Ed Bock of APEX Electrical Interconnection Consultants explains how to increase contact performance by pre-stressing to achieve greater strength and expanded dynamic range.
In examining the consequences of residual stress that develop in a contact beam subjected to stresses beyond the material’s elastic limit, we can see how residual stress may be beneficial, or if not properly configured, may actually be very harmful to a contact spring.In examining the consequences of residual stress that develop in a contact beam subjected to stresses beyond the material’s elastic limit, we can see how residual stress may be beneficial, or if not properly configured, may actually be very harmful to a contact spring.
Definitions of both the elastic limit and residual stress are important in understanding how this increase or decrease in performance occurs.
- Elastic limit – This defines a range of stress characterized by the absence of a permanent deformation upon release of the load. In essence, we have complete springback. A stress in excess of the elastic limit will lead to a permanent set upon release of the load.
- Residual stress – Residual stresses are induced whenever a material is plastically deformed (pushed beyond the elastic limit). The resulting permanent strain prevents the complete recovery of the elastic component upon unloading so that residual stresses are generated within the material.
Inducing residual stresses can be advantageous and has been incorporated into a number of applications to increase the strength of the system, extend the dynamic range, or both. The method is defined as “pre-stressing for strength.” In essence, the technique incorporates residual stress to increase the effective yield strength of the component. The trick is to produce a residual stress in the component that is in opposition to the stress that will be developed by the service load. Examples outside the connector industry that highlight this concept include cannon barrels, hydraulic cylinders, and, of course, pre-fabricated (“pre-stressed”) concrete beams.
Turning the discussion back to electric contacts, let’s look at the internal stress history of an electric contact beam. The following figures show the internal stress distribution of a beam that is initially bent beyond the elastic limit (it becomes fully plastic), and the internal stress pattern that results as the beam is unloaded. The net stress distribution after complete unloading is shown; this is the residual stress distribution that remains in the beam.
Consider that this pre-stressed beam may now be subjected to a service load – actual use such as a contact spring. Depending on the residual stress pattern and the bending moment generated by the service load, the effective yield strength – the point at which permanent set occurs – may be either increased or decreased.
In the best case scenario, the effective yield strength can be expanded to 1.5X the material yield strength. However, in the worst case scenario, the effective yield will be but 0.5X the materials yield strength. And so, residual strength may be extremely beneficial (by increasing the useful operating range of the spring) or extremely harmful (by limiting spring performance to less than desired).
By examining the residual stress distribution, additional insight for the increased/decreased strength can be seen. Note that in the example, the outer fiber is in compression (-0.5yield stress). If a bending moment in the same direction as that which produced the initial fully plastic condition is now applied, a reversal in the direction of stress is achieved. In that case, the effective yield strength would be yield stress plus 0.5 yield stress, or effectively 1.5X the material’s yield stress. However, by the same reasoning, if the applied bending moment is counter to the bending moment that took the beam to plastic, the effective yield stress will be 0.5X yield stress – a reduction in active range.
Setting up the proper conditions to increase strength may be somewhat complicated and likely would require a number of additional pre-forming operations; generally, to achieve improvement, the direction of force for the final forming operation should be in the same direction as the direction of spring bending in its intended use.
The following figure shows both the “correct” and “incorrect” forming operations, in order to benefit from residual stress.
The concept of residual stress, and in particular pre-stressing for increased strength and dynamic range, offers additional options that increase performance. It also lends insight into why performance may sometimes be less than expected, such as residual stress acting in the wrong direction, decreasing the active range of the contact spring.
Read more articles by Ed Bock:
- Contact Design Considerations for Long-Term Reliability
- Silver: A Superior Finish for High-Current Applications
- Why Minor Increases in Contact Resistance Matter
Ed Bock is a senior consultant at APEX Electrical Interconnection Consultants. He has more than 44 years experience in the connector industry (33 with AMP) in the areas of contact physics, electrical contact phenomenon, fretting corrosion, contact lubrication, and wear studies (tribology). His experience includes the areas of electric contact behavior and contact performance at both signal and power levels while subjected to various environmental conditions.