Identification of damage in composite materials using thermoelastic stress analysis

Abstract

A quantitative damage assessment methodology for composite materials has been
achieved using Thermoelastic Stress Analysis (TSA). The TSA technique provides fullfield
data which is collected in a non-contacting and real time manner. The damage
assessment methodology proposed requires a means of calibrating and temperature
correcting the thermoelastic signal; these are developed and presented in this thesis.

The thermoelastic theory for calibrating thermoelastic data from orthotropic bodies has
traditionally been based on a stress formulation. There are difficulties in calibrating
orthotropic materials in this manner and an alternative calibration routine has been
devised and validated. The calibration routine provides the thermoelastic theory as a
function of strain and permits a simplified calibration route as the laminate strains are
the basis and can be measured in a straightforward manner.

During damage propagation in laminated structures the specimen heats. The increase in
temperature has a significant effect on the thermoelastic data and necessitates that the
thermoelastic data be corrected to remove the effect of temperature from the data. A
routine is developed that enables the correction of the thermoelastic data in a point-bypoint
manner.

By combining the strain calibration and temperature correction procedures a damage
assessment methodology has been devised. The application of the methodology is
demonstrated on glass / epoxy laminate specimens that are fatigue damaged and the
damage state assessed using this method; the extent and type of damage is verified
qualitatively using visual inspection methods. The work described is applicable to any
orthotropic material. The effect of fatigue damage is assessed by periodically collecting
thermoelastic data during the specimen life. This data is analysed using damage metrics
based on the calibrated strain obtained from the TSA.

The wider application of the TSA damage assessment methodology is considered by
assessing the ability to locate subsurface damage. A complementary IR technique is used
in conjunction with TSA known as Pulse Phase Thermography (PPT). Initial studies
demonstrate the ability to resolve the spatial extents of subsurface damage. The purpose
of this step is to guide TSA to areas of concern that can subsequently be assessed using
the damage metrics to characterise the effect of damage on the residual life of the
component.

The strain calibration and temperature correction methods that enable TSA to be
applied quantitatively to damaged composite materials have not been accomplished prior
to this work. They provide novel methods by which TSA data can be assessed, and their
application is not restricted to damage studies alone. The ability to temperature correct
TSA data has been shown to be of vital importance if thermoelastic data is to be
compared in a quantitative fashion. The strain calibration procedure presented will
enable thermoelastic studies to be reported quantitatively and expand the application of
TSA particularly in validation studies. The damage assessment methodology presented
represents a step forward in the application of TSA to the damage assessment of
composite materials.

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