Could you do an explanation in a future post of the differences between a part (say, a bracket on a car) failing due to a migrating stress fracture versus a total, quick failure where it just breaks in two? Is that the same stress dynamic in play with different outcomes, or are they different processes entirely?
@Alex: Usually, a crack which grows over time is due to fatigue, which I'll cover in my next installment. (In plastic parts, cracks which grow over time could also be due to environmental stress cracking, which I've written about before. In metals, there is a phenomenon called stress corrosion cracking, which is analagous to environmental stress cracking; I might write about this later).
Based on what I've seen in my career, fatigue failures are actually far more common than overload failures. However, overload failures are the easiest to understand, which is why I wanted to cover them first.
The mechanics of fatigue are a little more complicated. As I'll discuss, a common mistake is to treat "fatigue strength" as though it's a property like yield strength or ultimate tensile strength. It's not. But the big picture is the same: you need to understand the forces that act on the part, and the properties of the material from which it is made -- as well as all of the variables which might cause either one to vary from its normal value.
Dave: In the kinds of parts that are mentioned here, such as the brake cam, are the parts typically designed in accordance with the yield strength of the material, or is there some "allowable stress" design method that's set forth that is not dependent on yield? If yield is not used as criteria, does it make any difference in terms of failure rates?
@Chuck: Other engineers should feel free to weigh in on this, but in my experience, it's most common to design to the yield strength, with an appropriate factor of safety. Doing this should protect you against overload failures, provided that (as I pointed out in the article) the loads are what you think they are, and the yield strength is what you think it is.
Dave, I think the weight is eventually distributed across the area, and then it can bear more weight than concentrate to particular points. I think in most of the industrial wing, the stress tests are doing for a mass areas rather than stress test in cubic/cm sqd.
@Mydesign: You're right that loads redistribute to a certain extent as a result of localized yielding, so that a linear FEA which predicts a stress greater than the yield strength in a small region doesn't necessarily indicate failure of the component. This is why designing to "get the red out" of a FEA model, without any insight into the physical situation, can result in overdesign. On the other hand, stresses below the yield strength can lead to fatigue failure if they are repeatedly applied. To get a handle on fatigue, it's important to know how the loads on a part vary over time. This is what I will discuss in the next installment.
As mentioned earlier, repeated stresses/cycles on an assembly are a major contributor to fatigue and crack propagation. The biggest contributor to repeated cycles is vibration. Sometimes it's difficult to observe, but even high frequency vibration (though very small displacements) can be a fatigue factor due to their high cycle rate.
Vibration can be an issue when attaching a component to a moving machine (frequency depends on the machine dynamics), when designed to handle siesmic vibrations near fault lines (relatively low frequency <10Hz), or just designing to handle transportation to the end user (between 2-500Hz). The frequency and amplitudes vary, but the main goal is to design components with resonant frequencies well above what the sample will see while in use or transport, and when designing machinery, to avoid stacking resonant frequencies so the components aren't exciting each other's resonant frequencies while in use.
My experience has been that operators have the unique ability to find every unintented use of a piece of machinery - causing real eningeering challanges when it comes time to find out what REALLY went wrong.
It's interesting to watch what happens when a part is redesigned, "beefed up" because it's been breaking in the field. If the redesign works correctly, the part is no longer the "weakest link", and something else now is.
The usual progression is a series of parts end up being redesigned, one at a time, as each becomes the weakest link in turn.
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