Laminates tensile failure mode

Fracture process in multidirectional composite laminates subjected to in-plane static or fatigue tensile loading involves sequential accumulation of damage in the form of matrix cracks that appear parallel to the fibres in the off-axis plies, edge delamination and local delamination long before catastrophic failure. These resin dominated failure modes significantly reduce the laminate stiffness and are detrimental to its strength. 

Many reports have shown that for a tc/4 quasi-isotropic laminate, when the interlaminar normal stress is tensile and is the dominant component among the free edge stresses, then open mode delamination may occur. Examples of such laminates include [ 45/90/0]s, [ 45/0/90]s, and [0/ 45/90]s laminates. In these laminates, the open-mode delamination crack may propagate into the laminate before final failure of the laminate. For these laminates, classical failure criteria are not suitable for laminate strength prediction. 

The EDT specimen shown in Fig 3.34(a) has been used to characterize the interlaminar failure of the composite in opening mode I (Whitney and Knight, 1985). The straight-sided tensile specimen has starter cracks placed along the free edges at the laminate mid-plane (Fig 3.34(a)). Due to the low interlaminar shear... 

Susceptibility to interlaminar failure is a major weakness of advanced laminated composite materials. It can occur by in-plane shearing (i.e., sliding) (mode II). and out-of-plane shearing (i.e.. tearing) (mode III) as well as by tensile (mode I) deformation. Mode II loading is of particular interest, as values have been shown to correlate with compression after impact data [142.143]. which is required for such purposes as civil aircraft certification. 

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