Abstract:
As global warming is a prime concern to the wellbeing of the planet, lighter
planes are a requirement to reduce CO2 emissions. Light structures made of carbon
epoxy composite materials are of particular interest but are sensitive to impacts such
as hail or bird strikes. Static and dynamic testing of composite specimens and
structures (from aeronautical standard) through novel testing methods was the first
aim of this PhD research. Subsequently it led to novel material characterisation and
material parameters calibration for which numerical simulations of impact responses
could be developed. In this research further investigation under static, dynamic and
impact loading of two certified aeronautical materials occurred. Carbon non crimp
fabric epoxy (tufted and untufted) response was investigated. Novel tests and testing
methods were developed for in-plane and for delamination focussing on the use of
optical analysis using digital image correlation (DIC) and high speed cameras. A
novel damage detection method was proposed using DIC. The experimental data set
was used to calibrate a damage model with imposed strain rate laws and added
delamination Mode I and II interface prior to a punch validation study. A novel
compression apparatus designed for DIC usage worked well in static and dynamic. A
novel intermediate strain rate tensile test worked better on bias direction lay-up than
on axial one. Dynamic DIC method proved of interest to record strains up to strain
rate achieved with a Split Hopkinson pressure bar apparatus. In quasi static tufting
reduced axial properties considerably but had little effect in shear loading, in addition
it increased significantly the resistance to delamination and reduced the crack speed in
dynamic. The damage fields generated allowed for the detection of damage
progression for various load cases. More damage occurred in compression and shear
than in tension as the tufted laminate showed more pronounced damage than the untufted one. The dynamic effect of tufting on in-plane and impact response was
reduced as it increased considerably delamination resistance in Mode I and II. For
both tufted and untufted NCF composites, strong strain rate effects were detected
from a low speed on the in–plane strength and failure strain as no or little effects were
recorded on the material stiffness. Novel dynamic delamination Mode I and II tests
combined with optical analysis provided possibilities to detect rate effects and crack
speed propagation while loaded in pure mode I and II. No strain rate effects were
recorded in delamination Mode I apart from a slight effect during crack initiation
which was stronger for the tufted material. In Mode II a slight rate effect was detected
for the tufted interface during crack propagation. During out-of-plane impact loading
at intermediate speed, a minor negative loading rate effect was detected.The model calibrated in damage, delamination and strain rate prove useful for
dynamic DCB representation and assessment of possible mix mode crack loading.
Modelling tufts as P-link was of interest but requires further investigation. Damage
and strain rate was well modelled in tension, compression and bias direction loading,
even if the strain rate shear law would require some modification. A Meso-scale
model was validated successfully by means that the model responses would follow the
experimental trends in quasi static loading but with the modification of 4 parameters
among ±50. Further research could extend its use for impact modelling. This research
showed the complexity of the failure mechanism occurring in composite materials,
modelling them at high speed in the plane and in out of plane impact remains a
difficult challenge. Carbon composites damage sensitivity is significant and invisible
to the naked eye for some load cases and lay-ups necessitating regular non destructive
testing on aging airframe.