Cold Expansion Of Holes Research Articles

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.

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.

Using XRD elastic and plastic strain data to evaluate the effectiveness of different cold-working techniques in aerospace materials

Generating compressive stresses in aerospace materials is an important consideration for enhancing fatigue life. Shot peening and cold expansion of holes are two techniques for imparting beneficial compressive stresses. X-ray diffraction is a direct method for measuring elastic strains. Diffraction peak widths are an indication of plastic strain. Elastic and plastic strains can be used to better assess the true condition of a component. This paper presents elastic and plastic strain information from shot peened and cold expanded aerospace materials. Examination of surface data showed which shot peened samples had the deeper layer of compressive stresses. Likewise, elastic and plastic strain data enabled successful ranking of the holes in terms of the maximum amount of cold working.

Practical Challenges Testing Coupons with Residual Stresses from Cold Expanded Holes

Abstract Cold expanding holes in metal components is widely used throughout the aerospace industry to extend fatigue life and damage tolerance. The beneficial residual stresses induced by hole cold expansion arrest or significantly retard the growth of fatigue cracks, and in doing so make fatigue or crack growth life prediction more difficult. Under low stress level/high cycle fatigue testing, residual compressive stresses have been shown to increase the fatigue strength of the material leading to an “infinite life” and consequent “run-out” of test coupons at the test hole or changing the location of failure away from the test hole. While advantageous to aircraft operators, it makes statistical life prediction difficult because the incomplete data have to be censored, leading to a knock-down of the beneficial life attained. Conversely, increasing stress levels toward material yield stress; life improvement factors are more definitive and tend toward unity. This paper reviews the derivation of residual stresses induced by hole cold expansion and discusses benefits to the structure/test coupon and how the residual stress field impacts fatigue life and damage tolerance/durability under cyclic load conditions. Challenges observed when testing coupons that have been processed using hole cold expansion including modification of the crack behavior and its effect on material microstructure, relocation of crack initiation, dynamics of the redistribution of stress and the crack growth mechanism as well as scatter of results, are also discussed with a focus on aluminum materials.

The effect of hole shape on the extent of fatigue life improvement by cold expansions

The cold expansion of circular holes is known to improve resistance to fatigue. In this study the effect of the cold expansion of a circular hole on fatigue life by means of a quasi-elliptical pin was investigated. Additional evaluations were conducted, including determinations of the effects of crack propagation from the hole. The major life extension was obtained through slower crack growth in the short-crack stage. The decrease in fatigue crack growth in cold-expanded specimens was related to higher crack-opening stresses which are a consequence of the presence of compressive residual stresses arising from cold expansion. In this study, an experimental investigation was carried out to quantify the effect of the cold expansion on the initiation and the propagation of the fatigue crack and was discussed. Fatigue life improvement of the cold-worked hole specimen was explained by determining the hardness results around the cold-worked hole. The results indicate that significant life improvements can be obtained through cold expansion applied with a quasi-elliptical pin in this work with the optimum results being obtained when the pin diameter is 4% larger than the diameter of the specimen hole. Also, a brief examination of the effect of the rivet shape on the fatigue life of a riveted specimen was carried out. To lengthen the fatigue life of a riveted plate which uses countersunk head rivets, the shape of the countersink and the rivet head were improved. The experimental results showed that the fatigue life of the riveted plate was improved where the improved rivet was used.

The use of digital image correlation in a parametric study on the effect of edge distance and thickness on residual strains after hole cold expansion

The paper presents the results of an experimental study that used digital image correlation to measure the residual strains created by hole cold expansion both before and after insertion of an interference fit fastener. The study used 7075-T6 aluminium test coupons with multiple fastener holes to more closely simulate a real aircraft structure where multiple fastener holes are often cold expanded sequentially and where the interactions between the holes are a function of both the hole-to-free-edge distance as well as the pitch between the fastener holes. Both hole-to-free-edge distance and coupon thickness were varied to measure their effects on residual strain after hole cold expansion and after hole cold expansion and interference fit fastener insertion. The results showed that as the edge distance was decreased, the tensile strains at the low edge side of the coupon increased exponentially for both thick (6.35 mm) and thin (1.59 mm) coupons.

The benefit from an adequate finite element simulation of the cold hole expansion process

The article presents outcomes from finite element and X-ray diffraction analyses of the cold hole expansion process fulfilled by means of solid mandrel gradually passing through the hole. Regardless of the manner of accomplishing – split sleeve; split mandrel; with a mating tapered split sleeve; spherical mandrelling – an axial gradient of the residual stresses is obtained due to the interaction between the layers caused by an axial force flow which passes through the mandrel, the workpiece and the support to forma closed circuit. To predict the fatigue life it is necessary to know the residual stresses on the entrance and the exit faces immediately to the hole periphery which are considered to be potential crack initiation sites. It has been proved that there are two key moments in the building of an adequate finite element model. The first is modeling of a realistic contact mandrel-workpiece with or without mediator and workpiece-support. The second is a suitable constitutive model of the workpiece material. Six hardening models of low-carbon steel obtained on the basis of a symmetric strain-controlled experiment and half-cycle test data from unidirectional tension experiment have been used consecutively in the finite element model of the process. On the basis of a comparison with experimental results obtained by means of an X-ray diffraction technique it has been established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle secures finite element results close to the experimental ones. The chosen finite element model has been compared with simplified ones following the residual circumferential normal stress criterion and its advantages have been proved.

A new 3D finite element model of the spherical mandrelling process

A new 3D finite element (FE) model of the cold hole expansion process by means of spherical mandrelling (SM) has been developed. The model has the following characteristics: the spherical motion of the tool has been defined by means of decomposition along static axes; interactions (normal and tangential contact with friction) defined in the tool–workpiece–supports system; nonlinear kinematic hardening of the workpiece material, defined on the basis of symmetric test with strain-controlled deformation to achieve a stabilized cycle. The actual spherical motion of the tool and the optimal value of the angle of nutation have been found through numerical minimization of the lost available work functional, which is equivalent to the minimum generated entropy. The FE model authenticity has been proved by means of comparison of the results for axial force variation with those obtained by experiment. The optimized FE model has been used for investigation of the residual displacements and stresses around cold expanded holes by means of SM. The FE results obtained have been compared with those obtained from other FE models.

Experiment and FEM Simulation on Residual Stress of Cold Expansion Hole

A 2D cylindrical plate model has been established to study the distribution of residual stress of cold expansion hole under different interference values. In addition, the effects of material models on residual stress fields are considered also. Experiments are carried out to measure the residual stress of cold expansion hole and verify simulation results. FEM results show, with interference values increasing, the higher residual radial and circumferential stresses are obtained. At same interference value, the residual stress of Hardening Material( HM ) model is much larger than that of Elastic Perfectly Plastic Material( EPPM ) model.

FEM Simulation and Experimental Test on Residual Stress of Cold Expansion Hole of Rectangular Plate

A 2D rectangular plate model has 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. The FEM results show, with interference values developing, the maximum values of circumferential residual compressive/tensile stress increase and higher positive stress values are obtained at the boundary of the rectangular plate. However, higher positive stresses at the boundary of plate are harmful to the capacity of fatigue resistance, the determinations of best interference value to a certain wide plate were proposed. Finally, the effect of the plate width on the residual stress distribution was analyzed.

Numerical study of double cold expansion of the hole at crack tip and the influence on the residual stresses field

Hole cold expansion (HCE) is an effective method to prolong the fatigue life of mechanical structures because it introduces residual compressive tangential stresses around the expanded hole. In the present work, experimental and numerical studies have been carried out in order to analyse the effect of the application of both single and double HCE procedures, on the subsequent fatigue life of 6082-T6 aluminium alloy. In the double HCE experiments, the second expansion was conducted both in the same direction (SESD) and in the opposite direction (SEOD) of the first expansion, employing a rigid ball of a diameter slightly larger than that used in the first expansion. The distribution of the residual tangential stresses resulting from the double expansion procedure is compared with that obtained after the application of a single expansion. The results obtained indicate that the SEOD method can give rise to an improvement in the fatigue life of the material.

Effect of cold expansion on fatigue performance of open holes

In this paper, residual stress measurements were made on aluminium alloy 2A12T4 open holes after cold expansion. By contrast, a finite element method (FEM) was also used to analyze the residual stress distribution around cold expansion hole. Then a series of uniaxial fatigue tests were carried out using specimens containing non-cold expanded holes and cold expanded holes to assess the effect of direct cold expansion process on fatigue performance of open hole. At last the fracture morphology of the specimens after fatigue test was examined by scanning electron microscopy. The results reveal that direct cold expansion technique could elevate about six times fatigue life. After cold expansion, fatigue crack of open holes always initiates on entrance face.

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

ABSTRACTMany 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.

Residual Stress Analysis Near a Cold Expanded Hole in a Textured Alclad Sheet Using X-ray Diffraction

In this paper we present the methods of determination and the stress obtained at the periphery of a cold expanded hole in a 2024-T3 Alclad aluminum alloy sheet. The measurements in the aluminum clad were performed by the sin2Ψ method, taking experimental precautions to deal with the texture effects. In the core aluminum a special method had to be implemented to determine the stress values in a direction not accessible to the X-ray diffraction. The strains were measured in sample orientations selected according to the texture characteristics and stress factorsF ij were used to calculate the stress tensor. TheF ij values were determined assuming a quasi-isotropic material behavior, after concluding that the stress results were not significantly affected by factors calculated for textured material. The residual stress profile, both in the clad and in the sheet, shows a nearly axisymmetric stress state. Compressive stresses were observed near the periphery of the hole, with values that are higher on the exit than on the entrance face. Residual stresses were also higher in the hoop direction than in the radial direction. They decreased with the radial distance to the hole and affected the previous stress state over a distance of 6 mm. The plastic deformation induced by the cold expansion is well evidenced by the FWHM values, which in the affected zone decrease with increasing distance from the hole edge.

Thermodynamic optimisation of cold expansion of holes by means of spherical mandrelling

The paper presents the outcomes of optimisation in the spherical mandrelling process (SM) employed for cold expansion of holes in order to create a zone of residual compression stresses around the worked holes. The multi-objective optimisation of the process is performed in two approaches: scalarisation of the vector optimisation criterion, performed by the reference approach (optimistic approach) and by an alternative (thermodynamic) approach. The vector optimisation criterion has been substituted by the generated entropy functional depending on the vector of controlling factors (process parameters). The generated entropy plays the role of a surrogate function of a feasible scalarised function, and therefore it is not necessary to scalarise the vector optimisation criterion. Optimisation is reduced to generated entropy minimisation. The effectiveness of the latter approach has been proved.

Editorial

Editorial

Progress in Thermoelastic Residual Stress Measurement

Abstract: The possibilities of using thermoelastic stress analysis to measure residual stresses are assessed, particularly in the context of the effect of plastic strain on the thermoelastic output. Components manufactured from aluminium and steel were deformed so that they experienced a plastic deformation and in some cases, a residual stress. Thermoelastic data from the components were compared with data from geometrically similar undeformed components. According to well‐accepted theory, the thermoelastic data from both components should be practically identical. The results from the deformed components show departures from those of the undeformed components; the possible reasons for this are discussed in detail. A practical application of cold‐expanded holes used in aircraft structures is described and recommendations are made for possible future work.

Elastic strains around cracked cold-expanded fastener holes measured using the synchrotron X-ray diffraction technique

In this work, the capability of the synchrotron X-ray diffraction technique to measure changes in the residual strains caused by fatigue crack growth from cold-expanded holes in an aluminium alloy plate has been investigated. The measurement technique was validated by comparing the synchrotron measurements around uncracked holes with those obtained on similar specimens using the neutron diffraction and Sachs' techniques. Measurements were performed with and without superimposed tensile mechanical loading and the results compared with analytical results. The improved spatial resolution of the synchrotron technique relative to the other methods has allowed the strain fields around a fatigue crack growing from a cold-expanded hole to be evaluated in the unloaded state and at two loading levels. Only at the higher load does the crack appear to open along its full length.

Residual Stress Creep Relaxation Around Cold Expanded Holes in an Aluminium Alloy

Residual stress distributions were measured in aluminum-alloy 2650 disks after cold expansion and after exposure to creep at 150°C for various times. Strains measured as a result of boring out the hole after cold expansion were found to exhibit significant angular variations around the hole. However, after exposure to 150°C residual strains and stresses showed less angular variation. To take account of the angular variation, the Garcia-Sachs technique for measuring residual stresses was employed. Surface residual stresses on the entrance and exit faces of the disks were measured using the x-ray method. The most significant relaxation of the residual stresses occurred within the first 100 h. Two simple models were used to predict residual stress relaxation, one using a uniaxial creep relaxation model and a second using tensile and isochronous stress-strain curves. A good comparison between predictions and experiments was observed.

Analysis of Residual Stresses Following Overloading of Cold Expanded Holes Using the X-ray Diffraction Technique and Finite Element Method

Cold working of holes is an effective method used to enhance the fatigue life of aerospace structures. The method introduces a zone of compressive residual stresses around the hole, which attenuates fatigue crack growth rates by reducing the effective stress intensity factor range. Therefore, the method reduces the tendency for fatigue cracks to initiate and grow under cyclic loading. Aircraft components are subjected to a complicated sequence of variable fatigue loading during service. Residual stresses around cold expanded holes can be relaxed because of the plastic deformation produced by in-service loading. We have studied the changes of residual stress distribution around a 4% cold expanded hole in 2024-T351 aluminium alloy, when the plate was subjected to different values of compressive overloading. Results from an FE model and X-ray diffraction measurements are presented and compared.