The main objective of this thesis is to study the
residual strength of notched polymer matrix composite laminates (PMCLs) and fibre
reinforced metal laminates (FRMLs). A new effective crack growth model based on failure
mechanisms and fracture behaviour of notched composite laminates is developed to simulate
the residual strength of these materials. The effects of various parameters such as notch
size, constituent properties, specimen geometry, stacking sequence and fibre orientation
on the residual strength of these composite laminates with a circular hole or a sharp
notch are also investigated.
Firstly, damage and failure mechanisms of notched composite laminates
including failure modes, notch tip damage zone and notch sensitivity of unidirectional and
multidirectional composite laminates are reviewed as well as stress distributions around
various cut-outs. The effect of notch size on residual strength of these composite
laminates is then addressed. Previous models used to evaluate the residual strength of
notched composite laminates are discussed individually and the limitations of these models
and the requirements for a new model are subsequently examined.
A new effective crack growth model (ECGM) is developed in this thesis.
Damage is assumed to initiate when the local normal stress reaches the tensile strength of
the unnotched laminate, and it propagates with an increase of the applied load. The damage
is modelled by a fictitious crack with cohesive stress acting on the crack surfaces, and
the damage growth is simulated by extension of the fictitious crack and reduction of the
cohesive stress with crack opening. The apparent fracture energy is used to define the
relationship between the unnotched strength and the critical crack opening. Based on the
equilibrium condition, an iterative technique is developed to evaluate the applied load
required to produce the damage growth. The residual strength of notched composite
laminates is defined by the unstable point of the applied load with damage growth.
The ECGM is unique because it involves some capabilities of the
progressive type models without finite element analysis. Based on the unnotched tensile
strength (so) and apparent fracture energy, this
model simulates the residual strength of notched polymer matrix composite laminates with
good accuracy and simplicity compared with the previous models, e.g. Point Stress and
Damage Zone Criteria. The effects of notch size, specimen width, stacking sequence, and
constituent properties on the residual strength are evaluated by the ECGM. The simulations
using this new model produce good correlations with experimental data for various laminate
configurations. The ECGM simulates damage growth in terms of effective crack extension
step by step. In such an approach, estimations of the damage zone size ahead of the notch
tip and real crack initiation/propagation are obtained.
The ECGM is also extended to simulate the residual strength of fibre
reinforced metal laminates (FRMLs) with a circular hole or a sharp notch. The residual
strength simulated from the ECGM correlated well with experimental data in the open
literature for various ARALL, GLARE and CARE notched laminates. Furthermore, the stress
redistribution with damage growth in notched composite laminates is also discussed.
Finally, using two carbon/epoxy [0/90]4S composite laminate
systems, experimental investigations were conducted to study the effect of fibre/matrix
interfacial adhesion on notched residual strength. The effect of hole size and notch
sensitivity of the laminates were also investigated. Fracture surfaces were examined by
Scanning Electron Microscopy (SEM) to characterise failure mechanisms of these laminates.
