Simulating stress in timber

[christopher.leung]

Hi All,

I am working on simulating the model of a simple tenon joint between sections of timber, I have been trying to use the physical materials properties in Fusion 360's built-in materials library. Timber (all species I can find in the library) appear red-lined as a "Nonlinear material" in the properties window, hence I have been working through the "Nonlinear static stress" study workflow. I have checked the list of properties, and those for a timber (e.g. Wood (Oak)) appear to have all the values that other material (e.g. Steel, mild) have. However, after specifying a load case and constraints, when trying solve I get "Cannot Solve" with the attached message.

Am I missing a value(s) of a property in the materials library? I cannot see any empty fields in the library. If there is something missing I have datasheets on the timber species I would like to try simulating. Or, is this an intrinsic issue with trying to simulate stress with non-linear materials? (Fusion 360, Student version 2.0.8176, Windows 10)

[John_Holtz · Accepted solution]

Hi @christopher.leung 

I think this is the reason for the message (from the Help):

Material Types that are NOT supported for simulation studies:

• Orthotropic materials: Examples of orthotropic materials are wood, reinforced plastic, or similar materials with physical properties that differ in each direction. The properties are individually defined in each of three coordinate axis directions. This material model is selected from the Physical tab of the Material Browser dialog. Choose the Orthotropic option from the Behavior drop-down list. There are also several grades of wood, which are based on orthotropic properties, predefined in the Fusion 360 Material Library.

  Note: Although you can choose predefined orthotropic materials or define materials with orthotropic properties, they are not currently supported for simulation studies within Fusion 360.

Wood is an orthotropic material: it has different material properties in different directions. The solution would be to enter your own customer material properties using an isotropic material. Naturally, this requires you to decide what properties (the modulus in the direction of the grain, perpendicular to the grain, or some combination) to use for the single modulus of elasticity that you enter for the isotropic material.

[christopher.leung · Accepted solution]

Hi @John_Holtz 

Many thanks for your response, the Help reference is clear. I will look to see if the representation of this specific joint (and other similar cases we are looking at) modelled as timber can be usefully approximated by simplifying its representation as isotropic. Thanks again.

[Anonymous]

Hi @John_Holtz 

That makes a lot of sense in terms of what the solver can handle. But I think that it would be better if the material was automatically converted to pseudo-isotropic for simulation with a warning telling you that this was done. The default could be the minimum parallel-to-fibre directions for example. 

Or there should be a possibility to tell the material to be pseudo-isotropic in the material properties settings I think, rather than have to create a whole new isotropic material and put in values from scratch. 

[lRR14]

I don't have a orthotropic option at all in the behavior tab.

[henderh]

Hi @lRR14,

I see the Behavior selector in the Advanced Properties of the Physical tab, but not a behavior tab. Could you provide a screenshot if it's something different? 

I believe at one time (~2020 during the previous discussion above) there was once the Orthotropic behavior type displayed in the Material Browser dialog when authoring a new material. I want to say there were several behavior types (I probably have the terminology wrong) and for the Wood category they weren't classified as isotropic or orthotropic, IIRC it was transverse isotropic (i.e. they had independent Axis1 and Axis2 properties to represent with and against the grain).

Since Fusion Simulation does not currently support orthotropic or transverse isotropic materials, you'll need to create a custom material that uses the material properties from the weakest direction if you're interested in simulating a worst case scenario load case.

Hope this helps!