Root Cause Failure Analysis - Understanding Mechanical Failures

Root Cause Failure Analysis - Understanding Mechanical Failures President of Sachs, Salvaterra & Associates It may sound a little far-fetched, but experts say that the causes for more than 90% of all plant failures can be detected with a careful physical examination using low power magnification and some basic physical testing. Inspection of the failure will show the forces involved, whether the load applied cyclically or was single overload, the direction of the critical load, and the influence of outside forces such as residual stresses or corrosion. Then, accurately knowing the physical roots of the failure, you can pursue both the human errors and the latent causes of these physical roots. Before explaining how to diagnose a failure, we should review the effects of stress on a part. When a load is put on a part, it distorts. In a sound design the load isn’t excessive, the stress doesn’t exceed the "yield point", and the part deforms elastically, i.e., when the load is released the part returns to its original shape. This is shown in , a "stress-strain" diagram that shows the relationship between loads and deformation. In a good design, the part operates in the elastic range, the area between the origin and the yield strength, the part will be permanently deformed. Even greater increases in load will cause the part to actually break. illustrates a very basic point of design, and applies when the load on a part is relatively constant, such as the load on the frame of a building or the stress in the legs of your desk. It is a very different case when fluctuating loads are applied, such as those in a hydraulic cylinder or in an automotive connecting rod. These fluctuating loads are