Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites

Importancia y uso:

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.

FIG. 3 Mixed-Mode Summary Graph

5.2 This test method can serve the following purposes:

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

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 (2) at the point at which the compliance has increased by 5 % or the load has reached a maximum value (5%/max) depending on which occurs first along the load deflection curve (see Fig. 4). Each definition of delamination initiation is associated with its own value of G_c and G_II/G calculated from the load at the corresponding critical point. The 5%/Max G_c value is typically the most reproducible of the three G_c values. The NL value is, however, the more conservative number. When the option of collecting propagation values is taken (see 5.3.2), a third initiation value may be reported at the point at which the delamination is first visually observed to grow on the edge of the specimen. The VIS point often falls between the NL and the 5%/Max points.

FIG. 4 Load-Displacement Curves

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 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.

Subcomité:

D30.06

Referida por:

D6856_D6856M-23, D4762-23

Volúmen:

15.03

Número ICS:

19.060 (Mechanical testing), 83.120 (Reinforced plastics)

Palabras clave:

composite materials; delamination; interlaminar fracture toughness; mixed-mode bending; Mode I–Mode II;