Prolonged crack propagation and increased fatigue lifetime of hybrid laminates compared to monolithic metals are key aspects for their use in, e.g., the aerospace industry. Especially for safety relevant components, detailed knowledge about the fatigue capabilities is necessary. Steady material degradation with progressive numbers of cycles affects the integrity of the laminate structure as well as the mechanical properties, which need to be understood to estimate lifetime expectancy up to the very high cycle fatigue (VHCF) regime. For thermoplastic-based hybrid laminates, which offer the possibilities of formability, recyclability, and mass production due to short consolidation cycle times, this knowledge up to the VHCF regime still needs to be established to enable these laminates for use in safety critical components.

In this study, thermoplastic-based hybrid laminates containing AA6082 aluminum alloy sheets and unidirectional glass and carbon fiber-reinforced polyamide 6 were investigated. Fatigue tests up to the VHCF regime (max. 1E9 cycles) were conducted under tension-tension loading on a resonant fatigue testing system offering a frequency of 1 kHz. Fatigue progress and accompanying damage were monitored through stress-strain hysteresis analysis, using high speed cameras with frame rates up to 100 kHz for deformation analysis with digital image correlation, and surface temperature measurement. Microscopic investigations of the damaged volume were conducted after defined decreases of stiffness and numbers of cycles to expand the microstructural observation of surface damage and deduce onto the volume inner damage mechanisms.

The test results show that during VHCF load the mechanical properties are influenced mostly by changes in microstructure within the aluminum alloy sheets due to damage initiation and propagation. As a result, the fatigue lifetime of the metal sheets differs greatly from the total fatigue lifetime of the hybrid laminate. Compared to the HCF regime the interface damage changes in terms of crack and delamination rate.