Evalution of Interlaminar Fracture Strength by Moire Interferometry

采用云纹干涉法研究了单向碳纤维/环氧复合材料双悬臂梁试件的I型层间断裂。实验测定了裂纹尖端附近的裂纹张开位移(COD)，以评估试件梁根处的变形。分析了梁根变形对断裂韧性(G_I)值估算的影响，建立了一个改进的解析模型，研究了COD、G_I和柔度(C)作为裂纹前缘材料刚度(Lambda)的函数。推导出了COD溶液，并与云纹干涉法测得的裂纹尖端附近的COD分布进行了比较。结果表明，COD溶液可以预测所测COD剖面的所有重要定性特征，并且在负荷点的定量一致性很好。为了研究碳纤维/环氧复合材料DCB试件的层间变形，将云纹干涉法测得的COD值与解析模型进行了比较。估算了裂纹前缘的层间拉伸弹性模数和应变分布。结果表明，应变分布的大小与试件的模数和厚度有关。裂纹尖端的弹性模量与应变之间也存在着特定的相关关系。

Moire interferometry was utilized to study mode I interlaminar fracture in double cantilever beam (DCB) specimens of unidirectional carbon fiber/epoxy composites. The crack opening displacement (COD) was experimentally determined in the vicinity of the crack tip to evaluate the distortion at the beam root of the specimen. The beam root distortion was found to make an important contribution to increasing the value of COD.The effect of the beam root distortion on the evaluation of fracture toughness (G_I) value was also examined.An improved analytical model was introduced to study COD,G_I, and compliance (C) as a function of the material stiffness (lambda) ahead of the crack front. The COD solution was derived and compared with the COD profiles near the crack tip measured by moire interferometry. Results showed that the COD solution can predict all the important qualitative features of the measured COD profiles ; also, the quantitative agreement at the loading point was very good. It was found that lambda plays an important role in evaluating the values of COD,C and G_I.To study interlaminar deformation in DCB specimens of carbon fiber/epoxy composites, the values of COD determined by moire interferometry was compared with the analytical model. Estimations were made on the interlaminar tensile modulus and strain distribution ahead of the crack front. The results showed that the size of strain distribution was dependent on both the modulus and thickness of the specimens. Also there existed a specific correlation between the modulus and strain at the crack tip.