Investigation of localized fatigue properties in unalloyed steels by infrared thermography: Fid-Bau Portal

Investigation of localized fatigue properties in unalloyed steels by infrared thermography

Medgenberg, Justus

One application field of infrared technology as non-destructive testing method constitutes the non-contacting and temporally resolved investigation of thermomechanical coupling processes in fatigue loaded structures. In the past numerous investigations have been performed which predominantly focus on the analysis of macroscopic damage processes under the assumption of homogenous material behaviour. By contrast the goal of the present thesis was the investigation of strongly localized fatigue damage processes on unalloyed steel S355 in the regime of high cycle fatigue using high resolution infrared thermography. The thesis is divided into a theoretical-numerical and an experimental part. In the first part of the thesis a numerical model for the investigation of thermomechanical coupling processes is presented which can be incorporated into existing finite element programs. After adjustment of the material parameters the numerical model exhibits a very good coincidence with experimental results. The second part of the thesis deals with the experimental assessment of local nonlinearities in the temperature evolution due to material damage. The analysis of the infrared measurements is carried out by a newly developed processing methodology. The results show that strongly localized damage processes as the development of cyclic Lüder’s bands and the damage progress at the bottom of geometric notches can be verified by analysis of the temperature signal. Further experiments on welded specimens prove the importance of crack propagation in the weld toe for the fatigue behaviour. The results of the thermographic investigations correlate with data from strain measurements and microscopic investigations of surface changes. Overall the results of this thesis demonstrate high potential of the thermographic detection of damage processes during fatigue testing und indicate starting points for further research activities.