Pultruded versus wound mandrel carbon tube
Quote:
Originally Posted by
Chuck C
I have to ask about your carbon tubes. I see words to the effect they are made on a mandrel. Does that also mean they are wound with some marvelous fiber. If you check the specifics for arrows you’ll see there are two ways to go carbon: the old cheap pultruded or high tech built up on a mandrel. I use the pultruded, they are significantly tougher. On my second boat I went hi-tech. I still use the boat but only after repairing/beefing up numerous times. There was a period there when it seemed every time I used it, some tube, some place, broke. They were easy to repair but a real pain. The hi-tech were a little lighter but larger diameter and overall a disaster. I have never had a problem with the pultruded. They are stronger, tougher and better performing for my use.
Hi Chuck, interesting to read your comments. My carbon tubes were made by C-Tech in Auckland, New Zealand. They are mandrel wound pre-preg carbon with a Young's modulus of about 95GPa. I have been entirely happy with their quality.
My understanding of the mandrel wound manufacturing process is as follows: They wrap cellophane around a steel rod mandrell. They then wrap unidirectional pre-preg carbon tape around the mandrell using a special machine. The amount of ovelap as they wrap the tape in a spiral along the length of the mandrel controls the wall thickness. Next they wrap more cellophane shrinkwrap to compress the layup, then they bake it at quite high temperatures to cure it.
Information I have found on the web has been that the mandrel wound tube is superior - see Small Carbon Fiber Tubes (at the bottom of a long page), so it is interesting that you have found the pultruded tube superior.
It may be that what you have found applies only to the small sizes of tube designed for arrows that you used for your trusses. My understanding is most larger tube is done mandrel wound and this is superior at larger diameters as they can build in some hoop strength so it is less vulnerable to splitting at the joins.
Pultruding results in all fibres running purely along the length so bigger sizes with thin walls can split quite easily. This wouldn't be a problem at smaller diameters as the wall thickness is a much bigger percentage of overall diameter, so hoop strength as a percentage of longitudinal strength will be plenty high enough for the small size pultruded tube.
The people who did my tubing do a lot of high tech boat masts, which are all made by this mandrel wound process. It's a reasonably hands on process, I was able to talk to them and discuss my application, and specify the OD of the ferrule tube to better than 0.1mm so it made a good slide fit in the joins between main tubes.
Quote:
Originally Posted by
Chuck C
Your question about shock loading now makes more sense to me.
My question about shock loading was more around the properties of the trusses. I guess I was concerned that the carbon tube although very strong is hard to join reliably at the corners of the trusses.
Bicycle frame manufactures have wrestled with this for a long time and I think it has slowed/limited the use of carbon in bike frames when on paper it is far superior to aluminum in strength to weight. (Shock loading on bicycle frames being very high compared to static loads)
I'd be curious as to the failure mode for the tube that was breaking on you - was it splitting on you or breaking/buckling at the joins?
I've just had another thought regarding your experience of pultruded versus wound tube in your trusses, so perhaps cancel most of the above:
Pultruded is presumably a greater wall thickness relative to overall diameter. This is probably ideal for trusses where the main thing that determines overall stiffness is the overall dimension of the truss. The greater wall thickness/thin tube will take the stresses that concentrate in the truss corners much better, whereas a larger diameter thin wall tube will fail due to buckling forces in the relatively thinner tube at these corners.
In situations where the main structural element is the tube itself, then thin wall larger diameter will be much stiffer, and the wound mandrel process will be superior as the unidirectional carbon fibre can be angled to provide enough hoop strength to prevent splitting at the corners. The C-tech people were able to tell me that hoop strength of the layup was 30% of the longitudinal strength, and that with overlaps at ferrule joins of 3 times wall diameter, the joins wouldn't be significantly weakening the tube lengths.
I think that in the end, we probably both got lucky and settled on the correct product for our different applications!
Perhaps the message from all this for others looking at adapting some product to a new application is to discuss your application with the manufacturer/supplier and or to get samples before proceeding. Being able to discuss technical details of their product and it's suitability for my application with C-Tech was a big confidence booster when it came to shelling out $1000 on something untried.
Maybe I've caught the bug on this as I'm going to order the first carbon mast and yard for a Goat Island Skiff from those guys...
Cheers, Ian
Sounds like fun... needz photos :-)
I've tried to make sponsons Tom's way, by hand. My first results were 'interesting'. I kept getting creases at the join which needed to be worked with heat gun and roller to get rid of potential (and realised) leaks.
I would recommend that you either use a pvc pipe or stringers to stretch out the mating edges (attached at multiple points) to assist bringing them together straight. I use two long flat stringers and tape them along the back of the join areas. Leave room for bulldog clips on the edge when the glue is drying. Paint the glue along the join and carefully position the edges together by means of the stringers. Clamp along the stringers and place bulldog clip along the edges. Wait for it to dry, then work along the edge with a heat gun and roller (nice solid flat surface underneath, of course), rolling from edge to inside, removing bulldog clips as you go. When you've gotten to the end, you can go back with the heatgun and roller, rolling along the edge.
If you use a PVC pipe you can fold over the join area, instead of matching along the longest edge. Just remember to protect the PVC pipe from the PVC glue. Remove the pipe before following up with the heat gun.
Use a pvc material for the sponsons without any extra 'solar protection' coating - or sand it down, judisciously. The sponsons require only the lightest weight PVC material. You can find
As per the australian management saying, "if at first you don't succeed, hide all evidence to the fact that you ever tried" :wink: Therefore practice making sponsons first, then you should be ready for the sponson holders and shell.
Of course, as this is a woodworking forum, your coaming will have once grown with the sun and rain :2tsup: