Interlaminar tensile stress

Fig. 5 has been prepared for laminated composite adherends. In this case, the peel stresses will develop interlaminar tensile stresses in both the splice plate and the skin. These will cause failure in the manner shown unless the splice plates are tapered sufficiently to reduce the peel stresses to insignificance. In the case of metallic structures, it would be the adhesive layer that would fail under this mechanism. Obviously, these peel stresses will be more severe for thicker adherends and will be negligible for extremely thin members, which is why the... 

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. 

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). 

Figure 6.14 Schematic representation showing interlaminar failure of fibre composite substrates arising from the transverse (out-of-plane) tensile stresses, o-ii, which occur due to the eccentricity of the loading path, and the relatively poor transverse strength of fibre composites.

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 ... 

Figures 9-12 give the interlaminax stresses for the different interfaces for ho = 0.1 hjn When comparing these results, the form of the distribution of

Many reports have shown that for a tc/4 quasi-isotropic laminate, when the interlaminar normal stress is tensile and is the dominant component among the free edge stresses, then open mode delamination may occur. Examples of such laminates include [ 45/90/0]s, [ 45/0/90]s, and [0/ 45/90]s laminates. In these laminates, the open-mode delamination crack may propagate into the laminate before final failure of the laminate. For these laminates, classical failure criteria are not suitable for laminate strength prediction. 

Fibers Tensile strength o UT GPa Tensile Young s strain modulus euT,% E, GPa Tensile stress, ouc, GPa Poisson s ratio, p Interlaminar shear strength, riLS, MPa Fatigue-endurance limit a/a 3T, %... 

The plot of diffusivity and interlaminar tensile strength obtained from the interlaminar tests is shown in Figure 5. As can be seen, the strength correlates well with the thermal diffusivity. Fot this case, the entire sample section is under uniform stress as was the case for the tensile test. 

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. 

In particular, the techniques based on the termination of certain plies within the laminate has also shown promise. Static tensile tests of [30°/-30°/30°/90°]s carbon-epoxy laminates containing terminals of [90°] layers at the mid-plane show that premature delamination is completely suppressed with a remarkable 20% improvement in tensile strength, compared to those without a ply terminal. Cyclic fatigue on the same laminates confirms similar results in that the laminate without a ply terminal has delamination equivalent to about 40% of the laminate width after 2x10 cycles, whereas the laminates with a ply terminal exhibit no evidence of delamination even after 9x10 cycles. All these observations are in agreement with the substantially lower interlaminar normal and shear stresses for the latter laminates, as calculated from finite element analysis. A combination of the adhesive interleaf and the tapered layer end has also been explored by Llanos and Vizzini, (1992). 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

Based on the failure mechanisms and stress distributions at the interface between steel and composite adherends of the co-cured single and double lap joints, tensile load bearing capacities of the two joints were evaluated. Since failure started at the edge of the interface between steel and composite adherends, it is important to consider the failure criterion using interfacial out-of-plane stress distributions at the interface. Three-dimensional Tsai-Wu and Ye-delamination failure criteria were used to predict partial cohesive failure or interlaminar delamination failure in the co-cured single and double lap joints. 

The interlaminar shear strength is the interfacial shear stress or shear strength of the matrix material and is measured with the 3-point bend test(ASTM D 2344), which is ideal for routine testing. Figure 17.43 shows a diagrammatic view of the test and shows how shear, tensile and compressive forces are involved. Shear failure will take place at the midplane in the form of delamination, whilst Figure 17.44 hows how a parabolic shear stress distribution occurs. 

Tensile properties of the HDPE/RET blend are shown in Table 8.2. The HDPE 100/0 carbon-fibre composite showed complete linear stress-strain behaviour up to its ultimate tensile strength and fracture at 10.3% strain. No definitive fracture was seen in the HDPE blends. This is due to the interfacial de-bonding between the constituents within the polymer. The apparent loss of cohesive strength of the matrix material resulted in fibre pull-out and interlaminar slip between the carbon-fibre plies. 

Stress distribution in the neighborhood of the free edge was calculated using the pseudo 3-D finite element program. In-plane stresses were Gn, G22, cji2 interlaminar stresses were Gzy, and Gzx- For ease of comparison, a tensile in-plane load (Nx) of 1000 Ib/in was used in the analysis. 

Average stresses over a distance of 2t (t= ply thickness) from the free edge were used in the criterion for failure prediction. For comparison, the interlaminar stresses listed in Tables were assumed to be loaded under a tensile load of 1000 Ib/in. 

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