Does anyone could help me with the definition and understanding about the creep ductility in uni-axial stress states and multi-axial stress states?
Any help is appreciate!
The creep ductility under uniaxial or multiaxial condition can be defined based on the % elongation and % reduction in area in specimen testing. The importance of uniaxial and multiaxial creep ductility to my understanding is that the component should have sufficient ductility to avoid premature/catastrophic/brittle fracture while handling it during operation or at the end of its life. Ductility may decrease due to continuous operation at high temperature resulting in metallurgical changes or environment.
As per my understanding….Percentage reduction in area in uniaxial constant load specimen is considered as the measure of ductility. This is used in ductility exhaustion method of damage calculation. Failure occurs when the cumulative strain reaches a limiting ductility.
In a notched specimen failure occurs at lower ductility i.e. failure occurs at lower elongation of material. Or we can say notch sensitivity is related to creep ductility. It is suggested that a minimum smooth bar creep ductility of about 10% in terms of reduction in area may be desirable for avoidance of notch sensitivity. Japanese codes give this value as 20%. In my opinion equivalent total strain at rupture can be used as measurement of ductility in mutiaxial case…
This question is both important as the answers are complex. In a simple loading condition tradition indicates that ductility can be measured by elongation and reduction of area. However it is well established that these are not true materials values and are influenced by the specimen geometry.
Notched bar tests introduce multiaxiality so as long as the stres state is know data can be assessed but often in this case results are considered as notch strengthening or notch weakening.
Please let me know if this thread is still of interest
Hello. I was wondering if you know what this means?
"It is assumed that all casts of Type 316 and Type 316L(N) steel have the same value of minimum ductility. Whilst creep ductility varies significantly with strain rate and temperature, there is no evidence to suggest that the minimum creep ductility for Type 316 steel is dependent on either of these parameters." from UK DEVELOPMENT OF A STRAIN BASED CREEP-FATIGUE ASSESSMENT PROCEDURE FOR FAST REACTOR DESIGN by C. PICKER
http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/28/042/28042997.pdf
How is minimum creep ductility found or defined and why doesn't it vary with strain rate and temp whereas, creep ductility does?
@Jonathan David Parkar...Please share your views sir....still following this thread..
Regards
Naveen
As per my understanding, if you plot % reduction of cross sectional area of a specimen vs. min creep rate for a number of specimens tested at various stress levels, you will get a DBTT type curve for creep. It contains two shelves i.e. upper and lower. This transition is due to change of fracture micromechanism from intergranular to transgranular manner. The upper shelf points are actually referred as ''creep ductile'' and the lower shelf points are referred as ''Creep brittle specimens''. So, unlike our purview as room temperature ductility, high temperature sustained load ductility is little complex relationship. Now, it is a thought experiment of mine, brittle to ductile transition can be seen also if notches of different curvatures can be introduced at the gage length as because the effective creep stress will vary with the cross sectional area, So if you have a good CNC facility and little weak creep testing facility (Not having enough weight values) you can get the DBTT curve by varying the cross sectional area under a Fixed load. As the creep stress faced by the specimen will defer if you change the area.
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