Thermoelastic Stress Analysis (TSA) has been used to detect and evaluate the severity of damage on a flat metallic plate. The damage takes the form of a semi-circular notch that represents a surface flaw. Thermoelastic data was gathered from the undamaged side of the plate. The experimental results show that shallow surface flaws can be detected by using phase information from thermoelastic data. This information can then be used to indicate the flaw severity in terms of the notch depth. It is shown that the phase data is dependent on the heat conduction effects around the notch, which enable an assessment of the damage. This is modelled using a simple finite element simulation of the effects of heat conduction on the thermoelastic response. A discussion on the potential of using phase variation across damaged regions to analyse damage severity is provided.