Fastener Load Research Articles

The influences of eccentricity on the fastener load and bearing strength of eccentric connection in aircraft structure

Purpose This paper aims to study the influences of eccentricity on the fastener load and bearing strength of the eccentric connection in the aircraft structure. Design/methodology/approach The special experiment is designed for the researches. The fastener loads of the eccentric connection are gained by using the derived formulas and numerical analysis, and the fastener load rules is verified by the experiment. The bearing strength of the eccentric connection is investigated by the experiments under different eccentricities compared with that gained from the experiment. Findings The study results are summarized as follows. Magnitude of the fastener load in the eccentric connection is greatly affected by distance from the fastener to the centroid of the fastener cluster and that from the fastener to the concentrated load. With the increase of eccentricity of the homolateral concentrated load, the fastener load increases, and difference of the fastener loads becomes larger, forming the short plate effect of the bucket. It means that fastener with the maximum load (the shortest plate of the bucket) leads to decrease of the bearing strength of the eccentric connection (the capacity of the bucket). Originality/value The investigation on the influence of eccentricity on the bearing strength of eccentric connection is firstly presented. The vector expression of the fastener load in eccentric connection is firstly derived. And the influencing mechanism of the fastener load on the bearing strengths of the different eccentric connections is demonstrated. The study results can provide guidance for the structure design of the eccentric connection.

Quantification of longitudinal fastener stiffness and the effect on fastening system loading demand

Over the past 20 years, there have been at least 10 derailments due to spike fastener fatigue failures in North America. These fatigue failures have been considered a moderate to severe challenge that require manual walking inspections that are both time and labor intensive. These fatigue failures have been found to result from spike overloading due to lateral and longitudinal loads. To date, there has been limited quantification of the vertical, lateral, and longitudinal fastener forces in track. This paper quantifies the effect of fastener type on fastener load to account for various track types and locations. Laboratory experimentation was performed to quantify the stiffness of multiple fastening systems and this data was input into a previously validated analytical model to quantify the effect of stiffness on fastener loading. Additional laboratory experimentation was performed to quantify the relationships between both fastening system type and vertical loading and spike strain. While the laboratory data indicate a significant variance in stiffness between fastening systems, the model results indicate that the load transferred to the fastening system is less sensitive. However, spike strain data indicate the load path was affected by fastener type and vertical load. The characterization of longitudinal stiffness of multiple fastening systems and the relationship to spike load as presented can be used to advance track mechanistic-empirical design and improve rail neutral temperature prediction and track buckling models.

Analytical Nonlinear Modeling of Rail and Fastener Longitudinal Response

Comparatively little attention has been given to the quantification of fastener demands, especially in the longitudinal direction. Research quantifying fastener demands is justified by the more than 250 FRA reportable derailments on mainlines and sidings in the United States caused by “defective or missing spikes or rail fasteners” over the last 20 years. Failed fasteners are rarely caused by loads acting from a single direction (vertical, lateral, or longitudinal); they occur from a combination of these loads. A literature review identified that though multiple models have been developed for analyzing track, they were not designed to quantify fastener demands, especially those in the longitudinal direction, and some make assumptions that could be improved on based on more recent research into the mechanics of fastening system behavior. This paper advances the mechanistic–empirical (M-E) track analysis and design approach through the development, validation, and application of a 3D nonlinear parametric track model that quantifies longitudinal fastener demands. Key research findings include: bilinear approximations, in combination with considering the interaction between vertical loads and slip, were necessary to accurately quantify fastener forces; ballast and fastener stiffness had a direct logarithmic relationship on fastener load; and for well-supported sleepers, changes in component resistance to slip produced minimal changes in fastener demands because the vertical applied load increased the required load to produce slip. Going forward, this validated model could be used to quantify fastener and track demands for additional loading and operational scenarios to further optimize component design for improved track safety and reliability.

Crack Propagation Life Prediction of a Single Lap Shear Joint: A Linear Elastic Fracture Mechanics Based Machine Learning Approach

Estimation of fatigue life in the aerospace industry plays a key role in terms of safety, certification, and inspection. Individual aircraft tracking programs aim to calculate the fatigue index for each aircraft based on its load history. Therefore, accurate estimation of fatigue life based on unique load spectra for each aircraft plays a crucial role. The purpose of this study is to predict the crack growth (CG) life of a single lap shear joint for an aircraft based on different spectral loads using random forest regression and k-nearest neighbors(k-NN) regression. A finite element model was built using the Huth fastener flexibility method to obtain fastener loads and stress values on the structure under maneuver loads. Then, finite element results were compared to analytical calculations. Based on Fighter Aircraft Loading Standard for Fatigue and Fracture (FALSTAFF) spectra, 90 different spectra were developed to be used for the calculation of CG life. The AFGROW software was used to calculate the CG life based on the load spectra and the findings of the finite element model. Analytical calculations of CG lives and features of individual load spectrums were fed to machine learning models. Finally, the CG life predictions of the machine learning models were compared with analytical calculations. According to the findings, a good correlation was observed between the analytical and predicted CG lives.

Analytical Elastic Modeling of Rail and Fastener Longitudinal Response

The rail fastening system plays a critical role in maintaining proper railroad track geometry by transferring vertical, lateral, and longitudinal forces from the rails to crossties. Broken spikes in elastic fastening systems have been linked to inadequate transfer of longitudinal loads, posing a safety risk for timber crosstie ballasted track. Longitudinal track demand caused by passing trains has been investigated in previous research, but the magnitude and distribution of longitudinal fastener loads is not well understood or documented. To address these track component failures and improve fastener design, this paper presents a validated analytical model that estimates longitudinal rail seat loads, advancing current formulations to focus specifically on the rail seat. The validated method was used to quantify the distribution and magnitude of longitudinal loads in both the rail and fastening system caused by passing trains. Further, this paper quantifies the effect of track stiffness, number of powered locomotives, and wheel spacing on these distributions and magnitudes. This information provides valuable insight into the specific type of spike failures that have led to at least ten derailments and the requirement of manual walking inspections on multiple North American heavy axle load railroads as detailed in this paper. Further, this method can be used to quantify the longitudinal fastener loads for different track conditions to advance the mechanistic-empirical track design philosophy for elastic fastening systems.

Failure prediction of irregular arranged multi-bolt composite repair based on finite fracture mechanics model

A numerical method based on the finite fracture mechanics model was proposed in this study to accurately predict the failure of irregular arranged multi-bolt composite repair. The stress distribution and the stress intensity factors required in the model were obtained using a two-stage analysis strategy, which can effectively lower the FE model scale and computational cost while keeping high computational accuracy. In the two-stage analysis strategy, the bolt load distribution was calculated using the global analysis method. After obtaining the fastener load from the first-level model, detailed stress and stress intensity factor analysis were conducted around individual fasteners and cutouts, which were then used to evaluate the failure behaviour of the composite panels. To validate the numerical method, tensile tests were carried out on three kinds of bolted composite repairs with circular, elliptical and oblong cutouts. Failure mode, failure location and failure strength predicted by the proposed numerical method were consistent with the experimental results, illustrating the effectiveness and applicability of the numerical method on predicting the failure behaviour of bolted composite repairs.

Research of fastener distribution for typical irregular connections of civil aircraft

In this paper, based on the analytical method of calculating simple and regular single shear connector distribution, based on the engineering formula, the flexibility coefficient of different fasteners, pin spacing, plate width and plate thickness is given. The deformation coordination equation between adjacent fasteners is established. The diameter of different fasteners is given by the equilibrium conditions of the binding force. The calculation equations of fastener load distribution under the spacing of fastener, plate width and plate thickness have been calculated. The finite element model for calculating fastener load is established in finite element software based on finite element method. The calculation results of finite element method and analytical method are compared, and the analytical method is proved to be used to calculate plate width, plate thickness, fastener diameter, and the analysis method. The effectiveness of pin load distribution for typical connection structures with constant spacing between Fasteners is also presented.

The Effects of Fatigue Cracks on Fastener Loads during Cyclic Loading and on the Stresses Used for Crack Growth Analysis in Classical Linear Elastic Fracture Mechanics Approaches

High strength threaded fasteners are widely used in the aircraft industry, and service experience shows that for structures where shear loading of the joints is significant, like skin splices, fuselage joints or spar caps-web attachments, more cracks are initiated and grow from the edges of the fastener holes than from features like fillets radii and corners or from large access holes. The main causes of this cracking are the stress concentrations introduced by the fastener holes and by the threaded fasteners themselves, with the most common damage site being at the edge of the fastener holes. Intuitively, it is easy to visualize that after the crack initiation, during the growth stages, some of the load transferred initially by the fastener at the cracked hole will decrease, and it will be shed to the adjacent fasteners that will carry higher loads than in uncracked condition. Using currently available computer software, the method presented in this paper provides a relatively quick and quantitatively defined solution to account for the effects of crack length on the fastener loads transfer, and on the far field and bypass loads at each fastener adjacent to the crack. At each location, these variations are determined from the 3-dimensional distribution of stresses in the joint, and accounting for secondary bending effects and fastener tilt. Two cases of a typical skins lap splice with eight fasteners in a two rows configuration loaded in tension are presented and discussed, one representative for wing or fuselage skins configurations, and the second case representative for cost effective laboratory testing. Each case presents five cracking scenarios, with the cracks growing from approx. 0.03 inch to either the free edge, next hole or both simultaneously.

Simulation of the Wing-to-Fuselage Assembly Process

The paper presents a simulation of the Airbus A350-900 wing-to-fuselage assembly process. The latter is a complex multistage process where the compliant parts are being joined by riveting. The current research analyzes the quality of the temporary fastener arrangement. The fastener arrangement is being checked to ensure that the residual gap between joined parts is small, and the fastener loads closing the gap are calculated. The deviations of the part shape from nominal are modeled via initial gaps. A cloud of initial gaps is generated based on the statistical analysis of the available measurements assuming the stochastic nature of local gap roughness. Through the reduction of the corresponding contact problem to a quadratic programming (QP) problem and the use of efficient QP algorithms together with the task-level parallelism, the mass contact problem solving on refined grids is accomplished.

Analysis of the influence of train load characteristics on the water damage of slab tracks

High-speed train load is one of the main factors that causes the failure of a slab track and is also the direct cause for producing water pressure in the cracks. Hence, choosing the right loading method is the key to analyze the water damage of slab tracks. In order to identify the distribution characteristics of the vertical force acting on the slab track, the analytic expression of fastener load was derived, and the field test was conducted to verify the theoretical model. According to the distribution characteristics of fastener load and the area of crack filled with water, the loading method was presented to analyze the water damage of slab tracks, and the influence of loading characteristics on the hydrodynamic pressure and crack propagation was analyzed. The results show that the distribution of fastener load in the main affected zone approximately accords with the Gaussian distribution under the single wheel load. When under the action of double wheels, the superimposed effect makes the second derivative of the loading function to increase by 99.93%. The superimposed effect of three or more wheels is less than 1% and can be ignored. Considering that the hydrodynamic pressure is highly sensitive to the second derivative of loading function, the load applied on the track slab should be in the form of a bogie, and different loads should be applied step by step based on the time series. The distribution of hydrodynamic pressure along the crack is approximately equal to the cubic polynomial, and the pressure peak appears at the crack tip. Also, the pressure is proportional to the square of the running speed of the train. In the region with abundant rainwater and a poor drainage system, reducing the train speed and taking timely measures such as grouting can effectively reduce the water damage of slab tracks.

Wind loads on fasteners used to attach flexible porous siding on multi-layer wall systems

Recent full-scale studies of wind loads on components of multi-layer wall systems have shown that maximum instantaneous pressure differences across individual layers can be a large fraction of the net load across the wall system, as noted in Cope et al. (2012) The results of these studies are considerably different from results obtained using dynamic pressure chambers where the entire section of the wall is exposed to the same quasi-steady pressure difference or time varying pressures. The largest differences in results between the full-scale wind tests at the Insurance Institute for Business & Home Safety (IBHS) Research Center and the pressure chamber tests occurred for the exterior flexible multi-part siding layer. In the pressure chamber tests, the pressures tend to equalize rapidly between the exterior and interior surfaces of the flexible siding resulting in very low net loads on the siding. In the full-scale wind tests, the response of the flexible siding to the temporal and spatial variations in wind loads imposed by the flow around the building could be observed as a wave moving along the wall surface.The study reported in this paper investigates the loads on the fasteners used to attach the siding to the wall. Specifically, the relationship between the instantaneous pressure differences across the siding and the loads on the fasteners is presented; with the goal of determining whether any systematic reduction or amplification of loads on the fasteners resulted from the very short duration peak instantaneous pressure differences applied to the siding. Specialty instrumentation was developed that allowed measurement of outward acting loads applied to the siding fasteners. This paper describes that specialty instrumentation and presents comparisons of pressure differences multiplied by the tributary area assigned to a fastener against the withdrawal loads measured using the instrumented fasteners.While there is significant scatter in the results, probably due to friction in the fastener load system among other things, results show a strong overall one to one correlation between the net outward loads calculated by applying the pressure load to the tributary area and the measured loads on the fasteners.

A Numerical Analysis of Riveted Lap Joint Containing Multiple-site Damage

The evaluation of stress intensity factors (SIFs) for cracks in mechanically fastened joints is one of the central issues in a damage tolerance analysis. Accurate stress intensity solutions may be difficult to determine due to geometric complexity along with variations in fastener load transfer and fastener interference and material thickness because of corrosion. Detailed finite element models that include specific aspects of fastener for lap joints analysis, but such representations are often impractical for large lap joints involving many fasteners. In order to reduce the number of degrees of freedom in a particular model, a methodology was implemented that efficiently depicts mechanical fasteners in lap joints using finite elements. Uncorroded and corroded lap joints with three crack scenarios were studied by the computationally efficient model. The effects of pillowing corrosion and fastener interference were also included in the model. The results show that the effect of MSD and thinning of the material is to increase substantially stress intensity factors (SIFs) values compare to that of a single crack without corrosion. For a given cyclic stress range, SIF decreases with increasing rivet interference level. This is particularly true for shorter crack lengths. SIF values for the longer crack are not sensitive to the length of shorter crack on the opposite side of the rivet hole. The effects of corrosion pillowing cause SIF values on the faying surface to be larger than those on the opposing surface, and the crack front distorts from the straight front. Moreover, stresses on the faying surface may exceed the yield strength of the material.

Force transfer mechanism on fastener section of mechanically anchored waterproofing membrane roofs under wind pressure during typhoons

The wind pressure, billow of a PVC sheet, and fastener load of a mechanically anchored waterproofing membrane system on a building rooftop were measured during a typhoon. The results showed that the waterproof sheet billowed upward, and the height of the billow was greater on the windward side that was under higher negative pressure. The axial and lateral forces acting on the fastener of the waterproof sheet were measured by a six-component force transducer, and it was found that the lateral force acting on the windward side of the fastener was larger. The tensile force acting on the fastener section was calculated using an equation for rooftop wind pressure. Finally, the method used to determine the fastener's position and the types of materials best-suited to cope with wind pressure on the roof were proposed, and the safety of this waterproof system was evaluated.

Influence of Edge Restraint on Clip Fastener Loads of Standing Seam Metal Roof Panels

Abstract This study investigates the effect of boundary conditions on the structural load paths within standing seam metal roof (SSMR) panel specimens subjected to uplift wind pressures. The objective is to determine the effect of boundary conditions (restrained or free) on observed load distributions among clip fasteners installed within a SSMR test specimen. Two roof specimens with instrumented clip fasteners were installed in a steel pressure chamber and subjected to static uplift pressures, while measuring panel deflection and clip fastener load distributions. The test specimen boundary conditions were modified in turn to represent field panel location (all free boundaries), gable panel (one restrained boundary, long dimension), eave panel (one restrained boundary, short dimension), and a corner panel (having two adjacent restrained boundaries). The load-displacement characteristics of the SSMR system were determined in a third specimen. The study found statistically significant differences in clip fastener loads, among the first three fastener rows away from the crosswise panel restraint, i.e., eave boundary condition. The gable boundary appeared to have less influence on load on the nearest fasteners. For the eave boundary condition, the measured load on the three exterior fasteners was about 40 % lower than the total fastener load on interior clips located just 3 m (10 ft) away. The results suggest why clip failures seldom occur in corner locations during uplift pressure tests. An empirical design method is proposed for predicting loads on clip fasteners installed near roof boundaries. More importantly, the design of SSMR roofing can be made more efficient, since it is shown that eave clip fasteners carry less than half the load of the interior fasteners, if other failure mechanisms (i.e., sheet tearing or boundary anchor failure) do not control the roof edge failures of installed SSMR.

Fracture mechanics analysis of row shear failure in dowelled timber connections

A quasi-nonlinear fracture mechanics model is presented for the analysis of row shear failure in timber connections with multiple fasteners in a row. A prerequisite for use of the model is a known distribution on the fasteners of the total applied load. It is shown that the ideal plastic and linear elastic fracture mechanics solutions appear as special cases of the quasi-nonlinear model. The model offers strength predictions that include the effect of among others the number of fasteners in a row, fastener spacing, row spacing, end-distance, edge-distance, fastener diameter, and material properties, such as shear strength, fracture energy, and modulus of elasticity. Simple explicit expressions are obtained from the analysis for any known fastener load distribution. For bolted connections, where bolt-hole gaps cause high loads on certain bolts at random, the model may be used in simulation procedures or in probabilistic models.

Wind Uplift Behavior of Mechanically Attached Single-Ply Roofing Systems: The Need for Correction Factors in Standardized Tests

In the United States, currently there are three consensus test methods for determining the wind uplift capacity of mechanically-attached single-ply roofing membrane systems. Each is based upon a specific specimen size and static loading regime. However, few studies have examined the relationship, if any, of failure loads among these methods. This study was conducted to determine the effects of varying specimen sizes on the static wind uplift performance or failure loads of mechanically-attached single-ply roofing membranes. Full-scale wind uplift pressure tests were performed in three phases, using six specimen sizes, and three membrane materials. The results were used to determine the minimum acceptable specimen sizes to be used for testing. In Phase 1, fastener failure loads were monitored, while in Phases 2 and 3, the failure experiments were repeated without measuring fastener loads. A significant difference in the apparent failure loads occurs among different specimen sizes for the same membrane materials. The results suggest that correction factors are necessary for comparing tests on different test beds and that the performance of membranes in the test chamber may not produce the same results on full scale structures.

Effect of load transfer on the cracking behavior at a countersunk fastener hole

The objective of this paper is to describe the effect of fastener load transfer on the cracking behavior at countersunk fastener holes. Both experimental and computational studies are described and compared when possible. Fatigue failures at previously tested mechanical joint specimens were investigated and characterized. Qualitative three-dimensional finite element analyses revealed that the degree of load transfer through the fastener could influence the cracking behavior at the countersunk fastener hole. This finding from the qualitative analysis provides one good explanation to the experimental results of the mechanical joint specimens. The cracking location’s dependence on the load transfer can be attributed to the non-uniform bearing load distribution along the straight bore caused by the load transfer.

Load transfer in multirow, single shear, composite-to-aluminium lap joints

A three-dimensional finite element model has been developed in order to determine the load transfer in multifastener single shear joints. The model is based on continuum elements and accounts for all important mechanisms involved in load transfer, such as bolt-hole clearances, bolt clamp-up and friction. In particular, member plates can be of different thickness and stiffness and with different coefficient of thermal expansion. An experimental programme was conducted in order to validate the finite element model through measurements of fastener loads, by means of instrumented fasteners. Good agreement between simulations and experiments was achieved and it was found that bolt-hole clearance is the most important factor in terms of load distribution between the fasteners. Any variation in clearance between the different holes implies that load is shifted to the fastener where the smallest clearance occurs. Sensitivity to this variation in clearance was found to be large, so that temperature changes could significantly affect the load distribution if member plates with different thermal expansion properties are used. It was found that good accuracy in load transfer predictions requires that all aforementioned factors are taken into consideration and that nonlinear kinematics is accounted for in the solution process.

Modeling Mechanical Fasteners in Single-Shear Lap Joints

One central issue in damage tolerance analysis is the evaluation of stress intensity factors for cracks at fastener holes in mechanically fastened joints. Accurate solutions for stress intensity factors may be difficult to determine due to geometric complexity, along with variations in fastener load transfer and fastener interference. Detailed finite element models that include specific aspects of fastener geometry may be developed for lap joint analysis, but such representations are often impractical for large lap joints involving many fasteners. A methodology is derived that efficiently depicts mechanical fasteners in lap joints using finite elements. Fastener material properties are determined, based on an empirical force-displacement relationship, for highly refined fastener models, as well as for idealized spring element representations of fasteners. The two fastener characterizations are used in combination to develop computationally efficient models for determining loads, stresses, and stress intensity factors for cracks in lap joints. Parametric studies involving single-shear lap joints with three rows of fasteners indicate that both fastener representations provide comparable load transfer and relative displacement between mating sheets for a variety of fasteners. These studies also suggest that the particular fastener material has relatively little effect on stress intensity solutions, whereas the fastener diameter has a significant effect. Residual strength predictions are obtained for large bolted lap joints with multiple site damage, which are consistent with experimental results from the literature.

Analysis of the fatigue life of riveted sheet metal helicopter airframe joints

This paper provides a brief review of a literature search on fatigue life of riveted airframe sheet metal joints and then a description of a test and analysis program conducted for common riveted joints used in rotorcraft airframe structures. Many papers dealing with fatigue test and analysis of fastened sheet metal have been published. These papers reveal a wide range of factors that may affect the fatigue life of such structures, and this paper provides a summary of this published literature. The test and analysis program was conducted on 4.8 mm diameter rivets which are common in rotorcraft airframe structure. The test specimens were manufactured, completed, and assembled in a production environment using production personnel, materials, and facilities. Various common fastener types, joint geometries, and materials were included in the testing. Static tests and three-dimensional finite element analysis were performed to evaluate fastener load distributions and local stress fields. Fatigue tests were conducted to obtain data for evaluating the effects of fastener type, joint geometry, and joint material. Limited fatigue tests were also conducted on ‘damaged’ and painted specimens.