Mechanical APDL Structural Analysis Guide

Figure 8.10: Stress Relaxation and Creep

ResultingForce

Applied

Displacement

(a)Stressrelaxation

Primary Secondary

Tertiary

Rupture

(b)Creepstraindueto

constantappliedstress

Time Time

ε

Creep is important in high temperature stress analyses, such as for nuclear reactors. For example, suppose
you apply a preload to some part in a nuclear reactor to keep adjacent parts from moving. Over a
period of time at high temperature, the preload would decrease (stress relaxation) and potentially let
the adjacent parts move. Creep can also be significant for some materials such as prestressed concrete.
Typically, the creep deformation is permanent.

The program analyzes creep using two time-integration methods. Both are applicable to static or tran-
sient analyses. The implicit creep method is robust, fast, accurate, and recommended for general use.
It can handle temperature dependent creep constants, as well as simultaneous coupling with isotropic
hardening plasticity models. The 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.

The terms implicit and explicit as applied to creep have no relationship to explicit dynamic analysis

or to any elements referred to as “e xplicit dynamic elements.”

The creep strain rat e may be a function of stress, strain, temperature, and neutron flux level. Built-in
libraries of creep strain rat e equations are used for primary, secondary, and irradiation induced creep.
(See Creep Equations for discussions of, and input procedures for, these various creep equations.) Some
equations require specific units. For the explicit creep option in particular, temperatures used in the
creep equations should be based on an absolute scale.

The following topics related to creep are available:

8.4.1.6.1. Implicit Creep Procedure

8.4.1.6.2. Explicit Creep Procedure

8.4.1.6.1. 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 is 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

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Modeling Material Nonlinearities