Creep Behavior of Materials

When a metal or alloy is under a constant load or stress, it may undergo progressive plastic deformation over a period of time, even though applied stress is less than the yield strength at that temperaure. This time dependent strain is called creep (above definition is taken from AMIE study material) More information is taken from Wikipdeia and shown below.

In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point. Creep always increases with temperature.

The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

Stages of Creep

In the initial stage, or primary creep, the strain rate is relatively high, but slows with increasing strain. This is due to work hardening. The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing (thermal softening). This stage is known as secondary or steady-state creep. This stage is the most understood. The characterized "creep strain rate" typically refers to the rate in this secondary stage. Stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain rate exponentially increases with strain because of necking phenomena.

General creep equation

where is the creep strain, C is a constant dependent on the material and the particular creep mechanism, mb are exponents dependent on the creep mechanism, Q is the activation energy of the creep mechanism, σ is the applied stress, d is the grain size of the material, k is Boltzmann's constant, and T is the absolute temperature. and

Creep in materials must be taken into consideration before designing machine components which work in high temperature / high stress environments. Other components in which creep is important design consideration include Bulb filaments, Crown Glass, Metal Paper clips, ...

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Histroy of Dutile Fractures

        Well, Hope everyone is preparing well for the Summer 2011 exams. Here are some of the cool facts about Ductile Fracture. The crack extension energy side of the Griffith equation applied only to "ideally brittle" materials. Believe me, it was not for lack of research that the materials research community failed to extend fracture theory into the very important field of ductile fracture. Some of the problems faced by Humanity due to Ductile fracture are given below:

Ships Break In Two!

This was an extremely serious problem in World War II, when over 250 ships fractured or cracked. Nineteen of these broke completely in two! Luckily, in some cases, fractures occurred in ships that were being outfitted and had never put to sea. All of the ship fractures and the two other examples that follow were in metals that were ductile, but just not tough enough.

The Great Boston Molasses Tank Disaster

One of the most famous brittle fractures was the Great Boston Molasses Tank Disaster in 1919. There was a tank of molasses, 90 ft in diameter and 50 feet high whose contents were supposed to have become rum. When the tank split, a wall of molasses advanced down the street. Many of the deaths and casualties occurred among people who were engulfed in their flats below the level of the street. There were 12 deaths and 40 injuries. Half a century later it was determined that the tank's steel was below its ductile/brittle transition temperature; the same problem as with the WWII merchant ships.

The Silver Bridge Collapse

A more recent brittle fracture disaster was the collapse of the Silver Bridge in West Virginia, in December 1967 in which 46 people perished as their cars plunged into the icy Ohio River. The National Bureau of Standards' metallurgists judged the bridge accident to be caused by stress-corrosion cracking resulting from long exposure to hydrogen sulfide vapor, H2S, from nearby paper mill digesters. The bridge failure is an example where the energy required to extend the fracture was reduced while the metal was in service.

With the benefit of 20/20 hindsight, the ship hull and molasses tank accidents occurred when the steel's energy required to extend the fracture at service temperatures was too low starting when the metal left the steel mills

The above information is taken from http://www.nhml.com/ Please do refer to them for more info.