Very high cycle fatigue testing and behavior of GFRP cross- and angle-ply laminates

Abstract

Compared to low cycle and high cycle fatigue of fiber-reinforced composites, the very high cycle fatigue behavior has only been explored in part. Underlying reasons are time consumption and the variety of difficulties of high-frequency testing. Therefore, a special high frequency bending test rig providing reasonable testing times and online damage monitoring has been developed within the DFG priority program 1466. Comprehensive test series have been conducted investigating the VHCF of two GFRP laminates, a cross-ply and an angle-ply lay-up. Several load levels have been applied to investigate the effect of stress amplitude. Whereas both laminates show a characteristic HCF behavior at higher load levels, damage initiation and progression are delayed at lower loads. Differences in VHCF and HCF damage behavior are revealed. As neither cracking nor delamination is found at lower loads, the existence of initiation thresholds and a fatigue limit is discussed. In fact, angle-ply specimens tested at low load levels do not fail within 1.5∙108cycles. A deeper understanding of the experimental results is achieved by additional modelling. Firstly, stiffness degradation is simulated by means of a 2D representative laminate element including cracking and delamination. Secondly, a statistical approach by Berthelot is adopted to analyze the role of weak areas. For the first time, a comprehensive investigation of VHCF of FRP including both fatigue life investigation and damage growth characterization is presented.