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The effect of delaminations on the stiffness-controlled behavior of laminated composites is investigated. Four panels of [±45,0,90]2s quasi-isotropic layup are fabricated from the T300/Narmco 5208 material system, three with prescribed delaminations of different rectangular shapes and orientations. Nondestructive buckling tests and vibration experiments are conducted on these panels with clamped-free and clamped-simple support boundary conditions to assess the stiffness-controlled behavior. Free-free vibration tests are also conducted to study the influence of delamiantions on higher modes. Finite-element analyses are performed to compare with the experimental results. The findings indicate that delaminations of the size considered can be tolerated for some service conditions without dire consequences.

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This paper presents the design analysis and results of an experimental program that was conducted to evaluate mixed-mode and Mode II interlaminar fracture behavior of a resin matrix composite material system. A double cracked-lap-shear specimen was designed utilizing a simple, new analysis method. The specimen was made of AS4/3502 graphite/epoxy material with a [±45, 0, 90]6s quasi-isotropic balanced symmetric layup. The lap interface studied was at ±45° orientations to the loading direction. A fundamental feature of the designed specimen is its ability to be tested under net tensile and compressive loadings. The specimen exhibits mixed-mode or Mode II behavior depending on the loading direction. This paper summarizes the tension testing only. The crack growth during testing was monitored by observing the isochromatic fringes in photoelastic coatings mounted on the two lap surfaces. Preliminary results indicate that the fracture behavior follows a resistance curve. In order to assess the double cracked-lap-shear specimen data and test, a comparison was made with results obtained with single cracked-lap-shear specimens. Less scatter in the data from the double cracked-lap-shear specimens was found.

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