Ultimate failure

In mechanical engineering, ultimate failure describes the breaking of a material. In general there are two types or failure: fracture and buckling. Fracture of a material occurs when either an internal or external crack elongates the width or length of the material. In ultimate failure this will result in one or more breaks in the material. Buckling occurs when compressive loads are applied to the material and instead of cracking the material bows. This is undesirable because most tools that are designed to be straight will be inadequate if curved. If the buckling continues the material will create tension on the inner side of the material and compression on the outer part, thus fracturing the material.

In engineering there are multiple types of failure based upon the application of the material. In many machine applications any change in the part due to yielding will result in the machine piece needing to be replaced. Although this deformation or weakening of the material is not the technical definition of ultimate failure, the piece has failed. In most technical applications pieces are rarely allowed to reach their ultimate failure or breakage point, instead for safety factors they are removed at the first signs of significant wear.

There are two different types of fracture brittle and ductile. Each of these types of failure occur based on the material's ductility. Brittle failure occurs with little to no plastic deformation before fracture. An example of this would be stretching a clay pot or rod, when it is stretched it will not neck or elongate merely break into two ore more pieces. While applying a tensile stress to a ductile material, instead of immediately breaking the material will instead elongate. The material will begin by elongating uniformly until it reaches the yield (engineering) point, then the material will begin to neck. When necking occurs the material will begin to stretch more in the middle and the radius will decrease. Once this begins the material has entered a stage called plastic deformation. Once the material has reached its ultimate tensile strength it will elongate more easily until it reaches ultimate failure and breaks.

There are numerous methods to improve the strength of a material and therefore increase its ultimate failure point. Cold working a material is done by plastically deforming a material below its recrystallization temperature. This is most commonly seen by manufacturers hammering a material at room temperature. Hot working is any plastic deformation that is done above the recrystallization temperature. Cold working remains less effective because it is done below the recrystallization factor, but also remains more accurate. Because of thermal expansion and contraction of a material when heated hot working adds additional strength but also adds a rougher surface due to oxidization.

Heat treatment of a material is when the material is heated to extreme temperatures and quenching the material in water to cool it quickly. By heating the material to these very high temperatures the materials atomic structure is capable of being altered into a stronger material. This will also hopefully remove any cracks or deformations that will weaken a material. These cracks will weaken a material because they focus the stress or strain of a material to a particular point.

ee also

* Failure mode
* Material Strength
* Fabrication (metal)

References

*Manufacturing Processes for Engineering Materials Fifth Edition