Failure modes composite laminates

Delamination represents the weakest failure mode in laminated eomposites, and is considered to be the most prevalent life-limiting crack growth mode in most composite structures. As such, ever-increasing attention has been directed toward the understanding and characterization of delaminations of various natures, and at... 

Because of the various characteristics of composite laminates, it is difficult to determine a strength criterion in which all failure modes and their interactions are properly accounted for. Moreover, the verification of a proposed strength criterion is greatly complicated by scatter in measured strengths caused by inconsistent processing techniques (that... 

One of the most important properties which control the damage tolerance under impact loading and the CAI is the failure strain of the matrix resin (see Fig. 8.8). The matrix failure strain influences the critical transverse strain level at which transverse cracks initiate in shear mode under impact loading, and the resistance to further delamination in predominantly opening mode under subsequent compressive loading (Hirschbuehler, 1987 Evans and Masters, 1987 Masters, 1987a, b Recker et al., 1990). The CAI of near quasi-isotropic composite laminates which are reinforced with AS-4 carbon fibers of volume fractions in the range of 65-69% has... 

Sohi. M.M., Hahn, H.T. and Williams, J.G. (1987). The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates. In Toughened Composites. ASTM STP 937 (N.J. Johnston cd.). ASTM. Philadelphia, PA. pp 37 60. 

Two types of composite physical property tests were conducted to measure properties which are sensitive to the degree of adhesion and failure mode of the fiber-matrix interphase. Short beam shear tests (ASTM D2344-84) were conducted on 18 ply unidirectional laminates. The support span-to-thickness ratio... 

Fracture process in multidirectional composite laminates subjected to in-plane static or fatigue tensile loading involves sequential accumulation of damage in the form of matrix cracks that appear parallel to the fibres in the off-axis plies, edge delamination and local delamination long before catastrophic failure. These resin dominated failure modes significantly reduce the laminate stiffness and are detrimental to its strength. 

In general, the notched Charpy (ISO 179) [26] and Izod (ISO 180) [27] tests are not meaningful for composites, and ISO 179 recommends that only unnotched specimens should be tested. The difficulty with these tests is that in the notched condition the majority of specimens tested perpendicular to the plane of the test panel delaminate at the root of the notch. This reduces the specimen to a thinner version of the unnotched specimen, which as described above for the interlaminar shear test (see Section 5.4) is susceptible to compression-initiated failures under complex local loads. Specimens cut in the plane of the laminate or sheet will be less susceptible to delamination at the notch tip, and crack growth will be possible from the notch tip. However, other compression shear failure modes are still possible in some composites, and they will not be loaded in this direction in most applications. 

Kondo A, Sato Y, Suemasu H, Aoki Y. Fracture resistance of carbon/epoxy composite laminates under mixed mode II and III failure and its dependence on fracture morphology. Adv Compos Mater 2011 20 405—18. http //dx.doi.org/10.1163/ 092430411X568197. 

Tension, compression, and shear are the three fundamental modes in which a composite lamina may fail. As the composite material is made up of multiple laminae (layers and plies) of various orientations, the stresses in the lamina s principal directions vary from lamina to lamina. As the load is increased, so do the various stresses in the laminae, and failure values may be attained in a certain lamina in a certain principal direction without the overall laminate experiencing actual failure in other words, the failure of the composite laminate is a progressive phenomenon. This progressive damage evolution is subcritical for a while, but eventually leads to ultimate failure of the composite laminate. 

Figure 5.11 Basic failure modes in bolted composite laminates (a) net-section failure (b) bearing failure ...

What was interesting in this study was that the short overlap composite joints did not fail via interlaminar failure and it was evident from the analysis that the stresses induced in the adhesive were sufficient to fail the adhesive before the transverse strength of the laminate was reached. The fact that a consistent failure mode was observed on test allowed the direct comparison of the analyses results for all of the different joint configurations. It has been shown that in all cases the... 

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