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.     

短波长特征X射线衍射法无损测定孔挤压强化A100钢内部径向残余应力分布

工件表层残余压应力场直接影响其疲劳性能。为研究孔挤压强化试样的内部残余应力状态,利用自主研发的短波长X射线衍射仪,无损地测定了2.4mm厚A100钢孔挤压件板厚中心层的内部径向残余应力场的分布。结果表明:孔挤压强化使工件内部引入了残余压应力,最大压应力位于距孔表面4.6mm处,为-302MPa;残余压应力场深度达30mm左右。     

不同背压方式对静液挤压过程影响的模拟研究

目的研究两种常用的背压方式在静液挤压工艺中对材料变形过程的影响。方法通过选取合理的背压工艺和压力数值,减小静液挤压过程中坯料的拉应力区域,降低材料开裂的风险。基于有限元分析(FEA)软件ANSYS,采用压力加载的方式对顶杆背压和静水背压静液挤压工艺中坯料内部应力应变分布以及材料流动性能的影响开展分析和讨论。结果两种背压方式均存在临界值,适当的压力范围可以抑制裂纹,并且可以获得较好的挤压样品;顶杆背压较静水背压更明显地抑制了轴向拉应力,但会导致其余两项拉应力的滋生;静水背压对三向拉应力都有一定的抑制效果。结论顶杆背压适合于在塑性材料的大挤压比加工中抑制表面纵向裂纹的萌生,也适用于塑性一般的材料的静液挤压变形;静水背压对三向拉应力都有一定的抑制作用,适用于加工变形能力较差的材料。     

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。     

孔强化对TC18钛合金疲劳寿命的影响

为提高TC18钛合金带孔零件的疲劳寿命,使用基体和焊缝上开孔的TC18钛合金试样,研究孔挤压和孔喷丸强化前后的表面残余应力,孔强化工艺对试样疲劳寿命的影响以及试样疲劳断口.研究表明,对基体和焊缝上的孔进行喷丸强化处理后,孔表面残余压应力值达到-300MPa以上,由于残余压应力和表面完整性的作用,孔喷丸强化效果比挤压强化...     

冷胀孔裂纹寿命增长效果估算模型

本文用含孔边穿透裂纹的冷胀孔、未胀孔剩余寿命之比来评价孔冷胀强化工艺延长裂纹扩展寿命的效果。本文提出把理论计算方法与实验测试结果相结合来确定残余应力场:根据实验测量结果确定冷胀孔塑性区半径:把塑性区半径代入残余应力场的理论计算式中求出残余应力场初形;根据最大残余应力值与材料屈服应力之间的规律关系(由冷胀孔残余应力测试结果得出的)修正已计算出的残余应力场。本文用冷胀孔单边穿透裂纹在等幅循环载荷作用下的裂纹扩展寿命试验结果检验了所提出的模型,此估算模型可较保守而又较充分反映孔冷胀强化工艺对孔边裂纹的增寿作用。     

发动机铸铁气缸盖热处理前后的残余应力对比

采用压痕应变法、全释放应变法和深孔(DHD)法分别检测两种材料4种型号的发动机铸铁气缸盖在热处理前后的残余应力,分析了热处理消除气缸盖残余应力的效果。结果表明：通过压痕应变法和全释放应变法测得热处理前后气缸盖表面的残余应力基本为压应力,两种方法测得的残余应力水平相当,除3号气缸盖外,其余气缸盖热处理后的表面压应力峰值基本没有下降;通过DHD法测得气缸盖深度方向存在拉应力,热处理工艺没有降低气缸盖深度方向的拉应力峰值;3号气缸盖可以采用热处理工艺消除表面残余应力,其余气缸盖热处理前残余应力的水平较低,无需进行热处理。     

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。     

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。     

电弧熔丝单层单道积材残余应力模拟分析及锤击消除研究

本工作利用SYSWELD有限元软件,建立了电弧熔丝单层单道积材三维模型,采用双椭球热源对熔丝积材过程的温度场、应力场进行了动态仿真模拟,分析揭示了工艺参数对残余应力的影响规律,。结果表明:在一定范围内,采用较低的线能量和熔丝积材速度、高预热温度,能有效降低残余应力。基于ABAQUS软件,将熔丝积材后的残余应力导入锤击模型中,模拟分析发现,锤击过程中材料产生的变形达到最大值后会发生微小的弹性回复,最后形成塑性接触变形。锤击后,焊缝及热影响区的残余应力极大减小,锤击表面一定深度范围内的残余拉应力转变为压应力。最后通过锤击试验,验证了锤击法能有效降低熔丝积材部位的残余应力,将一定深度范围内局部残余拉应力转为压应力。     

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.     

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.     

Identifying the residual stress field developed by hole cold expansion using finite element analysis

The split sleeve cold expansion process is a cost effective method of enhancing the fatigue performance of aircraft fastener holes. However, the 3‐D nature of the induced residual stress fields is not fully understood. For this research, 2‐D and 3‐D models with uniform hole expansion and 3‐D models with expansion produced by contact with a rigid mandrel were developed. The models’ relative capabilities of capturing the residual stress fields were then evaluated. The residual stress profiles varied significantly through the thickness of the workpiece and were also strongly influenced by the direction of mandrel motion. Therefore the uniform expansion models were inadequate. The 3‐D contact models indicate that the mandrel entry face is the critical fatigue location, reporting the lowest circumferential compressive stresses adjacent to the hole. The effect of varying the frictional coefficient and plastic hardening laws were also investigated using the 3‐D contact models.     

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.     

Numerical simulation of residual stress relaxation around a cold‐expanded fastener hole under longitudinal cyclic loading using different kinematic hardening models

Cold expansion of fastener holes creates compressive residual stresses around the hole. This well‐known technique improves fatigue life by reducing tensile stress around the holes. However, cyclic loading causes these compressive residual stresses to relax, thus reducing their beneficial effect. Estimation of the fatigue life without considering the residual stress relaxation might lead to inaccurate results. In this research, numerical studies were carried out using 2D finite element (FE) models to determine the initial tangential and radial residual stress distributions generated by cold expansion and their relaxation under cyclic loading. To predict the stress relaxation, four nonlinear kinematic hardening models were applied in simulation of stress/strain path. The results obtained from the FE analysis were compared with available experimental results. A good agreement between the numerical and experimental results was observed.     

A comparative numerical study of combined cold expansion and local torsion on fastener holes

Cold expansion and local torsion processes provide controllable strengthening mechanisms for a fastener hole and therefore have engineering significance. They rely on the residual stress and the accumulated shearing strain, respectively, which are difficult to measure. Due to the complex closed form solutions for these mechanisms, their numerical study is of great importance. In this work, a combination of the cold expansion and the local torsion on a fastener hole has been investigated numerically to evaluate the amount and nature of the total accumulated residual stresses around a fastener hole. Different cases of the cold expansion and the local torsion processes were modelled and studied by finite element simulation to investigate the existence of a loading case which produces a beneficial compressive residual stress field in the vicinity of a hole. Sensitivity of the final residual stress with respect to a range of process parameters including adequate diametral interference and angle of rotation was investigated.     

Effect of expansion technique and plate thickness on near-hole residual stresses and fatigue life of cold expanded holes

Cold expansion of fastener holes is a common way of improving fatigue performance of airframes. Among the several techniques applicable, the split-sleeve method is the most accepted in creating beneficial compressive residual stresses around expanded holes. In the present work, residual stresses at expanded holes in several types of aluminium plates produced by two different techniques, split-sleeve and roller burnishing, have been evaluated by the novel destructive Sachs method and then compared. It was found that stress distribution particularly at the vicinity of the hole was sensitive to the method of expansion and plate thickness, due to differing characteristics of the plastic material flow. Thus, secondary reverse yielding after cold expansion found to reduce residual hoop stresses at the edge of the hole, and excessive expansion above a limit, was thought to increase reverse yielding. S–N data revealed that no benefit was gained from expanding beyond this limit. It was suggested that the reduction in the number of cycles to crack initiation or more often to crack growth was due to increased reverse yielding at the vicinity of the expanded hole.     

Correlation of one-dimensional fatigue crack growth at cold-expanded holes using linear fracture mechanics and superposition

This work assesses the ability of linear elastic fracture mechanics (LEFM) with superposition to correlate the growth of one-dimensional fatigue cracks at cold-expanded open holes under constant amplitude loading. Care is taken in the work to accurately: control the test setup to ensure one-dimensional crack growth, determine residual stress in the coupons, measure crack growth, determine the fatigue crack growth rate (FCGR), compute stress intensity factors, and correlate fatigue crack growth rate with stress intensity factor range ΔK and stress ratio R. The work used long dog-bone coupons having a gage section 38.1 mm wide and a centrally located 7.09 mm diameter hole. The coupons were fabricated from 2.03 mm thick 7075-T6 sheet. The small coupon thickness and alignment of the loading fixture to eliminate bending resulted in one-dimensional crack growth. Residual stress due to cold expansion (CX) was measured using the contour method, as a function of position on the crack plane. Residual stress measurements gave typical results for the average residual stress field, with near-yield compression at the hole giving way to tension further out. Measurements on multiple coupons showed ±10% variability in residual stress. Crack growth behavior of multiple as-machined (AM) coupons (without CX) tested at R of 0.1 or 0.5 agreed with earlier results published in the literature. The scatter in lifetime, defined as the range of lifetime divided by the average lifetime, was less than 30% in the AM coupons. Crack growth behavior of multiple CX coupons tested at the same two applied stress ratios was consistent with predictions by linear superposition, where the predictions used a correlation for fatigue crack growth rate as a function of ΔK and R based on crack closure concepts and a piecewise log–log fit to FCGR versus ΔK_eff data from tests of non-residual stress bearing material and from the literature. Scatter in lifetime of CX coupons was 152% at R = 0.1 and 69% at R = 0.5. While the scatter in CX coupon lifetime is considerably greater than for AM coupons, it is found consistent with the observed 10% variability in residual stress. The work therefore demonstrates the ability of LEFM with superposition to accurately correlate the behavior of coupons with and without residual stresses.     

Effect of residual stress around cold worked holes on fracture under superimposed mechanical load

Cold working is one method used to enhance the fatigue life of holes in aerospace structures. The method introduces a compressive stress field in the material around the hole and this reduces the tendency for fatigue cracks to initiate and grow under superimposed cyclic mechanical load. To include the benefit of cold working in design the stress intensity factors must be evaluated for cracks growing from the hole edge. Two-dimensional (2D) finite element analyses have been carried out to quantify the residual stresses surrounding the cold worked hole. These residual stresses have been used in a finite element calculation of the effective stress intensity factor for cracks emanating from the hole edge normal to the loading direction. The results of the 2D analysis have been compared with those derived using a weight function method. The weight function results have been shown always to underestimate the stress intensity factor. A three-dimensional (3D) FEA has been carried out using the same technique for stress intensity factor evaluation to investigate the effect of through thickness variation of residual stress. Stress intensity factors calculated with the 3D analysis are generally higher than those calculated using the 2D analysis.