Confronting Fatigue – Attack and Defense
From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss a few of the many factors that can initiate fatigue failure in the service environment.
The Service Environment
Once a product leaves the factory you, the manufacturer, have lost control of the many factors that can initiate a fatigue failure once it is placed in service. Abuse and inadequate maintenance are leading preliminaries of failure by fatigue, as well as other failure modes. Failures of components or assemblies “up stream” from your product may introduce higher loads than the product or component was designed to sustain. Harsh service environments, such as road salts or ocean front installations may instigate corrosive attack, with corrosion pits providing a fatigue initiation sight. Analysis and identification of the root cause of fatigue failures in service is critical to educating your customer in the appropriate use and maintenance of your product and getting them back on track as a satisfied customer.
Identifying the root cause of service environment initiated fatigue failures can be challenging, and sometimes obscure, as the following example illustrates. Some years ago, we provided analytical support on a lawsuit which was filed after a person sustained a back injury when the metal leg of a “stacking chair” fractured. Stacking chairs are the type of institutional chairs you often see in school auditoriums and other public buildings and are designed to be stacked, one upon the other, for more compact storage when not in use. This particular chair came from a college in Ohio. Our analysis proved that low stress, high cycle fatigue was the failure mode. In other words, low magnitude stresses applied at high frequencies, in this case over a million cycles.
The chair had been in use for a relatively brief time, and even if it had seen longer service, it seemed unlikely that it could have been subjected to the number of load cycles indicated by the fracture morphology. This presented something of a mystery, as the failure mode was indisputable. Investigation of the service environment revealed that the chairs were used sporadically and when not in use, were stacked in a storeroom. The college staff was methodical in setting up the chairs in orderly rows in an adjacent auditorium, then stacking them from the same end of the same rows when they were no longer required, with the same chair ending up on the bottom of the stack before going back into storage. The stack was higher than the maximum specified by the manufacturer, providing a load in excess of the design limit. A survey of the area revealed that the storeroom was immediately above the main HVAC installation, the final and key piece of the puzzle. Vibration from the HVAC system, transmitted through the storeroom floor, and loads from the weight of chairs stacked in excess of the design limit provided the stresses required to initiate the fatigue crack. Once the crack grew to the point at which the remaining intact tubular leg could no longer sustain the load of a sitting person, final fracture occurred.
As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure. They are: 1. How did it fail? 2. Why did it fail? and 3. What will prevent future failures? If you have commissioned a failure analysis, and all three of these questions are not answered, all you have paid for is some interesting pictures and a possible lawsuit when your product fails again.
In Failure Analysis of Fatigue – Part 6 we will discuss some examples of fatigue failures and features that are characteristic of fatigue fracture.