Failure In Service

1.- Failure in Service
The failure of a material is a situation in wich that material fail under the action of external loads. It depends on time, temperature, stress, load, type of material, etc.We could describe 3 failure in service cases:

Case 1 : Fracture We could define fracture as the local separation of 2 parts of a material. There is 2 types of fractures : Brittle fracture: ◦ There'sno plastic deformation before material fracture in the brittel type. ◦ This kind of fracture is characterized by a fast crack propagation and shows a granular appearance. These fractures aregenerally of the flat type. ◦ The fracture usually occurs a long cristalographic planes, wich are perpendicular to the applied forces. ◦ The crack stops growing either when it reaches the end of the part (thepart breaks), or the energy required for crack extension exceeds the strain energy.



Brittle Fracture example on War ship

Dynamic Tests: Impact Tests (Charpy-Izod tests): The objective oftheses impact tests is to determine the amount of energy absorbed by a material during the fracture, in other words, to determine the impact strength.  Ductile fracture: This kind of fracture occursunder an extreme plastic deformation before fracture. May have a gray and fibrous appereance.

Differences between ductile and brittle facture:

In ductile fractures, the cracks moves slowly,grow and spreads to the edges of the piece. The basic steps that describes the ductile materials failure are:     Void formation void coalescence (cracks formation) Crack propagation Failure

Thefailure results in a cup-cone shaped failure surface Case 2 : Fatigue A fatigue in some material is the progressive damage before material brokes. It occurs when a material is subjected to a cyclicload. It depends on time and it's related to a number of stress cycles. Can occur under axial, flexoral or torsional stress/strain. Fatigue cracks are brittle and the crack grows incrementally. Failure...