Fatigue Life Of Riveted Joints Research Articles

Fatigue properties of riveted joints under different hole perpendicularity errors and squeeze forces.

Riveting is the most important method of joining sheet metal and is widely used in the assembly of aircraft components. The perpendicularity error of the holes is unavoidable during automatic drilling and riveting, which has a significant impact on the quality of the assembly. In this paper, the effects of hole perpendicularity error and squeeze force on the interference fit size, interface contact state, microstructure morphology, fatigue life, and fracture form of riveted joints were investigated experimentally. The results show that the interference fit size increases with a greater tilt angle. When the tilt angle is 0°, the rivet shank is in close contact with the inner and outer sheets, and there is no obvious gap at the interface between the rivet and the sheets. As the tilt angle increases to 2° and 4°, a gap appears at the interface of regions 1, 2, and 3, while the rivet shank at region 4 is in close contact with the outer sheet. The fatigue life decreases when the tilt angle increases from 0° to 4°. For the same tilt angle, the fatigue life of riveted joints with a 0° tilt direction is higher than that of riveted joints with a 180° tilt direction. Increasing the squeeze force can to some extent reduce the adverse effect of the tilt angle on the fatigue life. The hole perpendicularity error does not affect the failure form, while the squeeze force has a significant effect on the failure form of the specimens. RESEARCH HIGHLIGHTS: The fatigue life of riveted joints decreases as the tilt angle increases. The size of the interfacial gap increases with increasing tilt angle. Higher fatigue life at 0° tilt direction than at 180° tilt direction. Increasing the squeeze force can somewhat reduce the negative effect of tilt angle on fatigue life.

Artificial neural network based fatigue life assessment of riveted joints in AA2024 aluminum alloy plates and optimization of riveted joints parameters

The objective of this paper is to provide the fatigue life of riveted joints in AA2024 aluminum alloy plates and optimization of riveted joints parameters. At first, the fatigue life of the riveted joints in AA2024 aluminum alloy plates is obtained by experimental tests. Then, an artificial neural network is applied to estimate the fatigue life of riveted lap joints based on the number of lateral and longitudinal holes, punch pressure, gap between the edge of hole and rivet, rivet shank diameter, and rivet shank length. Also, meta heuristic optimization algorithm is applied to calculate the riveting process parameters. Finally, sensitivity analysis is used to obtain the influence of parameters affecting the riveting process on the fatigue life.

EFFECT OF THE RIVET-HOLE TOLERANCE ON THE STRESS-SEVERITY FACTOR

This work is focused on a quantitative procedure for estimating the generally unfavourable effects that incorrectly drilled holes, characterized by the initial clearance between a rivet and a hole, have on the fatigue life of riveted joints. The solution is based on an analytical approach using the stress-severity-factor concept. An experimental programme with riveted-joint specimens characterized by low-load transfer factors was realized in the Czech Aerospace Research Centre (VZLU) test lab under constant amplitude loading. The holes for rivet joints with 4-mm diameters were prepared with the clearance in a range of 0.0–0.16 mm. Force-controlled riveting was applied using a constant pressure force to form the driven head. To prevent fretting events between the joined parts, their anodized contact surfaces were lubricated with MOLYKA, plastic grease with molybdenum disulphide and graphite. The experimental data showed that the load-transfer factor and the fatigue life depend on the initial clearance between a rivet and a hole. The presented procedure introduced the hole-filling factor, integrated in the stress-severity-factor concept as a function of the initial clearance between a rivet and a hole.

Improvement of fatigue life of riveted joints in helicopter airframes

Using original cold-formed rivets in repairs of airframes of helicopters is difficult due to no access to inside parts of the airframe. Thus, the main aim of the study was to investigate the possibility to use the blind rivets or hybrid joints by verification the fatigue performance of such joints that must be better than with original rivets. Riveted and hybrid joints have been experimentally tested under static and fatigue loads. Furthermore, numerical calculations of stress distribution for strapped joint have been conducted. The test results covered fatigue life of lap joints and models of repaired airframe sheets using ordinary mushroom head rivets ref. 3558A-4-10, titanium driven blind bolts with pin, ref. MBF2110AB-05-150 and modified hybrid joints. Using titanium driven blind bolts with pin instead of ordinary hammer-bucked rivets, can improve the fatigue life of element made of aluminum alloy AW 2024T3. There are advantages of replacing riveted joints with modified hybrid (rivet & adhesive) joints in threefold increase in fatigue life of repaired airframe structures.

ИССЛЕДОВАНИЕ ВОЗДЕЙСТВИЯ АГРЕССИВНЫХ ФАКТОРОВ ВНЕШНЕЙ СРЕДЫ НА РАЗВИТИЕ КОРРОЗИОННЫХ ПОРАЖЕНИЙ НА ОБРАЗЦАХ СЛОИСТОГО МЕТАЛЛОСТЕКЛОПЛАСТИКА КЛАССА СИАЛ

The article presents the results of examination of the influence of aggressive environmental factors during tests in a salt fog chamber and moisture saturation on the strength characteristics and low-cycle fatigue of riveted joints of laminated glass-reinforced plastic with and without an integrated protection system. It is shown that the multilayer structure of SIAL prevents the development of corrosion damage in layers along the depth of the material. The mechanical properties of samples made of SIAL material with three-layer and five-layer structure after moisture saturation practically do not change. It was found that the lack of corrosion protection of the laminated material leads to a decrease in the fatigue life of riveted joints.

Experimental and numerical investigation of fatigue crack growth behavior and optimizing fatigue life of riveted joints in Al-alloy 2024 plates

The purpose of this paper is to investigate the crack growth and fatigue life estimation of the optimized rivet joints in aluminum 2024 sheets numerically and experimentally. For this point, the optimal model is obtained using the Taguchi and finite element method (FEM), with respect to the cost functions and the defined levels for the parameters. The relevant geometrical parameters such as rivets length, holes diameter, and dimensional tolerances as well as pattern of where the rivets are and the material of the rivet joints are optimally determined and subsequently the fatigue life of the aluminum joint is obtained by experimental tests. The Taguchi method with its own algorithm reduced the number of experiments to 32 design points. These experiments are simulated by the FEM with a three-dimensional (3D) elastoplastic model. After the determination of optimal joint, a 3D boundary element model (BEM) of fatigue crack growth is performed on the optimal model and its fatigue life is evaluated. Then, to validate the numerical results of the fatigue life of this optimized joint, the samples are fabricated and subjected to constant-range alternative loading by a servo-hydraulic machine. The results showed that the mode of failure in the samples is the fracture at the front plate, where the punch was applied to the rivets shank and occurred in the first row. Also, the results of experimental tests for fatigue crack growth are compared with the numerical method that results in acceptable achievements.

Effect of load transfer by friction on the fatigue behaviour of riveted lap joints

An experimental investigation of load transmission throughout a riveted lap joint is presented with a focus on the role of friction. The main objective was to generate results to be utilized in a semi-empirical model for predicting the fatigue life of riveted joints, which is being developed by the authors. The test variables accounted for in the experiments were the rivet squeeze force and the faying surface condition. Triple-row riveted lap joint coupons of aluminium alloy 2024-T3 Alclad sheets intended to be a simple representation of longitudinal connections in the aircraft fuselage skin structure were assembled with two different rivet squeeze forces. For either force half of the joints had a thin Teflon interfoil in the overlap area in order to minimize friction between the sheets. Friction measurements reveal that, contrary to the friction coefficient between the Alclad mating surfaces, the friction coefficient between the Teflon and Alclad contact surfaces is considerably lower and does not increase with the cycle number.A convenient method of the determination of load transmission throughout a lap joint is proposed utilizing measurements by strain gauges mounted on only outer surfaces of the overlapping sheets supplemented by the secondary bending analysis according to a simple theoretical model. The results obtained indicate that the effect of the squeeze force and the faying surface condition on axial forces in the overlapping sheets is quantitatively insignificant. It is noted that an analytical procedure commonly used to compute axial forces in the sheets of joints with mechanical fasteners considerably underestimates load transfer by the critical, outer rivet rows.Comparative fatigue tests on the riveted coupons with the Teflon interlayer and without such a layer reveal superior fatigue lives of the former. These results demonstrate, contrary to the prevailing opinion, an overall detrimental influence of frictional load transfer at applied stress values representative of those experienced in service by lap splices of the aircraft fuselage.Post-failure examinations of the riveted joints provide evidence that the crack initiation locations and crack paths depend on the test variables and the stress level. For the uncoated joints, the effect of the squeeze force and applied load on the area of fretted regions around the rivet holes was noted, whilst the total roughness heights did not exhibit such a dependency.It is shown that the present and reported in other works experimental results on the effect of coatings on the fatigue performance of riveted joints cannot be explained based on FE stress analyses available in the literature due to simplifications involved in modelling the faying surface condition. A major conclusion of the present work is that because of the extremely complex determinants of frictional load transfer, its effect on the fatigue behaviour of riveted joints can only be predicted using a semi-empirical approach.

Fatigue life prediction model for riveted lap joints

Applicability of currently available approaches to predict the fatigue life of riveted joints in aircraft structures is limited to cases when the actual joint, for which the predictions are made, and the reference joint, for which the prediction model has been tuned, differ only in the geometry. It is required that the riveting process should be similar for the actual and reference case, but the similarity criterion cannot be formulated in a precise way. In the prediction model developed by the authors and presented in this paper the influence of riveting on the fatigue life of a joint is unambiguously characterized by measurable quantities, namely rivet hole expansion and the load transfer distribution. Hence, the similarity of the riveting processes for the reference and actual joint is no longer required, which considerably extends the transferability of the reference results. A validation of the model is performed by comparing fatigue lives computed and observed in over 80 fatigue tests on aluminium alloy, lap joint specimens with three rivet rows, typical for aircraft fuselage skin connections in the longitudinal direction. Various combinations of production variables, such as sheet material and thickness, the squeeze stress and rivet type, were involved. A significant improvement in the present model prediction accuracy compared to a model which disregards the effect of riveting has been noted. Specifically, underestimates or overestimates of the fatigue life observed for the latter model in the case when hole expansion of the actual joint is larger or smaller respectively than for the reference joint are avoided with the present approach.

Applicability of Empirical Formulae for the Fatigue Notch Factor to Estimate Riveted Lap Joint Fatigue Strength

A semi-empirical fatigue life prediction model under development by the present authors for riveted lap joints used in aircraft structures is outlined. In contrast to existing models, it will account for the influence of the rivet squeeze force on the fatigue life of riveted joints. To determine the effect of rivet-hole interference on the fatigue behaviour of a riveted joint, a series of fatigue tests on filled hole coupons with different amounts of interference will be carried out under loading conditions representing the bypass load, transfer load and secondary bending. These experiments will allow evaluating of the dependency of the fatigue notch factors on rivet hole expansion. Preliminary results obtained so far are presented in this paper.

Local Phenomena During Riveting Process

Abstract The paper presents experimental and numerical study of the local phenomena during the riveting process. It is commonly accepted that technological factors of the riveting process has a strong influence on the fatigue life of riveted joints. The authors analysed the papers concerned the experimental researches of the riveting force influence on fatigue life. The magnitude of the life increase caused by the riveting force increase suggests the authors that this is not only the result of beneficial stress system but the change of the joint formation mechanism has taken place. This was an inspiration to undertake more detailed researches of the riveting process. The strain progress during the riveting process has been experimentally investigated for four types of aluminium rivets used in airframes. Measurements confirm very high strains near the driven head. For some types of rivets the reversal strain signal has been recorded. Several FE model has been use to investigate the riveting process. The axisymmetric and solid models were used. The agreement of experimental and numerical results in some cases were good, in other cases the numerical models demand further development. In any calculations, the reversal strain effect has not been obtained, This suggest that it is result of the phenomenon which has not been taken into account in numerical modelling. The working hypothesis has been assumed that during the riveting process adhesive joints (called cold welding) were formed and destroyed during the process, what was the reason of the observed reversal strain signal. The authors are going to continue this investigation.

The Analysis of the Influence of Riveting Parameters Specified in Selected Riveting Instructions on Residual Stresses

Abstract The riveting parameters strongly affect residual stresses induced during riveting, which in turn have an impact on the fatigue life of riveted joints. Since rivets are established as critical from the fatigue point of view, the fatigue life of riveted joints often determines the life of the whole structure. The authors were able to become acquainted with three riveting instructions (process specifications) used by the aerospace companies from western Europe. This work presents the analysis of the riveting parameters' influence on residual stresses around the rivets. The impact of the clearance between a rivet shank and a hole as well as driven head dimensions and a rivet length were investigated based on the numerical simulations. The aim of the analysis was to determine the range of stresses variation when the requirements of the riveting instructions are fulfilled. For the purposes of comparison, the calculations were performed also with the parameters as specified in the Polish industry standards. For all calculations, the geometry of the universal rivet MS20470 was used. The results show that residual stresses can vary strongly depending on the parameters in the instructions and standard requirements.

Improving Fatigue Life of Riveted Joints by Rivet Hole Sizing

The aim of this paper is to assess the impact of the rivet hole sizing process on the fatigue life based on the example of the structural connections characteristic for riveted joints used in aviation industry. Test specimens reflected the structural connection consisting in a riveted lap joint of an airplane plating stiffened with a T-bar. Connected plates and the T-bar are made of D16CzATW aluminum alloy. 3 mm diameter oval head solid rivets for aviation-related purposes were made of PA24 aluminum. During fatigue tests, individual specimens with non-sized holes and with sized holes were subjected to uniaxial, one-sided, fixed-amplitude loading (R = 0). It can be concluded from the fatigue life comparison that introduction of an additional operation in the riveting process, i.e. the hole sizing, results in significant, about two-fold increase of the fatigue life of the riveted structural connection, even at slight sizing degree. The difference of the specimen damage nature was observed between specimens with sized and non-sized holes.

Corrosion treatments and the fatigue of aerospace structural joints

Corrosion Inhibiting Compounds (CICs) retard or prevent degradation by corrosion, and are used widely in aviation, particularly in large aircraft with well-developed Corrosion Prevention and Control Programs. The diverse General Aviation sector, including small airliners, also benefit, but are poorly supported in terms of guidelines for CIC selection and application. One risk is that lubricating chemicals (like CICs) might affect the fatigue performance of structural joints, and potentially affect the overall economic or safe life of the aircraft.This paper discusses the effect of the application of Corrosion Inhibiting Compounds (CICs) on the fatigue life of riveted joints, and on the failure mechanisms involved. Single lap joints treated and untreated with CICs were tested under constant amplitude fatigue loading, with multiple load levels. Two different CICs, one oil-based, and one a soft-waxy CIC, were applied. The results showed that the presence of CICs had a significant influence on the fatigue life, and also on the failure mode of the joints. At high load levels, the application of CICs caused a reduction in the fatigue life of the joint by more than a factor of two. In this load range, the CICs appeared to cause the failure mode to change from tensile failure of the sheet (the prevalent mode at medium load levels) to shear failure of the rivets (observed at the highest load levels). Specimens that failed by rivet shearing showed some fatigue cracks propagating along the critical rivet row. In treated specimens tested at medium load levels, a reduction in the fatigue life still occurred, with all specimens failing in the sheet. At low load levels, there was little difference in fatigue life for the three conditions, although specimen test run-outs meant that further testing will be needed.The results are believed to have significance for managing the small aircraft in which these joints are common.

Riveting Process Induced Residual Stresses Around Solid Rivets in Mechanical Joints

The interference fit provided by solid rivets introduces a residual stress field beneficial to the fatigue life of riveted joints. Evolution in riveting technology has led to force-controlled riveters which provide greater consistency over the rivet installation process and the resulting residual stress field. By reexamining the rivet installation process and its effects on the formation of residual stresses, the fatigue benefits of rivets could be further exploited. Using a 3-D finite element model, installation of universal and countersunk rivets in monolithic aluminum sheet has been studied. Aspects of accepted riveting practice, including the degree of rivet flushness and the rivet squeeze force were found to play significant roles in the formation of residual stresses. Residual stresses beneath the rivet head were also found to be influenced primarily by through-thickness compression of the joined sheets during riveting, challenging the traditional analogy of riveting to radial expansion processes.