Let's face it: things break. In every manufacturing company, chances are good that at some time or another, you'll find a group of engineers and technicians standing around a broken part, scratching their heads, and asking, "Why?"
Sometimes parts break during testing under extreme conditions. These failures help us to understand the limits of a design. Other times, parts fail unexpectedly in service. These are the failures we would all like to avoid. At best, they lead to unhappy customers. At worst, they can lead to people being hurt or even killed. The good news is that a solid understanding of potential failure modes can help us prevent service failures.
The simplest failure mode to understand is overload. This is what happens when the load on a part exceeds the material's strength. Exceeding the yield strength will cause the part to permanently deform. Exceeding the ultimate strength will cause it to break. In either case, the failure occurs immediately.
Assuming you've done your stress analysis right, parts shouldn't fail in overload under normal operating conditions. So an overload failure means one of two things: either the load was too high, or the material's strength was too low.
As an example of the latter, the cam lever of a mechanical brake bent permanently the first time the brake was applied. The bend reduced the lever's stroke, preventing the brake from fully actuating. Fortunately, all of the brakes were tested prior to being shipped to the customer; otherwise, this could have been a dangerous situation. (This was an emergency brake for heavy equipment.) It turned out that the lever supplier had substituted hot-rolled, low-carbon steel for the cold-rolled, medium-carbon steel which had been specified. The steel the supplier used had a much lower yield strength than the specified steel. All of the levers had to be replaced, at the supplier's expense. The attempt to substitute a cheaper material turned out to be quite costly for the supplier.
Substitution of one material for another is not the only reason why a material's strength might be low. For example, improper heat treatment -- usually too high a temperature, too long of a time, or too slow of a cooling rate -- can lead to low strength. (For carburizing or nitriding heat treatments, too low of a temperature or too short of a time can also lead to low strength.) But even if a part is made from the correct material and receives the correct heat treatment, things can happen in service, which may reduce its strength. For example, exposure to high temperatures will generally decrease the strength of metal parts. For plastic parts, add ultraviolet light and exposure to certain chemicals to the list of things that can reduce strength.
If a part breaks in overload, and the material's strength is not too low (which can often be determined by a simple hardness test), then the load must have been too high. In many cases, examination of the failed part can provide clues as to which direction the load came from, and what type of load -- tension, compression, torsion, or bending -- it was. The question then becomes: Where did the load come from? Answering this question requires careful consideration of the mechanical system as a whole. Often, the broken part may turn out to have been an innocent bystander. For example, misalignment of another part might have produced abnormally high loads.
Avoiding overload failures, then, is a matter of understanding the forces that act on a part and the properties of the material from which it is made. It's also important to understand variables, which may cause either the loads or the material properties to differ from their normal values. As we'll see, these principles are also the key to understanding other failure modes. In my next post, I'll discuss how to apply these principles to fatigue failures.