|
Lester Populaire
|
|
|
Group: Forum Members
Posts: 311,
Visits: 13K
|
While i agree on many things said here, like that balsa can be hard to machine and resin uptake is hard to control in infusion, i would strongly disagree that the mechanical properties of foams are the same or better.
Rohacell 71A has a compressive strength of 1.5 MPa, a compressive modulus of 92 MPa, a shear strength of 1.3 MPa and a shear modulus of 29MPa
Balsa endgrain SBC.100 has a compressive strength of 9.2 MPa, a compressive modulus of 2526 MPa, a shear strength of 2.6 MPa and a shear modulus of 187MPa
On topo of that, Balsa can handle much higher processing and operating temperatures. So there is certainly a place for balsa!
As mentionned before, i would recommend to use a viscous coating resin or a gel-coat with a short pot life, mixed with microballoons to seal the core.
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
I absolutely agree with that (although there are foams wich top that numbers). But this shear strength and compression strength only really comes into account for things that deal with serious forces, like wind turbine blades, or parts for heavy industrial machines. For hobbyist and even for most structural car applications you could consider airex as just as strong as balsa.
The higher possible processing temperatures come in handy for prepreg for shure. But with infusion, the high moisture affinity is a huge drawback. I ended up heating the pieces for several hours, to get the water out. And letting it rest just a couple of days at room temperature brought back most of the humidity.
|
|
|
|
|
Lester Populaire
|
|
|
Group: Forum Members
Posts: 311,
Visits: 13K
|
Serious question - which foam tops that? They don't even get close to those numbers according to my understanding. Rohacell is one of the very best foams out there.
I think for sport applications like kiteboards and such balsa endgrain is way better suited than foams. I never had problems with moisture uptake either, as most epoxy systems are rather forgiving with that.
All of this aside i think a rohacell or airex PVC foam would be way better suited for an UAV in most cases.
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
The fibre reinforced airex versions. Strange, especially for kiteboards I would see foam far in front in terms of performance. Even bow limbs are made with foam core. And this is an application where every use stresses the material close to it's absolute limits.
|
|
|
|
|
John Hansen
|
|
|
Group: Forum Members
Posts: 42,
Visits: 232
|
+xWhile i agree on many things said here, like that balsa can be hard to machine and resin uptake is hard to control in infusion, i would strongly disagree that the mechanical properties of foams are the same or better. Rohacell 71A has a compressive strength of 1.5 MPa, a compressive modulus of 92 MPa, a shear strength of 1.3 MPa and a shear modulus of 29MPa Balsa endgrain SBC.100 has a compressive strength of 9.2 MPa, a compressive modulus of 2526 MPa, a shear strength of 2.6 MPa and a shear modulus of 187MPa On topo of that, Balsa can handle much higher processing and operating temperatures. So there is certainly a place for balsa! As mentionned before, i would recommend to use a viscous coating resin or a gel-coat with a short pot life, mixed with microballoons to seal the core. Thank you Lester. What have you created with End Grain Balsa? Have you seen any links to destructiuve testing of CF with balsa core? The question of need for strength is not the same for every product. While designing my special purpose UAV, I watch a lot of videos. And when a UAV crashes, they are often damaged or destroyed. Damaged or destroyed is not a good thing. So regardless of the weight, strength is paramount.
A Lifelong Learner
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
This is what I am telling you the whole time. When little sandwich composite things crash, they are not getting damaged because the pressure strength or the shear strength of the core material is surpassed. Raising that numbers doesn't make this any better. The key would be to increase the failure free energy absorption propertys of the whole structure. In fact the balsa version would be heavier than the foam version, therefore carrying more momentum when crashing. Your 100g quasi isothropic skin will never hold the forces to cause shear failure within the core. This is even more true for 3 dimensional shapes. Extreme core strength is either needed for high punctual pressure on the surface like bolt trough applications, or high shear forces in beam like structure. But as my example with bow limbs demonstrates, foams can endure even the highest tensions carbon fibre can come up with.
why do you think foams are like they are? The people who invented them did choose the density and strengh compromise the way they did for reasons.
|
|
|
|
|
John Hansen
|
|
|
Group: Forum Members
Posts: 42,
Visits: 232
|
+xWhile i agree on many things said here, like that balsa can be hard to machine and resin uptake is hard to control in infusion, i would strongly disagree that the mechanical properties of foams are the same or better. Rohacell 71A has a compressive strength of 1.5 MPa, a compressive modulus of 92 MPa, a shear strength of 1.3 MPa and a shear modulus of 29MPa Balsa endgrain SBC.100 has a compressive strength of 9.2 MPa, a compressive modulus of 2526 MPa, a shear strength of 2.6 MPa and a shear modulus of 187MPa On topo of that, Balsa can handle much higher processing and operating temperatures. So there is certainly a place for balsa! As mentionned before, i would recommend to use a viscous coating resin or a gel-coat with a short pot life, mixed with microballoons to seal the core. Lester, Thank you for your input. Have you seen any specific formulas (ratios of resin to microballoons) that would seal the end grain and result in the lowest weight? NOTE, it must still provide adequate adhesion properties to result in the proper shear strength. And would there be better results from using phenolic microballoons or glass microballoons? Which would result in the lowest weight gain for the end grain balsa? At this time, I am unable to find this kind of data.
A Lifelong Learner
|
|
|
|
|
John Hansen
|
|
|
Group: Forum Members
Posts: 42,
Visits: 232
|
+xThis is what I am telling you the whole time. When little sandwich composite things crash, they are not getting damaged because the pressure strength or the shear strength of the core material is surpassed. Raising that numbers doesn't make this any better. The key would be to increase the failure free energy absorption propertys of the whole structure. In fact the balsa version would be heavier than the foam version, therefore carrying more momentum when crashing. Your 100g quasi isothropic skin will never hold the forces to cause shear failure within the core. This is even more true for 3 dimensional shapes. Extreme core strength is either needed for high punctual pressure on the surface like bolt trough applications, or high shear forces in beam like structure. But as my example with bow limbs demonstrates, foams can endure even the highest tensions carbon fibre can come up with. why do you think foams are like they are? The people who invented them did choose the density and strengh compromise the way they did for reasons. I understand that the optimum skin/core design would strike a balance between the shear strength of the core and the tensile strength of the skin. I will have to do some in-use destructive testing to actually know if the design will work for the strength and stiffness that my UAV will need. I will very likely build more than one sample with balsa and similar samples with foam materials. My unique design will work best with higher stiffness and less flexibility and this is rather different than many other current UAV designs that make their frames from tubes which hold the motors.
A Lifelong Learner
|
|
|
|
|
oekmont
|
|
|
Group: Forum Members
Posts: 550,
Visits: 27K
|
A stronger core material doesn't result in greater stiffness. For that, you have to increase the stiffness of the skin or tge thickness of the core.
|
|
|
|
|
John Hansen
|
|
|
Group: Forum Members
Posts: 42,
Visits: 232
|
+xThat's the core of the problem. You will not find much data for the resin uptake, because it varies more than 100%. No two pieces are the same. I have had balsa, wich was almost like birch, and I had balsa, wich was like xps foam. I have seen balsa wood with fine structure, and wood with a sponge like structure. The small influence of the core to the bending characteristics results directly out of the principles of mechanics. And the pressure resistance of a sandwich is proportional to the pressure resistance of the pure core material. It's as easy as that. Have you ever had airex or rohacell in your hands? Maybe you picturing it like insulation eps. Real core foams are incredibly solid. You can drive your car across an airex board without damaging it. I am not saying you should not use balsa. I am just saying that it seems to me that you are glorifying for some reason. And your image of it being an top end ultralight option ("as light as possible") and much stronger than foams doesn't correspond with my experience. And resin uptake is a tricky beast itself. other than the used wood, it depends on your resin, the thickness, the surface preparation, infusion aiding preparation and many more. If you want to seal it, use a quick setting epoxy, or a 2k laquer. Oekmont There are many different vendors out there. Some of them specify the density of the end grain balsa and sell a variety of densities. And some vendors will write things like:"density may vary by as much as 100% So if I bought a balsa that was intended to be a specific density, some vendors try very hard to give me what I want, and other vendors do not try at all. The questions I have relate to a product that is widely used in a number of industries for many years. If you have experience in making two composites where one is balsa core, and one is foam core, with the same skins on the same thickness of core, what did they end up with in terms of stiffness and weight? When you bought the balsa, did you order one weight and not receive the weight you ordered? It seems you are saying that the density of balsa is like a roll of the dice at the casino? And the outcome regarding resin uptake cannot be predicted by even the most carefully followed processes?
A Lifelong Learner
|
|
|
|