【作者】 张亮；

【导师】 方洪渊；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 疲劳性能是工程材料最重要的力学性能之一，也是进行工程结构件设计及可靠性评估的基本数据。为了获得材料的疲劳性能需要进行耗时、昂贵的疲劳试验，这不仅延长了工程结构的设计和制造周期，同时增加了成本。针对疲劳试验耗时、昂贵的特点，在分析高周疲劳过程能量耗散特点的基础上，提出了高周疲劳寿命的预测方法。针对焊接接头组织和力学性能不均匀的特点，利用缺口疲劳试验获得了A7N01-T4铝合金焊接接头疲劳寿命的构成特点。试验结果表明，母材、焊缝和热影响区的疲劳断裂寿命差别较大，而疲劳裂纹萌生寿命差别不大，各微区疲劳裂纹萌生寿命占疲劳断裂寿命的比例不同，焊缝区内疲劳裂纹萌生阶段占据了疲劳断裂的大部分时间，疲劳裂纹萌生寿命不可忽略。针对焊接接头微区疲劳裂纹萌生寿命占疲劳总寿命比例较高的特点，为了准确预测疲劳寿命，基于连续损伤力学，提出了一个考虑载荷频率影响的高周疲劳损伤模型。该模型考虑了应变速率对高周疲劳损伤的影响。疲劳试验结果表明，该模型适用于对疲劳载荷频率敏感和不敏感材料的疲劳寿命预测。为了捕捉载荷频率增加引起的试件温度的变化，采用精密集成温度传感器AD592CN自行研制了一套疲劳试件温度实时测量装置，该装置配有4个AD592CN温度传感器，可以同时实现两路绝对温度和一路相对温度测量。该温度测量系统消除了测量过程中外界环境温度变化对测量结果的影响，可以对疲劳过程中试件温度的微小变化进行准确、稳定的实时测量和记录。根据高周疲劳过程中材料温度演化曲线的特点，从宏观和微观两个尺度分析了循环加载过程中疲劳试件温度演化曲线各阶段的能量耗散特点。分析发现，疲劳过程中材料内部缺陷的运动引起了试件温度的升高。绝热条件下，温度演化曲线第一阶段的储能变化较小，机械能大部分用于试件温度的升高，从而表现出较大的温度上升速率。随着循环周次的增加，试件内位错密度随之增大，从而导致储能的增加，引起了温度演化曲线第二阶段温升速率的减小。当试件进入失稳扩展阶段，裂纹尖端的能量快速释放，引起了试件温度的再次快速上升。在对温度演化曲线分析的基础上，提出了基于能量耗散的高周疲劳寿命预测方法和模型，所提出的模型具有明确的物理意义。该模型中的唯一参数——高周疲劳断裂极限温升，是一个与材料有关的常数，表征了材料抵抗高周疲劳断裂的能力。该常数表示绝热状态下，完美晶体材料高周疲劳断裂时试件所能达到的最高温度。利用所提出的试验方法，只需数千周的加载，通过测量试件的初始温升速率即可预测材料的高周疲劳寿命。利用自行研制的实时温度测量装置测量了A7N01-T4铝合金母材及其焊接接头试件的温度演化曲线。试验结果表明：温度上升速率随着应力幅和载荷频率的增加而增大，换热条件对温度演化曲线第二阶段的温度上升速率有较大影响。利用所得到的温度演化曲线及疲劳试验结果验证了所提出的基于能量耗散理论的高周疲劳寿命预测模型，并发现对于A7N01-T4铝合金母材及其焊接接头试件，在两种载荷频率下得到的模型参数接近，与理论分析结果吻合良好。疲劳断口分析表明，载荷频率的变化对A7N01-T4铝合金母材及其焊接接头的疲劳断裂特征没有显著影响。更多还原

【Abstract】 Fatigue performance, as one of the most important mechanical properties ofengineering materials, is foundmental data for design and reliability assessment forengineering component. Fatigue test is required to obtain fatigue data, however,the test procedure is time-consuming and costly, which will prolong the design andproduction cycle as well as increasing cost. In the thesis, experimental andtheoretical methods for fatigue life prediction were studied based on the analysisof energy dissipation of high cycle fatigue.Aiming at the inhomogeneity in microstructure and mechanical performancefor welded joint, fatigue crack initiation characteristics of A7N01aluminium alloywelded specimen were investigated by notch fatigue test. The experimental resultsshow that the differences of fatigue life to failure among base metal, weld metaland heat affected zone (HAZ) are significant, but the difference of fatigue crackinitiation life is slight. There are distinguished differences on the ratio of fatiguecrack initiation life to fatigue life to failure for the three microzones. The stage offatigue crack initiation expends most of the whole fatigue life. Therefore，thefatigue crack initiation life cannot be ignored.Due to the high percentage of fatigue crack initiation life for fatigue failure, amodified high cycle fatigue model based on continuum damage mechanics wasproposed, which takes into account the influence of loading frequency on fatigueproperties. In the proposed model，the effect of strain rate on high-cycle fatiguedamage was considered. Fatigue test results indicate that the model can be appliedto both frequency-sensitive and frequency-insensitive materials.In order to monitor the temperature rise due to increasing of loadingfrequency, a real-time temperature detecting system based on accurate integratedtemperature sensor AD592was developed. The system consists of four AD592CNtemperature sensors, which can measure two actual temperatures and one relativetemperature simultaneously. The influence of variations of environmentaltemperature can be removed by the developed system. Meanwhile, the system,which has advantages of high precision and stability, is suitable to detect the sightchange of temperature during fatigue test in real time.According to the feature of temperature evolution curve, the characteristics ofenergy dissipation during high cycle fatigue were analyzed in macroscopic andmicroscopic scale. It is found that the temperature rise of the specimen is attributedto the movement of defects in materials. Under adiabatic conditions， the variationof internal storage energy in the first phase of the temperature evolution is slight and most of the expended mechanical energy transforms into heat which causestemperature of the specimen to increase rapidly. Thereafter, the increasing incycles causes a greater mobile dislocation density and an increasing in storageenergy. Correspondingly, the increasing rate of temperature falls in the secondphase of the temperature evolution. In the phase of crack instability propagation,the local energy at the crack tip releases quickly which causes a further rapidincrease immediately prior to failure. On the basis of the analysis of temperatureevolution, prediction method and model based on energy dissipation for high cyclefatigue life were proposed. The model has an explicit physics meaning. In addition,the only one parameter in the model is a material dependent constant called"limiting temperature rise for high cycle fatigue failure", which characterizes thecapacity for resisting high cycle fatigue failure. The physics meaning of theparameter can be expressed as the maximum temperature rise of material whenfatigue failure occurs for perfect crystal in adiabatic condition. Consequently, highcycle fatigue life can be determined by measuring the initial slope of thetemperature in several thousands of cycles using the proposed experimentalmethod.Temperature evolution of A7N01-T4aluminium alloy and the weldedspecimens under high-cycle fatigue load were obtained by the developedtemperature detecting system. Experiment results show that the temperature riserate increases with increasing stress amplitude and cyclic frequency. Thetemperature rise rate in the second phase of the temperature evolution is influencedby the heat transfer conditions. The proposed fatigue life prediction model wasverified by the measured temperature evolution and fatigue test data. The resultsshow that the parameters at frequency10Hz and128Hz are almost equal, which isin accord with analyses in theory. Fractographic observations of fatigue specimensshow that the influence of cyclic frequency on fatigue failure mode is insignificantfor both A7N01-T4aluminium alloy and the welded specimens.更多还原