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. Two testing geometries are covered by these test methods, single-edge-notch bending (SENB) and compact tension (CT). The scheme used assumes linear elastic behavior of the cracked specimen, so certain restrictions on linearity of the load-displacement diagram are imposed. A state-of-plane strain at the crack tip is required. Specimen thickness must be sufficient to ensure this stress state. The crack must be sufficiently sharp to ensure that a minimum value of toughness is obtained. The significance of these test methods and many conditions of testing are identical to those of Test Method E 399, and, therefore, in most cases, appear here with many similarities to the metals standard. However, certain conditions and specifications not covered in Test Method E 399, but important for plastics, are included. This protocol covers the determination of GIc as well, which is of particular importance for plastics. These test methods give general information concerning the requirements for KIc and GIc testing. As with Test Method E 399, two annexes are provided which give the specific requirements for testing of the SENB and CT geometries. Test data obtained by these test methods are relevant and appropriate for use in engineering design. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.There is currently no ISO standard that duplicates these test methods. Pending ISO/CD 13586 covers similar testing and references this test method for testing conditions.