Examination of the Essential Work of Fracture Method for Paper Material by Use of Thermography

The "essential work of fracture" method was applied to various machine-made papers. The deforming and fracturing processes of the paper during the testing is examined by means of the thermographic observation.
Plastic deformation zone appears in three ways when deep double edge notched tension specimens are strained under in-plane stress, i.e. type(i); appearing whole through the ligament in a vague manner and developing into a circular (or oval) zone even before or at the maximum load point, type(ii); appearing from notch tips and amalgamating into a circular (or oval) zone after the maximum load point, and type(iii); appearing frorn notch tips and not amalgamating together until the sheet failure. Specimens with small ligament length (L) are likely to belong to type(i), while those with large L to type(ii)&(iii).
Among these three types, type(i) fulfills the original assumption of the EWF method in terms of the complete ligament yielding before crack initiation. Thus the specific essential work of fracture determined using the linear relation of type(i) should be correct, although the estimated work is a little smaller than that from the linear relation of type(ii)&(iii).