Effect of adding Ni–Ti shape memory alloy wire on the fatigue behavior of aluminum-glass fibers/epoxy laminated composites

Fiber-metal laminate (FML) composites are among the industry's top materials priorities due to their high fatigue resistance and low weight. In this study, the effects of adding nickel-titanium shape memory alloy wire (SMAW) (0, 1, 3, and 6 wires) and pre-straining these wires (0, 1, 3, 5, and 7% pre-strain) on the fatigue behavior of FML composites were investigated. For this purpose, tensile and fatigue tests were performed and then the fracture surface of the samples was studied with a scanning electron microscope (SEM). The FMLs used included glass fibers, epoxy resin, and T3-2024 aluminum. The FMLs were tested under variable fatigue loadings of 65, 75, and 85% of the ultimate tensile strength (UTS). Increasing the number of SMAWs increased the strength of the FML composites; in other words, the FML containing 6 wires has a fatigue life that is 140% higher than that of the FML without the wires. Also, increasing the SMAW pre-strain percentage showed that applying 1-5% pre-strain increased the fatigue life of the FMLs, but applying a 7% pre-strain reduced their fatigue life by about 5% compared to the sample containing 5% pre-strained wire. It was found that fatigue life increased with decreasing load intensity. This means that the sample containing 6 SMAWs, which was pre-strained about 3% and loaded 65% UTS, has 200% more fatigue life than this sample with 85% loading. SEM images show debonding at the SMAW-FML interface, fiber fracture, and matrix cracking, as the most important FML’s fracture.