Bonded crack retarders are a novel technology for providing improved damage tolerance in aerospace structures. They can be applied either in locations where mechanical fastening is being replaced by integral construction, or simply as a means of improved crack growth resistance in fatigue critical locations. In previous studies we have measured the residual stresses arising from the coefficient of thermal (CTE) mismatch between the reinforcing strap and the substrate. We are now moving in to the next stage and aim of this proposed experiment is to study the interaction of the thermal residual strains with the fatigue crack growth in a middle crack tension (MT) specimen. The measurements will be performed in situ on a pre-cracked MT specimen using conventional diffraction technique.