Abstract:
Composite materials have been increasingly used in the past two decades since they
offer significant potential weight reduction, part design flexibility and improved specific
mechanical performance compared to traditional metals. For specific applications, braid
reinforced composites offer better near net shape part and manufacturing flexibility than
conventional unidirectional laminates, albeit at the expense of slightly lower in-plane
stiffness and strength. Furthermore, for impact and crash applications, which is the
emphasis of this thesis, their tow waviness and interlocking can offer excellent damage
tolerance and energy absorption.
In this work, heavy tow (24k) biaxial carbon and glass braided preforms were used to
manufacture coupons and beam structures to undertake an extensive testing campaign to
characterise different damage and failure mechanisms occurring in braided composites.
Due to large shear deformation and surface degradation, non conventional measurement
techniques based on marker tracking and Digital Image Correlation were successfully
used to measure strains in the damaging material.
The modelling of braided composites was conducted using the meso-scale damage
approach first proposed by P. Ladevèze for unidirectional composites. The calibration
of an equivalent braid unidirectional ply was achieved using the experimental results
obtained for different braided coupons. Furthermore, failure mechanisms observed
experimentally, such as tow stretching and fibre re-orientation occurring during loading
history, were integrated into the model. A new unidirectional ply formulation was
subsequently implemented into the explicit finite element code PAM-CRASHTM.
Validation of the new model using single element, coupons and beams were conducted
that provided a satisfying correlation between experimental tests and numerical
predictions.
