MSE 2016 - Full Program

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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.

Speaker:
Gordon Just
TU Dresden
Additional Authors:
  • Ilja Koch
    Technical Univerity of Dresden
  • Maik Gude
    Technical Univerity of Dresden