Observations and analyses of secondary bending for riveted lap joints

The phenomenon of secondary bending in riveted lap joints of the configuration representative of connections of the aircraft fuselage sheets in the longitudinal direction is investigated experimentally and analytically. The experiments involved strain gauge measurements of secondary bending stresses carried out in close proximity to the fatigue critical section of the riveted lap joint and fatigue tests performed to study the effect of secondary bending on the riveted joint fatigue life. The strain gauge measurement results allowed validation of a simple analytical model proposed by Schijve to estimate secondary bending moments induced in mechanically fastened joints with eccentricities. Variables considered in the fatigue tests were several joint geometry related parameters known to influence the magnitude of secondary bending in the fatigue critical location. It was shown that the fatigue test data for joints of various geometries, which were considerably scattered if the fatigue lives were presented against the applied stress amplitude, could become consolidated within common scatter bands when the lives were plotted in terms of the combined tensile stress amplitude including the bending stress computed from the model by Schijve.     

Fatigue life estimation of coach peel riveted joints using multi-axial fatigue criteria

This study investigates fatigue behaviour of riveted joints in a coach peel configuration under various load ranges and load ratios. Three well-known multi-axial fatigue criteria of Smith–Watson–Topper, Glinka and Fatemi–Socie are used to predict the fatigue life of the joint based on the stress and strain components obtained from a finite element analysis. A three-dimensional finite element model was developed with considering elastic–plastic behaviour of aluminium alloy 2024-T3 for the joint plates as well as frictional contact between all the contacting parts. Fatigue life estimation results agreed well with existing experimental data for the same joint. This verifies the finite element results and shows how well the multi-axial fatigue equations can predict fatigue life of the riveted joints. The finite element modelling approach may be then used with confidence for fatigue life estimations of different configurations of riveted joints.     

Fatigue behaviour of riveted Glare lap joints

Fibre metal laminates, such as Glare, are a family of materials with very interesting properties for fatigue-critical applications. This article describes the results of a research programme carried out to evaluate the fatigue and damage tolerance characteristics of riveted Glare lap joints, representative of fuselage longitudinal joints. The comparison with the behaviour of metallic joints shows that different contributions are made to the total fatigue life, with the crack propagation life being by far the longer one for the Glare material whereas the crack nucleation life covers almost the entire fatigue life for metallic joints. Design rules should take this peculiar behaviour into consideration, to achieve the maximum benefit in the use of this class of materials in fatigue-critical applications.     

Implication of unequal rivet load distribution in the failures and damage tolerant design of metal and composite civil aircraft riveted lap joints

Riveted lap joints are being used widely in civil aircraft structures. Conventional design procedures assume that the joint can be designed as if all rivets carry load equally. As found in literature associated with fatigue and fracture, forensic studies on structural failures, this assumption is not entirely valid. In this paper, the regulatory codes for civil aircraft as applicable to riveted joints in the form of FAR 25 regulations are briefly reviewed. The regulatory code discusses safety factors in an implied way, but has little specific recommendations for riveted joints. However, studies on the failures of specific aircraft illustrated in this paper add to the argument that both static strength and life are affected by the initial design procedures for the riveted joints. In this paper, finite element models for metal–metal, composite–metal, composite–composite lap joints are studied. A three row lap joint used in commercial aircraft and which was part of failure studies is also examined. Unequal rivet loads and in cases, nearly 50% more than conventional design has been seen in linear finite element analysis. Elasto-plastic analysis using rivet flexibility shows re-distribution of loads. Based on these observations, the effect of rivet loads on life estimation including the use of concepts such as by-pass stresses is discussed. These results have implications for static strength at ultimate load, damage tolerance and fail safety and are discussed in this paper. Next, in a composite–composite lap joint, the influence of ply-angle on the rivet loads is studied. Also, a composite–metal lap joint is studied for the rivet load distribution and life estimation. It is found that the load shared by the rivet rows in a composite–metal lap joint are not symmetric and therefore are more susceptible to cracking and subsequent failure as the unequal distribution can cause some of the rivet loads to be high. In conclusion, the issue of fail safe and damage tolerant design of civil aircraft structures with riveted joints are addressed, especially the implication of unequal load distribution on the failures of such joints and it is suggested that these unequal rivet load distributions be catered for at the early design stage itself via finite element analysis and the possibility of an over-arching safety factor could be considered that incorporates both ultimate load and damage tolerance conditions.     

Fatigue behaviour of double lap riveted joints assembled with and without interfay sealant

Fatigue tests on ‘double shear’ riveted specimens were carried out. Material was aluminium alloy, 7075‐T73, thickness 3 mm. Specimen surfaces were protected by chromic acid anodization and epoxy primer. The mating surfaces of some specimens were sealed by a two‐component manganese‐dioxide cured, polysulfide compound. Specimens were pre‐assembled by inserting a temporary spring fastener in a pilot hole. The holes were reamed to the final diameter after sealant curing; then the Hi‐Loks were installed. Comparative fatigue tests were carried out on dry assembled specimens. Quite surprisingly, the fatigue resistance of sealed specimens was very low when compared with the results of dry assembled specimens. Hysteresis cycles measured in sealed and un‐sealed specimens clearly indicated a lubricant effect of the sealing layer, which justified the results obtained. Additional tests were performed on specimens assembled by applying different clamping force during the sealant curing time. The results obtained indicated an independence of the fatigue resistance on this parameter. Different failure modes were observed in sealed and un‐sealed specimens: fatigue cracks nucleated outside the hole at a location in the shadow of the rivet in the un‐sealed joints, while fatigue cracks nucleated at both sides of the holes in sealed joints. Finite element calculations carried out under different values of the friction coefficient between the mating surfaces confirmed the different location of the fatigue critical areas in sealed and un‐sealed specimens.     

Fatigue resistance and residual strength of riveted joints in FML

Static and fatigue tests have been carried out on 190-mm-wide fibre metal laminate (FML) riveted lap joints. The specimens were made with a Glare 3-3/2-0.3 material, i.e. a laminate composed of three layers of 2024-T3 aluminium alloy, thickness 0.3 mm and two double layers (0/90) of pre-preg FM-94-27%-S2 glass fibre. Seven configurations were tested which differed in the number of rivet rows (two or three), in the rolling direction of metal layers (perpendicular or parallel to the load direction), in the rivet type (solid or special, such as Hi-Lok or Lock-Bolt), in the rivet material for solid rivets (7050-T73 or 2017-T3), in the rivet diameter (4.8 or 5.5 mm) and in the presence of interlaminar doublers in the overlap area (titanium, aluminium, glass fibre). An additional difference was in the pre-formed rivet head: solid rivets had countersunk head, while Hi-Loks and Lock-Bolts had protruding head. The fatigue tests demonstrated the efficiency of a selective local reinforcement in the overlap area; in some cases, the fatigue resistance was so high that fatigue cracks nucleated in the laminates, rather than in the overlap area, as commonly expected.     

The driven rivet head dimensions as an indication of the fatigue performance of aircraft lap joints

Solid aluminium alloy rivets are used in joints of aircraft structures. Riveting implies a squeezing process of the rivet stem with large plastic deformations to form the driven rivet head. The dimensions of the driven rivet head (diameter D and height H) depend on the applied squeeze load F_sq. High squeeze loads are beneficial for the fatigue properties of riveted joints. The present investigation is focused on the relation between F_sq and the rivet head dimensions during controlled squeezing. Measurements of D or H will then allow estimation of the applied squeeze load for quality control of the riveting process with respect to the fatigue properties of the joint. Extensive test series were carried out with five rivet materials, three rivet diameters and two sheet materials (2024-T3 and Glare). An equation was derived for F_sq(D) and F_sq(H) based on assuming a zero volume change of the rivet during plastic deformation. The two material constants in the equations were empirically determined for the rivet materials. A satisfactory correlation was obtained.     

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S–N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.     

Fatigue strength evaluation of self-piercing riveted Al-5052 joints under different specimen configurations

In this study, static and fatigue tests were conducted using coach-peel, cross-tension and tensile–shear specimens with Al-5052 plates for evaluation of the fatigue strength of the SPR joints. For the coach-peel, cross-tension and tensile–shear geometries, the ratios of the fatigue endurance limit to static strength were 11%, 14% and 34%, respectively, assuming fatigue cycles of 10⁶ for an infinite lifetime. The equivalent stress intensity factor range can properly predict the current experimental fatigue lifetime. Fatigue crack initiation occurred due to fretting damage between the upper and lower sheets and between the rivet and these sheets.     

Experimental and analytical investigation of fatigue and fracture behaviors for scarfed lap riveted joints with different lap angle

Fastener load-transferred experiments and fatigue tests of the scarfed lap riveted joints with different lap angle were carried out. The fracture surfaces were observed by optical microscope (OM) in this paper. Both experimental and computational studies were described and compared when possible. Based on the qualitative finite element analysis (FEA), the multi-axial fatigue life of the scarfed lap riveted joints has been predicted by Smith–Watson–Topper (SWT) method and Wang–Brown (WB) method respectively. Both of the test results and predicted results show that fatigue life of scarfed lap riveted joints is remarkably increased after introducing lap angle into the faying surface. 8 mm-thick specimens with the lap angle of 1.68 °C exhibit the best fatigue performance, and 20 mm-thick with the lap angle of 3.37 °C do in the present study. Compared with the result of WB theory, the result of SWT theory is more conservative and reliable. For structures’ reliability designs, SWT theory and WB theory are all fallibility.     

Fatigue of single- and double-rivet self-piercing riveted lap joints

The fatigue behaviour of single‐ and double‐rivet aluminum alloy 5754‐O self‐piercing riveted (SPR) lap joints has been investigated experimentally and analytically. With the single rivet, the experimental program involves a set of 27 cyclic tension tests on joints with 1‐, 2‐ or 3‐mm‐thick sheet coupons. In most cases (85%), fatigue cracks are found to initiate in the gross section on the faying surface of the upper sheet. With two rivets (installed in two rows), the experimental program consists of nine cyclic tension tests, three for each of the three combinations of riveting orientation possible, on SPR joints consisting of 2‐mm‐thick sheet specimens. The fatigue life of double‐rivet joints is found to be strongly dependent on the orientation combination of the rivets. Monotonic tests with the double‐rivet joints also reveal an influence of orientation combination. In addition to experiments, values of local stress and rivet‐sheet microslip in the single‐rivet joints have been evaluated through three‐dimensional elastic finite‐element analysis. The analyses are used to interpret experimental observations of fatigue crack initiation location, life and fretting damage severity.     

Experimental and numerical investigations into the effect of an interference fit on the fatigue life of double shear lap joints

In this research the effect of bolt interference fit on the fatigue life of lap joints in double shear was investigated by conducting experimental fatigue tests and also analytically by FE simulation. In the experimental part, fatigue tests were carried out on specimens made from aluminium alloy 2024-T3 plates joined together as double lap joints and secured using bolts having fits ranging from zero clearance to different levels of interference. The results demonstrate how the failure is affected using different levels of interference fit. In the numerical study, 3-D FE models were used to simulate the different pin in hole fits considered and the results have been used to help explain the trends which were observed in the experimentally obtained S–N curve behaviour.     

High cycle fatigue mechanisms of aluminum self‐piercing riveted joints

A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.     

An experimental study on the failure of carbon/epoxy single lap riveted joints after thermal exposure

The bearing strength of a blind riveted single lap joint of a carbon/epoxy composite after thermal exposure was studied. Considering the maximum service temperature based on material specifications and glass transition temperature, six types of specimens were prepared and tested in accordance with ASTM D5961. Failure loads of specimens exposed to a 177 °C environment were above 90% compared to sound specimens. Failure loads of 77% were obtained for specimens exposed to a 232 °C environment for 4 h. Bearing and rivet pulling-through failure modes were observed in all specimens. Delamination caused by degradation of the matrix property was found in specimens exposed to temperature higher than 204 °C.     

Plasticity induced crack closure in adhesively bonded joints under fatigue loading

The mean load of a cyclic loading has a large effect on fatigue crack growth rates in metallic materials and bonded joints. In metallic structures, this effect has been attributed to plasticity-induced crack closure, but little is known about the mechanism responsible for this mean load effect on fatigue crack growth in adhesively bonded joints. This paper presents a computational investigation of the plasticity-induced crack closure mechanism affecting disbond growth in adhesively bonded joints under fatigue loading. The results show that the ratios of crack-opening and crack-closure are approximately independent of the level of plastic constraint, indicated by the ratio between the plastic zone size and the adhesive thickness. An effective strain-energy release rate parameter, which accounts for the crack closure behaviour, has been developed as a new correlating parameter for disbond growth. Comparisons with the experimental results pertinent to four different adhesive bonded joints reveal that this new correlating parameter is capable of unifying the fatigue growth rates by eliminating the effect of mean loads.     

Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.     

Fatigue behaviour of laser repairing welded joints

This paper presents a fatigue study in Nd-YAG laser surface repairing welded joints in specimens of two base materials used in mould production. The tests were carried out in a servo-hydraulic machine in tension, under constant amplitude loading, with two stress ratios R = 0 and R = 0.4. Welded specimens were prepared with U notches and filled with laser welding deposits. The fatigue results are presented in the form of S–N curves obtained in welded and non-welded conditions. Complementary measurements of hardness and residual stresses profiles were carried out along the surface of laser welded specimens to understand the observed fatigue behaviour. The melted material was the weaker region, with lower values of hardness and higher tensile residual stresses, presenting also a high number of defects that are potential failure sites. The presence of such defects can explain the relatively poor fatigue strength of the laser repairing joints in comparison to base materials.     

Effect of torque tightening on the fatigue strength of bolted joints

The effect of tightening torques on the life of plates bolted using single and double lap joints was investigated. The effect of plate thickness using an aircraft grade aluminium alloy with double lap joints was also studied. Constant amplitude fatigue tests, under load control, were carried out, with a near zero stress ratio, on plain specimens (for bench mark purposes) and on both single and double lap joint specimens, for which several torque levels were applied on the bolted joints. The objective of the fatigue tests was to demonstrate failure trends for each joint type, material thickness and torque loading, rather than the generation of comprehensive S–N curves. Possible factors that affected the fatigue life of the bolted joints are discussed and conclusions are drawn with respect to the beneficial effects of tightening torques on the bolted single and double lap joints.     

开缝衬套孔挤压残余应力场数值计算研究

本文基于有限元技术模拟了开缝衬套挤压和直接芯棒挤压两种孔挤压工艺,对比研究了两种不同工艺导致的孔壁材料轴向流动和孔壁残余应力场。结果表明：开缝衬套挤压可有效抑制材料向挤出端流动,这在飞机夹层孔结构挤压中可减小夹层间隙尺寸;衬套开缝对应孔壁区域残余应力有突变,但仍然是对抗疲劳有利的压应力,而非拉应力;开缝衬套挤压挤入端孔边是压应力,而直接芯棒挤压是拉应力;相同干涉量条件下两种工艺引入的残余压应力峰值近似,但开缝衬套挤压残余压应力场域较直接芯棒挤压增大了约1 mm.