2013 ANSYS, Inc. July 31, 2014 1

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ANSYS Mechanical Advanced Nonlinear Materials

Workshop 2A Stress Relaxation

2013 ANSYS, Inc. July 31, 2014 2

Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation

Model Description

C-Clamp assembly consisting of a 3D solid body meshed with tetrahedral elements to represent the clamp and a line body meshed with beam elements to represent the bolt

Bolt tied to the clamp body via fixed joints at each end.

Materials:

Rate dependent structural steel for clamp

High strength rate independent steel for bolt

Goal

Predict rate dependent stress relaxation of structure using an implicit time hardening creep model.

C-Clamp

Bolt

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Model Description (contd)

Materials:

Bolt: Rate independent structural steel Clamp: Rate dependent structural steel that relaxes according to the following

Modified Time Hardening formula:

Where: C1 = 2e-10, C2=1, C3=1.5, C4 = 0, metric units (mm,mm-s^-2)

Load specifications:

10,000N bolt pretension loading for 150 seconds

13

1

1432

C

etC

/TCCC

cr

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Steps to Follow:

Restore Archive browse for file WS2A-creep.wbpz

Save as

File name: WS2A-creep Save as type: Workbench Project Files (*.wbpj)

Workshop 2A: Stress Relaxation

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation The project Schematic should look like the picture to the

right.

From this Schematic, you can see that Engineering (material) Data and Geometry have already been defined (green check marks).

It remains to define the rate dependent creep material properties, set up and run the FE model in Mechanical

Highlight the Engineering Data Cell and open by clicking on the Right Mouse Button (RMB)=>Edit.

Verify that the units are in Metric(Tonne,mm,..) system. If not, fix this by clicking on

Utility Menu=>Units=>Metric(Tonne, mm,..)

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation

Define the creep material model

Highlight the Default Structural Steel material and RMB=>Duplicate to create a separate material model. Rename this new material Creep

With the new Creep material model highlighted, go to the Toolbox and insert a Modified Time Hardening creep model from the Creep folder.

The Engineering Data page should now look as shown

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Define the creep material model (contd)

Set the Reference Length Unit to mm,s,K,tonne,mm s^2 Fill in the data table accordingly (constant values from page WS2A-3).

Workshop 2A: Stress Relaxation

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation

Return to the Project Schematic and Refresh project

Double click on the Model Cell to open the FE Model (Mechanical Session) (or RMB=>Edit)

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Once inside the Mechanical application, verify the working unit system

Unit > Metric (mm,kg,N,s,mV,mA)

Open the folders beneath the model branch to become familiar with the model set-up.

Expand Geometry branch and note the two different body types (solid body for the clamp, line body for the bolt).

Expand the Connections branch and note the two fixed joints designed to connect the ends of the bolt to the faces of the clamp by way of constraint equations automatically. The pinball is sized to represent the extent of bolt head and nut interface with the clamped surfaces.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation

Highlight the Solid body representing the clamp and change material assignment to the Creep model

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Highlight the Analysis Settings branch, set number of load steps to 2 and define the Solution Control specifications as shown for each load step.

Note:

LS1 establishes the static stress state in one substep at time zero (=1e-006 s) with the Creep Effects turned Off.

LS2 activates the creep calculation (end time =150 s) over several substeps, allowing bisections if/when creep strain exceeds 10 times the elastic strain.

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Loads and Boundary conditions.

The back face of the clamp body is fixed. A bolt pretension load is applied to the line body over two load steps

LS 1 calculates the adjustment needed in bolt shank length to produce user defined bolt load

Set LS2 to lock the adjustment into the bolt via constraint equations .

Workshop 2A: Stress Relaxation

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Execute the solve

This run should converge without trouble, but might take a few minutes depending on hardware.

Note the development of creep strain as indicated by nonzero values reported for CREEP RATIO in the solver output during the run.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation After solution is complete, post process results (displacement, stress, strain,etc)

Insert Equivalent Creep Strain Notice how the Equivalent Creep Strain increases

(as expected) over time, while bolt pretension and equivalent stress dissipates as the structure relaxes.

Animate various results over time to confirm these effects.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation To test the accuracy of the creep calculation, tighten the creep ratio from 10 to 0.5, but keep the same autotime stepping specifications.

The solution still converges, but the tighter creep ratio causes bisections and many more iterations.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Notice the difference in results as compared with original run.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation

Try running the model once more, with a tighter time increment and a further reduced creep ratio.

The solution converges, more iterations and a few bisections toward the end of the run.

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Customer Training Material Customer Training Material Customer Training Material Workshop 2A: Stress Relaxation Compare results with previous run.

Notice how results are beginning to converge to same answer.

This underscores the importance of making sure that the path dependent nature of the nonlinear creep calculation is captured with an adequately refined time increment.