Self-piercing Riveting Research Articles

Experimental investigation on the pre-holed screw-drilling riveting process for joining dissimilar lightweight sheets

The use of lightweight sheets such as carbon fiber reinforced composites (CFRP), magnesium alloys, and aluminum alloys is of great significance to realize the lightweight of automobiles. However, the differences between the sheets make the joining processes of dissimilar sheets a great challenge. A pre-holed screw-drilling riveting (PH-SDR) process is proposed based on a study of existing joining processes, the rivet with threads is riveted into the sheets by rotating, and the effective joining between the sheets is achieved by embedding the sheets into the rivet threads. The joint strength of the PH-SDR and self-piercing riveting (SPR) was compared through the lap-shear test. It was found that in the case of 1 mm Al6061-T6 + 2 mm AZ31B, the strength of the PH-SDR joint was 3.09 kN, which is 30.9% higher than the SPR of 2.36 kN. In the case of 2 mm CFRP + 2 mm Al6061-T6, when the spindle speed was 600 rpm, the strength of 5.43 kN and the energy absorption of 20.3 J were obtained for the PH-SDR joint, which is 51.2% and 89.7% higher than the optimal SPR joint. The results show that the proposed PH-SDR process provides a feasible solution for the joining of dissimilar lightweight sheets.

Study on the forming process and shear mechanical behavior of CFRP/Al self-piercing riveting employed 3D modeling

In this work, a 3D finite-element model (FEM) was developed to accurately predict the mechanical behavior of CFRP/Al self-piercing riveting (SPR) during the forming and shearing process. A continuous damage model based on the modified Hashin failure criterion was proposed to model the CFRP, and a cohesive model considering the traction-separation criterion was used to capture the delamination between CFRP layers. The simulation results were in good agreement with the experimental results. Based on the developed FEM, the damage of CFRP in the SPR forming process, the shear mechanical properties and damage of joints in the SPR shearing process, the effects of CFRP plate thickness and rivet length on the SPR forming cross-section and shear strength were investigated. The results showed that, the maximum interlock value of 0.46 mm was obtained when the rivet length was 6.5 mm and the CFRP plate thickness was 1.6 mm. The maximum shear peak load of 3611.98 N was obtained when the rivet length was 6.5 mm and the CFRP plate thickness was 2.2 mm. The interaction between rivet length and CFRP plate thickness should be considered in the actual SPR connection.

Mechanical Properties of B1500HS/AA5052 Joints by Self-Piercing Riveting

Self-piercing riveting (SPR) is a suitable technology to join various materials and has attracted more attention in the automotive industry. In this work, the effects of forming parameters on the forming qualities and mechanical properties of B1500HS steel/AA5052 aluminum alloy SPR joints were analyzed. The results show that the sheet stack sequence has little influence on the peak tensile load and rigidity of SPR joints. When the steel sheet is placed on the aluminum sheet, the failure displacement, energy absorption, and ductility factor are, respectively, 2.77, 2.13, and 2.28 times larger than those of the joints with the aluminum sheet placed on the steel sheet. The SPR joints with steel sheets placed on aluminum sheets have better mechanical stability. Meanwhile, when the steel sheet is placed on the aluminum sheet, the fatigue life of the joint can be increased by about 98.4%, 88.3%, and 118.1%, respectively, under high, medium, and low fatigue loads. A joint with opposite riveting direction has the optimal fatigue performance and the fatigue life is 1.64 and 2.14 times those of the other two-rivet joints. Generally, the fatigue fractures of aluminum alloy sheets in SPR joints occurred in fatigue tests. The fatigue fracture of a joint with a steel sheet stacked on an aluminum sheet extends uni-directionally to the edge of the sheet from the riveting point, while a symmetric fatigue crack of aluminum occurs for joints with the opposite sequence. The distribution of fatigue cracks is related to fatigue load, and fatigue cracks mainly originate in the fretting wear area of the contact interface between the rivet leg, upper sheet, and lower sheet.

Joining Strategies for Double-Sided Self-Pierce Riveting.

Double-sided self-pierce riveting (DSSPR) has been presenting itself as a proper alternative to self-pierce riveting (SPR) with many advantages for joining geometries of different thicknesses and cross-sections. To ensure its successful future industrial application, this paper presents a detailed comparison between different strategies to produce mechanical joints by means of the DSSPR process and discusses its performance and feasibility. Results show that the use of flat-bottom holes in both sheets provide interesting results, since they allow for a precise positioning of the tubular rivet in specific pre-defined locations, thus avoiding an incorrect joining procedure. This strategy tightens the tolerances of the process, while keeping a suitable level of destructive performance as demonstrated by the lap shear tests. Pre-riveting of the sheet has also been shown to produce suitable results in combination with or without a flat-bottom hole in the opposite sheet. This strategy comes at a cost of a slightly lower performance than that obtained with flat-bottom holes in both sheets, although the requirements of force and energy to complete the joining process are smaller. The conclusions of this research work are essential for selecting the joining strategy with DSSPR according to the requirements of the intended application.

Neural network modelling of mechanical joints for the application in large-scale crash analyses

This paper presents an artificial neural network (NN) modelling approach to represent a connector model in large-scale finite element explicit crash simulations. The NN model was established to describe the local force–deformation response of point connectors in automotive applications, namely self-piercing rivets and flow-drill-screws. The study is limited to two-sheet connections and to the use of feedforward NNs. Successive loading and unloading of the joints is not studied. Various architectures and complexities of the feedforward NNs were evaluated and trained based on data generated from a constraint model found in the literature. This forms a proof of concept for implementing a modelling technique not based on physics-motivated constitutive equations. The impact of the network complexity and training data diversity was investigated. The NN model was implemented as a cohesive zone model for incremental force prediction in an explicit finite element code. In order to have a wide selection of joint types, five different joint configurations including self-piercing rivets (SPR) and flow-drill screws (FDS) were investigated. Numerical results from the NN model were compared to physical tests from all joint configurations. It was shown that a rather basic machine learning technique like a feedforward NN was able to reproduce path-dependent force–deformation behaviour for the application in explicit FE solvers.

Performance analysis of similar and dissimilar self-piercing riveted joints in aluminum alloys

In this work, similar (2A12) and dissimilar (6061) aluminum alloy sheets are validly joined using self-piercing rivet process. A quasi-static experiment is proposed to investigate the mechanical behaviors, failures mode, and mechanism of the different joints. Moreover, a method based on deep learning algorithm is anticipated to detect the appearance defects of the SPR welded joints. The results indicated that 2A12 joints of similar sheets contained the advantageous static strength and 6061 similar sheet joints had superior anti-vibration performance conducts. The joints with 6061-2A12 sheets introduced the most decent and comprehensive mechanical properties. The main failure mode of 2A12 similar sheet joints was substrate fracture. The performance of the substrate affects the failure mode of the joint and the plasticity of the substrate is better. When the time comes, the failure mode is mostly pull-off failure. Poor plasticity of the substrate can easily lead to substrate breakage. The reason for joint pull-off and button fall-off failure is that there is large plastic deformation in the lower plate of the joint and the mechanical internal locking structure is damaged. 2A12 substrate breakage belongs to a composite fracture that combines intergranular fracture and microvoid aggregation type fracture. The area of the 6061 substrate near the edge of the sample is shear fracture and the area near the center of the sample thickness is dominated by microvoid aggregation type normal fracture. The effectiveness of the method was verified by conducting a series of experiments and the detection accuracy of the method can reach about 90%. The detection speed was as high as 50 frames per second (FPS), which can effectively solve the problem that the rivet quality was difficult to monitor.

Mechanical joining of high-strength multi-material systems − trends and innovations

In conjunction with mechanical joining processes. Mechanical joining processes play a key role for the realization of multi-material lightweight structures, which are essential with regard to environmental protection. However, joining of dissimilar high-strength materials is challenging due to the varying properties of the joining partners and due to their high flow stresses and often limited ductility. Thus, the evolution of established processes as well as the development of innovative and highly productive joining technologies are necessary. Requirements for a highly volatile production environment are versatility, flexibility, resilience and robustness. Within this contribution, current trends and innovations related to selected mechanical joining processes for enabling the material mix are outlined in order to point out opportunities to address these requirements in the future. In this context, joining using cold formed pin structures is presented as a promising approach for connecting dissimilar materials like metals to fibre-reinforced plastics. Furthermore, it is shown how the shear-clinching technology can be combined with a process-adapted application of locally limited heat treatment in order to promote the joinability and control the material flow during joining. A novel approach for reducing process forces and expanding process windows is the use of ultrasonic assistance for mechanical joining operations, which is demonstrated by the example of a nut staking process with superimposed high frequency oscillation. As concerns the widely used self-piercing riveting technique, current research activities relate not only to the further development of the joining process itself, for example by combining self-piercing riveting and tumbling, but also to the use of new rivet materials like high strain hardening stainless steels. In addition, the evolution towards mechanical joining 4.0 against the background of data-based process control in conjunction with of mechanical joining processes is also subject of the considerations.

Computer vision for automatic defect detection of self-pierce rivet joints

Rising costs for raw materials and energy require the development of lightweight construction concepts that are suitable for the load. Especially in the transportation sector, lightweight construction of structural components leads to improvements in resource- and energy-efficiency. The joining process of self-pierce riveting with semi-hollow pierce rivet is often used to join these lightweight materials. To ensure the product properties, the quality of each joint must be guaranteed. Currently, checking the joint quality is done manually through cyclical visual inspections and destructive tests, which costs considerable resources and time. Hence, a timely defect detection during the production process, based on computer vision, can save costs and enables better quality documentation of the rivet joints.In this paper a computer vision approach, based on the object detection algorithm YOLOv5, for automated defect detection of self-pierce riveting joints is presented. To this end, current methods for quality control of rivet joints in automotive production are discussed, as well as existing research on computer vision for defect detection on self-pierce rivet joints. Images from rivet joints on two different sheet metal pairings have been gathered and labelled, containing five different defect classes that can be found in automotive production. The images comprise a 90 ° and 45 ° viewing angle of the rivet joints to compare the algorithm's performance for these angles. To estimate the influence of training data samples on the defect detection performance the algorithm was trained and tested on different training data sizes. Furthermore, the transferability of the learned model from an aluminium sheet metal to an aluminium die cast pairing is assessed. It has been found that computer vision delivers sound results at the task of detecting defects in rivet joints, even with small training data size and hence has the potential to improve quality control in automotive riveting processes.

Study of Convolution Neural Network Based Deep Learning to Classify the Quality of Self-Piercing Riveting Joint

The SPR(Self-Piercing Riveting) process is a mechanical joining process that is mainly applied to assembling multimaterial parts to reduce the weight of the car body. Because the quality of SPR joints is mainly evaluated through cross sectional inspection, which is a type of destructive inspection, it is expensive and time-consuming. Machine learning technology is being proposed as a non-destructive testing because it can predict the quality based on the signals generated during the process. However, research result on the quality prediction modeling of SPR joints have not yet been reported. In this study, the prediction accuracy according to the signal combination was compared and evaluated by applying the CNN algorithm using the displacement and load signals generated during the SPR process and the acoustic signal obtained from the acoustic sensor. The materials used in the experiment were SGAFC 1180Y, CFRP, and SPFC 590 and the thickness were 1.4 mm, 1.8 mm, and 1.0 m respectively and a three-layer SPR process was performed. After evaluating joining was performed by selecting the abnormal process conditions, with 12 conditions that may occur during the process. Consequently, in the case of the first model in which the CNN algorithm was based on displacement and load signals, the quality prediction accuracy was estimated to be 90.0%. In the case of the second model in which the CNN algorithm added acoustic signals to the displacement and load signals, the quality prediction accuracy was estimated to be 77.5%.

Effect of Top Sheet Materials on Joint Performance of Self-Piercing Riveting

Three types of self-piercing riveting (SPR) joints, i.e., steel/aluminum, carbon fiber reinforced polymer (CFRP)/ aluminum, and aluminum/aluminum, were constructed using three different top sheet materials with the same aluminum alloy as the bottom sheet. The effects of the top sheet material on the joint quality and mechanical behavior were evaluated. The top sheet materials’ characteristics dominate the rivet piercing process and the consequent interlock distance. The high-strength steel top sheet requires a comparatively higher rivet setting force and induces early flaring of the rivet tail, resulting in a larger interlock distance. Though the CFRP needs the highest rivet setting force to penetrate the rivet through the CFRP fibers, the CFRP/aluminum joint exhibits the smallest interlock distance because of the SPR process-induced damages to the CFRP and subsequently less flaring of the rivet tail. In strength tests, the damaged CFRP sheet resulted in rivet head pullout of the CFRP/aluminum joints, which exhibited the lowest lap-shear and cross-tension strengths. In contrast, the steel/aluminum joints demonstrated the highest strengths because of their comparatively larger interlock distances. In addition to the experimental analysis, simulations revealed the rivet penetration and flaring mechanisms with various top sheet materials, and their respective joint quality and strengths.

Simplified modelling of self-piercing riveted joints and application in crashworthiness analysis for steel-aluminium hybrid beams

In this paper, an equivalent simplified model of self-piercing riveting (SPR-ESM) validated by tests is constructed to characterize the macroscopic mechanical responses of SPR joints under quasi-static and dynamic loading conditions. The application of SPR-ESM in the crashworthiness analysis of steel-aluminium hybrid beam is investigated. The results reveal that the failure of the riveting point in the test specimen is accurately simulated by the numerical model connected by SPR-ESM, and relative errors of mechanical responses between simulation and test are controlled within 7 %. Parametric study is performed to explore the effect of the riveting point failure on the mechanical performance of steel-aluminium hybrid beam under different loading angles. The results indicate that: (1) a severe instability deformation mode of steel-aluminium hybrid beam is easily to happen when the loading angles increases; (2) the effect of riveting point failure on the mechanical performance of steel-aluminium hybrid beam is limited when the beam has a stable folding deformation; and (3) a significant decrease in the ability of resisting deformation and energy absorption is easily to happen when the steel-aluminium hybrid beam has a severe instability deformation mode.

Insight of Salt Spray Corrosion on Mechanical Properties of TA1-Al5052 Self-Piercing Riveted Joint

Self-piercing riveted (SPR) joints in automobiles inevitably suffer from corrosion damage and performance reduction. In this work, the influence of salt spray corrosion on the mechanical properties of TA1-Al5052 alloy SPR joints was studied. The TA1-5052 SPR joints were prepared and salt spray tests were carried out for different durations. The static and fatigue strengths of the joints after salt spray corrosion were tested to analyze the effect of salt spray duration on the performance of the joints. The results show that the joints' static strength and fatigue strength decrease with prolonged salt spray time. The salt spray duration affects the joint's tensile failure mode. The tensile failure without corrosion and with a short salt spray time is the fracture failure of the lower aluminum sheet, and the tensile failure of the joints after a long time of salt spray corrosion is the failure of the rivets. The fatigue failure form of the SPR joint is the formation of fatigue cracks in the lower aluminum sheet, and salt spray time has little effect on the fatigue failure form. Salt spray corrosion can promote the initiation and propagation of fatigue cracks. The fatigue crack initiation area is located at the boundary between the lower aluminum sheet and the rivet leg. The initiation of cracks originates from the wear zones among the sheet metal, rivets, and salt spray particles.

Mechanical and geometrical characterisation of tumbling self-piercing riveting joints in multi-material-systems

Increasing requirements on lightweight construction to preserve resources necessitate new processes and methods in the production of weight-optimised multi-material-systems. Common joining processes such as the conventional semi-tubular self-piercing riveting are reaching their limits because of the rigid process designs. To extend this process, the linear punch movement is superimposed by an orbital forming process with a tumbling kinematic and provides new possibilities to influence the joining process. The configuration of the tumbling strategy consisting of the parameters tumbling angle and tumbling kinematics enables a targeted control of the material flow for the production of tailored joints. Within the scope of the investigations, the influence of the two parameters is examined both for the joining process and for the resulting joint formation. Force-displacement diagrams, micrographs and rivet head geometries are analysed to determine the influence on the process itself. Further, shear tensile tests and cross section tests are carried out to characterise the load-bearing capacities of tumbled joints and to identify the effects of the tumbling strategy on the mechanical properties. The tested materials in form of a dual-phase steel HCT590X+Z and a precipitation-hardening aluminium alloy EN AW-6014 have widely varying mechanical properties to represent multi-material-systems.

Study on the mechanical properties and fatigue failure mechanism of 7075-T6 aluminum alloy joints under different joining processes

To obtain the theoretical basis and reference basis for the selection of low ductility aluminum alloy joining methods, three kinds of joints were obtained using 7075-T6 aluminum alloy sheets for clinching, self-piercing riveting (SPR), and resistance spot welding (RSW) joints, and the single-lap shear tests were performed on the three kinds of joints, and fatigue tests were conducted on the clinched joints and SPR joints. The fatigue fracture of the SPR joint was analyzed using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The results showed that the SPR joint demonstrated the best static performance, followed by the clinched joint and the RSW joint. In the case of the fatigue test, the SPR joint showed better fatigue performance than that of the clinched joint. The fatigue failure of the SPR joint and clinched joint is associated with fretting wear. Fretting wear of the SPR joint is more severe than that of the clinched joint. The debris produced by fretting led to microcracking phenomena in the plate. Under the action of alternating load, the microcracks continued to expand and generate macrocracks, and the joint was broken and finally failed. This fretting wear is the root cause of the failure of the SPR joint. Thus, the SPR joints exhibited the best mechanical properties.

Determining the influence of different process parameters on the versatile self-piercing riveting process using numerical methods

Owing to the increasing use of multi-material construction and the resulting material incompatibilities, mechanical joining technologies are gaining importance. The reasons for this are the variety of joining possibilities as well as the high joint load-bearing capacities. However, the currently rigid tool systems cannot react flexibly to changing process conditions, such as varying sheet thicknesses or strengths as they require a change in the tool configuration. Therefore, a novel, versatile self-piercing riveting process (V-SPR) is proposed to improve the applicability of joining several sheet material combinations by considering only one rivet-die combination. Although previous studies have demonstrated the potential of V-SPR for joining equal materials, the present understanding of the process lacks detailed investigations regarding optimal tool configurations and sheet-joining tasks, such as varying the total and individual blank thicknesses. In this regard, a systematic parameter study in combination with statistical analyses is conducted to determine the applicability and feasibility of V-SPR using EN AW-6014 as an example. The process recommendations and limitations discussed in this paper pave the way for meaningful use of novel rivet geometry in versatile process chains.

Research on the Simplified Method of Nonlinear Finite Element Analysis for CFS-SPR Connections

This study reviewed some simplified methods of finite element analysis (FEA) for connections in cold-formed steel (CFS) structure, and summarized eight simplified methods divided into three categories. Shear performance tests were performed for six groups of self-piercing riveted (SPR) connection in CFS. A constitutive model of shear behavior for SPR connections was proposed, which was simplified from the load–displacement curve of shear performance test results. The models of SPR connection were established in ABAQUS by the eight simplified methods, and then the FEA results and the test results were compared. The applicable scope of each simplified model was explored, and a simplified method of FEA that was most suitable for the shear behavior of the CFS-SPR connection was proposed. Moreover, the shear performance test of the CFS shear wall with SPR was conducted by considering the rivet spacing, and failure modes and load–deformation curves were obtained. On this basis, numerical models of the CFS-SPR connection shear wall were established. By comparing the test results and the FEA results for the CFS-SPR connection shear wall, the feasibility of a simplified method of FEA applied to the CFS-SPR connection was verified. The main failure modes of the CFS-SPR connection were that the rivet tail pulled out from the bottom sheet and the rivet head pulled over from the top sheet. The SPR connection of the CFS frame could be simplified with a pin or a fastener element, and the SPR connection between the steel frame and the sheathing could be simulated by a Cartesian connector or a Spring2 element. The FEA results were highly similar to the test results for the CFS-SPR shear wall.

A study on non-destructive testing of geometrical parameters of self-piercing riveting joints using an acoustic microscope with a scanning focusing converter

ABSTRACT A non-destructive technique was proposed for geometrical parameters’ measurement of self-piercing riveting (SPR) joints based on ultrasonic C-scan image. Firstly, in order to enhance the detection precision, the characteristics of the ultrasonic echo signal at the edge of internal structure were analysed, and a theoretical formula on its sound intensity distribution was established. Then, an image segmentation process for the detection of SPR expansion diameter was suggested. Results showed that taking the greyscale value at the ‘inflection point’ from the C-scan image as the threshold of the segmentation, the non-destructive test results can get good correlation with the metallographic cross section, which provides a new method on the quality evaluation of SPR by using non-destructive test.

Equivalent characterization of pre-strained material properties and mechanical behavior prediction of steel/aluminum self-piercing riveted joints

Self-piercing riveting formation results in additional straining and property variations of the sheet material and rivet of the joint. As the riveted forming strain was concentrated within a limited area, it was difficult to characterize the properties of the pre-strained materials through conventional tensile tests. The present study dealt with the effects of considering riveted forming strain on the mechanical behavior prediction accuracy of the self-piercing riveted (SPR) joints of DP590 high-strength steel and AC43500-T7 cast aluminum alloy sheets. Main efforts were put on the evaluation of the failure parameters and behaviors of the pre-strained sheet materials. Equivalent specimens of the two sheet materials with different pre-strain levels were prepared via a flat rolling process for the characterization of the sheet deformation during the riveting process. Based on the material parameters obtained from the tensile tests of the rolled sheets and the compression tests of the rivet, a three-dimensional simulation model of the SPR joints was established. The mechanical behaviors of the joints were investigated under three loading conditions of 0° pull-out, 90° lap shear, and 45° tension–shear. The prediction accuracies of pull-off displacement under the three loading conditions reached 94.07%, 91.61% and 94.02%, and were 5.57%, 26.43% and 6.18% higher than those predicted by the model without considering the riveted forming history, respectively. The prediction accuracies of the peak force under the three loading conditions were 98.43%, 89.70% and 95.35%, respectively. The error difference of predicted peak force of the two models was less than 3%.

Geometric and mechanical joint characterization of conventionally and tumbled self-piercing riveting joints

In view of economic and ecological trends, the concepts for lightweight construction in transport systems are becoming increasingly important. These are frequently applied in the form of multi-material systems, which are characterized by the selective use of materials and geometries. One major challenge in the manufacturing of multi-material systems is the joining of the individual components to form a complete system. Mechanical joining processes such as semi-tubular self-piercing riveting are frequently used for this application but reach their limits concerning the number of combinations of geometry and material. In order to react to the requirements and to increase the versatility of semi-tubular self-pierce riveting, a process combination consisting of a tumbling process and a self-pierce riveting process has been presented previously. This process combination is used in this work to investigate the versatility and to identify the influencing parameters on it. For this purpose, experiments are conducted to identify process-side influence possibilities. The tests are performed with a dual-phase steel aluminum alloy to represent the varying mechanical characteristics of multi-material systems. Furthermore, the initial sheet thicknesses of the joining partners are varied in several steps. In addition to the geometric joint formation used to describe the undercut, the rivet head end position and the residual sheet thickness, the joining process, is also analyzed during the investigations. Further, the innovative joining process is evaluated by comparing it with a conventional self-piercing riveting process. The knowledge obtained represents a basis for the identification and evaluation of the versatility of the process combination.

Joining mechanism and damage of self-piercing riveted joints in carbon fibre reinforced polymer composites and aluminium alloy

The joining mechanism and damage of self-piercing riveted joints in carbon fibre reinforced polymer (CFRP) composites and aluminium alloy were investigated. The damage constitutive model for composite materials, considering the shear effect, was proposed to predict the mechanical behaviour and damage evolution of the CFRP. Self-piercing riveting (SPR) tests were performed to determine the damage patterns of joints with different laminate structures. Furthermore, numerical simulations of the SPR process were performed to understand the joining mechanism and analyse the damage evolution process of the joints. The results show that matrix cracks and fibre cracks existed in the SPR joints in the CFRP and aluminium alloy, located around the rivet head. With the increase in CFRP thickness, the damage degree of the joint surface decreases. Therefore, the proposed constitutive model can predict the complex damage behaviours of a CFRP subjected to large deformations. The matrix cracks are mainly caused by the matrix tensile damage, with the ply angle influencing the damage evolution trend. In addition, delamination exists between each ply of the SPR joint; the propagation direction of delamination damage is consistent with the fibre direction of the lower layer, especially for the joints with [0/90/0]s CFRP or [45/-45/45]s CFRP.