Creep Loading - CAESAR II - Help

Creep is the slow, permanent deformation of a solid material under mechanical stresses. Creep occurs at elevated temperatures during long-term exposure to high levels of constant stress below the yield strength of the material. Power boiler piping is an example of the type of piping subjected to creep loading.

Materials

At elevated temperatures, creep governs the allowable stress properties for a material. Allowable values controlled by creep, also called time-dependent allowables, are a function of duration of loads.

Default material allowables for ASME B31.3 and ASME B31.1 are based on 100,000 hours.

Creep life varies for EN-13480 materials, as shown in the following examples. CAESAR II includes creep life in the material name.

For material 1.0345S-16-100 (material number 406 in CAESAR II):

• 16 - Indicates a maximum thickness of 16 mm.

• 100 - Indicates the allowable is for a load duration of 100,000 hours, which is the default value for CAESAR II.

For material 1.0345S-16-200 (material number 468 in CAESAR II):

• 16 - Indicates a maximum thickness of 16 mm.

• The allowable is for a load duration of 200,000 hours.

If the material allowable corresponding to the load duration you need is not available, then you can create a new custom material with the required data.

Calculations

CAESAR II creep stress (CRP) is a scalar combination of one sustained case (SUS) and one expansion case (EXP). The software implements EN-13480 creep methodology for many supported piping codes.

Creep stress calculations follow EN-13480 paragraph 12.3.5:

(1) EN-13480-3:2017 does not provide an explicit stress equation for the directional SIF approach. CAESAR II applies a conservative approach for EXP stress.

(2) EN-13480-3:2017 applies the 0.75 factor to the entire EXP stress value. EN-13480-3:2017/A4:2021 applies the 0.75 factor only to the EXP moment stress value.

(3) EN-13480-3:2017/A4:2021 applies a 0.75 factor to the bending moment. Axial and torsional stress components do not include 0.75 factor.

You can control the use of directional SIFs by setting EN-13480/CODETI Use In-Plane/Out-Plane SIF to True in the Configuration Editor

You do not need to specify additional load multipliers to implement creep according to the equation, as shown in the following Load Case Editor example. If you specify additional load multipliers, the software applies these as additional scale factors.

Load Case Editor

CAESAR II does not automatically generate creep load cases, so you must manually create a creep load case using the CRP stress type. You can define a creep stress range for each strain range as needed.

The following load set example contains two pressures (P1 and P2) and two temperatures (T1 and T2). T2 is in the creep range.

• Two creep load cases (L9 and L10) correspond to P1 and P2 operating conditions at T2.

• The final load case (L11) is the maximum combination case from all the CRP cases to obtain maximum stress or loads in all the creep cases.