Composites Interlaminar shear stress

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 25. Translation of fiber strength and interlaminar shear stress (ILSS) of the carbon/carbon composites of Figure 24 after four impregnation and carbonization (1000°C) cycles with coal-tar pitch and 12.5% sulfur (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.

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. 

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,... 

Stress concentrations for linear elastic matrices and fibres have been calculated in [5] and [6]. The stresses are maximum at or near the interface. The composite interlaminar shear strength (ILSS) is SCF times lower than the interface strength x. when fracture initiates at the interface. The composite strength predicted by such calculations decreases with increasing fibre volume fraction. The decrease is very strong near the maximum packing density. These predictions may be too pessimistic because any yield behaviour will reduce the stress concentrations. 

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 ... 

Various surface pretreatments, often referred to as primers, are put on fibers and other textiles by the manufacturers to enhance subsequent bonding. Depending on the subsequent use of the textiles, the change in adhesion can be negative, nonexistent, or positive. In interlaminar shear strength tests of untreated and oxidative surface-treated polyacrylonitrile-based carbon fiber/epoxy composites the shear stress went from 14.9 to 22.1 MPa. 

Composite materials typically have a low matrix Young s modulus in comparison to the fiber modulus and even in comparison to the overall laminae moduli. Because the matrix material is the bonding agent between laminae, the shearing effect on the entire laminate is built up by summation of the contributions of each interlaminar zone of matrix material. This summation effect cannot be ignored because laminates can have 100 or more layersi The point is that the composite material shear moduli and G are much lower relative to the direct modulus than for isotropic materials. Thus, the effect of transverse shearing stresses. 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

Composite structures in service are often subjected to complex 3-D load paths. In general, a delamination will be subjected to a crack driving force with a mode I opening, a mode II forward shear and a mode III anti-plane shear, as illustrated in Fig 3.29. Because delamination is constrained to grow between individual plies, both interlaminar tension and shear stresses are commonly present at the... 

A H Puppo and H A Evensen, Interlaminar shear in laminated composites under generalized plane stress , J Compos Mater 1970 4 204-220. 

In research as well as there in industry, delamination during drilling FRP is recognized as one of the most critical problems. As defined by various researchers, it is an interlaminar or inter-ply failure phenomenon or behavior. When occurred at the topmost surface ply around the drilled hole periphery, it is called peel-up delamination or simply hole entry delamination of the composite workpiece. In addition, this interlaminar shearing of the last ply at the bottommost surface of the FRP composite material is usually more severe and is generally called push-out delamination or in simple words hole exit delamination. Delamination can be introduced by three mechanisms peeling up of the topmost layer, pushing out of the bottommost layer, and an additional cause, called thermal stress mode. 

However, a severe and persistent problem in laminated composites made of anisotropic fiber-reinforced plies is delamination. High interlaminar peel and shear stresses near edges start delamination cracks that grow along the non-reinforced interlaminar planes with little resistance. Delamination substantially reduces the load-bearing capacity and durability of advanced composites and has led to disastrous structural failures. Since the discovery and explanation of the mechanisms of delamination in advanced composites in the early 70s, many researchers have tried to... 

Under three point bend loading of a composite (beam), cracks may be developed due to tensile stresses at the lower stratus of the specimen as well as compression stresses at the upper one, or due to interlaminar shear. The type of failure depends on the ratio of span to depth (L/D). Short beam specimens usually fail in shear and long ones by tensile or compression stresses. For interlaminar shear strength (ILSS) tests, a L/D = 5 was chosen (ASTM-D-2344-76). In case of flexural strength tests, this ratio was fixed to 40 (DIN 29971). 

The ILSS of a u.d. composite may be calculated by two extreme approaches. The most optimistic estimate for ILSS is obtained by neglecting stress concentrations. Consider a triangular array of fibres that is subject to a longitudinal shear stress equal to the interlaminar shear strength in a horizontal plane. On the assumption that at fracture the horizontal resin bridges between the fibres bear the matrix yield stress X and that an effective fraction a of the fibre surface bears the interface strength x, it can be derived that ... 

Generally a span/thickness ratio of 16 is used but, for materials which tend to fail in an interlaminar shear mode (such as unidirectionally reinforced composites), a ratio of 20 is to be preferred. Flexural strength is a convenient method for comparing properties, because it involves a stress/deformation mode that is often encountered under service conditions, and the test specimens are relatively small. 

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