Fracture, Fatigue and Damage Evolution of Coposites

通过模量损失、内摩擦变化（机械阻尼）和声发射（AE）对SiC纤维增强钛合金复合材料（SCS-6/Ti- 6al - 4v）的损伤演化进行了评价。采用恒循环幅加载对单向增强无缺口试件进行四点弯曲损伤。在预定循环次数后，通过弯曲共振法测量动态杨氏模量和内耗，并在试验过程中监测声发射信号。复合材料损伤主要随最大应力水平（sigma_<max>）的变化而逐渐变化。纤维断裂主要发生在sigma_<max, >大于0.5sigma_B（其中sigma_B为复合材料的抗弯强度）。观察到与纤维断裂相关的高简单声发射信号，并且在试样破坏之前，内部测量迅速增加。而在sigma_<max> < 0.35 sigma_B时，基体开裂占主导地位。随着这些复合材料中基体裂纹（由完整纤维桥接）的增长，内摩擦测量值增加。小振幅声发射信号与基体裂纹扩展有关。当sigma_<max>值在0.35 ~ 0.5sigma_B之间时，桥接区发生纤维断裂。在所有的测试中，杨氏模量只下降了其初始值的1%，直到试样破坏之前。然而，在本研究中研究的所有应力水平下，内摩擦测量在试样灾难性破坏之前迅速增加。这一结果表明，内摩擦的增加与复合材料的损伤积累有关，并建议测量内摩擦的变化可能适用于这种复合材料的损伤评估。

Damage evolution in a SiC fiber reinforce Ti alloy composite (SCS-6/Ti-6Al-4V) has been assessed by modulus loss, change in internal friction (mechanical damping) and acoustic emission (AE). Damage was introduced into unidirectionally reinforced unnotched specimens by four point bending using constant cyclic amplitude loading. Dynamic Young's modulus and internal friction were measured by a flexural resonant method after a predetemined number of cycles, and AE signals were also monitored during the tests. Damage induced in the composites changed progressively primarily as a function of the applied maximum stress levels (sigma_<max>). Fiber fracture dominated at sigma_<max> values greater than 0.5sigma_B (where sigma_B is the flexural streength of the composite). High smplitude AE signals associated with fiber fracture were observed, and internal fiction measurements increased rapidly just before failure of the specimen. In contrast, matrix cracking dominated at sigma_<max> values lower than 0.35 sigma_B. As matrix cracks (bridged by intact fibers) grow in these composites, internal friction measurements increase. Small amplitude AE signals were observed to be associated with matrix crack grwth. Also, fiber fracture occurred in the bridged zone at sigma_<max> values between 0.35 and 0.5sigma_B. In all tests, the Young's modulus decreased by only 1% of its initial value until just before failure of the specimen. However, internal friction measurement increased repidly prior to catastrophic failure of specimen at all stress levels investigated in this study. This result shows that an increase in internal friction is related to damage accumulation in the composites, and suggests that measurement of changes in internal friction may be applicable to the assessment of damage in such composites.