13th International Conference on Fracture June 16–21, 2013, Beijing, China -5lengths, c, of 5 mm, 7 mm, 9.5 mm, 14 mm and 20 mm in laminates numbered 1 to 5 respectively. Refer Figure 2a). As expected, the cracks developed in all soft aluminum layers and not in stronger fibres of prepregs. No interfacial crack growth (Delamination) was observed at aluminum fibre interfaces. Each cracked laminate was then subjected to load-extension test in a hydraulic test rig to measure the load line stress applied over it at fracture, fr y applied σ ) ( , . Refer Figure 3 for the test arrangement. At fracture, aluminum layers failed first by critical crack growth followed by stretching of fibre layers till the laminate separated. fr y applied σ ) ( , values varied in the laminates due to different crack sizes in them. As explained in Section 3, induced stress in aluminum layers at fracture, al fr y induced σ ) ( , , , was different from fr y applied σ ) ( , . The values are provided in Table 2. Refer Figure 2b). Five cracked specimens of plain aerospace aluminum 2014-T6 alloy, also numbered 1 to 5, with small thickness, p, but same crack sizes and other dimensions similar to the laminates, were also tested for fracture. For same crack length, fr y applied σ ) ( , values of plain aluminum alloy specimens were found to be greater than that of the laminates due to smaller thickness of the former. Thickness effects were however nullified by obtaining d C,al K . Sample load-extension curves of laminate and of plain aluminum specimen at 20 mm crack length are displayed in Figure 4. All the curves, including those at other crack lengths also, were mostly found to be linear even near fracture loads, because of low ductility of aluminum alloy. The principles of LEFM were therefore valid and the use of parameter K for fracture characterization was justified. c w Load d Dimensions not per scale z y x Load c w p z y x a) Glare Laminate b) Plain aluminum alloy specimen Cracks in all aluminum layers Figure 3. Fracture test arrangement Gripper (Gripping pressure = 5MPa) Laminate b) Laminate on test rig c) Tested laminate Laminate Notch Fatigue crack a) Cracked laminate Figure 2. Crack configuration in Glare laminate and in plain aluminum alloy specimen
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