Deriving creep constants from creep curve data Article ID: x394 Product(s): COSMOSWorks, COSMOSDesignSTAR Version: All Versions Category: Nonlinear Created: 07/17/2007 Last Revised: 07/19/2007 Discussion
How to enter creep in COSMOS COSMOSWorks and COSMOSDesignSTAR use the Classical Power Law for Creep (Bailey-Norton law). The creep strain at time t is :
where: T = Temperature (Kelvin) (= input temperature + reference temperature + offset temperature) CT = A material constant defining the creep temperature-dependency temperature-dependency C0, C1 and C2 are the three creep constants If there is no temperature variation, then:
Note: ●
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In the COSMOSWorks and COSMOSDesignSTAR Material dialog box, the C0, C1 and C2 constants are labelled: C0 = CREEPCONST1 C1 = CREEPCONST2 C2 = CREEPCONST3 CT = CREEPTC Check Include Creep Effect to c reep effect in the selected Effect to activate the inclusion of creep material model. Creep effect is considered only for nonlinear studies. Among all material models available with nonlinear studies, creep effect is not available with linear elastic orthotropic and viscoelastic material models. For more information, please refer to Creep Models in Models in the COSMOSWorks and COSMOSDesignSTAR online help. C1 = CREEPCONST2 is unitless. Therefore, its value doesn't change when you change the time unit of the stress unit. C0 = CREEPCONST1, though, changes. C0 depends on the time unit AND the stress unit.
Example Suppose you have the creep data below (for Grades of Stainless Steel - Grade 310, reference: http://www.fanagalo.co.za/tech/tech_grade_310.htm). Creep data Stress to develop a creep rate of 1% in the indicated time at the indicated temperature.
Time
Temperature (°C)
550
600
650
700
750
800
10 000 h
Stress (MPa)
110
90
70
40
30
15
100 000 h
Stress (MPa)
90
75
50
30
20
10
Let s say we pick two stress points from the data above: ’
Temperature = 550., Creep Strain(Ec)=0.01 ->
Time(hr)
Stress(MPa)
10,000.
110.
100,000.
90.
In The Creep Strain Formulation:
It is usually safe to assume C2=1., so we need to find C0 & C1. 0.01 = C0 (110.)^C1 (10,000.) 0.01 = C0 (90.)^C1
(100,000.)
Equating the two equations, and using the Log function: C1 Log(110) = Log[(90.^C1)(10.)] = C1 Log(90.) + 1. From here you can calculate C1 = 11.47 And either equation can be used to get C0=2.5988e-29 Note: ●
The above constants have been calculated using non-standard units (MPa and hours). If you use instead N/m² and seconds, like in the SI system, the data is changed. Actually, C1 is unitless. Therefore, its value doesn't change when you change the time unit of the stress unit. C0, though, changes. C0 depends on the time unit AND the stress unit. If you use N/m² and seconds, then: C0 = 0.01 / (100000*3600 * (90E6)^11.47 = 1.616 E-102 Therefore, for the above material, if you are using the SI system and seconds as the time unit, you should enter:
CREEPCONST1 = 1.616 E-102 CREEPCONST2 = 11.47 CREEPCONST3 = 1
Now if you do this calculation for another temperature, and then add to the formulation the temperature term TC, you can also calculate the last constant for thermal variations.
