Mode I fracture resistance characteristics of graphite/epoxy laminated composites

Abstract

Mode I crack resistance behavior of fiber‐reinforced (graphite/epoxy) composites laminated unidirectionally and anti‐symmetrically was studied. Double cantilever beam (DCB) specimens of stacking sequences, [O12//O12] and [(O/90)3s//(90/O)3s] were used where // represents the initial crack location. Resistance curves (R‐curves) were constructed for three initial crack lengths in order to determine the effects of initial crack length on the resistance behavior. The resistance force, GR, for a crack increment was determined from the compliance calibration method. The results show that for the case of [(O/90)3s//(90/O)3s], the initial crack deviated from the midplane and propagated in a zigzag fashion within 13th(90‐deg), while the crack propagated along the midplane for a [O12//O12] case. The results also show that for both cases, GR was affected by the initial crack length before GR was stabilized. However, GR was not affected by initial crack length when GR was stabilized for each case.

Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross‐ply laminates

“…It is clear that initiation G Ic values were quite close for both 0°/90° and 0°/0° specimens, even from the generated pre‐cracks. This contrasts with other studies, e.g., Rhee et al 10 . and Morais et al 11 .…”

“…On the other hand, maximum propagation values of the present 0°/90° specimens were close to two times those of 0°/0° specimens. Again, this ratio was significantly smaller than the 3 to 4 value reported in previous studies 10,11 …”

“…While recent studies involved 3–4 mm thick 0°/90° specimens, 10,11 it was decided to use thick specimens in the present study. The exact stacking sequences were [(0° 2 /90°) 6 /0° 2 //90°/(0° 2 /90°) 6 /0° 2 ] and [(0° 2 /90°) 6 /0° 2 //(0° 2 /90°) 6 /0° 2 ], where // stands for the position of the starter delamination.…”

“…Therefore, it is of particular interest to obtain G Ic values of specimens with delaminations in θ 1 /θ 2 interfaces, where θ 1 ≠θ 2 . Unfortunately, there seems to be a fundamental problem in DCB tests of such specimens: instead of pure self‐similar interlaminar propagation, the starter crack rapidly jumps inside one of the interface plies, giving rise to a mixed inter‐intralaminar growth, which frequently involves significant fibre bridging 3–11 . This complex damage morphology is believed to be responsible for G Ic values that are typically three to four times higher than those of [0°] n laminates.…”

Interlaminar fracture studies in Portugal: past, present and future

“…Several studies have already been presented on DCB testing of MDs 6 , 14–22 . The first difficulty with MDs is the selection of appropriate stacking sequences.…”