This thesis details an experimental study on the determination of the fracture
parameters for a crack located at the interface between two dissimilar materials using
the method of photoelasticity. The interface is potential1y an inherent weak spot of
any composite material, structure"or adhesively bonded joint. Accurate description of
the state of stress at the crack tip is required for strength prediction. The concept of
the complex stress intensity factor is used to characterise the elastic crack tip stress
field for an interface crack. Complex stress intensity factors and their moduli have
been measured experimental1y for standard bimaterial crack geometries using the wel1
established technique of photo elasticity.
Bimaterial specimens comprising aluminium al10y and epoxy resin components were
used. This creates a large material mismatch at the interface and al10ws data to be
col1ected from the epoxy component of the specimen using transmission
photoelasticity. An automated ful1 field photoelastic technique was developed to
significantly reduce the data col1ection time. The technique comprises elements from
the approaches of three wavelength and phase stepping photoelasticity and is a
significant improvement on techniques previously available.
Stress intensity factors were determined by fitting a theoretical stress field solution
for the bimaterial crack to the experimental data. A computational routine
automatical1y selects the region of best fit between the experimental data and the
theoretical solution. This data is then used to determine the complex stress intensity
factor and its modulus value. In order to provide a robust fit between the experimental
data and the theoretical field solution a weighting function was incorporated into the
The measured bimaterial stress intensity factors are compared with those determined
experimental1y for equivalent homogeneous specimens made from epoxy resin. The
differences between the two are then discussed. The experimental results agree with
the wel1 known concept that tension and shear effects are inherently coupled at the
crack tip. However, the effects of changing the load angle with respect to the interface
also demonstrate that some contrasts exist with known numerical solutions.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.