Interlaminar stresses near the free-edge of composite laminates play an important role in the initiation of free-edge delamination. Most works have been reported on the effect of interlaminar normal stress and mode Ⅲ interlaminar stress on the free-edge delamination. And the effect of mode II interlaminar stress was neglected under static load because the value of the stress component is zero at the free-edge.
In the present study, experimental and analytical works are conducted to investigate various delamination modes at the laminate free-edge. Especially, the effect of mode II interlaminar stress on the free-edge delamination is examined. Based on the classical laminate theory, ($\pm\theta$/90), where $\theta$=30 and 45 degrees, laminate families are selected and tested under compressive load to eliminate the effect of transverse cracking. Tested laminates are subjected to microscopic examination to observe delaminated locations. The fracture surfaces of delamination are further examined for the determination of the predominent mode of failure using a scanning electron microscope. Test specimens are analyzed by a global-local variational model. With the analyzed results, the average stress failure criterion is introduced to predict delamination onset.
From the experimental and analytical study, it is shown that the mode II interlaminar stress also plays an important role on the initiation of free-edge delamination. The average stress over one ply thickness from the free-edge is unreasonable to evaluate test results of the specimens in which the effect of mode II interlaminar stress is predominant. And the average over one ply thickness at a distance of one half ply thickness from free-edge is found to be more reasonable to predict the onset of free-edge delamination.
