Effect of detail design on fatigue performance of fastener hole

In the present study, a series of tests were conducted on aluminum alloy 2024 to investigate the effect of detail design on the fatigue behavior of fastener holes in specimens. Two types of detail designs were concerned. One was the mode of fastener holes (countersunk rivet or countersunk bolt), the other was drilling process (traditional air-drilling process or one step compound cutting process). The fracture surfaces were observed by means of an optical microscope. Finite element method (FEM) was employed to analyze the distribution of stress around the fastener holes. The results showed that crack always initiated at the hole edge where the stress concentration occurred. Crack initiation was induced by stress concentration. Crack initiation life accounted for 80% of total fatigue life of fastener holes. The fatigue life of fastener hole using countersunk rivet was longer than that using countersunk bolt. Contrasted to traditional air-drilling process, the fatigue life of fastener hole could be improved by 44–55% using one step compound cutting process. However, the dispersibility of fatigue life became increasingly severe when fatigue life was prolonged.     

Effect of defects on fatigue life of plates with fastener holes

The behavior of defects (inclusions and cavities) on fatigue life of plates with fastener holes under tensile loading has been analyzed. Special attentions have been paid on the influence of the size and location of defects on fatigue life of plates with fastener holes. Thirty‐five different finite element models of plates with different size and location of defects are established and the nominal stress method is used to estimate the fatigue lives of this models. The results show that there is a region whose center is the maximum stress point of the hole without defects. When the defect is located in this region, the influence of defects on fatigue life of plates with fastener holes is obvious. When the defects are far away form this region' center, the defects hardly influence the fatigue life of plates with fastener hole. The larger the size of the defect is, the bigger this region is. In this region, the larger the size of the defect and the shorter the distance between the defect and the region's center, the shorter the fatigue life of plates with fastener hole is.     

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.     

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S–N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.     

激光冲击处理对LY12CZ铝合金疲劳性能的影响EI

本文研究了激光冲击处理对铝合金疲劳寿命的影响，研究表明激光冲击处理可以大幅度地提高带有类似紧固孔铝板的疲劳寿命，初步讨论了提高寿命的原因。     

Fatigue crack propagation from a cold-worked hole

The cold expansion process is widely used to enhance the fatigue life of structures with fastener holes. Various studies assert that the cold expansion improves the fatigue strength of fastener holes; however, the improvement of fatigue life is difficult to quantify. Therefore, the influence on fatigue life of cold-worked process was studied by numerical and experimental tests. Then, a parametric study on material hardening behavior and Bauschinger’s parameter was performed for several loading conditions in order to determine their effect on crack growth propagation. The results of the numerical tests have exhibited a good prediction of the fatigue life of the component.     

The development of fatigue damage around fastener holes in thick graphite/epoxy composite laminates

This paper discusses the mechanisms by which damage develops and grows around countersunk fastener holes in composite laminates under fatigue loading conditions. Experiments have shown that the erosion of material between ply layers nucleates delaminations which then grow through the laminate under the action of the fatigue loads. The damage at the hole bore surface produces sites for the nucleation of delaminations. The morphology of the delaminations and ply cracking was mapped extensively and from these maps it was found that the volume of material around the fastener hole, damaged by the fatigue loading, adopted a characteristic shape; the volume of damaged laminate increased towards the faying surface of the laminate and (metal) fixture. This characteristic damage volume was generated by the fastener rocking under the fatigue loads. Growth of the delaminations has been shown to be preceded by intraply cracking and, as fatigue loading proceeded, more delaminations were generated at the hole bore surface. The interfacial region between the composite laminate and the fixture also provided the nucleation site for fatigue cracks in the fasteners. The effects of initial fastener-hole clearances on damage nucleation and growth did not appear to follow any clear pattern. However, coupons with excessive initial hole clearances did appear to exhibit greater than expected damage growth after only one sequence block.Several techniques for the measurement of damage growth (development) were investigated. Stiffness measurements of the test system were found to show only small changes with hole wear and fastener rocking (using shadow moire techniques) also showed only small changes with hole wear. Ultrasonic C-scanning methods were used to map the extent of damage around the fastener holes with fatigue loading.The experimental work has shown that damage development around fastener holes is a complex process, usually producing several delaminations in the region of the fastener hole which grow and may ultimately lead to the failure of the coupon.     

Investigation of the fatigue life of pre- and post-drilling hole in dog-bone specimen subjected to laser shot peening

The influence of processing sequence of laser shot peening (LSP) on the fatigue properties of fastener hole was investigated with finite element method and experiments. The results show that different processing sequences lead to different residual stress distributions and different fatigue lives. The compressive residual stresses (CRS) are squeezed into two-sided surface layers of fastener hole by two sided laser shot peening, and the ellipse CRS fields are found on both sided surfaces of sample. However, when the pre-drilling hole in dog-bone specimen is subjected to LSP, the tensile stresses appear at its mid-thickness region, while the CRS distribute in the entire thickness region of the post-drilling hole after LSP. The fatigue crack initiation of specimens treated by LSP stems from the subsurface layer of hole edge. The fatigue striation spacing of specimen with post-drilling hole after LSP is narrower in comparison with that of case with pre-drilling hole before LSP. The fatigue life of post-drilling hole is longer than that of the pre-drilling hole.     

Investigation of cold expansion of short edge margin holes with pre‐existing cracks in 2024‐T351 aluminium alloy

The United States Air Force has requirements to inspect and cold expand potentially thousands of fastener holes for an aircraft fleet, and the presence of existing cracks at those fastener holes is expected. Fatigue experiments were performed to investigate the resulting fatigue crack growth life of a fastener hole that contained a representative ‘unknown’ crack at the time of inspection (approximately 0.050 in. in length) at a short edge margin hole that was then cold expanded and compare that to a non‐cold expanded hole and a cold expanded hole with no pre‐existing cracks. The United States Air Force analytical approach used to account for the benefit due to cold expansion was compared to the experimental data and does not consistently provide conservative predictions.     

Mixed mode fatigue growth of curved cracks emanating from fastener holes in aircraft lap joints

The finite element alternating method is extended further for analyzing multiple arbitrarily curved cracks in an isotropic plate under plane stress loading. The required analytical solution for an arbitrarily curved crack in an infinite isotropic plate is obtained by solving the integral equations formulated by Cheung and Chen (1987a, b). With the proposed method several example problems are solved in order to check the accuracy and efficiency of the method. Curved cracks emanating from loaded fastener holes, due to mixed mode fatigue crack growth, are also analyzed. Uniform far field plane stress loading on the plate and sinusoidally distributed pin loading on the fastener hole periphery are assumed to be applied. Small cracks emanating from fastener holes are assumed as initial cracks, and the subsequent fatigue crack growth behavior is examined until long arbitrarily curved cracks are formed near the fastener holes under mixed mode loading conditions.     

RELAXATION OF RESIDUAL STRESSES AT COLD-WORKED FASTENER HOLES DUE TO FATIGUE LOADING

Abstract— Cold-expansion of fastener holes is now commonly used within the aerospace industry to increase the fatigue endurance of airframes. Although a number of methods of cold expansion are possible, the split-sleeve cold-expansion process is the most widely accepted and is frequently used in the repair and manufacture stages of both military and civil aircraft. In the present work, the redistribution of residual hoop stresses due to the application of constant amplitude fatigue loading at 4% cold-expanded holes has been studied. A modified Sachs method was adopted to evaluate the residual stress profiles and a replication technique was used to quantify crack growth. It was found that the decay of the residual hoop stress profile near the bore of the hole was due to the initiation and growth of small fatigue cracks. Cracks were found to initiate both near and below the fatigue limit, but subsequently arrested so stabilising the overall residual stress profile.     

Self-sensing time-domain reflectometry for detection of the bearing failure of a CFRP laminate fastener hole: effect of peeling of the insulator

Carbon-fiber-reinforced polymer (CFRP) composites are used in aerospace and automobile structures. For many CFRP structures, mechanical metallic fasteners are adopted. In internal structures such as a wing box, damage to CFRP structures around the fastener holes is difficult to see. A simple method of finding damage around fastener holes is thus required for CFRP structures. This study applies self-sensing time-domain reflectometry to detect bearing failure around fastener holes of CFRP structures. In a previous paper, the effect of fasteners was minimized when the micro-strip line (MSL) was placed apart from the fasteners. This study numerically and experimentally investigates the peeling of glass-fiber-reinforced polymer (GFRP) insulator. A bearing failure results in embossing at the edge of a fastener hole. The embossing produces a wedge that causes the peeling of GFRP, generating a gap between the GFRP and CFRP. The gap strongly affects the characteristic impedance of the MSL.     

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.     

A SIMPLIFIED RESIDUAL STRESS MODEL FOR PREDICTING FATIGUE CRACK GROWTH BEHAVIOR AT COLDWORKED FASTENER HOLES

This paper presents an investigation of the fatigue crack growth (FCG) behavior at fastener holes in a high strength steel, 30CrMnSiNi2A, before and after a cold-expansion process. The fatigue life of coldworked specimens was significantly increased compared to non-coldworked ones and increased the lower applied stress level. From a study of the residual stress distribution near the edge of the hole, it was found that the experimental residual compressive stresses in absolute values were much less than the calculated values derived by previous analytical methods. Thus, a simplified residual stress model for describing the FCG behavior at coldworked fastener holes is proposed, from which, the δK_eff and fatigue lives of specimens after cold-expansion can be predicted satisfactorily for engineering applications.     

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

In this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.     

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.     

Three-dimensional stress distribution around cold expanded holes in aluminium alloys

Aluminium alloy 6082 (HE30) and aluminium-lithium alloys 8090 and 2091 were examined after holes were cut and cold-expanded by means of an oversized mandrel. The extent of cold working was analysed by two techniques, namely neutron diffraction (ND) and the Sachs cutting method (SM). While the former requires neutron diffraction facilities and is a non-destructive measurement technique, the latter is based on the removal of small layers of material from the inside of the expanded hole while recording the apparent change in strain by the removal of the layer. The stress distributions analysed by ND are compared with the method of Sachs and the results are discussed with respect to distances over which the state of stress changes from compression to tension. These changes are significant in explaining the improvement of service life in mechanical fastener holes from the process of cold expansion.

In all alloys examined it was found that the improved fatigue performance was a function of the degree of expansion. All alloys exhibited a maximum in expansion beyond which fatigue life deteriorated. This was due to crack initiation during excessive hole enlargement exceeding the yield strength of the alloys. Crack initiation usually started from inside the hole of the fastener but always perpendicular to the applied load. The initial stages of growth occurred perpendicularly over a short distance but further growth occurred on a plane with normals inclined at about 55–60° to the tensile axis. This behaviour was due to the presence of texture development in the alloys which is beneficial to the resistance of both fatigue crack initiation and propagation.     

An experimental investigation on the effect of short time exposure to elevated temperature on fatigue life of cold expanded fastener holes

An experimental study has been carried out to investigate the effect of short time exposure to elevated temperature on fatigue life of cold expanded fastener holes. When cold expanded holes are subjected to temperature variations their fatigue life changes remarkably. However the exact governing mechanism has not been clearly addressed before. In this study, another stress relaxation mechanism rather than creep has been introduced which happens at a short time due to temperature rise. Results of fatigue tests on Al 7075-T6 show that the effect of this mechanism can be beneficial or detrimental. This research has sought to clarify this issue.     

Fatigue life prediction of friction stir spot welds based on cyclic strain range with hardness distribution and finite element analysis

The main aim of the present study is to investigate the fatigue behavior of single friction stir spot welds (FSSW) using strain-based modified Morrow’s damage equation. The correlation between microhardness, cyclic material constants, and mechanical strength of different zones around the FSSW are assumed to be proportional to the base material hardness. Experimental fatigue tests of friction stir spot welded specimens have been carried out using a constant amplitude load control servo-hydraulic fatigue testing machine. ANSYS finite element code has been used to simulate a single tensile shear friction stir spot welded joint, and non-linear elastic-plastic finite element analysis has been employed to obtain the values of local equivalent stress and strain near the notch roots of the joints. The results based on the numerical predictions have been compared with the experimental fatigue test data. It has been shown that the strain-based approach does a very good job for estimating the fatigue life of friction stir spot welded joints.