Flow curve
The term flow curve is used in rheology and materials science .
Rheology
The flow curve is a diagram that shows the relationship between shear stress (or alternatively viscosity ) and shear rate for a given substance .
For this purpose, a sample is loaded in a viscometer or rheometer at an ever increasing speed and the viscosity is recorded. Many substances such as water or silicone oil show a horizontal line on the viscosity-shear rate diagram, i.e. no dependence of the viscosity on the stress (the shear rate). Such substances are called Newtonian fluid .
In addition, the following models and typifications exist:
However, all models are subject to certain restrictions, so that the assumption of a model for a certain substance only applies with sufficient accuracy in a certain range (temperature, shear rate).
See also: yield point
Materials science
The flow curve shows the relationship between flow stress and degree of deformation . B. be determined with the help of the compression , bulge or tensile test . In addition to the degree of deformation, the yield stress is also dependent on the process parameters temperature , hydrostatic pressure and deformation speed as well as the material and its microstructure .
The yield stress is a measure of the force required per unit area ( pressure ) to plastically deform a body .
Modeling
The characteristics of the flow curve have, in addition to the above The following material-specific factors also have an influence:
- Blocking of dislocation movements by grain boundaries , precipitates , inclusions and impurities
- Mutual obstruction of movement from dislocations
- Mutual cutting of dislocations
- Gradual blockage of dislocation sources
- Multiple glides
- Interaction with interstitially dissolved atoms
The modeling of flow curves takes place according to different approaches:
- Physical: Function theoretically derived from known physical relationships
- Semi-empirical: approximation functions derived from physical considerations and measurement data
- Empirical: pure mathematical approximation using experimental measurement data
The most frequently used approach to modeling flow curves to calculate the required forming forces and the energy consumption of industrial forming systems is the Hensel-Spittel approach, developed in 1978, which can be assigned to the semi-empirical approaches. The Hensel-Spittel approach enables the rapid calculation of yield stresses in FEM simulations and is also frequently used for FEM calculations for rolling flat products and wire. An extension of the Hensel-Spittel approach is the Freiberg approach based on it, which enables an even more realistic representation of the yield stress, especially with higher degrees of deformation.
See also
swell
- ↑ Schmidtchen, M. / Spittel, M. (2011): Flow curves for cold and hot forming, in: MEFORM2011 - Material parameters for the simulation of forming processes, TU Bergakademie Freiberg.
- ↑ Hensel, A. / Spittel, T. (1978): Kraft- und Arbeitsbedarf bildsamer Formgebungsverfahren, Verlag Grundstoffindindustrie.
- ↑ Spittel, M. / Spittel, T. (2009): Metal Forming Data of Ferrous Alloys, in: Landolt-Börnstein, Group VIII Advanced Materials and Technologies Volume 2C.