【作者】 王小莉；

【导师】 上官文斌；

【作者基本信息】 华南理工大学 ， 车辆工程， 2014， 博士

【摘要】 橡胶隔振器是汽车动力总成等系统中常见且重要的隔振元件，由金属和橡胶复合而成，其疲劳特性对汽车的安全舒适性、操作稳定性和其他零部件的疲劳特性有重要的影响。对橡胶隔振器的疲劳寿命预测，主要是对其主体隔振橡胶材料的疲劳特性进行试验和建模研究。本文基于疲劳设计的理论框架，完善和进一步发展了疲劳寿命计算的基本理论和方法，研究了大变形隔振器橡胶材料在单轴拉伸载荷、单轴拉伸/压缩载荷和多轴载荷作用下的疲劳寿命预测方法等一系列相关问题。现将这些工作归纳如下：(1)研究了不同的疲劳损伤参量和橡胶试件的几何形状与橡胶材料疲劳寿命之间的关系。开展了隔振器橡胶材料的单轴拉伸疲劳试验，采用的试件是三种几何特征不同的橡胶试件（哑铃型试片、哑铃型试柱和圆环试件）；基于疲劳试验数据，对比了多种常用于橡胶疲劳寿命预测的损伤参量与橡胶拉伸疲劳寿命之间的关系；研究和讨论了基于这三种橡胶试件获取的疲劳寿命之间的相关性。结果表明，最大主应变(Green-Lagrange应变、Almansi-Euler应变、工程应变、对数应变和伸长率)峰值、八面体切应变峰值和应变能密度峰值为损伤参量的寿命预测模型均能很好地以幂法则来建立损伤参量与疲劳寿命之间的关系，预测寿命与实测寿命的相关系数均达到0.9以上。其中以最大主Green-Lagrange应变峰值为损伤参量的寿命模型预测效果最佳。橡胶材料拉伸疲劳寿命与所有橡胶试件的几何形状无关，是一种材料属性。在工程实践中可用试验成本较低的哑铃型试片来代表哑铃型试柱和圆环试件进行橡胶拉伸疲劳试验，可节省成本和缩短试验周期。(2)研究应变比R对隔振器橡胶材料单轴疲劳寿命的影响，提出了考虑应变比R效应的疲劳损伤参量。对由汽车橡胶隔振器常见的橡胶材料制作的哑铃型试柱，进行不同应变比R下的单轴拉伸/压缩疲劳试验，并分析应变比R对疲劳寿命的影响。以应变峰值、应变幅值为损伤参量，建立相应的疲劳寿命预测模型，并对R≥0和R<0两种工况下模型的预测结果进行比较。结果表明：对于应变比R改变的疲劳载荷工况，以应变幅值为损伤参量的寿命预测模型、和以应变峰值为损伤参量的寿命预测模型这两种常规的模型都有一定的局限性。为了考虑应变比R对橡胶疲劳寿命的影响，提出了等效应变幅值。基于等效应变幅值的寿命模型可较好地预测R在较大范围内变化的载荷工况下的疲劳寿命(特别是对R≤0的载荷工况)，预测的疲劳寿命可为该橡胶材料在疲劳设计阶段提供有效的数据支持。(3)开展了隔振器橡胶材料的裂纹扩展试验，并建立了橡胶材料裂纹扩展特性的统一模型。利用带单边缺口的纯剪试件，开展了隔振器橡胶材料的裂纹扩展试验，其中疲劳载荷是变幅加载的位移。对这种加载工况下的裂纹扩展数据，不宜采用传统的割线法或七点多项式法计算裂纹扩展速率。研究结果表明：采用插值函数来表征实测的裂纹扩展长度与循环次数的关系进而得到橡胶材料的裂纹扩展统一模型，这种方法是准确合理的。(4)基于连续介质力学的理论，推导了有限变形和多轴应力状态下开裂能密度的求解方法，进而计算多轴载荷下橡胶材料的撕裂能。在多轴疲劳载荷下，有效用来驱动裂纹的扩展的那部分应变能密度称为开裂能密度。基于开裂能密度和橡胶材料的裂纹扩展特性预测橡胶材料的多轴疲劳寿命时，须计算在外载荷下橡胶隔振器的开裂能密度。为了由有限元软件(如ABAQUS)默认输出的应变计算在有限变形下的开裂能密度，推导了常见6种超弹性模型下开裂能密度的计算公式，并说明了所需的积分方法。结果表明：本文开裂能密度的计算方法是准确合理的，基于该方法计算的开裂能密度，可以将橡胶等双轴疲劳和单轴疲劳寿命较好地统一起来。(5)基于开裂能密度和隔振器橡胶材料的裂纹扩展特性实测数据，预测了橡胶隔振器的多轴疲劳寿命。基于开裂能密度这一连续介质力学参数和隔振器橡胶材料裂纹扩展试验获得的裂纹扩展特性(裂纹扩展速率与撕裂能之间的关系)，得到了橡胶隔振器多轴疲劳寿命的计算公式，并计算了某一典型结构形式的橡胶隔振器的疲劳寿命。结果表明：采用上述方法对橡胶隔振器疲劳的预测结果(寿命、开裂位置和开裂方向)与实测结果较为一致。其中预测的疲劳寿命分布在实测疲劳寿命的2倍分散线之内，满足工程疲劳寿命预测的要求。本文橡胶隔振器的多轴疲劳寿命预测方法，用试验效率较高和投入较少的材料裂纹扩展试验代替需要耗费大量时间的材料疲劳破坏试验，可为橡胶隔振器前期的疲劳设计提供参照的同时，还可大幅地缩短产品疲劳设计的周期。更多还原

【Abstract】 Rubber isolators are usually subjected to substantial static and dynamic loads, and oftenfail due to nucleation and growth of defects or cracks. Prevention of such mechanical fatiguefailures necessitates thorough understanding of the deformation mechanisms of the rubbermaterials during cyclic loading, so as to predict the fatigue life of rubber components moreaccurately. The investigation on the mechanical fatigue of rubbers has become one of thecurrent international hot issues. In this thesis，a series of fatigue behaviors for filled natrualrubbers under uniaxial tension, uniaxial tension/compression, and multiaxial loads, areinvestigated in the frame of mechanics of fatigue design. The major studies can besummarized as follows:(i) Two factors that might influence the tension fatigue model of filled natural rubbersused in rubber isolators are investigated. One is the damage parameter and the other is thespecimen geometry used in the fatigue experiment. The uniaxial tension fatigue experimentsare carried out for three typical types of filled natural rubber specimens: a dumbbell simpletension specimen (STS), a dumbbell cylindrical specimen (DCS), and a hollow cylindricalspecimen (HCS). The commonly used damage parameters (Green-Lagrange strain,Almansi-Euler strain, engineering strain, logarithmic strain, stretch ratio, etc.) for fatigue lifeprediction are described and discussed. The fatigue life prediction models using the measuredtension fatigue life of the STS and different damage parameters are developed, and acorrelation coefficient is used to characterize the error between the measured fatigue life andthe estimated one using the developed fatigue life prediction model. It is concluded that alldamage parameters discussed in this paper can be used to estimate tension fatigue life withcorrelation coefficient greater than0.9. The fatigue life model with the STS is appropriate topredict the fatigue life of the DCS and the HCS, which shows that the relationship betweenthe tension fatigue life and the damage parameters is independent of the geometry of thespecimens. One can thus carry out tension fatigue test using only a STS to model the tensionfatigue of rubbers.(ii) The effect of strain ratio R on the fatigue life of filled natural rubbers used in rubberisolators is investigated experimentally and numerically. A uniaxial tension/compressionfatigue experiment is conducted on DCS rubber specimens subjected to loads representingdifferent R-ratios. The experimental fatigue data are used to formulate two preliminary fatiguemodels based on peak strain and strain amplitude as the damage parameters. The deficienciesof these two models in predicting fatigue life over a wide range of R ratios are discussed, and an alternative life prediction model is proposed. The proposed model incorporates the effectof R-ratio using an equivalent strain amplitude. It is shown that the proposed model couldeffectively predict fatigue life over a wide range of R-ratios with an improved accuracy,praticually for loads of negative R ratio.(iii) The fatigue crack growth (FCG) experiment and modeling method for filled naturalrubbers used in rubber isolators under variable amplitude loads are carried out using anedge-crack pure shear specimen. Variable amplitude loads are imposed on the edge-crack pureshear specimen, and such load provides a more effective way of obtaining the measured FCGdata under different load levels than the conventional constant amplitude load. The commonlyused data processing techniques for getting the crack growth rate (crank length versus numberof cycles) in metal materials, the secant method or the local incremental polynomial method,are not applicable for computing the crack growth rate of rubber material, since the FCG dataunder the variable amplitude loads embodies large fluctuating locally. Based on the knownFCG law and the measured FCG data under variable amplitude loads, a power function isproposed to fit the measured crack growth length and number of cycles using the least-squarestechnique. The crack growth rate is thus calculated from the determined power function, and aFCG prediction model for filled natural rubbers is established from the crack growth data andthe associated tearing energy. To validate the developed FCG model, a dumbbell specimenmade of the same rubber compound as the pure shear specimen is manufactured and is used tocarry out the tensional fatigue experiment. The comparsions between the measured tensionalfatigue life of the dumbbell specimen and that evaluated from the established FCG modelvalidates the proposed data processing method for FCG data of filled natural rubbers underthe variable loads.(iv) A new method for calculation of cracking energy density (CED), which is related tothe tearing energy of rubbers under multiaxial loads, is proposed in the frame of continuummechanics. Using the measured fatigue crack growth characteristic of rubber materials and thecalculated CED of rubbers under external loads, one can predict fatigue life of rubbercomponents. To calculate CED using the output strain from the finite element software(ABAQUS) as inputs, the formula of CED under the principal coordinate system is derivedand the required integral technology is given. Six hyperelastic constitutive models (Ogden,Mooney-Rivlin, Neo-Hookean, Yeoh, Arruda-Boyce and Van der Waals) for rubber materialsare used in the method for calculating CED. It is shown that the CED calculated from theproposed method is valid and effective to unify the uniaxial and equiaxial fatigue life ofrubber materials. (v) The multiaxial fatigue life of rubber isolators is predicted by combing the calcualtedCED and the FCG model of the studied rubber material. Using the measured FCGcharacteristic of rubber material and the calculated CED of rubbers under external loads, aformula for calculating the fatigue life of rubber components under multiaxial loads isestablished, and is applied for predicting a typical rubber isolator. It is shown that thepredicted fatigue life of the rubber isolator agree well with the measured fatigue life within afactor of two, and the predicted crack location and orientation are comparable with themeasured results. The fatigue life prediction method for rubber isolators under multiaxialloads can thus be used as an effective and low cost tool for up-front knowledge of rubbercomponents in the design stage.更多还原