ASTM D6671/D6671M-13e1

5.2.1 To establish quantitatively the effects of fiber surface treatment, local variations in fiber volume fraction, and processing and environmental variables on G_c of a particular composite material at various mode mixtures,

5.2.2 To compare quantitatively the relative values of G_c versus mode mixture for composite materials with different constituents, and

5.2.3 To develop delamination failure criteria for composite damage tolerance and durability analyses.

5.3.1 Delamination Initiation—Two values of delamination initiation shall be reported: (1) at the point of deviation from linearity in the load-displacement curve (NL) and (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 (3-5). Since they are often believed to be artificial, propagation values must be clearly marked as such when they are reported. One use of propagation values is to check for problems with the delamination insert. Normally, delamination toughness values rise from the initiation values as the delamination propagates and fiber bridging develops. When toughness values decrease as the delamination grows, a poor delamination insert is often the cause. The delamination may be too thick or deformed in such a way that a resin pocket forms at the end of the insert. For accurate initiation values, a properly implanted and inspected delamination insert is critical (see 8.2).

5.3.3 Precracked Toughness—Under rare circumstances, toughness may decrease from the initiation values as the delamination propagates (see 1.1 This test method describes 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 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 may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.

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.