Hot rolling

Investigation of the creep characteristics of a polycrystalline material

Introduction
When a tensile test is carried out, an increasing load is applied to a test specimen. In ductile metals, weobserve elastic behavior up to yield point, successively followed by yielding, plastic deformation, necking and finally fracture. Polycrystalline materials can exhibit plastic deformation below theyield stress. The rate of deformation is dependent upon load and temperature.
Creep is a phenomenon which takes on unique significance in high temperature applications, notably with the turbine bladesin jet engines. It is however a cause of thinning and tearing in the lead flashing around chimneys and roof joints on buildings. It can occur in soldered electrical joints if only a small loading ispresent. Only small strain rates are necessary.
There are several stages:
* The initial elastic extension.
* Primary creep in which the strain rate decreases with time.
* Secondary orquasi-viscous steady state creep shows constant strain in relation to time.
* Tertiary or accelerating creep leads to fracture ( creep rupture or stress rupture)
If we compare the elongation atstress rupture with the elongation to failure, or a tensile test specimen, the latter will have deformed plastically to a much lesser degree. Unloading will allow elastic recovery (Hooks law) but fullrecovery will not take place.
The mechanism by which creeps takes pace is complex but related to the behavior of dislocations. Grains boundaries also play in the mechanism, related to dislocationactivity. In the final stages of creep, cavities appear at grain boundaries, which eventually coalesce and lead to the failure. Grain size is therefore of critical importance in the performance of a gasturbine blade; tungsten lights filaments may be made of a single crystal to enhance their lives.
Aim of the Lab
The principal objectives of this lab exercise are learning how to proceed the...