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The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed stress relief. [citation needed] The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow ...
Stress-relieving is a technique to remove or reduce the internal stresses created in metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress-relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly. [21]
The stress-free temperature that is used is dependent on environmental extremes and thus varies with location. In the United Kingdom, CWR is stressed to 27 °C (81 °F), the mean summer rail temperature. In the US, standard stress free temperatures vary from 35 to 43 °C (95 to 109 °F). [7]
Experimentally, stress relaxation is determined by step strain experiments, i.e. by applying a sudden one-time strain and measuring the build-up and subsequent relaxation of stress in the material (see figure), in either extensional or shear rheology. a) Applied step strain and b) induced stress as functions of time for a viscoelastic material.
The thermal method involves changing the temperature of the entire part uniformly, either through heating or cooling. When parts are heated for stress relief, the process may also be known as stress relief bake. [13] Cooling parts for stress relief is known as cryogenic stress relief and is relatively uncommon. [citation needed]
[2] [6] The specified holding temperature is one that is at a high enough temperature to relieve high residual stress levels, yet is still below the lower transformation temperature. [ 1 ] [ 2 ] In addition to the reduction of stress, high hold temperatures below the transformation temperature allow for microstructural transformations, therein ...
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This is a combination of a large temperature gradient due to low thermal conductivity, in addition to rapid change in temperature on brittle materials. The change in temperature causes stresses on the surface that are in tension, which encourages crack formation and propagation. Ceramics materials are usually susceptible to thermal shock. [2]