4.8.1. Constant Composite Cross Section with a Straight Beam

The base to compute the equivalent beam properties is a generic straight box-like composite structure with constant cross section and constant composite layup along the length. A consequent equivalent beam model is built, and its frequencies are compared against the 3D model frequencies (SOLID185 elements).

The fully configured Workbench project is contained in a compressed file and can be downloaded here. This configured project requires Release 2024 R2 or more recent versions.

The section is shown in the following figure with the coarse mesh used to access the methodology. The material properties are listed in the table.

Figure 4.63: Cross Section and Layup of the Box

Table 4.1: Material Properties

	Foam	Glass UD	Carbon Woven
Ex [Mpa]	70	45000	61340
Ey [Mpa]	70	10000	61340
Ez [Mpa]	70	10000	6900
Gxy [Mpa]	26.92	5000	3300
Gxz [Mpa]	26.92	5000	2700
Gyz [Mpa]	26.92	3846.15	2700
νxy	0.3	0.3	0.04
νxz	0.3	0.3	0.3
νyz	0.3	0.4	0.3
Density [t/mm3]	6E-11	2.0E-09	1.42E-09

For this example, the reference point considered for the cross section properties is the Origin. The following shows the calculation result.

Figure 4.64: Computation Result of the Equivalent Beam Properties

Figure 4.65: Mesh of the Cross Section and its Principal Properties

A simple straight beam of the desired length is created in SpaceClaim and meshed in the Mechanical application. The equivalent beam model is built in the Mechanical system Modal: Beam model (C) by adding an APDL snippet under the Geometry object in the Tree View. The results from the Section Cut computation in ACP are directly pasted here as shown below. Alternatively, a Python script could be inserted in ACP which automatically generates the APDL inputs (APDL file). This file can then be loaded by the Command Snippet object in the Mechanical. The Python script option enables further automation of the workflow.

Note: SpaceClaim is no longer included in the unified installer. Ansys recommends using Discovery Modeling to create geometries, or its alternative DesignModeler.

Figure 4.66: APDL Snippet to Apply the Beam Properties

The beam is fixed on one end and the first six frequencies are calculated. The table below compares the results of the 3D solid model and equivalent beam model.

Table 4.2: Comparison of the 3D Solid Model and Equivalent Beam Model Results

Frequency	3D Model	Equivalent Beam	Diff. %
1	82.574	82.152	-0.511
2	95.958	95.484	-0.494
3	509.19	504.95	-0.833
4	592.73	589.07	-0.617
5	1384.1	1372.7	-0.824
6	1619.6	1610.5	-0.562

The following figure shows a comparison of the mode shapes for the first and fifth frequencies.

Figure 4.67: Comparison of Mode Shapes for the First and Fifth Frequencies

You may compare the data of the inertia properties for the 3D solid model and equivalent beam model using the reports below.