Self-pierce riveting is a single-action, dual-sided fastening process, using the pressed insertion of a semi-tubular or tubular rivet shaped specifically to create the joint. The design of the rivet and die used; along with the sheet material being riveted determines the strength and corrosion qualities of the joint produced.
To apply SPR as a joining method, some basic design rules should be set:
• A ductile material should be on the bottom;
• Thin-to-thick ratio: The bottom sheet must exceed 33% of the total thickness of the joint.
Measuring some easily identifiable variables allows a record to be made at the evaluation stage that can then be compared as a quality check through the lifetime of the operation.
Non-destructive external checking can give a good indication that the joint is within the set specification. Changes such as the shape of the button or level of head flushness is a sure sign that something in the process is changing.
Sectioning of a joint at the evaluation stage gives an important story into the joint’s characteristics. This indicates how strong the joint will be; its resistance to corrosion; and future stability of the process.
The primary non-destructive method of in-process checking uses head flushness. As a manual check, a flushness gauge may be used to take readings at agreed intervals.
When a joint is designed correctly, variations in head flushness are an indicator of something has changed in the fastening process and failures may be caused with the strength or corrosion resistance of the joint.
During the design stage, a joint will produce a nominal head flushness reading indicating that the rivet head may be slightly above the surface of the top sheet. This is acceptable as long as contact between the under-head of the rivet and the sheet is good. The inspection criteria should be set to the appropriate reading in this stage. The flushness gauge is used as a comparison tool only, detecting change from the accepted design.
Process monitoring can give some real time security that there has been no process variation from that designed at the start. Computer process monitoring can also record historical failure detail and trends during production.
Process monitoring uses feedback during insertion to trace the process over its duration. The system is ‘taught’ the specifications of a good joint during the evaluation stage, this data is then used to compare against the inspection stage. It does this by using limits that are applied to a curve. Each joint produces its own curve, which is overlaid onto the base. If it falls outside the limits set it is flagged.
The best indication of determining joint strength is to conduct a tensile test of the joint in question. It is recommended that the material from the application components are used whenever possible to avoid misleading results.
The most meaningful tensile tests are are a single rivet subjected to tension in shear and peel where the bottom sheet is pulled off the other sheets. In comparing the integrity of riveted joints, the most relevant results to consider are F-max and energy absorption in shear, since this is the most common loading experienced; and in peel, as this is the worst loading case for the rivet, and will expose any weakness in the joint.
The tensile test pulls the riveted joint to destruction. By looking at the resultant debris of the coupon, the failure mode of the joint can be determined. The most common modes of failure when a joint has been tested to destruction are:
Macroscopic examination of cross-sectioned joints is the only method capable of directly evaluating sub–surface defects and the inter-facial features of a mechanically fastened joint. After an SPR joint has been designed, it is important to check that during production the rivet setter and material being riveted are behaving as originally specified. To check this, the base joint should be measured for important parameters so the production joints have a number of figures to uses for comparison. These parameters are as follows:
It is important to ensure the right joint is designed for the your application. Involving us in the development phase of your project will ensure you get it right the first time.