Yield point phenomenon during strain rate change in nanocrystalline Ni–Fe

2011 ◽  
Vol 65 (3) ◽  
pp. 217-220 ◽  
Author(s):  
S. Van Petegem ◽  
J. Zimmermann ◽  
H. Van Swygenhoven
Keyword(s):  
Strain Rate ◽  
Yield Point ◽  
Rate Change ◽  
Strain Rate Change ◽  
Yield Point Phenomenon

A Theory of the Yield Point and the Transition Temperature of Mild Steel

1956 ◽  
Vol 23 (2) ◽  
pp. 219-224
Author(s):  
F. Forscher
Keyword(s):  
Transition Temperature ◽  
Mild Steel ◽  
Strain Rate ◽  
Yield Point ◽  
Structural Model ◽  
Yield Criterion ◽  
State Of Stress ◽  
Yield Point Phenomenon ◽  
Stress And Strain Rate ◽  
Temperature Strain

Abstract Experimental results indicate the dependence of the yield-point phenomenon of mild steel on temperature, strain rate, duration of stress, and stress state. This paper proposes a yield criterion which can account for these variables. The theory is developed on the basis of a “structural” model, by which the behavior of microscopic and submicroscopic elements is idealized. The theory postulates as yield criterion a critical number of relaxation centers (active Frank-Read sources) or, equivalently, a critical size of relaxation centers. The transition-temperature phenomenon is considered to be the result of an inhibition of yielding (upper yield point) by means of geometry, temperature and/or strain rate. A relation is given which expresses its dependence on the state of stress and strain rate.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

2016 ◽  
Vol 716 ◽  
pp. 121-128 ◽  
Author(s):  
Jens Dierdorf ◽  
Johannes Lohmar ◽  
Gerhard Hirt
Keyword(s):  
Flow Stress ◽  
Dynamic Recrystallization ◽  
Strain Rate ◽  
Metal Forming ◽  
Elevated Temperatures ◽  
Relaxation Behavior ◽  
Rate Change ◽  
Strain Rates ◽  
Flow Curves ◽  
Strain Rate Change

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Extrapolation of Creep Curve and Creep Life Prediction From Secondary Creep by Evaluation of Strain Rate Change

2010 ◽  
Author(s):  
Hiroyuki Sato
Keyword(s):  
Strain Rate ◽  
Creep Curve ◽  
Life Prediction ◽  
Temperature Reconstruction ◽  
Rate Change ◽  
Secondary Creep ◽  
Creep Life ◽  
Strain Rate Change ◽  
Creep Life Prediction ◽  
Creep Curves

New method of creep life prediction by Strain-Acceleration-Parameter, SAP, is presented. Sato has found that shapes of creep curves can be characterized by the SAP that reflects magnitude of strain-rate change in secondary creep [1–4]. The SAP values are defined at minimum creep rates, and show the shapes of a creep curve, that depends on stress and temperature. Reconstruction of creep curves by a combination of SAP and a minimum-creep rate is successfully performed, and the extrapolated curves agree well with experiment. The predicted life times also reasonably agree with that obtained by experiment. The possibility of precise life prediction by SAP is pronounced. One of an important advantage of the proposed method is that the required parameters evaluated by individual creep curve are simpler than that are used in methods previously proposed, i.e., the theta projection concept, for example. Possibilities of wide application on many kinds of heat resistant materials should be investigated with the method of SAP.

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