Free edge

At the free edges of a laminate (sides of a laminate or holes), the interlaminar shearing stresses and/or interlaminar normal stress are very high (perhaps even singular) and would therefore cause the debonding that has been observed in such regions. 

The interlaminar shear stress, t, has a distribution through half the cross-section thickness shown as several profiles at various distances from the middle of the laminate in Figure 4-54. Stress values that have been extrapolated from the numerical data at material points are shown with dashed lines. The value of is zero at the upper surface of the laminate and at the middle surface. The maximum value for any profile always occurs at the interface between the top two layers. The largest value of occurs, of course, at the intersection of the free edge with the interface between layers and appears to be a singularity, although such a contention cannot be proved by use of a numerical technique. 

The existence of interlaminar stresses means that laminated composite materials can delaminate near free edges whether they be at the edge of a plate, around a hole, or at the ends of a tubular configuration used to obtain material properties. In all cases, delamination could cause premature failure so must be considered in specimen design because othen/vise the specimen does not represent the true physical situation. 

The significance of interlaminar stresses relative to laminate stiffness, strength, and life is determined by Classical Lamination Theory, i.e., CLT stresses are accurate over most of the laminate except in a very narrow boundary layer near the free edges. Thus, laminate stiffnesses are affected by global, not local, stresses, so laminate stiffnesses are essentially unaffected by interlaminar stresses. On the other hand, the details of locally high stresses dominate the failure process whereas lower global stresses are unimportant. Thus, laminate strength and life are dominated by interlaminar stresses. 

Figure 4-61 Free-Edge Delamination-Suppression Concepts...

Boundary conditions used to be thought of as a choice between simply supported, clamped, or free edges if all classes of elastically restrained edges are neglected. The real situation for laminated plates is more complex than for isotropic plates because now there are actually four types of boundary conditions that can be called simply supported edges. These more complicated boundary conditions arise because now we must consider u, v, and w instead of just w alone. Similarly, there are four kinds of clamped edges. These boundary conditions can be concisely described as a displacement or derivative of a displacement or, alternatively, a force or moment is equal to some prescribed value (often zero) denoted by an overbar at the edge ... 

The boundary conditions for these equilibrium equations are more complicated than for classical lamination theory. However, they are more logical because the Kirchhoff shear force or free-edge condition, in which... 

Thus, we reject the mechanism of scissoring and try to look near the free edges in the boundary layer to evaluate the stresses. Then, in Section 4.6, we predict very large stresses that in practical situations cause premature static failure and adversely influence the fatigue life of a laminate as well. Our problem is the quantitative prediction of those... 

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... 

Whitney, J.M. and Knight, M. (1985). A modified free-edge delamination specimens. In Delamination and Debonding of Materials, ASTM STP 876 (W.S. Johnson, ed.), ASTM, Philadelphia, PA, pp. 298-314. 

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