Non-destructive Evaluation of Self-piercing Riveted Joints
Creators: name: Jessadatavornwong, Papangkorn
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401607
subject: Metals and alloy materials

description: The use of lightweight materials has been growing in demand in the automotive industry to reduce vehicle weight and fuel consumption. However, the main challenge of using these materials is the joining method. For example, a large amount of energy is required for spot-welding of aluminum alloys due to the presence of its oxide layer. Self-piercing riveting (SPR) is an alternative solution that mechanically joins automotive structures. It allows for more layers in the joint which can be difficult when welding. One challenge that the SPR poses is that it requires the optimization of several joining parameters, which typically occurs during a joint development phase. During this phase, cross-sections are analyzed to provide an assessment of joint quality. With the use of sectioning, which has been traditionally used for joint evaluation, only a single cross-section can be observed resulting in missing important features, e.g. defects. Hence, non-destructive evaluation (NDE) approaches are generally preferable in the quality inspection area. However, existing NDE methods cannot visualize the internal structure and provide a 3D image of the workpiece. In this thesis, X-ray computed tomography (CT) is evaluated as a method for being able to do this.

Chapter 2 of this thesis provides a comprehensive literature review, covering both self-piercing riveting and X-ray CT technology. Although the X-ray CT is widely applied in many engineering fields, a lack of practical application of this NDE technique to the SPR process is found. 

The first objective of this PhD project is, therefore, to investigate the capability of the X-ray CT to evaluate the quality of SPR joints. It corresponds to Chapter 3 of the thesis. The SPR joints in this study were chosen to investigate a wide variation of substrate materials typically observed in industry. The processing parameters for the X-ray CT are firstly optimized for each sample configuration to reduce human bias when deciding the X-ray CT setting before the joint inspection. From the X-ray CT images, many important features can be identified, e.g. sheet boundaries, sheet-rivet junctions and gaps under the rivet head. The X-ray CT image visualization can also be improved by the adhesives between the substrates. Furthermore, several defects can be captured including cracked substrates, circumferential cracks, rivet cracks and joint asymmetry which would not be captured using two-dimensional sectioning. 

X-ray CT has been adapted to provide quantitative information about defects in many applications. However, the small defects, e.g. porosity, are challenging to observe, and the detection limit has not yet been addressed. This is important for SPR joints because it sets a limit on the defect size that is detectable with the technology. This is investigated in Chapter 4 of this thesis. Defined size defects were accurately created on the rivet surface, replicating a cracked rivet. Although the small defects cannot be captured, some meaningful results can be drawn about the detection limit of the X-ray CT, which is primarily driven by the distance between the X-ray source and the sample. Defects of twice the size of the voxel size can be detected, and those of three times the voxel size can be measured. Defect detectability can also be improved by increasing scan duration so that noise is reduced.

Whilst the optimum use of riveting process parameters is expected to provide defect-free joints, some defects can exist in either the sheets or rivets. The effect of these defects on the mechanical performance of the SPR joints is not fully understood. Chapter 5 studies one of the common defects in SPR joints – rivet cracks. Notches were created on the rivet tail at different sizes, locations and numbers prior to the rivet insertion process. This is referred to as ex-situ defects. The presence of ex-situ defects can modify the joint geometrical factors leading to poorer joint strength. It was found that the head height of the SPR joints can be used to indicate the sign of internal defects without internally inspecting the cross-sections. When the rivet is more protruded than 15%, the joint was found to have limited interlock which reduced load-bearing capacity, displacement at maximum lap-shear force and energy absorption. When the defect is closer to the rivet tip and where there are a greater number of defects, a further reduction in joint strength is seen. In addition, the location of the defects is critical, as they are most problematic when they are subjected to the tensile force. 

Finally, the effect of ex-situ defects on the mechanical properties of SPR joints is studied using numerical simulation. A finite element parametric study is conducted in Chapter 6 to capture a greater variety of cracked rivets. It was demonstrated that numerical analysis can create a predictive model to determine not only the effect of defect depth and the number of defects on lap-shear behavior but also coach-peel properties. The joint strength is weakened when the ex-situ defects are deeper leading to loosened SPR joints. As such, the rivet is removed from the lower sheet earlier than the baseline model. In respect of the coach-peel property, it is significantly impacted when the interlock is not well formed on both sides. This is seen when more defects are added. Nonetheless, the defective SPR joints may be acceptable if the interlock is shown. A much smaller impact on structural performance is observed. It is helpful to build the quality criterion when defects exist in the SPR joints. 

This work has shown that X-ray CT is a powerful tool for obtaining an understanding of the quality of SPR joints. It has been demonstrated that it can be used as a non-destructive evaluation technique to provide a much more comprehensive assessment of joint quality than sectioning techniques. There is a limit on the size of the defects that can be detected, being about two to three times the size of the voxels. Defects were found to often reduce the strength of joints although the size and position of the defects affect how much the joint strength is affected by the defect. Interestingly, the head height can give a good indication of the joint quality without sectioning or X-ray CT. Furthermore, it was found that there was a good correlation between modelling of the riveting and mechanical testing process, suggesting that a robust numerical modelling of the process can be developed which could further support the quality control process for riveted joints. 

To summarize, the knowledge of this PhD study expands the application of X-ray CT on SPR joints that can be integrated into the production line, providing possible real-time quality evaluation. This also helps eliminate the need for cross-section analysis during production validation which can reduce the production cost.