better metric for CAI strength. For the Q1 facesheet, there is a high propensity for delamination buckling mode of failure. The Q2 has an increased occurrence of fiber failure mode and the Q3 facesheet has fiber failure as the dominant mode. The high level of delamination buckling for the Q1 facesheet could be due to the close proximity of the 0 ∘ plies to the free surface. The reduced support from the surrounding plies will encourage delamination buckling as the ultimate compressive strength is reached. The CAI results for the 76.2 mm diameter indentor are shown in Fig. 6.2. From these results, there again is a decreasing trend in CAI strength with increasing planar delamination area. For the large diameter indentor the global instability failure mode appears for specimens with a relatively large residual dent depth. The results also show that the resulting failure mode 0 100 200 300 400 500 600 700 800 0 50 100 150 200 250 300 350 400 Planar Delamination Area (mm²) Ultimate Compressive Strength (MPa) Q1−C1 Q1−C2 Q1−C3 Q2−C1 Q3−C1 Fiber Failure (FF) (closed) Delam. Buckling (DB) (open) Global Instability (GI) (closed - circled) Fig. 6.1 Compression after impact (CAI) strength against planar delamination area for specimens with 25.4 mm diameter indentor 600 800 1000 1200 1400 1600 1800 2000 0 50 100 150 200 250 300 350 400 Planar Delamination Area (mm²) Ultimate Compressive Strength (MPa) Q1−C1 Q1−C2 Q1−C3 Q2−C1 Q3−C1 Fiber Failure (FF) (closed) Delam. Buckling (DB) (open) Global Instability (GI) (closed - circled) Fig. 6.2 Compression after impact (CAI) strength against planar delamination area for specimens with 76.2 mm diameter indentor 6 Compression-After-Impact of Sandwich Composite Structures: Experiments and Simulation 49
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