Nearly all engineering designs serve a purpose. When they fail to serve the purpose for which they were designed, they are considered to have failed. Failure is an expectation, as nothing has an infinite lifetime. As such, all engineering designs have an expected or intended lifetime after they are built and delivered, which may be thought of as a best-before date.
Things with moving parts, like cars, may have a lifetime of fifteen years and 200,000 km. That is because a very popular mode of failure is fatigue. The strength of a material actually decreases when it is stressed, even at low values. On the other hand, static objects, like chairs, tend to survive longer than is required – typically, the paint will fade long before a leg breaks. A chair tends to go out of style before it falls apart. This is not surprising: whenever it is possible, engineers apply a high safety factor to their designs.
