Almost all defect mechanisms that will fail eventually in the field with normal stresses will fail much faster in the laboratory at higher stresses. You can identify those problem areas quickly and improve the design before thousands of products are in the hands of customers. The Basics of Reliability Let s look at the strength of a product and the effect of lifetime stresses. However, we know from experience that this illustration is idealistic. Strength varies among products in a bell-shaped distribution.
Some products are extra strong, and some are weaker than the average.
Similarly, the stresses applied to a group of products in their lifetime will vary in a bell-shaped distribution. A few loads will be quite severe, while others are extra gentle. Figure 2 illustrates typical real-world strength and stress distributions. As long as the strength of every product, even the weakest ones, is greater than the applied stresses, even the greatest, there will be no problem. It might be a seemingly innocuous design change, a change of assembly personnel, or a change in a component.