Strain fields around knots in Norway spruce specimens exposed to tensile forces

Two-dimensional strain fields around knots in two Norway spruce specimens subjected to tension loading were detected using a contact-free measuring technique based on white-light digital image correlation. The first specimen included a traversing Edge knot, and the second one, a Centric knot. The development of strain fields as a function of load level was measured by consecutive cyclic load tests where one side of the specimen was studied during each test. The objectives were to examine to what extent the strain fields could be detected, to investigate the correlation between strain fields measured on different sides of a specimen and to analyse the strain distributions around the knots. The results show that the applied technique is useful for catching both overall and detailed information about the behaviour of knots in wood members exposed to loading. Clear wood defects that could not have been detected by neither visual inspection nor scanning were observed, and conclusions could be drawn regarding the release of internal stresses. The correlations between strain fields on different sides of the specimens were excellent, and the correspondence between measurement results and comparative finite element calculations was surprisingly good considering the fact that the employed FE models were fairly simple.