Identification of damage in composite materials using thermoelastic stress analysis

A quantitative damage assessment methodology for composite materials has been<br/>achieved using Thermoelastic Stress Analysis (TSA). The TSA technique provides fullfield<br/>data which is collected in a non-contacting and real time manner. The damage<br/>assessment methodology proposed requires a means of calibrating and temperature<br/>correcting the thermoelastic signal; these are developed and presented in this thesis.<br/><br/>The thermoelastic theory for calibrating thermoelastic data from orthotropic bodies has<br/>traditionally been based on a stress formulation. There are difficulties in calibrating<br/>orthotropic materials in this manner and an alternative calibration routine has been<br/>devised and validated. The calibration routine provides the thermoelastic theory as a<br/>function of strain and permits a simplified calibration route as the laminate strains are<br/>the basis and can be measured in a straightforward manner.<br/><br/>During damage propagation in laminated structures the specimen heats. The increase in<br/>temperature has a significant effect on the thermoelastic data and necessitates that the<br/>thermoelastic data be corrected to remove the effect of temperature from the data. A<br/>routine is developed that enables the correction of the thermoelastic data in a point-bypoint<br/>manner.<br/><br/>By combining the strain calibration and temperature correction procedures a damage<br/>assessment methodology has been devised. The application of the methodology is<br/>demonstrated on glass / epoxy laminate specimens that are fatigue damaged and the<br/>damage state assessed using this method; the extent and type of damage is verified<br/>qualitatively using visual inspection methods. The work described is applicable to any<br/>orthotropic material. The effect of fatigue damage is assessed by periodically collecting<br/>thermoelastic data during the specimen life. This data is analysed using damage metrics<br/>based on the calibrated strain obtained from the TSA.<br/><br/>The wider application of the TSA damage assessment methodology is considered by<br/>assessing the ability to locate subsurface damage. A complementary IR technique is used<br/>in conjunction with TSA known as Pulse Phase Thermography (PPT). Initial studies<br/>demonstrate the ability to resolve the spatial extents of subsurface damage. The purpose<br/>of this step is to guide TSA to areas of concern that can subsequently be assessed using<br/>the damage metrics to characterise the effect of damage on the residual life of the<br/>component.<br/><br/>The strain calibration and temperature correction methods that enable TSA to be<br/>applied quantitatively to damaged composite materials have not been accomplished prior<br/>to this work. They provide novel methods by which TSA data can be assessed, and their<br/>application is not restricted to damage studies alone. The ability to temperature correct<br/>TSA data has been shown to be of vital importance if thermoelastic data is to be<br/>compared in a quantitative fashion. The strain calibration procedure presented will<br/>enable thermoelastic studies to be reported quantitatively and expand the application of<br/>TSA particularly in validation studies. The damage assessment methodology presented<br/>represents a step forward in the application of TSA to the damage assessment of<br/>composite materials.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Emery, Trystan Ross

Advisor dc:contributor.advisor

• Barton, Janice