Influence of motion compensation on lock-In thermographic investigations of fatigue crack propagation

Lock-in thermography measurements are influenced by rigid body motion of the specimen due to the mechanical loading. These motion affects asymmetries in the measured temperature fields in the vicinity of the crack. This effect can be eliminated by a motion compensation procedure. Unfortunately, the results of the lock-in thermography are affected by this procedure, too. In this work the influence of the motion compensation procedure on fatigue crack propagation experiments, its gains and disadvantages are studied in a qualitative and quantitative manner. A new method for an automatic determination of the crack length in thermographic measurements is introduced and compared with potential drop measurements. Beside the known thermoelastic and dissipative effects, higher harmonic responses in the discrete Fourier transformation are analyzed and discussed.