Investigation of Fatigue of Glass Fiber–Reinforced Plastic Tubes Under Multiaxial In‐Phase and Out‐Of‐Phase Loading

ABSTRACT In order to investigate fatigue in fiber‐reinforced plastics under in‐phase and out‐of‐phase multiaxial loading conditions, tube specimens were designed and tested. Initially, tension–compression and torsional moments were individually applied, followed by their superposition of in‐phase and with a 90° phase shift of the amplitudes. With a special clamping device, an inside illumination was possible and backlight images were taken to investigate the specific damage mechanisms for each scenario. For a better understanding of the layer‐wise stress situation in both scenarios, a classic laminate theory approach was conducted. From a 3D digital image correlation system, the strain fields were analyzed and a significant interlaminar effect in the out‐of‐phase testing was identified. The analysis of the measurements and the dissipated energies revealed a significantly lower fatigue life of the out‐of‐phase tested specimens compared with the in‐phase case, associated with a more severe interlaminar damage development.