0x00D Part Name Doesn't Control Load Path
Physics doesn't care about engineering intent
Naming a part “non-load-bearing cover” does not magically give it an exemption from structural loading. To ignore a part during structural analysis, the part must have substantially less stiffness compared to all its neighbors, or be disconnected in a way that prevents load transfer across it.
What's all the fuss about?
If ignored during analysis, the “cover” or its attachments may be too weak and fail under use. I've seen this happen - a “cover” failed, and suddenly robot operators were having problems with the limb having low stiffness.
You're spending too much mass somewhere; on an overbuilt “cover”, and/or on a core structure that's overbuilt because it's not utilizing the “cover” to it's full advantage.
A Beam Bending Example
A hollowed out plate forms a part with a lower face, three ribs, and potentially a top face. An applied load bends the beam upward.
Without the top face the neutral axis is close to the bottom face of the part. When bent upwards, the top face of the part experiences significant compression. No opinion, naming convention, or falsely “conservative analysis” will save the top face "cover" from needing to survive that compressive (in this example) effect. There are three ways to proceed:
Make the top face (and its attachments) stiff and strong enough to resist this compression; contributing to the overall structure and changing the neutral axis. In most cases this is the best option.
Make the top face have a very low effective stiffness that can endure the compressive strain. This can commonly be seen as flexible plastic covers on mass-insensitive equipment (lawn mowers, snow blowers, etc.).
Isolate a high stiffness top face from the compressive strain. Pull from the bag of tricks used to handle materials with different CTE (0xTBD Coefficient of Thermal Expansion) in extreme temperatures, or woodworking techniques used to handle wood movement.
Quantify Using the Area Moment of Inertia
Substantial changes in moment of inertia are a clear indicator that the “cover” is a significant participant in the structure of the assembly and must be analyzed accordingly.
For this example, set the overall width to 200mm, overall thickness to 30mm, lower face thickness to 3mm and the width of each rib also to 3mm. Varying the thickness of the top face confirms this structure must be analyzed with the top face “cover” installed.
With no cover, the total moment of inertia is 6.3e-8 m^4.
With just a 0.5mm cover, the moment of inertia doubles to 12.0e-8 m^4.
With a 1.0mm cover, the moment of inertia has almost tripled; 16.7e-8 m^4.