Detection of short cracks in riveted connections using Lock‐In‐Thermography

Abstract

Although many riveted bridge structures made of old structural steel from the 19th and early 20th centuries are still in use today, there is usually no need to replace them. When assessing these steel structures with regard to necessary refurbishment or strengthening measures, engineers need information about the remaining fatigue lifetime in particular. Often no sufficient remaining fatigue life can be determined based on normative S‐N‐curve assessment. An initiation of fatigue cracks has to be assumed. These cracks occur primarily at the edge of the rivet holes. In the early phase of crack growth below the rivet head, the crack cannot be found in a visual inspection of the structure. For this reason, the remaining cyclic lifetime based on fracture mechanics is determined assuming a start length of the crack beyond the rivet head. This paper reports investigations using thermoelastic stimulated Lock‐In‐Thermography as a NDT method for detection of hidden cracks in riveted joints. The results have shown that cracks below rivet heads can be detected. This is possible despite the presence of a corrosion protection coating. The detectable crack length depends mainly on the level of stress and the loading frequency.