Create a margin of safety calculation using an allowable method

This example uses a piston rod as the model. In this example, you use the Stress allowable and Displacement allowable methods, respectively, to create the following margin of safety calculations:

• Margin of safety against yielding for the entire piston rod.Use the von Mises criterion as the method to predict when yielding initiates.Assume that the yield strength is 280 MPa.

• Margin of safety against excessive axial elongation at the tip of the piston rod.Assume that the maximum allowable axial elongation at the tip is 2 mm.

Assume that the units for the model are kilogram, millimeter, and second. Thus, the units for stress and deflection results that the software uses in the margin of safety calculation are kPa and mm, respectively.

Note:

This example assumes that you created load case sets. It begins with the Calculation Creation dialog box open.

Evaluating margin of safety for yielding

1. Next to Method, click Select a method .

2. In the Calculation - Method Selection dialog box, from the Method list, select Stress allowable and click OK.

3. On the Inputs tab, in the Allowable box, type 280 and change the units to MPa.The yield strength of the material is 280 MPa. The von Mises criterion predicts that yielding begins when the von Mises stress equals the yield strength.

4. Click Load Extraction .

5. In the Load Extraction dialog box, do the following:From the Result Type list, select Stress - Element-Nodal.From the Component list, select Von-Mises.From the Combine At list, select Nodes.

6. In the Nodal Combination group, do the following:From the Nodal Combination list, select Average.Select the PID, MID, Element Type, and Feature Angle check boxes.Select the Include Internal Elements check box.At each node, the software averages each stress component, and then calculates the von Mises stress from the averages of the stress components.

7. In the Selection group, do the following:From the Type list, select Node.In the graphics window, drag a selection box around the piston rod.The software creates a set of results that contains the von Mises stress for all the nodes in the model.Click Confirm source edit .The source appears in the Sources table.

8. In the Operations group, do the following:From the Filtering list, select None.From the Absolute Values list, select No.By definition, von Mises stress is positive.From the Aggregation list, select Maximum.The software uses the maximum value in the set of results as the working load in the margin of safety calculation.In the Factor box, type 0.001.The value in the Factor box converts the unit for the results that you selected in step 7, which is kPa, to the unit of the value that you entered in step 3, which is MPa.Note: 1 kPa = 0.001 MPa

9. Click OK (both dialog boxes).

Evaluating margin of safety for deflection

1. Next to Method, click Select a method .

2. In the Calculation - Method Selection dialog box, from the Method list, select Displacement allowable and click OK.

3. On the Inputs tab, in the Allowable box, type 2 and change the units to mm.The maximum allowable axial elongation at the tip is 2 mm.

4. Click Load Extraction .

5. In the Load Extraction dialog box, do the following:From the Result Type list, select Displacement - Nodal.From the Component list, select Y.For this model, the axial direction coincides with the absolute Y-direction.From the Coordinate System list, select Absolute Rectangular.From the Combine At list, select None.

6. In the Selection group, do the following:From the Type list, select Polygon Face.In the graphics window, select the topmost face of the piston rod.The software creates a set of results that contains the Y-component of the displacement for all the nodes in the selected face.Click Confirm source edit .The source appears in the Sources table.

7. In the Operations group, do the following:From the Filtering list, select None.From the Absolute Values list, select No.In this example, positive elongation is equivalent to positive displacement in the Y-direction. However, if the positive sense of the Y-axis was in the opposite direction, positive elongation would be equivalent to negative displacement. For such a situation, from the Filtering list, select Only Negative Values (< = 0). Doing so excludes any nodes that have positive displacements from the margin of safety calculation. Then, from the Absolute Values list, select Yes to change the sign of the remaining displacements so that they are compatible with the sign of the value you entered for the maximum allowable axial elongation at the tip.From the Aggregation list, select Maximum.The software uses the maximum value in the set of results as the working deflection in the margin of safety calculation.In the Factor box, type 1.For this case, because the unit for the results that you selected in step 6 is the same as the unit for the value that you entered in step 3, a conversion factor is not required.

8. Click OK (both dialog boxes).

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Create a margin of safety calculation using an allowable method, Simcenter 3D 2021.1 Series

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