Demystifying Orthotropic Material Definition

The heck with the groundhog and his weather prediction!  The NFL season recently ended in spectacular fashion and spring is right around the corner.  And spring means baseball!  With hardball season fast approaching, now is a good time to clear up the mystery of defining orthotropic material properties in SOLIDWORKS Simulation.  Why?  Well, you might want to solve a few Finite Element Analysis studies with baseball bats and if your material properties aren’t set up properly the results will be invalid.

Most of us are used to working with materials that are isotropic in nature.  Isotropic means the material properties are the same in each direction.  But an orthotropic material is unique in that material properties are dependent upon orientation.  The most common example of an orthotropic material is wood.  Other orthotropic material examples include composite laminates and some heavily processed metals.

For an isotropic material model, you could use hand calculations to calculate elongation (δ=P*L/E*A) and bending (δ=P*L^3/3*E*I).  If you only consider elongation, the hand calculation for an orthotropic material will closely match that of an isotropic material if the elastic modulus along the (orthotropic) longitudinal axis is equal to the Isotropic material elastic modulus.  The bending calculations for an orthotropic material, however, are more complex.  Rather than bore you with additional formulae, let’s look at setting up and understanding the orthotropic material definition in SOLIDWORKS Simulation.

Let’s consider a square profile cantilever beam that is 10 mm wide and 100 mm in length.  I’m making the Front plane and a reference coordinate system visible in the picture.  I will use these two types of reference geometry to demonstrate how an orthotropic material can be defined for analysis.

Defining an orthotropic material in SOLIDWORKS Simulation requires two additional steps.  First, change the material model type from Linear Elastic Isotropic to Linear Elastic Orthotropic.  Second, select a Reference Geometry entity to define material orientation.  While an isotropic material has a single value for Elastic Modulus, an orthotropic material has different values of Elastics Modulus with respect to the X, Y, and Z-axes.  Notice that Poisson’s Ratio and Shear Modulus also depend upon material orientation for an orthotropic material.  In reviewing the defined ‘Front Plane’ reference, how do you know which direction in your SOLIDWORKS CAD model corresponds to X, Y, and Z for the material model?

There is a simple rule to follow when using reference planes to define orthotropic materials.  When looking at the positive-normal side of a reference plane, the X-axis is defined from left to right as you read text.  The Y-axis orientation for is from the bottom to the top of the text.  Finally, the Z-axis of the material is out of the screen, perpendicular to the X- and Y-axes (following the right-hand rule), i.e. the positive normal of the Front Plane.  I will verify that this is true using the SOLIDWORKS Simulation Load Case Manager.

In the first Finite Element Model “Load Case Study – Plane” I chose the Front Plane as the reference.  I applied a fixed geometry at the left end of the cantilever beam and three 100N loads on the right end of the beam – a tensile load, a vertical bending load, and a horizontal bending load.  I also set up three sensors to track the maximum displacement in the X, Y, and Z directions for the study.  The results are shown in the picture below.

To verify that the simple rule for orthotropic reference planes is correct, I created a coordinate system in the SOLIDWORKS model that matches the global coordinate system of the part.  Then I copied the “Plane” study to a “Coordinate System” study, modified the orthotropic material reference, and re-solved the finite element model, again using the Load Case Manager.  The results for that study are below.  The sensors used to track displacement values are an exact match for results between the two studies.

Now that you have a handle on defining orthotropic material orientations in SOLIDWORKS Simulation, you can get to work on your baseball swing.  Even better, maybe now you’ll pay more attention and point the Louisville Slugger logo on your wooden bat towards the sky!  (Yes, there is such a thing as the ‘logo up’ rule.)  Now go make your products better with SOLIDWORKS Simulation!