Mechanical properties and damage onset stress in 24 or 25-ply unidirectional CFRP laminates that contain different thicknesses and gap lengths of fiber discontinuity are investigated by tensile testing and analytical model. Same damage behavior, as with previously reported, that interlaminar delamination between fiber continuous and discontinuous plies follows after crack initiation at the edge of the discontinuous fibers is observed even if thinner and longer gap of the fiber discontinuities are introduced. It has been shown that the laminates with long gap fiber discontinuity show higher stress of crack onset than that with short gap, and the crack onset stress decreases with the number of the discontinuous plies. A similar trend can be seen for the onset stress of the interlaminar delamination though no delamination has been observed in the short and long gap 1-ply and long gap 2-ply discontinuous laminates. The crack onset stress is evaluated by representing the energy release rate with crack initiation by using stress change in shear-lag model. Due to the fact that the relation between the number of discontinuous plies and crack onset stress can be predicted for both short and long gaps of fiber discontinuity by assuming a certain value of critical energy release rate, it is shown that the crack onset behavior is not affected by thickness and gap length of the fiber discontinuity. Another analytical model with an assumed critical energy release rate has successfully predicted the delamination onset stress regardless of sizes of fiber discontinuities by applying the exact thicknesses of continuous and discontinuous plies. By comparing the predicted delamination onset stress and fracture stress of the laminates, it is concluded that two and more fiber discontinuous plies can affect the overall damage behavior of the laminates.
In fabricating complicated laminated composite structures by using prepregs, various kinds of fiber discontinuities such as ply drop-off and seams can be formed. Special care must be taken for the fiber discontinuity since they can induce stress concentration, and damage onset and growth originated from them may lead to catastrophic failure of the structure. The present study evaluates mechanical behaviour in unidirectional CFRP laminates that contain centered or dispersed fiber discontinuities fabricated by using interlaminar-toughened CFRP prepregs. Tensile testing has shown that higher onset stress of the interlaminar delamination between continuous and discontinuous plies can be achieved by interlaminar-toughened CFRP with centered fiber discontinuity compared to that for conventional CFRP prepregs. As for the specimen with dispersed fiber discontinuities, fracture stress keeps almost constant with more than 20 mm of interval of the dispersed fiber discontinuities. By applying the onset stress of the delamination that is experimentally obtained using centered discontinuity to an analytical model, critical energy release rate by the delamination onset is calculated. Then it is indicated that one can predict the fracture stress of the specimen with the dispersed fiber discontinuities with more than 20 mm interval of the discontinuities by using the critical energy release rate. This indicates that fracture of the laminate is dominated by the interlaminar delamination between discontinuous plies with more than 20 mm of interval.
This paper investigates the effect of the fiber discontinuity, caused by the prepreg cut in fabricating laminated structures, on the mechanical properties in CFRP laminates. Stress-strain relations under tensile loadings are obtained for unidirectional laminates that contain the fiber discontinuity, and damages in the laminates have also been observed. Nonlinear stress-strain relations at the center of the laminates have been found to be due to the interfacial debonding at discontinuous edge. In addition, it has been revealed that strains at distant from the fiber discontinuity increase sharply because the interlaminar shear stress is released by the interlaminar delamination that grows from the discontinuity. Finite element (FE) analysis model has been developed in order to calculate the stress distribution in the plates. The FE results have shown that stress concentrates in the fiber discontinuous edge with thicker discontinuous layer. Furthermore, shear-lag analysis model has also been proposed, and it has been shown that obtained stress distributions agree well qualitatively with the FE results.
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