1. Scope 1.1 This test method describes the determination of the opening Mode I interlaminar fracture toughness, GI , of unidirectional fiber-reinforced polymer matrix composites using the double cantilever beam (DCB) specimen (Fig. 1). 1.2 This test method is limited to use with composites consisting of unidirectional carbon fiber tape laminates with brittle and tough single-phase polymer matrices. This limited scope reflects the experience gained in round robin testing. This test method may prove useful for other types and classes of composite materials, however certain interferences have been noted (see 6.5). 1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only. 1.4 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.