【作者】 吴富强；

【导师】 姚卫星；

【作者基本信息】 南京航空航天大学 ， 飞行器设计， 2008， 博士

【摘要】 复合材料因其优良的材料性能而被广泛应用于航空领域。为提高飞机性能,降低使用成本,一个必然途径是提高复合材料的设计许用应变/强度,如此,则材料原先被静强度所覆盖的疲劳问题将逐渐暴露出来,复合材料的抗疲劳设计将成为飞机结构设计的重要关注点。而现阶段有关复合材料疲劳行为研究的理论结果与实际情况相差较大,仍处于初级阶段,需要对复合材料疲劳行为展开广泛而深入的研究。鉴此,本文从以下几个方面对复合材料疲劳行为进行研究:首先研究了复合材料的疲劳损伤演化规律,并通过可检测的材料剩余刚度定义一个反映损伤大小的参量,由此构建了含两未知参数的宏观唯象损伤模型。再依据复合材料刚度衰减速率在寿命全区域内呈“快→缓→快”的变化特点,并结合损伤模型的函数特征,推导出两模型参数成线性关系,以及参数与外加载荷水平成反比,与材料静强度、疲劳寿命成正比。算例结果表明该模型能对变幅载荷下复合材料的剩余寿命给出较好预测结果,与试验值的误差基本限于3倍范围内;其次研究了给定疲劳寿命下层合板疲劳强度间的相互联系。采用疲劳强度比描述任意铺角单向板与纵、横向板间疲劳强度的联系,并分析知疲劳强度比随材料疲劳寿命增加而递减、随铺设角度增加而先递减后递增;通过分析刚度退化规律来反映层合板与纵、横向板间疲劳强度的联系,并详细分析了以纤维影响为主和以基体影响为主的层合板的刚度退化的不同模式。再结合复合材料疲劳寿命曲线方程后,构建一套利用纵、横向板疲劳寿命预测任意铺设单向板、层合板疲劳寿命的方法。算例结果表明该方法预测的疲劳寿命值与试验值相吻合;再次研究了复合材料静强度和其疲劳寿命分散性来源的特征,二者的共性是均受材料内部的初始缺陷影响,不同点是不同的加载方式引起不同特征的损伤。结合一个材料疲劳寿命曲线方程,且依据材料在等置信度等存活疲劳寿命曲线中任一点处的疲劳强度与疲劳寿命的失效概率相等的统计规律,构建了一个基于复合材料静强度概率分布描述任意载荷下其疲劳寿命概率分布的模型。5组算例结果表明模型值与试验值比较吻合。接着研究了复合材料剩余强度的衰减规律,即随加载次数增加,其衰减速率先较大,再变慢,后再次增大至材料断裂,由此提出一个两参数宏观模型对其进行描述。在进一步分析剩余强度衰减特点后得两参数与层合板的铺设角度、加载水平等成正比,而与层合板疲劳寿命成反比。并以此模型为基础推导了复合材料静强度分散性与剩余强度分散性间的联系。10组算例结果表明该模型合理描述了复合材料剩余强度的衰减规律。最后研究了复合材料缺口件剩余强度的演化规律,即随加载次数增加,其剩余强度先单调递增,达到最大值后,剩余强度再单调递减,降至最大外加载荷时材料断裂,则寿命前期内材料剩余强度大于其静强度,分析原因是不同加载方式造成缺口根部应力集中的变化规律不同,本文构建一个宏观模型对其进行描述,算例表明该模型能对复合材料缺口件剩余强度演化规律合理描述。更多还原

【Abstract】 Composite material has been widely used in aircraft structures because of its excellent mechanical characteristics. It is an inevitable path to increase the design allowable strain or the design allowed strength of composite for strengthening the aircraft property and reducing the use cost. Then, the fatigue trouble will be gradually exposed in structures, which had been concealed by the higher design allowable strength. And the anti fatigue design will be the focus on structure design for aircraft. However, the research on fatigue behavior of composite is still primary and imperfect, now. There are very difference between the fatigue theoretical value and experimental result for laminate. It is necessary and valuable to research deeply the fatigue behavior of composite. The major content and achievements in this dissertation are as follows.The stiffness degradation rule of laminate has been studied, and then a macro phenomenal model has been presented to describe the rule, which has been defined by the residual stiffness of laminate. The change characteristic of stiffness degradation rate is from quick to slow and to quick again in the whole fatigue life for laminate. To analyze the characteristics of stiffness degradation and the function characteristics of model, it can be obtained that the relation of model parameters is linear and the parameters value get bigger with the decrease of loading or the increase of static strength and fatigue life. A series fatigue experimental data of laminate were used to verify the model, and the results showed that the predicted life is in good agreement with the experimental ones.The relation of fatigue strengths among different laminates has been studied. Then, a fatigue strength ratio has been employed to describe the relation between arbitrary angle ply fatigue strength and longitudinal or transverse ply fatigue strength. To analyze the characteristics of fatigue strength ratio, it is obtained that the ratio gets bigger with the increasing of fatigue life, and changes with the increasing of ply angle from decreasing to increasing. The relation between arbitrary laminate fatigue strength and longitudinal or transverse ply fatigue strength has been deeply studied by analyzing stiffness degradation of laminate. And there are two stiffness degradation modes in laminate. One is mainly controlled by fibre property and the other is mainly controlled by matrix property. Then, a method to predict the fatigue life of unidirectional ply or arbitrary laminate by the fatigue life of longitudinal or transverse ply has been presented. The verifying results showed that the predicted life is in good agreement with the experimental ones. The characteristic of dispersion source of static strength and fatigue life in composite laminates had been studied. It is concluded that the inner original defects are the common and major reason of causing the random distribution of static strength and fatigue life. And the difference between the static strength dispersion and the fatigue life dispersion derives from the different loading mode. Based on a S-N curve model and the statistical rule, which the failure probabilities between the static strength and the fatigue life in the uniform confidence level and the same survival S-N curves of material are same, a model that is used to describe the distributions of fatigue life of laminate, based on their distributions of static strength, is set up. Five kinds of verifying results show that the predicted values are in good agreement with the experimental ones.The degradation rule of the residual strength about composite laminates has been studied. The degradation ratio changes with the increasing of fatigue life from increasing to decreasing and to increasing again, until material fracturing. And a two- parameters model has been presented to describe this rule. To analyze deeply the characteristics of residual strength degradation, it can be obtained that the model parameters are proportional to the sequence angle and loading and are inverse proportional to the fatigue life. According to the model, the quantitative relationship between static strength dispersion and residual strength dispersion has been derived. The experimental data of ten kinds of laminate are employed to verify this model, and the results show that the model describes the degradation rule of residual strength fairly well.The residual strength degradation rule of notched laminate has been analyzed. It is concluded that the residual strength increases with the increasing of cycle number and then decreases when the residual strength arrives the maximum value. When the residual strength decreases to the maximum cycle loading, the notched laminate occurs to fracture. The different charge rule of stress concentration at the notch root, which derived from the cycle loading, is the major reason of causing that the residual strength is bigger than the static strength at the beginning period of fatigue life of notched laminate. And then, a model that is used to describe the rule is set up. The verifying results show that the model can describe the residual strength degradation rule of notched laminate fairly well.更多还原