Study of Interfacial Creep Strength of Polymer Matrix Composites

研究了碳纤维增强聚合物基复合材料（CFRP）的时效行为。主要研究结果如下：1.单向CFRP的时间相关行为：蠕变变形在所有离轴角度（包括纤维方向）都能清楚地观察到。蠕变应变的大小主要取决于蠕变应力和离轴角。与大多数常规金属材料不同，复合材料的蠕变速率随着蠕变应变的发展而迅速降低直至消失。卸载复合材料的蠕变应力后，复合材料的蠕变应变随时间部分恢复，但对于每个离轴角，无论先前的蠕变应力水平如何，最终都会保留一定量的永久应变。蠕变应变在应力阶梯式降低后随时间逐渐降低。这种部分卸载后的蠕变回复表明，存在与外加应力方向相反的内应力， ...更多信息 它已经发展得比爬行更快了.角铺CFRP的时效行为：本文还研究了由单向CFRP层板组成的对称角铺层[±30]_<3s>、[±45]_<3s>和[±60]_<3s>层板在高温下的应力松弛效应。应力松弛效应在所有角铺设层合板中都被清楚地观察到。应力在总应变保持恒定后的短时间内迅速松弛，然后松弛速率趋于消失，而与持续的总应变水平和铺层方向无关。当相对于保持总应变的瞬间的最大应力对松弛应力进行归一化时，所得到的相对应力松弛曲线趋于彼此接近。角铺设层合板应力松弛效应的这些特征与单向CFRP离轴试件的应力松弛效应相似。宏观力学建模：基于带内变量的不可逆热力学理论，建立了描述单向复合材料时间和速率相关行为的多轴本构模型。这个模型可以扩展到包括损伤在内的广泛的非弹性。该模型能较好地描述单向CFRP的蠕变和松弛行为.微机械建模：采用单元法对单向纤维复合材料进行均匀化处理，采用粘塑性模型对基体材料进行均匀化处理，建立了描述单向纤维复合材料弹粘塑性行为的细观力学模型。该细观力学模型能够适度描述单向CFRP高温离轴拉伸行为的非线性和率相关性.应用于多向层合板：经典的层合板理论不能准确地预测角铺设层合板的变形与小的α-取向角，因为增强层间应力的影响。为了克服这个问题，三维均匀化方法和一个完整的多轴本构模型组成层的适当组合应被考虑。少

Time-dependent behavior of carbon fiber-reinforced polymer matrix composites (CFRP) has been studied. The results obtained are summarized as follows :1. Time-Dependent Behavior of Unidirectional CFRP : The creep deformation is clearly observed for all off-axis angles, including the fiber direction. The magnitudes of the creep strains depend significantly on the creep stress and the off-axis angle. Unlike most conventional metallic materials, the creep rate of the composite rapidly decreases to disappear as the creep strain develops. The composite creep strain partly recovers with time after unloading the creep stress of the composite, but a certain amount of permanent strain eventually remains for every off-axis angle, regardless of the prior creep stress level. The creep strain gradually decreases with time after the stepped decrease in stress. Such a creep recovery after partial unloading reveals that the internal stress acting in the opposite direction of the applied stress exists a … More nd it has developed witlt prior creep.2. Time-Dependent Behavior of Angle-Ply CFRP : The stress relaxation effects at high temperature in the symmetric angle-ply laminates [±30]_<3s>, [±45]_<3s> and [±60]_<3s> consisting of unidirectional CFRP laminae have also been examined. The stress relaxation effects are clearly observed in all angle-ply laminates. The stress rapidly relaxes in a short period just after the total strain is kept constant, and then the relaxation rate tends to vanish, irrespective of the sustained total strain level and of the ply orientation. When the relaxation stress is normalized with respect to the maximum stress at the instant when the total strain is held, the resulting relative stress relaxation curves tend to come close with each other. These characteristics of the stress relaxation effects in the angle-ply laminates are similar to those in the off-axis specimens of unidirectional CFRP.3. Macromechanical Modeling : A multiaxial constitutive model to describe the time- and rate-dependent behavior of unidirectional composites has been developed on the basis of the irreversible thermodynamics with internal variables. This model can be extended to cover a wide range of inelasticity including damage. The proposed model can moderately describe creep and relaxation behaviors of unidirectional CFRP.4. Micromechanical Modeling : A simple micromechanics model to describe the elastic-viscoplastic behavior of unidirectional fiber composites has been developed using the method of cells for composite homogenization and a viscoplastic model for matrix material. This micromechanics model can moderately describe the non-linearity and rate-dependence of the off-axis tensile behavior of unidirectional CFRP at high temperature.5. Application to Multidirectional Laminates : The classical laminated plate theory fails in accurately predicting the deformation of the angle-ply laminates with small ply-orientation angles because of the effect of enhanced interlaminar stresses. To overcome this problem, appropriate combination of a three-dimensional homogenization method and a complete multiaxial constitutive model for constituent plies should be considered. Less