基于冲击层数的铝合金紧固孔激光冲击强化抗疲劳研究

发布时间：2018-06-07 07:35

本文选题：激光冲击强化 + 紧固孔　；参考：《江苏大学》2016年硕士论文

【摘要】：飞机结构中存在大量的小孔结构是飞机的薄弱环节,激光冲击强化技术能有效延长小孔的使用寿命,然而单层冲击存在应力分布不合理、疲劳增益不稳定的问题,为解决这一问题增加冲击层数对紧固孔的残余应力场进行调控,达到稳定提高紧固孔疲劳寿命的目的。在满足疲劳增益稳定性的前提下,对冲击层数进行优化可以起到简化工艺、节约成本的作用。本文首先分析激光冲击层数研究现状,借助ABAQUS软件分析冲击层数等参数对紧固孔残余应力的影响,并对冲击层数等参数进行优化处理,设计了不同冲击层数的激光冲击试验,最后进行疲劳验证试验。本文的主要研究工作与成果如下:(1)从应力波角度分析了单次激光冲击残余应力形成和估算,从弹性极限和材料硬化的角度分析多次冲击残余应力场的叠加,最后根据残余应力对紧固孔应力强度因子的影响规律,揭示激光冲击处理延长紧固孔服役周期的机理。结果表明:冲击层数的增加可以提高残余应力,但每次增加幅度会减小;残余压应力能有效降低紧固孔的应力强度因子幅度。(2)利用ABAQUS软件分析冲击层数等参数对紧固孔残余应力的影响。结果表明:冲击层数的累加造成残余应力增大,但每次增加比例减小,双层、三层的结果相差不大,中间孔壁拉应力区不断压缩;与载荷半高宽和冲击层数相比,峰值压力对残余应力的影响更为显著;峰值压力较小时,载荷半高宽和冲击层数可以弥补峰值压力的不足,但峰值压力较大时,这种弥补作用不明显。(3)计算均匀拉伸下紧固孔的应力强度因子,在仿真残余应力的基础上,计算均匀拉伸和残余应力场综合作用下紧固孔的应力强度因子,分析激光冲击处理对应力强度因子的修改。结果表明:激光冲击处理能有效降低紧固孔附近的应力强度因子。均匀拉伸下三维无量纲和有量纲应力强度因子趋势一致,随着θ角的增大先减小后增大,θ=00和900时应力强度因子最大;裂纹沿任意方向扩展时应力强度因子不断增大。(4)结合断口与残余应力结果,分析不同冲击层数对紧固孔疲劳寿命、裂纹源与裂纹扩展速率的影响;最后通过分析激光冲击层数对韧窝微观特征的影响,分析激光冲击对铝合金塑性的影响规律。结果表明:单层冲击疲劳增益较小,双层和三层冲击疲劳增益为单层的两倍,双层、三层强化端寿命相等。未强化试样是单点角裂纹,强化试样是孔壁多点疲劳源,冲击层数的累加导致裂纹源沿厚度方向转移。相比于未处理试样,强化端疲劳辉纹间距明显变窄,冲击层数的累加导致辉纹间距变窄,双层和三层的疲劳辉纹间距基本相等。强化层数的累加效应使韧窝更大、更深,说明激光冲击处理能提高试样的塑性变形能力。建立了冲击层数-残余应力-疲劳寿命三者之间的关系,激光冲击对应力强度因子幅度的修改是裂纹扩展寿命增大的原因,冲击层数对拉应力区的压缩是疲劳源转移的诱因,研究冲击层数对紧固孔疲劳寿命的影响,为工程实际应用提供参考。
[Abstract]:The existence of a large number of small hole structures in aircraft structure is the weak link of aircraft. Laser impact strengthening technology can effectively prolong the service life of small holes. However, there is a problem of unreasonable stress distribution and instability of fatigue gain in single layer impact. In order to solve this problem, the residual stress field of the fastening holes can be adjusted to achieve stability by increasing the number of impact layers. In order to improve the fatigue life of the fastening holes. On the premise of satisfying the stability of the fatigue gain, the optimization of the number of impact layers can simplify the process and save the cost. This paper first analyzes the status of the research on the number of laser shock layers and analyses the impact of the impact layer on the residual stress of the fastening holes with the help of the ABAQUS software, and the impact layer. The number of parameters are optimized and the laser shock test with different impact layers is designed. Finally, the fatigue verification test is carried out. The main research work and results of this paper are as follows: (1) the residual stress formation and estimation of single laser shock are analyzed from the angle of stress wave, and the residual stress of multiple shocks is analyzed from the angle of elastic limit and material hardening. At last, according to the influence of the residual stress on the stress intensity factor of the fastening hole, the mechanism of the laser shock treatment to prolong the service period of the fastening holes is revealed. The result shows that the increase of the number of shock layers can increase the residual stress, but the amplitude of the increase will decrease each time; the residual pressure stress can effectively reduce the stress intensity factor of the fastening hole. (2) (2) the effect of the number of impact layers on the residual stress of the fastening holes is analyzed by using the software of the impact layer. The results show that the addition of the number of shock layers increases the residual stress, but the ratio decreases each time, the result of the double layer and the three layer is not quite different, the tensile stress zone in the middle hole wall is continuously compressed, and the peak pressure is compared with the load half width and the number of the impact layers. The influence of the residual stress is more significant; the peak pressure is small, the load half width and the number of shock layers can make up the shortage of peak pressure. But when the peak pressure is large, this compensatory function is not obvious. (3) the stress intensity factor of the tightened hole under uniform tension is calculated and the uniform tensile and residual stress are calculated on the basis of the simulated residual stress. The stress intensity factor of the fastening hole is analyzed by field synthesis, and the modification of the force intensity factor is analyzed by laser shock treatment. The result shows that the stress intensity factor near the fastening hole can be reduced effectively by laser shock treatment. The stress intensity factor is maximum at theta =00 and 900. The stress intensity factor increases continuously when the crack propagates in any direction. (4) the effects of the number of different impact layers on the fatigue life of the fastening holes, the crack source and the crack growth rate are analyzed with the results of fracture and residual stress. Finally, the influence of the number of laser shock layers on the microcosmic characteristics of the dimple is analyzed. The effect of laser impact on the plasticity of aluminum alloy shows that the single layer impact fatigue gain is smaller, the double layer and the three layer impact fatigue gain is two times, the double layer and the three layer are equal. The unreinforced specimen is a single point angle crack, the reinforced specimen is the multipoint fatigue source of the hole wall, and the number of impact layers leads to the crack source along the thickness direction. In contrast to untreated samples, the fatigue striation spacing of the reinforced end is narrower and the number of impact layers narrowed, and the fatigue striation spacing between the two layers and the three layers is basically equal. The cumulative effect of the number of strengthened layers makes the dimples greater and deeper, indicating that the laser shock treatment can improve the plastic deformation ability of the specimen. - the relationship between the three of the residual stress and fatigue life, the modification of the amplitude of the force intensity factor of the laser shock is the reason for the increase of the crack propagation life. The compression of the number of impact layers on the tensile stress zone is the cause of the fatigue source transfer. The study of the impact of the number of impact layers on the fatigue life of the fastening holes provides a reference for the practical application of the engineering.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：V215.5;V229

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