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

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.