Fastener Holes Research Articles

An FE analysis for assessing the effect of short-term exposure to elevated temperature on residual stresses around cold expanded fastener holes in aluminum alloy 7075-T6

A previous experimental study revealed fatigue life reduction in Al 7075-T6 cold expanded fastener holes exposed to 120 °C for 1 h. The obtained experimental evidence indicated a residual stress reduction associated with material softening at elevated temperatures, termed as thermo-mechanical stress relaxation. In order to identify and characterize the potential features of this phenomenon, FE analysis is carried out in this study and a detailed body of evidence is provided for occurrence of a time-independent thermo-mechanical residual stress relaxation around cold expanded fastener holes due to exposure to elevated temperature. The results of FE simulation demonstrate a good agreement with experimental results obtained earlier.

Observations of fatigue crack initiation in 7075-T651

This paper reports the results of a study of the initial stages of fatigue crack formation in the commercial aluminum alloy 7075-T651. Direct observations were made of the evolution of cracking in a notch under moderately high fatigue loads in a scanning electron microscope. The experimental conditions were designed to mimic the behavior of fastener holes in the lower wing covers of military attack aircraft, thus the loading parameters were such that local plasticity was expected. Constituent particle cracking was observed to be the sole cause of fatigue crack initiation in these experiments; particle debonding was not a factor. The iron-bearing constituent particles were the sites of nearly all of the cracks initiated by fatigue. Mg 2Si particles were not effective crack initiation sites. This was attributed to the difference in the moduli of the particles: the iron-bearing particles were harder than the matrix while the Mg 2Si particles were softer. A large fraction of the cracks in the constituent particles were present before any loading was applied, but a certain amount of additional constituent particle cracking was caused by the fatigue loading. A cracked constituent particle eventually gave rise to a crack in the surrounding matrix, but this process required the application of fatigue loads. No matrix cracks were observed prior to fatigue loading. Subsequent propagation of the matrix cracks also required additional fatigue cycling. These observations are being used in the development of detailed computer models of the fatigue process in 7075-T651.

The influence of the environment and corrosion on the structural integrity of aircraft materials

ABSTRACTFatigue results of 11/2 dog‐bone jointed specimens manufactured from 7075‐T6 aluminium alloy indicated that the application of corrosion preventative compounds (CPCs) at the faying surfaces slightly decreased the log mean fatigue life at 144 MPa, while the effect was not statistically significant at the higher stress level (210 MPa). The addition of the CPC also reduced fretting corrosion at the faying surfaces and shifted the fatigue initiation sites closer to the edge of fastener holes. Scatter in fatigue life was found to be associated with the location and size of the intermetallics at the initiation site. The presence of corrosion in the bores of the countersunk fastener holes reduced the fatigue life by up to one order of magnitude. Fatigue test results for dog‐bone specimens manufactured from 7075‐T651 and 2024‐T351 aluminium alloys indicated that the presence of exfoliation corrosion reduced the fatigue life under dry conditions, with a greater reduction under humid conditions. The application of a CPC to the corroded region eliminated the influence humidity had on fatigue life.

Study for the Design of an Eddy Current Array Probe for the Imaging of Aeronautical Fastener Holes

The design of an eddy current imaging array probe dedicated to the inspection of aeronautical fastener holes is presented. The probe aims at enhancing the characterisation of defects such as surface fatigue cracks as well as at simplifying the inspection procedures (limited probe displacements). In this paper, a general probe design featuring separate inducing and sensing functions is proposed and studied thanks to 3D finite element computations. Then, a first experimental validation of the structure is given. Finally alternative pickup sensor technologies are considered. A quantitative study of the influence of both the nature of the used sensors and their arrangement as part of a multisensor array is proposed and discussed.

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.

Computational methods in eddy current crack detection at fastener sites in multi-layer structures

Reliable detection of both surface and subsurface cracks around fastener sites continues to be a need for maintaining ageing aircraft structures. In this work, a comprehensive overview of both experimental and modelling work addressing eddy current crack detection around fastener holes in multi-layer structures is presented. This overview consists of a strategy for the application of computational methods along with case studies that demonstrate the role of modelling for this particular class of problems. The state-of-the-art on eddy current modelling applicable to low-frequency eddy current problems is reviewed. The role of modelling in feature extraction algorithm development, crack characterisation, and probability of detection evaluation is highlighted.

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.

Residual-Strength Assessment of Modified Transport Aircraft Fuselages

In this study, several combinations of fail-safe features were investigated to assess their effect on the damage-tolerance characteristics and residual strength of a typical military transport aircraft fuselage section composed of floating-frame construction. Detailed nonlinear finite element models were developed for a key area of a representative aft fuselage. These local fuselage models were refined to include longitudinal lead and multiple-site-damage cracks of various sizes. A global-to-local modeling procedure was used whereby kinematic boundary conditions applied to the edges of the local fuselage models were developed using internal field quantities from a finite element simulation of a structurally complete (global) fuselage for a given load case. A residual-strength assessment of the relevant aft section for load cases involving a combination of limit internal pressure and flight loads was conducted using highly refined finite element models for cases involving 1) no fail-safe features (i.e., the basic fuselage configuration), 2) inclusion of shear ties as part of a modified fuselage design, 3) inclusion of tear straps as part a modified fuselage design, and 4) inclusion of both shear ties and tear straps. The influence of discrete source damage and small-scale cracking at fastener holes (i.e., multiple-site damage) was considered in the analysis. The critical crack-tip opening-angle fracture criterion and plane-strain core concept were employed in performing large-deformation elastic-plastic stable-tearing analyses using three-dimensional fracture-analysis code and structural analysis of general shells code to predict the fuselage section's residual strength. These results suggest that shear ties and tear straps may be effectively used to increase the structural integrity of the aft fuselage. Key results from this study may provide useful information regarding the efficiency of incorporating fail-safe design modifications in the existing aircraft to enhance the residual strength of cracked sections of the fuselage.

A strain criterion for pull-through failures in crest-fixed steel claddings

Crest-fixed steel claddings made of thin, high strength steel often suffer from local pull-through failures at their screw connections during high wind events such as storms and hurricanes. Adequate design provisions are not available for these cladding systems except for the expensive testing provisions. Since the local pull-through failures in the less ductile steel claddings are initiated by transverse splitting at the fastener holes, numerical studies have not been able to determine the pull-through failure loads. Numerical studies could be used if a reliable splitting criterion is available. Therefore, a series of two-span cladding and small scale tests was conducted on a range of crest-fixed steel cladding systems under simulated wind uplift loads. The strains in the sheeting around the critical central support screw fastener holes were measured until the pull-through failure occurred. This paper presents the details of the experimental investigation and the results including a strain criterion for the local pull-through failures in crest-fixed steel claddings.

Feasibility of digital image correlation for detection of cracks at fastener holes

At present, detecting structural defects such as cracking and corrosion before they become critical is largely achieved by time consuming techniques such as eddy current and ultrasonic testing. These techniques require point-by-point scanning over the area to be tested. Digital image correlation could provide a cheaper and quicker testing technique. It works by correlating images of the structure surface in unloaded and loaded states taken with a standard digital camera, giving the displacement and strain fields. The specific case of a crack at a hole in an aluminium plate was investigated. It was found that the strain concentration around the crack tip is too localised to detect; however, the displacement jump across the crack could be seen. This technique allows the cracks to be detected and would allow rapid testing of a structure if it can easily be loaded.

Re-cold expansion process simulation to impart the residual stresses around fastener holes in 6061 A-T6 aluminium alloy

Cold expansion processes are widely used in aerospace structures to eliminate or delay fatigue crack nucleation and to improve fatigue life. Fastener holes, in which the fatigue cracks initiate from stress concentrations, are plastically expanded using a mandrel pulled through the hole. Cold expansion technology has been applied to enhance low-cycle fatigue performance in repair as well as production applications. Repair of aircraft structures is a key component to extend aircraft service life. Re-cold expansion process conditions such as the degree of cold expansion should be determined to impart the beneficial compressive residual stresses around the holes under tensile loadings. In this paper, a process simulation using three-dimensional finite element analysis is conducted to determine the residual stress imparted by re-cold expansion in the fastener holes under the external loading conditions. Three levels of re-cold expansion under three external loading levels are performed in this numerical investigation. It is shown that the re-cold expansion process with at least 6 per cent of the degree of cold expansion imparts deep residual stresses around the hole so that the resulting stress levels on the hole entry side remain compressive under applied external stress levels between 100 and 200 MPa. In addition, residual stress redistribution under various applied external stresses is discussed.

Mechanic treatment of fastener holes: The influence of compressive residual stress on the corrosion behaviour of aluminium alloys

AbstractCorrosion has a significant impact on the life cycle cost of military aircraft. Fleet experiences show that structural areas around joining elements (rivets, bolts) and faying surfaces are particularly sensitive to corrosion. During the repair currently performed the corrosion usually is blended out, the surface protection applied, the fastener inserted and a new top coat applied. Blending out corrosion often results in a reduction of fatigue life. To restore fatigue life the induction of compressive stress in the area affected is common practice. It is generally accepted that the induction of compressive residual stress extends fatigue life but its impact on the overall corrosion behaviour of alloys is discussed controversially in the literature. For this reason the influence of compressive residual stress on the corrosion behaviour was investigated in a joint experimental project. The compressive residual stress was introduced in different ways on laboratory scale and the specimens artificially weathered to simulate the effect of corrosion.

Simulating fretting contact in single lap splices

Cracks in riveted lap splices commonly nucleate in regions of fretting damage around the fastener holes. The crack location is dependent upon the rivet squeeze force and the clamped faying surface area around the holes since fretting damage occurs at the edge of contact. Being able to predict the amount of faying surface clamping from rivet forming and the peaks in edge-of-contact stresses associated with fretting contact will help in the understanding of splice fatigue failure. This paper describes the development of a 3D finite element splice model that predicts the variation of contact area and edge-of-contact stresses in a loaded splice. The prediction is made for four rivet squeeze force levels and both universal and countersunk rivet styles.

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.

Study of a Crack at a Fastener Hole by Digital Image Correlation

In this work the efficacy of using digital image correlation to determine stress intensity factors for a crack emanating from a fastener hole has been investigated. To this end a fatigue crack was grown in pure mode I from a 50 mm diameter hole in an Al 7010 alloy plate test-piece. This crack was then loaded elastically under several combinations of mixed mode (I + II) displacements. In each case, images of the sample surface before and after the deformation were recorded using a high resolution digital camera. The surface preparation consisted only of scratching the surface lightly with silicon carbide abrasive paper. The crack location and resulting displacements were then calculated using digital image correlation. The analytical displacement fields for a traction free crack under arbitrary loading conditions based on the Muskhelishvili’s complex function approach were fitted to the experimentally measured displacement fields and the mixed mode stress intensity factor was determined in each case. Good agreement with the nominal applied values was obtained. The uncertainty of the crack tip position has a major influence on the accuracy of the stress intensity factors and so the Sobel edge finding filter was successfully applied to experimental displacement fields to establish precisely the crack tip location.

Evaluating Drainage Characteristics of Water Resistive Barriers as Part of an Overall Durable Wall Approach for the Building Enclosure

Abstract The most recent model residential building code has been modified to require increased use of water/weather resistive barrier (WRB) materials in construction, and to require some means of draining water from the building enclosure. However, “drainage” performance is not defined, and the Code is unclear about which currently available WRB products and design approaches help provide a durable water-resistant exterior wall enclosure. The present work is a qualitative, “order of magnitude” study of the drainage characteristics of various types of WRB materials (felts, housewraps, drainage wraps, drainage boards, and furring strips), and is a “first step” toward developing an overall durable wall approach for the building enclosure. The ASTM E2273 drainage efficiency test was used in 40 wall assembly mockups to evaluate 11 WRB materials in 8 design configurations. Traditional WRBs and housewraps provided little or no drainage capability to the exterior wall designs tested. Drainage-enhanced housewraps provided an improved level of drainage, but they still retained water. The retained water can migrate through fastener holes to the underlying construction. Best drainage performance was obtained by using WRBs with furring, drainage mats, and profiled sheets (drainage boards). These overall results are in general agreement with similar research by others. These results can guide designers and builders in the proper selection and use of such materials, which should be used as part of an overall durable wall approach to protecting the building enclosure.

배열 압전 능동 센서를 이용한 볼트 구멍의 층간분리 탐지

영구히 장착된 배열 압전 능동 센서를 사용하여 복합재 보강판의 볼트 구멍에 있는 층간분리 손상을 탐지하였다. 다양한 신호처리 기법을 사용하여 국부적인 수직하중에 의하여 발생한 볼트 구멍 주위의 눈에 보이지 않는 작은 층간분리를 탐지하였다. 배열 압전 센서를 사용하여 진단신호를 생성하였으며, 응답신호를 측정하였다. 응답신호를 신호 처리하여 손상에 민감한 특성들을 추출하였다. 이 특성들을 사용하여 손상 지수를 계산하였고, 손상 지수를 사용하여 손상의 유무와 위치를 추정하였다. Delamination damage at a bolt hole in a composite stiffened panel was detected using a built-in piezoelectric active sensor array. Various signal processing techniques were used to detect an invisible small scale delamination around a fastener hole due to localized transverse loading. A built-in piezoelectric sensor array was used to generate diagnostic signals and to measure response signals. Then, the response signals were processed to extract damage-sensitive features. Damage indexes were calculated to estimate the severity and location of the damage from the features.

A Simplified Modeling Approach for Predicting Global Distortion in Large Metallic Parts Caused by the Installation of Interference Fit Bushings

Abstract One technique for improving the fatigue life of a fastener hole in a built-up metallic structure is to use an interference fit bushing. The installation of these bushings produces residual compressive stresses around the hole that improve the fatigue life. It also causes residual strains and therefore distortion of the part. The distortion is both in-plane growth of the part and out-of-plane deflection, or cupping. The resulting distortion can cause difficulty in downstream assembly, such as misalignment of fastener holes or a misfit of mating surfaces. For small parts with only a few bushings, it is possible to predict the distortion by explicitly modeling the installation process by using a finite element approach. However, for a large part with many bushings, it is not practical to build a model with a detailed finite element representation of each bushing installation. In this work, a simplified approach is presented for predicting the global distortion of large shell-like parts that is caused by installing many interference fit bushings. The simplified approach consists of representing the bushings with equivalent springs in a shell-based finite element model of a large part. In this approach, the equivalent springs are not constructed with spring elements that are commonly found in finite element codes. Rather, the equivalent springs are constructed with a number of shell elements in two layers. The properties of the elements that make up the equivalent springs are estimated from experiments on small test coupons. The same springs, with the same calibrated parameters, are then applied to the larger part of interest. Results indicate that this approach can be used to estimate the distortion that can be expected in a large part that is cause by the installation of interference fit bushings.

The effect of bolt clamping force on the fracture strength and the stress intensity factor of a plate containing a fastener hole with edge cracks

Research has been completed to establish the effect which the clamping force, resulting from torque tightening a nut and bolt, has on the fracture strength and the stress intensity geometry factor of a fastener hole containing a symmetrical pair of edge cracks. The work has involved the carrying out of a programme of experimental tests and also the conducting of a numerical study. The tests were carried out using specimens made from aluminum alloy, grade AL7075-T6 rectangular plate containing a central hole with fatigue propagated edge cracks. Three batches of specimens were produced, one without a bolt inserted (for the purpose as a benchmark) and two with nut and bolts fitted but with different tightening torques applied. The joint fracture strengths were obtained using a tensile testing machine. In the numerical investigation, a finite element package was used to model the three test specimen variants used and thereby establish their stress intensity geometry factors. The numerical analyses considered the effect of parameters such as the coefficient of friction between contacting surfaces and the magnitude of the remotely applied axial load. The results show that the bolt tightening torque, and hence the plate clamping force, has a significant effect on reducing the stress intensity factor, and thus the joint fracture strength compared to bolt-less specimens.

Ultrasonic detectability of potentially closed cracks from cold-worked holes under loaded conditions

Many non-destructive inspections for cracks rely on detecting ultrasound scattered or reflected from the crack. Recently, several methods for second-layer ultrasonic crack detection have been developed, sometimes using automated analysis methods to interpret the complex ultrasonic responses from multiple layers. In the validation of these techniques, the assumption is often made that fatigue cracks will reflect as much ultrasound as an artificially introduced EDM notch or saw cut. However, this is not always the case for 'closed' cracks, which can have a very low ultrasonic reflectivity. Of particular concern is the case of cold-worked holes, where a residual compressive stress field exists, extending to approximately one radius from the fastener hole. In addition, if a crack has grown under fatigue loads that include high peak stresses, there can be a closed region in the plastic zone at the crack tip. To complicate matters further, interference-fit fasteners apply an additional tensile stress, which may or may not counteract the cold-worked stress field near to the fastener, depending on the amount of interference. This problem was encountered whilst validating a two-layer ultrasonic inspection technique on real cracks in a real fatigue-test wing containing cold-worked holes. Inspections were performed at four different remote load levels by stress-jacking the wing. Whilst the ultrasonic indications from some of the cracks grew with increasing remote load, others did not. Some ultrasonic indications grew both towards the fastener and away from it, suggesting that both crack-tip closure and cold-worked crack-root closure were occurring. A possible reason for some cracks not growing is that they were already being held open by interference-fit fasteners. The aim of this project was to investigate and understand the effects of cold working, interference-fit fasteners and crack-tip closure on the ultrasonic detectability and sizing of cracks. In particular, information was required about the potential improvement in defect detectability by applying a remote tensile load to the structure to overcome the residual compressive stresses. Results will be reported of an experiment designed to grow fatigue cracks to a range of lengths at cold-worked holes in a single specimen, with interference-fit fasteners installed. These cracks could then be inspected ultrasonically at various remote tensile-load levels and the results compared with EDM notches in a similar specimen. Finally, the interference-fit fasteners could be removed and the ultrasonic inspections at the various loads repeated. Initial results have confirmed that ultrasonic indication size and amplitude both increase with remote tensile load but further results will be required to determine the dependence of defect detectability on remote load.