1.1 These test methods are designed to characterize the toughness of plastics in terms of the critical-stress-intensity factor, KIc, and the energy per unit area of crack surface or critical strain energy release rate, GIc, at fracture initiation. 1.2 Two testing geometries are covered by these test methods, single-edge-notch bending (SENB) and compact tension (CT). 1.3 The scheme used assumes linear elastic behavior of the cracked specimen, so certain restrictions on linearity of the load-displacement diagram are imposed. 1.4 A state-of-plane strain at the crack tip is required. Specimen thickness must be sufficient to ensure this stress state. 1.5 The crack must be sufficiently sharp to ensure that a minimum value of toughness is obtained. 1.6 The significance of these test methods and many conditions of testing are identical to those of Test Method E399, and, therefore, in most cases, appear h