Interlaminar tension

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... 

In addition to influencing the anisotropic response of the laminate, the stacking sequence can cause interlaminar through-thickness stress to occur at the free edge of the laminate. Interlaminar tension stresses can cause delamination under static, cyclic and impact loading. 

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

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. 

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... 

The composite s resin-dominated material properties are in-plane shear, interlaminar shear, transverse tension/compression, and mode I and II fracture toughness. These properties are... 

Macedo FS, Pereira AB, de Morais AB. Mixed bending-tension (MBT) test for mode I interlaminar and intralaminar fracture. Compos Sci Technol 2012 72 1049-55. http // dx.doi.org/10.1016/j.compscitech.2012.03.023. 

The crack plane in tension remained generally perpendicular to the direction of the applied load, without any deviation into the interlaminar plies. Oxidation of the composites did not change this general trend. SEM was performed on the tensile fracture surfaces to characterize the nature of the tensile failure. Micrographs of the fracture surfaces of the 5X and 8X as-prepared composites show a relatively rough fracture surface, but with very limited fiber pull-out. This corresponds well with the relatively low tensile failure strains of 0.2% seen in both of these composites. 

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,... 

Tensile strength at room temperature Uniaxial compression at room temperature Interlaminar shear at room temperature Open hole tension at room temperature (see Fig. 19)... 

Tension Compression Shear Flexure Bearing Interlaminar ... 

Critical ply termination When a laminate with a 90° ply at mid-plane is subjected to tension, peak interlaminar stresses are generated at the interface between the ply or within it. This is because the Poisson s ratio is so different from the surrounding plies. One solution has been to terminate the 90° ply away from the free edge. 

As expected, the moduli of in-plane tension/compression and in-plane shear turned out to be very different, while the interlaminar shear moduli are almost identical. 

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