rehman, like Gokul said ( ) could you elaborate a bit, the failure & fracture mechanisms of metals are an immense field of research, we'll be able to help you once we know what you're after.
The most prominent fractures in metals are known to be fatigue and creep. Aluminium frames of aircrafts serves as the perfect epitome for fatigue whereas coat hangers' cracks or indeed fractures is a good example of creep. What perennial stated is also perfectly correct. It is you I'm afraid that hasn't asked the question properly!