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

在复合型载荷作用下，DCB胶接接头的断口表面形成凹凸不平的表面，伴随着裂纹的内聚扩展。根据裂纹尖端的应力分析，结合银纹产生的判据，可以解释这种凹凸不平的断裂面的形成机理。在循环复合型载荷作用下，裂纹扩展阻力随II型分量的增加而增大。随着能量释放率范围？Gi的减小，I类载荷作用下环氧胶粘剂的裂纹由粘结扩展到界面扩展。复合型破坏出现在粘结破坏和界面破坏之间。裂纹一旦在某一GI处止裂，其大小取决于表面条件(表面处理)以及其他条件，如循环加载频率。在正常的实验室条件下，裂纹在一段时间后恢复扩展，称为“Second…More Rary裂纹扩展”。在极端干燥的条件下进行疲劳试验时，不会发生二次裂纹扩展。疲劳裂纹扩展特性受加载方式的影响较大，如I型、II型或混合型。在室温下，即使在相对湿度为65%的条件下，也会发生二次裂纹扩展，少量的吸水率对碳纤维/环氧复合材料的破坏也有较大影响。在实验室条件下制备的复合材料，其拉伸强度比水分含量较低的样品降低了20%。在高周疲劳时，空气中的少量水分对复合材料的疲劳破坏也有很大影响。在断裂的纤维上可以识别出少量的环氧树脂。对于承受拉剪载荷的单搭接接头，裂纹出现在存在应力奇异性的粘结端。随着外加载荷的增加，裂纹界面向内扩展，并向粘结层方向转变。它发生在裂纹的稳定扩展阶段。总能量释放率准则可以用来估计裂纹偏转发生的地点和时间。固结过程中粘结收缩引起的残余应力的影响可能是粘结层变薄时单搭接变硬的主要原因之一。较少

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