Interlaminar shear stress

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. 

Figure 4-54 Interlaminar Shear Stress Distribution through...

Goel VK et al (1995) Interlaminar shear stresses and laminae separation in a disc finite element analysis of the L3-L4 motion segment subjected to axial compressive loads. Spine (Phila Pa 1976) 20(6) 689-698... 

Fig. 8.13. (a) Distribution of interlaminar normal stress, <7 , and interlaminar shear stress, r-,. in (0°/90°]s laminate under axial tension, (b) Effect of stacking sequence on through-the-thickness distribution of... 

Joo, J.W. and Sun, C.T. (1992). A failure criterion for laminates governed by free edge interlaminar shear stresses. J. Composites Mater. 26, 1510-1522. 

Figure 24. Translation of fiber strength and interlaminar shear stress (ILSS) of carbon/carbon skeletons rigidized with a phenolic resin, as a function of the carbonization temperature (34).

Figure 35. Mechanical properties of carbon-carbon epoxy-resin hybrid composites, compared with the properties of the composite skeletons before resin impregnation (61,62). The composite skeletons were prepared from Sigrafil HM 3 PAN based fiber, rigidized with a phenolic resin, and densified by four cycles with coal-tar pitch plus sulfur the carbonization temperature was 1000 C. (b) Flexural strength. (c) Interlaminar shear stress, measured with two sample thicknesses.

A high proportion of aerospace structures consist of relatively thin panels that are stiffened by stringers and frames or ribs. These panels are prone to buckle under compressive load and, in order to prevent delamination due to interlaminar shear stress, buckling is usually prevented up to ultimate levels of load. 

At a distance of about twice the specimen thickness away from the edge, O , and x y approach the uniform stress, which can be calculated from laminate theory, whereas the interlaminar shear stresses x vanish. The finite-difference method does not allow the conclusion that the shear stress contains a singularity at the interface, although there are indications that this might be the case. 

Temperature, °C Interlaminar shear stress. MPa Failure mode... 

Shear Properties - Double notch shear testing is preferred to short-beam shear, because of the multiple stress condition for the short beam shear geometry. In the double notch shear (DNS) test, the compression load translates into an interlaminar shear stress along the midplane connecting the two notches in the composite (See Figure 3). Table 3 lists the ultimate double notch shear strengths for the two sets of composite test bars. 

P(l) Special attention shall be paid to the through-thickness properties of the adherends. Delamination due to peel and interlaminar shear stresses shall be considered. 

Secondly, the layered structure causes interlaminar shear stresses at each layer interface together with other in-plane shear stresses. Therefore, it should also be verified that the allowable in-plane shear stress of the adherend is not exceeded. This is not included, however, in the conditions above, as the interlaminar shear failure is typically preceded by a throughthickness tensile failure. The required value of the interlaminar shear strength is also seldom available, and there are no standardised test procedures to determine such a value. As a result, bonded joint induced interlaminar shear stresses are not calculated in the design procedures presented. 

Figure 22.3 shows the large in-plane anisotropy for the elastic constants E and G of a Type I carbon fiber in an epoxy matrix tested at various angles to the fiber direction. The interlaminar shear stress is shown as a function of ply angle in Figure 22.4. 

Figure 22.4 Interlaminar shear stress as a function of ply angle for Type I high modulus carbon fiber epoxy resin laminate tested in uniaxial tension in x-direction. The maximum interlaminar shear strength occurs at 0 = 35° and the stresses are zero at 0, 60 and 90°. Source Reprinted with permission from Pipes RB, Pagano NJ, Interlaminar shear stress in composite laminates under axial tension, J Composite Mater Sci, 13, 2131-2136, 1978. Copyright 1978, Sage Publications.

Pipes RB, Pagano NJ, Interlaminar shear stress in composite laminates under axial tension,... 

Interlaminar shear stress n. Shear stress between layers of a laminate, an important cause of laminate failure and delamination. 

Goel VK, Monroe BT, Gilbertson LG, Brinckmann P (1995) Interlaminar shear stresses and laminae separation in the disc. Spine 20 689-698 Rao AA, Dumas GA (1991) Influence of material properties on the mechanical behaviour of the L 5-S1 intervertebral disc in compression. Journal of Biomechanics 13 139-151... 

Carbon fibres exhibit rather poor adhesion to matrix polymers, so it is usual to carry out oxidative treatments of various t3qjes to modify the morphology and chemical structure at the surface, in particular to increase its polarity and introduce reactive groups. Such treatments greatly improve the adhesion as measured, for example, by the interlaminar shear stress. 

International Electro-technical Commission. Interlaminar shear stress. 

The strengths in these three modes were given above for a type II-S (treated) unidirectional c u bon fibre composite. Rotem and Hashin (1975) suggested that matrix failure depended on the combined effect of the tensile stress perpendicular to the fibre direction and the interlaminar shear stress and that failure occurs when ... 

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