Pin-loaded Lugs Research Articles

Contact pressure analysis of pin-loaded lug with clearance

In engineering design, none of the existing models for calculating the contact pressure in pin-loaded lug is fully compatible with the actual working conditions. In order to obtain a more accurate formula for calculating the maximum contact pressure in pin-loaded lug, after verifying the reliability of the finite element model with PRESCALE pressure measurement film, single-parameter and multi-parameter analysis of two types of factors affecting the contact pressure distribution in the compressive surface of the pin-loaded lug were conducted in the fully elastic state. The influence of the relevant design parameters of the pin-loaded lug on its maximum contact pressure was obtained, and a formula for calculating the maximum contact pressure of the pin-loaded lug based on the Hertz stress distribution model was introduced. It is verified by a large number of finite element samples that the predicted values of the formula are in good agreement with the finite element solutions in the range of common scales for engineering design, and have high accuracy, which can be used for engineering design reference.

Prognostic analysis of fastener joints in straight attachment lugs

PurposeMany failures of aircraft structural components in the past were attributed to cracks emanating from joints, which are identified as the most critical locations. In cases using the recently emerging structural health monitoring (SHM) systems, continuous monitoring needs be carried out at many major joint locations. The purpose of this paper is to develop computational techniques for fastener joints, including the possible change in contact conditions and change in boundary values at the pin-hole interface. These techniques are used for the prognostic analysis of pin-loaded lug joints with rigid/elastic pin subjected to fatigue loading by estimating the residual life of the component at any given instance to assist the SHM systems.Design/methodology/approachStraight attachment lug joints with rigid/elastic push-fit pin and smooth pin-hole interface are modelled in commercial software MSC PATRAN. In each case, the joint is subjected to various types of fatigue load cycles, and for each type of cycles, the critical locations and the stress concentrations are identified from the stress analysis. Later, for each type of fatigue cycle, the number of cycles required for crack initiation is estimated. A small crack is located at these points, and the number of cycles required to reach the critical length when unstable crack growth occurs is also computed. The novelty in the analysis of life estimations is that it takes into account possible changes in contact conditions at the pin-hole interface during load reversals in fatigue loading.FindingsThe current work on fastener joints brings out the way the load reversals leading to change in contact conditions (consequently changing boundary conditions) are handled during fatigue loading on a push-fit joint. The novel findings are the effect of the size of the hole/lug width, elasticity of the material and the type of load cycles on the fatigue crack initiation and crack growth life. Given other parameters constant, bigger size hole and stiffer pin lead to lesser life. Under load controlled fatigue cycles, pull load contributes to significant part of fatigue life.Originality/valueThe analysis considers the changing contact conditions at the pin-hole interface during fatigue cycles with positive and negative stress ratios. The results presented in this paper are of value to the life prediction of structural joints for various load cycles (for both pull-pull cases, in which the load ratios are positive, and pull-push cycles, where the load ratios are negative). The prognostic data can be used to effectively monitor the critical locations with joints for SHM applications.

Computational Fatigue Analysis of the Pin-Loaded Lug with Quarter-Elliptical Corner Crack

In the present paper, mathematical models based on a new fracture mechanics methodology are developed for the failure analysis of an attachment lug with one/two quarter-elliptical crack(s) emanating from a hole. The strength of lug subjected to cyclic loading is theoretically examined through the following issues: the stress analysis, the estimation of life up to failure and the crack path evolution. The nonlinear stress field along the crack front is simulated by using the [Formula: see text]-integral method together with the finite element method. Furthermore, analytical and numerical methods are employed for the stress intensity factor calculation. The stress-ratio dependence crack growth model is applied in order to evaluate both the life up to failure and the crack growth path. The proposed models are validated through available crack growth data, and the comparison between different results is satisfactory.

Fatigue damage analysis of wing-fuselage attachment lug

In the present paper, the fatigue behavior of failure critical pin-loaded lug with the through-the-thickness crack at a hole is analyzed. Such an investigation examines the stress analysis and the residual life estimation through a computational model here proposed in which suitable fracture mechanics concepts are employed. Hence, the stress ratio dependence crack growth model is taken into account to assess the fatigue strength of damaged lug. Further, in order to calculate the stress intensity factor, fracture mechanics based analytical and numerical approaches are applied. Finally, to demonstrate the compatibility of developed model, relevant experimental data are used. Such crack growth observations show a fairly good correlation with obtained failure assessments.

Fatigue strength assessment of initial semi-elliptical cracks located at a hole

In the present paper, the residual strength of a pin-loaded lug and a finite plate, both with a semi-elliptical crack emanating from a hole, is examined. A new analytical methodology, based on Fracture Mechanics concepts, is proposed to analyse the crack propagation process in terms of life estimation and crack front evolution. Firstly, the stress field and the stress intensity factor are computed by applying both analytical and numerical approaches. Then, the two-parameter driving force model proposed by Kujawski is implemented for the fatigue life estimation and the crack front evolution. The validity of model here employed is assessed through the comparison between crack growth calculations and experimental data available in the literature, such comparison shows a quite good correlation for the crack and geometrical configurations here examined.

Fatigue failure analysis of pin-loaded lugs

In the present paper, mathematical models are proposed in order to analyze the strength of pinloaded lugs with semi-elliptical crack and through-the-thickness crack. The crack growth investigation of considered crack configurations tackles the fatigue life evaluation and the crack path simulation of semi-elliptical crack. The residual strength is estimated by applying the two-parameter driving force model. The numerical and analytical approaches are employed for the stress intensity factor calculation. Experimental fatigue crack growth data are used in order to verify efficiency of the developed models. A good correlation between fatigue crack growth estimations and experimental observations is obtained.

A study of contact non-linearities in pin-loaded lugs: Separation, clearance and frictional slipping effects

Pin-loaded lugs with bush fitting are widely encountered in industrial applications to connect parts and transmit loads and motions. Due to their mechanical function, frictional contact inevitably takes place on the pin–bush and lug–pin interfaces, and can lead to non-linear behaviours occurrence under monotonic or periodic loading, such as bush–lug contact separation, pin–bush conforming contact effects in the presence of initial clearance, or bush–lug frictional slipping mechanisms. The aim of this paper is not to present new results of finite element simulations for lugs involving contact with friction but to provide a comprehensive study of those contact non-linearities through a dedicated analytical contact model.

Maximum equivalent stress in a pin-loaded lug in the presence of initial clearance

Various design charts of ample validity and prompt access are presented, which permit the contact stresses within the lug of a pinned connection to be forecast in the presence of an initial clearance between the pin periphery and the lug bore. To cover the range of the practically encountered geometries and loadings, round-ended lugs of various widths and with a variously tapered shank are considered, and several inclinations of the applied load are addressed. The charts are compiled with the aid of finite elements. The employment of the recently proposed load factor Φ allows the combined effects on the peak contact stresses of the load intensity and of the initial clearance to be predicted.

Three-dimensional stress concentrations at circular pin holes in clearance-fit lugs

The influences of thickness and bonding clearance on stress concentration factors (SCFs) at circular holes in pin-loaded straight lugs are systematically investigated using the finite element method. The three-dimensional effect on stress concentration at pin hole is strong when the thickness B of lug is higher than the radius R of pin hole. The maximum tensional SCF K max normalized by its corresponding plane stress solution K p―σ increases with increasing B/R when B/R is higher than 2 for all of r/R (the radius of pin to that of lug), and also increases with decreasing r/R for a given B/R. It is also found that the plane stress SCF K p―σ nearly remains a constant when r/R < 0.98, but is strongly sensitive to r/R and increases by 30% with r/R changing from 0.98 to 1. On the other hand, the friction coefficient, Young's modulus and the load level have also influences on stress concentrations, which should not be neglected in design of structures. An empirical formula of the maximum SCF is obtained for convenience of engineering applications.

Analytical modelling for static stress analysis of pin-loaded lugs with bush fitting

Pin-loaded connections are widely used in the industry. Connecting rods of reciprocating engines in automotive applications or pin-loaded lugs of landing gears and airframe structures can be cited as examples. These systems are subject to thermomechanical cyclic loadings and fatigue mechanisms are often observed. The calculation of stresses in the lug is crucial in view of a fatigue lifetime analysis. Analytical modelling may be very helpful in the early design stage, by allowing fast parametric studies and the identification of the most influent parameters on the response in stresses. In this paper, we propose analytical models for bush fitting and pin-loading conditions, leading to the complete calculation of the stress distribution in the lug. In each case, the analytical solutions are compared with finite element simulations. In the last part, we perform a sensitivity analysis as an application of the presented analytical tools.

Parametric study of the elastic stress distribution in pin-loaded lugs modelled in two and three dimensions and loaded in tension

This paper describes a numerical study that investigates the effects of loading a lug with a pin. In this work, the loading direction is axial with respect to the lug. The models are both two-dimensional (2D) and three-dimensional (3D). The effects of a change in the level of pin interference, the amount of material around the pin hole, and the thickness of the lug are demonstrated and discussed. Through-thickness effects are shown to be appreciable, and a simple 2D model can give a potentially serious underestimation of the true stress levels which may be particularly significant when a joint is subjected to fatigue loading.

Maximum Equivalent Stress in a Pin-Loaded Lug Subject to Inclined Loading

The elastic maximum equivalent stress in a pin-loaded, straight- or taper-shanked, round-ended lug subject to inclined loading is investigated using the finite element and the boundary element methods, and, partly, photoelasticity. The influence of the load direction, of the ratio between the bore diameter and lug width, and of the shank taper angle are particularly addressed. In addition, the effects of the axial length of the lug trunk and, partially, of the initial clearance between the lug bore and pin are numerically examined in the presence of a loading variously inclined to the lug axis. An interpolating expression for the maximum equivalent stress as a function of the ratio between the bore diameter and lug width, taper angle, and load inclination angle is proposed.

Fatigue crack growth at stress concentrators under spectrum loading

Fatigue cracks initiate at stress raisers such as notches, discontinuities, and surface defects. Many of the field failures that indicate the presence of a fatigue crack at failure can be traced to crack initiation from one or more crack initiation sites and merger of cracks over a period of service. Substantial service life is spent in the growth of small cracks from an initial size of few micrometres before they coalesce and grow to critical dimensions that cause fracture. This paper summarizes research that was carried out in order to understand the kinetics of crack growth of small cracks at notches under simulated FALSTAFF service loading. This paper also presents a method used to understand crack growth kinetics in a pin-loaded lug joint through a crack-front-mapping technique.

Elastic Analysis of A Pin Loaded Lug

A numerical study is described of the elastic deformation of a pin-loaded lug. A boundary element model for engineering analysis of elastic deformation of the round ended lug is proposed. Computational results on the deformation pattern, stress distribution, effect of pin‐ligament interface, influence of pin interference, effect of the ratio of lug width W and height of lug, H ,t ohole diameter D are obtained and discussed.

A parametric study of the elastic stress distribution in pin-loaded tubes

This paper describes a numerical study of the stress distribution in pin-loaded tubes. The effects of (a) the distance from the pin to the open end, (b) the variation in pin diameter, (c) the variation of tube thickness and (d) the variation in pin fit on the radial and circumferential stresses around the pin hole have been found and discussed. It is found that there are considerable differences between the stresses at the inner and outer surfaces of the tube which arise because the region around the pin and towards the open end of the tube ovalizes. This ovalization is more pronounced for a small head distance, a smaller diameter pin, a thinner tube and a clearance fit. The results for the pin-loaded tube are compared with those for a pin-loaded lug and it is shown that there are many similarities in the stress patterns.

Boundary element contact mechanics analysis of pin-loaded lugs

In this paper, the boundary element method (BEM) for two-dimensional contact mechanics analysis is employed to obtain the elastic stresses in a straight-shank symmetric attachment lug subjected to single pin-loading in the direction of 0°, 22.5° and 45° from the symmetric lug axis. Several lug geometries are considered and the effects of friction and the different conditions of fit between the pin and the lug are investigated. Some results for the contact stress distribution around the periphery of the pin-hole are presented, as are numerical values for the stress concentration factor obtained for the cases considered. Where possible, the BEM results are compared with those available in the open literature and the agreement between them is excellent. The BEM study shows that for a given lug configuration and direction of loading, the normalized stresses, and hence the stress concentration factor, depend only on the friction condition and the extent of the contact area between the lug and the pin. It is shown further that if this contact area is also fixed, the stress concentration factor varies linearly with the coefficient of friction for the range of values considered.

A parametric study of the elastic stress distribution in pin-loaded lugs

This paper describes a numerical study of the stress distribution in pin-loaded lugs. The effects of : (i) the ratio of lug width ( w) to hole diameter ( d), (ii) the ratio of the distance of the hole centre from the free end of the lug ( h) to hole diameter ( d), and (iii) the pin interference/clearance, on the magnitude and position of the maximum circumferential stress around the hole in the lug are established and discussed. Pin fit is shown to be a particularly important detail and the beneficial effects of an interference fit are shown to be considerable. It is also shown that for ratios w/d greater than 4 and h/d greater than 3 there is little variation in the maximum circumferential stress for a given applied load.

Two-dimensional boundary element contact mechanics analysis of pin-loaded lugs with single cracks

The boundary element method for frictional contact mechanics analysis of elastic structures is applied to the problem of a pin-loaded, straight, symmetric attachment lug with a single edge crack at the pin hole. Stress intensity factor solutions are obtained for several lug geometries and relative crack sizes under a wide range of frictional contact conditions between the pin and the lug. In the case of the lug being subjected to an axial pin load, these solutions are presented for four different angular extents of contact, φ, between the two components at the pin hole. These values of φ cover the range of areas of contact which may be reached from clearance pin fit situations, to those which may be attained in initial interference pin fit cases. In addition, stress intensity factor results are obtained for the cracked lug when the pin load is applied in two inclined directions to the symmetric lug axis.

Cracks emanating from pin-loaded lugs

Pin-loaded lugs were analysed in the presence of cracks emanating from circular holes. The analysis presents a unified treatment of interference, push or clearance fit pins. Both metallic (isotropic) and composite (orthotropic) plates were dealt with. The finite element model used special singular six-noded quadrilateral elements at the crack tip. The non-linear load contact behaviour at the pin-hole interface was dealt with by an inverse technique. A modified crack closure integral (MCCI) technique was used to evaluate the strain energy release rates (SERRs) and stress intensity factors (SIFs) at the crack tips. Numerical results are presented showing the non-linear variation of SIF with applied stress, and the influence of the amount of interference or clearance and the interfacial friction on SIF.

The use of fundamental fields to obtain weight functions for mixed-mode cracks

A boundary element analysis has been devised for calculating mixed-mode weight functions applicable to crack problems. The weight functions are obtained as the solutions of highly singular numerical problems resulting from the application of fundamental fields to a crack configuration. Special subtraction techniques are described for dealing with the singularities so that accurate Mode I and Mode II weight functions can be obtained efficiently. Some results were obtained for a central slant crack and an edge slant crack in a rectangular sheet, and compared with those of other workers; they show good agreement. Finally, results obtained for a pin-loaded lug demonstrate the importance of considering the detailed load distribution in the evaluation of both Mode I and Mode II stress intensity factors.