Fatigue strength evaluation of self-piercing riveted Al-5052 joints under different specimen configurations
Introduction
One of the main objectives in the automotive industry currently is to reduce the weights of automobiles. To achieve this goal, a new joining technology as a replacement for spot welding in lightweight metals, such as aluminum and magnesium alloys, is required in the automotive industry. Riveting methods are often considered as substitutes for spot welding. Among the several types of riveting methods available, the self-piercing riveting (henceforth SPR) process is gaining in popularity due to its many advantages. SPR does not require a pre-drilled hole, and this method can be used to join a wide range of materials, including combinations of similar or dissimilar materials.
SPR is essentially a cold-forming joining process. During the SPR process, a semi-tubular rivet is pressed by a punch into the sheets. The rivet pierces the upper sheet and flares into the bottom sheet under the influence of an upset die. A mechanical interlock is formed between the two sheets, which is key to the structural strength of the joints.
The fatigue strength of the SPR joints has been investigated by a number of authors for a number of materials [1], [2], [3], [4], [5], [6]. For example, Mori et al. [2] examined the static and fatigue strengths of spot-welded and self-piercing rivet joints in aluminum alloy sheets under tensile–shear and cross-tension configurations. They observed that while the static strength of the self-piercing rivet joint was about 1.5 times as large as that of the spot-welded joints, the fatigue strength was increased by about three times in the tensile–shear configuration. He et al. [3] investigated the strength, stiffness, impact resistance, failure modes and failure mechanisms of SPR joints with similar and dissimilar metal sheets consisting of an aluminum alloy and a copper alloy. They reported that the fatigue strength of SPR joints was largely affected by the properties of the sheets and that both the static and fatigue strength of SPR joints increased with an enhancement of the joint stiffness. Xing et al. [4] investigated the static and fatigue strength of multiple-rivet SPR joints. They reported that these levels are influenced by the rivet number and rivet distribution pattern. Franco et al. [5] investigated the possibility of joining aluminum alloys and carbon fiber composites using SPR. They reported large values of the fatigue resistance of SPR joints, even for load amplitudes close to the maximum static resistance of the joint and a fairly large range of fatigue strengths. Su et al. [6] investigated the fatigue behavior of SPR and clinch joints in tensile–shear specimens of aluminum sheets. They reported that the experimental fatigue lives of these joints can be estimated using structural stress solutions.
However, fatigue lifetime data of a SPR joint is normally reported as a function of the applied load range [7], [8], [9]. The reported fatigue strength data are not high enough to apply the other types of specimens due to the obscurity of the various factors that govern their fatigue strengths. The fatigue lifetime of a SPR joint specimen is generally dependent on the load magnitude, the loading type, the dimensions and configuration of the specimen, the sheet material, and other factors. Even with the same rivet diameter, sheet material, and sheet thickness, the load range amplitude representing the fatigue strength can differ from one specimen type to another due to different loading types. Therefore, the fatigue strength data for the SPR joints under several types of loading are needed in order to design a structure with SPR joints. To solve this problem, it is desirable to adopt general structural parameters, such as the stress, strain, and multiaxial fatigue criteria, to assess the fatigue lifetimes of these joints. Thus far, there has not been any report on appropriate fatigue strength parameters to correlate the fatigue lifetimes of SPR joints with different specimen configurations.
Therefore, in this study, fatigue tests under constant amplitude loads are conducted using coach-peel, cross-tension and tensile–shear specimens of Al-5052 aluminum alloy sheets to evaluate the fatigue strength of SPR joints under different specimen configurations. The experimental fatigue lifetimes of SPR joint specimens are also estimated using fatigue strength parameters. Finally, appropriate parameters for evaluating the fatigue lifetimes of three types of specimens are proposed.
Section snippets
Specimen preparation and fatigue test
Al5052-H32 aluminum alloy sheets with a thickness of 1.5 mm were joined by SPR. Tensile tests on the sheet material were conducted in order to obtain the tensile stress–strain curve for a FEM structural analysis. The tensile specimen was machined to a uniform gage length and width of 70 mm and 12.5 mm, respectively. Fig. 1 shows the engineering stress–strain curve for the Al5052-H32 alloy. The mechanical properties of the material are summarized in Table 1.
Coach-peel, cross-tension and
Optimal punching force for SPR joining
For SPR specimens, the joint strength is dependent on the sheet thickness, rivet diameter, die geometry, joining force, and other factors. In this study, a series of monotonic tensile tests was conducted on tensile–shear specimens with different amounts of punching force in an effort to determine the optimal punching force.
Fig. 6 shows the punching force against the maximum tensile–shear force for the SPR specimens in this study. Each data point is the average value from two specimens. As the
Conclusion
In this study, static strength and fatigue tests were conducted with coach-peel, cross-tension and tensile-shear specimens with Al-5052 plates for an evaluation of the fatigue strength of SPR joints. A structural analysis of the three types of specimens was carried out using the finite element code ABAQUS. For the tensile–shear specimen with Al-5052 plates, the optimal applied punching force for the SPR joining process was found to be 21 kN using the current sheet thickness of 1.5 mm and the
Acknowledgements
This study was financially supported by Seoul National University of Science & Technology. The authors would like to acknowledge Monotec Korea for providing the rivets and the fixture for the SPR process.
References (17)
- et al.
Fatigue of self-piercing riveted joints in aluminum alloy 6111
Int J Fatigue
(2003) - et al.
Mechanism of superiority of fatigue strength for aluminum alloy sheets joined by mechanical clinching and self-pierce riveting
J Mater Process Technol
(2012) - et al.
Self-piercing riveting of similar and dissimilar metal sheets of aluminum alloy and copper alloy
Mater Des
(2015) - et al.
Fatigue analyses of self-piercing rivets and clinch joints in lap-shear specimens of aluminum sheets
Int J Fatigue
(2015) - et al.
Fretting behavior of self-piercing riveted aluminium alloy joints under different interfacial conditions
Mater Des
(2006) - et al.
Strength of adhesive aided SPR joint for AM50 magnesium alloy sheets
Proc Eng
(2011) - et al.
The behaviour of a self-piercing riveted connection under quasi-static loading conditions
Int J Solids Struct
(2006) - et al.
Fretting fatigue and wear in bolted connections: a multi-level formulation for the computation of local contact stresses
Tribol Int
(2009)
Cited by (40)
Effect of process parameters on joint performance in self-piercing riveted dissimilar automotive steel joints
2022, Materials Today: ProceedingsCitation Excerpt :Skoruapa et al. [5] developed the model for predicting the fatigue life of aluminium alloys. Kang et al. [6] evaluated the fatigue life of aluminium alloys for different samples geometry e.g. shear tensile, cross tension and coach peel. He reported that fatigue life in shear tensile geometry was higher compared to cross tensile and coach peel geometries.
Fatigue characterization and crack propagation mechanism of self-piercing riveted joints in titanium plates
2020, International Journal of FatigueCitation Excerpt :They thought that the fatigue cracks initiated at the bottom surface of the upper sheet at high load amplitudes, and then developed in transverse direction and along the sheet thickness direction to the top surface of the upper sheet; while at low load amplitudes, the fatigue cracks initiated at the top surface of the lower sheet. Kang and Kim [18] did a brief analysis using the partial enlarged images of the fractures in AA5052 joints and found a similar situation as Li et al. [17] at high-loading range, while the fatigue cracks initiated on the lower sheet near the rivet tail at low-loading range and then propagated along the sheet thickness direction. Moraes et al. [19] and Huang et al. [20] investigated the fatigue crack initiation mainly based on a detailed analysis on the fretting wear in dissimilar aluminium/steel and similar aluminium joints.
Fatigue strength evaluation of self-piercing riveted joints of AZ31 Mg alloy and cold-rolled steel sheets
2020, Journal of Magnesium and AlloysCitation Excerpt :In addition, the fatigue lifetimes of the specimens were evaluated by applying the equivalent stress intensity factor of spot-welded specimens as proposed by Zhang [24]. For the fatigue lifetime evaluation at various loading angles of Al-5052 SPR joints, the equivalent stress intensity factor is reported to be the most suitable evaluation parameter [23,25]. In the current study, which evaluated the fatigue lifetimes of Mg/SPCC SPR joints, the maximum principal stress was found to be most appropriate.
Self-piercing riveted-bonded hybrid joining of carbon fibre reinforced polymers and aluminium alloy sheets
2019, Thin-Walled StructuresCitation Excerpt :The results indicated that the fatigue life was significantly influenced by the sheet thickness under a low fatigue load. Fatigue tests of three types of SPR specimens were conducted by Kang et al. [29], and the results indicated that the SPR joints were less vulnerable to tensile-shear loading compared with cross-tension and coach-peel loading. Zhang et al. [30] recently aimed at evaluating the mechanical performance and fretting mechanism of SPR joints in 1420 Al–Li alloys, they concluded that fretting wear occurred in different regions with different load levels.
Fracture mechanism of titanium sheet self-piercing riveted joints
2019, Thin-Walled StructuresEffect of process deformation history on mechanical performance of AM60B to AA6082 self-pierce riveted joints
2019, Engineering Fracture MechanicsCitation Excerpt :These results highlight the importance of including the residual stresses and deformation history when simulating the mechanical performance of a joint. Regarding fatigue performance, Kang and Kim [16] created SPR models of tensile-shear, coach-peel and cross tension based on experimental geometry. However, no residual stresses and strains due to the manufacturing history were included.