Extrapolation of Sigmoidal Creep Curve by Strain Acceleration Parameter

2013 ◽  
Vol 592-593 ◽  
pp. 606-609 ◽  
Author(s):  
Hiroyuki Sato ◽  
Kosuke Omote ◽  
Akira Sato ◽  
Kouki Ueno
Keyword(s):  
Strain Rate ◽  
High Temperatures ◽  
Creep Curve ◽  
Primary Creep ◽  
Microstructural Change ◽  
Rate Change ◽  
Strain Rate Change ◽  
Acceleration Parameter ◽  
Creep Characteristics ◽  
Creep Curves

It has been widely accepted that the creep characteristics at high temperatures are mainly evaluated by a minimum creep rate and a time to fracture. Although, a shape of creep curve may vary depending on deformation conditions, the apparent minimum creep rates may become the same value. Thus, for detailed analysis and prediction of creep behavior, other values should be considered which reflects the shape of each creep curve. For the purpose, authors have proposed Satos Strain-Acceleration-Parameter (SAP) which reflects strain rate change during creep. Based on the concept of SAP, the whole creep curve can be represented by a set of small numbers of numerical parameters, and can be extrapolated from a part of creep curve [. It is also well accepted that the creep rates depend on microstructures, and microstructural changes cause strain rate change. The SAP would reflect stability and magnitude of microstructural change during deformation at high temperatures. In this paper, application of the concept of SAP to creep curves that show sigmoidal type primary creep is presented. The creep curve can be divided into two regime based on the SAP values. The sigmoidal creep curve is reasonably reproduced by the concept of Strain-Acceleration-Parameter, and reasonably agrees with experiment. Whole creep curve can be reasonably represented by a few numerical values which reflect shape of creep curve in each regime. The concept of SAP is applicable for quantitative evaluation of both normal and sigmoidal type of creep curves.

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.

Extrapolation of Creep Curve and Creep Rate by Strain Acceleration Parameter in Al-Mg Solid Solution Alloys

2014 ◽  
Vol 794-796 ◽  
pp. 307-312
Author(s):  
Hiroyuki Sato ◽  
Kosuke Omote ◽  
Akira Sato
Keyword(s):  
Solid Solution ◽  
Steady State ◽  
Creep Rate ◽  
Creep Curve ◽  
Creep Behavior ◽  
Minimum Creep Rate ◽  
Strain Rate Change ◽  
Acceleration Parameter ◽  
Creep Characteristics ◽  
Creep Curves

It has been widely accepted that the creep characteristics at high temperatures are mainly evaluated by a minimum creep rate. Although, a shape of creep curve may vary depending on deformation conditions, the apparent steady state or minimum creep rates be the same. Thus,for detailed analysis and prediction of creep behavior, other values which reflect the shape of each creep curve should be considered. For the purpose, authors have proposed Sato’s strain- acceleration-parameter (Strain Acceleration and Transition Objective index, SATO-index) which reflects strain rate change during creep deformation. Based on the concept of SATO-index, the whole creep curve can be represented by a set of small number of numerical parameters, and can be extrapolated from a part of creep curve. In this paper, application of the concept of SATO-index to the creep curves of aluminum-magnesium solid solutions that the creep behavior of the alloys are well investigated and analyzed. The creep curve can be extrapolated by the concept from transient part of creep curve, and the extrapolated creep rates at the minimum creep rate agree well with experiment. Efficiency of the concept of SATO-index to creep experiments is pronounced.

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.

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