The initiation ofdelamination in graphite/epoxy specimens was investigated both analyt ically and experimentally. Three specific laminate families were investigated: [±15 n] s, [± 15 n /0n]s, and [0n / ± 15n] s with the normalized effective ply thickness, n, varying from one to five. Delamination initiation was determined to have occurred when a drop in load was detected and a delamination was observed on a replication of the specimen edge. Measured initiation stresses were correlated using the strain energy release rate approach and the Quadratic Delamination Criterion introduced herein. The strain energy release rate approach was unable to accurately correlate data for two of the three laminate fami lies. The data clearly shows that the critical value of strain energy release rate is dependent on ply thickness. The Quadratic Delamination Criterion, an average stress criterion which compares the calculated out-of-plane interlaminar stresses to their related strength param eters, is proposed and shows excellent correlation with the delamination initiation stress data.
A simple and efficient method is presented to determine the interlaminar stresses in a symmetric composite laminate under uniaxial loading. Expressions for the interlaminar stresses are assumed in terms of exponentials based on shapes that the interlaminar stresses must take in order to assure overall (integral) force and moment equilibrium. The boundary conditions and the traction continuity between plies are satisfied exactly. The exponential terms in the stress expressions are determined by minimizing the laminate complementary energy. Typical results are presented and compared with previous results found in the literature. The current method is shown to efficiently deal with the problem including the ability to perform the analysis of thick laminates (100 plies or more) with relative ease and cost-effectiveness.
The impact resistance of composite sandwich plates has been studied ex perimentally. Several panels with [±45/0] s Hercules AS4/3501-6 graphite/epoxy face- sheets were manufactured with different types of cores: aluminum honeycomb, Nomex honeycomb and Rohacell plastic foam with three different core thicknesses, 3.2 mm, 6.4 mm and 9.6 mm. These panels were impacted at a range of low energy levels (less than 10 joules). X-ray photos were taken to determine the extent of delamination damage, and the panels were sectioned so the location and lengths of delaminations, debonds, and core damage could be determined. Extensive delamination and core damage was found in spec imens with no visible surface damage. Core damage in the Rohacell consisted of cracks in the foam while the damage in the honeycomb consisted of the buckling of cell walls. The largest delaminations were found in the interface between the bottom two plies (-45°/+45°) of the top facesheet.
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