Fastener Holes Research Articles

Fatigue life estimation of bolt clamped and interference fitted-bolt clamped double shear lap joints using multiaxial fatigue criteria

In this paper seven different multiaxial fatigue criteria based on stress, strain and energy were employed to estimate the fatigue lives of double shear lap joint specimens of aluminum 2024-T3 with bolt clamped and interference fitted-bolt clamped fastener holes. Detailed finite element (FE) simulations were conducted to obtain the stress and strain distributions in the joint to be used to as basic data in estimating the fatigue lives using multiaxial fatigue criteria. The estimated lives were compared with available experimental fatigue test results to investigate the capability of the criteria in predicting the fatigue lives. The results showed that the accuracy of life estimation for any criterion varies for different batches of specimens and applied load ranges. However, including the fatigue crack growth life can improve the fatigue estimation life and reduce the absolute error of multiaxial fatigue criteria.

Estimating crack growth in rotorcraft structures subjected to mission load spectrum

This paper is an extension of the work originally reported on the international conference of experimental mechanics. In this extended work, numerical and experimental results are presented from a project aimed at predicting the fatigue life of a rotorcraft structural component subjected to flight load spectrum. The structural component is a riveted joint used in cabin frame cap splices of several civilian and military helicopters modeled herein as a lap-joined nested angle assembly. This component is fatigue sensitive due to the highly cyclic and vibratory nature of a rotorcraft mission load spectrum and as such accurate prediction of its fatigue life is of major importance in the design cycle. In this work, numerical studies are conducted using constant amplitude and mission spectrum loads. The rate of fatigue crack growth for through-the-thickness crack initiated from the critical fastener hole is computed using 2D standard and weight function models with the crack plane stress field obtained from 3D finite element analysis. Effect of load interaction due to tensile overload is included using strip-yield retardation model and the effect of surface shear arising from contact friction between the plates and the fasteners is studied using parametric models. Finally, results of the numerical simulations are compared with representative experimental data obtained under similar spectrum loading condition.

A comparative numerical study of combined cold expansion and local torsion on fastener holes

ABSTRACTCold expansion and local torsion processes provide controllable strengthening mechanisms for a fastener hole and therefore have engineering significance. They rely on the residual stress and the accumulated shearing strain, respectively, which are difficult to measure. Due to the complex closed form solutions for these mechanisms, their numerical study is of great importance. In this work, a combination of the cold expansion and the local torsion on a fastener hole has been investigated numerically to evaluate the amount and nature of the total accumulated residual stresses around a fastener hole. Different cases of the cold expansion and the local torsion processes were modelled and studied by finite element simulation to investigate the existence of a loading case which produces a beneficial compressive residual stress field in the vicinity of a hole. Sensitivity of the final residual stress with respect to a range of process parameters including adequate diametral interference and angle of rotation was investigated.

A computer-aided specimen design to induce shearing strain around a hole

The local torsion process which involves an axisymmetric torsional loading around the design position of a hole in a structure has recently found important applications for grain refinement in tubes and fastener holes. The specimen design is a pre-requisite for any experimental investigation of the local torsion process. The purpose of this study is to investigate a number of potential designs which enable the implementation of the local torsion. These designs include a mandrel to twist the specimen and a range of connection configurations between the mandrel and the specimen including threaded, hexagonal and spline bindings. In order to understand the process requirements and for estimating the effect of design parameters, a coupled numerical analysis of the specimen is utilized, involving a non-linear finite element analysis using Abaqus. A sequence of computational steps with different boundary conditions at the interface is devised to simulate the coupled problem. The proposed configurations are compared for their abilities to perform the local torsion and produce the shearing strain within an annular zone around the hole. This allows investigation of the effectiveness of each design for the implementation of the local torsion. The distribution of strain accumulated around the hole in the radial direction is studied after carrying out the process. Finally, a specimen–mandrel assembly design is recommended to ensure that the process can be successfully implemented.

Numerical Analysis of Fastener Delamination/Disbond Arrest Mechanism in Aircraft Composite Structures

The effectiveness of using a fastener as a fail-safe delamination/disbond arrest mechanism in aircraft composite structures is investigated. This is accomplished by simulating the crack propagation behavior of a split-plate model with a fastener in the commercial finite element software Abaqus. In mode I, the fastener is capable of completely stopping the crack propagation; in mode II, the fastener provides resistance to propagation via three main mechanisms: 1) the fastener joint stiffness, 2) fracture mode transition, and 3) crack-face friction generated by fastener preload. The fastener hole clearance significantly reduces the effectiveness of the arrest mechanism by delaying the engagement of the fastener joint. Thisworkprovides the basis for the development of analyticalmethods for the design and optimization of crack-arrest features in composite aircraft.

Solution to the eddy-current induction problem in a conducting half-space with a vertical cylindrical borehole

The eddy-current induction problem in a conducting half-space with a vertical borehole is studied and a solution based on modal techniques is presented. Owing to the particular characteristics of the specific geometry, the straightforward application of the truncated region eigenfunction expansion method (a well-established tool for the analytical solution of such problems) presents significant difficulties. Therefore, a modified truncation approach has been adopted that treats the problem in two steps, each one based on the superposition principle. The approach proves to be very efficient and numerically robust. Results for the coil impedance are in excellent agreement with those obtained with the finite-element method. From the engineering point of view, the geometry simulates the eddy-current inspection of fastener holes in aircraft structures when coil scanning is performed from the surface. The presented solution constitutes an important step towards the development of a complete model for fastener hole crack inspections.

Structural health monitoring of fatigue crack growth in plate structures with ultrasonic guided waves

Fatigue crack growth in plate structures is monitored with ultrasonic guided waves generated from piezoelectric transducers. Cracks initiate in the vicinity of fastener holes due to cyclic in-plane loading. Ultrasonic guided waves that are partially obstructed by the fastener holes are investigated. Since fatigue crack growth increases the obstruction, these waves are effective for monitoring fatigue crack growth in a pitch-catch mode. The transmission coefficient (TC), which is defined essentially as the current-to-baseline amplitude ratio, and the transmission coefficient ratio (TCR), which is based on amplitude ratios from a single wave, are signal features used for crack characterization. The TCR is well suited for structural health monitoring. The excellent agreement between experimental results and finite element analysis of wave propagation corroborates the experiments. A sparse array of transducers is shown to effectively monitor a multifastener joint. The approach using obstructed ultrasonic guided waves has strong potential for prognostics-based structural health management due to the linear relationship between crack size and the TC.

Investigated on Fatigue Properties of Aluminum Ally 7050T7451 with Fastener Holes by Laser Shock Processing

Based on the FEM code ABAQUS and MSC.Fatigue, the process of LSP before hole-drilling was adopted to study the residual stress field of aluminum alloy7050T7451 with Fastener Holes after Laser shock processing (LSP), and the fatigue life of the specimens by LSP was analyzed in this paper. The results indicate that multiple laser shock processing can improve the residual compressive stress and fatigue life to a certain degree, and with the increasing number of shot, the strengthening effect gradually decreases. The ratio of the fatigue life of specimens treated by LSP to the fatigue life of untreated specimens is gradually decreased as the mean stress σm increases, and when the σm is 67.3MPa, the fatigue life of specimens treated by LSP advances 719%, compared with that of untreated specimens.

Evaluation of Eddy-Current Probe Signals Due to Cracks in Fastener Holes

Cracks in conductors are detected through changes in the impedance of a coil that induces current in the material. In order to gain insight into the physics of the inspection, we have developed a theoretical and computational model that predicts the signals due to cracks in circular cylindrical holes using a boundary element calculation. In formulating the problem, the electromagnetic field is decomposed into transverse electric and transverse magnetic scalar modes. The effect of a planar crack in an electromagnetic field is represented by an electric current dipole layer orientated normal to the crack surface. The dipole density is determined by the integral equation whose dyadic kernel ensures that the tangential electric and magnetic fields are continuous at the surface of the hole. Instead of solving this equation, a numerical approximation is found in the form of a discrete system of linear algebraic equations formed using either boundary and volume elements depending respectively, on whether the crack opening is negligible or not. Because the kernel embodies the interface conditions at the surface of the hole, a discrete approximation of the field is only necessary in the flaw domain which means that relatively few unknowns are needed. The probe impedance variation has been computed for both ideal cracks, defined as having negligible opening but impenetrable to current, and open cracks/slots. Open crack model predictions of coil impedance variations with position relative to a semi-elliptical axial crack are in good agreement with measurements.

A study of the effect of CPCs on fatigue crack propagation in a representative fuselage lap joint specimen

This paper investigates the effect of corrosion preventive compounds (CPCs) on the growth of cracks emanating from fastener holes in a typical (pressurised) fuselage lap joint. To this end we present the results of a series of fatigue tests in room temperature dry conditions as well as tests where the specimen has been treated with CPCs. The results of these tests reveal that although CPCs do not significantly affect the stresses in the joint they can have a detrimental effect on fatigue crack growth in a representative structural joint when tested in laboratory air.

Development of a Probabilistic Linear Damage Methodology for Small Aircraft

Arisk assessment evaluation of the continued operational safety of the general aviationfleet canprovide important insight to the criticality/severity of a potentially serious structural issue. As such, the methodology and a computer code, SMART|LD, were developed to address risk assessment and risk management of general aviation structural issues. This information will provide a proactive approach to enable a nonbiased review of data to assure airworthiness. To address variability and uncertainties in loading and material properties, a probabilistic methodology was developed and implemented that considers the random loading and probabilistic stress-life material behavior developed from constant amplitude tests. The stress severity factor is used to account for the geometry effects (notches, fastener holes, etc.). Failure is determined using the Miner damage index with the index calibrated from simulations of variable amplitude tests. Monte Carlo sampling is used to calculate the structural probability of failure, or mean and standard deviation of flights and hours to failure, the hazard function, and sensitivity analysis. A numerical example is presented to demonstrate the methodology.

The Surface Roughness Modeling on Turn-Milling Process and Analysis of Influencing Factors

Surface roughness plays an important role in product quality and has received serious attention for many years. It has formulated an important design feature in many situations such as parts subject to fatigue loads、precision fits、fastener holes and aesthetic requirements. In addition to tolerances, surface roughness imposes one of the most critical constraints for the selection of machines and cutting parameters in process planning. This paper focuses on developing a geometric model of turn-milling process in order to predict the surface roughness effectively so that we can offer reference or consulting in the practical machining process. The surface roughness model takes into account both cutting parameters and movement parameters such as cutter radius feed per tooth 、 cutting speed 、number of teeth 、cutters' eccentricity、 depth of cutting、spindle rotating speed and so on. Through this model, we discussed the influencing effects of various cutting parameters on the surface roughness and analyze which one has the most important impact.

Numerical Simulation of the Cold Expansion and Residual Stress Relaxation for Aluminum Alloy 7050

Cold expansion is a well-known technique for improving the fatigue life of fastener holes in aeronautical structures by introducing a compressive residual stress field around them. In this paper, a 3-D finite element model is used to analyze the residual stress distribution and relaxation around an expanded hole for aluminum alloy 7050. The results reveal that the cutting process of split sleeve cold expansion and creep are main reason for residual stress relaxation in room temperature, which may limit the beneficial effects of cold expansion.

Identifying the residual stress field developed by hole cold expansion using finite element analysis

ABSTRACTThe split sleeve cold expansion process is a cost effective method of enhancing the fatigue performance of aircraft fastener holes. However, the 3‐D nature of the induced residual stress fields is not fully understood. For this research, 2‐D and 3‐D models with uniform hole expansion and 3‐D models with expansion produced by contact with a rigid mandrel were developed. The models’ relative capabilities of capturing the residual stress fields were then evaluated. The residual stress profiles varied significantly through the thickness of the workpiece and were also strongly influenced by the direction of mandrel motion. Therefore the uniform expansion models were inadequate. The 3‐D contact models indicate that the mandrel entry face is the critical fatigue location, reporting the lowest circumferential compressive stresses adjacent to the hole. The effect of varying the frictional coefficient and plastic hardening laws were also investigated using the 3‐D contact models.

A combined upper-bound and elastic-plastic finite element solution for a fastener hole subjected to internal torsion

Fastener holes used in the mechanical joints are vulnerable to failure due to development of stress concentration at their edges. Inducing compressive residual stresses by different techniques has been the most common method to reinforce the holes to date. In this work, a new reinforcement technique called internal torsion, which can be classified as a localized severe plastic deformation process, is proposed as an alternative to the cold expansion pre-stressing. A special specimen is designed to represent the behavior of a typical fastener hole during the internal torsion process. The deformation of the specimen in the vicinity of its hole surface is studied by introducing a parametric kinematically admissible velocity field (PKAVF) within the deformation affected zone (DAZ). Calibration of the parameters in relation to the deformation of the material during the process is done by an elastic-plastic finite element solution that was performed in ABAQUS for a specimen made of interstitial free (IF) steel. Numerical analysis of the deformation is carried out to understand the process and to estimate the optimum process parameters. Subsequently, the calibrated model is used in an upper-bound solution of the problem to estimate the torque–twist response of the specimen during internal torsion. Finally, the results of upper-bound solution are compared with those of finite element analysis. There is a good agreement between the upper-bound solution and finite element results, which verifies validity of the calibrated velocity field model and the upper-bound solution based on the model for the internal torsion problem.

Fatigue crack growth monitoring in aluminum using acoustic emission and acousto-ultrasonic methods

SUMMARY Damage monitoring, failure prognostics, and remaining service life prediction represent great technological challenges for reliable maintenance of aeronautical structures. Consequently, there is a whole variety of non-destructive evaluation techniques that are available to ensure the quality of the metallic structures. The majority of these techniques are able to detect defects such as discontinuities of surface or volume and the variations of section and are applied to discrete intervals. The application of these techniques proves too long and generates high maintenance costs. This paper proposes experimental methodologies to monitor fatigue damage growth in real time and also use a physics-based model for fatigue life prediction. The aluminum alloy samples, with inserted pre-cracks in the fastener holes, was tested mechanically in fatigue tension–tension cyclic loading with follow-up of two complementary health monitoring techniques such as acoustic emission (AE) and acousto-ultrasonic (AU). The approach uses AE to detect fatigue crack initiation and crack growth in aluminum samples and also applies AU measurements in order to assess global health conditions and damage accumulation during tension–tension cyclic loading. Nasgro analytical fracture mechanic model was used to predict crack growth and to determine the number of the load cycles Nf required to grow the initial crack to final crack size ac. The results indicate that exploiting health monitoring data such as AE signals coupled with analytical physics-based models provides a convenient methodology to determine the fatigue life and to estimate safety factor on life of the materials tested. Furthermore, this paper demonstrates that AU flexural Lamb wave (A0) offers high potential to track damage before the occurrence of the first crack, such as fatigue damage caused by plastic deformation, and quantitatively to assess fatigue damage stages of the material. Copyright © 2011 John Wiley & Sons, Ltd.

Stress Intensity Factor Solution for Cracks at an Offset Loaded Fastener Hole

Stress intensity factors were obtained for radial and diametrical cracks at an offset (noncentered) loaded fastener hole. The new closed-form solution was derived by superposing and compounding several stress intensity factor solutions, including a new key stress intensity factor solution developed for cracks in a finite plate with in-plane bending loads. The accuracy of the new closed-form solutions was verified for various configurations against results found in the literature as well as finite element analysis results. Nomenclature = half crack length from tip-to-tip = length of crack i from the edge of the hole to the tip = distance between the edge and the center of the crack = edge distance of the hole = bearing to bypass stress ratio (

Automated Crack Detection for Digital Radiography Aircraft Wing Inspection

Radiography testing is one of the most important nondestructive testing (NDT) techniques for aircraft wing inspection. Traditionally, radiography inspection is time and manpower consuming work. In addition, human inspection of cracks and damage based on film radiography is very subjective, inconsistent, and sometimes biased. Digital radiography allows all the benefits of producing significant increases in magnification and image clarity, as well as efficient management of digital images. Therefore, it is desirable to develop a computer aided system to assist interpretation of radiographic images to increase the objectivity, accuracy, and efficiency of radiographic inspection. In this article, an automated crack detection algorithm using region of interest (ROI) minimization and improved Canny edge detection techniques is developed for aircraft wing inspection. Cracks of up to 2 mm long on the aircraft wing around the fastener holes are detected automatically.

Numerical analysis of adhesively bonded T-joints with structural sandwiches and study of design parameters

Adhesively bonded T-joints are extensively used in assembling sandwich structures. The advantage of adhesive bonded joints over bolted or riveted joints is that the use of fastener holes in mechanical joints inherently results in micro and local damages to the composite laminate during their fabrication. One type of adhesive joint in such structures is the T-joint between sandwich panels. The aim of this research paper is to study, by numerical analysis, the effect of fillet geometry and core material of sandwich panels on the performance of T-joints. The base angle of the core triangle (fillet) is the most important geometry parameter of the triangular T-joint. Nine geometrical models with different base angles of the core triangle are made to investigate the effect of the base angle on the performance of the T-joints. It should be mentioned that the base angle in the triangular foam is changed, so that the final volume of the filler is kept constant in all the cases. Different foams with different stiffness are used to model the core of the panels to study the effect of the core material of sandwich panels. To model the adhesive between joint components, contact elements and cohesive zone material models are used. Therefore, failure of adhesive and separation of joint elements can be modeled. Damage and core shear failure of the base panel are modeled by using a written macro-code in the ANSYS finite element method (FEM) program. The ultimate strength of the joint in each case is calculated by modeling adhesive failure and core shear failure of the sandwich panels. Finally, the results of FEM are validated by experimental results available in the literature. In general, the failure load predicted by the FEM is within 5% of the experimental results. The best angle of the core triangle was found to be 45°. Also, the results showed that by changing the core material of the sandwich panel, the joint failure load is also changed.

Investigated on Residual Stress Field of Aluminum Alloy 7050T7451 with Fastener Holes by Laser Shock Processing

Laser shock processing (LSP) is a new technique for surface strengthening of fastener holes. The process of LSP before hole-drilling was adopted. A finite element model was established to study the effects of laser shock parameters on the residual stress field of aluminum alloy7050T7451 with Fastener Holes. The results indicate that increasing the laser power density until a fixed value results in a large peak in the hole-edge surface residual compressive stress. The hole-edge surface residual compressive stress and the depth of residual compressive stress are both increased with the increase of laser pulse width. Multiple laser shock processing can improve the residual compressive stress greatly, and with the increasing number of shot, the strengthening effect is gradually diminished.