Fracture, Fatigue and Damage Evolution of Coposites

采用模量损失、内耗变化（机械阻尼）和声发射（AE）等方法研究了SiC纤维增强钛合金复合材料（SCS-6/Ti-6Al-4V）的损伤演化。采用等幅循环加载的四点弯曲方法对单向增强无缺口试件进行了损伤试验。在预定的循环次数后，通过弯曲共振法测量动态杨氏模量和内耗，并且在测试期间还监测AE信号。在复合材料中引起的损伤主要作为所施加的最大应力水平（σ_）的函数逐渐变化<max>。纤维断裂主要发生在σ_<max>值大于0.5 σ_B（其中σ_B是复合材料的弯曲强度）时。观察到与纤维断裂相关的高信号声发射信号，内部摩擦测量值在试样失效前迅速增加。相比之下，基体开裂在σ_<max>值低于0.35 σ_B时占主导地位。随着基体裂纹（由完整纤维桥接）在这些复合材料中生长，内部摩擦测量增加。小振幅AE信号被观察到与基体裂纹的生长。此外，纤维断裂发生在桥接区中，σ_<max>值在0.35和0.5 σ_B之间。在所有的测试中，杨氏模量仅下降了1%，其初始值，直到之前的试样失败。然而，在本研究中研究的所有应力水平下，在试样发生灾难性失效之前，内摩擦测量值迅速增加。这一结果表明，在复合材料中的损伤积累，并建议，测量的内部摩擦的变化可能适用于评估在这种复合材料的损伤的增加。

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.