|
speed_demon
|
|
|
Group: Forum Members
Posts: 23,
Visits: 163
|
I would alternate the cloth orientation, How many layer and of what weight was the real question. I never used foam as a core material for a high load/floor material( only ever wood) and didn't know how much flex it would have, needing more glass.
|
|
|
|
|
Chris Scott
|
|
|
Group: Forum Members
Posts: 20,
Visits: 235
|
As Warren pointed out, there is no 'rule of thumb' to use for every scenario. People may have their own Golden Rule that they follow, and it may work for their specific applications. However in this world of composites, things are approached VERY differently than with conventional metallic components. There is only one way to be certain.. that's testing. Metallic components provide very predictable characteristics. Composites for the most part, do not (at least not to the same degree). For instance, me and you can have the same materials, layup schedule and resin.... but depending on our techniques and abilities, we will have two very different results. For this reason, in aerospace we even have a specific federal regulatory requirement that adds another multiplier for factor of safety for composite parts. Designing composite parts from an engineering perspective is a very involved matter. I say all this as an aerospace engineer that develop and engineers aircrafts for a living. My advice to a non-engineer? It's not the best advice, but it's just experience, testing, and getting your hands dirty. There's so much versatility and options, it's impossible for a simple equation to envelope it all. Start building and have fun doing it! If initially your part is too weak or flimsy, you can just add reinforcement! Beautiful part of composites.
|
|
|
|
|
Trace Elliott
|
|
|
Group: Forum Members
Posts: 4,
Visits: 30
|
As an engineer myself, I join Chris Scott's view. To calculate the required thickness of a particular part, you'll need to compute shape, fibers orientation, and loads. You'll need software that is anything but user-friendly, not to mention ridiculously expensive for an individual (thousands if not tens of thousands of euros). And the months it will take you to learn to use the software could be put to use by making prototypes and testing them. My advice: start by matching the thickness (1mm steel => 1mm CF), and try to understand the way the loading works, to best orient the fibers. For a car bonnet, I'd use mutliple payer of bi-axial (0/90 and 45/-45), changing the orientation for every layer i order to have an near-isotropic part (same strength in all directions, like metallic plates). For a part that is under a tensile load, I'd put more fibers along the length than the width of the part (uni-directional layers interspaced with 45/-45, for instance) When you have a point load, try to include an insert, something to spread the load before it reaches the carbon fiber. Keep in mind that the shape of the part gives it part of its rigidity. A 1mm thick plate might be flexible, whereas a complex shape of the same thickness will be much more rigid. And like all of us, scour the net to find people who have made a similar part, and are kind enough to share the detail of their layup (you hear that McLaren? Tell us how you make your chassis!!)
|
|
|
|
|
speed_demon
|
|
|
Group: Forum Members
Posts: 23,
Visits: 163
|
I think people are getting off track... with the shapes and variables (twill/plain, low/high humidity, post cure, etc.) because sheet metal's rigidity is affected by shaping. If "you" had to make/replace a given (your choose) steel hood and make it the same...I.E. shape and hood frame out of 100% CF AND one out of 100%E glass what would be the wt cloth and # of layers you would use. I know with the variables no one will/can give an "exact" # for a 1 to 1 ratio, so lets say a range or minimum( do in a lab by a machine) to a DYI guy in his backyard for comparison sake. For example thickness of fiberglass to make a 2"x 4" box tubing 4ft long that would as strong or stronger (in every load direction) than one made of 1/8"mild steel. There "is" an equivalent # out there that would always give an "atleast as strong" for an equivalent of a different material.
|
|
|
|
|
tomzi1234
|
|
|
Group: Forum Members
Posts: 40,
Visits: 500
|
as many said. starting point is match the thickness. i personally with CF would go 3/4 thickness of steel. especially if you are looking for saving weight and saving material. CF is still 3x stronger as steel. and to be on safe side that 1/4 would be for safety. can not say much of e-glass since i haven't done much with it. but that part i think Hanaldo cowered more than in detail (thanx for explaining)  the rest is trial and error
|
|
|
|
|
ChrisR
|
|
|
Group: Forum Members
Posts: 363,
Visits: 3K
|
" If "you" had to make/replace a given (your choose) steel hood and make it the same..."My choice? for street us or race, CF exposed or painted? Race - maybe 1 layer of 200gsm each side of a foam / cork core - it's not structural , its a cover, just need the stiffness to prevent deflection - if it was for street use I'd use quite a bit more to minimise the chance of penetrations/shattering from a pedestrian impact "...I.E. shape and hood frame .... to make a 2"x 4" box tubing 4ft long that would as strong or stronger (in every load direction) than one made of 1/8"mild steel..."
These two terms (in bold) make me think you are still thinking in terms of a homogeneous isotropic material. You create the hood shape AND frame as ONE if designing to minimise weight.
If you are designing a box, you need to know how the load is being transferred in to/out of and along the box. This is how you orientate the fibres.
|
|
|
|
|
speed_demon
|
|
|
Group: Forum Members
Posts: 23,
Visits: 163
|
ChrisR (22/09/2015)
" If "you" had to make/replace a given (your choose) steel hood and make it the same..."My choice? for street us or race, CF exposed or painted? Race - maybe 1 layer of 200gsm each side of a foam / cork core - it's not structural , its a cover, just need the stiffness to prevent deflection - if it was for street use I'd use quite a bit more to minimise the chance of penetrations/shattering from a pedestrian impact "...I.E. shape and hood frame .... to make a 2"x 4" box tubing 4ft long that would as strong or stronger (in every load direction) than one made of 1/8"mild steel..."
These two terms (in bold) make me think you are still thinking in terms of a homogeneous isotropic material. You create the hood shape AND frame as ONE if designing to minimise weight.
If you are designing a box, you need to know how the load is being transferred in to/out of and along the box. This is how you orientate the fibres. Thanks for the reply, I wouldn't be worried about pedestrian impact, LOL. I was concerned about the load from a layer of heavy/wet snow causing it to fail. What weight/layers would you use if you made it out of 2 x2 twill say 340 g/m E-glass as I would want to start with a cheaper material first? I put in that thing about 2x4 4ft long tubing think I would get a "high-side" conversion number vs. a "low-side" number for Laboratory/auto-clave/pre-preg CF, so I could get a general range/ guesstimate.
|
|
|
|
|
Chris Scott
|
|
|
Group: Forum Members
Posts: 20,
Visits: 235
|
The reason why this is an impossible comparison is that even within the 'carbon fiber' family, there is no conversion. Take a simple three point flexural test... for every different possible way you can manufacture a CFRP beam, you will get different results. Also, while you might find that a beam made of uni-directional construction with 0 angle (longitudinal) might produce impressive results on a three point flexural test, but when you start adding in torsional forces(torque) it won't perform as well. Simply by changing the angle of the tows you will develop completely different characteristics. Most metals are incredibly isotropic materials, so you gain strength in all directions automatically. With CFRP you have to design your own properties. Think of CFRP as offering infinite customization! I could probably just type for pages on this subject, but instead I'll offer some tips and considerations to make when designing your parts and be helpful! :] - Vacuum bagging/infusion will significantly remove/reduce voids in your product. Be mindful of resin content % as well. Voids/dry spots will significantly reduce your stiffness while excess resin will brittle your part. Also using vacuum will help in producing consistent product. - Composites gain their strength with geometry. One of the ways to improve geometric strength is by adding a lightweight core. Also stay away from sharp corners. - Orientate your fabric weaves appropriately. - Remember you can add partial layers to areas of concern, - Not all tows and not all fabrics are made equal...also not all resin are equal too. I've spent my fair share of time and money wasting it on cheap stuff and trying to cut corners, not worth it. - Anything exposed to the sun, recommend protecting it with a good automotive clear or a specific gel coat that will help protect your product from UV rays which will yellow and embrittle the resin. This is not a concern for steel components. I hope this helps. Use test pieces to practice any new processes, or if you are unsure of the result. Worst case scenario, you manufacture a part that does not have the required stiffness/strength you desire, you can easily just scuff up the back side of the part and continue to lay more materials until you are satisfied. Also allow your parts to post cure, you'll find that resins will reach full strength over time.
|
|
|
|
|
speed_demon
|
|
|
Group: Forum Members
Posts: 23,
Visits: 163
|
I've found a comparison in a new book I'm reading that showed composites that were made "AS STIFF" as a steel panel with their thickness and weight ratio versus the steel. The graph used a factor of 1 for the steel. I thought is was interesting since I hardly find any info in print on the subject. I will add the other material/ laminate values for those interested (food for thought). relative wt. relative thickness material 1.00 1.00 steel .65 1.22 titanium .55 2.29 E-glass/epoxy .48 1.44 aluminum .36 1.66 magnesium .31 1.88 Aramid/epoxy .21 2.97 5-ply Douglas fir .19 3.77 aramid/foam core panel
|
|
|
|