Minimum Fibreglass Tube Engagement
Minimum Fibreglass Tube Engagement
(OP)
What is the minimum Tube Engagement that is necessary for a connection between a Fibreglass Tube into an aluminium tube that is in cantilever.
Fibreglass tube is 76.5mm OD 6mm Wall
Aluminium Tube is 77.0 ID 6mm wall
Fibreglass tube is 76.5mm OD 6mm Wall
Aluminium Tube is 77.0 ID 6mm wall





RE: Minimum Fibreglass Tube Engagement
The fibreglass is quite long (about 5-6m) out of the aluminium tube in a cantilever situation, and has a load due to wind acting upon it. This generates a bending moment down at the connection into the aluminium tube.
My question is how far into the aluminium tube is the connection required to be before it is of no greater benefit?
RE: Minimum Fibreglass Tube Engagement
Is the contraption vertical, horizontal or somewhere inbetween?
What grade of aluminum?
What type of glass? What is the layup?
Epoxy or polyester?
How is the fiberglass tube joined to the aluminum?
RE: Minimum Fibreglass Tube Engagement
RE: Minimum Fibreglass Tube Engagement
At some point the stress riser ( in the Fibreglass ?)at the edge of the aluminum tube will be the limiting factor. Generally making the edge flexible by thinning the material is a help. Fancy shapes to avoid a "staright across" edge can help.
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RE: Minimum Fibreglass Tube Engagement
As a fictitious example, if L = 5500 mm, FS = 2, and w = 0.0841 N/mm, then b = 678.4 mm.
You'll also need to check the bending stress on the fibreglass tube (sigma = M*c/I) at the aluminum tube mouth, and the bending stress on the aluminum tube at its base or support, to ensure the tubes can support the applied bending moment (with a factor of safety).
You'll also need to check the compressive stress on the fibreglass tube at the aluminum tube mouth (and I don't have an equation for that currently), to see if it governs.
Do you have a wind speed yet?
RE: Minimum Fibreglass Tube Engagement
If so then one of your major issues is local shear strength/crush at the lip of the tube. There are several solutions.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Minimum Fibreglass Tube Engagement
Orientation is vertical.
Target Wind Speed is 240 km/h
Fixed into aluminium 6061 using a silicone adhesive.
Fibreglass tube is pultruded matting in polyester matrix.
Wind Speed: 240 kmh, 66.67 m/s
Wind Pressure: (0.6x(Vel2)x1.2) = 3200.00 Pa
Fibreglass Projected Area: (76.5mm x 5500mm) =0.421 m2
Total Force Due To Wind: (Pressure x Area) 1346.40 N
This gives me a value for W as (1346.4 N / 421000 mm)= 0.00306 n/mm.
When used in "Vonluekes" equation, b = L/{0.5 + [3.12/(FS*w)^0.5]} gives a B = 136.2mm.
Max Bending Moment: (Dist from Centroid to base connection) 3702.60Nm.
Any further sugestions based on this data.
RE: Minimum Fibreglass Tube Engagement
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Minimum Fibreglass Tube Engagement
RE: Minimum Fibreglass Tube Engagement
Aside from that, I've decided my above equation might be inaccurate, especially if FS > 1, because it somewhat uses an assumption of rigid-body mechanics. If I can, I'll try to investigate further.
RE: Minimum Fibreglass Tube Engagement
=(0.5xDensityAir)xVdes2xCfig
=(0.5 x 1.2kg/m3) x 66.66^2 x 1.2
=3199.36Pa
Multiplied by Area Gives a Force(N):
Area 76.5 x 5500mm = 0.421 m2
Area x Pressure = 1346.93 N
Is this not correct?
RE: Minimum Fibreglass Tube Engagement
Without a detailed analysis, my best guess for required engagement length is currently 6*D, though 7*D would of course be better.
RE: Minimum Fibreglass Tube Engagement
RE: Minimum Fibreglass Tube Engagement
1. In the aluminium tube?
2. In the Fibreglass tube?
RE: Minimum Fibreglass Tube Engagement
I investigated further and found that when one tube is eight times more flexible (per unit length) than the other tube (as in your case), the required engagement length appears to be 5*D. Greater than 5*D seems to exhibit little or no change, theoretically. (Note that 6*D might be effective if both tubes are relatively stiff and of comparable stiffness, which isn't the case in your assembly.)
The peak longitudinal shear stress on your fibreglass tube is 1.47 MPa. I think the longitudinal shear strength of the polyester resin, alone, is no less than 8 MPa (maybe higher), so this indicates your fibreglass tube wouldn't split longitudinally. And, your pultruded tube has E-glass matting (fabric), so the weft fibre adds additional longitudinal shear strength, as well as some hoop strength, since there would be significant reinforcement fibre in the hoop direction.
It appears that the contact pressure exerted on your fibreglass tube by the lip of your aluminum tube is concentrated in only a small, 12-mm-wide band, and peaks at roughly 53 MPa on the fibreglass at the top 5 mm of your aluminum tube. (The aluminum tube is below yield.) I'm not sure, but I think slightly over 20 MPa of uniform external pressure exerted over a larger area of the fibreglass tube might cause a local stability problem (?), assuming the fibreglass tube modulus of elasticity is 14 GPa. I don't know if the smallness of the 53 MPa spot will (a) precipitate a stability problem, or (b) be able to sufficiently redistribute causing no problem. If you're concerned, you might consider gluing a 15 mm high, 5 or 6 mm thick piece of aluminum tube inside the fibreglass tube adjacent to the outer aluminum tube top edge, which might prevent the fibreglass tube from being able to collapse inward locally at that contact pressure point.
Regarding your last question, hoop stress is typically calculated as force per unit area on a pipe wall projected to a plane and divided by wall thickness; e.g., p*D/(2*t) for uniform internal or external pressure. But the stress in your tubes is due to bending, not uniform pressure, so the stress changes to mostly shear stress as you go around the tube 90 deg to the neutral axis. In other words, it may not be straightforward (nor meaningful) to calculate a hoop normal stress. The contact stress at the aluminum tube lip is almost a point load (or cosine contact band) exerted on the fibreglass tube, and dissipates as you move away from that point.
RE: Minimum Fibreglass Tube Engagement
Also I noticed, the contact stress on that 53 MPa spot, mentioned earlier, drops to 26.5 MPa by the time it gets to the wall midsurface, just 3 mm below the contact surface. The hoop stress as you go around 90 deg on the fibreglass tube from there is about -22 MPa. The hoop stress on the aluminum tube top edge at the neutral axis is about 55 MPa.