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Influence of sheet thickness on fatigue behavior and fretting of self-piercing riveted joints in aluminum alloy 5052
Materials and Design 87 (2015) 1010–1017
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Influence of sheet thickness on fatigue behavior and fretting of self-piercing riveted joints in aluminum alloy 5052 Lun Zhao a, Xiaocong He a,⁎, Baoying Xing a, Yi Lu a, Fengshou Gu b, Andrew Ball b a b
Innovative Manufacturing Research Centre, Kunming University of Science and Technology, Kunming 650500, P R China Centre for Efficiency and Performance Engineering, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
a r t i c l e
i n f o
Article history: Received 16 June 2015 Received in revised form 22 August 2015 Accepted 24 August 2015 Available online 29 August 2015 Keywords: Self-piercing riveting Sheet thickness Failure positions Fretting Fretting failure mechanism
a b s t r a c t This article deals mainly with the influence of sheet thickness on the fatigue behavior and fretting of single-lap self-piercing riveted joints in aluminum alloy 5052. The fatigue lives, failure modes, failure positions, fretting and fretting failure mechanism of the joints were investigated. A statistical analysis was performed to analyze the rationality of fatigue test data. The results showed that the fatigue life of the joints increases with increasing sheet thickness, but the increase of the fatigue life is limited. And, with increasing sheet thickness the fatigue failure positions transferred from the pierced sheet to the locked sheet. In all the joints fretting resulted in the initiation and propagation of fatigue cracks at the interfaces. Increasing sheet thickness could decrease the size and degree of fretting at the interface between the two riveted sheets. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The most direct approach to manufacture more energy-efficient vehicles is to reduce the weight of the vehicle bodies. Consequently, the number of light-weight metal alloys such as aluminum alloy being used in the automobile manufacturing industry is increasing [1,2]. However, there are issues with the conventional spot-welding of aluminum alloys, for example surface sensitivity and short electrode tip life. Considerable effort has been expended to develop various joining processes and assess their suitability for use in light-weight metal alloys. In engineering design the use of friction stir welding [3], laser welding [4], and mechanical clinching [5] are among the most interesting joining technologies. Self-piercing riveting (SPR) is also a quite new technique which is suitable for joining new light-weight sheet materials and especially for joining aluminum sheets. SPR joining techniques possess lots of advantages, such as environmental friendliness, safety, dependability and simplicity [6]. In the past several years, many papers reported investigations into the mechanical behavior and fretting of SPR aluminum joints. The purpose of research reported in Mori et al.’s paper [7] was to join multiple steel and aluminum alloy sheets using the SPR technique. The steel sheets ranged from mild steel to ultra-high strength one having a tensile strength of 980 Mpa. The join-ability for three steel and aluminum alloy sheets in various combinations was examined using
⁎ Corresponding author. E-mail address: [email protected] (X. He).
http://dx.doi.org/10.1016/j.matdes.2015.08.121 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
experiments and finite element simulation. The join-ability was improved by setting the softer sheet uppermost, due to smoother piercing. In addition, the joining range for the SPR of three high strength steel and aluminum alloy sheets was extended by optimizing the shape of the die. These measures enabled ultra-high strength steel, mild steel and aluminum alloy sheets to be joined successfully. An article by Wood et al. [8] reported studies into the performance of SPR joints in aluminum sheet (A5754) at typical automotive crash speeds. Significant findings relate to the design of the fixture and the force transducer to test the U shaped tension specimens over the speed range of interest. A finite element model of the fixture and the test measurement system was developed to ensure a near optimal design. A simple and effective model was developed to determine rivet flaring in SPR joints [9]. Both interrupted and un-interrupted SPR experiments were performed with recording of force and punch displacement data. Based on the analysis of experimental SPR process data and the examination of cross-sections of SPR samples, two key points of the force-displacement curves were identified that mark the start and end of rivet flaring, respectively. It was found that these two events were able to be related to the path of the tip of the flaring rivet and hence the internal characteristics of the SPR samples via a simplified geometric approach. 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 [10]. The results showed that 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.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
Mucha's recent paper presented the analysis of residual stress origin and its effect on rivet failure in SPR joints [11]. Finite element (FE) method was used to analyze the residual stress distribution in the rivet. The effect of maximum joint forming force value on its maximum strength and residual stress distribution in the rivet was described. The theoretical considerations on safety coefficient determination for the rivet material were also presented. The fracture mechanics of elastic–plastic model body was characterized by using the energy state equations. The aging of steel/aluminum SPR joints in salt spray environment was studied by Calabrese et al. to evaluate their mechanical degradation in these critical environmental conditions [12]. The investigation was carried out on symmetrical or unsymmetrical joints at varying the total thicknesses. The experimental results evidenced that the corrosion degradation phenomena influence significantly the performances and the failure mechanisms of the joints, inducing premature failure of the joint at lower stress level. In a similar study, galvanic corrosion of steel/aluminum SPR joints was investigated [13]. Potentiodynamic polarization tests, performed in 3.5 wt.% NaCl solution, evidenced the anodic and cathodic behavior of metal constituent of the joints. Furthermore, long term aging tests were performed to evaluate the relationship among failure mechanism, joint configuration and corrosion damage in salt spray environment. A theoretical model was proposed to forecast failure modes. Based on this model a simplified map of failure mechanisms promoted by corrosion phenomena has been drawn. Li et al. [14] investigated the influence of distance between the sheet edge and the rivets on the fatigue performance of SPR joints in lap-shear and coach-peel specimens. Static and fatigue tests in lap-shear specimens were carried out on SPR aluminum joints with different edge distances to investigate the effect of edge distance on joint quality, fatigue strength and failure modes. The results showed that to obtain an acceptable lap shear and coach-peel fatigue performance, the edge distance has to be more than 8 mm. The length of the developing crack before joint specimens lost strength was the primary determinant of fatigue life in various specimens during the coach-peel fatigue tests. For lapshear specimens under fatigue tests, the degree of concentration of stress and the subsequent crack initiation time were the dominant factors determining fatigue life. Calabrese et al. [15] reported on research into the durability behavior and performance of symmetrical or asymmetrical SPR joints in aluminum alloy sheets. Long term aging tests were performed to evaluate the durability of the mechanical joint in critical environmental conditions. The experimental results showed that the performance and failure mechanisms of SPR joints are significantly affected by degradation due to corrosion. A theoretical model was proposed to predict the failure modes. A simplified failure map of the failure mechanisms and the effects of corrosion were investigated. The shearing strength analysis of double joints made using various joining techniques was studied by Mucha et al. [16]. The capabilities of S350 GD sheet metal joining using the Clinch-rivet technique were presented. The results achieved for joints arranged in parallel and perpendicularly to the load for specified joint spacings were discussed. An assessment of joint effectiveness was performed for both homogenous double joints and different combinations of these joints. The main reason for the decrease in fatigue life is the initiation and propagation of cracks. Although there is no external fatigue stress in fretting, the local contact load may give rise to the formation and propagation of cracks. Zhou and Leo [17] investigated fretting on different materials combinations. The results proved that fretting could accelerate the initiation and propagation of cracks or even change the direction of fatigue cracking, resulting in fretting damage which can decrease the fatigue life of the specimens. Carlos et al. [18] found that fretting fatigue arising from the cyclic sliding of two contacting surfaces with small relative displacements between them is detrimental to the materials. One or both of the components in contact may be subject to bulk stresses caused by cyclic loads. The assessment of the fretting fatigue strength and life of any component is complicated by the many parameters involved, the complexity of the stress field cyclic variation during fretting
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and uncertainties about the contact conditions. Some singular aspects of fretting fatigue related to strength analysis and testing were described. A procedure developed by the authors to estimate the fretting fatigue strength and life were presented. The assessment outcomes with the results of tests performed by different authors were also compared. Chen et al. [19] investigated fretting wear in aluminum alloy AA5754 SPR joints. Various degradation zones were characterized in the contact surfaces between the aluminum sheets, as well as the rivet and the locked sheet. Subsequent examination of the fretting scars at the contact surfaces and through the cross-section was carried out using Scanning Electron Microscope (SEM). The fretting patterns in the SPR joints were identified. It was noted that fretting wear was initially patchy and layers of compacted debris were created as fretting continued. An Energy Dispersive Xray (EDX) analysis of the fretting debris from the interface between the rivet and the aluminum sheet revealed the presence of Al, Fe, Si, C and O indicating that intimate mixing of debris from the aluminum alloy and the steel rivet had occurred. The effects of testing conditions such as the magnitude of the load and the number of loading cycles on the fretting wear patterns were also studied. Han et al. [20] characterized fretting fatigue in single-lap SPR joints of aluminum alloy 5754 sheets. The results demonstrated that fretting occurs at three different positions in the SPR joints. It was established that fretting led to surface work-hardening and crack initiation as well as early stage crack propagation. Cracking began at the surface of the riveted sheets as a result of high stress concentration and propagated obliquely to the mating surface under the effect of fretting fatigue. The depth of damage due to fretting depended on the magnitude of the applied load and the cycle time. In present paper, fatigue experiments of single-lap SPR joints in aluminum alloy 5052 (AA5052) were conducted. F–N curves were obtained to characterize the fatigue life of the joints. The impact of sheet thickness on failure modes, failure positions, fretting and fretting failure mechanism of the joints was also studied. The failure surfaces were observed by an optical microscope and a SEM. The fretting debris was tested by an EDX detector. The results were used to investigate the influence of sheet thickness on fatigue behavior and fretting of the joints. 2. Experimental procedure 2.1. Materials The nominal compositions and the mechanical properties of AA5052 sheet and the rivet are listed in Tables 1 and 2 respectively. The mechanical properties of AA5052 were measured using an extensometer with 20 mm gage length on an MTS servo hydraulic test machine. The mechanical properties of the rivet were obtained using an AG-IS10KN mechanical test machine. 2.2. Specimen preparation Specimens were made up as lap-shear joints, a common type of joint in vehicle structures. Specimen geometries and dimensions are
Table 1 Compositions and mechanical properties of AA5052 sheet. Compositions (balance Al) wt% Si
Cu
Zn
Mn
Cr
Mg
Fe
0–0.25
0–0.10
0–0.1
0–0.1
0.15–0.35
2.2–2.8
0–0.40
Mechanical properties of AA5052 Young's Modulus (GPa)
Tensile strength(MPa)
Yield strength (MPa)
Elongation (%)
69.45
234.18
185.26
13.78
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Table 2 Compositions and mechanical properties of the rivet. Compositions (balance Fe) wt% Mn
C
Si
Cr
Cu
P
S
B
0.8–1.10
0.33–0.38
0–0.30
0–0.30
0–0.025
0–0.025
0–0.025
0.0008–0.005
Mechanical properties of the rivet Young's modulus (GPa)
Poisson's ratio
Yield strength (MPa)
Compressive strength (MPa)
Rockwell hardness (HRC)
206
0.3
1719.7
1595.6
44
presented in Fig. 1. In order to obtain satisfied qualities of the results in the static and fatigue experiments, all specimens design referred to welding standard (GB-2649). The specimens were produced using RIVSET VARIO-FC (MTF) systems. The specimens were prepared using three sheet thicknesses 1.5, 2.0 and 2.5 mm, corresponding to rivets of 5, 6 and 7 mm length respectively. All rivets were 5.3 mm in diameter. In order to make it easy to describe the single lap aluminum SPR joints with different sheet thicknesses, the following nomenclature is used: S1515 joint: SPR joints with similar 1.5 mm thickness aluminum alloy sheets; S2020 joint: SPR joints with similar 2.0 mm thickness aluminum alloy sheets; and S2525 joint: SPR joints with similar 2.5 mm thickness aluminum alloy sheets. Table 3 shows the dimensions of the specimens and Table 4 presents the satisfied joining parameters of the specimens. The typical crosssections of S1515, S2020 and S2525 joints are shown in Fig. 2. H is the head height of the rivet, I is the interlock of the rivet, S is the spread of the rivet and T is the remaining bottom thickness of the joint. These parameters are normally used as quality criteria for SPR joints.
glued to all specimens on both ends. A test was performed for more than 2 million cycles or until visible failure occurred, whichever came first. Five fatigue load levels from 70% to 30% were chosen to demonstrate the fatigue behavior of joints and two or three specimens were tested for each load level. The statistical parameters of the fatigue life are shown in Table 5. Specimens were labeled as ‘n-t’, where ‘n’ denoted the load level and ‘t’ referred to the type of joints. Weibull distribution is widely applied to analyze the rationality of fatigue life data. Because the number of specimens used at each load level were quite small, it was hard to determine the Weibull parameters by a common approach. But the coefficient of variation (CV) can be used to estimate the parameters [21]. In this paper, a two-parameter Weibull Distribution was used to analyze the rationality of the fatigue life data. The two-parameter Weibull probability density function can be written as [21]: f ðxÞ ¼
In engineering practice, the SPR joints are likely to perform under complicated and unstable load conditions. The joints must meet the demanding requirements with respect to static strength and fatigue life. Therefore, the fatigue performance of SPR joints with three different sheet thicknesses was investigated in this study. According to previous static tests, the average of failure loads for S1515, S2020, S2525 specimens was 3115.39 N, 4488.07 N and 5442.82 N respectively. The fatigue tested parameters were determined based on the average of failure load obtained from static tests. A load-controlled cyclic tension-tension fatigue test was performed on the MTS servo hydraulic testing machine using a sine waveform with load ratio R = 0.1. The frequency f = 20 Hz was carried out for all tested specimens. In purpose of decreasing bending deformation of the specimens during the tests and ensuring alignment of load paths, spacer with corresponding dimension were
ð1Þ
α¼
1:2 CV
ð2Þ
β¼
m Γ 1þ1 α
ð3Þ
3. Influence of sheet thickness on fatigue behavior and fretting 3.1. Fatigue test and data analysis
α α x α−1 x exp − β β β
where f(x) is the probability density function, α is the value of shape parameter, β is the corresponding scale parameter, CV is the coefficient of variation, Г is the gamma function and m is the mean of fatigue life. The analytic results of the fatigue life of SPR joints are shown in Table 5. The values of β is defined as 63.2% of the failure probability and it can be seen from Table 5 that values of β are greater than those of mean fatigue life and thus all of the data fitted a two-parameter Weibull Distribution, and the rationality of the fatigue data was analyzed. The least square lines of the fatigue load and fatigue life on lg–lg coordinates are fitted by least squares method (LSM). The fatigue loadfatigue life (F–N) curves derived from test data and the fitted lines are shown in Fig. 3. The calculated equations of F–N curves are NS2020 = 106.780F−4.691, NS1515 = 106.148F−4.038, NS1515 = 107.532F−5.703.
Fig. 1. Geometry of the specimen.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017 Table 3 Dimensions of the specimens. Specimen S1515 S2020 S2525
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Table 5 Statistical parameters of fatigue life.
Sheet material
Sheet thickness/mm
Rivet diameter/mm
Rivet length/mm
AA5052 AA5052 AA5052
1.5 2.0 2.5
5.3 5.3 5.3
5 6 7
Table 4 Joining parameters of the specimens. Specimen
Punch travel/mm
Material check pressure/bar
Riveting pressure/bar
Compressing pressure/bar
S1515 S2020 S2525
132.46 130.38 128.50
50 50 50
130 180 220
130 180 110
n-t
Mean of life
CV of life
α of life
β of life
5- S1515 4- S1515 3- S1515 2- S1515 1-S1515 5- S2020 4- S2020 3- S2020 2- S2020 1- S2020 5- S2525 4- S2525 3- S2525 2- S2525 1-S2525
38,861 72,046 221,718 383,088 2,059,561 66,380 108,340 175,140 660,025 1,765,636 43,854 135,417 335,908 1,066,047 2,098,812
0.030 0.118 0.450 0.106 0.0309 0.028 0.173 0.201 0.140 0.335 0.210 0.228 0.223 0.180 0.067
40.213 10.153 2.665 11.314 39.790 42.981 6.933 5.958 8.534 3.583 5.711 5.274 5.381 6.656 18.023
39,403 75,683 249,450 400,670 2,088,600 67,248 115,880 188,860 698,710 1,960,000 47,401 147,034 364,306 1,142,649 2,161,780
3.2. Influence of sheet thickness on fatigue life Fig. 3 clearly shows that the fatigue life is highly enhanced under any fatigue load when sheet thickness is increased from 1.5 mm to 2.0 mm. The main reason for the decrease of fatigue life is the initiation and propagation of the fatigue cracks. Fatigue life normally consists of three stages, crack initiation, crack propagation and rapid failure. The stages of crack initiation and propagation account for the most of fatigue life. Increasing the sheet thickness delayed crack initiation, because the thicker sheet had higher resistance to external load. Also, with an increase in sheet thickness the length of the crack propagation path is increased. In the propagation phase, the fatigue cracks need to traverse grain boundaries and with increasing sheet thickness the grain boundaries are also increased. Therefore, the fatigue crack takes longer to traverse the grain boundaries. These reasons explain why an increase in sheet thickness could increase the fatigue life of the joints. However, the fatigue lives under high fatigue load and low fatigue load are quite different when sheet thickness is increased from 2.0 mm to 2.5 mm. It can be seen from Fig. 3 that S2525 and S2020 specimens possess comparable fatigue life under high fatigue load. But with the decline of the fatigue load, the fatigue life of S2525 specimens are slowly greater than that of S2020 specimens. Therefore, fatigue life is influenced significantly by sheet thickness under low fatigue load, but it is less influenced by sheet thickness under high fatigue load. Besides, Fig. 3 shows when the sheet thickness is increased from 1.5 mm to 2.0 mm, the fatigue life of specimens is enhanced more than when sheet thickness is increased from 2.0 mm to 2.5 mm. This means with an increase in sheet thickness the increase of fatigue life is limited. 3.3. Influence of sheet thickness on fatigue failure modes and positions Fatigue failure modes of all specimens with different sheet thicknesses at five various types of load amplitudes during fatigue tests are
shown in Fig. 4. All specimens of S1515 group failed in the pierced sheet next to the rivet head. One S2020 specimen failed in the pierced sheet next to the rivet head, but all the other 11 specimens failed in the locked sheet along joint button. All specimens of S2525 group also fractured in the locked sheet along joint button. It can be seen that sheet thickness does not influence fatigue failure modes of SPR joints. All specimens failed in the sheet materials since the rivets were not pulled out from the locked sheet by the maximum loads during the fatigue tests. Also, the riveted sheets were subjected to cyclic fretting which led to fatigue cracks in the fretting area and eventually resulted in fatigue failure. Fig. 4 shows that the failure positions transferred from the pierced sheet to the locked sheet with increasing sheet thickness. Normally, the main fatigue failure position is located at the locked sheet along the joint button. The material of the locked sheet in the joining area was subjected to greater deformation than that of the pierced sheet during the riveting process because the locked sheet was deformed into the die cavity but the pierced sheet was simply punched with a hole [14] and this might cause some flaws in the locked sheet in contact with the joint shank. Besides, the fretting where the locked sheet was in contact with the rivet shank accelerated the initiation and propagation of fatigue cracks. This is the reason why 11 specimens of S2020 group and all specimens of S2525 group were initially fractured at the locked sheet along the joint button. However, the initial failure position for all specimens in S1515 group was in the pierced sheet next to the rivet head. This is mainly because more severe fretting took place at the interface of the two riveted sheets in S1515 joints (see Fig. 5). The sheets of S1515 group were thin and the fastening of the two riveted sheets were relatively loose, which caused the severe fretting. This could have accelerated the initiation and propagation of fatigue cracks in the pierced sheet and finally led to the failure of S1515 specimens.
Fig. 2. Typical cross-section of SPR joints with different sheet thicknesses.
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Fig. 3. F–N curves of SPR joints with different sheet thicknesses.
3.4. Influence of sheet thickness on fretting Fretting is one of the most significant causes of fatigue failure in SPR joints and it normally takes place in two places in the joints as shown in Fig. 5. Position A was at the interface between the pierced sheet and the locked sheet. Position B was at the interface between rivet shank and the locked sheet. In order to prove that increasing the sheet thickness could decrease the amount and degree of fretting on the interface between the two riveted sheets and this could explain that with increasing sheet thickness the fatigue failure positions transferred from the pierced sheet to the locked sheet. Fretting at position A on the bottom surface of the pierced sheet of SPR joints with three different sheet thicknesses are
shown in Fig. 6. It can be clearly seen that the size and degree of fretting differed depending on the thickness of the sheet. As evident in Fig. 6 (a), there was extremely severe fretting in S1515 specimens. A large fretting debris area about 3.3 mm long and 0.4–0.6 mm wide was found at position A. The fretting debris area was dark in color close to the fatigue crack. For S2020 specimens, the fretting debris area was around 2 mm long and about 0.4–0.6 mm wide, as shown in Fig. 6 (b). Fig. 6 (c) shows that there was very little fretting debris area at position A of S2525 specimens. It is evident that the fretting between the two riveted sheets gradually decreased with increasing sheet thickness. Micrographic images at 2000 times magnification were captured in order to further study the influence of sheet thickness on the fretting at position A. The micrograph of the fretting region in a S1515 pierced sheet is shown in Fig. 6 (d), which presented severe delamination and pitting. Fig. 6 (e) shows many micro-cracks in the area of light-colored debris. These micro-cracks distributed interlacely and became the form of closed loop. Fig. 6 (f) is the micrograph of an S2525 specimen showing reduced delamination, debris and no obvious micro-crack. It is believed that increasing the sheet thickness could enhance the resistance to failure by reducing or even eliminating the relative movement between the two riveted sheets. This would in turn reduce or eliminate the degree of fretting at position A. Therefore, it can be concluded that increasing the sheet thickness could decrease the size and degree of fretting at the interface between the two riveted sheets. 3.5. The analysis of fretting failure mechanism Samples of fretting debris from positions A and B of S1515 specimens were examined by EDX to reveal the chemical composition. The spectrum for the fretting debris at position A of an S1515 specimen is shown in Fig. 7(a). The chemical composition was largely made up of aluminum and oxygen with a small quantity of magnesium so the main ingredient of the fretting debris is deemed to be Al2O3.
Fig. 4. Fatigue failure modes of SPR joints with different sheet thicknesses.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
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Fig. 5. Illustration of fretting positions.
Fig. 7(b) shows the energy spectrum for the fretting debris at position B of an S1515 specimen. The dominant elements were aluminum and oxygen. Again, the main ingredient of the fretting debris is considered to be Al2O3. The fretting debris on the rivet shank surface contained some zinc, tin and other elements in smaller quantities. This is mainly because the rivet was processed to provide an antioxidant function. During the fretting process, the metallic oxide at the contact interface was caused by the continuous cyclic load and micro-movement. During the early stage of fretting, the white layers were generated on the local contact surfaces and larger size aluminum particles were stripped from the contact surface under adhesive and fatigue effects. Subsequently, the white layers and the aluminum particles were broken down by mechanical processes and oxidized to become the metallic oxide (Al2O3) [15]. All specimens in S1515 group failed in the pierced sheet next to the rivet head. Fig. 8 shows the fatigue failure mode after 3 × 105 fatigue cycles. An S1515 specimen was cut along the rivet to show the fretting
debris and joint separation at position A. Fig. 8(a) exhibits a main fatigue crack and fretting debris appeared on the bottom surface of the pierced sheet. Fig. 8(a) also shows the direction of the fatigue crack in relation to the direction of the fretting. The fatigue crack propagated along the width direction of the pierced sheet by approximately 15.8 mm and in the thickness direction by 1.5 mm. The pierced sheet was penetrated by the fatigue crack along the thickness direction of the sheet and finally led to the fracture of S1515 specimen. The failure surface of an S1515 specimen was observed using an optical microscope to further investigate the failure mechanism of S1515 specimens. The micrograph of fretting debris on the bottom surface of the pierced sheet is shown in Fig. 8(b). It shows an obvious fatigue crack at the fretting area and the crack propagated along the width and thickness. This suggests that the fatigue crack of the pierced sheet started at the black fretting regime. Fretting at this position is mainly due to repeated loading which caused a relative micro-movement at the interface between the pierced sheet and the locked sheet. With the micro-movement of the two riveted
Fig. 6. Comparison SEM of fretting on the bottom surface of the pierced sheet with different sheet thicknesses: (a) S1515, (b) S2020, (c) S2525, (d) micrograph of S1515, (e) micrograph of S2020, (f) micrograph of S2525.
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Fig. 7. Spectrum of the fretting debris.
Fig. 8. Fatigue failure mode of S1515 specimen after 3 × 105 fatigue cycles: (a) fatigue failure on the bottom surface of the pierced sheet, and (b) micrograph of fretting debris on the pierced sheet.
sheets, a severe plastic deformation was generated in the surface of fretting area. This could lead to decrease plasticity and increase fragility in the area. Subsequently, lots of micro-cracks initiated in the fretting debris domain under repeated loading and friction. The main microcrack propagated continually along the width and thickness direction and finally formed the main fatigue crack. Eleven of the 12 specimens in S2020 group and all 12 specimens in S2525 group fractured in the locked sheet along the joint button. One of S2020 specimens that failed in the locked sheet after 3 × 105 fatigue cycles was chosen to represent S2020 and S2525 group specimens in further investigations into the fretting failure mechanism. Fig. 9(a) shows that the specimen fractured at the button radiating out from the 3 o'clock and 10 o'clock positions. Fig. 9(b) shows dark fretting
debris on the locked sheet surface in contact with the rivet shank. This suggests that fretting took place at the interface between the locked sheet and the rivet shank because of cyclical sliding movements between them. The fretting debris on the locked sheet surface in contact with the rivet shank was examined using an optical microscope to further understand the failure mechanism of S2020 and S2525 specimens. The micrograph of the central part of the locked sheet surface in contact with rivet shank is shown in Fig. 9(c). Two fatigue cracks on the internal surface of the button can be seen clearly. These were caused by the fretting between the surfaces that resulted in the generation of fretting debris and the initiation of micro-cracks at the internal surface of the button. Also, combining the effect of the tangential force and the applied load, these micro-cracks propagated along the button radiating out from
Fig. 9. Fatigue failure mode of S2020 specimen after 3 × 105 fatigue cycles: (a) top surface of the locked sheet, (b) locked sheet surface in contact with rivet shank, and (c) micrograph for the central part of the locked sheet surface in contact with rivet shank.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
the 3 o'clock position, the similar phenomenon also occurred at the button radiating out from the 10 o'clock position and finally caused the failure of the specimen. 4. Conclusions This paper reports experimental investigations into fatigue behavior and fretting of SPR joints with different sheet thicknesses. F–N curves were obtained to characterize the fatigue life of the joints. The influence of sheet thickness on failure modes, failure positions, fretting and fretting failure mechanism of the joints was also studied. The major findings of this paper can be summarized as follows: (1) overall, the fatigue life of the joints increased with increasing sheet thickness of the specimens, but the increase of fatigue life was limited. Fatigue life was influenced significantly by sheet thickness under low fatigue load, but it was less influenced by sheet thickness under high fatigue load; (2) all specimens failed in the sheet materials in fatigue experiments. Fatigue failure modes of the joints were not affected by sheet thickness. But the fatigue failure positions transferred from the pierced sheet to the locked sheet with increasing sheet thickness; (3) increasing the sheet thickness decreased the amount and degree of fretting on the interface between the two riveted sheets; and (4) the joints fractured because fretting occurred at the interface and this resulted in the initiation and propagation of cracks at the interface during the fatigue experiments.
Acknowledgments This study is partially supported by the National Natural Science Foundation of China (Grant No. 50965009). References [1] N.H. Hoang, O.S. Hopperstad, M. Langseth, I. Westermann, Failure of aluminum selfpiercing rivets: an experimental and numerical study, Mater. Des. 49 (2013) 323–335.
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[2] X. He, L. Zhao, C. Deng, B. Xing, F. Gu, A. Ball, Self-piercing riveting of similar and dissimilar metal sheets of aluminum alloy and copper alloy, Mater. Des. 65 (2015) 923–933. [3] X. He, F. Gu, A. Ball, A review of numerical analysis of friction stir welding, Prog. Mater. Sci. 65 (2014) 1–66. [4] X. He, Finite element analysis of laser welding: a state of art review, Mater. Manuf. Process. 27 (2012) 1354–1365. [5] J. Mucha, The analysis of lock forming mechanism in the clinching joint, Mater. Des. 32 (10) (2011) 4943–4954. [6] J. Mucha, The effect of material properties and joining process parameters on behavior of self-pierce riveting joints made with the solid rivet, Mater. Des. 52 (2013) 932–946. [7] K. Mori, Y. Abe, T. Kato, Self-pierce riveting of multiple steel and aluminum alloy sheets, J. Mater. Process. Technol. 214 (2014) 2002–2008. [8] P.K.C. Wood, C.A. Schley, M.A. Williams, A. Rusinek, A model to describe the high rate performance of self-piercing riveted joints in sheet aluminum, Mater. Des. 32 (2011) 2246–2259. [9] R. Haque, N.S. Williams, S.E. Blacket, Y. Durandet, A simple but effective model for characterizing SPR joints in steel sheet, J. Mater. Process. Technol. 223 (2015) 225–231. [10] S.H. Kang, H.K. Kim, Fatigue strength evaluation of self-piercing riveted Al-5052 joints under different specimen configurations, Int. J. Fatigue 80 (2015) 58–68. [11] J. Mucha, The failure mechanics analysis of the solid self-piercing riveting joints, Eng. Fail. Anal. 47 (2015) 77–88. [12] L. Calabrese, E. Proverbio, G. Di Bella, G. Galtieri, C. Borsellino, Failure behaviour of SPR joints after salt spray test, Eng. Struct. 82 (2015) 33–43. [13] L. Calabrese, E. Proverbio, E. Pollicino, G. Galtieri, C. Borsellino, Effect of galvanic corrosion on durability of aluminium/steel self-piercing rivet joints, Corros. Eng. Sci. Technol. 50 (1) (2015) 10–17. [14] D. Li, L. Han, M. Thornton, M. Shergold, Influence of rivet to sheet edge distance on fatigue strength of self-piercing riveted aluminum joints, Mater. Sci. Eng. A 558 (2012) 242–252. [15] L. Calabrese, L. Bonaccorsi, Durability on alternate immersion test of self-piercing riveting aluminum joint, Mater. Des. 46 (2013) 849–856. [16] J. Mucha, W. Witkowski, The Experimental Analysis of the Double Joint Type Change Effect on the Joint Destruction Process in Uniaxial Shearing Test, Thin-Walled Struct. 66 (2013) 39–49. [17] Z. Zhou, V. Leo, Fretting Wear, Science Press, Beijing, 2002 [in Chinese]. [18] N. Carlos, V. Jesús, D. Jaime, Life assessment in fretting fatigue, Key Eng. Mater. 618 (2014) 99–122. [19] Y. Chen, L. Han, A. Chrysanthou, J. O'Sullivan, Fretting wear in self-piercing riveted aluminum alloy sheet, J. Wear 255 (2003) 1463–1470. [20] L. Han, A. Chrysanthou, K. Young, J. O'Sullivan, Characterization of fretting fatigue in self-piercing riveted aluminum alloy sheets, Fatigue Fract. Eng. Mater. Struct. 29 (2006) 646–654. [21] X. He, S. Oyadiji, Application of coefficient of variation in reliability-based mechanical design and manufacture, J. Mater. Process. Technol. 119 (2001) 374–378.
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Influence of sheet thickness on fatigue behavior and fretting of self-piercing riveted joints in aluminum alloy 5052 Lun Zhao a, Xiaocong He a,⁎, Baoying Xing a, Yi Lu a, Fengshou Gu b, Andrew Ball b a b
Innovative Manufacturing Research Centre, Kunming University of Science and Technology, Kunming 650500, P R China Centre for Efficiency and Performance Engineering, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
a r t i c l e
i n f o
Article history: Received 16 June 2015 Received in revised form 22 August 2015 Accepted 24 August 2015 Available online 29 August 2015 Keywords: Self-piercing riveting Sheet thickness Failure positions Fretting Fretting failure mechanism
a b s t r a c t This article deals mainly with the influence of sheet thickness on the fatigue behavior and fretting of single-lap self-piercing riveted joints in aluminum alloy 5052. The fatigue lives, failure modes, failure positions, fretting and fretting failure mechanism of the joints were investigated. A statistical analysis was performed to analyze the rationality of fatigue test data. The results showed that the fatigue life of the joints increases with increasing sheet thickness, but the increase of the fatigue life is limited. And, with increasing sheet thickness the fatigue failure positions transferred from the pierced sheet to the locked sheet. In all the joints fretting resulted in the initiation and propagation of fatigue cracks at the interfaces. Increasing sheet thickness could decrease the size and degree of fretting at the interface between the two riveted sheets. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The most direct approach to manufacture more energy-efficient vehicles is to reduce the weight of the vehicle bodies. Consequently, the number of light-weight metal alloys such as aluminum alloy being used in the automobile manufacturing industry is increasing [1,2]. However, there are issues with the conventional spot-welding of aluminum alloys, for example surface sensitivity and short electrode tip life. Considerable effort has been expended to develop various joining processes and assess their suitability for use in light-weight metal alloys. In engineering design the use of friction stir welding [3], laser welding [4], and mechanical clinching [5] are among the most interesting joining technologies. Self-piercing riveting (SPR) is also a quite new technique which is suitable for joining new light-weight sheet materials and especially for joining aluminum sheets. SPR joining techniques possess lots of advantages, such as environmental friendliness, safety, dependability and simplicity [6]. In the past several years, many papers reported investigations into the mechanical behavior and fretting of SPR aluminum joints. The purpose of research reported in Mori et al.’s paper [7] was to join multiple steel and aluminum alloy sheets using the SPR technique. The steel sheets ranged from mild steel to ultra-high strength one having a tensile strength of 980 Mpa. The join-ability for three steel and aluminum alloy sheets in various combinations was examined using
⁎ Corresponding author. E-mail address: [email protected] (X. He).
http://dx.doi.org/10.1016/j.matdes.2015.08.121 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
experiments and finite element simulation. The join-ability was improved by setting the softer sheet uppermost, due to smoother piercing. In addition, the joining range for the SPR of three high strength steel and aluminum alloy sheets was extended by optimizing the shape of the die. These measures enabled ultra-high strength steel, mild steel and aluminum alloy sheets to be joined successfully. An article by Wood et al. [8] reported studies into the performance of SPR joints in aluminum sheet (A5754) at typical automotive crash speeds. Significant findings relate to the design of the fixture and the force transducer to test the U shaped tension specimens over the speed range of interest. A finite element model of the fixture and the test measurement system was developed to ensure a near optimal design. A simple and effective model was developed to determine rivet flaring in SPR joints [9]. Both interrupted and un-interrupted SPR experiments were performed with recording of force and punch displacement data. Based on the analysis of experimental SPR process data and the examination of cross-sections of SPR samples, two key points of the force-displacement curves were identified that mark the start and end of rivet flaring, respectively. It was found that these two events were able to be related to the path of the tip of the flaring rivet and hence the internal characteristics of the SPR samples via a simplified geometric approach. 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 [10]. The results showed that 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.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
Mucha's recent paper presented the analysis of residual stress origin and its effect on rivet failure in SPR joints [11]. Finite element (FE) method was used to analyze the residual stress distribution in the rivet. The effect of maximum joint forming force value on its maximum strength and residual stress distribution in the rivet was described. The theoretical considerations on safety coefficient determination for the rivet material were also presented. The fracture mechanics of elastic–plastic model body was characterized by using the energy state equations. The aging of steel/aluminum SPR joints in salt spray environment was studied by Calabrese et al. to evaluate their mechanical degradation in these critical environmental conditions [12]. The investigation was carried out on symmetrical or unsymmetrical joints at varying the total thicknesses. The experimental results evidenced that the corrosion degradation phenomena influence significantly the performances and the failure mechanisms of the joints, inducing premature failure of the joint at lower stress level. In a similar study, galvanic corrosion of steel/aluminum SPR joints was investigated [13]. Potentiodynamic polarization tests, performed in 3.5 wt.% NaCl solution, evidenced the anodic and cathodic behavior of metal constituent of the joints. Furthermore, long term aging tests were performed to evaluate the relationship among failure mechanism, joint configuration and corrosion damage in salt spray environment. A theoretical model was proposed to forecast failure modes. Based on this model a simplified map of failure mechanisms promoted by corrosion phenomena has been drawn. Li et al. [14] investigated the influence of distance between the sheet edge and the rivets on the fatigue performance of SPR joints in lap-shear and coach-peel specimens. Static and fatigue tests in lap-shear specimens were carried out on SPR aluminum joints with different edge distances to investigate the effect of edge distance on joint quality, fatigue strength and failure modes. The results showed that to obtain an acceptable lap shear and coach-peel fatigue performance, the edge distance has to be more than 8 mm. The length of the developing crack before joint specimens lost strength was the primary determinant of fatigue life in various specimens during the coach-peel fatigue tests. For lapshear specimens under fatigue tests, the degree of concentration of stress and the subsequent crack initiation time were the dominant factors determining fatigue life. Calabrese et al. [15] reported on research into the durability behavior and performance of symmetrical or asymmetrical SPR joints in aluminum alloy sheets. Long term aging tests were performed to evaluate the durability of the mechanical joint in critical environmental conditions. The experimental results showed that the performance and failure mechanisms of SPR joints are significantly affected by degradation due to corrosion. A theoretical model was proposed to predict the failure modes. A simplified failure map of the failure mechanisms and the effects of corrosion were investigated. The shearing strength analysis of double joints made using various joining techniques was studied by Mucha et al. [16]. The capabilities of S350 GD sheet metal joining using the Clinch-rivet technique were presented. The results achieved for joints arranged in parallel and perpendicularly to the load for specified joint spacings were discussed. An assessment of joint effectiveness was performed for both homogenous double joints and different combinations of these joints. The main reason for the decrease in fatigue life is the initiation and propagation of cracks. Although there is no external fatigue stress in fretting, the local contact load may give rise to the formation and propagation of cracks. Zhou and Leo [17] investigated fretting on different materials combinations. The results proved that fretting could accelerate the initiation and propagation of cracks or even change the direction of fatigue cracking, resulting in fretting damage which can decrease the fatigue life of the specimens. Carlos et al. [18] found that fretting fatigue arising from the cyclic sliding of two contacting surfaces with small relative displacements between them is detrimental to the materials. One or both of the components in contact may be subject to bulk stresses caused by cyclic loads. The assessment of the fretting fatigue strength and life of any component is complicated by the many parameters involved, the complexity of the stress field cyclic variation during fretting
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and uncertainties about the contact conditions. Some singular aspects of fretting fatigue related to strength analysis and testing were described. A procedure developed by the authors to estimate the fretting fatigue strength and life were presented. The assessment outcomes with the results of tests performed by different authors were also compared. Chen et al. [19] investigated fretting wear in aluminum alloy AA5754 SPR joints. Various degradation zones were characterized in the contact surfaces between the aluminum sheets, as well as the rivet and the locked sheet. Subsequent examination of the fretting scars at the contact surfaces and through the cross-section was carried out using Scanning Electron Microscope (SEM). The fretting patterns in the SPR joints were identified. It was noted that fretting wear was initially patchy and layers of compacted debris were created as fretting continued. An Energy Dispersive Xray (EDX) analysis of the fretting debris from the interface between the rivet and the aluminum sheet revealed the presence of Al, Fe, Si, C and O indicating that intimate mixing of debris from the aluminum alloy and the steel rivet had occurred. The effects of testing conditions such as the magnitude of the load and the number of loading cycles on the fretting wear patterns were also studied. Han et al. [20] characterized fretting fatigue in single-lap SPR joints of aluminum alloy 5754 sheets. The results demonstrated that fretting occurs at three different positions in the SPR joints. It was established that fretting led to surface work-hardening and crack initiation as well as early stage crack propagation. Cracking began at the surface of the riveted sheets as a result of high stress concentration and propagated obliquely to the mating surface under the effect of fretting fatigue. The depth of damage due to fretting depended on the magnitude of the applied load and the cycle time. In present paper, fatigue experiments of single-lap SPR joints in aluminum alloy 5052 (AA5052) were conducted. F–N curves were obtained to characterize the fatigue life of the joints. The impact of sheet thickness on failure modes, failure positions, fretting and fretting failure mechanism of the joints was also studied. The failure surfaces were observed by an optical microscope and a SEM. The fretting debris was tested by an EDX detector. The results were used to investigate the influence of sheet thickness on fatigue behavior and fretting of the joints. 2. Experimental procedure 2.1. Materials The nominal compositions and the mechanical properties of AA5052 sheet and the rivet are listed in Tables 1 and 2 respectively. The mechanical properties of AA5052 were measured using an extensometer with 20 mm gage length on an MTS servo hydraulic test machine. The mechanical properties of the rivet were obtained using an AG-IS10KN mechanical test machine. 2.2. Specimen preparation Specimens were made up as lap-shear joints, a common type of joint in vehicle structures. Specimen geometries and dimensions are
Table 1 Compositions and mechanical properties of AA5052 sheet. Compositions (balance Al) wt% Si
Cu
Zn
Mn
Cr
Mg
Fe
0–0.25
0–0.10
0–0.1
0–0.1
0.15–0.35
2.2–2.8
0–0.40
Mechanical properties of AA5052 Young's Modulus (GPa)
Tensile strength(MPa)
Yield strength (MPa)
Elongation (%)
69.45
234.18
185.26
13.78
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Table 2 Compositions and mechanical properties of the rivet. Compositions (balance Fe) wt% Mn
C
Si
Cr
Cu
P
S
B
0.8–1.10
0.33–0.38
0–0.30
0–0.30
0–0.025
0–0.025
0–0.025
0.0008–0.005
Mechanical properties of the rivet Young's modulus (GPa)
Poisson's ratio
Yield strength (MPa)
Compressive strength (MPa)
Rockwell hardness (HRC)
206
0.3
1719.7
1595.6
44
presented in Fig. 1. In order to obtain satisfied qualities of the results in the static and fatigue experiments, all specimens design referred to welding standard (GB-2649). The specimens were produced using RIVSET VARIO-FC (MTF) systems. The specimens were prepared using three sheet thicknesses 1.5, 2.0 and 2.5 mm, corresponding to rivets of 5, 6 and 7 mm length respectively. All rivets were 5.3 mm in diameter. In order to make it easy to describe the single lap aluminum SPR joints with different sheet thicknesses, the following nomenclature is used: S1515 joint: SPR joints with similar 1.5 mm thickness aluminum alloy sheets; S2020 joint: SPR joints with similar 2.0 mm thickness aluminum alloy sheets; and S2525 joint: SPR joints with similar 2.5 mm thickness aluminum alloy sheets. Table 3 shows the dimensions of the specimens and Table 4 presents the satisfied joining parameters of the specimens. The typical crosssections of S1515, S2020 and S2525 joints are shown in Fig. 2. H is the head height of the rivet, I is the interlock of the rivet, S is the spread of the rivet and T is the remaining bottom thickness of the joint. These parameters are normally used as quality criteria for SPR joints.
glued to all specimens on both ends. A test was performed for more than 2 million cycles or until visible failure occurred, whichever came first. Five fatigue load levels from 70% to 30% were chosen to demonstrate the fatigue behavior of joints and two or three specimens were tested for each load level. The statistical parameters of the fatigue life are shown in Table 5. Specimens were labeled as ‘n-t’, where ‘n’ denoted the load level and ‘t’ referred to the type of joints. Weibull distribution is widely applied to analyze the rationality of fatigue life data. Because the number of specimens used at each load level were quite small, it was hard to determine the Weibull parameters by a common approach. But the coefficient of variation (CV) can be used to estimate the parameters [21]. In this paper, a two-parameter Weibull Distribution was used to analyze the rationality of the fatigue life data. The two-parameter Weibull probability density function can be written as [21]: f ðxÞ ¼
In engineering practice, the SPR joints are likely to perform under complicated and unstable load conditions. The joints must meet the demanding requirements with respect to static strength and fatigue life. Therefore, the fatigue performance of SPR joints with three different sheet thicknesses was investigated in this study. According to previous static tests, the average of failure loads for S1515, S2020, S2525 specimens was 3115.39 N, 4488.07 N and 5442.82 N respectively. The fatigue tested parameters were determined based on the average of failure load obtained from static tests. A load-controlled cyclic tension-tension fatigue test was performed on the MTS servo hydraulic testing machine using a sine waveform with load ratio R = 0.1. The frequency f = 20 Hz was carried out for all tested specimens. In purpose of decreasing bending deformation of the specimens during the tests and ensuring alignment of load paths, spacer with corresponding dimension were
ð1Þ
α¼
1:2 CV
ð2Þ
β¼
m Γ 1þ1 α
ð3Þ
3. Influence of sheet thickness on fatigue behavior and fretting 3.1. Fatigue test and data analysis
α α x α−1 x exp − β β β
where f(x) is the probability density function, α is the value of shape parameter, β is the corresponding scale parameter, CV is the coefficient of variation, Г is the gamma function and m is the mean of fatigue life. The analytic results of the fatigue life of SPR joints are shown in Table 5. The values of β is defined as 63.2% of the failure probability and it can be seen from Table 5 that values of β are greater than those of mean fatigue life and thus all of the data fitted a two-parameter Weibull Distribution, and the rationality of the fatigue data was analyzed. The least square lines of the fatigue load and fatigue life on lg–lg coordinates are fitted by least squares method (LSM). The fatigue loadfatigue life (F–N) curves derived from test data and the fitted lines are shown in Fig. 3. The calculated equations of F–N curves are NS2020 = 106.780F−4.691, NS1515 = 106.148F−4.038, NS1515 = 107.532F−5.703.
Fig. 1. Geometry of the specimen.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017 Table 3 Dimensions of the specimens. Specimen S1515 S2020 S2525
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Table 5 Statistical parameters of fatigue life.
Sheet material
Sheet thickness/mm
Rivet diameter/mm
Rivet length/mm
AA5052 AA5052 AA5052
1.5 2.0 2.5
5.3 5.3 5.3
5 6 7
Table 4 Joining parameters of the specimens. Specimen
Punch travel/mm
Material check pressure/bar
Riveting pressure/bar
Compressing pressure/bar
S1515 S2020 S2525
132.46 130.38 128.50
50 50 50
130 180 220
130 180 110
n-t
Mean of life
CV of life
α of life
β of life
5- S1515 4- S1515 3- S1515 2- S1515 1-S1515 5- S2020 4- S2020 3- S2020 2- S2020 1- S2020 5- S2525 4- S2525 3- S2525 2- S2525 1-S2525
38,861 72,046 221,718 383,088 2,059,561 66,380 108,340 175,140 660,025 1,765,636 43,854 135,417 335,908 1,066,047 2,098,812
0.030 0.118 0.450 0.106 0.0309 0.028 0.173 0.201 0.140 0.335 0.210 0.228 0.223 0.180 0.067
40.213 10.153 2.665 11.314 39.790 42.981 6.933 5.958 8.534 3.583 5.711 5.274 5.381 6.656 18.023
39,403 75,683 249,450 400,670 2,088,600 67,248 115,880 188,860 698,710 1,960,000 47,401 147,034 364,306 1,142,649 2,161,780
3.2. Influence of sheet thickness on fatigue life Fig. 3 clearly shows that the fatigue life is highly enhanced under any fatigue load when sheet thickness is increased from 1.5 mm to 2.0 mm. The main reason for the decrease of fatigue life is the initiation and propagation of the fatigue cracks. Fatigue life normally consists of three stages, crack initiation, crack propagation and rapid failure. The stages of crack initiation and propagation account for the most of fatigue life. Increasing the sheet thickness delayed crack initiation, because the thicker sheet had higher resistance to external load. Also, with an increase in sheet thickness the length of the crack propagation path is increased. In the propagation phase, the fatigue cracks need to traverse grain boundaries and with increasing sheet thickness the grain boundaries are also increased. Therefore, the fatigue crack takes longer to traverse the grain boundaries. These reasons explain why an increase in sheet thickness could increase the fatigue life of the joints. However, the fatigue lives under high fatigue load and low fatigue load are quite different when sheet thickness is increased from 2.0 mm to 2.5 mm. It can be seen from Fig. 3 that S2525 and S2020 specimens possess comparable fatigue life under high fatigue load. But with the decline of the fatigue load, the fatigue life of S2525 specimens are slowly greater than that of S2020 specimens. Therefore, fatigue life is influenced significantly by sheet thickness under low fatigue load, but it is less influenced by sheet thickness under high fatigue load. Besides, Fig. 3 shows when the sheet thickness is increased from 1.5 mm to 2.0 mm, the fatigue life of specimens is enhanced more than when sheet thickness is increased from 2.0 mm to 2.5 mm. This means with an increase in sheet thickness the increase of fatigue life is limited. 3.3. Influence of sheet thickness on fatigue failure modes and positions Fatigue failure modes of all specimens with different sheet thicknesses at five various types of load amplitudes during fatigue tests are
shown in Fig. 4. All specimens of S1515 group failed in the pierced sheet next to the rivet head. One S2020 specimen failed in the pierced sheet next to the rivet head, but all the other 11 specimens failed in the locked sheet along joint button. All specimens of S2525 group also fractured in the locked sheet along joint button. It can be seen that sheet thickness does not influence fatigue failure modes of SPR joints. All specimens failed in the sheet materials since the rivets were not pulled out from the locked sheet by the maximum loads during the fatigue tests. Also, the riveted sheets were subjected to cyclic fretting which led to fatigue cracks in the fretting area and eventually resulted in fatigue failure. Fig. 4 shows that the failure positions transferred from the pierced sheet to the locked sheet with increasing sheet thickness. Normally, the main fatigue failure position is located at the locked sheet along the joint button. The material of the locked sheet in the joining area was subjected to greater deformation than that of the pierced sheet during the riveting process because the locked sheet was deformed into the die cavity but the pierced sheet was simply punched with a hole [14] and this might cause some flaws in the locked sheet in contact with the joint shank. Besides, the fretting where the locked sheet was in contact with the rivet shank accelerated the initiation and propagation of fatigue cracks. This is the reason why 11 specimens of S2020 group and all specimens of S2525 group were initially fractured at the locked sheet along the joint button. However, the initial failure position for all specimens in S1515 group was in the pierced sheet next to the rivet head. This is mainly because more severe fretting took place at the interface of the two riveted sheets in S1515 joints (see Fig. 5). The sheets of S1515 group were thin and the fastening of the two riveted sheets were relatively loose, which caused the severe fretting. This could have accelerated the initiation and propagation of fatigue cracks in the pierced sheet and finally led to the failure of S1515 specimens.
Fig. 2. Typical cross-section of SPR joints with different sheet thicknesses.
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Fig. 3. F–N curves of SPR joints with different sheet thicknesses.
3.4. Influence of sheet thickness on fretting Fretting is one of the most significant causes of fatigue failure in SPR joints and it normally takes place in two places in the joints as shown in Fig. 5. Position A was at the interface between the pierced sheet and the locked sheet. Position B was at the interface between rivet shank and the locked sheet. In order to prove that increasing the sheet thickness could decrease the amount and degree of fretting on the interface between the two riveted sheets and this could explain that with increasing sheet thickness the fatigue failure positions transferred from the pierced sheet to the locked sheet. Fretting at position A on the bottom surface of the pierced sheet of SPR joints with three different sheet thicknesses are
shown in Fig. 6. It can be clearly seen that the size and degree of fretting differed depending on the thickness of the sheet. As evident in Fig. 6 (a), there was extremely severe fretting in S1515 specimens. A large fretting debris area about 3.3 mm long and 0.4–0.6 mm wide was found at position A. The fretting debris area was dark in color close to the fatigue crack. For S2020 specimens, the fretting debris area was around 2 mm long and about 0.4–0.6 mm wide, as shown in Fig. 6 (b). Fig. 6 (c) shows that there was very little fretting debris area at position A of S2525 specimens. It is evident that the fretting between the two riveted sheets gradually decreased with increasing sheet thickness. Micrographic images at 2000 times magnification were captured in order to further study the influence of sheet thickness on the fretting at position A. The micrograph of the fretting region in a S1515 pierced sheet is shown in Fig. 6 (d), which presented severe delamination and pitting. Fig. 6 (e) shows many micro-cracks in the area of light-colored debris. These micro-cracks distributed interlacely and became the form of closed loop. Fig. 6 (f) is the micrograph of an S2525 specimen showing reduced delamination, debris and no obvious micro-crack. It is believed that increasing the sheet thickness could enhance the resistance to failure by reducing or even eliminating the relative movement between the two riveted sheets. This would in turn reduce or eliminate the degree of fretting at position A. Therefore, it can be concluded that increasing the sheet thickness could decrease the size and degree of fretting at the interface between the two riveted sheets. 3.5. The analysis of fretting failure mechanism Samples of fretting debris from positions A and B of S1515 specimens were examined by EDX to reveal the chemical composition. The spectrum for the fretting debris at position A of an S1515 specimen is shown in Fig. 7(a). The chemical composition was largely made up of aluminum and oxygen with a small quantity of magnesium so the main ingredient of the fretting debris is deemed to be Al2O3.
Fig. 4. Fatigue failure modes of SPR joints with different sheet thicknesses.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
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Fig. 5. Illustration of fretting positions.
Fig. 7(b) shows the energy spectrum for the fretting debris at position B of an S1515 specimen. The dominant elements were aluminum and oxygen. Again, the main ingredient of the fretting debris is considered to be Al2O3. The fretting debris on the rivet shank surface contained some zinc, tin and other elements in smaller quantities. This is mainly because the rivet was processed to provide an antioxidant function. During the fretting process, the metallic oxide at the contact interface was caused by the continuous cyclic load and micro-movement. During the early stage of fretting, the white layers were generated on the local contact surfaces and larger size aluminum particles were stripped from the contact surface under adhesive and fatigue effects. Subsequently, the white layers and the aluminum particles were broken down by mechanical processes and oxidized to become the metallic oxide (Al2O3) [15]. All specimens in S1515 group failed in the pierced sheet next to the rivet head. Fig. 8 shows the fatigue failure mode after 3 × 105 fatigue cycles. An S1515 specimen was cut along the rivet to show the fretting
debris and joint separation at position A. Fig. 8(a) exhibits a main fatigue crack and fretting debris appeared on the bottom surface of the pierced sheet. Fig. 8(a) also shows the direction of the fatigue crack in relation to the direction of the fretting. The fatigue crack propagated along the width direction of the pierced sheet by approximately 15.8 mm and in the thickness direction by 1.5 mm. The pierced sheet was penetrated by the fatigue crack along the thickness direction of the sheet and finally led to the fracture of S1515 specimen. The failure surface of an S1515 specimen was observed using an optical microscope to further investigate the failure mechanism of S1515 specimens. The micrograph of fretting debris on the bottom surface of the pierced sheet is shown in Fig. 8(b). It shows an obvious fatigue crack at the fretting area and the crack propagated along the width and thickness. This suggests that the fatigue crack of the pierced sheet started at the black fretting regime. Fretting at this position is mainly due to repeated loading which caused a relative micro-movement at the interface between the pierced sheet and the locked sheet. With the micro-movement of the two riveted
Fig. 6. Comparison SEM of fretting on the bottom surface of the pierced sheet with different sheet thicknesses: (a) S1515, (b) S2020, (c) S2525, (d) micrograph of S1515, (e) micrograph of S2020, (f) micrograph of S2525.
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Fig. 7. Spectrum of the fretting debris.
Fig. 8. Fatigue failure mode of S1515 specimen after 3 × 105 fatigue cycles: (a) fatigue failure on the bottom surface of the pierced sheet, and (b) micrograph of fretting debris on the pierced sheet.
sheets, a severe plastic deformation was generated in the surface of fretting area. This could lead to decrease plasticity and increase fragility in the area. Subsequently, lots of micro-cracks initiated in the fretting debris domain under repeated loading and friction. The main microcrack propagated continually along the width and thickness direction and finally formed the main fatigue crack. Eleven of the 12 specimens in S2020 group and all 12 specimens in S2525 group fractured in the locked sheet along the joint button. One of S2020 specimens that failed in the locked sheet after 3 × 105 fatigue cycles was chosen to represent S2020 and S2525 group specimens in further investigations into the fretting failure mechanism. Fig. 9(a) shows that the specimen fractured at the button radiating out from the 3 o'clock and 10 o'clock positions. Fig. 9(b) shows dark fretting
debris on the locked sheet surface in contact with the rivet shank. This suggests that fretting took place at the interface between the locked sheet and the rivet shank because of cyclical sliding movements between them. The fretting debris on the locked sheet surface in contact with the rivet shank was examined using an optical microscope to further understand the failure mechanism of S2020 and S2525 specimens. The micrograph of the central part of the locked sheet surface in contact with rivet shank is shown in Fig. 9(c). Two fatigue cracks on the internal surface of the button can be seen clearly. These were caused by the fretting between the surfaces that resulted in the generation of fretting debris and the initiation of micro-cracks at the internal surface of the button. Also, combining the effect of the tangential force and the applied load, these micro-cracks propagated along the button radiating out from
Fig. 9. Fatigue failure mode of S2020 specimen after 3 × 105 fatigue cycles: (a) top surface of the locked sheet, (b) locked sheet surface in contact with rivet shank, and (c) micrograph for the central part of the locked sheet surface in contact with rivet shank.
L. Zhao et al. / Materials and Design 87 (2015) 1010–1017
the 3 o'clock position, the similar phenomenon also occurred at the button radiating out from the 10 o'clock position and finally caused the failure of the specimen. 4. Conclusions This paper reports experimental investigations into fatigue behavior and fretting of SPR joints with different sheet thicknesses. F–N curves were obtained to characterize the fatigue life of the joints. The influence of sheet thickness on failure modes, failure positions, fretting and fretting failure mechanism of the joints was also studied. The major findings of this paper can be summarized as follows: (1) overall, the fatigue life of the joints increased with increasing sheet thickness of the specimens, but the increase of fatigue life was limited. Fatigue life was influenced significantly by sheet thickness under low fatigue load, but it was less influenced by sheet thickness under high fatigue load; (2) all specimens failed in the sheet materials in fatigue experiments. Fatigue failure modes of the joints were not affected by sheet thickness. But the fatigue failure positions transferred from the pierced sheet to the locked sheet with increasing sheet thickness; (3) increasing the sheet thickness decreased the amount and degree of fretting on the interface between the two riveted sheets; and (4) the joints fractured because fretting occurred at the interface and this resulted in the initiation and propagation of cracks at the interface during the fatigue experiments.
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