Page 102 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 3. Measurements of interfacelinterlaminar properties 85

stresses in the mid-plane, the crack propagation will be driven by the normal tensile
stress to the laminar interface. At the onset of crack propagation, there may be an
abrupt change in the stress-strain curve (Fig 3.34(b)). The strain energy release rate,
G,, associated with edge delamination growth has been analyzed based on the rule
of mixtures and laminated plate theory, assuming the absence of the residual
thermal stresses. The constant G, is derived as a function of the critical applied
tensile strain, E~, and the laminate thickness 2h:
G, = t:h(E - E’) , (3.36)

where E’ is the effective Young’s modulus of the laminate in the longitudinal
direction during edge delamination. By testing specimens made from laminates of
different stacking sequences, different percentages of mode I and mode I1 can also be
obtained (O’Brien, 1984). Although the EDT specimen with starter cracks produces
a mode I response, the data reduction scheme is complicated due to the presence of
lamination residual stresses (see Section 7.5.1). These residual stresses induce an
initial outward curvature along the specimen edges, as schematically depicted in
Fig 3.34(c), due to asymmetric nature of the specimen above and below the
centerline. Such residual stresses can be very significant and the equation must be
modified accordingly when residual stresses are included (Whitney, 1989).

References

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test for fiber composite materials. In Test Methods for Design AIlowable for Fibrous Composites: 2nd
Vol. ASTM STP 1003, (C.C. Chamis ed.), ASTM, Philadelphia, PA, pp. 231-260.
Adams, D.F. (1990). The Iosipescu shear test method as used for testing polymers and composite
materials. Polym. Composites 11, 286290.
Adams, D.F. and Walrath, D.E. (1982). Iosipescu shear properties of SMC composite materials. In
Composite Materials: Testing and Design (Sixth Con$). ASTM STP 787 (I.M. Daniel ed.), ASTM,
Philadelphia, PA, pp. 19-33.
Adams, D.F. and Walrath, D.E. (1987a). Current status of the Iosipescu shear test method. J. Composite
Mater. 21, 49&507.
Adams, D.F. and Walrath, D.E. (1987b). Further development of the Iosipescu shear test method. Exper.
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ASTM D 2344 (1989). Test method for apparent interlaminar shear strength of parallel fiber composites
by short-beam method.
ASTM D 3039 (1982). Test method for tensile properties of fiber-resin composites.
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