Thermoelastic Stress Analysis (TSA) is developed to provide a prediction of fatigue life in glass reinforced polymers. A test specimens has been designed to promote cracking and a methodology is defined that allows the measurement of the strain in the damaged region. It is shown that a TSA approach can evaluate fibre breakage, matrix cracking and delamination damage. A strain based metric is established based on calibrated data obtained from the TSA, which can be used to assess the condition of a component throughout its fatigue life