Part strength can mean different things subject to the conditions under which it is required to function. Ultimate strength could be based on the load a part is expected to carry – for example a plastic chair. This part would be tested well beyond what might be considered its maximum potential load to ensure that it would not fail.
It may be that strength relates to deformation where no or very little deformation of a part is permitted. For example, a clip for retaining the cable of, say a vacuum cleaner. This needs to ‘deform’ enough to enable the cable to be removed and replaced but permanent deformation would render it useless.
For plastics parts the nature of stress is an important consideration. Plastic parts have a ‘memory’, that is to say that if the stress is applied for a relatively short period of time the part will return to its original shape once the stress is removed. However, if the part is stressed for a long time then ‘creep’ occurs – when the stress is removed the part will not return to its original shape.