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|Title: ||Modelling residual stresses and environmental degradation in adhesively bonded joints|
|Authors: ||Jumbo, Furopanyekirim S.|
|Issue Date: ||2007|
|Publisher: ||© F.S.Jumbo|
|Abstract: ||The aim of this research was to develop predictive models for residual stresses and
environmental degradation in adhesively bonded joints exposed to hot/wet
environments. Different single lap joint configurations and a hybrid double lap joint
with dissimilar adherends (CFRP/AIIFM73 double lap joint), were exposed to
different ageing environments in order to determine the durability of the joints and the
effects of ageing on the failure load.
Thermal residual stresses in bonded joints were investigated with analytical solutions
and finite element modelling, first with a bimaterial curved beam to validate the
modelling process and determine the most suitable method for calculating thermal
stresses in bonded joints. It was found that none of the analytical solutions and 2D
geometric approximations was fully able to describe the 3D stress state in the strip.
The incorporation of geometric and material non-linearity into the models was
necessary to obtain accurate results. The validated methods were then used predict the
thermal residual stresses in bonded lap joints. The thermal stresses were found to be
highest in joints with dissimilar adherends.
Moisture uptake in bonded joints was investigated using Fickian diffusion modelling.
Gravimetric experiments were used to determine the Fickian diffusion parameters for
the bulk adhesive and composite adherends. Transient diffusion modelling was used
to predict the uptake in bonded joints. It was seen that moisture diffusion is a fully
three dimensional process, and the effects of moisture absorption can only be
adequately studied using 3D FEA.
The effects of swelling from moisture absorption in bonded joints were investigated
using coupled stress-diffusion FEA models. Coupled stress-diffusion 3D FEA was
used to predict the transient and residual hygroscopic stresses that develop in bonded
lap joints as a function of exposure time in accelerated ageing environments, taking
into account the effects of moisture on the expansion and mechanical properties of the
adhesive and CFRP substrate. It was seen that moisture absorption induces significant
stresses in the joints and markedly different behaviour was seen in the cases of
absorbent and non-absorbent adherends.
Hygro-thermo-mechanical stresses arising from the exposure of single and double lap
joints with thermal residual stresses to hot/wet environments were investigated. In the
single lap joints, a reduction in the stresses present in the adhesive was predicted,
owing to swelling of the adhesive from moisture absorption. In the double lap joint
with dissimilar adherends, exposure to hot/wet environments initially reduced the
stresses in the joint when dry, followed by an increase in the magnitude of some stress
components and reductions in others with increasing levels of moisture absorption.
This led to a higher equivalent stress state in the adhesive than when dry.
Thermal residual and mechanical strains predictions were validated with internal
strains measured by neutron diffraction and surface strains measured by moire
interferometry. Comparisons of predicted and measured thermal residual strains
showed low levels of strain in joints with similar adherends. The magnitude of strains
in the CFRP/AI double lap joint was significant, with the same spatial distribution and
magnitude in both measured and predicted strains. The comparison of mechanical
strains predicted by FEA and measured strains by moire interferometry showed good
agreement. High magnification moire interferometry also confirmed the location of
strain concentrations predicted by FEA.
A path independent cohesive zone model (CZM) and a coupled continuum damage
model were used to predict and characterise damage and failure initiation in bonded
joints. Progressive failure prediction was calibrated in the cohesive zone model using
the moisture dependent cohesive fracture energy of FM73. There was a reasonably
good agreement with the experimental failure loads. This implementation of the
cohesive zone model is limited by the ability of the interface elements used, thereby
creating mesh dependency.
The Gurson-Tvergaard-Needleman (GTN) coupled damage model was used to predict
the effects of residual stresses on failure loads. However, this method is difficult to
implement, given the numerous parameters required. The failure loads predicted by
the GTN model were comparable with the experimental data when the joints were dry
or wet. The damage models were capable of predicting the sudden crack growth and
propagation seen experimentally.|
|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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