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Article

Low-Speed Impact and Residual Mechanical Performance of the BR1200HS/AA6082 Self-Piercing Riveted Joints

by
Ying-Lian Jia
1,2,
Zhi-Chao Huang
1,* and
Yu-Qiang Jiang
1
1
Key Laboratory for Conveyance and Equipment, Ministry of Education, East China Jiaotong University, Nanchang 330013, China
2
School of Electromechanical Engineering, Jiangxi V&T College of Communications, Nanchang 330013, China
*
Author to whom correspondence should be addressed.
Metals 2023, 13(11), 1800; https://0-doi-org.brum.beds.ac.uk/10.3390/met13111800
Submission received: 26 September 2023 / Revised: 18 October 2023 / Accepted: 19 October 2023 / Published: 26 October 2023
(This article belongs to the Section Welding and Joining)
Browse Figures
Versions Notes

Abstract

:
To investigate the low-speed impact response of the BR1200HS steel and AA6082 aluminum alloy self-piercing riveted (SPR) joint, low-speed impact tests with impact energies of 2.5 J, 5.0 J, 7.5 J, 10.0 J, 12.5 J, 15.0 J, 20.0 J, and 30.0 J were conducted utilizing a drop hammer impact tester. The results show that with the increased impact energy, the visual breakages of the SPR joints become more severe. The maximum impact energy the joints can sustain ranges from 10.0 J to 12.5 J. When the impact energy is less than 12.5 J, the contact force/energy–time curves show similar variations. Moreover, as the impact energy increases, total uptake energy value (Et), maximum uptake energy value (Ef), and maximum contact force (Fm) of the joints increase gradually. The low-speed impact energy has little effect on the maximum static tensile force of the impacted joints. However, the residual energy values decrease with the increase in impact energy. The tensile failure form of the joints is the pulling out of the rivets from the lower plates, and the low-speed impacts have no significant effect on the tensile failure form of the joints.

1. Introduction

Because of the growing energy crisis and the stricter requirements of vehicle emission regulations, the automotive manufacturing industry is facing unprecedented challenges. A lightweight automobile body is currently the key technology for decreasing energy use, while guaranteeing the safety and comfort of users [1,2,3]. With the development of automobile lightweight technology, the mixing of steel/aluminum and other multi-materials for the automobile body structures has already been the unavoidable tendency of automobile design technology development [4,5]. Because of the advantages of no heat input and noise pollution, and easy realization of the heterogeneous material connection, self-piercing riveting is widely used in the connection of automobile hybrid body structures [6,7,8]. The body of the domestic automobile may be subjected to various types of external loads during its service life, and it is necessary to study the mechanics of the SPR joints at different operating conditions, especially the impact destructions suffered by the SPR joints.
The main sources of impact wear on the SPR joints come from the impact of stones or gravel, hail, and raindrops during the driving of the automobile [9]. The speed of this type of impact breakage is usually between 1 and 10 ms−1, and it is often referred to as the low-speed impact [9,10,11]. Generally, the low-speed impact wear causes a minimal effect on component surface quality. However, if the impacted components are bearing load in their engineering life cycle, the impact breakage will expand, causing a wider range of breakage to the interior of the components. The existing research on low-speed impact is mainly focused on the investigation of composite materials subjected to low-speed impact breakage [12,13,14]. These investigations include the breakage evolution of the material that is subjected to low-speed impact [15,16], and the uptake of impact energy by the composite [17,18]. The effects of impedance of composite materials subjected to low-speed impact on the compressive and interlaminar shear load are also reported [19,20,21]. In particular, the residual tensile strength [22,23], residual flexural strength [24,25], residual fatigue life [26,27], and residual kinetic properties [28] of impact-damaged composite laminates are investigated. Zhao et al. [9] studied the impact behavior of self-piercing riveted joints made of dissimilar materials (DP590/AA6061) at impact energies of 5 J, 10 J, 20 J, and 30 J, and the results show that low-speed impact does not change the tensile failure mode of the joint, but significantly reduces its mechanical self-locking performance. Khoramishad et al. [29] studied the effect of stacking order on the low-speed impact behavior of metal laminates, and revealed that the impact performance of metal laminates is mainly influenced by the number of metal layers and the material properties of the first and last metal layers. Pärnänen et al. [30] reported that the steel surface condition does not significantly affect the impact response by drop-weight impact, and the debonding between the lower metal plate and the composite part proceeds as a mixed-mode fracture. Lu et al. [23] investigated the residual tensile properties of fiber–metal laminates by experimental tests and numerical simulations, and the residual tensile strength decreased with the increase in incident energy. Sedaghat et al. [27] predicted the residual fatigue life of the laminates under the different low-energy impacts, and the results showed that the fatigue life of the same laminates after a larger-energy low-velocity impact is shorter, and the numerical modelling simulation and experimental study showed the same variation.
For the low-speed impacted joined parts, Borba et al. [31] investigated the sensitivity of CF-PEEK friction-riveted joints to impact breakage and damage extension in the fatigue and quasi-static mechanical tests, and noted that the impact breakage did not affect the fatigue life of the riveted joints. Ghanbari et al. [32] studied the impact load response of composite plates bonded lap joints, and the results showed that the loading capacity of the single lap joints varies dramatically with the type of adherent as well as with the temperature. Andréa et al. [33] investigated the resistance of AA2024-T3 and carbon fiber-reinforced polyphenylene sulfide joints utilizing a drop-hammer test, and found that the joints exhibit rebound behavior at all energy levels. Silva et al. [34] showed that the impact energy is completely absorbed by the hybrid adhesive joint. Zhao et al. [9] reported that low-speed impact does not affect the form of failure of the joint, and the fatigue life of SPR joints is reduced. Huang et al. [35] studied the effect of a low-speed impact position on the strength of the SPR joints using a drop-hammer test, and indicated that the joint strength severely decreases when the center of the rivet is impacted by the punch. Liu et al. [36] discussed the effects of impact energy and impact surface on Al/PEEK/CFRTP joints at various relative adhesive lengths.
Although many studies have been published on low-speed impact, there were few reports of low-speed impacts on dissimilar material SPR joints. The impact performance is reported for BR1200HS and AA6082 SPR joints when subjected to low-speed impacts, and the residual mechanical behavior and failure mechanisms for impact-damaged SPR joints are investigated using tensile and fatigue tests as well as scanning electron microscope (SEM) inspections.

2. Material and Methods

2.1. Materials and SPR Connection

The jointing plates are made of 1.2 mm BR1200HS hot forming steel plate produced by Baoshan Iron & Steel Co., Ltd. (Baoshan, China), and the 2.0 mm AA6082-T6 plate (hereafter referred to as AA6082) produced by Aluminum Corporation of China Co., Ltd. (Beijing, China). The chemical compositions of the plates are shown in Table 1. The material properties of the plates are shown in Table 2. The rivet material is 36MnB4 high-strength galvanized steel, and the hardness of the rivets is H5 (46HRC ± 2HRC). The length of semi-hollow rivets is 6.5 mm and the diameter of the rivet tail is 5.3 mm. The BR1200HS and AA6082 plates were cut into 135 mm × 36 mm following the rolling direction. The SPR joints are joined by a single-lap joint, and the overlap is 36 mm in length, as shown in Figure 1a. The riveting equipment with a maximum riveting pressure of 80 kN is manufactured by TUCKER, Giessen, Germany.

2.2. The Low-Speed Impact Test

The low-speed impact test according to the ASTM E2298-2015 test standard was performed to study the impact resistance of SPR joints [37]. The CEAST 9340 drop-hammer impact test system was used in the impact test. The impactor in the impact test was a steel hemispherical impactor head with a diameter of 12.7 ± 0.1 mm and a total mass of 5.337 ± 0.001 kg. The impact test was carried out at 23 °C ± 2 °C and a humidity of 50% ± 10%. As for the service environment of the home car, the impact was mainly the low-speed impact, and the impact energy was generally less than 30.0 J [9]. Furthermore, the impact can only be considered as a low-speed impact test if the impact speed becomes lower than 10 m/s [10,11]. Therefore, it is known from Equation (1) that the impact speed and impact energy value of the impactor before impacting the joints can be adjusted by changing the fall height of the striker, as shown in Table 3.
v = 2 g h
where  v  represents the impact speed, g is the gravitational acceleration, and h represents the falling height of the impactor. The impact speed in the low-speed impact test ranges from 0.97 m·s−1 to 3.35 m·s−1 [38,39].
Before the impact test, the SPR joints were clamped with the fixture, as shown in Figure 2a. The center of the impactor was aligned with the center of the rivet to ensure that the SPR joints were exactly impacted by the impactor, as shown in Figure 2b. The impact tests are repeated five times for each group of joints, and the mean impact responses are calculated in the following section.

2.3. The Mechanical Tests

To evaluate the influence of mechanical performance on the SPR joints by low-speed impact, three SPR joints with different incident energies (including the SPR joints without impact) were subjected to the tensile test. In the tensile tests, the spacers were clamped to the end of the joints to align the loading shaft with the center line of the joints (Figure 1a). The thickness of the spacer at the end of the upper plate was 2.0 mm, and the 1.2 mm thick spacer was clamped at the end of the lower plate. The RGM-4030 electronic universal testing machine produced by Shenzhen REGER Instruments Co., Ltd. (Shenzhen, China) of China was used in the tensile tests. The tensile speed was 2 mm/min, and at least three tensile tests were performed for the joints impacted at the same impact energy.

2.4. The Surface Microstructure Characterization

To explore the influences of impact energy on the joints, the microscopic morphological characteristics of the tension-failed joints were observed through scanning electron microscopy (SEM). The acceleration voltage and current of the field emission scanning electron microscope (Hitachi SU8010, Tokyo, Japan) are 10.0 kV and 10.0 μA, respectively. After the tensile test, the overlap of the upper and the lower plates on the SPR joints was selected for inspection to analyze the tensile failure mechanism.

3. Results and Discussion

3.1. Low-Speed Impact Characteristics of the SPR Joints

In this work, the low-speed impact tests of the SPR joints impacted at different impact energies were performed. The contact force/energy–time curves, speed/displacement–time curves, and the characteristics of the dynamic response of the SPR joints were obtained.

3.1.1. Critical Impact Energy of the SPR Joints

A trial impact test is conducted to identify the critical impact energy that the joints can withstand for studying the joints’ resistance against impact. The impact energy values of the SPR joins are set as 30 J and 20 J, respectively, at first, and the impact test reveals that the SPR joints are completely broken, and large cracks can be found at the locking area, as shown in Figure 3a,b. Also, it can be observed from the upper plate of the joint in Figure 3a that the marks formed by the impact of the punch appear. Further, the impact experiments are conducted at the 15.0 J energy. The results show that visible cracks of different sizes and similar crack forms at the latch of the SPR joints can be found in four of five joints, and only one sample has no visible macroscopic cracks, as shown in Figure 3c. Subsequently, the SPR joints are tested at a 10.0 J impact energy, and no macroscopic visible cracks appear in the five joints, as shown in Figure 3e. To further refine the critical impact energy range that the joints can bear, the joints are subsequently tested at 12.5 J impact energy, and macroscopic cracks of different sizes are formed in three joints, as shown in Figure 3d. Consequently, from these tests, the critical impact energy that the joint can withstand can be deduced to be between 10.0 J and 12.5 J.
From the above trial impact, rivets and the undersurface of the lower plate of the SPR joint show no visible cracks or breakage when the joint is subjected to an impact energy of less than or equal to 10.0 J. However, the joint fractures at impact energies above 10.0 J. Therefore, to further investigate the SPR joint’s mechanics, the joints are subjected to five energies, namely 2.5 J, 5.0 J, 7.5 J, 10.0 J, and 12.5 J. Six joints are impacted joints at each impact energy level for the section profile and static tensile tests, respectively.

3.1.2. The SPR Joint for Dynamical Feedback

The change in load applied to the punch impactor throughout the impact process is captured by the force transducer mounted on the punch. By using a data collection system connected to a PC to record the load/time curve throughout the impact process, the full details of the impact event are provided from the initial contact to the final fracture of the specimen.
Typical energy uptake–time and contact force–time curves are depicted in Figure 4, where F is the contact force. The characteristic points are depicted in the curves. The energy uptake energy is shown by double-dotted lines, and the contact force is depicted by solid lines in Figure 4. The breakage of the SPR joints can be summarized by the curves [9,16,17]. It can be found from Figure 4 that the trends of contact forces are the same as the impact energy increased, and the absorbed energy value–time curves likewise have the same characteristics. Generally, the fluctuations of energy uptake values and discontinuities in impact force indicate that the breakage nucleation and extension are formed through delamination or a cracked matrix [9,40,41,42,43,44]. Therefore, the shapes of joint energy uptake–time curves and contact force–time curves in Figure 4 at small impact energies seem to be alike.
Generally, the impact feedback of the SPR joint can be evaluated by the Initial Breakage Point (IBP), Maximum Contact Force Point (MCFP), Joint Failure Point (CFP), and Total Impact Time Point (TITP). These characteristic points at different impact energies are shown in Table 4. Moreover, the impact procedure is categorized into four main phases, depending on the degree of breakage to the SPR joint at low-speed impacts.
The first phase is the time interval between 0 and ti, where the impactor begins to touch the joint, and the breakage of the joint occurs. When the contact force is dropped or the slope changes at this time interval, the Initial Breakage Point (IBP) is obtained [17]. The incipient force and energy in Figure 4 are acquired at Fi and Ei points, respectively. It can be found from Table 4 and Figure 4 that the values of Fi and Ei are very small when the IBP appears. Therefore, the initial breakage in the SPR joint is probably very slight. Due to the high-frequency oscillations of the contact force, the first plunge or slope variation of the contact force at the IBP cannot always be determined from the impact tests [17,45]. Therefore, the first sudden plunge or variation in the slope of the contact force can only be used as a warning to determine the early breakage of the SPR joint. According to Table 4, the Fi and Ei values are almost unchanged with the increase in impact energy.
The second phase is the ti-tm time interval. When the impact contact force sustained by the SPR joint appears at the maximum value, the maximum contact force characteristic point (MCFP, tm, Fm) occurs. The MCFP is represented by the point, at which the force is sudden or gradually decreased on the contact force–time curve [32,46]. This force is also the peak force that the SPR joint can withstand before the occurrence of the critical breakage. According to Figure 4 and Table 4, the corresponding value of Em at tm has not reached the final energy value Et that the SPR joint can absorb during the impact test. Taking the impact test at 2.5 J (Figure 4a) as an example, it can be found that the value of Em is 0.851 J, and the value of Et is 2.402 J. The value of Em is only 35.4% of the value of Et. Therefore, it can be presumed that the joint can continue to absorb energy when the contact force reaches the maximum value. That also can be verified from Figure 4e that the contact force is almost unchanged after MCFP. When the impact energy is larger from Table 4, the value of Fm is also larger. However, the Em value is increased at first and then decreased. This is mainly due to the extremely absorbed energy of the SPR joint before the severe breakage.
The third phase starts from the impact time tm to the termination of tf. The characteristic point at this phase is the largest energy uptake value during the whole impact test (expressed by Ef), referred to as the Joint Failure Point (CFP). At this phase, the SPR joint loses the original structural integrity due to the perforation of the joint, the dimpling of the rivet head, the bulging or cracks of the lower plate, and other breakages caused by the impact. Therefore, the joint is no longer able to bear more impact energy. The corresponding contact force at CFP is denoted by Ff. For example, when the joint is impacted at 2.5 J, Figure 4a, the energy uptake value Ef at CFP is listed as 2.606 J, which is the largest energy value point during the impact test. Moreover, the contact force at CFP is decreased from 4.774 kN at MCFP to 3.013 kN. It can be seen from Table 4 that the values of Ff and Ef gradually increase as the impact energy increases.
The fourth phase is from the failure point to the termination of the entire impact test, i.e., the tf-tt interval. At this phase, the contact force is gradually decreased to the minimized value (denoted by Ft), and the characteristic point is called TITP (Total Impact Time Point), also known as the point where the impactor loses contact with the joint. The energy uptake value corresponding to the TITP point is represented by Et, as shown in Figure 4. Furthermore, the energy uptake value at the TITP point is a constant, Figure 4. Taking Figure 4a as an example, at this time, the Ft at the TITP point is approximately 0.016 kN, and the absorbed energy value Et is 2.402 J, which is also the total energy value absorbed by the whole joint. From Table 4, it is observed that the value of Ft is very small, and gradually decreased to zero at large impact energy, while the Et value is increased all the time. This is caused mainly by the severe breakage of the SPR joint, as shown in Figure 4.
From Figure 4, the contact force fluctuates during the impact process, especially in the initial phase of the impact process. Due to the small impact energy, the joints are elastic–plastic deformed, and the elastic load is canceled out. Therefore, the contact force is fluctuant. As the impact test continues, the plastic deformation of the joints is significant, and the fluctuation of contact force is not obvious. The impact energy affects the critical phase parameter values. As depicted in Figure 4a,b, as the impact energy increases at the relatively low impact energy, the current impact speed of the impactor is small, and the breakage of the joint and value of ti is increased. As the impact energy keeps increasing, as shown in Figure 4c–e, the impact speed of the impactor is accelerated, the impact time becomes smaller, and the degree of breakage of the joint is increased. Meanwhile, by increasing the energy of the impact, the elastic deformation of the joints is transferred into elastic–plastic deformation, and the value of Fi is first increased and then decreased.
According to the measured contact force of the impacted joint, the contact force F is 8.839 kN and 9.481 kN at 10.0 J and 12.5 J, respectively. The diameter of the rivet leg after opening can be approximated by the inner diameter of the riveting mould, which is about φ10 mm, and the wall thickness of the rivet leg is 1 mm, as shown in Figure 1b,c. The force area at the rivet leg at the time of impact should be estimable as 28.2 mm2. Therefore, the critical incident energy of cracking in the lower plate of the impacted joint is 10.0~12.5 J, and the stress at the rivet leg can be calculated by F/S, within the range of 3.134~3.362 MPa.
The speed–time curves in the impact experiments appear in Figure 5a. The impact incidence speeds corresponding to impact energies at 2.5 J, 5.0 J, 7.5 J, 10.0 J, and 12.5 J are 0.97 m·s−1, 1.37 m·s−1, 1.68 m·s−1, 1.94 m·s−1 and 2.16 m·s−1, respectively. When the impactor is disengaged from the SPR joint, the disengagement velocities are −0.27 m·s−1, −0.41 m·s−1, −0.55 m·s−1, −0.69 m·s−1, and −0.70 m·s−1 (the negative signs indicated that the impactor is bounced in the opposite direction of impact speed). From Figure 5b, it can be found that the maximum displacement (i.e., deflection) of the joint after impact is 0.820 mm, 1.181 mm, 1.475 mm, 1.679 mm, and 1.855 mm at five different impact energies, separately.
The speed–time and displacement–time curves of the SPR joints at five different impact energies are very similar, as depicted in Figure 5. The potential energy and kinetic energy of the impactor are completely diverted into the SPR joint where the value of absorbed energy reaches the maximum value, which is proved by the energy uptake value Ef of the FP point in Figure 4. The energy uptake value of the joint after the FP point is decreased to a constant in Figure 4, which also reflects that the energy begins to convert back to the impactor by the sample, i.e., the mark of the impactor rebound. It can be evidenced that the speed of the impactor presents a direction opposite to the impact speed after the FP point, Figure 5a, and the displacement is decreased from the maximum point in Figure 5b.
According to Table 5, the Fm/tm ratio becomes larger as the impact energy increases. This indicates that with increasing impact energy, the contact load per unit time of the sample at the MCFP point increases, and the sample is susceptible to suffer from impact breakage, as shown in Figure 4. Moreover, the total absorbed energy Et increases with the increase in the impact energy due to the decrease in the impactor rebound energy. According to Table 4 and Figure 6, the total impact duration tt increases as the impact energy increases. The longer the total impact duration, the longer the interaction time between the SPR joint and the impactor, and the greater the breakage of the sample. The time to reach the maximum contact force during the impact test is firstly increased and then decreased, which is mainly due to the increasingly large incidence speed of the impactor with the increase in impact energy, thus leading to the shorter time to reach the maximum contact force. Similar variations of Et, Fm, and Ef can be found in Table 4.
The Et/E ratio in Table 5 represents the energy uptake rate. The large ratio indicates that more impact energy has been absorbed within the joint. The peak value of the Et/E ratio is 0.931 with an energy of 5.0 J. As shown in Table 5, the relative absorbed energy value remains around 0.90 with increasing impact energy. Thus, it can be presumed that there is a critical value of the relative absorbed energy. As shown in Table 5, the variation of the bounce energy ΔE (ΔE = E-Et) is also the same as the total absorbed energy. However, the relative bounce energy (E-Et)/E reaches the critical value of 0.108 at an impact energy of 10.0 J with increasing the impact energy, and then the relative bounce energy decreases. Then, with increasing impact energy, the impacted sample is pierced, more energy is consumed, and there is enough touch time for the sample and the impactor, leading to more serious breakage of the sample. So, the relative rebound energy decreases with the increase in impact energy.

3.1.3. Section of the Impacted Joints

Three SPR joints with different incident energies (SPR joints before impact were defined as the incident energy of 0.0 J) are selected for profiling along the center axis of the rivets. Figure 7 shows the cross-section of the SPR joint after impact at various impact energies. The cross sections for impact energies from 2.5 J to 30.0 J are shown in Figure 7a–h, respectively. In Figure 7a, the gaps occur between the upper/lower plates and the rivet/the upper plate (tagged with No. 1), the gaps between the lower plate and the rivet foot of the joint (tagged with No. 2) also appears, and the maximum gap width is 0.035 mm. Compared with Figure 7a, with increasing the impact energy, a significant crack can be found in the area for the plates of the joint (tagged with No. 3) in Figure 7b,c, and the gap width continues to increase, as shown in the area marked by No. 2. In Figure 7c, the gaps in the area marked by No. 2 and 3 are wider and longer when compared with Figure 7b. As the impact energy rises to 10.0 J, as shown in Figure 7d, the gaps in areas 1, 2, and 3 of the SPR joint are much more pronounced, and wider and longer gaps appear.
A fracture occurs in the area of the rivet foot (tagged with No. 4) in the self-hold zone of the joint, and the visible cracks appear in the lower plate, as shown in Figure 7e. Moreover, the gaps in the areas 1, 2, and 3 also emerge in Figure 7e. By increasing the impact energy, the lower plate of the SPR joint shows a thicker rupture crack, as shown in area 4 in Figure 7f, the gap for the plates and the rivet of the joint continues to increase, and the maximum gap width reaches 0.2 mm. When the impact energy reaches 20.0 J or even 30.0 J, the plates of the joints are detached from the rivets, and the joint is disabled absolutely, as shown in Figure 7g,h.
It is clear that with increasing impact energy, the degree of breakage of the SPR joint increases until the joint connection fails. No obvious breakage can be observed at an energy less than 10.0 J in Figure 3. However, it is noticed in Figure 7 that even at the impact energy of 2.5 J, a clear gap caused by the impact is still visible in the section of the joint. Therefore, a small impact can cause impact breakage to the joints in the practical use of the SPR joints in engineering.

3.2. The SPR Joint’s Mechanical Characteristics

3.2.1. The SPR Joint’s Residual Strength

One of the more frequent service conditions of the joints is shear loading [43]. Consequently, quasi-static tensile tests are carried out to study the effects of impact breakage on the shear strength of SPR joints. The correlations between the residual Max. tensile static load/residual energy uptake values at different impact energies are displayed in Figure 8a. The average Max. tensile force and energy uptake of the SPR joints with the impact energy of 0 J are taken as the reference for better comparison.
From Figure 8a, the mean Max. tensile static load of the unimpacted sample is 10.984 kN. With increasing the impact energy, the average Max. tensile static load is decreased slowly. When the impact energy is 10.0 J, the min value appears. The mean Max. tensile static load at an impact energy of 10.0 J is 9.312 kN. The mean Max. tensile static load is reduced by 14.9% compared to the unimpacted sample, as shown in Figure 8b. With increasing impact energy, as shown in Figure 8a, the impacted SPR joints also absorb progressively less energy in the tensile test. The unimpacted joint absorbed a total energy of 37.887 Joules during stretching. The SPR joint can absorb only 25.860 J at the impact energy of 10.0 J before it is completely peeled off. Compared to the unimpacted joint in Figure 8a, the average uptake energy decreases by 31.7%.
From Figure 8a, when the impact energy is 10.0 J, the tensile load and energy uptake values of the joint are lower than that of 7.5 J and 12.5 J. The main reason is that when the impact energy is 10.0 J, no significant cracking occurs in the self-hold zone of the joint. Therefore, with increasing impact energy, the joint may absorb even more energy, and more severe breakage inside the joint is formed. Therefore, the shear strength is reduced. When the impact energy is within the interval of 10.0–20 J, some cracks appear in the self-hold zone of the joint, and the crack width increases with the increased impact energy. At this time, large plastic deformation has emerged in the plates and rivets, and the direction of breakage is vertical to the tensile load in the self-hold zone. Therefore, the fluctuations of the tensile load and energy uptake values can be found in Figure 8a; the impact affects the tensile resistance of the SPR joints very little. Similar results can be found in Tai’s research [44].

3.2.2. The Tension Failure Forms of the SPR Joints after Low-Speed Impacts

The forms of failure of the SPR joints after the low-speed impact and tensile experiments are shown in Figure 9. From Figure 9a,b, it can be found that the low-speed impact has no significant effect on the forms of failure of the joints at the impact energy of 0.0 J and 2.5 J, and the self-hold failure is the main failure form, i.e., the rivet is removed from the lower plate, the upper and lower plates of the joint are separated, and the joint fails. However, an impact energy of 2.5 J is sufficient to destroy the structural integrity of the rivet head of the tensile SPR joint in Figure 9a,b. Compared with Figure 9a, a small segment at the rivet head rim is separated from the rivet head as in Figure 9b. However, the rivets in Figure 9a,b remain on the upper plate. This result is mainly because the impact energy is only 2.5 J, the small impact energy causes less breakage inside the head of the rivet and the riveted joint. Therefore, the small energy impact on the head of the rivet is not enough to alter the form of tensile breakage of the SPR joints during the tension tests.
The tensile failure of the SPR joints is still the form of the rivet pulling off from the attached lower plate as the impact energy increased in Figure 9c–f. However, the rivet is removed from the upper plate in the failed joints. The head of the rivet is torn, resulting in an incomplete rivet head, as shown in Figure 9c–f. With increasing impact energy, the contact load caused by the low-speed impact has a greater impact on the rivet head, and the rivet makes it difficult to withstand greater load during the tensile test in Figure 9c–f, resulting in joining failure.

3.3. The Tension Failure Mechanism of the SPR Joints

The lower plates of the failed joints after tensile tests at impact energies of 0.0 J and 7.5 J are chosen for scanning electron microscopy tests to investigate further the effect of low-speed impact on the tensile breakage mechanism of the SPR joints in Figure 10 and Figure 11. Figure 10b and Figure 11b (area 1#) show the self-hold zone of the SPR joint. The rivet is pulled out of the self-hold zone due to the external tensile force, as shown in Figure 10a and Figure 11a, causing the tearing of the aluminum alloy plate in the self-hold zone. This can be reflected by the presence of many tough dimples referred to in Figure 10c and Figure 11c (area 3#). For the impacted joint, in addition to more tensile dimples, there are some microcracks in Figure 11c.
No obvious cracks or microcracks are present in area 4# in Figure 10d. In contrast, area 4# in Figure 11d shows the presence of obvious cracks; moreover, the widest crack here is up to 0.256 mm. This was primarily because the joint in Figure 11 is undergoing a low-speed impact experiment with an impact energy of 7.5 J, which results in the lower plate of the SPR-7.5 J joint tearing in the tensile test. Although, there is no obvious breakage in the impacted SPR joint, the upper and lower plates within the SPR joint have gaps between them and the rivet, as shown in Figure 7c. The SPR joints are incapable of withstanding greater tensile loads, and the cracks shown in Figure 11d appear. Consequently, the maximum tension static load is reduced by 8.1% for the SPR-7.5 J joint when compared with the SPR-0.0 J joint, as shown in Figure 8a. As for area 4# in Figure 10d, no cracks are evident because the joint is not low-speed-impacted.
Figure 10e and Figure 11e show the microstructure morphology of area 5# in Figure 10d and Figure 11d, separately. The SPR-0.0 J SPR joint shows more severe scraping between the rivet leg in the 5# area of the lower plate when the rivet is extracted from the lower plate, resulting in deep scratches and microcracks in the 5# area of the lower plate in Figure 10e. Compared with Figure 10e, only minor scratches appear in Figure 11e, and no significant microcracks are observed. This is mainly because the Max. static load of the SPR-7.5 J SPR joint is smaller than that of the SPR-0.0 J joint. Figure 10f and Figure 11f show the microstructure morphology of the 6# area in Figure 10d and Figure 11d, separately. There are many grindings and microcracks in Figure 10f, indicating that more serious micro-motion wear is generated between the rivets and the lower and upper plates during the tensile test. Wear chip oxidation of the aluminum alloy plate and steel plate is caused by the long duration of fretting. However, as shown in Figure 11f, the SPR-7.5 J SPR joint experienced an impact energy of 7.5 J, the bearing capacity is decreased, and the fretting wear duration is short. Therefore, there are microcracks in Figure 11f, and no obvious wear chips appear.

4. Conclusions

The low-speed impact and residual mechanical behaviors of the BR1200HS/AA6082 SPR joints are investigated in this work. The impact characteristics, the static property, and the tension failure fracture are researched. According to the results of the experiment, the following conclusions are reached.
(1)
There are no obvious cracks in the SPR joint at impact energy between 10.0 and 12.5 J, and the internal breakage becomes more serious with increasing the impact energy.
(2)
For the impacted joints, when there is no visible crack in the appearance of the joints, the variations of the Contact Force/Energy uptake value–time curves of the SPR joints are similar. The parameters, such as total uptake energy value (Et), maximum uptake energy value (Ef), and maximum contact force (Fm), of the joints are increased gradually with increasing the impact energy. When the incident energy is 12.5 J, the total value of Et is 11.254 J, the maximum value of Ef is 12.547 J, and the maximum value of Fm is 9.481 kN.
(3)
The residual Max. static loads of the SPR joints after low-speed impact do not differ greatly, but the residual energy uptake values are decreased. When the impact energy is 10.0 J, the minimum residual static load is 9.312 kN. When the impact energy is 20.0 J, the minimum residual uptake energy is 23.906 J. The low-speed impact does not have a significant effect on the tensile failure form of the joints, and the joint failures are all in the form of rivets pulling off from the lower plates.

Author Contributions

Software, Y.-Q.J.; Validation, Z.-C.H.; Investigation, Y.-L.J.; Resources, Z.-C.H.; Project administration, Z.-C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 52375333), the Major Scientific and Technological Achievements Ripening and Engineering Research Projects of Jiangxi province (Grant No. 20232AEI92001).

Data Availability Statement

All the raw data supporting the conclusion of this paper were provided by the authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 13 01800 g001
Figure 1. The size of plates, riveting mold, rivet, and the jointing mode of the SPR joints [5]: (a) The size of plates and the jointing mode of the SPR joints. (b) The size of riveting mold. (c) The size of the rivet.
Figure 1. The size of plates, riveting mold, rivet, and the jointing mode of the SPR joints [5]: (a) The size of plates and the jointing mode of the SPR joints. (b) The size of riveting mold. (c) The size of the rivet.
Metals 13 01800 g001
Metals 13 01800 g002
Figure 2. The sample clamping fixture (a) and the critical dimensions of the fixture (b).
Figure 2. The sample clamping fixture (a) and the critical dimensions of the fixture (b).
Metals 13 01800 g002
Metals 13 01800 g003aMetals 13 01800 g003b
Figure 3. The macroscopic appearance of the joints impacted at impact energies of (a) 30.0 J; (b) 20.0 J; (c) 15.0 J; (d) 12.5 J; (e) 10.0 J.
Figure 3. The macroscopic appearance of the joints impacted at impact energies of (a) 30.0 J; (b) 20.0 J; (c) 15.0 J; (d) 12.5 J; (e) 10.0 J.
Metals 13 01800 g003aMetals 13 01800 g003b
Metals 13 01800 g004aMetals 13 01800 g004b
Figure 4. Variation of energy uptake and contact force at impact energies of (a) 2.5 J; (b) 5.0 J; (c) 7.5 J; (d) 10.0 J; (e) 12.5 J. (I, II, III and IV in the figure indicate the different stages of impact action).
Figure 4. Variation of energy uptake and contact force at impact energies of (a) 2.5 J; (b) 5.0 J; (c) 7.5 J; (d) 10.0 J; (e) 12.5 J. (I, II, III and IV in the figure indicate the different stages of impact action).
Metals 13 01800 g004aMetals 13 01800 g004b
Metals 13 01800 g005
Figure 5. The speed/displacement–time curves of SPR joints at various impact energies: (a) The speed–time curves. (b) The displacement–time curves.
Figure 5. The speed/displacement–time curves of SPR joints at various impact energies: (a) The speed–time curves. (b) The displacement–time curves.
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Metals 13 01800 g006
Figure 6. The tm-impact and tt-impact at different impact energies.
Figure 6. The tm-impact and tt-impact at different impact energies.
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Metals 13 01800 g007
Figure 7. The sections of the joints after the impact test at impact energies of (a) 0.0 J; (b) 2.5 J; (c) 5.0 J; (d) 7.5 J; (e) 10.0 J; (f) 12.5 J; (g) 15.0 J; (h) 20.0 J; (i) 30.0 J. (The numbers in the figures indicate where the gaps appear in the joints).
Figure 7. The sections of the joints after the impact test at impact energies of (a) 0.0 J; (b) 2.5 J; (c) 5.0 J; (d) 7.5 J; (e) 10.0 J; (f) 12.5 J; (g) 15.0 J; (h) 20.0 J; (i) 30.0 J. (The numbers in the figures indicate where the gaps appear in the joints).
Metals 13 01800 g007
Metals 13 01800 g008
Figure 8. The residual strength of the SPR joints: (a) Maximum static load–impact energy curve and energy uptake value–impact energy histogram. (b) The residual strength–impact energy curve.
Figure 8. The residual strength of the SPR joints: (a) Maximum static load–impact energy curve and energy uptake value–impact energy histogram. (b) The residual strength–impact energy curve.
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Metals 13 01800 g009
Figure 9. The forms of the tension failure of the SPR joints after the low-speed impact experiments: (a) 0.0 J; (b) 2.5 J; (c) 5.0 J; (d) 7.5 J; (e) 10.0 J; (f) 12.5 J.
Figure 9. The forms of the tension failure of the SPR joints after the low-speed impact experiments: (a) 0.0 J; (b) 2.5 J; (c) 5.0 J; (d) 7.5 J; (e) 10.0 J; (f) 12.5 J.
Metals 13 01800 g009
Metals 13 01800 g010
Figure 10. The tensile failure surface of the lower plate at the impact energy of 0.0 J: (a) macroscopic morphology; (b) area 1#; (c) area 3#; (d) area 2#; (e) area 5#; (f) area 6#.
Figure 10. The tensile failure surface of the lower plate at the impact energy of 0.0 J: (a) macroscopic morphology; (b) area 1#; (c) area 3#; (d) area 2#; (e) area 5#; (f) area 6#.
Metals 13 01800 g010
Metals 13 01800 g011
Figure 11. The tensile failure surface of the lower plate at the impact energy of 7.5 J: (a) macroscopic morphology; (b) area 1#; (c) area 3#; (d) area 2#; (e) area 5#; (f) area 6#.
Figure 11. The tensile failure surface of the lower plate at the impact energy of 7.5 J: (a) macroscopic morphology; (b) area 1#; (c) area 3#; (d) area 2#; (e) area 5#; (f) area 6#.
Metals 13 01800 g011
Table 1. Chemical composition of BR1200HS steel and AA6082-T6 (wt.%).
Table 1. Chemical composition of BR1200HS steel and AA6082-T6 (wt.%).
MaterialsCMnPSiBTiSAlFeMgCuZnCr
BR1200HS0.031.580.0080.0410.0051.040.0060.02Bal.----
AA6082-0.56-1.00-0.03-Bal.0.331.000.030.060.03
Table 2. Material properties of the plates.
Table 2. Material properties of the plates.
MaterialsUltimate Tensile Strength (MPa)Yield Strength (MPa)Elongation (%)
BR1200HS1230115022
AA6082-T630027513
Table 3. Impactor drop height and impact speed for various impact energies.
Table 3. Impactor drop height and impact speed for various impact energies.
Impact Energy (J)2.55.07.510.012.515.020.030.0
Total mass (kg)5.3375.3375.3375.3375.3375.3375.3375.337
Falling height (mm)4896143191239287382573
Impact speed (m·s−1)0.971.371.681.942.162.372.743.35
Table 4. The characteristic points at the different impact energies.
Table 4. The characteristic points at the different impact energies.
The Impact ProcessImpact Energy Level E (J)
2.55.07.510.012.5
0~titi (ms)0.022 ± 0.0140.033 ± 0.0160.024 ± 0.0210.012 ± 0.0010.013 ± 0.001
Fi (kN)0.348 ± 0.3470.676 ± 0.3360.833 ± 0.5800.382 ± 0.0020.449 ± 0.013
Ei (J)0.007 ± 0.0080.021 ± 0.0140.026 ± 0.0330.006 ± 0.0010.008 ± 0.001
ti~tmtm (ms)0.859 ± 0.0381.022 ± 0.0541.059 ± 0.0181.061 ± 0.0860.779 ± 0.033
Fm (kN)4.979 ± 0.1786.530 ± 0.1707.638 ± 0.0708.727 ± 0.2409.481 ± 0.011
Em (J)2.032 ± 0.0934.359 ± 0.2136.628 ± 0.0538.839 ± 0.4888.387 ± 0.414
tm~tftf (ms)1.447 ± 0.0451.534 ± 0.0321.570 ± 0.0181.545 ± 0.0261.547 ± 0.008
Ff (kN)3.425 ± 0.3574.460 ± 0.0475.487 ± 0.2067.121 ± 1.8116.511 ± 0.168
Ef (J)2.641 ± 0.0305.145 ± 0.0027.700 ± 0.00110.131 ± 0.00112.547 ± 0.001
tf~tttt (ms)2.643 ± 0.0702.681 ± 0.0552.760 ± 0.0292.846 ± 0.0432.853 ± 0.042
Ft (kN)0.018 ± 0.0020.027 ± 0.0050.029 ± 0.0050.038 ± 0.0030.043 ± 0.003
Et (J)2.308 ± 0.0824.653 ± 0.0386.817 ± 0.0598.919 ± 0.07211.254 ± 0.053
Table 5. The dynamic response characteristic parameters of SPR joints at different impact energies.
Table 5. The dynamic response characteristic parameters of SPR joints at different impact energies.
Characteristic ParametersImpact Energy Level E(J)
2.55.07.510.012.5
Fm/tm (kN/ms)5.352 ± 0.4656.416 ± 0.5057.215 ± 0.1898.298 ± 0.89912.194 ± 0.531
Et/E0.923 ± 0.0330.931 ± 0.0080.909 ± 0.0080.892 ± 0.0070.900 ± 0.004
E-Et0.192 ± 0.0820.347 ± 0.0380.683 ± 0.0591.081 ± 0.0721.246 ± 0.053
(E-Et)/E0.077 ± 0.0330.069 ± 0.0080.091 ± 0.0080.108 ± 0.0070.099 ± 0.005
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Jia, Y.-L.; Huang, Z.-C.; Jiang, Y.-Q. Low-Speed Impact and Residual Mechanical Performance of the BR1200HS/AA6082 Self-Piercing Riveted Joints. Metals 2023, 13, 1800. https://0-doi-org.brum.beds.ac.uk/10.3390/met13111800

AMA Style

Jia Y-L, Huang Z-C, Jiang Y-Q. Low-Speed Impact and Residual Mechanical Performance of the BR1200HS/AA6082 Self-Piercing Riveted Joints. Metals. 2023; 13(11):1800. https://0-doi-org.brum.beds.ac.uk/10.3390/met13111800

Chicago/Turabian Style

Jia, Ying-Lian, Zhi-Chao Huang, and Yu-Qiang Jiang. 2023. "Low-Speed Impact and Residual Mechanical Performance of the BR1200HS/AA6082 Self-Piercing Riveted Joints" Metals 13, no. 11: 1800. https://0-doi-org.brum.beds.ac.uk/10.3390/met13111800

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