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.     

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.     

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.     

A new conception for enhancement of fatigue life of large number of fastener holes in aircraft structures

A new conception for increasing fatigue life of large number of fastener holes in aircraft structures is developed. It is accomplished by a new method, called friction stir hole expansion (FSHE). This method not only reduces labour and time consumption, but it also decreases the overall cost for processing a large number of holes in structures made of aerospace grade 2024‐T3 aluminium alloy. FSHE combines the advantages of friction stir processing with these of mandrel cold working methods in two ways: a micro effect, expressed in hole surface modification, and a macro effect, expressed by the introduction of beneficial compressive residual macro stresses. The effectiveness of the method has been assessed by fatigue tests. Finite element simulations have been carried out. It has been proven that the greater fatigue life of fastener holes, processed by FSHE, is a consequence of the obtained micro effect.     

Analysis of the Residual Stress around a Cold-expanded Fastener Hole in a Finite Plate

Abstract:  In this paper, we present results of residual stresses around a cold-worked fastener hole in a thin, finite plate. An analytical model, with plane stress conditions, elastic nonlinear–plastic loading and unloading taken into account, is explicitly presented and has been developed from existing analytical models and methodologies. The solution also accounts for elastic deformation of the mandrel, and is applicable to different sizes of hole and mandrel. A parametric study of the strain-hardening exponent, plate size, Bauschinger parameter and expansion ratio was carried out in the context of the present solution. A sample study of a plate subjected to 4% hole-cold expansion was performed to compare this model with other analytical predictions, finite element simulation and X-ray diffraction experimental results. A brief review of related issues is given. This work is intended to present completely the closed-form equations able to generate residual stress profiles with ease around a cold-expanded hole in a thin, finite plate where boundary conditions are at issue.     

孔挤压过程中孔边应力应变演化规律的试验与模拟研究

该文采用挤压棒直接挤压的方法对LY12CZ铝合金进行孔挤压试验,测试了在不同过盈量、不同润滑条件下挤压时挤压力和孔边应变变化曲线。建立三维有限元模型,对挤压过程开展了数值模拟。结合试验和模拟结果,揭示了挤压过程中孔边应力应变的演化和分布规律;挤压力的变化特征以及摩擦系数和过盈量对挤压力的影响;孔的深度方向切向应力在挤压过程中的分布及孔边残余压应力的来源与演变。模拟值与试验测量值吻合良好,验证了有限元模拟的可靠性。     

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.     

Theory of stress coining technique

This paper describes the process of stress coining that improves fatigue performance at the hole. In stress coining, residual compressive stresses are induced at the edge of the hole, the elastic-plastic boundary in the plate material is located slightly away from the hole, the minimum and maximum pressures which establish the residual stresses are determined, the residual stresses and their distributions in the partially elastic zone and partially plastic zone are analyzed, and the minimum and maximum mandrel sizes are determined as well as the lower and upper limits of the mandrel-hole interference in the mandrel design. Numerical solutions are given for varieties of different mandrel materials, plate materials, and hole sizes.     

Optimization of a cold-working process for increasing fatigue life

This paper introduces a method which allows the calculation of the optimal radial interference and the optimal mandrel shape on a cold-expanded bushing–hole connection commonly used in aerospace structures, in order to obtain the desired values of the residual stresses on the hole surface. This method has been developed by an extended campaign of FE analysis, planned and evaluated with statistical techniques on the basis of a previous presented closed-form method to determine the residual stress field. The method will give the possibility to reduce the zone of the hole surface subject to negligible residual radial stresses, obtaining also compressive residual circumferential stresses on the entire hole surface.An experimental axial fatigue test plan on aeronautical components with optimized cold-expanded bushing–hole connections and subjected to cyclic loads showed a substantial improvement in fatigue life.     

EFFECTS OF COLD EXPANSION OF A HOLE ON FATIGUE CRACK INITIATION LOCATION AND LIFE OF AN LY12CZ ALLOY

Abstract— In the present study the effects of cold expansion of a hole on the location of fatigue crack initiation and life of LY12CZ alloy sheets are experimentally investigated and a quantitative expression of fatigue crack initiation life presented. Test results and analysis show that there may exist an optimum amount of cold expansion, for which both the coefficient of the resistance to fatigue crack initiation and the threshold are increased, and that the direction of cold expansion has no appreciable effect on fatigue crack initiation life. Observations show that, after cold expansion the fatigue crack nearly always initiates on the final entry face from which the mandrel is introduced into the hole during the final cold expansion process. Therefore it may be thought that the entry face is less resistant to fatigue crack initiation. In addition, the assumption of equivalence of residual stresses and also the strain hardening conditions on both the entry and exit faces may be questionable.     

A literature survey on the stability and significance of residual stresses during fatigue

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near‐surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.     

Contact stress and residual stress in the nose rail of a high manganese steel crossing due to wheel contact loading

Fatigue failure of a high manganese steel crossing is related to its internal crack initiation and growth, which is affected significantly by the magnitude and distribution pattern of contact stress and residual stress in the crossing. Considering the actual service conditions of a crossing and the accuracy requirement for numerical calculation, a whole model of wheel/crossing/ties and a partial model of wheel/crossing are established using elastic‐plastic finite element method. The distributions of contact stress fields and residual stress fields due to wheel contact loading are studied. The effect of train speed on the residual stress in the nose rail is discussed. The contact stress field shows regular contours in the cross‐section of nose rail and decreases remarkably with increasing distance of the wheel‐crossing contact position. The maximum contact stress is located at the contact surface between wheel and crossing. The maximum residual stress is located at a position of 1.5‐2.0 mm below the surface of the nose rail, rather than at the contact surface of wheel and crossing. In a failed high manganese steel crossing, the dense cracks mainly were observed neither at the position of maximum contact stress (the contact surface between the wheel and the crossing), nor at the position of maximum residual stress (1.5‐2.0 mm below the surface of the nose rail), but around the depth of 0.8‐1.0 mm from the worn surface, which is between the position of maximum contact stress and the position of maximum residual stress. It indicates that the combined effects of the maximum contact stress and the maximum residual stress play important roles in fatigue crack initiation in the nose rail. The size of high residual stress region increases with the increase of the train speed. The maximum residual stress in the nose rail increases remarkably with the increase of the train speed.     

Reconstitution of fatigue crack growth in Al-alloy 2024-T3 open-hole specimens using microfractographic techniques

Fatigue is one of the main problems in the provision of service life and safety of aircraft structures. The menace of fatigue cracking is accentuated in areas of stress concentration, e.g., joints of structural components. An example is the fuselage where riveting is used. One of the techniques for improving the fatigue life of these connections is the cold expansion of the rivet hole. As part of a larger project on the fatigue behaviour of aeronautical structures, an experimental study of open-hole specimens in Al-alloy 2024-T3, with and without hole expansion, is presented. The residual stress field created by the cold expansion was experimentally assessed by using the X-ray technique and predicted by FEA. Fatigue tests were supplemented by SEM measurements of fatigue striation spacing along longitudinal and transverse directions in the crack surface of each specimen. Empirical models and fractographic techniques developed by Nedbal et al. are used for the analysis of the experimental data, and results of quantitative microfractography are presented. Crack tunnelling was quantified based on the reconstituted crack history and on the surface crack growth measurements.     

The effect of residual stress due to interference fit on the fatigue behavior of a fastener hole with edge cracks

The fatigue property of riveted lap joint is greatly related to the riveting-induced residual stress. However, an accurate study of the fatigue property considering the influence of residual stress quantitatively can be very difficult. A 3D interface element based on the virtual crack closure technology (VCCT) was developed to calculate the stress intensity factor (SIF) for through cracks at the hole edge. The riveting process was analyzed prior to the tensile test in the finite element code, so that the residual stress can be taken into account to get the eventual value of SIF. The result shows that, with the presence of fatigue cracks, the initial stress-strain state in the structure would change, especially near the crack tip, where great compressive stress can be found. The eventual residual stress cannot be derived by simply superimposing the riveting-induced residual stress with the crack-induced residual stress. The 3D-VCCT interface element shows strong ability to solve the SIFs. The FE analysis results agree well with the reported models both in the fatigue crack growth rate (FCGR) and in the shape of the crack front. However, when the crack is extremely short, not only the reported models, but also the present numerical model would fail. Besides, unlike Elber's model and Schijve's model, this study shows that the crack opening stress should not be a function of the stress ratio solely, but also with the crack length included.     

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.     

Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy – An experimental study

This paper presents the experimental results of fatigue life enhancement and the residual stresses around the cold expanded holes in Al 2024, a widely used aerospace alloy. Two techniques for cold expansion of holes, namely split-sleeve with taper pin technique and split-sleeve with ball technique were considered for comparison, as the former involves surface contact and the latter has line contact during expansion. The techniques were compared based on the fatigue life enhancement in the expanded holes, the induced and the residual stresses due to expansion. The holes were expanded by 2%, 3%, 4%, 5%, and 6% using INSTRON machine in both the techniques. While both the techniques resulted in improvement in fatigue life of the expanded holes, the taper pin technique yielded 200% higher fatigue life improvement than that obtained by ball technique. The induced residual stresses were measured by mounting strain gages of 0.2 mm gage length. These are drawn as a function of induced strain. In both the techniques residual stresses increased with increase in percentage of expansion until 5% and then decreased for 6% expansion. The increase in fatigue life at 5% expansion was 1.88 times and 5.3 times higher than that of the non-expanded holes for ball and tapered method, respectively. The corresponding improvement in taper method was greater than the non-expanded holes. While, it was observed that the residual stresses decreased with respect to the distance from the hole in both the techniques, the ball technique resulted in lower residual stresses than that of taper pin technique.     

Moiré Interferometry Assessement of Residual Stress Variation in Depth on a Shot Peened Surface

Abstract: The main goal of this work was the development of experimental techniques to measure in depth non‐uniform residual stresses, as alternative to the more conventional hole‐drilling method with strain gauges. The proposed experimental methodology is based on moiré interferometry. This high resolution field technique allows in‐plane displacement assessment without contact. Grating replication techniques were also developed to record high quality diffraction gratings onto the specimen’s surface. A laser interferometry setup was implemented to generate the master grating (virtual). The stress relaxation was promoted by blind hole‐drilling and the obtained fringe patterns were video recorded. Image processing techniques were applied to assess the in‐plane strain full‐field. A finite elements code (FEM), ANSYS^®, was used to simulate the stress relaxation process and to calculate the hole‐drilling calibration constants.     

Interference fit effect on improving fatigue life of a holed single plate

This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates made of a medium‐carbon forging steel (35 KB2), heat treated by quenching followed by tempering, up to a hardness of about 350 BH, obtaining a sorbitic microstructure. Tensile and impact tests showed an ultimate tensile strength of about 1100 MPa, a yield strength of about 1000 MPa, an elongation to failure of 15% and an impact toughness KV of 43 J at room temperature. Axial fatigue tests were performed on holed specimens with or without a pin, made of the same material, press fitted and still left into their central hole. The tension–tension fatigue tests have been performed with a stress ratio R = 0.1. The effect on fatigue strength was investigated both experimentally and numerically. Three different conditions were investigated by using open hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress‐life (S–N) curves pointed out an increased fatigue life of the interference fit specimens, compared with the open hole ones. The numerical investigation was performed in order to analyse the stress field by applying an elastic plastic 2D simulation, with commercial finite element software. The stress history and distribution around the interference‐fitted hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) because a residual and compressive stress field is generated by the pin insertion.