A thermal imaging technique for studying crack development in wood under torsional loading

A thermal imaging system has been used for monitoring fracture in wood under both static and fatigue torsional loading. The thermal images of softwood test-pieces containing a knot under torsional loading predicted the cracking time and crack position that agreed well with visual observation. The thermal images obtained under torsional fatigue loading indicated a temperature increase during the unloading part of a loading cycle, which meant that thermal energy was dissipated during the relaxation stage of the loading cycle. The maximum temperature reached also increased as the loading cycles increased. Results from thermal images of a softwood indicated that the earlywood exchanged more thermal energy than latewood. Optical microscopy and SEM confirmed that in earlywood the region near a growth ring is the weaker area. For all the test pieces, whether softwood or hardwood, with or without a knot, the hotspots revealed during thermal imaging appeared before the load dropped sharply and these were confirmed to be the positions for crack initiation. This shows that it is possible to predict and depict failure and its progress using thermal imaging techniques. C 2005 Springer Science + Business Media, Inc.