Mode II R-curve of wood measured by 4-ENF test

A mode II R-curve, which depicts fracture toughness a function of crack length, can be determined by a four-point bend end-notched flexure (4-ENF) fracture test alone without having to observe the crack propagation when measuring the load, loading line displacement, and longitudinal strain at a certain point of the specimen. This is possible because the fracture toughness and crack length can be calculated by the load–strain compliance and the combination of load–loading line and load–strain compliances, respectively. Based on this concept, a 4-ENF fracture test of spruce was conducted, and its validity was examined by comparing the results with those obtained by the 4-ENF test using the beam theory and compliance calibration methods of data reduction, as well as three-point bend end-notched flexure (3-ENF) tests. The R-curve could be determined properly by the 4-ENF fracture test alone if the strain was measured in the region where a crack-free condition was maintained during the test.