Crack detection in pillars using infrared thermographic imaging

In this paper, we describe a series of crack-detection tests on scale models of cavern supported by pillars performed in the laboratory to find out where and when crack initiation occurs. Crack initiation was detected by two different methods, thermographic camera imaging and strain-gauge measurements, and comparisons were drawn. For the crackdetection test, three physical models of pillars were made out of gypsum with different pillar widths (25 mm, 50 mm, and 100 mm). When cracks begin to develop in the pillar models, a thermographic camera can detect temperature changes around the cracks that are induced by friction at the contact areas. Whereas the strain-gauge measurement indicates only local strains, the thermographic imaging can cover overall strain variations. The authors did not correlate the increase in temperature variations with strain. With the 50-mm and 100-mm pillar widths in the laboratory test, the crack-induced failure naturally occurred in three steps: (1) first crack initiation, (2) crack propagation, and (3) failure. But for the 25-mm pillar width, the crack-induced failure occurred immediately after the first crack initiation; propagation was not observed.