Without a doubt, the most common failure mode I see in my lab is fatigue. Unlike overload failures, which occur more or less immediately when a load is applied, fatigue failures are progressive in nature. In other words, they develop over time as a result of repeated loads. If you've ever bent a paperclip back and forth until it snapped, you've seen fatigue at work.
The word fatigue implies that the material is somehow getting tired, or that its properties are becoming exhausted by repeated application of loads. Of course, this is not actually the case. If you want to convince yourself of this, you can cut a small sample from of a part that failed in fatigue, and perform mechanical tests on the sample. In almost all cases, you will find that the strength is the same as it was before the part was put into service. This demonstrates that fatigue is not a matter of the material losing its strength or degrading over time. So why does fatigue occur?
Fatigue failures start with the formation of a small crack, which grows with repeated loads. Once the crack gets big enough, the part breaks.
Crack growth can be understood as an energy balance. When you apply stress to a part, you increase its strain energy. You can think of this as the energy stored in a stretched spring. Now let's think about a part with a crack in it. The surface of the crack has a surface energy associated with it. Making the crack larger increases its surface energy. So a part that has a crack in it can lower its strain energy by increasing the size of the crack by converting strain energy into surface energy. This is why a crack can grow under repeated loads. (Incidentally, it also explains why you can cut glass by scribing a line in it with a glass cutter, then bending it.)
But what causes the initial small crack to form, especially if the overall stress is low? Most parts contain stress risers, where the local stress may be significantly higher than the overall stress. Cracks initiate at these stress concentrations.
Some stress risers are geometric in nature, and can be minimized by good design. These include sharp corners, notches, and holes, among others. Other stress risers may come from manufacturing processes, and can be minimized by good manufacturing practices. These include casting porosity and other kinds of defects. Still, other stress risers may be internal to the material itself. For example, most steels contain nonmetallic inclusions, which act as internal stress risers. Bearing quality and aircraft quality steels are produced using special steelmaking techniques to minimize their inclusion content.
Dave, I understood what you meant about wanting to have your own internal database and why. What I was trying to find out was, at a broader view, isn't it more or less redundant with everyone else's internal database, and why can't all of this data be maintained in (one or more) centralized repositories, which might be accessible to the software tools? Perhaps the answer is there's too much data, or perhaps the answer is it's too product-specific to a manufacturer's/service provider's own products/services. Is it one of these or something else?
@vimalkumarp: Thanks for the reference to McDonough and Braungart's Cradle to Cradle. It looks like an interesting book. I will try to find it.
Just to illustrate the point about how the lack of information affects design, right now I'm working on a die cast aluminum part which has an electroless nickel coating and operates at high temperatures (400 - 500°F).
We know something about how the die cast aluminum alloy behaves in fatigue at room temperature, but we don't know much about how it behaves at elevated temperatures. We know the fatigue strength will be lower, but by how much? We also don't know much about how the electroless nickel coating affects the fatigue strength. Again, we expect that the coating will reduce the fatigue strength, but we're not sure by how much.
Because we don't have the capability to do high temperature fatigue testing in-house, we might not fully answer these questions in the course of this project. Instead, we will have to make educated, conservative assumptions which will probably result in the component being somewhat overdesigned. As resources (hopefully!) become available in the future, we will try to do further testing to fill in the gaps in our knowlege.
By the way, it's very important to consider the effect of coatings on the mechanical properties of a material. In general, coatings which are more brittle than the substrate tend to reduce ductility, impact strength, and fatigue life. This is particularly true if the coatings apply tensile residual stresses to the substrate. These principles apply not only to plating of metals, but also painting of plastics. (For plastics, solvent attack on the substrate is another concern; in metals, the parallel to this is hydrogen embrittlement). You should never assume that you can apply a coating to a material without affecting its mechanical behavior.
Over-design is a good thing if cost and weight is not a deciding factor. Things that are impossible to inspect after being built must be "over-designed" or designed for no post inspection.
Many systems are over designed. It is just difficult to predict what aspects will be critical in actual applications. It is amazing the products and systems that last well beyond their design life (while others don't make it).
McDonough and Braungart explain this point of things getting overdesigned in their cradle to cradle design concept. The lack of data leading to overdesigning is really a point worth noting
@Ann: I think TJ's response sums up why it's often advantageous for companies to build up their own internal databases of fatigue data: "close enough" often isn't.
Dave, loved the article, and this response. I think you should qualify the use of software further though. You pointed out you need to use the right material, in the right condition. Anyone who reaches for fatigue software is likely NOT to find the correct material for their analysis at one point or another. Using a similar material as "close enough" is also likely to lead to erroneous and dangerous results.
Thanks, Dave, for such a complete intro to fatigue failures. I also find it especially interesting to read about all the CAE tools for fatigue analysis. Beth's second question and your response is also intriguing. It sounds like there's a need for more centralized fatigue databases of materials and/or parts made with them. Each company doing all this on its own and building up its own database seems awfully wasteful of time and energy. Is this info just too hard to centralize and keep updated?
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