The perfect joint
There are a number of measures that can be taken to assess the quality of a self-pierce riveted joint – both in the early design phase and during the process.
There are two basic rules for self-pierce riveting: the riveting process should go from the harder to the softer material and from the thinner to the thicker material layer. The bottom layer should always account for at least a third of the total thickness. Various procedures can be used for determining whether a riveted joint is made in a secure process, is reliable and is protected against corrosion.
Cross section analysis
The interior of a riveted joint can be assessed by the microscopic analysis of cross sections. This method is used both in the design phase and during production to ensure that the material and the process meet the specified requirements. The following criteria are relevant in cross section analysis:
- Rivet head height (position of the rivet head relative to the material surface)
- Interlock or spreading of the rivet
- Minimum material thickness in the bottom layer
In the design phase, minimum requirements for these parameters which ensure a sufficiently stable joint are defined. During cross section analysis itself, it is essential to cut precisely through the centre of the rivet in order to ensure a correct measurement.
Flushness of the rivet head
The external appearance of the joint also gives an indication of its quality. A key indicator is the flush positioning of the rivet head relative to the material surface. Inadequate or excessive penetration of the surface may indicate a quality issue. Although the measurement can be made manually, modern self-pierce riveting systems can also automatically detect any deviations during the process. If the rivet position varies while other process parameters remain unchanged, this indicates that the factors influencing the process may have changed and that the joint may possibly be weaker and more vulnerable to corrosion. During the design phase, a tolerance for the rivet position is defined; this is normally of the order of ±0.2 millimetres. The rivet head may therefore be slightly raised or countersunk with respect to the material surface. However, it is important for the underside of the head to be in contact with the material over its entire surface. Flushness measurements are comparison measurements that identify deviations from the defined design parameters.
In a tensile test, an individual riveted joint is made up using sample materials and then exposed to shear or tensile stress until it fails. If the joint does not reach the required strength, the failure pattern provides an indication of possible causes:
- Rivet tail breakout: the rivet tail is torn out of the bottom layer. This occurs frequently in the case of multi-layered joints, especially if the bottom layer is as thick as or thinner than the upper layers or if the rivet is too short.
- Rivet head breakout: the top layer breaks away from the rivet and tears off. This means that the riveted joint has reached maximum strength for the rivet head diameter. This problem occurs mainly in the case of two-layer joints with a top layer which is significantly thinner than the bottom layer.
There are two main factors which influence the riveting process: force (F) and travel (s). Reference data for perfect joining results can be determined by measuring these parameters in the evaluation phase. The force feedback is presented in the form of an envelope taking permitted deviations into consideration. During operation, process parameters are monitored on a real-time basis to identify any deviations from the defined values. Each joining process generates an individual curve. If the deviation between this individual curve and the reference curve is too severe and the individual curve is outside the envelope, the system signals an error. This way, trends and changes during production can be detected and documented at an early stage.
A strong partner
We develop the ideal self-pierce riveted joint for your requirements and fully integrate all the quality assurance measures required. If you involve us in your design phase from an early stage, you can benefit from our experts’ extensive experience.