PSI - Issue 61

Ahmet Çevik et al. / Procedia Structural Integrity 61 (2024) 291–299

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Cevik et al. / Structural Integrity Procedia 00 (2019) 000 – 000

The crack tip speed results can also be influenced by the directionality of the warp and fill yarns. Especially the observed fluctuations in the crack tip speed can be attributed to the crack jump phenomena as the crack travels warp and fill yarns that lie along the direction parallel and perpendicular to the crack propagation, respectively. Representation of the crack-jumping phenomena during the propagation of a crack in a plain weave fabric material is shown in Figure 10 (Kim and Sham (2011)). Considering this phenomenon, warp yarns speed up the crack propagation, whereas the fill yarns slow down it. In our case, 5 Harness satin weave pattern allows for fast crack growth in four warp yarns measuring 10mm which approximately agrees with crack jump distance in the specimen Fabric 2c where fluctuations are more apparent.

Fig. 10. Propagation of a crack front through the warp and fill yarns (Kim and Sham (2011)).

In Figure 11, the stress contours for   ,  r ,  r  in the curved region are shown, obtained from the finite element analysis of the curved composite laminate loaded to a displacement of 17 mm taken from the average of the failure displacements of specimens Fabric 2c and 3c. According to these stress contours, the maximum tangential stress of 466 MPa occurs as tensile stress on the inner side of the curved region and a minimum tangential stress of 417 MPa on the outer side of the curved region. The radial stress reaches its maximum value of 43 MPa at the center of the curved region at the 8 th ply and stays almost level across a wide area from the 6 th ply to the 11 th ply. The maximum shear stress of 16 MPa occurs in the transition region from curved region to the arms. As mentioned previously, in the experiments, for specimens Fabric 1c and 2c, the main failure occurs in the 6 th and 11 th plies, respectively. In the experiment for Fabric 3c, the main crack initiates in the 6 th ply. At these failure locations, the tangential and shear stresses are not high compared to the material strength in the warp yarn direction to initiate failure. However, the radial stresses in this region are about 40-44 MPa, which is close to the material interlaminar strength of 53 MPa. From this analysis and the post-mortem microscope pictures, we can conclude that radial stress-dominated inter-laminar and intra-laminar failure occurs for curved fabric specimens subjected to pure shear loading.

(a) (c) Fig. 11. The stress field on the curved region, obtained from the FEA at 17 mm displacement, the average failure displacement of the experiment Fabric 2c and 3c, (a) tangential stress, (b) radial stress and (c) shear stress. (b)

4. Conclusions This study examines the failure of fabric curved composite laminates experimentally under quasi-static shear loading. Novel experimental loading fixture design that allows for the free sliding of the bottom arm creates pure shear loading of curved beams. The force-displacement stiffness behavior is consistent with the finite element analysis under pure shear loading solving the discrepancy between experimental and computational behavior observed in the literature. In the experiment of fabric specimens, a high-speed camera is utilized at 420,000 fps to capture the dynamic failure event that occurred in the curved laminates. Detailed fractography of the tested specimens is carried out with an optical microscope. The following conclusions are drawn: • The main failure mode of [(45/0) 7 /45/45/0/45] woven composites under pure shear loading is observed as delamination as also shown by Tasdemir and Coker (2022) under axial/moment loading, and the initiation of the failure is dominated by the contribution of the radial stresses.