One of the most common terms you will hear our technicians cite within a root cause failure analysis report is something called a stress riser. We are often asked what this term means and how did it result in a failure. In this article we will go over the definition of a stress riser and how it relates to mechanical or even electrical failures.
A stress riser is a location of stress concentrated to a specific area of a material. Whether it be the metal of an electric motor’s shaft, a bit of solder joining printed circuit board’s power connector or even the glass underneath the surface of a touch screen, all materials have a certain amount of force that they can withstand before they break. Now if you take the amount of force needed to cause damage to a material and apply it evenly across the entire surface you will find that the material can withstand much more than if you concentrate that force to one specific point. Any fracture or bend – really anything that weakens the material – creates what is called a stress riser. This place is one where concentrated stress will propagate resulting in a failure of the material’s structure.
The most common cause of mechanical stress risers that we see in the service center are radial and axial loads. A radial load is one that acts on the material at a 90 degree angle. Most often when we see radial load failures it is on the front end bearing on a motor shaft or gearbox. This is caused by the excessive load causing a stress riser where the edge of the bearing or the outer race gouges the shaft material. As the load increases and the metal weakens and the shaft can suddenly bend or even break. An axial load is the one that runs parallel to the shaft material. Excessive loads axially acting on the shaft can also cause damage either to the internal mechanical structure of the motor or even cause bends of the shaft as well.
Another well documented stress riser we see is found in the HMI or Human-Machine-Interface department. Here we will often see a single stress riser in glass touchscreens lead to a catastrophic failure that is caused by a single simple stress riser. Have you ever been driving down the road and have a small rock get kicked up by a semi ahead of you and nick your windshield? What starts as a small little scratch in the glass soon gives way to a larger crack across the whole windshield. Similarly we see the same sorts of stress risers be caused by operators who incorrectly jab at the screens with pens, screw drivers, etc. This leads to cracks that spread along these risers and lead to complete failure of the touchglass.
In the electronics lab, the most common stress riser we see is from connectors taking mechanical stress. This is often from simply too much force being applied while connecting and disconnecting the plug. If the plug does not have some sort of securing method, such as a screw or mounting pin, often all that holds the connector on is solder. Just a little bit of too much wiggle and we see stress risers begin to form where the solder meets the pad and pin underneath. Enough of this and the joint will break leading to a shorted connection. In some cases this can cause electrical arching and lead to a bad thermal failure on the board.
We hope this article helps in locating and lessening the amount of stress risers impacting your equipment- this can be the key to success in reducing costly downtime. Please comment and tell us what stress risers you identify and overcome in your facility. We would like to hear your input! Be sure to visit us online at www.gesrepair.com or call us at 1-877-249-1701 to learn more about our services. We’re proud to offer Surplus, Complete Repair and Maintenance on all types of Industrial Electronics, Servo Motors, AC and DC Motors, Hydraulics and Pneumatics. Please subscribe to our YouTube page and Like Us on Facebook! Thank you!