New to FEA, designing a product. Are my results valid? [Solidworks]
New to FEA, designing a product. Are my results valid? [Solidworks]
(OP)
I am designing a plastic skateboard part in Solidworks and could use some help determining whether my testing is valid.
Link to photo album: http://imgur.com/a/1FAY8
I am trying to determine whether the board is thick enough. My rated load is 300 lbs since people will be jumping around on them though there not too many overweight skateboarders. The material properties were inputted for flexural modulus and strength (rather than tensile) as I saw those values are better to test plastics loaded in bending.
The test was set up with the load placed on a face that is around the middle. The boards and its trucks are bonded using a contact set since adding bold contacts made the program go haywire with large deformations and each failed test took an hour. The fixtures are on the inside edge of each of the trucks that way they could bow out as if they had wheels.
My questions/concerns are:
1) Did I set this up correctly? Should I make some makeshift wheels? If so does anyone have any good resources/tutorials on how to set up rolling supports?
2) I have a minimum FOS of 0.86 though this is right near the trucks where it is supported. Should I be concerned with this? The FOS in the middle of the board is around 1.5 which is great.
With these kind of boards it is desirable to have some flex to it in order to carve and corner so I would not want to make this too rigid. I am looking for a sweet spot but these stress concentrations are throwing me off.
Thank you for your help!
Link to photo album: http://imgur.com/a/1FAY8
I am trying to determine whether the board is thick enough. My rated load is 300 lbs since people will be jumping around on them though there not too many overweight skateboarders. The material properties were inputted for flexural modulus and strength (rather than tensile) as I saw those values are better to test plastics loaded in bending.
The test was set up with the load placed on a face that is around the middle. The boards and its trucks are bonded using a contact set since adding bold contacts made the program go haywire with large deformations and each failed test took an hour. The fixtures are on the inside edge of each of the trucks that way they could bow out as if they had wheels.
My questions/concerns are:
1) Did I set this up correctly? Should I make some makeshift wheels? If so does anyone have any good resources/tutorials on how to set up rolling supports?
2) I have a minimum FOS of 0.86 though this is right near the trucks where it is supported. Should I be concerned with this? The FOS in the middle of the board is around 1.5 which is great.
With these kind of boards it is desirable to have some flex to it in order to carve and corner so I would not want to make this too rigid. I am looking for a sweet spot but these stress concentrations are throwing me off.
Thank you for your help!





RE: New to FEA, designing a product. Are my results valid? [Solidworks]
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
The minumum FoS is far from acceptable though should I be concerned if it is only such a small area? Is it relatable to when using bolts there is a stress concentration in the surrounding hole?
I would really appreciate some insight on this.
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
Another is to ignore the little red bits if they are small.
Neither is totally satisfactory.
FoS is better termed factor of ignorance, in your case you really don't know the loads. But for many applications a FoS of 10 would be sensible for a man rated device.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
You've loaded the part in the center, and while that certainly results in an extreme loading, I don't think I've seen many people on a skate board on one foot, or with booth feet planted in the middle. Seems like the feet should be right over the trucks. You should be looking at multiple load cases. Go to a park and see how kids are actually using them.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
You're right. Doing this test is somewhat moot since the loading is dynamic and will be centered on the trucks though this is the best option I have. With my limited resources I can't make a real protoype.
I am using a natural fiber reinforced plastic for plastic injection which has some great properties. It behaves like a glass fiber reinforced material. My design has centered a lot around what others have done. Knowing their material (ABS) and their dimensions I think my design should work. As it stands now the board is 0.50" thick. The izod/charpy test values are higher than what's currently being used so to my understanding notch sensitivity and impact should be alright. Only concern with the material would be the elongation before break is 3%.
For reference I have attached part of my material data sheet.
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
1/2" thick, really? Is anybody out there injection molding a solid half inch skateboard? Have you talked to your molder about this? A half inch wall will take forever to cool, so your cycle time, which is mostly cooling, will be huge. Plus, that's what, 4 lbs of material? This will be an expensive part. And good luck controlling shrinkage and warp.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
You've got me thinking a lot...and worried. From your expertise do you think my product will work anywhere near what I am simulating?
I put some wheels and bolts on it with the same material properties as before and was getting FoS of 1.2 in tiny specs around the bolts. Though what you are saying about the fiber orientation has got me worried. Should I be running this with just standard ABS properties? The molding gates will likely run coming from the bottom of the board which screws with the fiber orientation, making it parallel to the force.
There are a lot of boards out there that are even thicker than mine. At my thickest point I am 0.5". My skateboard that I own is around 0.7" all around. This just requires to be done before on large machines. Each board is about 1.6 pounds and quotes I have gotten are a few dollars per part which is fine, I am still projecting great margins, though its not pennies on the dollar like other parts. The tolerances on my part are not a huge issue so unless the warping is too much I am hoping that it is alright.
I appreciate the help.
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
I think why you're saying it's wrong is probably because I am loading it by placing the load in the middle? Normally riders would have their feet over the trucks which would make compressive forces but I don't think that failure would be an issue for that type of circumstance. I'm trying to test some extreme loading conditions. You never know how people are going to use these things.
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
W = dens * V = dens * W * L * t = spgr(abs) * dens(h20) * W * L * t = 1.05 * .03611 lbf/in3 * 8" * 32" * .7" = 6.79 lbs.
That assumes density of unfilled ABS. If your board is only 1.6 lbs then it is either highly filled with a low density filler or its foamed. Take the trucks off and throw it in water. I'll bet it floats (maybe even with the trucks on!)
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
My board should be lighter than my competitors and have ecological benefits. I thought that I would have better strength but I am concerned about the effect fiber orientation now. If it's "highly filled" would that result in more random disbursement or even more anisotropic?
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: New to FEA, designing a product. Are my results valid? [Solidworks]
https://i.ytimg.com/vi/XWBmuqNzBCE/maxresdefault.j...
This is the "Bantam" by Globe I was referring to earlier where its max thickness is about 0.7". Similarly you can find boards by "Penny" which are even smaller though they have great load taking capability. There are videos of cars rolling over the Penny board without it breaking, but they use their own special compound material.
My board dimensions are 24" x 6.5" x (0.5 max, the geometry has curvature so you could say about) 0.4" x 0.0368 = 2.29lb compared to solidworks estimate of 1.64lb which includes all of the cuts for texture on top and other geometry. I'm quite sure this weight calc is right. The model should be good, it's the FEA that I'm new to and trying to learn.