Error-proofing techniques to achieve the perfect SPR joint quality, every time.
You need to know that your self-pierced rivets are correctly inserted. You can visually inspect the rivet head which should indicate that you have a sound joint, although this is only part of the story.
Components of a strong, reliable joint
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.
Evaluation of self-pierce riveted joints
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.
Flushness gauge
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.
Flushness gauge
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.
Tensile testing
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:
- Rivet tail tearing out of bottom sheet – most common in multi stack joints or when lower sheet is similar or thinner than upper sheets or the rivet is shorter than ideal due to the process being rationalized across many joints.
- Rivet head bearing out of top sheet – the ultimate failure mode, here the joint is as strong as it can be without increasing the diameter of the rivet head. This mode is most common in 2-stack joints when the top sheet is the significantly thinner component.
Cross-sectioning
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:
- K – Head flushness to top sheet
- i – Interlock into bottom sheet
- t-min – minimum material remaining of bottom sheet
Tolerances to these measured parameters may be added if testing shows a secure joint will allow for a small amount of variance. For example if a joint is designed with 0.4mm of interlock; but enough margin is placed in the design that the joint can be effective with only 75% of its joint strength, then the lower tolerance could be seen to be 0.3mm should testing confirm this. It is important for the joint to be cut down its center so that accurate measurements of “i” and “t-min” are taken. This can be proven by taking a measurement of the head width and checking it against the known diameter of the head of the rivet being checked.
The right joint for your application
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.