Lecture
Semi-analytic modelling strategy for the initiation and evolution of transverse cracking of CFRP with and without nano-modified matrix
Wednesday (28.09.2016) 12:30 - 12:45 Part of:A new experimental approach based on a shear force free cyclic bending test stand, avoiding
non preferable specimen heating, has previously been developed to asses the fatigue behaviour
of endless carbon fibre reinforced cross-ply laminates at very high load cycles (N > 10^8).
For investigating the influence of carbon nanotube (CNT) modified matrices on the initiation
and propagation of fatigue damage nanoparticle-modified specimens were tested against nonmodified
specimens. Both material types were tested at six different strain levels with a load
ratio of R = -1. Additionally static four-point bending tests have been performed stepwise to
examine the crack initiation and development under quasi-static loading. In quasi-static and
fatigue tests transverse cracking was the first damage mechanism observed, underlining its
importance for subsequent damage mechanisms e.g. delamination or fibre breakage.
From the static experiments it can be shown, that a fast initial increase of cracks takes place
for the non-modified as well as the modified specimens when reaching the transverse failure
strain. The failure strain of the CNT-modified specimens is significantly (0.2 % strain) lower
compared to the unmodified specimens.
From the fatigue experiments at VHCF relevant loadings a crack initiation and growth is still
found for both specimen types. In case of the CNT-modified specimens, transverse crack evolution
is more pronounced resulting in early crack initiation and higher crack growth rates
during fatigue.
Both, quasi-static and cyclic experimental findings indicate that a CNT-modification of the
matrix system does not improve the transverse crack resistance of CFRP, which might be attributed
to additional stress concentrations caused by nanoparticles. Recent results in small
scale matrix testing encourage these findings by a diminishing size effect by incorporation of
nanoparticles.
The experimental results are used to develop a physically based material model capable to
describe the initial cracking state after the first load cycle and the development during fatigue
loading. Therefore, a strain energy release rate (SERR) master curve for relevant cracking
states is calculated by means of finite element analysis (FEA). The master curve and experimental
results are combined to describe the strain dependent initial cracking behaviour. Furthermore,
fatigue crack density propagation curves are formulated in terms of SERR range for
describing the fatigue crack density growth during fatigue.
The developed approach is promising to predict the crack density growth for different load
levels with respect to sequence-effects and will be adopted for different load ratios and specimen
lay-ups.
