Investigation of the damage-dependent material damping in carbon fibre-reinforced polymers

Carbon fibre-reinforced polymers (CFRPs) are established construction materials due to their superior specific strength and stiffness. Structural health monitoring aims to increase the safety of components by detecting damage at an early stage for which the material damping has been identified as a sensitive indicatior. However, given the numerous different forms of damage that occur in CFRP, a consistent relationship has yet to be reported.

The objective of this work was to investigate the damage-dependent material damping in CFRP and thereby contribute to the knowledge in this field. The focus was on the effect of barely visible matrix cracks, which often denote the beginning of developing damage, to examine the potential of damping for an early indication. An existing experimental methodology was adapted to examine the relation between mode I matrix cracks and the damping of flexural modeshapes. The presented empirical approach was thoroughly evaluated before being applied to measure the amplitude-dependent frequency and damping in coupon-level specimens. The experimental setup was modelled using the finite element method, accompanied by modal testing to validate the numerical representation.

Analysing the free decay data, no correlation between damage and damping was found for
the investigated matrix cracks. The empiricism challenges the hypothesis of a positive
relation and reemphasises the notion that two mechanical conditions dominate the relationship
between damage and damping in CFRP: The extent and relative arrangement
of cracks on the one hand and the stress-strain far field, which causes fictional relative
motion of fracture surfaces on the other hand. The experiments yield the conclusion that
modal damping may not be a good indicator for the investigated matrix cracks."