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. (C) 2003 Elsevier Ltd. All rights reserved.