So, why did it fail? If the failure is due to mechanical causes, including fracture, wear and deformation, a true answer is always “because the stress exceeded the strength.” Tying that down to exactly where that was the case, and what contributed to that being the case, is the work of the failure analyst.
I always try to encourage my clients to understand some level of human factors related to the failure. I often find that relates to poorly defined specifications.
Who writes most material specifications? In the case of my work, it is mainly mechanical or design engineers, not materials engineers, and they often leave off something critical.
I am a fan of using industry-wide specifications when possible. If it’s a load-bearing application, I’m a fan of using a mechanical-property requirement that, in the best of cases, has been confirmed as adequate by some sort of competent calculation or computer modeling. It’s also important for the design function (best performed by a design and a materials engineer working together!) to specify the test locations and methods.
A recent (not unique!) project I worked on specified a Rockwell C hardness of 40-45 on a 3-mm-diameter through-hardened steel cylinder. I performed the test using a Knoop 500-gram test load micro-indentation hardness tester. Nobody seemed to know how the manufacturer performed the test. They showed direct HRC values. The only POTENTIALLY valid way to do that on such a small diameter would be to cut the part and stand it on end and test it in the core. Good luck getting it not to tip when you apply the force!
You would have to look at the resulting indentation to make sure that it was at least 2.5 diameters away from the edge of the cross section in order to declare it a valid test. There would likely be room for only one test per cross section. This method would not reveal any surface-hardness variations, either on the high or low end.
If, out of ignorance, the supplier tested the material on the cylindrical surface, it would provide a very low false reading. These parts, according to my test, were on the high end of the spec, while their tests were in the middle of the range. Maybe they were running it hard to make it test above the minimum.
This gets them into the range where hydrogen damage can be a bigger problem. The parts were treated with a process that allows hydrogen to diffuse into the part. Some mechanical and design engineers know something about hydrogen embrittlement, but not all of them. Heat-treated steel specifications are among the most challenging to create well.
This begs two questions:
- Who is writing your material specifications?
- Who is writing your heat-treated steel specifications?