Examination of the Essential Work of Fracture Method for Paper

The "Essential Work of Fracture (EWF)" method to estimate fracture toughness was examined by means of infrared thermography and video-microscopy. Plastic deformation zone appears in three ways when deep double edge notched tension (DENT) specimens are strained under in-plane stress as follows: 1.type(i) - appearing whole through the ligament in a vague and scattering manner, and developing into a circular (or oval) zone even before or at the maximum load point; 2.type(ii) - appearing from the notch tips and amalgamating into a circular (or oval) zone after the maximum load point; 3.type(iii) - appearing from the notch tips and not amalgamating into a circular (or oval) zone until the sheet failure. Specimens with small ligament are likely to belong to type(i), while those with large ligament to type(ii)&(iii). The analysis using video-microscopy showed that crack propagation starts at the maximum load point for the specimens with small ligament length, while it starts earlier before the point with an increase of the ligament length. And it turned out that type(i) specimens belong to the former. According to the original prerequisite, the EWF method is valid if there is a complete yielding of the ligament before the crack propagation. Therefore only type(i) specimens are concluded to be valid for the EWF method.