Every design approach, we must ensure that the stress level is below the yield in ductile materials, automatically ensuring that the part will not break under a static load.
For brittle materials, we must ensure that the stress levels are well below the ultimate tensile strength.
Two other failure modes that apply to machine members are fatigue and wear. Fatigue is the response of a part subjected to repeated loads. Wear often happens where two parts are in contact with each other such as gears, bearings, and chains, for which it is a major concern.
source: Machine Elements in Mechanical Design, Robert L. Mott