Moving crack (metalworking)
A moving crack is a crack that propagates with some speed due to loading and unloading of a metal work material.[clarification needed] When loading and unloading is being done, a large fraction of irreversible energy associated with those actions is eventually dissipated as heat and other part is being stored in the work material due to change in material structure and constraints caused dislocation pile-ups, etc. The fraction of heat dissipated in material () depends on type of material of fracture model is being used there. By conducting some experiments, it is found that the fraction dissipated as heat may be as large as 0.85-0.95 for metals, but this value doesn't depend on the magnitude and rate of deformation. So according to work of Mason, can be as small as 0.5 for aluminium and steel at low value of strains only and for titanium at both and low value of strains.
Heat generation and temperature increment
In general, the amount of loading and unloading energy which is converted into heat is for unit volume having high value. So this large value of heat generation per unit of volume results in substantial rise in temperature of the tip of the moving crack. This temperature rise can be of several hundred degrees Celsius as found in experiment done by Mason and Rosakis and others. The process region near tip of moving crack is the zone for maximum temperature.
Mostly, for measurement of temperature at tip of moving crack mode-I of fracture is being preferred. But maximum temperatures are expected for other two modes-(I, II)of fracture due large deformations with shear banding, particularly with high confining pressure, where impact generated shear bands is due to the impact load itself. According to a study by Zhou et al. and Rosakis et al.(1997) on impact produced shear bands of mode-II gives a result of temperature rises of over 1650 K for C-300 steel. These temperatures are being measured by special purpose high precision thermocouple.
Mathematical formulation of temperature of moving crack tip
If one applies the cell model of material and given total energy supply as to a central cell (this can be found by the area of a cohesion-decohesion curve), with heat generated = , where = fraction of energy supplied converted into heat energy and considering adiabatic temperature rise i.e. no heat is going out by conduction, temperature rise becomes:
where = density, = specific heat, = volume of the cell. The above calculation of T conduction of heat from the body has been neglected and this assumption is not valid.
Revising this for conduction using the moving crack tip governing equation for heat conduction for the upper half of the crack (, where is the plane passing through crack) in -direction is
where is temperature at time , is conductivity of material, is the diffusivity of material , and is the heat per unit volume. Solving this heat conduction equation using Laplace transform, one gets
where, is the error function.
Using the final equation of , one can calculate the temperature at the tip of the moving crack.
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- Yunus A. Cengel - Thermodynamics
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