ASTM D6671/D6671M-19

5.1 Susceptibility to delamination is one of the major weaknesses of many advanced laminated composite structures. Knowledge of the interlaminar fracture resistance of composites is useful for product development and material selection. Since delaminations can be subjected to and extended by loadings with a wide range of mode mixtures, it is important that the composite toughness be measured at various mode mixtures. The toughness contour, in which fracture toughness is plotted as a function of mode mixtures (see Fig. 3), is useful for establishing failure criterion used in damage tolerance analyses of composite structures made from these materials.

5.2 This test method can serve the following purposes:

5.3 This method can be used to determine the following delamination toughness values:

5.3.2 Propagation Option—In the MMB test, the delamination will grow from the insert in either a stable or an unstable manner depending on the mode mixture being tested. As an option, propagation toughness values may be collected when delaminations grow in a stable manner. Propagation toughness values are not attainable when the delamination grows in an unstable manner. Propagation toughness values may be heavily influenced by fiber bridging which is an artifact of the zero-degree-type test specimen 5.3.3 Precracked Toughness—Under rare circumstances, toughness may decrease from the initiation values as the delamination propagates (see 5.3.2). If this occurs, the delamination should be checked to ensure that it complies with the insert recommendations found in 8.2. Only after verifying that the decreasing toughness was not due to a poor insert, should precracking be considered as an option. With precracking, a delamination is first extended from the insert in Mode I, Mode II, or mixed mode. The specimen is then reloaded at the desired mode mixture to obtain a toughness value.

1.1 This test method covers the determination of interlaminar fracture toughness, G_c, of continuous fiber-reinforced composite materials at various Mode I to Mode II loading ratios using the Mixed-Mode Bending (MMB) Test.

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 test method is further limited to the determination of fracture toughness as it initiates from a delamination insert. This limited scope reflects the experience gained in round robin testing. This test method may prove useful for other types of toughness values and for other classes of composite materials; however, certain interferences have been noted (see Section 6). This test method has been successfully used to test the toughness of both glass fiber composites and adhesive joints.

1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.

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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.