Lock-In Thermography:: A tool to analyse and locate thermo-mechanical mechanisms in materials and structures

This paper aims to illustrate the advantages of Lock-In Thermography (LIT) as a non-destructive, real time and non-contact technique to analyse and to locate thermo-mechanical mechanisms in materials and structures. Due to the first and second principles of thermodynamics, there is a relationship between temperature and mechanical behaviour laws, LIT is classically used to measure linear thermo-elastic effect to evaluate stresses in structures under periodic, random or transient loading. The new digital processing D-MODE presented allows extracting non linear coupled thermo-mechanical effects (dissipated energy) cycle by cycle during a fatigue test on specimens and on real structures. This quantity much smaller than thermoelastic source needs a high sensitive thermal imaging camera and a dedicated algorithm to separate Dissipated energy from Thermo-elastic source. On other hand, it is known for a long time that there is a correlation between plasticity in materials and the appearance of heat dissipation [2]. More recently, it was shown there is a clear relationship between fatigue limit and occurrence of dissipated energy. We propose to improve these two methods by using LIT instead of temperature rise measurement to predict crack occurrence in real structures. At last we present some industrial applications in automotive and aircraft industries.