Study on the fatigue crack and secondary crack growth in the adhesive layer under Modes I, II and Mixed mode loadings

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Bumpy surface is formed on the fractured surface of adhesively bonded DCB joints under mixed mode loading in conjunction with a crack cohesively propagates. The mechanism for formation of such bumpy fractured surface can be explained based on the stress analysis at the crack tip in conjunction with the criterion for an occurrence of crazes. The crack propagation resistance increases under cyclic mixed mode loading with an increase of mode II component. An increase in threshold is appreciable.The crack for epoxy adhesive under mode I loading propagates from cohesively to interfacailly as the energy release rate range, ?G_I decreases. Mixed mode failure appears between to cohesive and interfacial failure modes. The crack once is arrested at a certain ?G_I. Its magnitude depends on the surface condition (surface treatment) as well as other conditions such as cyclic load frequency. The crack resumes to propagate after a while under the normal laboratory condition, which is named as "Second … More ary crack growth". No secondary crack growth occurs when the fatigue tests are conducted under extremely dry condition. The characteristics of fatigue crack growth are strongly influenced by loading mode, such as Mode I, Mode II or mixed mode. The secondary crack growth occurs even at 65 % relative humidity at room temperature.Small amount of water absorption also affects the failure of carbon fiber epoxy composites appreciably. When the composite is fabricated under the laboratory condition, the tensile strength decreases 20 % lower than that of the sample fabricated with less moisture. At high cyclic fatigue, a small amount of moisture in air also strongly affects the fatigue failure of the composite. Little epoxy resin can be identified on the broken fibers. For single lap joints subjected to a tensile shear load, cracks occur at the bond termini where the stress singularity exists. With increasing the applied load, the crack interfacailly grow inward, then, they change their direction into the adhesive layer. It happens during the stable crack propagation period. The total energy release rate criterion can be used to estimate where and when such crack deflection occurs. The effect of residual stress due to adhesive shrinkage during consolidation could be one of major reasons why single lap joins become strong when the adhesive layer becomes thin. Less