Non-Linear Effects in Stress Measurement by Thermoelastic Techniques

This chapter discusses nonlinear effects in stress measurement by thermoelastic techniques. The phenomenon by which a body changes temperature as it undergoes a change in stress has been used to measure stresses in a vast range of applications. One of the main advantages of stress measurement by thermoelastic techniques is that it allows full field stress measurement with minimal surface preparation. The only surface preparation that is required is to paint the surface of the specimen with a thin layer of high emissivity paint. Using infrared techniques to measure the temperature distribution over a body, it is relatively easy to produce a good qualitative image of the distribution of surface stresses over a body. The chapter discusses the effects of thermal conduction on surface temperature measurements. It shows that an area that was experiencing changes in surface stresses that were lower than the far-field stresses, actually appeared to be undergoing higher stress changes due to the effects of thermal conduction from within the body. The chapter suggests that metallic specimens must usually be painted with a high-emissivity paint in order to both increase the amount of infrared radiation emitted, and to provide a uniform emissivity over the observed surface.