With the aim of decelerating crack development and extending fatigue life, hybrid laminates have been introduced to complement the use of monolithic materials, e.g. aluminum in the aerospace industry. Especially for safety-relevant components, the use of durable materials and lasting components is important to meet the ever-increasing relevance of sustainability. But equally important is the knowledge of the mechanical properties and the damage behavior in the application scenarios, which are indispensable for the estimation of fatigue life and thus for the design. Compared to thermoset-based hybrid laminates, thermoplastic-based hybrid laminates allow improved possibilities in forming, recyclability, shorter production times, and more sustainable use. However, the mechanical properties are not yet sufficiently known, especially regarding fatigue loads.

Thermoplastic-based hybrid laminates, consisting of unidirectional glass and carbon fiber-reinforced polyamide 6 and AA6082 aluminum alloy sheets, are investigated in this work. Laminates are investigated in the state as consolidated and after post-stretching to evaluate the significance of the consolidation process-related residual stresses present regarding the fatigue capabilities. As such, the fatigue-induced crack propagation behavior of the laminate is investigated under tension-tension loading up to the VHCF range of 1E8 cycles. The damage evolution is examined using 3D digital image correlation for deformation measurements (fatigue testing setup in Fig. 1), accompanied by computed tomographic in situ characterizations of the integral damage states.

The results show that the fatigue life of the thermoplastic-based laminates depends significantly on the residual stress states present due to the consolidation process, which in combination with the microstructural properties of the aluminum alloy sheets influence the damage initiation and propagation. Post-stretching can increase fatigue life by more than 60%, resulting in significant performance improvements, highlighting the potential within thermoplastic-based laminates.