Temperature changes of defects in glued joints of laminated lumber and finger joints and also defects or knots in lumber were observed in bending and in longitudinal compression using infra-red thermo-graphical technique. Rises of temperature were observed at defects or knots where stresses were concentrated or cracks often existed. Ends of glued joints or beginning of delamination were also detected by the rise of temperature. The temperature rises were caused by the hysteresis loop of stress-strain curves at the stress concentrated portions under repeated bending and/or compression. The amount of heat generation can be increased with increase of number of repetitions of loading.
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