Lug Joint Research Articles

An Advanced Nonlinear Framework for Early Detection and Prognosis of Fatigue Cracks in Plate-Like Structures

Plate-like structures are prone to the initiation of fatigue cracks during operation, primarily due to the harmonic loading imposed by the external environment. These micro-cracks possess the potential to progress into more severe forms, compromising structural integrity and, in extreme cases, leading to catastrophic accidents. The primary focus of this research is to introduce an online nonlinear framework for the early diagnosis and prognosis of fatigue cracks. This framework aims to detect nascent fatigue cracks in plate-like structures, facilitating proactive measures to prevent structural failure and preserve the overall integrity. The initial phase of this study involves capturing Lamb waves generated by subjecting the inspected plate-like structures to an excitation signal. The excitation signal, along with the S0 mode Lamb wave, is employed to identify a Nonlinear Autoregressive with Exogenous inputs (NARX) model, serving as a representation of the underlying nonlinear dynamic mapping intrinsic to the inspected specimen. Subsequently, the Nonlinear Output Frequency Response Functions (NOFRFs) are identified based on the established NARX model. An early fatigue crack damage index, denoted as optimal NOFRFs weighted contribution rate based on the KL divergence (KLRm), is extracted from the NOFRFs and employed for estimating crack lengths. Additionally, the Particle Filter (PF) is utilized for predicting the Remain Useful Life (RUL). To validate the effectiveness of the proposed framework, fatigue experiments are conducted on aluminum lug joint specimens. The experimental results demonstrate the effectiveness and precision of the framework in estimating crack lengths and predicting RUL for nonlinear early fatigue cracks.

Experimental Study of the Role of Gap Size on Bolt Failure in Clevis and Lug Joints

Abstract A common standard for conservatively evaluating the required bolt strength in a clevis–lug joint is to assume that the bolt must survive under an interaction equation that combines loading caused by bending, shear, and axial preload–induced stresses. In practice, this leads to unnecessarily large bolts because the true failure mode is typically closer to simple shear with some bending stresses. In this experimental study, a clevis and lug joint with a variable gap between the clevis arms was used to determine the force necessary to break high strength bolts with various gaps between the clevis and the lug, ranging from pure shear (no gap) to a gap 2.8 times the diameter of the bolt. The testing results clearly show that failure occurs at forces that are often significantly higher than would be predicted by classic, closed-form analysis of the combined loading. In particular, the bolt failed at the force predicted by simple double shear failure theory for gap sizes up to half of the bolt diameter, and for larger gaps, the failure force was consistently higher than would be predicted by classic interaction equations. Thus, although assuming failure that is due to combined bending, shear, and axial stresses is safely conservative, the necessary bolt size is actually significantly smaller than this method predicts.

Effect of cold-working on corrosion induced damage in lug joints

Lug joints are preferred joineries for transferring heavy loads to parent components in aerospace vehicles. They experience corrosion due to environmental conditions, improper surface finishes and rubbing displacement between the pin and lug-hole. This causes damage of different sizes and shapes near the lug-hole. Stiffness degradation due to corrosion-induced damage is modelled as a through-pit at one of the identified critical locations through stress analysis. The effect of this pit on fatigue crack initiation life is estimated. Lug-hole is pre-stressed by cold-working and the benefits of inducing plastic wake on the intended performance of the lug joint during the damages due to corrosion are brought out and compared with non-cold-worked lug-hole. Numerical analysis is performed on this lug joint with press-fit. The results obtained highlight the benefits of cold-working and the methodology can be extended to damage growth and analyse the effect of surface treatments for better structural integrity of components of aerospace vehicles.

Analysis of separation mechanism and life enhancement study of lug joint with interference fit fastener

Analysis of separation mechanism and life enhancement study of lug joint with interference fit fastener

Study on Non-linear Contact Stress and Prognosis Analysis of Lug Joints With Misfits

Components of aircraft and other large scale structures are generally made in parts in order to facilitate easy manufacturing, assembly and dis-assembly. Lug joints with fasteners are preferred in several aerospace applications where their purpose is to connect primary load carrying members. Study on structural integrity of these types of joints becomes mandatory from the safety point of view. This requires in-depth understanding of science and technology of Structural Health Monitoring (SHM) at these joints. Analysis presented here deals with the critical locations of interference and clearance fit lug joints which continuously operate under fatigue loading. Both interference and clearance fits exhibit non-linear contact-stress variation above certain load level and this effect needs to be addressed for both fatigue and monotonic loading. The maximum Pull or Push loads causes high stress concentrations at critical locations along the pin-hole interface. With continuous usage of the component, crack like damages initiate at these locations and they grow till failure of the component. The non-linear prognosis analysis is carried out to generate the data required for SHM at any stage which can be used to estimate remaining life of the component.

Trajectory planning method for docking of large aircraft components

PurposeIn the digital assembly system of large aircraft components (LAC), the docking trajectory of LAC is an important factor affecting the docking accuracy and stability of the LAC. The main content of docking trajectory planning is how to move the LAC from the initial posture and position to the target posture and position (TPP). This paper aims to propose a trajectory planning method of LAC based on measured data.Design/methodology/approachFirst, the posture and position error model of the wing is constructed according to the measured data of the measurement points (MPs) and the fork lug joints. Second, the particle swarm optimization algorithm based on the dynamic inertia factor is used to optimize the TPP of the wing. Third, to ensure the efficiency and stability of posture adjustment, the S-shaped curve is used as the motion trajectory of LAC, and the parameters of the trajectory are solved by the generalized multiplier method. Finally, a series of docking experiments are carried out.FindingsDuring the process of posture adjustment, the motion of the numerical control locator (NCL) is stable, and the interaction force between the NCLs is always within a reasonable range. After the docking, the MPs are all within the tolerance range, and the coaxiality error of the fork lug hole is less than 0.2 mm.Originality/valueIn this paper, the measured data rather than the theoretical design model is used to solve the TPP, which improves the docking accuracy of LAC. Experiment results show that the proposed trajectory method can complete the LAC docking effectively and improve the docking accuracy.

Finite Element Analysis of Lug Joint

Finite Element Analysis of Lug Joint

Structure design and reliability analysis of Al-Ti lugs

Aircraft lug joint is the key part of load transfer. In order to improve the safety of lug joint, on the premise of meeting the design requirements of static strength and fatigue, the composite connection lug structure design technology of different metal materials is proposed in this paper. Firstly, the damage safety design and life reliability analysis of the lug structure are studied theoretically. Secondly, based on the concept of damage safety design and the design principle of deformation coordination, the design method of composite connection lug with deformation coordination is proposed, and the thickness ratio of single ear is 0.8:1:0.8. Finally, the reliability of the composite lug is analyzed. The results show that the structural design scheme of aluminum-titanium composite ear piece can meet the requirements of static strength and damage tolerance, and compared with the conventional ear structure, the failure probability of structure mission life is greatly reduced when the weight of the composite connection lug is only increased by 4.9%. The proposed method can effectively guide the structural design of composite ear piece.

A study on prognostic analysis of attachment lugs under off-axis loading

PurposeLug joints with fasteners play a crucial role in connecting many major components of the aircraft. Most of the failures in the past were credited to the damages initiating and progressing from these types of joints. Ensuring the structural integrity of these fastener joints is a major issue in many engineering structures, especially in aerospace components, which would otherwise lead to fatal failure. The purpose of this paper is to adopting the prognostic approach for analysing these lug joints with fasteners subjected to off-axis loading by estimating the crack initiation and crack growth life of these joints. This data will be useful to estimate the remaining life of these joints at any given stage of operations, which is mandatory in structural health monitoring (SHM).Design/methodology/approachStraight and tapered lug joints are modelled using the finite element method in MSC PATRAN and analysed in MSC NASTRAN. These lug joints are analysed with a push fit fastener. The contact/separation regions at the pin–lug interface are carefully monitored throughout the analysis for various loading conditions. Critical locations in these lug joints are identified through stress analysis. Fatigue crack initiation and fatigue crack growth analysed is carried out at these locations for different load ratios. A computational method is proposed to estimate the cycles to reach crack initiation and cycles at which the crack in the lug joint become critical by integrating several known techniques.FindingsAnalysis carried out in this paper describes the importance of tapered lug joints, particularly when subjected to non-conventional way of loading, i.e. off-axis loading. There is a partial loss of contact between pin and lug upon pin loading, and this does not change further with monotonically increasing pin load. But during load reversals, there is a change in contact/separation regions which is effectively handled by inequality constraints in the boundary conditions. Crack growth in these lug joints pertains to mixed-mode cracking and is computed through the MVCCI technique.Originality/valueMost of the earlier works were carried out on in-plane pin loading along the axis of symmetry of the lug. The current work considers the off-axis pin loading by loading the lug joints with transverse and oblique pin load. The significance of taper angle under such loading condition is brought in this paper. The results obtained in this paper through prognostic approach are of direct relevance to the SHM and damage tolerance design approach where the safety of the structural components is of foremost priority.

Stress concentration and optimal design of pinned connections

A pinned connection or lug joint is a common connection type used both in civil engineering and mechanical engineering. In civil engineering, this connection is used for assembling truss members, and in mechanical engineering, this connection type is widely used in machine elements. The standard design is with a circular pin. The stress concentration factor size depends on the tolerances between pin and assembled parts and also by the three-dimensional design. Relatively different maximum stress values are seen depending on the modelling being done in two dimension (with assumptions) or in full three dimension. The focus in the present article is on the two-dimensional design and minimizing the maximum stress. It is shown that not only the contact geometry is important for reducing the stress, the external design is equally important. By finite element analysis including contact modelling, it is shown that reduction in the stress concentration factor of up to 18% is possible.

Fatigue Life Benefits of Cold Worked Holes in Fastener Joints

Lug joints with fasteners play a vital role in connecting major components of flight vehicles by acting as primary load carrying members. The structural integrity of these fastener joints and in particular fatigue life is of prime concern from the design point of view. Cold working of holes is one of the methods of achieving enhancement of fatigue life of these joints and this aspect is computationally addressed in this paper. The process of cold working is modeled, consists of introduction of a large diameter mandrill into the hole and withdrawing it leaving behind plastically stretched material around the hole periphery. Later one can use this hole with interference or clearance fit pins for better fatigue life. Material non-linear analysis is employed during the cold working process. These issues are addressed in this paper through computational approach using FEM software MSC NASTRAN/PATRAN. The fatigue life is computed at the identified critical locations through stress analysis using modified Basquin’s equation with Morrow’s mean stress effect. Life of the lug joints before and after cold working of the hole are estimated and compared.

Fatigue Analysis of Wing-Fuselage Lug section of a Transport Aircraft

Lug joints are special type of pin joints, which are widely used for joining various parts of an aircraft, especially for joining wings to fuselage. During service, the lug type joints are subjected to fatigue loading and complete load transfer takes place through the pin. At the pin and lug interface, the combination of high stress concentration and fretting could potentially lead to crack initiation and then crack propagation under cyclic loading. Because of this reason the wing-fuselage lug joints are considered as most fracture critical components in the aircraft structure. To appraise the safety level of lugs under working conditions, fatigue crack growth and residual life data are required.In the present work, a computational model for estimating the residual fatigue life of attachment lugs has been proposed. The pin is assumed to have a larger stiffness than the lug. The pin is assumed to be fit in the lughole with zero clearance and no frictional restraint. Initially stress concentration effects in the loaded lug were determined by applying analytical and numerical methods. Stress intensity factor for the pin loaded lug with through-the-thickness emanating from the lug hole has been determined. Both analytical and numerical methods have been used for obtaining the stress intensity factor. Further, fatigue crack growth life for the cracked lug subjected to constant amplitude cyclic loading was estimated using the Walker’s crack growth model. Also the effect of different radius ratios (lug geometry) on the number of cycles to failure is studied. For the FE analysis MSC Nastran/Patran softwares have been used.

Tension failure assessment at lug hole edges

The lug is the critical component of lug joints, and one of its failure mode is the tension failure at hole edges. In the present paper, the fatigue crack growth simulation of a semi-elliptical crack located in the lug net cross-section, at one of the two hole edges, is carried out by employing a theoretical model based on the Paris law. The lug sizes, loading conditions and crack configurations are assumed in accordance with those related to some experimental tests available in the literature, performed on lugs of PolyMethylMethAcrylate under pulsating tension.

Finite Element Analysis of Fretting Fatigue Fracture in Lug Joints Made of High Strength Steel

Many structural applications are aiming for weight reduction by using high strength steel. In a lug joint the load is transmitted by a pin, which leads to a pressure distribution on the hole in the lug. When a lug joint is subjected to axial cyclic loading conditions, the stress distribution becomes multiaxial, i.e. a combination of normal and tangential stresses. In such loading case, a fretting crack initiates at the contact interface between the pin/lug connection which is followed by a fatigue crack propagation up to the final rupture of the lug. In this study, the fretting fatigue crack initiation and propagation in a pin/lug joint are simulated using multiaxial fatigue criterion and fracture mechanics, respectively. To do so, first a 2D finite element model is developed for obtaining stresses and strains at the contact interface in a pin/lug joint. Using the extracted data, fretting fatigue failure parameters are analysed. Next, the obtained stresses and strains are used to estimate the crack initiation lifetime using a fatigue multiaxial critical plane model. A 3D model is set-up to simulate the crack propagation using eXtended Finite Element Method (XFEM). Eventually, the predicted total fatigue lifetimes are compared against experimental observations taken from literature.

Fatigue crack monitoring of aerospace structure based on lamb waves and binary tree support vector machines

To overcome the difficulty in identifying the fatigue crack in key parts of aerospace structure, a kind of methods aimed to monitor the crack length based on matching pursuit (MP) method and binary tree support vector machines (BT-SVM) classification algorithm was developed. In this method, Lamb wave signals were decomposed into a linear combination of several Chirplet atoms by MP method, and then the matching parameters were extracted as feature vectors for training and testing in BT-SVM classification algorithm. At the same time, a lug joint model was created with a certain ratio and the effect of crack extension on Lamb wave signals propagation was studied. At last, fatigue loading experiments were carried out in lug joints and tail reinforced frames of aircraft. The results showed that this method can monitor the length of fatigue crack effectively, which presents a new approach for monitoring the fatigue crack.

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.

Prognosis analysis of tapered lugs with interference and clearance fits under fatigue loading

In aircraft structures, a variety of lug joints are casted extensively to couple primary structural components. These lug joints are normally fabricated by a single pin or bolt type of fasteners, which leads to a simple joint that is easy to assemble and disassemble. The lug can act as a pivot since clamping of the joint is normally not allowed or acceptable. However, the elastic stress concentration for attachment lugs is very high, and this high stress concentration at the edge of the hole influences crack initiation and crack growth life. The analysis of these problems becomes more complex when there is a loss of contact between the pin and the lug which ends up in contact stress non-linear moving boundary value problems. Integrity of these lug joints becomes primary concern, since their failure can cause a catastrophic failure of the structure. This has led to mandatory implementation of structural health monitoring (SHM) concept at these censorious locations during the design and operational phase of the flight vehicles. The interference fits in particular, tend to reduce the alternating part of the stress variation at critical locations at the expense of increase in mean stress associated with pre-loading condition. There is a reasonable premise for expecting a better fatigue strength and fatigue life from interference fits. This paper addresses prognostic analysis of both interference and clearance fit lug joints to estimate the residual life of these joints and feed into the analysis of SHM methodology.

Fatigue Analysis of Lug Joint in the Main Landing Gear

Fatigue Analysis of Lug Joint in the Main Landing Gear

A novel probabilistic approach for damage localization and prognosis including temperature compensation

The development of a reliable structural damage prognostics framework, which can accurately predict the fatigue life of critical metallic components subjected to a variety of in-service loading conditions, is important for many engineering applications. In this article, a novel integrated structural damage localization method is developed for prediction of cracks in aluminum components. The proposed methodology combines a physics-based prognosis model with a data-driven localization approach to estimate the crack growth. Specifically, particle filtering is used to iteratively combine the predicted crack location from prognostic model with the estimated crack location from localization algorithm to probabilistically estimate the crack location at each time instant. At each time step, the crack location predicted by the prognosis model is used as a priori knowledge (dynamic prior) and combined with the likelihood function of the localization algorithm for accurate crack location estimation. For improving the robustness of the localization framework, online temperature estimation is carried out. The model is validated using experimental data obtained from fatigue tests preformed on an Al2024-T351 lug joint. The results indicate that the proposed method is capable of tracking the crack length with an error of less than 1 mm for the majority of the presented cases.

Application of EWK and Modified X-W Fracture Models to Load-Bearing Lug Joints

In the aeronautic field, ductile structure usually undergoes local fracture under complicated service loading conditions, which may triggers collapse of a structure. As a result, it is very essential to study the mechanism of fracture initiation and propagation of ductile materials especially under complex stress states. In this paper, a comparative study of fracture patterns of a load-bearing lug joints structure under several loading conditions with two ductile fracture models is performed by using the commercial finite element platform ABAQUS/ Explicit through a user material subroutine VUMAT. The numerical comparisons using two ductile fracture models in predicting failure of load-bearing lug joints structure shows that modified X-W fracture model agrees well with the experimental observation under complex stress states.