2 Experimental and Computational Investigation of Out-of-Plane Low Velocity Impact Behavior of CFRP Composite Plates 13 Fig. 2.4 High speed camera images of [07/904]s specimen taken at 15,000 fps. (a) Just before the impact and (b) after impact subjected to loading with no damage, (c) after damage initiation Fig. 2.5 Micrograph images of two faces of damaged specimen showing uniformity of damage Fig. 2.6 High speed camera images of [07/904]s specimen taken at 210,000 fps showing progression of matrix cracking and delamination Fig. 2.7 Micrograph of laminate after the impact experiment showing the final failure pattern and the location of the field of view of the high speed camera To look at the failure sequence at a higher time resolution, an impact test was conducted under the same conditions and the results were captured with the high speed camera at 210,000 fps, 4.76 s interframe time. In the first frame the composite is loaded and initial delamination in the bottom 0/90 interface can be seen. 38 s later a matrix crack starts from the lower interface and is followed by a delamination at the upper interface growing towards the impact point. Five microseconds 5 s later, a second matrix crack initiates at the right hand side from the bottom interface and propagates towards the upper interface. In the last picture, 24 s later, upper delamination grows from the matrix crack towards the impact point and joins with the upper left delamination front. This illustrates the failure sequence of [07/904]s laminates under low velocity impact (Fig. 2.6). In order to observe the strain fields at the crack front during impact, DIC analysis is performed on the high speed camera images captured at 20,000 fps. The post-impact micrograph is shown in Fig. 2.7 with the final failure pattern similar to the previous cases. Because of the resolution, framing rate and shutter speed limitations, the field of view of the high speed camera is limited to the area just under the impact site, highlighted with red frame in Fig. 2.7. The high speed camera pictures and the Tresca strain fields using DIC method during the impact event are shown in Fig. 2.8a and b, respectively. At 100 s, initial delamination fronts are observed at upper interface from both sides and lower interface from only the left side. One frame (50 s) later, the delaminations at the upper interface propagate towards the center from both sides, however the lower delamination is found to have arrested. In the final frame, the upper delaminations from both sides combine to form a single
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