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|Title: ||Experimental and numerical analysis of damage in CFRP laminates under static and impact loading conditions|
|Authors: ||Tsigkourakos, George|
|Keywords: ||Impact fatigue|
|Issue Date: ||2013|
|Publisher: ||© George Tsigkourakos|
|Abstract: ||Engineering composites and especially long fibre carbon composites have been in
high demand not only in aerospace and automotive applications, but also in high end
everyday applications. In aerospace, carbon composites are used predominantly for
secondary structures attached by joints or fasteners to various alloys or even
different composites, and are exposed to service loads and repetitive impacting.
Impact fatigue (IF) is not studied adequately for long cycles and relevant literature is
investigating mainly drop weight tests and high speed projectile experiments.
The main aim of this research was to investigate the behaviour long fibre CFRP’S
exposed to repeated low-velocity, low energy impacts, and to observe the damage
effects of this regime on the structural integrity of these materials.
Two types of specimen configurations using CFRPS’s were used and exposed to
loading conditions relevant to the Izod impact fatigue test (IIFT), and the tensile
impact fatigue test (TIFT), in order to determine the fatigue behaviour of the
specimens for each of these load conditions. For the IIFT, the fatigue life was
investigated using IM7/8552 unidirectional specimens and T700/LTM45 cross-ply
specimens were utilised for the TIFT. The specimen thicknesses were altered in both
cases and parametric studies were carried out, where it was seen that IF results in
high level of scatter and the apparent decrease in life was seen at relatively modest
levels of maximum force after relatively few cycles. In the case of the IIFT, a
durability limit was not apparent which increases the complications when designing
against IF. In the case of the TIFT the stiffness deterioration was reflected as an
increase of the loading time, in the force vs time graph, over the total fatigue life
Fatigue crack growth was investigated using fractography and X-ray micro-CT at the
micro and macro level. It was seen, that IF had the potential to initiate cracks and to
cause their propagation at low levels of loading. For the IIFT, a single crack was
growing substantially in the fibre direction and across the sample width causing
matrix cracking and probably breaking of some fibres, which acted as impact wave
guides since matrix cracks were propagating initially along the length of the fibres. In
the case of the TIFT multiple damage modes were presented (matrix cracks, axial
splits and delaminations). Their sequence and progression was successfully
captured and contrasted against the number of impacts. Axial splits governed the
damage scenario, with delaminations extending between them and the free edges.
For the TIFT, IF was studied using the force-life (F-Nf) and energy-life (E-Nf) curves.
The tests undertaken showed that when halving the thickness of the laminates the
fatigue life presented a 10-fold decrease as well as higher scatter.
Finite element modelling was undertaken to validate the experimental data of the
TIFT test. Successful simulation of a single impact was carried out using a fully
transient 3-D model of the actual experiment configuration which involved geometric
non-linearities in addition to the multiple contact conditions. The analysis was
undertaken using the Abaqus 6.11 explicit solver. Since the numerical single impact
results (force vs time response) was in agreement with the experimental results, the
crack modes, experimentally observed, were also incorporated in the model utilising
the use of the cohesive zone elements (CZE).|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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