75 AFD specimens was dominated by this microvoid-based mechanism. The experimental results indicated that the strain-hardening exponent (n-value) and electrical resistivity ($\rho$-value) were consistently modified by the AFD in all the specimens, with an inverse linear relationship for the n-value and a nonlinear increasing behavior for the $\rho$-value.

The localized compressive deformation (LCD) effect generated by an indentation process at the crack tip on the fatigue crack growth of the 7075-T651 aluminum alloy is reported. Eccentrically loaded single-edge crack tension specimens (ESE(T)) were pre-cracked at a crack length of about 20 mm by applying a constant amplitude fatigue loading. Subsequently, the LCD process was performed by using a semi-spherical indenter with a radius of 16 mm to compress the crack tip zone at different forces (5.0, 7.0, 12.5, 13.5, 15.5 kN), applied on the opposite surfaces of the specimens. The fatigue cracking process was continued on the compressed samples until an overall crack length of about 30 mm was obtained. The compressive load and the number of delayed cycles is discussed in terms of crack length and crack tip opening displacement (CTOD). A direct relationship between the compressive force induced by the LCD process and the delay of the crack propagation due to the crack arrest was observed. This effect became evident at a compressive force of 5.0 kN, where the crack propagation was arrested for about 9000 cycles in comparison with the non-LCD sample. However, when the force increased, the crack arrest also increased. The crack was considered to be completely arrested at a compressive load of 15.5 kN, since the crack did not grow after the application of more than 3 × 10⁶ cycles.

The Charpy impact resistance of monolithic high-strength and dissimilar structured steel was studied. A gas metal arc welding process was used to fabricate the structured steel by depositing a layer of austenitic stainless steel, followed by a layer of hardfacing material over the high-strength steel plate. ANSYS LS-DYNA^TM was used to simulate pendulum–striker impacts on steel Charpy samples. A Cowper–Symonds constitutive model was employed to capture the strain rate behavior. The corresponding material constitutive model parameters were obtained from the literature for the monolithic materials; an iterative numerical optimization method was used to couple the parameters of the structured steel simulation and experimental results. Numerical simulation results showed close agreement with experimental ones. Simulation is a valuable tool for explaining the fracture mechanism in the Charpy impact test and can be used to efficiently design parts made of structured steel that will be subjected to impacts or high-speed deformations.