2024铝合金喷丸试件疲劳寿命试验及仿真研究

现有的喷丸材料疲劳性能研究扩展有限元模型没有考虑残余应力对裂纹扩展的影响。对2024铝合金的喷丸与未喷丸试样进行三弯疲劳试验,以明确喷丸工艺对试件疲劳寿命的强化作用。通过ABAQUS建立试件的二维平面应力模型,导入残余应力并利用扩展有限元法模拟循环载荷下裂纹的萌生与扩展,对比试验结果来验证该扩展有限元数值模型的正确性。最后基于该数值模型,改变载荷工况,研究不同载荷工况下残余应力对疲劳寿命的影响,得到喷丸残余应力强化作用与载荷工况的关系。结果表明:喷丸引入的残余应力可以有效地增强试件的疲劳寿命;过大的循环载荷可能造成喷丸残余应力发生松弛;在最大载荷不变的前提下,应力比越小,试件疲劳寿命越短;应力比越大,残余应力对疲劳寿命强化效果越明显。     

多轴复杂应力形式下TB6高强钛合金耳片的微动疲劳断裂研究

本实验研究了TB6高强钛合金在拉-扭多轴应力条件下的微动疲劳断裂机理,并结合宏微观观察、残余应力测试、计算机模拟分析了不同工艺条件的耳片在拉-扭疲劳试验应力下的微动疲劳寿命差异,研究了微动疲劳裂纹的萌生及扩展特性,揭示了耳片裂纹萌生机制。结果表明:二次挤压耳片的断口扩展区面积比例较大,说明其拉-扭疲劳扩展更为充分。经二次挤压后,耳片的拉-扭微动疲劳总寿命得到延长,且裂纹萌生寿命均占总寿命的95%以上;二次挤压后耳片的裂纹萌生驱动力较低,呈现较高的裂纹萌生抗力。各断口均起源于耳片内孔微动磨损印迹较重处,大面积连续片状的微动磨损印迹表明二次挤压后的钛合金微动疲劳敏感性大大降低。经挤压强化后,孔壁形成了较高幅值的残余压应力层和组织强化层,可以有效地抑制裂纹的萌生和扩展。耳片内孔微动磨损层的形成是机械诱导机制和热诱导机制共同作用的结果,其最终磨损形式为疲劳磨损。     

残余应力场中疲劳裂纹闭合模型的扩充

在残余应力场中一般采用残余应力强度因子法预测疲劳裂纹的扩展,但它未考虑残余压应力对裂纹的闭合效应。本文以八十年代初J、C、Newman提出的疲劳裂纹闭合模型为基础,加以修正和扩充使之适用于单边裂纹体,并在原有模型的裂纹面接触应力上迭加残余应力参量。通过实验测定了40Cr钢高温回火态试样在有和无残余应力场时的裂纹闭合力和扩展速率,结果表明,计算值与实验值基本一致。扩充模型尚存在些问题有待改进。     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

涡轮盘篦齿裂纹扩展的有限元数值模拟

为了详细考察篦齿裂纹的扩展规律,对篦齿裂纹从齿尖一直扩展到即将完全穿透篦齿环的过程进行了数值模拟.含篦齿裂纹的涡轮盘有限元模型采用子模型法建立,使用M积分计算裂纹前沿的应力强度因子;在确定篦齿裂纹前沿每一节点处的局部扩展方向及距离后,通过样条曲线拟合出新裂纹前沿,并依靠自适应网格划分实现裂纹区有限元网格的更新.数值模拟结果表明,篦齿根部过渡圆角顶部可以视为裂纹缓慢扩展阶段与快速扩展阶段的分界点,在此之前篦齿裂纹以穿透型裂纹的形态以较低的速度进行扩展,在此之后篦齿裂纹开始向表面裂纹进行转化,并且平均扩展速度大大增加,分界点前的裂纹扩展寿命是之后的数倍.此外,由数值模拟结果还可以发现,增大篦齿根部过渡圆角半径以及减小相邻篦齿的间距,均有助于延缓篦齿裂纹的扩展.     

考虑大变形和排水条件时柱孔扩张问题统一解析

将柱孔周围土体中的应力分布分为弹性区和塑性区,在弹性区中采用小变形理论,在塑性区中采用大变形理论和统一强度准则。根据柱孔扩张问题排水条件时的应力平衡方程、应力和应变连续的边界条件,推导出考虑土体剪胀、大变形、排水条件和中主应力等因素的柱孔扩张问题塑性区半径和极限扩孔压力的理论解答,同时也获得了弹塑性区中的应力和应变场分布规律。理论计算结果与现场实测结果较为接近,初步说明该理论具有一定的工程应用价值。对比分析表明:扩孔压力和塑性区半径受土体剪胀的影响要比b值显著。     

断裂故障研究中裂尖塑性区的影响及处理原则

通过对K判据的适用区与裂纹尖端塑性区的比较,在塑性区一定要小于K判据适用区的条件下,给出了K判据的适用边界线和判断脆性断裂的判断式;确定了K判据在结构静强度计算中塑性修正的基本原则,在此基础上讨论得出了塑性区尺寸小于裂纹长度,裂纹扩展寿命的估算方法仍适用,塑性对裂纹扩展寿命估算影响不大,而选用不同的经验公式估算裂纹扩展寿命所出现的误差却很大的结论.     

7850-T7451铝合金板材孔挤压工艺性能研究

铝合金产品在目前的市场上已经屡见不鲜,也是金属制造行业中一项极为重要的分支。本文采用挤压棒直接冷挤压的方法对7850-37451铝合金厚板进行了孔挤压强化,对比分析了其孔挤压前后疲劳寿命状态原因;并与第三代高纯7850-T7451铝合金厚板孔挤压强化效果进行对比。通过扫描电镜（SEM）、透射电子显微镜（TEM）以及X射线应力（XRD）等方法,研究了两种合金的疲劳断口形貌特征、微观组织变化以及孔表层的残余应力场。结果表明,采用4％～6％的挤压量对7850-T7451厚板进行挤压强化可取得较好的疲劳强化效果,试件的疲劳寿命是未挤压强化前的29倍;而7850-T7451铝合金厚板疲劳寿命仅是未挤压强化的5．5倍。孔挤压后,7850-T7451厚板在强化层产生位错缠结及残余压应力,压应力层深度约为7．3mm,最大残余压应力出现在距孔边约1m处,应力值为387MPa。强化层内形成的位错胞状结构和残余压应力可有效延缓疲劳裂纹的扩展速率,从而提高试件的疲劳寿命。     

谱载荷下裂纹扩展计算模型及其在真实飞行谱中的应用

本文讨论了复杂谱载荷作用下裂纹扩展寿命预测的问题,选择了到目前为止功能较强的两种裂纹扩展模型:修正的广义的Willenborg模型和修正的Maarse模型;并对这两个模型就塑性区计算与超载延缓区计算等方面作了改进。 将本文改进后的上述两个模型应用于歼击机真实飞行谱载荷下裂纹扩展寿命的计算,计算结果与实验结果的比较表明:经本文改进后的模型功能强、精度较高、使用方便,可推广应用于各种复杂谱载荷下疲劳裂纹扩展预测的工程实践中去。     

II型III型动态裂纹尖端断裂过程区近似评估方法

动态裂纹尖端断裂过程区轮廓的确定问题仍然是一个没有得到完全解决的问题。基于弹性动力学的理论和复应力函数方法,提出一种伪应力函数方法,用于近似评估动态裂纹尖端应力场。通过与已知应力场计算结果对比,验证了伪应力函数的正确性。利用此近似方法通过Von Mises强度准则和Tresca强度准则,分别确定了不同强度准则条件下、不同裂纹扩展速度下断裂过程区的轮廓。计算结果表明:II型和III型动态裂纹尖端断裂过程区关于裂纹面对称分布,随着裂纹扩展速度增大而增大。当裂纹传播速度接近瑞利波速时,断裂过程区变化加剧。利用Tresca强度准则计算得到的动态裂纹尖端断裂过程区面积比利用Von Mises强度准则计算得到的断裂过程区的面积大。     

Analysis of cold expanded fastener holes subjected to short time creep: Finite element modelling and fatigue tests

The beneficial effects of cold expansion have been well documented in previous studies, yet the performance of cold expanded plates exposed to elevated temperatures is an area of technical interest. In this research, finite element (FE) simulations along with experimental fatigue tests have been carried out to investigate the effect of exposure to elevated temperature on residual stress distribution and subsequent fatigue life of cold expanded fastener holes. According to the obtained results, creep stress relaxation occurs due to exposure to 120 °C for 50 h. FE results demonstrate a non-uniform residual stress relaxation regime through the plate thickness around the cold expanded hole and the fatigue test results show that the subsequent fatigue lives have significantly decreased.     

Residual stress/strain evolution due to low‐cycle fatigue by removing local material volume and optical interferometric data

Three experimental methods, based on optical interferometric measurements of deformation response to local material removing, have been implemented for residual stresses determination. Two first techniques are employed to characterize initial residual stress values and their evolution near welded joints of aluminium plates under low‐cycle fatigue. The hole‐drilling method gives high‐accurate dependencies between residual stress components and number of cycles. The second approach comprises cracks modelling by narrow notches to describe residual stress distributions in more wide spatial range near the weld. The results demonstrate residual stress evolution is of complex character and cannot be uniquely qualified as a gradual relaxation. Besides, the secondary hole drilling method is developed and used as a fast and reliable tool to quantify the redistribution of residual strains near cold‐expanded holes due to low‐cycle fatigue. Dependencies of circumferential residual strains along the secondary hole edge versus number of cycles are constructed.     

Edge distance effects on residual stress distribution around a cold expanded hole in Al 2024 alloy

Cold expansion process is a well-known technique for improving the fatigue life of aerospace structures by introducing a compressive residual stress around the fastener holes. However, there are concerns about the residual stress distribution around those holes which are located near the free edges of structure. The purpose of this study is to investigate the influence of edge distance ratio (e/D) on the residual stress distribution around a cold expanded hole in Al 2024 alloy. A two-dimensional finite element simulation was carried out with various degrees of cold expansion and various values of e/D. It was found that for edge distance ratios less than e/D = 3, there are considerable effects on the residual stress profile. Also, the dependency of residual stress distribution on the degree of expansion was reduced significantly for small e/Ds. The results revealed that the bulging of the plate free edge increases with reduction of e/D but in worse cases the maximum bulging at the plate free edge was lower than 3% of the hole radius. The weight function method was then used for determining stress intensity factors for a crack emanating from a cold expanded hole.     

Numerical and experimental investigation of the cold expansion process with split sleeve in titanium alloy TC4

A series of uniaxial fatigue tests were carried out using specimens containing non-cold expanded and cold expanded holes to assess the effect of split sleeve cold expansion on fatigue behavior of titanium alloy TC4. The fracture surfaces of specimens were observed by scanning electron microscope (SEM). 3D finite element models were also used to analyze the residual stress fields around cold expanded holes. Based on the qualitative finite element analysis (FEA), the multi-axial fatigue lives of the non-cold and cold expanded holes have been predicted by Smith–Watson–Topper (SWT) method and Wang–Brown (WB) method respectively. The effects of the friction between the split sleeve and the hole’s surface were also considered. The results reveal that crack of cold expanded specimen always initiates near entrance face and the crack propagation speed along transverse direction is faster than along axial direction. The lowest compressive stress occurs at the entrance face where crack is preferentially initiated. The mandrel entrance face is the most sensitive region to friction between the split sleeve outer surface and the hole. After cold expansion, fatigue life of TC4 open hole was increased to 1.7–2.2 times. Compared with the result of SWT theory, the result of WB theory is more conservative and reliable.     

Characterization of residual stresses from cold expansion using spatial statistics

Residual stress fields from cold expansion have been widely used to extend the fatigue life of aircraft structures. However, the spatial statistical character of these residual stress fields has not been established and has not been incorporated in current analysis methods. The objective of this study was to establish a spatial statistical method to quantify the residual stress field around a cold expanded hole. A framework called the Spatial Analysis of Residual Stress (SpARS) was developed utilizing spatial correlation, response surface modelling techniques and statistical resampling methods to characterize the residual stress field. Our results showed that tolerance bounds on residual stress can be quantified using this method. We also demonstrated the SpARS method using recently published round robin case studies. The newly developed model will be useful for aircraft structural fatigue crack growth analyses to incorporate residual stress fields for extending inspection intervals for fatigue and fracture critical structures.     

芯棒锥面结构对孔冷挤压强化残余应力场的影响

为了在紧固孔周引入均匀的残余压应力,以延长紧固孔构件的疲劳寿命、提高其抗应力腐蚀性能,利用ANSYS有限元软件,建立了轴对称弹塑性有限元模型,对直接芯棒冷挤压强化过程进行了仿真,特别是对芯棒的前锥段曲线结构形式进行了设计与分析,研究了前锥段曲线形式对残余应力场分布的影响.结果表明:孔壁表面的周向残余应力分布复杂且不均匀,比较而言,外凸型正弦曲线型芯棒所产生的残余压应力沿孔壁深度方向分布更加均匀;几种曲线形式的芯棒在上表面近孔边区域均产生了径向残余拉应力,在孔的挤入段产生了轴向残余拉应力,但外凸型正弦曲线型芯棒在上述区域所产生的残余拉应力较小,且分布区域也较小.     

Finite element method and experimental investigation on the residual stress fields and fatigue performance of cold expansion hole

Cold expansion is an efficient way to improve the fatigue life of an open hole. The residual stress fields of cold expansion holes are vital for key components designing, manufacturing and fatigue properties assessment. In this paper, three finite element models have been established to study the residual stress fields of cold expansion hole, experiments were carried out to measure the residual stress of cold expansion hole and verify simulation results. Three groups of specimens with different cold expansion levels are examined by fatigue test. The fracture surfaces of specimens are observed by scanning electron microscope. The finite element method (FEM) results show, with interference values develop, the maximum values of circumferential residual compressive/tensile stresses increase in “infinite” and “finite” domain, and a higher positive stress values are obtained at the boundary of “finite” domain. The effects of the friction between the mandrel and the hole’s surface and two cold expansion techniques on the distribution of residual stress is local, which only affects the radial residual stress around the maximum value and the circumferential residual stress near the hole’s edge. Crack always initiates near entrance face and the crack propagation speed along transverse direction is faster than that along axial direction.     

A novel method of cold expansion which creates near-uniform compressive tangential residual stress around a fastener hole

A recognized way of improving the fatigue resistance of a fastener hole is to introduce compressive tangential residual stress around it. This can be achieved by using a cold expansion method in which an oversized pin or ball is forced through the hole to produce a local plastic region surrounded by an elastic one. Once the pin or ball is removed allowing the elastic region to spring back it results in compressive tangential residual stress around the hole. In practise, however, it is found that such a cold expansion method creates a non‐uniform residual stress distribution through the plate thickness and even tensile residual stress can be created at the entrance and exit faces. In this paper a new method of cold expansion is proposed. It uses a tapered pin with a mating tapered split sleeve and creates an almost uniform compressive residual stress around the hole as shown by FE method. Also, fatigue tests were carried out to verify that the method does significantly improve fatigue life. Finally the tangential residual stress distribution and fatigue life improvement of this new method were compared with those of a well‐established cold expansion method and it was shown that the new method is more efficient in improving fatigue life.     

开缝衬套孔挤压残余应力场数值计算研究

本文基于有限元技术模拟了开缝衬套挤压和直接芯棒挤压两种孔挤压工艺,对比研究了两种不同工艺导致的孔壁材料轴向流动和孔壁残余应力场。结果表明：开缝衬套挤压可有效抑制材料向挤出端流动,这在飞机夹层孔结构挤压中可减小夹层间隙尺寸;衬套开缝对应孔壁区域残余应力有突变,但仍然是对抗疲劳有利的压应力,而非拉应力;开缝衬套挤压挤入端孔边是压应力,而直接芯棒挤压是拉应力;相同干涉量条件下两种工艺引入的残余压应力峰值近似,但开缝衬套挤压残余压应力场域较直接芯棒挤压增大了约1 mm.     

Measurement of in plane residual displacement due to interference fitting by TV-holographic interferometry

Experimental measurements of the in plane residual displacement field surrounding the cold expanded holes in aluminium plates of 100mm × 100mm × 10mm were made using the technique of TV-holographic interferometry. A special cold expansion fixture was designed to produce precise radial interferences of 0.015mm and 0.025mm at the hole walls. The experimentally determined displacement fields were compared with those obtained by finite element predictions. The experimental results showed good agreement with those obtained by finite element analysis. These results demonstrate that the TV-holographic interferometry technique can be used as an effective technique for general deformation investigations.