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Fretting behaviour of self-piercing riveted aluminium alloy joints under different interfacial conditions
Materials & Design Materials and Design 27 (2006) 200–208 www.elsevier.com/locate/matdes
Fretting behaviour of self-piercing riveted aluminium alloy joints under different interfacial conditions L. Han a
a,*
, A. Chrysanthou b, J.M. OÕSullivan
b
Warwick Manufacturing Group, International Automotive Research Centre, University of Warwick, Coventry CV4 7AL, UK b Department of Aerospace, Automotive and Design Engineering, University of Hertfordshire, AL10 9AB, UK Received 7 May 2004; accepted 7 October 2004 Available online 26 November 2004
Abstract The fretting behaviour of self-piercing riveted aluminium alloy joints with three different interfacial conditions has been investigated in this study. The fatigue life of the joints was observed to be dependent on the fretting behaviour under different interfacial conditions. The presence of a wax-based solid surface lubricant on the surface of the aluminium alloy sheet could delay the onset of fretting damage leading to longer fatigue life. Inserting PTFE tape at the interface between the two riveted sheets led to the reduction and even elimination of fretting damage in a self-piercing riveted joint. However, the presence of PTFE tape at the interface resulted in a significant reduction in the fatigue life and led to a change in the failure mode. The effect of the frictional force during the fretting fatigue process of a self-piercing riveted joint is also discussed. 2004 Elsevier Ltd. All rights reserved. Keywords: Mechanical fastening (D); Fatigue (E); Wear (E)
1. Introduction Self-piercing riveting is a relatively new joining method in the automotive field, where it can be used to join thin sheet material [1,2]. The technique involves clamping the sheets to be joined between a blank-holder and an upset die and forcing a rivet to pierce the upper sheet and flare into the bottom sheet under the influence of an upset die. The technique has many advantages. Unlike conventional riveting, it does not require a pre-drilled hole, thus saving labour and time. Apart from this, a wide range of materials can be joined, including combinations of similar or dissimilar materials. In comparison to spotwelding, the process is environmentally friendly due to the low energy requirement, low-noise and no fume emissions and involves no heating. In addition, the process is *
Corresponding author. Tel.: +44 247 657 5385; fax: +44 247 657 5366. E-mail address: [email protected] (L. Han). 0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.10.014
simple and automated, while the capital and operating costs are low. Self-piercing rivets were originally designed for the construction industry and were subsequently used in domestic products including washing machines and ventilation systems. Research in this area has shown that self-piercing riveting of aluminium alloys gives joints of comparable static strength and superior fatigue behaviour to spot-welding [3,4]. Therefore, the technique offers a solution to the automotive industry as the increasing use of lightweight aluminium alloys for automobile body-in-white applications needs to be balanced against the well-known problems of spot-welding of aluminium alloys [5,6]. Audi A8, the first generation of aluminium space frame vehicle first adopted this joining technique in its assembly followed by other automotive companies including Mackenzie, Jaguar and Volvo. Fretting has been observed and well documented in conventional riveted joints [7–9], where it was reported to lead to the initiation of fatigue cracks and multi-site damage. However, self-piercing riveting is relatively
L. Han et al. / Materials and Design 27 (2006) 200–208
201
Fig. 1. Section of a self-piercing riveted joint.
new and there are very few data available in the public domain concerning the behaviour of self-piercing riveted joints. In a recent publication [10], the present authors reported the observation of fretting damage during fatigue testing of AA5754 aluminium alloy sheets joined by self-piercing riveting. Fretting was observed at positions A, B and C as marked in Fig. 1 that presents a typical section of a self-piercing riveted joint. The investigation showed that fretting damage at position A, that is, at the interface between the two riveted sheets, led to crack initiation and propagation during fatigue testing. As the use of self-piercing rivets is likely to become more widespread within the automotive industry, it is important to gain an understanding of their behaviour and to obtain sufficient data to enable the development of predictive models of this behaviour. To date there have been very few previous investigations that have examined the fatigue performance of self-piercing riveted joints [11,12]. The aim of the work that is reported here was to examine the effect of various surface conditions on the fretting behaviour and fatigue life of self-piercing riveted samples. In this paper, there will be reference to fretting at position A only, since the variation of the alloy surface condition mainly affected the fretting behaviour at this position. Three different interfacial conditions were used: (i) as-rolled uncoated 5754 aluminium alloy sheets, (ii) wax-based lubricant-coated 5754 aluminium alloy sheets and (iii) samples of uncoated 5754 with a PTFE insert which was applied in order to prevent direct contact between the riveted sheets.
2. Experimental procedure 2.1. Materials and specimens The mechanical properties and the nominal alloy composition of aluminium alloy sheet 5754 that was used in this study are listed in Table 1. The thickness of all the sheets was 2 mm. The aluminium alloy sheet was supplied by Alcan International Limited in two different surface conditions; lubricant-coated and uncoated 5754. One set of self-piercing riveted samples was pre-
Table 1 Compositions and mechanical properties of 5754 Mechanical properties YoungÕs modulus (GPa)
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
70
240
22
68.5
Nominal composition in wt% (Balance Al) Si
Fe
Cu
Mn
Mg
0–0.40
0–0.40
0–0.10
0–0.50
2.60–3.60
pared using as-rolled 5754 alloy without the application of any coating. The other 5754 sheet material was coated with a chromate-free wax-based solid lubricant of 1 lm thickness. A third set of samples was prepared by joining uncoated 5754 alloy sheets with a PTFE-insert placed at the interface between the two riveted sheets, in order to prevent direct contact between them. The PTFE tape had a 1.9 mm width and a 0.2 mm thickness [13]. All samples were produced by Textron Fastening Systems using 36MnB4 rivets with a countersunk head [14]. The rivet had a shank diameter of 4.8 mm and a shank length of 7.0 mm. The setting pressure was 240 bar for all the samples. The specimens for fatigue testing conformed to the testing standard for resistance spot-welds by the International Organisation for Standardisation [15]. This standard is expected to be adopted for testing self-piercing riveted joints. As shown in Fig. 2, a typical specimen had an effective length of 200 mm, a grip length of 70 mm on both ends and a width of 60 mm. The sample was loaded in the longitudinal direction that was parallel to the longitudinal grain direction of the aluminium alloy sheet. 2.2. Measurement of coefficient of friction The coefficient of friction at the interface between the two riveted sheets for the three types of samples was measured using a direct shear apparatus. The essential features of the apparatus are shown diagrammatically in Fig. 3. The equipment operates by applying a vertical
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Fig. 2. Specimen geometry and the direction of the applied load.
Fig. 3. Direct shear apparatus.
(normal) force, N, to a pair of sheets through a loading plate. A shear stress was gradually applied on a horizontal plane by causing the two halves of the box to move against each other. The shear force, T, was automatically measured when a relative movement occurred and reached a steady state. The coefficient of friction l was then calculated by using Eq. (1) below: l¼
T : N
were stopped after a number of cycles. After testing, the wear scars on the two riveted sheets and the PTFE tape were examined by scanning electron microscopy/ energy dispersive X-ray analysis (SEM/EDX). Elemental analysis was carried out at a beam voltage of 20 kV.
3. Results ð1Þ
Pairs of sheets measuring 60 mm · 60 mm were used for these measurements. Three different normal loads N were applied leading to three different shear forces, T, for each interfacial condition. A plot of T against N yielded a straight line. The coefficient of friction l was then determined by calculating the slope of the line for each interfacial condition. 2.3. Test parameters All specimens were tested using a close-loop servohydraulic universal test machine. A cyclic tension–tension load with a sinusoidal waveform and a frequency of 20 Hz was employed as the applied load. Five cyclic load levels having a different maximum load and a minimum load of 0.5 kN were tested. The values of the maximum load ranged from 2.7 to 4.5 kN. In addition to fatigue tests leading to fracture of samples, some tests
3.1. Coefficient of friction measurements The results of the coefficient of friction measurements for the three different types of samples are listed in Table 2. The uncoated and the wax-based lubricant-coated samples had similar coefficient of friction values of 0.26 and 0.24, respectively. The coefficient of friction for the PTFE-insert samples was around 0.03 which was about an order of magnitude lower than the un-
Table 2 Coefficient of friction at different interfaces Samples
Interface
Coefficient of friction
Lubricant-coated Plain-surface PTFE-insert
Pierced sheet/locked sheet Pierced sheet/locked sheet Pierced sheet/PTFE/locked sheet
0.26 0.24 0.03
L. Han et al. / Materials and Design 27 (2006) 200–208
Maximum applied load (kN)
5 Lubricant-coated Uncoated
4.5
PTFE-insert 4 3.5 3 2.5 4.5
5
5.5
6
6.5
7
LogN
Fig. 4. Fatigue test results of the three groups of samples.
coated and the lubricant-coated pairs of the 5754 alloy sheet. 3.2. Fatigue endurance The fatigue test results for the three groups of samples are shown in Fig. 4. The lubricant-coated samples exhibited the longest fatigue life of the three types of samples, while the PTFE-insert samples had the shortest fatigue life. Fig. 4 also indicates that the difference in the fatigue life between the wax-based lubricant-coated sample and the uncoated sample was wider at low fatigue load values than at higher ones. 3.3. Observation of fretting Examination of fractured samples following fatigue testing indicated the presence of fretting scars at the interface between the riveted aluminium alloy sheets for all the lubricant-coated samples at all applied loads. As shown in Fig. 5, both the pierced and the locked sheets had suffered from fretting damage and the size of the fretting area differed as the value of the applied load changed. Fig. 6(a)–(d) show typical fretting scars and fretting damage at the interface between the two riveted sheets for the lubricant-coated samples. Clearly defined fretting scars were noticeable on the surface of both the riveted sheets at a peak load of 4.5 kN after 21,000 cycles, with the central region appearing to be
203
bright, while the surrounding scars were dark, as shown in Fig. 6(a). After 89,000 cycles at the same peak load, the fretting area increased and the bright central region had vanished, as shown in Fig. 6(b). As the fretting process continued, after 173,200 cycles at the same load, material delamination occurred, as shown in Fig. 6(c). In addition, the figures also indicate the effect of the magnitude of the applied load on the fretting behaviour. The fretting regimes, shown in Fig. 6(a), vary from 5 to 8 mm in length and from 2 to 4 mm in width. Fig. 6(d) presents the fretting scars on both sheets around the rivet after 850,000 cycles at 2.7 kN. It is evident that the fretting area of the joint was much smaller than that tested at 4.5 kN for 21,000 cycles even though the former had been tested for a much higher number of cycles. The development of fretting scars during fatigue testing of the uncoated samples was very similar to that of the lubricant-coated samples. Fretting scars and evidence of delamination during fatigue testing of the riveted uncoated sheets are shown in Fig. 7(a) and (b). Inserting a PTFE tape at the interface between the two riveted sheets prevented direct contact between them. Examination of samples following fatigue testing indicated that fretting damage occurred only for samples that were tested at a maximum fatigue load of 4.5 kN. Fretting scars were observed on both the pierced and the locked sheets as presented in Fig. 8(a) and (b). The size of the fretting regions had grown to around 5 mm in length and 1–2 mm in width after testing for 97,890 cycles. The size of the fretting scars was much smaller than that observed for the lubricant-coated and the uncoated samples at the same load level. In spite of the fact that the two sheets had been separated by the PTFE layer, surface damage of the riveted sheets characterised by delamination was also observed as shown in Fig. 8(c) and (d). There was no visible evidence of any fretting scars at all on the alloy surface at lower fatigue loads as indicated in Fig. 9(a) which presents the surface of the pierced sheet of a sample that was tested at a peak load of 3 kN. However, SEM examination of the PTFE tape in Fig. 9(b) revealed damage of the PTFE tape. Following fatigue testing, the failure mechanisms were also examined. The lubricant-coated and the uncoated
Fig. 5. The interface between the two riveted sheets for the lubricant-coated samples fractured following fatigue testing at: (a) 4.5, (b) 2.7 kN.
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Fig. 6. Fretting regions at the interfaces between the two sheets for the lubricant-coated samples: (a) after 2.1 · 104 cycles at 4.5 kN; (b) after 8.9 · 104 cycles at 4.5 kN; (c) delamination after 137,320 cycles at 4.5 kN, (d) after 8.5 · 105 cycles at 2.7 kN.
Fig. 7. Fretting damage on the riveted sheets for an uncoated sample after 215,060 cycles at 4.0 kN, (a) fretting debris on the pierced sheet, (b) delamination on the locked sheet.
samples failed in a similar way. As shown in Fig. 5(a) and (b), at the higher loads, rivet fracture led to failure of the samples, whilst at the lower fatigue loads the samples failed by fracture of the locked sheet. In the samples where PTFE tape was inserted at the interface between the two sheets, rivet fracture was the only means of failure that was observed at all fatigue loads, as shown in Fig. 10(a) and (b).
4. Discussion 4.1. Effect of the wax-based lubricant coating on fretting wear As shown in Fig. 6(a), two distinct fretting regions developed during the early stages of the fatigue test. EDX analysis of these fretting regions revealed the pres-
ence of high carbon content within the central region. This suggested that the central region was still covered by the solid lubricant. The outer region had turned black and represented those parts where the lubricant had been removed and the alloy surface had been exposed. Further examination indicated that ploughing took place during the early stages due to the cyclic micro-sliding movement, leading to plastic deformation and loss of material, as shown in Fig. 11(a). For lubricant-coated samples, the fretting process initially involved removal of the solid lubricant followed by fretting damage of the alloy surface. Once the alloy surface had been exposed, detachment of the alloy material occurred as a result of a continuous wear process. The debris produced by the wear process accumulated at the contact interface and formed a third-body particle to promote further three-body fretting. EDX analysis
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Fig. 8. Contact surfaces of a PTFE-insert sample after 97,890 cycles at 4.5 kN: (a) fretting scars on the pierced sheet, (b) on the locked sheet, (c) damaged surface of the pierced sheet, (d) damaged surface of the locked sheet.
Fig. 9. Contact surfaces of a PTFE-insert sample after 604,403 cycles at 3.0 kN: (a) no fretting scars on the pierced sheet, (b) damaged surface of the PTFE tape.
Fig. 10. The interface between the two riveted sheets for the PTFE-insert samples fractured following fatigue testing at: (a) 4.5, (b) 2.7 kN.
of the debris, shown in Fig. 11(b), indicated that the debris consisted of mainly aluminium oxide and a small amount of magnesium oxide as well as fragments of the lubricant, as listed in Table 3. Previous work by the present authors [10] showed that fretting contributed to the formation of fatigue cracks and failure of the
samples. The presence of the solid surface lubricant was effective in reducing fretting damage during the early stages of the fatigue test. The durability of the coating was rather low and decreased rapidly as the peak applied load increased. At the lower applied load values, the durability was higher since solid lubricants
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Fig. 11. (a) Surface roughening occurred on the locked sheet after 2.1 · 104 cycles at 4.5 kN. (b) Accumulated debris on the locked sheet after 173,200 cycles at 4.5 kN.
Table 3 EDX analysis of the debris shown in Fig. 11(b) Element
wt%
at.%
Al Mg Cl Si C Na O
007.25 000.44 000.19 000.45 071.77 000.25 019.65
003.57 000.24 000.07 000.21 079.43 000.15 016.33
Total
100.00
100.00
are more effective under conditions of partial slip and at low slip amplitudes [16] as well as low applied loads. For this reason, the difference in the fatigue life between the lubricant-coated and the uncoated samples was much higher at the lower fatigue load values. It was concluded that the solid surface lubricant led to an incubation period particularly at the lower fatigue load values where its durability was higher. This delayed the onset of fretting wear at the alloy surface and led to a slightly higher fatigue life for the lubricant-coated samples. On the other hand, it appears that for the uncoated samples, fretting damage started to take place on the alloy surface virtually straight at the beginning of the dynamic test, thus resulting in a reduction in the fatigue life of the joint. 4.2. Effect of the PTFE insert on fretting wear The PTFE tape was sandwiched between the two riveted sheets thus preventing direct contact between them at the critical zone where fretting damage was likely to take place. The use of the PTFE layer was successful in preventing fretting damage at all applied loads except at 4.5 kN which was the highest load used in the study. Under the highest applied load of 4.5 kN, the contact pressure at this interface was higher leading to a greater contact area [17]. The possibility of contact between asperities of the alloy sheets was higher and therefore this could cause more fretting damage under the effect of the cyclic movement. In addition, at a maximum applied load of 4.5 kN, the cyclic micro-sliding movement
led to two-body fretting at the alloy sheet/PTFE interface during the initial stages of testing and to possible wear damage to the PTFE layer. As the wear process continued this damage could have become severe enough for the PTFE to be torn away and allow direct contact between asperities of the alloy sheets. Fretting damage therefore took place at the alloy surface. At the lower load levels, two-body fretting led to surface damage of the PTFE tape, as shown in Fig. 9(b). However, under these conditions, the applied load (contact pressure) was too low to cause fretting wear of the aluminium sheets. 4.3. Effect of the frictional force on fretting wear In a self-piercing riveted joint, the normal load is interrelated to the applied load. The lower the applied load the lower the normal load leading to a smaller fretting area [17], as shown by comparing Fig. 6(b) and (d). This suggested that the lower the frictional force the smaller the fretting area, since the frictional force was directly proportional to the normal load. The inserted PTFE tape induced a very low coefficient of friction at the interface between the PTFE tape and the riveted sheet, as shown in Table 2. Consequently, at an identical normal load, the frictional force at this interface for the PTFE-insert samples was about 10 times lower than that for the coated and uncoated samples. This, along with the fact that the PTFE layer prevented direct contact between the aluminium alloy sheets, prevented fretting damage at the lower fatigue loads. At 4.5 kN which was the highest fatigue load tested in the study fretting damage did take place. Because of the low coefficient of friction, a much smaller fretting area resulted in the PTFE-insert samples. In addition, the frictional force is also directly related to the tangential force in the case of incipient sliding [18]. The lower frictional force therefore led to a lower tangential force at the contact surface. When the tangential stress was small, the amount of plastic deformation of micro-contact would be expected to be less since the probability of asperity interactions drops
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life. The low frictional force at the interface between the riveted sheets therefore led to a significant increase in the bearing load on the rivet. As a result all the PTFE insert samples failed by fracture of the rivet.
5. Conclusions Based on the results of the present investigation, the following conclusions can be drawn: Fig. 12. Debris that accumulated on the PTFE tape of a PTFE insert sample after 97,890 cycles at 4.5 kN.
and the amount of adhesive transfer is less [19]. However, in a fretting fatigue degradation process, plastic deformation is important in establishing a surface morphology which is long-lasting and therefore influences the future course of wear. The third-body of fretting is mainly produced at this stage. At the highest applied load of 4.5 kN, the normal load was higher resulting in a higher frictional force at the interface between the PTFE tape and the riveted sheet causing damage to the PTFE layer. It was possible that the damage to the PTFE layer led to direct contact between the asperities of the alloy sheets and therefore the frictional force at this interface increased slightly. Fretting wear of the aluminium sheets was initiated and the fretting debris became trapped at the alloy sheet/PTFE interface leading to three-body fretting, as shown in Fig. 12. As a result, fretting scars were observed, but the fretting area was much smaller in comparison with the samples without the PTFE insert. In contrast, at lower applied loads the normal load and the frictional force were also lower and therefore there was not sufficient damage to the PTFE layer and to the riveted sheets. The overall effect of the PTFE tape was therefore to reduce or eliminate the fretting damage. However, the fatigue results recorded a decrease in the fatigue life of the PTFE-insert samples. Moreover, examination of the fractured samples indicated that all the samples with the PTFE insert failed only by rivet fracture. As stated previously, this failure mechanism was very different compared to the samples where PTFE was not applied. This was because the much lower frictional force that was induced at the interface between the riveted sheets altered the load transfer mechanism of the joint. In a lap-joint, part of the cyclic applied load is transferred by friction between the contact surfaces, whilst the rest is transferred by bearing between the fastener shank and the sheet adjacent to the rivet. If the fraction of the load transferred by friction is reduced, the bearing load on the fastener is consequently increased and this could lead to earlier fatigue initiation and propagation of cracks at the fastener, resulting in a reduced fatigue
1. The fretting behaviour of a self-piercing riveted joint was affected by the interfacial conditions leading to different fatigue life. 2. The presence of a wax-based solid surface lubricant could delay the onset of fretting damage on the alloy surface leading to extended fatigue life. The solid lubricant was particularly more effective at lower fatigue load levels. 3. The application of a PTFE insert at the interface between the riveted sheets eliminated fretting at the lower fatigue loads and significantly reduced fretting damage at a peak fatigue load of 4.5 kN. However, this led to a reduction in the fatigue life of the selfpiercing riveted joints. 4. The failure mechanism of a self-piercing riveted joint was affected by the interfacial condition which could alter the load transfer mechanism of the joint.
Acknowledgements The authors thank Alcan International for providing financial support for the project. Thanks are expressed to Mr. Dean Homewood of Textron Fastening Systems for the preparation of samples.
References [1] Han L, Chen YK, Chrysanthou A, OÕSullivan JM. Self-pierce riveting – a new way for joining structures. ASME 2002;PVP-vol. 446(2):123–7. [2] King RP. Analysis and quality monitoring self-pierce riveting process, Ph.D. dissertation, University of Hertfordshire, 1997. [3] Krause AR, Chernenkoff ARA. A comparative study of the fatigue behaviour of spot welded and mechanically fastened aluminium joints, SAE Technical paper, 1995, 950710. [4] Booth GS, Olivier CA, Westgate SA, Liebrecht F, Braunling S. Self-piercing riveted joints and resistance spot welded joints in steel and aluminium, SAE Technical paper, 2000/01, 2681. [5] Barnes TA, Pashby IR. Joining techniques for aluminium spaceframes used in automobiles, Part I – solid and liquid phase welding adhesive bonding and mechanical fasteners. J Mater Process Technol 2000;99:62–71. [6] Barnes TA, Pashby IR. Joining techniques for aluminium spaceframes used in automobiles, Part II – adhesive bonding and mechanical fasteners. J Mater Process Technol 2000;99:72–9.
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[7] Iyer K, Xue M, Kasinanduni R, Bastias PC, Rubin CA, Wert JJ, Hahn GT. Contribution of fretting to the fatigue and corrosive deterioration of a riveted lap joint. ASME 1995;ADvol. 47:35–61. [8] Szolwinski MP. Linking riveting process parameters to the fatigue performance of riveted aircraft structures. Aircraft 2000:37. [9] Schijve J. Multiple-site-damage fatigue of riveted joints. Durability of metal aircraft structures. In: Atluri SN, Harris CE, Hoggard A, Miller N, Sampath SG, editors. Atlanta Technology Publications; 1992. p. 2–27. [10] Chen YK, Han L, Chrysanthou A, OÕSullivan JM. Fretting wear in self-piercing riveted aluminium sheet. Wear 2003;255: 1463–70. [11] Fu M, Mallick PK. Effect of process variables on the static and fatigue properties of self-piercing riveted joints in aluminium alloy 5754. SAE Technical paper, 0825; 2001. p. 117–29. [12] Fu M, Mallick PK. Fatigue of self-piercing riveted joints in aluminium alloy 6111. Int J Fatigue 2003;25:183–9.
[13] British Standard BS7786:1995, Specification for unsintered PTFE tape. General requirements, British Standard Publishing Limited, 1995. [14] British Standard BS EN 10263-1, Steel rod, bars and wire for cold heading and cold extrusion. General technical delivery conditions, British Standard Publishing Limited, 2001. [15] Draft International Standards, Resistance spot welding – Destructive tests of welds – method for the fatigue testing of spot-welded joints, ISO/TC44/SC5. [16] Zhou ZR, Vincent L. Lubrication in fretting – a review. Wear 1999;225–229:962–7. [17] Waterhouse RB. Fretting wear. ASM Met Handbook 1992;18:242–55. [18] OÕConnor JJ. The role of elastic stress analysis in the interpretation of fretting fatigue failures. In: Waterhouse RB, editor. Fretting fatigue. London: Applied Science; 1981, ISBN 0-85334932-0. p. 23–40. [19] Aldham D, Warburton J, Pendlebury RE. The unlubricated fretting wear of mild steel in air. Wear 1985;106:177–201.
Fretting behaviour of self-piercing riveted aluminium alloy joints under different interfacial conditions L. Han a
a,*
, A. Chrysanthou b, J.M. OÕSullivan
b
Warwick Manufacturing Group, International Automotive Research Centre, University of Warwick, Coventry CV4 7AL, UK b Department of Aerospace, Automotive and Design Engineering, University of Hertfordshire, AL10 9AB, UK Received 7 May 2004; accepted 7 October 2004 Available online 26 November 2004
Abstract The fretting behaviour of self-piercing riveted aluminium alloy joints with three different interfacial conditions has been investigated in this study. The fatigue life of the joints was observed to be dependent on the fretting behaviour under different interfacial conditions. The presence of a wax-based solid surface lubricant on the surface of the aluminium alloy sheet could delay the onset of fretting damage leading to longer fatigue life. Inserting PTFE tape at the interface between the two riveted sheets led to the reduction and even elimination of fretting damage in a self-piercing riveted joint. However, the presence of PTFE tape at the interface resulted in a significant reduction in the fatigue life and led to a change in the failure mode. The effect of the frictional force during the fretting fatigue process of a self-piercing riveted joint is also discussed. 2004 Elsevier Ltd. All rights reserved. Keywords: Mechanical fastening (D); Fatigue (E); Wear (E)
1. Introduction Self-piercing riveting is a relatively new joining method in the automotive field, where it can be used to join thin sheet material [1,2]. The technique involves clamping the sheets to be joined between a blank-holder and an upset die and forcing a rivet to pierce the upper sheet and flare into the bottom sheet under the influence of an upset die. The technique has many advantages. Unlike conventional riveting, it does not require a pre-drilled hole, thus saving labour and time. Apart from this, a wide range of materials can be joined, including combinations of similar or dissimilar materials. In comparison to spotwelding, the process is environmentally friendly due to the low energy requirement, low-noise and no fume emissions and involves no heating. In addition, the process is *
Corresponding author. Tel.: +44 247 657 5385; fax: +44 247 657 5366. E-mail address: [email protected] (L. Han). 0261-3069/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.10.014
simple and automated, while the capital and operating costs are low. Self-piercing rivets were originally designed for the construction industry and were subsequently used in domestic products including washing machines and ventilation systems. Research in this area has shown that self-piercing riveting of aluminium alloys gives joints of comparable static strength and superior fatigue behaviour to spot-welding [3,4]. Therefore, the technique offers a solution to the automotive industry as the increasing use of lightweight aluminium alloys for automobile body-in-white applications needs to be balanced against the well-known problems of spot-welding of aluminium alloys [5,6]. Audi A8, the first generation of aluminium space frame vehicle first adopted this joining technique in its assembly followed by other automotive companies including Mackenzie, Jaguar and Volvo. Fretting has been observed and well documented in conventional riveted joints [7–9], where it was reported to lead to the initiation of fatigue cracks and multi-site damage. However, self-piercing riveting is relatively
L. Han et al. / Materials and Design 27 (2006) 200–208
201
Fig. 1. Section of a self-piercing riveted joint.
new and there are very few data available in the public domain concerning the behaviour of self-piercing riveted joints. In a recent publication [10], the present authors reported the observation of fretting damage during fatigue testing of AA5754 aluminium alloy sheets joined by self-piercing riveting. Fretting was observed at positions A, B and C as marked in Fig. 1 that presents a typical section of a self-piercing riveted joint. The investigation showed that fretting damage at position A, that is, at the interface between the two riveted sheets, led to crack initiation and propagation during fatigue testing. As the use of self-piercing rivets is likely to become more widespread within the automotive industry, it is important to gain an understanding of their behaviour and to obtain sufficient data to enable the development of predictive models of this behaviour. To date there have been very few previous investigations that have examined the fatigue performance of self-piercing riveted joints [11,12]. The aim of the work that is reported here was to examine the effect of various surface conditions on the fretting behaviour and fatigue life of self-piercing riveted samples. In this paper, there will be reference to fretting at position A only, since the variation of the alloy surface condition mainly affected the fretting behaviour at this position. Three different interfacial conditions were used: (i) as-rolled uncoated 5754 aluminium alloy sheets, (ii) wax-based lubricant-coated 5754 aluminium alloy sheets and (iii) samples of uncoated 5754 with a PTFE insert which was applied in order to prevent direct contact between the riveted sheets.
2. Experimental procedure 2.1. Materials and specimens The mechanical properties and the nominal alloy composition of aluminium alloy sheet 5754 that was used in this study are listed in Table 1. The thickness of all the sheets was 2 mm. The aluminium alloy sheet was supplied by Alcan International Limited in two different surface conditions; lubricant-coated and uncoated 5754. One set of self-piercing riveted samples was pre-
Table 1 Compositions and mechanical properties of 5754 Mechanical properties YoungÕs modulus (GPa)
Tensile strength (MPa)
Elongation (%)
Hardness (HV)
70
240
22
68.5
Nominal composition in wt% (Balance Al) Si
Fe
Cu
Mn
Mg
0–0.40
0–0.40
0–0.10
0–0.50
2.60–3.60
pared using as-rolled 5754 alloy without the application of any coating. The other 5754 sheet material was coated with a chromate-free wax-based solid lubricant of 1 lm thickness. A third set of samples was prepared by joining uncoated 5754 alloy sheets with a PTFE-insert placed at the interface between the two riveted sheets, in order to prevent direct contact between them. The PTFE tape had a 1.9 mm width and a 0.2 mm thickness [13]. All samples were produced by Textron Fastening Systems using 36MnB4 rivets with a countersunk head [14]. The rivet had a shank diameter of 4.8 mm and a shank length of 7.0 mm. The setting pressure was 240 bar for all the samples. The specimens for fatigue testing conformed to the testing standard for resistance spot-welds by the International Organisation for Standardisation [15]. This standard is expected to be adopted for testing self-piercing riveted joints. As shown in Fig. 2, a typical specimen had an effective length of 200 mm, a grip length of 70 mm on both ends and a width of 60 mm. The sample was loaded in the longitudinal direction that was parallel to the longitudinal grain direction of the aluminium alloy sheet. 2.2. Measurement of coefficient of friction The coefficient of friction at the interface between the two riveted sheets for the three types of samples was measured using a direct shear apparatus. The essential features of the apparatus are shown diagrammatically in Fig. 3. The equipment operates by applying a vertical
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Fig. 2. Specimen geometry and the direction of the applied load.
Fig. 3. Direct shear apparatus.
(normal) force, N, to a pair of sheets through a loading plate. A shear stress was gradually applied on a horizontal plane by causing the two halves of the box to move against each other. The shear force, T, was automatically measured when a relative movement occurred and reached a steady state. The coefficient of friction l was then calculated by using Eq. (1) below: l¼
T : N
were stopped after a number of cycles. After testing, the wear scars on the two riveted sheets and the PTFE tape were examined by scanning electron microscopy/ energy dispersive X-ray analysis (SEM/EDX). Elemental analysis was carried out at a beam voltage of 20 kV.
3. Results ð1Þ
Pairs of sheets measuring 60 mm · 60 mm were used for these measurements. Three different normal loads N were applied leading to three different shear forces, T, for each interfacial condition. A plot of T against N yielded a straight line. The coefficient of friction l was then determined by calculating the slope of the line for each interfacial condition. 2.3. Test parameters All specimens were tested using a close-loop servohydraulic universal test machine. A cyclic tension–tension load with a sinusoidal waveform and a frequency of 20 Hz was employed as the applied load. Five cyclic load levels having a different maximum load and a minimum load of 0.5 kN were tested. The values of the maximum load ranged from 2.7 to 4.5 kN. In addition to fatigue tests leading to fracture of samples, some tests
3.1. Coefficient of friction measurements The results of the coefficient of friction measurements for the three different types of samples are listed in Table 2. The uncoated and the wax-based lubricant-coated samples had similar coefficient of friction values of 0.26 and 0.24, respectively. The coefficient of friction for the PTFE-insert samples was around 0.03 which was about an order of magnitude lower than the un-
Table 2 Coefficient of friction at different interfaces Samples
Interface
Coefficient of friction
Lubricant-coated Plain-surface PTFE-insert
Pierced sheet/locked sheet Pierced sheet/locked sheet Pierced sheet/PTFE/locked sheet
0.26 0.24 0.03
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Maximum applied load (kN)
5 Lubricant-coated Uncoated
4.5
PTFE-insert 4 3.5 3 2.5 4.5
5
5.5
6
6.5
7
LogN
Fig. 4. Fatigue test results of the three groups of samples.
coated and the lubricant-coated pairs of the 5754 alloy sheet. 3.2. Fatigue endurance The fatigue test results for the three groups of samples are shown in Fig. 4. The lubricant-coated samples exhibited the longest fatigue life of the three types of samples, while the PTFE-insert samples had the shortest fatigue life. Fig. 4 also indicates that the difference in the fatigue life between the wax-based lubricant-coated sample and the uncoated sample was wider at low fatigue load values than at higher ones. 3.3. Observation of fretting Examination of fractured samples following fatigue testing indicated the presence of fretting scars at the interface between the riveted aluminium alloy sheets for all the lubricant-coated samples at all applied loads. As shown in Fig. 5, both the pierced and the locked sheets had suffered from fretting damage and the size of the fretting area differed as the value of the applied load changed. Fig. 6(a)–(d) show typical fretting scars and fretting damage at the interface between the two riveted sheets for the lubricant-coated samples. Clearly defined fretting scars were noticeable on the surface of both the riveted sheets at a peak load of 4.5 kN after 21,000 cycles, with the central region appearing to be
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bright, while the surrounding scars were dark, as shown in Fig. 6(a). After 89,000 cycles at the same peak load, the fretting area increased and the bright central region had vanished, as shown in Fig. 6(b). As the fretting process continued, after 173,200 cycles at the same load, material delamination occurred, as shown in Fig. 6(c). In addition, the figures also indicate the effect of the magnitude of the applied load on the fretting behaviour. The fretting regimes, shown in Fig. 6(a), vary from 5 to 8 mm in length and from 2 to 4 mm in width. Fig. 6(d) presents the fretting scars on both sheets around the rivet after 850,000 cycles at 2.7 kN. It is evident that the fretting area of the joint was much smaller than that tested at 4.5 kN for 21,000 cycles even though the former had been tested for a much higher number of cycles. The development of fretting scars during fatigue testing of the uncoated samples was very similar to that of the lubricant-coated samples. Fretting scars and evidence of delamination during fatigue testing of the riveted uncoated sheets are shown in Fig. 7(a) and (b). Inserting a PTFE tape at the interface between the two riveted sheets prevented direct contact between them. Examination of samples following fatigue testing indicated that fretting damage occurred only for samples that were tested at a maximum fatigue load of 4.5 kN. Fretting scars were observed on both the pierced and the locked sheets as presented in Fig. 8(a) and (b). The size of the fretting regions had grown to around 5 mm in length and 1–2 mm in width after testing for 97,890 cycles. The size of the fretting scars was much smaller than that observed for the lubricant-coated and the uncoated samples at the same load level. In spite of the fact that the two sheets had been separated by the PTFE layer, surface damage of the riveted sheets characterised by delamination was also observed as shown in Fig. 8(c) and (d). There was no visible evidence of any fretting scars at all on the alloy surface at lower fatigue loads as indicated in Fig. 9(a) which presents the surface of the pierced sheet of a sample that was tested at a peak load of 3 kN. However, SEM examination of the PTFE tape in Fig. 9(b) revealed damage of the PTFE tape. Following fatigue testing, the failure mechanisms were also examined. The lubricant-coated and the uncoated
Fig. 5. The interface between the two riveted sheets for the lubricant-coated samples fractured following fatigue testing at: (a) 4.5, (b) 2.7 kN.
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Fig. 6. Fretting regions at the interfaces between the two sheets for the lubricant-coated samples: (a) after 2.1 · 104 cycles at 4.5 kN; (b) after 8.9 · 104 cycles at 4.5 kN; (c) delamination after 137,320 cycles at 4.5 kN, (d) after 8.5 · 105 cycles at 2.7 kN.
Fig. 7. Fretting damage on the riveted sheets for an uncoated sample after 215,060 cycles at 4.0 kN, (a) fretting debris on the pierced sheet, (b) delamination on the locked sheet.
samples failed in a similar way. As shown in Fig. 5(a) and (b), at the higher loads, rivet fracture led to failure of the samples, whilst at the lower fatigue loads the samples failed by fracture of the locked sheet. In the samples where PTFE tape was inserted at the interface between the two sheets, rivet fracture was the only means of failure that was observed at all fatigue loads, as shown in Fig. 10(a) and (b).
4. Discussion 4.1. Effect of the wax-based lubricant coating on fretting wear As shown in Fig. 6(a), two distinct fretting regions developed during the early stages of the fatigue test. EDX analysis of these fretting regions revealed the pres-
ence of high carbon content within the central region. This suggested that the central region was still covered by the solid lubricant. The outer region had turned black and represented those parts where the lubricant had been removed and the alloy surface had been exposed. Further examination indicated that ploughing took place during the early stages due to the cyclic micro-sliding movement, leading to plastic deformation and loss of material, as shown in Fig. 11(a). For lubricant-coated samples, the fretting process initially involved removal of the solid lubricant followed by fretting damage of the alloy surface. Once the alloy surface had been exposed, detachment of the alloy material occurred as a result of a continuous wear process. The debris produced by the wear process accumulated at the contact interface and formed a third-body particle to promote further three-body fretting. EDX analysis
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Fig. 8. Contact surfaces of a PTFE-insert sample after 97,890 cycles at 4.5 kN: (a) fretting scars on the pierced sheet, (b) on the locked sheet, (c) damaged surface of the pierced sheet, (d) damaged surface of the locked sheet.
Fig. 9. Contact surfaces of a PTFE-insert sample after 604,403 cycles at 3.0 kN: (a) no fretting scars on the pierced sheet, (b) damaged surface of the PTFE tape.
Fig. 10. The interface between the two riveted sheets for the PTFE-insert samples fractured following fatigue testing at: (a) 4.5, (b) 2.7 kN.
of the debris, shown in Fig. 11(b), indicated that the debris consisted of mainly aluminium oxide and a small amount of magnesium oxide as well as fragments of the lubricant, as listed in Table 3. Previous work by the present authors [10] showed that fretting contributed to the formation of fatigue cracks and failure of the
samples. The presence of the solid surface lubricant was effective in reducing fretting damage during the early stages of the fatigue test. The durability of the coating was rather low and decreased rapidly as the peak applied load increased. At the lower applied load values, the durability was higher since solid lubricants
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Fig. 11. (a) Surface roughening occurred on the locked sheet after 2.1 · 104 cycles at 4.5 kN. (b) Accumulated debris on the locked sheet after 173,200 cycles at 4.5 kN.
Table 3 EDX analysis of the debris shown in Fig. 11(b) Element
wt%
at.%
Al Mg Cl Si C Na O
007.25 000.44 000.19 000.45 071.77 000.25 019.65
003.57 000.24 000.07 000.21 079.43 000.15 016.33
Total
100.00
100.00
are more effective under conditions of partial slip and at low slip amplitudes [16] as well as low applied loads. For this reason, the difference in the fatigue life between the lubricant-coated and the uncoated samples was much higher at the lower fatigue load values. It was concluded that the solid surface lubricant led to an incubation period particularly at the lower fatigue load values where its durability was higher. This delayed the onset of fretting wear at the alloy surface and led to a slightly higher fatigue life for the lubricant-coated samples. On the other hand, it appears that for the uncoated samples, fretting damage started to take place on the alloy surface virtually straight at the beginning of the dynamic test, thus resulting in a reduction in the fatigue life of the joint. 4.2. Effect of the PTFE insert on fretting wear The PTFE tape was sandwiched between the two riveted sheets thus preventing direct contact between them at the critical zone where fretting damage was likely to take place. The use of the PTFE layer was successful in preventing fretting damage at all applied loads except at 4.5 kN which was the highest load used in the study. Under the highest applied load of 4.5 kN, the contact pressure at this interface was higher leading to a greater contact area [17]. The possibility of contact between asperities of the alloy sheets was higher and therefore this could cause more fretting damage under the effect of the cyclic movement. In addition, at a maximum applied load of 4.5 kN, the cyclic micro-sliding movement
led to two-body fretting at the alloy sheet/PTFE interface during the initial stages of testing and to possible wear damage to the PTFE layer. As the wear process continued this damage could have become severe enough for the PTFE to be torn away and allow direct contact between asperities of the alloy sheets. Fretting damage therefore took place at the alloy surface. At the lower load levels, two-body fretting led to surface damage of the PTFE tape, as shown in Fig. 9(b). However, under these conditions, the applied load (contact pressure) was too low to cause fretting wear of the aluminium sheets. 4.3. Effect of the frictional force on fretting wear In a self-piercing riveted joint, the normal load is interrelated to the applied load. The lower the applied load the lower the normal load leading to a smaller fretting area [17], as shown by comparing Fig. 6(b) and (d). This suggested that the lower the frictional force the smaller the fretting area, since the frictional force was directly proportional to the normal load. The inserted PTFE tape induced a very low coefficient of friction at the interface between the PTFE tape and the riveted sheet, as shown in Table 2. Consequently, at an identical normal load, the frictional force at this interface for the PTFE-insert samples was about 10 times lower than that for the coated and uncoated samples. This, along with the fact that the PTFE layer prevented direct contact between the aluminium alloy sheets, prevented fretting damage at the lower fatigue loads. At 4.5 kN which was the highest fatigue load tested in the study fretting damage did take place. Because of the low coefficient of friction, a much smaller fretting area resulted in the PTFE-insert samples. In addition, the frictional force is also directly related to the tangential force in the case of incipient sliding [18]. The lower frictional force therefore led to a lower tangential force at the contact surface. When the tangential stress was small, the amount of plastic deformation of micro-contact would be expected to be less since the probability of asperity interactions drops
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life. The low frictional force at the interface between the riveted sheets therefore led to a significant increase in the bearing load on the rivet. As a result all the PTFE insert samples failed by fracture of the rivet.
5. Conclusions Based on the results of the present investigation, the following conclusions can be drawn: Fig. 12. Debris that accumulated on the PTFE tape of a PTFE insert sample after 97,890 cycles at 4.5 kN.
and the amount of adhesive transfer is less [19]. However, in a fretting fatigue degradation process, plastic deformation is important in establishing a surface morphology which is long-lasting and therefore influences the future course of wear. The third-body of fretting is mainly produced at this stage. At the highest applied load of 4.5 kN, the normal load was higher resulting in a higher frictional force at the interface between the PTFE tape and the riveted sheet causing damage to the PTFE layer. It was possible that the damage to the PTFE layer led to direct contact between the asperities of the alloy sheets and therefore the frictional force at this interface increased slightly. Fretting wear of the aluminium sheets was initiated and the fretting debris became trapped at the alloy sheet/PTFE interface leading to three-body fretting, as shown in Fig. 12. As a result, fretting scars were observed, but the fretting area was much smaller in comparison with the samples without the PTFE insert. In contrast, at lower applied loads the normal load and the frictional force were also lower and therefore there was not sufficient damage to the PTFE layer and to the riveted sheets. The overall effect of the PTFE tape was therefore to reduce or eliminate the fretting damage. However, the fatigue results recorded a decrease in the fatigue life of the PTFE-insert samples. Moreover, examination of the fractured samples indicated that all the samples with the PTFE insert failed only by rivet fracture. As stated previously, this failure mechanism was very different compared to the samples where PTFE was not applied. This was because the much lower frictional force that was induced at the interface between the riveted sheets altered the load transfer mechanism of the joint. In a lap-joint, part of the cyclic applied load is transferred by friction between the contact surfaces, whilst the rest is transferred by bearing between the fastener shank and the sheet adjacent to the rivet. If the fraction of the load transferred by friction is reduced, the bearing load on the fastener is consequently increased and this could lead to earlier fatigue initiation and propagation of cracks at the fastener, resulting in a reduced fatigue
1. The fretting behaviour of a self-piercing riveted joint was affected by the interfacial conditions leading to different fatigue life. 2. The presence of a wax-based solid surface lubricant could delay the onset of fretting damage on the alloy surface leading to extended fatigue life. The solid lubricant was particularly more effective at lower fatigue load levels. 3. The application of a PTFE insert at the interface between the riveted sheets eliminated fretting at the lower fatigue loads and significantly reduced fretting damage at a peak fatigue load of 4.5 kN. However, this led to a reduction in the fatigue life of the selfpiercing riveted joints. 4. The failure mechanism of a self-piercing riveted joint was affected by the interfacial condition which could alter the load transfer mechanism of the joint.
Acknowledgements The authors thank Alcan International for providing financial support for the project. Thanks are expressed to Mr. Dean Homewood of Textron Fastening Systems for the preparation of samples.
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