Kink-band propagation in a multidirectional carbon fiber-polymer composite
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ultidirectional composite specimen geometry and strain-gage configuration. Loading direction is top to bottom.
the vicinity of the notch. Each gage was placed at a distance of 4 mm above and below the expected trajectory of damage. The average elastic strains multiplied by the distance between the gages gave the corresponding elastic displacements. These elastic displacements, which amounted to about 20 m, were deducted from the measured in-plane overlap displacements to obtain the actual in-plane overlap displacements sustained by the composite material in compression. The fractured specimens were examined with an optical microscope and a scanning electron microscope (SEM). Optical microscopy observations in a through-thickness section showed the presence of out-of-plane multiple kink bands in the 0 deg plies of the [(⫾45 deg/0 deg2)3]s laminate (Figure 2); this is a consequence of band broadening just as in the case of a unidirectional laminate. The average kink band angle is 25 deg to the horizontal. The wider end of the overall bandwidth is about 250 m with a total of 5 kink bands, each about 50-m wide. Microbuckling is also VOLUME 32A, DECEMBER 2001—3157
Fig. 2—Optical micrograph showing out-of-plane multiple-kink bands in the unidirectional plies and microbuckling of 45 deg off-axis plies within a multidirectional laminate. Loading direction is top to bottom.
Fig. 4—SEM micrograph showing microbuckled and delaminated fracture surfaces. Loading direction is top to bottom.
Fig. 3—SEM micrograph of an edge view of a composite laminate (in the thickness direction) just after catastrophic fracture showing microbuckling in the central unidirectional layers and attendant delamination between the 0 deg/45 deg layers. Loading direction is top to bottom.
Fig. 5—SEM micrograph showing the predominance of hacklelike features on the delaminated surfaces.
observed in the adjacent ⫾45 deg off-axis plies (Figures 2 and 6). The SEM photomicrograph (Figure 3) depicts an edge view of the [(⫾45 deg/0 deg2)3]s laminate in the thickness direction just after catastrophic fracture. The out-ofplane multiple kink bands in the central 0 deg layer along with the attendant 45 deg/0 deg delaminations on either side of the kink band strongly suggests that the laminate fails in a tunnel like fashion with consequent attendant damage comprising microbuckling of off-axis plies (Figure 2) and/ or delamination (Figure 3). The SEM examination revealed a combination of microbuckled and delaminated fracture surfaces, as shown in Figure 4. “Region A” is a microbuckled fracture surface of the central 0 deg layer, “region B” is a delaminated surface within the adjacent ⫺45 deg off-axis layer, “region C” is a ⫾45 deg delaminated surface, “region D” is a ⫹45 deg delaminated surface, “region E” is a microbuckled fracture surface of the next innermost 0 deg layer, and “region F” is the next innermost ⫺45 deg layer. Our inference is that delamination has occurred across the various inter- and intralaminar planes, continuously deviating away from on
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