Fastener Holes Research Articles

Experimental Research on Tensile Strength of Out-of-Plane Composite Joints for Integrated Composite Cabins

An experimental study of tensile failure behaviors of L-shaped adhesive-bonded and bolted hybrid composite joints, which are designed to connect composite cabins, is performed. A set of rigid jigs was designed to fix the specimens and ensure to apply an axial tensile load. The damage processes, failure loads and ultimate failure patterns of the joints are obtained from the tests. Some indentation around the fastener hole and remarkable discontinuity departure between the top and bottom L-shaped laminates which leads to the catastrophic failure can be observed on the specimens. It follows that the discontinuity is the weak spot and required to be strengthened.

An analytical model for strength prediction in multi-bolt composite joints at various loading rates

This paper presents an enhanced analytical model to determine the complete load displacement curve of single- and multi-fastener composite joints. It is an extension of a previous spring-based method, which included the effects of bolt pre-load and clearance, but not local bearing damage. The model has advanced beyond the state-of-the-art as loading rate effects, in addition to bearing damage, are accounted for through a novel conic damage approximation function. Using the developed model, an accurate prediction of the load displacement response of single- and multi-fastener joints to complete failure can be obtained in a matter of seconds. The method is validated against experimental data and excellent correlation was observed. Further studies carried out using the model suggest that slight variations in the energy absorption characteristics at each fastener hole in a multi-fastener joint can significantly alter the bolt-load distribution in the joint.

Cold expansion effects on cracked fastener holes under constant amplitude and spectrum loading in the 2024-T351 aluminum alloy

The increased number of aging aircraft in operation today requires a deeper understanding of fatigue life improvement methods. This research focused on the fatigue life benefit from cold expanded holes with preexisting cracks approximately 1.270mm (0.050in.) long under constant amplitude and wing spectrum loading. Holes with preexisting cracks were tested to simulate the worst case scenario of a hole with a crack the size of the detection threshold, 1.270mm (0.050in.), present before cold expansion that was not found by Non Destructive Inspection. Test results were compared to crack growth models generated in AFGROW. At high stress levels the AFGROW models yielded non conservative results greater than 150% of the test demonstrated fatigue life.

A fundamental investigation on rotating tool cold expansion: numerical and experimental perspectives

In this study, we present the development of a novel technique for cold expansion using a rotating tapered mandrel that friction processes the cylindrical wall of the fastener hole and simultaneously cold expands it. The developed technique, named as rotating tool cold expansion (RTCE), is experimentally and numerically investigated. A 3D thermomechanical finite element model for predicting the compressive residual stress, responsible for delaying crack propagation from the edges of the holes, is introduced. The efficacy is that RTCE is assessed for varying degrees of cold expansion under different lubricating conditions at the tool–workpiece interface, such as dry, metal working fluid, and nanopowder. The plastic deformation combined with friction stirring at the tool–workpiece interface helps the RTCE in controlling the surface damage at entry and exit of the hole that is most often observed with the conventional cold expansion technique. Enhanced friction due to the nanopowder at the tool–workpiece interface helps in sustaining efficacy of the RTCE even at a higher degree of cold expansion which otherwise leads to surface damage with other mediums.

An experimental study of initial flaws in thin sheets of 2024 aircraft aluminium alloy

Purpose – One of the challenges in the prediction of fatigue crack growth is to identify representative initial flaws and defects that can cause fatigue crack initiation and subsequent crack growth. Representative initial flaws identified from this experimental study provided an essential input for the fatigue life assessment programme of the PC-9/A training aircraft currently in service. The paper aims to discuss these issues. Design/methodology/approach – This paper addresses this challenge with a critical literature review and experimental assessment of initial flaw types that may cause fatigue crack initiation, by fatigue testing and fractography analysis using optical microscope and scanning electron microscopy (SEM). Findings – With a focus on aluminium alloy (AA) 2024-T3 thin sheet, the results cover various discontinuities from microstructural constituent particles inherent from the material process to macrostructural defects and surface discontinuities (such as burrs and machining marks) introduced during the production of airframes. It was found that most fatigue cracks originated from the bore surface discontinuities of rivet holes in the PC-9 vertical stabiliser thin panels rather than microstructural material defects of AA2024-T3 inherent from the material process. Research limitations/implications – The experimental study has found that quantifying fatigue initial flaw sizes which resulted from poorly finished fastener holes with arbitrary discontinuities at the surface is a challenging topic. This topic is under the current investigation using a statistics based analysis of initial flaws in the prediction of fatigue crack growth. Practical implications – The results obtained from this experimental study provided an essential input for the empennage and aft fuselage recertification and life assessment programme for the PC-9/A training aircraft currently in service. Originality/value – This experimental study examined AA2024-T3 thin skin panels from two different PC-9/A aircraft. The post-test failure analysis using optical microscope and SEM found that machining defects dominate fatigue crack initiation that can result in subsequent crack propagation.

Combined and interactive effects of interference fit and preloads on composite joints

The combined and interactive effects of the bolt-hole fit conditions and the preloads of the fasteners on the load carrying capacity of single-lap composite-to-titanium bolted joints have been investigated both experimentally and numerically. Quasi-static tests of the hybrid joints with different fit conditions are implemented, and a three dimensional finite element progressive failure analysis model is proposed to predict the influences of the bolt-hole fit conditions and fastener’s preloads on the mechanical behaviors of the joints. Based on the experimental validated simulation method, a multi-factor, mixed levels orthogonal design table and the analysis of variance method are used to arrange the simulation conditions and to further study the interactive effects of preloads and fit conditions. Through the analysis of the results, for the researched double bolt, single-lap composite-titanium joints, it is found that: the effects of both the interference fit and the preloads change from positive into negative mode with the increase of the interference fit values or preload values; appropriate bolt-hole fit conditions and preloads can improve the bolt-hole contact conditions of the loaded joints, and then retard the fiber failures around the fastener holes, and increase the load carrying capacity of the joints eventually; the interactive effect of the bolt-hole interference fit conditions and preloads cannot be ignored and the parameters need to be considered together and synthetically as the joints are being optimized.

On the fatigue crack growth analysis of spliced aircraft wing panels under sequential axial and shear loads

The spliced wing panels on a surveillance aircraft are subject to sequentially applied axial and shear load cycles. This paper aims to propose an approach to address the fatigue crack growth under this scenario. Current understanding on fatigue crack growth behaviours was reviewed, focusing on crack surface interferences and fatigue crack growth mechanisms under non-proportional mixed-mode loads. A generic spliced plate was analysed using non-linear finite element modelling. The analysis revealed shear-induced-bearing at fastener holes, and predicted both shear-induced-KI and shear-induced-KII at a 0° crack emanating from the fastener hole edge. When the crack is short relative to the fastener hole radius, the shear-induced-KI is dominant therefore it has the potential to cause shear-induced mode I overload. As the crack grows longer, the shear-induce-KII increases, and depending on the load level, this may lead to both short-range acceleration and long-range retardation. Through the above analysis, the engineering problem of fatigue crack growth in spliced wing panels under sequential axial and shear loads was converted into a more generally understood problem of fatigue crack growth under cyclic mode I plus intermittent cyclic mixed-mode loads.

Kinematic modeling and parameter identification of a new circumferential drilling machine for aircraft assembly

Automated fastener hole drilling is a key technology for low-cost and high-quality assembly of aircrafts. In this paper, a new circumferential drilling machine for fuselage assembly of large aircrafts is introduced. In order to meet the required position accuracy of drilled holes, this paper focuses on the kinematic calibration of the machine in order to improve its positioning accuracy. A modeling strategy, which combines the Denavit-Hartenberg (D-H) method and a modified version of the Hayati-Mirmirani (H-M) method, is proposed to deal with the special kinematic structure of the arc-base drilling unit of the machine. Main issues in kinematic parameter identification such as definition of objective function, calibration data selection, acquisition of initial values, and setting of convergence criteria are also discussed. Experiments of repeatability testing and kinematic calibration have been performed, and the results show that the positioning accuracy of the arc-base drilling unit is comparable to its repeatability after calibration. This suggests that the proposed kinematic calibration method is effective. Actual drilling tests have been performed on a simulated aircraft fuselage after implementing the identified kinematic model in the machine’s control software. Position errors of drilled holes are within ±0.5 mm, which meets the requirement for fastener hole drilling in the fuselage assembly of large aircrafts.

Effect of laser shock processing on fatigue life of fastener hole

The fatigue properties of laser shock processing (LSP) on both side surfaces of fastener hole with diameter of 3 mm in the LY12CZ aluminum alloy specimens were investigated. The superficial residual stress was measured by X-ray diffraction method. Fatigue experiments of specimens with and without LSP were performed, and the microstructural features of fracture of specimens were characterized by scanning electron microscopy (SEM). The results indicate that the compressive residual stress can be induced into the surface of specimen, and the fatigue life of the specimen with LSP is 3.5 times as long as that of specimen without LSP. The location of fatigue crack initiation is transferred from the top surface to the sub-surface after LSP, and the fatigue striation spacing of the treated specimen during the expanding fatigue crack is narrower than that of the untreated specimen. Furthermore, the diameters of the dimples on the fatigue crack rupture zone of the specimen with LSP are relatively bigger, which is related to the serious plastic deformation in the material with LSP.

A Model for Predicting the Stress Concentration of Intergranular Corrosion around a Fastener Hole

This paper presents a model that predicts the stress field around intergranular corrosion. The stress analysis is conducted in ABAQUS via a Python input script, which is written in Igor Pro. The intergranular corrosion path is described using a Monte-Carlo Markov Chain based on the materials grain size distribution and probability that the corrosion will turn at a grain boundary junction. The model allows a complete analysis of the stresses resulting from intergranular corrosion around a fastener hole of any size. As fatigue initiation is most likely to occur at the highest stress concentration, this model gives an understanding of which of the features of intergranular corrosion are most critical and can allow for the development of beta solutions for crack growth. This model has been applied to 7075-T651 extruded aluminum alloy from a legacy era aircraft but can be readily applied to any material where the microstructure is known and can be described using a statistical distribution.

The Influence of Corrosion Pits and Cold Expanded Fastener Holes on the Fatigue Life Aluminium 7075-T651

The Royal New Zealand Air Force (RNZAF) utilised the split sleeve cold expansion process to increase the fatigue life of fastener holes in the wings of the C130 transport fleet. As part of the validation of the fatigue improvements offered by the process the Defence Technology Agency conducted a series of fatigue tests on cold expanded fastener holes in aluminium 7075-T651, including specimens with corrosion induced after the cold expansion process had been performed. This research conducted an analysis of fatigue crack origins and modelled the stress concentration factors generated as a result of the corrosion pits. These results were used to explain the differing fatigue life and s-n curves produced by corroded and non-corroded fatigue specimens and the location of crack initiation sites around corroded cold expanded fastener holes.

Review on the Fatigue of Composite Hybrid Joints Used in Aircraft Structures

The use of composite materials as a replacement for commonly used metals such as aluminium and steel are increasing in the engineering industry, particularly in the aerospace sector. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal to composites. This change allows for further flexibility in design and fabrication of various components and joints. There are three main categories of joints used in composite materials – mechanically fastened joints, adhesively bonded joints and the combination of the two called hybrid joints. In order to adequately understand the effectiveness of these joints, substantial testing and validation is required, particularly in the use of hybrid joints for real life applications. Static testing, load distribution and parametric studies of hybrid joints have been investigated by various researchers; however further work is still required in understanding the durability and fatigue of hybrid joints and ensuring that both the adhesive and mechanical fasteners can work together effectively in producing an optimum joint. Mechanical fastening alone in composite laminates is not a preferred joining method as they create high stress concentrations around the fastener holes. Adhesive bonding although has numerous benefits it is difficult to detect the bond defect particularly in cases where weak bonds can occur during applications and it is sensitive towards the environmental conditions. Thus hybrid joints are seen arguably as being more effective in joining composite components together and offer greater residual strength. Hence the performance, strength and long-term durability of these joints need to be further investigated and be applied to practical situations whilst assisting in repair certification.

Fatigue of Cold Expanded Open Hole Coupons with Pre-Existing Cracks

Fastener holes have a high stress concentration at the edge of the hole and are primary sources of fatigue crack initiation, resulting in widespread fatigue damage leading to fatigue failures in airframe structures. The split-sleeve cold expansion (SsCx) technology is a simple and cost-effective way to improve the fatigue resistance of fastener holes by the introduction of compressive residual stresses around the holes. An investigation was carried out by DSTO to quantify the effectiveness of this technology, in terms of fatigue life improvement factors on a typical airframe aluminium alloy. Open hole (zero load transfer) coupons were tested to failure in non-cold expanded and cold expanded conditions. Coupons were also pre-cracked to specified crack lengths at the open hole, and cold expanded or left non-cold expanded, and tested to failure. This paper will present the results of the initial phases of the experimental program, involving constant amplitude fatigue loading of open hole coupons with and without cracks. The fatigue life improvement achieved by the use of hole cold expansion technology will be presented.

A Benchmark Problem for Eddy Current Nondestructive Evaluation

Currently, there are three TEAM Workshop benchmark problems involving eddy current nondestructive evaluation configurations, Nos. 8, 15, and 27. Regarding the first two, they have ceased to constitute challenging problems since they can be routinely solved with existing commercial software. In this paper, we present precision impedance measurements in a configuration involving a system of plates with through holes and a crack and propose them for benchmarking numerical codes. The benchmark represents a simplified version of eddy current inspection of fastener holes in aircraft structures, provides a lot of experimental data, and overall constitutes a more challenging problem.

The Use of Cavitation Peening to Increase the Fatigue Strength of Duralumin Plates Containing Fastener Holes

An effective method for improving the fatigue life of Duralumin plates with fastener holes, such as those used in the construction of aircraft, is to introduce a compressive residual stress around the fastener holes. Cavitation peening is a novel peening method that uses the cavitation impact produced when a high-speed water jet is injected into a water-filled chamber. In this paper, Duralumin plate specimens with holes were treated by cavitation peening under various conditions, and the fatigue strength of the specimens was determined using a plate bending fatigue test. It was revealed that a compressive residual stress was introduced not only on surfaces perpendicular to the axis of the cavitating jet but also on the walls of holes which were parallel to this. It was found that a 51% improvement in fatigue strength could be achieved by cavitation peening. Note that this is first report demonstrating an improvement in the fatigue life of Duralumin plates with fastener holes by cavitation peening.

Optimization of C/D Ratio of Adjacent Pre-stressed Fastener Holes Using Finite Element Analysis

Pre Stressing is a process of introducing residual compressive stresses zone around fastener holes which minimizes adverse effects of cyclic tensile stresses and retards the growth of fatigue cracks originating from the material flaws or surface imperfections. The present investigation is aimed at optimizing the inter hole distance between two adjacent cold expanded holes on a plate, limiting deformation of the material between holes to the elastic range by applying stress below the compressive yield strength of the material. It is found that ratio of central distance between holes to diameter of the holes (C/D) has a significant effect on the stress contours induced for a given percentage of expansion. The plate is modelled as a thick cylinder and equations are derived for radial stress. Graphs for these equations are plotted in MATLAB. ANSYS is used to conduct finite element analysis(FEA) of the cold expanded Mild Steel plate with two holes. To find the optimum centre distance between the two holes, the distance between the two holes was varied from 2D to 3.6D in steps of 0.2D and its effects, studied. The stress patterns in the vicinity of the holes from the analytical and Finite Element Methods were found to be comparable, proving the correctness of the approach. The relationship between C/D and percentage of expansion is analyzed, thus, optimizing the percentage of expansion required for a given C/D.

Fatigue Crack Growth Life Prediction around Cold Expanded Hole Using Finite Element Method

Cold expansion process is widely used to enhance fatigue life of fastener holes in aircraft industry. The benefit derived from cold expansion treatment is attributed to residual stresses induced around hole. This work is aimed at establishing a simple procedure for predicting Fatigue Crack Growth (FCG) life around cold expanded holes. Cold expansion process is simulated for uncracked and cracked holes using Finite Element method. The effect of residual stresses is accounted to predict FCG life using Willenborg's model. As a result of cold expansion, significant retardation in FCG rate and life improvement is achieved.

Modelling the effects of intergranular corrosion around a fastener hole in 7075-T651 aluminium alloy

Intergranular corrosion (IGC) is potentially a major concern for the structural integrity of aerospace aluminium alloys; the crack paths and networks formed make it particularly difficult to predict its impact on structural integrity, even when the likely location of the corrosion is known. IGC is a particular problem for the Royal Australian Air Force AP-3C Orion aircraft, where it is causing a significant increase to maintenance loads due to the unknown impact on structural integrity.In this paper a brick wall-style computational analysis is combined with a python script to create a finite element (FE) model of a plate-with-hole containing a random IGC path. The brick wall style analysis was chosen over measuring actual corrosion paths as analysing a large number of paths takes significant amounts of material, which is at a premium as it needs to be taken from the aircraft. The FE model produced realistic stress concentrations at the bore of the hole and showed that an IGC path that deviates vertically along its length will produce a small but noticeable change in the local stress concentration.

A study into the interaction of intergranular cracking and cracking at a fastener hole

This paper describes the results of a study into a new class of multi-site damage problems, viz: the interaction between multi-layer intergranular cracks and cracks that grow from small naturally occurring material discontinuities at a hole. The nature of the intergranular cracks, which are perhaps better thought of as delaminations rather than cracks, are chosen so as to approximate those found at fastener holes in RAAF AP3C (Orion) aircraft which arise due to ingress of the environment down the holes in the wing structure. The particular form of the multi-site damage studied involves two intergranular cracks (delaminations) that (in the vicinity of the hole) divide the plate thickness into thirds) interacting with cracks that emanate from the bore of the hole in a 7075-T6 aluminium plate.

Mechanical and Microstructural Characterization of Aluminum 6061 Processed by Plane Stress Local Torsion

The process of plane stress local torsion (PSLT) has been recently proposed as localized severe plastic deformation to enhance the mechanical properties of the fastener holes. The PSLT process can create large shear strains and, consequently, substantial grain refinement in the vicinity of the hole with a gradient in the radial direction. It is of great interest and importance to investigate the effect of the PSLT process on a range of materials commonly used in the industry. An introductory investigation of mechanical and microstructural properties of Al6061 samples processed by the PSLT is performed in this work. The PSLT is simulated using the finite-element method, and the torque twist curves obtained from finite-element simulations are compared with experimental measurements. Microstructural changes and the extent of grain refinement are investigated by means of optical microscopy. It is shown that the PSLT is responsible for a considerable improvement in the mechanical properties within the deformation zone. The microhardness tests were used to measure the degree of enhancement in ultimate and yield strengths of the material processed by the PSLT.