Using ANSYS for Creep Analysis.docx

Using ANSYS for Creep Analysis Creep is a rate dependent material nonlinearity in which the material continues to deform under a constant load. The three stages of creep are shown in the figure below. ANSYS has the capability of modeling the first two stages (primary and secondary). The tertiary stage is usually not analyzed since it implies impending failure (gross element distortion).

Creep strain due to constant applied stress ANSYS analyzes creep using two time integration methods. Both are applicable to static or transient analyses.  Implicit creep method is robust, fast, accurate, and recommended for general use. It handles temperature dependent creep constants, as well as simultaneous coupling with isotropic hardening plasticity models.  Explicit creep method is useful for cases where very small time steps are required. Creep constants cannot be dependent on temperature. Coupling with other plastic models is available by superposition only.

Implicit Creep Implicit creep refers to the use of backward Euler integration for creep strains. This method is numerically unconditionally stable, which means that it does not require as small a timestep as the explicit creep method, so it is much faster overall. For implicit creep plus rate-independent plasticity, the plasticity correction and creep correction are done at the same time, not independently. Consequently, implicit creep is generally more accurate than explicit creep, but it is still dependent on the time-step size. A small enough time-step must be used to capture the path-dependent behaviour accurately. You can simultaneously model implicit creep and BISO, MISO, NLISO, BKIN, and HILL plasticity. RATE command can be used to turn implicit creep on and off. Useful for setting up initial conditions prior to a creep analysis (plasticity is often active during the ramp up phase of a creep analysis). Enforce a creep limit ratio using the creep ratio control option in commands CRPLIM or CUTCONTROL, CRPLIMIT. A recommended value for a creep limit ratio ranges from 1 to 10.

Explicit Creep Explicit creep means that the forward Euler method is used for the calculation of creep strain evolution. The creep strain rate used at each time step corresponds to the rate at the beginning of the time step and is assumed to be constant throughout that time step,”t. Because of this, very small time steps are required to minimize error. For explicit creep with plasticity, plasticity correction is performed first followed by creep correction. These two corrections occur at different stress values; therefore, it may be less accurate. Explicit creep is no longer recommended for creep analysis. If you are learning to use creep in ANSYS, learn to use the implicit creep method.

Implicit Creep Procedure The basic procedure for using the implicit creep method involves issuing the TB command with Lab = CREEP, and choosing a creep equation by specifying a value for TBOPT. The following example input shows the use of the implicit creep method. TBOPT = 2 specifies that the primary creep equation for model 2 will be used. Temperature dependency is specified using the TBTEMP command, and the four constants associated with this equation are specified as arguments with the TBDATA command. TB,CREEP,1,1,4,2 TBTEMP,100 TBDATA,1,C1,C2,C3,C4

You can input other creep expressions using the user programmable feature and setting TBOPT = 100. You can define the number of state variables using the TB command with Lab = STATE. The following example shows how five state variables are defined. TB,STATE,1,,5

You can simultaneously model creep [TB,CREEP] and isotropic, bilinear kinematic, and Hill anisotropy options to simulate more complex material behaviors. See Material Model Combinations in the Elements Reference for the combination possibilities. Also, see Material Model Combinations in this chapter for sample input listings of material combinations. To perform an implicit creep analysis, you must also issue the solution RATE command, with Option = ON (or 1). The following example shows a procedure for a time hardening creep analysis (See Figure 8.16: "Time Hardening Creep Analysis").

Figure 8.16 Time Hardening Creep Analysis

The user applied mechanical loading in the first load step, and turned the RATE command OFF to bypass the creep strain effect. Since the time period in this load step will affect the total time thereafter, the time period for this load step should be small. For this example, the user specified a value of 1.0E-8 seconds. The second load step is a creep analysis. The RATE command must be turned ON. Here the mechanical loading was kept constant, and the material creeps as time increases. /SOLU RATE,OFF TIME,1.0E-8 ... SOLV RATE,ON TIME,100 ... SOLV

!First load step, apply mechanical loading !Creep analysis turned off !Time period set to a very small value !Solve this load step !Second load step, no further mechanical load !Creep analysis turned on !Time period set to desired value !Solve this load step

The RATE command works only when modeling implicit creep with either von Mises or Hill potentials. When modeling implicit creep with von Mises potential, you can use the RATE command with the following elements: LINK180, SHELL181, SHELL281, PLANE182, PLANE183, SOLID185, SOLID186, SOLID187, SOLSH190, BEAM188, BEAM189, SHELL208, and SHELL209. When modeling anisotropic creep (TB,CREEP with TB,HILL), you can use the RATE command with the following elements: PLANE42, SOLID45, PLANE82, SOLID92, SOLID95, LINK180, SHELL181, SHELL281, PLANE182, PLANE183, SOLID185, SOLID186, SOLID187, SOLSH190, BEAM188, BEAM189, SHELL208, and SHELL209. For most materials, the creep strain rate changes significantly at an early stage. Because of this, a general recommendation is to use a small initial incremental time step, then specify a large maximum incremental time step by using solution command DELTIM or NSUBST. For implicit creep, you may need to examine the effect of the time increment on the results carefully because ANSYS does not enforce any creep ratio control by default. You can always

enforce a creep limit ratio using the creep ratio control option in commands CRPLIM or CUTCONTROL,CRPLIMIT. A recommended value for a creep limit ratio ranges from 1 to 10. The ratio may vary with materials so your decision on the best value to use should be based on your own experimentation to gain the required performance and accuracy. For larger analyses, a suggestion is to first perform a time increment convergence analysis on a simple small size test. ANSYS provides tools to help you determine the coefficients for all of the implicit creep options defined in TB,CREEP. The TBFT command allows you to compare your experimental data with existing material data curves and visually “fit” your curve for use in the TB command. All of the TBFT command capability is available via either batch or interactive (GUI) mode. See Material Curve Fitting (also in this manual) for more information.