【作者】 杨冰；

【导师】 赵永翔；

【作者基本信息】 西南交通大学 ， 载运工具运用工程， 2011， 博士

【摘要】 疲劳短裂纹的扩展过程,占光滑表面结构服役寿命的70%-90%。在有关理论尚不完善与成熟的情况下,研究这一课题具有重要的科学意义与工程指导意义。本学位论文通过完成铁路LZ50车轴钢材料及旋削一滚压维修工艺试样的疲劳短裂纹复型试验,开展了如下研究工作：1.通过对表面未经滚压处理试样(未滚压试样)的试验研究,揭示了LZ50钢的两种微观结构障碍疲劳损伤机制。研究表明,疲劳短裂纹萌生于试样表面的铁素体。在微观结构短裂纹(MSC)阶段,主导有效短裂纹(DESFC)在扩展过程中出现两次较明显降速。第一次降速出现在短裂纹萌生后,扩展遇到晶界。第二次降速出现在DESFC突破铁素体晶界障碍约束后,扩展遇到富珠光体带状结构。而DESFC突破富珠光体带状结构约束,它的主导地位迅速确立,疲劳短裂纹扩展进入物理短裂纹(PSC)阶段。从此时起,直到出现常规观测手段可见的长裂纹及最终试样断裂,裂纹扩展速率持续随机增加,无明显障碍特征。两次降速最低点对应的平均DESFC表面长度分别为14.77μm和107.11μm,分别接近材料平均铁素体直径(14.61μm)和富珠光体带状结构间距(109.09μm),这说明LZ50钢的疲劳损伤具有两种微观结构障碍机制。2.引入微观结构障碍阻力系数函数,发展了新的多微观结构障碍疲劳短裂纹扩展率模型及其概率表征方法；并结合长裂纹试验结果,引入当量应力强度因子,提出了疲劳长、短裂纹扩展率统一模型及其概率方程。阻力系数反映了裂尖距离微观障碍越近,裂纹所受扩展阻力越强的特点。裂纹一旦突破前一障碍的约束,立即进入下一障碍影响范围内,重复受阻过程,具有周期性特征。随着裂纹尺度增大,扩展驱动力不断增强,微观障碍作用的影响效果持续减弱。本文提出的短裂纹扩展率模型继承有效短裂纹准则思想,以远场总循环应变能密度△W1与DESFC尺度α的乘积为裂纹驱动力参量；模型包含材料多种微观结构障碍特征尺度参量,利用阻力系数函数反映裂纹扩展过程中的减速现象。通过拟合试验数据,获得了LZ50钢短裂纹扩展率模型的概率参量及曲线,验证了模型的有效性。进一步,考虑到从短裂纹到长裂纹的扩展,其行为在客观上是一个连续发展的物理过程,通过拓展小范围屈服条件下J积分与应力强度因子的关系,引入能同时描述长、短裂纹扩展的弹塑性驱动力——当量应力强度因子△Keq,在短裂纹扩展率模型基础上,发展了包含多微观结构障碍的长、短裂纹扩展率统一模型。对未滚压短裂纹试样和长裂纹试样试验数据的处理结果,验证了统一模型的有效性。3.通过对不同时刻旋削与滚压试样及未滚压试样的试验对比研究,发现对未经滚压的试样,越早实施滚压处理,获得的延寿效果越好。通过引入滚压效应函数M(f),对未滚压试样短裂纹扩展率模型进行修正,发展了可考虑滚压时刻对扩展速率影响的新模型。滚压在试样表层和近表层产生了约235～100 MPa的周向残余压应力,和约316～132MPa的轴向残余压应力,可降低试验时的有效疲劳应力；同时,提高了材料表面的显微硬度,滚压后铁素体、珠光体的硬度分别较未滚压试样提高了10.81%和3.15%,抑制了短裂纹的萌生与扩展。比较几组不同时刻进行旋削与滚压处理的试样在滚压后的延寿率,发现随着处理时刻从f=0.7提前至f=0.0,滚压后平均延寿率从379%上升至641%。这表明,为获得更长的疲劳寿命,对未经滚压的试样,应当在条件具备的情况下,尽早实施旋削与滚压处理。滚压效应函数M(f)反映了滚压时刻与延寿率之间的关系,用它对疲劳短裂纹扩展率模型进行修正,能体现滚压时刻选择的不同对裂纹扩展率的显著影响。更多还原

【Abstract】 The propagation process of short fatigue crack can occupy 70%～90% of the service life for smooth surface structure. While the relevant theories are still not mature and complete, research on this topic is of important scientific and practical significance. Based on the replica test observations on surface short fatigue cracks of smooth hourglass shaped specimens with and without rolling maintence technology of railway LZ50 axle steel, following studies are performed:1. The fatigue damage mechanism of two kinds of micro-structural barriers for LZ50 axle steel is revealed through the experimental study on specimens without surface rolling.Research shows that the short fatigue cracks initiate in the ferrite grain on specimen surface. In the micro-structural short crack (MSC) stage, the crack growth rate of the dominant effective short fatigue crack (DESFC) exhibits decelerations twice clearly. After the initiations of short cracks, the first deceleration occurs while crack tips are close to the ferrite grain boundary. The second deceleration happens while DESFC breaks the limit of grain boundary, and the crack tips meet the pearlite banded structure. Once DESFC overcomes the resistance of the pearlite banded structure, its predominance is established quickly and the propagation of short fatigue crack enters the physical short crack (PSC) stage. From this point on, the crack growth rate increases continuously till the long crack that can be observed by conventional methods occurs, and keeps this trend to the final fracture of specimen. No obvious micro-structural barriers can be observed. Above two decelerations are corresponding to the average DESFC size of 14.77μm and 107.11μm, respectively. These two values are separately close to the average diameter of ferrite grain (14.61μm) and the average interval of rich pearlite banded structures (109.09μm) for present material. Thus it is clear that the fatigue damage of LZ50 axle steel includes two kinds of micro-structural barriers mechanisms.2. By introducing a resistance coefficient function of micro-structural barriers, a new short fatigue crack growth model that includes multiple micro-structural barriers is presented. The probabilistic description of this model is also proposed. Combined with the result of long crack test, a unified growth model and its probabilistic equation for short and long crack are developed. An equivalent stress intensity factor is applied in the unified model.The resistance coefficient function can reflect the relationship between the crack size and the resistance of micro-structural obstacle. That is the shorter the distance between DESFC tip and the micro-structural barrier is, the stronger the constraint force is. Once DESFC breaks the previous obstacle, it enters the influence range of the next barrier. Above process repeats again and again, and has a periodic feature. With the increase of crack size, the growth driving force becomes greater and greater. At the same time, the effect of micro-structural barrier starts to weaken continuously. Present short fatigue crack growth model inherits the idea of effective short fatigue crack criterion. The product of the total cyclic strain energy density of remote fields,△Wt, and the DESFC size, a, acts as the crack driving force of the model; characteristic size parameters of different micro-structural barriers are also included, and the resistance coefficient function is utilized to reflect the periodic deceleration phenomena during the propagation process. Probabilistic parameters of present model and corresponding short fatigue crack growth curve are obtained through fitting the test data. Description to the test results indicates the availability of the model. Furthermore, considering that the behavior of short and long crack propagation is a continuous and evolving physical process, the relationship between J-integration and stress intensity factor under the condition of small scale yielding is extended. An elastoplastic driving force, equivalent stress intensity factor△Keq, is introduced to describe the growth of short and long crack. Based on previous short fatigue crack growth model, a unified short and long crack growth model is developed. Fitting effects to the short crack and the long crack test results indicate the availability of the unified model.3. Comparative study is carried out on specimens with rotary turning and surface rolling at different moments and specimens without surface rolling. The results reveal that to obtain the better prolonging effect of fatigue life, surface rolling should be applied to specimens without rolling as early as possible. A rolling effect function, M(f), is introduced to revise the short crack growth model of specimens without surface rolling. The new model that can consider the effect of different rolling moments on crack growth rate is then developed.About 235～100 MPa circumferential residual compressive stress and 316～132 MPa axial residual compressive stress are engendered in specimen surface and sub-surface after rolling treatment, which can decrease the effective fatigue stress during test. Meanwhile, surface rolling increases the micro-hardness of material surface. For example, the hardness of ferrite and pearlite increases 10.81% and 3.15% than that of specimens without surface rolling. This treatment can restrain the initiation and growth of short cracks. Comparisons to the test results of five group specimens with different turning and rolling moments, indicate that the prolonging rate of life increases from 379% to 641% while the treating moments are advanced from f=0.7 to f=0.0. This shows that to obtain longer fatigue life, rotary turning and surface rolling should be applied to specimens without rolling as early as possible. The rolling effect function, M(f), reflects the relationship between the prolonging rate of life and the rolling moments. It can be used to revise previous short fatigue crack growth model and indicate the significant impact of rolling moment on crack growth rate.更多还原