Carbon fibre reinforced polymers (CFRP) are used in numerous lightweight structures in aerospace, automotive or sports equipment, where high specific strength and stiffness are essential for a reliable
lightweight design. However, due to their multi-scale nature and various constituents, failure behaviours of composite materials are very complex. Generally, composites are characterised by a
brittle behaviour, whereby fatigue loads can lead to sudden failure. Due to their specific failure mechanisms - including matrix cracks, delaminations and fibre failures - the stiffness and strength of
composites degrade under fatigue load. An increasing crack length and the associated stress maxima at
the crack tip result in delaminations, which lead to a significant stiffness decrease during fatigue. The failure mechanisms of CFRP are strongly dependent on the layer thickness, which can changes the
failure mode from a complex delamination dominated one towards a brittle, thus structural health monitoring is essential.
A promising NDT method sensitive to damages and other material non-linearities is the vibro-acoustic modulation (VAM) method. It combines a high-frequency lamb wave with a low-frequency structural
vibration which results in a stress and stiffness-dependent non-linear modulated signal. The high sensitivity in fatigue damage detection of metals and impact detection of composites has already
been demonstrated.
This study focuses on a systematic change of the CFRP laminate structure and a variation of local defect size to detect the damage degree and the development of composite typical failure mechanisms
with VAM. Two sets of commonly employed laminate layups were manufactured for tensile-tensile loaded (R=0.1) specimens. Firstly, in a cross-ply (CP) layup, the number of 90° layers was varied,
with a constant areal weight (268 g/m2), using Hexcel HexPly M21/T800S prepreg. Secondly, quasi-isotropic (QI) specimens were tested, with areal weight from 30 to 268 g/m2, using NTPT ThinPreg
402 prepreg. VAM was measured on impact tests with 5, 6.5 and 8 J impact energy both initially and under fatigue load. The ultrasonic lamb waves were applied with piezo-ceramic actuators disks in a frequency range of 185-230 kHz. The low-frequency was induced by a cyclic testing machine with a frequency of 3 Hz. The resulting damage was assessed using ultrasonic C-Scans and radiography.
The tensile fatigue tests demonstrate a correlation between stiffness decreasing damages such as inter-fibre fractures and delaminations and the VAM. Inter-fibre fractures, both in 90° and in 45° layers, lead to a sideband amplitudes increase. In contrast, large-scale, pronounced delaminations cause an abrupt stiffness-proportional decrease in the sideband amplitudes due to secondary vibrations caused by friction of the separated individual layers. Hence, before large delaminations occur, it is possible to define a VAM threshold that allows failure prediction. Varying ply thickness of QI laminates results in a lower modulation for brittle material behaviour with multiple inter-fibre fractures and small crack diameters. Initially, VAM increases linearly with increasing size of the delamination. In addition, the propagation of the local delamination (impact) during fatigue load can also be
monitored using VAM
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