Structural Creep is a temperature-sensitive mechanical behavior. At room temperature, the creep effect of steel is extremely weak. Under high temperature conditions, creep accelerates dramatically. At room temperature, when a structure is subjected to
sustained high stress near the yield strength
, and works for a long period of time, significant creep will also occur. In ANSYS Workbench, there are many creep material models. Through creep models, the stress relaxation phenomenon can be simulated, where the structure undergoes creep and stress changes with time after being subjected to external loads.
02
Case Study
Use the Static Structural module in the ANSYS Workbench platform.
Open Geometry, create a C-clamp and bolt model in SpaceClaim, with the bolt simplified as a beam.
In the material library, you can see that Creep contains many creep models. Here, the Modified Time Hardening model is used. The creep rate decreases with time and is strongly correlated with current stress. It converges easily and is suitable for stress relaxation scenarios.
Enter Model, assign the creep material to the C-clamp, and use ordinary structural steel for the bolt. This calculation considers the creep of the C-clamp.
The connection between the C-clamp and the bolt uses the Fixed function in Joints.
Through Fixed, the top vertex of the bolt beam can be bonded to the upper and lower surfaces of the C-clamp. This bonded connection automatically uses
constraint equations
to connect both ends of the bolt to the end faces of the clamp.
The Pinball region size setting is used to simulate the actual contact range between the bolt head, nut, and the clamped surface. Here it is set to a spherical region with a radius of 15mm.
Meshing, here the beam cross-section of the bolt is also displayed.
For creep calculation, the most important thing is to set the analysis steps. There are two time steps in total. In the first time step, the calculation time is 1e-6s. Through the first time step, a static stress state is established, and creep is not considered in this time step.
In the second time step, set the calculation time to 1500s, enable the creep effect, and use the default creep limit ratio of 1.
Fixed constraint on the left side of the C-clamp.
Set a bolt preload force for the bolt beam.
In the first load step, apply the preload force through Load, and the software calculates the bolt length adjustment required for this preload force. In the second load step, through constraint equations, the length adjustment calculated in the first step is locked in the bolt, so the Lock function is used here.
Calculation converges.
By viewing the solution information, you can see that the 2nd creep analysis load step completed a total of 19 substeps. The total number of iterations for the entire analysis up to this point is 58, indicating good iteration efficiency. The time reached the end time of the 2nd step we set
1500s, indicating that the total duration of the creep analysis has been completed and the calculation is finished. The last time step size is
87.1875s, which is much larger than the initially set 15s, indicating that the automatic time stepping continued to enlarge the step size in the later stages of calculation, significantly improving computational efficiency. Creep ratio = single-step creep strain / elastic strain = 0.3181,
which is less than our set limit of 1.0
, therefore the solver did not trigger time step bisection, and the calculation converged stably. This value also indicates that during the 1500s creep process, the structural creep strain has developed significantly but is still within a controllable range, consistent with the expectations of the creep analysis.
The equivalent creep strain shows expected growth over time. Meanwhile, as the structure undergoes stress relaxation, the equivalent stress of the C-clamp decreases with time.
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