Experimental methods to determine model parameters for failure modes of CFRP
The focus of this thesis is to develop methods to predict the damage response of Carbon
Fibre Reinforced Polymers (CFRP). In the pursuit of reducing the manufacturing cost
and weight of CFRP components, it is crucial to enable modelling of the non-linear
response associated with various failure modes. Two failure modes are considered in
this thesis: fibre compressive failure and interlaminar delamination. Multidirectional
laminated composites are commonly used when a low weight is desired due to their high
specific strength and stiffness. In a carbon/epoxy composite, almost exclusively the fibres carry the load. However, along the fibre direction, the compressive strength is
considerably lower than the tensile strength. With the same reasoning, the transverse
strength is considerably lower than the in-plane strength. This makes delamination and fibre compressive failure two of the major concerns in structural design. Moreover, the
presence of delaminations severely reduces the compressive strength of a laminate. This
can cause catastrophic failure of the structure.
In Paper A, we suggest a test method for determining fracture properties associated
with fibre compressive failure. A modified compact compression specimen is designed
for this purpose and compressive failure takes place in a region consisting exclusively of fibres oriented parallel to the loading direction. The evaluation method is based on a
generalized J-integral and full field measurements of the strain field on the surface of the
specimen. Thus, the method is not restricted to small damage zones.
Paper B focuses on measuring cohesive laws for delamination in pure mode loading.
The cohesive laws in mode I and mode II are measured with the DCB- and ENF-specimen,
respectively. With a method based on the J-integral, the energy release rate associated
with the crack tip separation is measured directly. From this, the cohesive laws are derived.
It is concluded that the nonlinear response at the crack tip is crucial in the evaluation of
the mode II fracture energy.
energy release rate