The Use of Vibrothermography for Monitoring Damage Evolution in Composite Materials

For the non-destructive evaluation of materials, several methods are well known, for instance the ultrasonic, the holographic, the radiographic method and others. For the visualization of stress concentrations on the other hand, Moire interferometry and photoelasticity are common practice. In the past few years however, a new method, called Stress Pattern Analysis by Thermal Emission (SPATE), is more and more known and used. A number of papers about the research on the value and interpretation of this method are already available but they are all especially concentrating on isotropic homogeneous materials. The basic thermoelastic theory for isotropic materials will be explained. However, because of the fact that the use of SPATE on composite materials has not been examined very well, the research being done at the Free University of Brussels is focusing on this domain. In this case, the deduction of the thermal emission — stress law must be reviewed and will be presented. A difficulty arises when at certain load levels, a phase shift is encountered between the upper and lower part of the hole. A basic theoretical explanation of this phenomenon will be put forward. Notification will be made that this is a general effect, valid for both isotropic and anisotropic materials. However, the magnitude of this effect is in general not noticeable on isotropic materials. On the other hand, this effect is in laminate materials enhanced and even of the same magnitude as the expected SPATE signal itself. As a result, SPATE scans of stressed laminated materials are much more difficult to interpret and evaluate correctly.