mandrel extraction force

[carbongr]

What should be approximately the required force that has to be applied to the polished stainless steel mandrel in order to extract it from the manufactured carbon fiber tube? The length of the tube is 2 meters with a wall thickness 1.2mm and the diameter of the mandrel 30mm. The used carbon fiber fabric is a braid 45/45 degrees.
Thanks

 

[Compositepro]

Anywhere from zero to greater than the strength of the tube.
Too many unspecified factors.
type of release
CTE of materials (depends on resin content)
45 degree braid is a Chinese finger puzzle. Its much harder to pull-off than push-off.

 

[carbongr]

Thank you for your reply Compositepro. However, what do you mean by this:
"Its much harder to pull-off than push-off."
?

 

[RPstress]

When you stretch a rod or a tube it decreases slightly in diameter. (If it's not a weird auxetic material.)

A +-45° carbon/polymer layup has a poisson's ratio of about 0.9, and if you pull on the tube and hold the tool the laminate will grip the tool harder if you manage to stretch it a bit.

If you hold the tool and push on the laminate the reverse will tend to happen.

Because before release the tube and tool won't really stretch much it may seem that it's not all that relevant, but practical considerations and part behaviour just make pulling a tube off of a tool much harder, as 'Pro says.

NB: If you have a decent temperature drop from an elevated temperature cure and an aluminium tool with a reasonable diameter the part will probably self-release due to the tool's thermal contraction. However, with steel (CTE ~12 microstrain/K) and a size of 30 mm the contraction will be low if cooling by (say) 70°C, and you'll need some sort of force to part the two.

 

[Compositepro]

I'm waiting for an aluminum mandrel right now because yesterday I could not remove the steel mandrel from a tube! 0.75" mandrel in a 2" O.D. fiberglass/BMI tube (0-90).

 

[Albigger]

'Pro -

With a fiberglass layup the shrink will just about match that of aluminum, depending on cure temp (but I'm sure you know that already). I'm not surprised you couldn't remove the steel mandrel.

(A while ago we built a tool for a customer who was going to manufacture a very large and complex RTM part. They thought something would go wrong on the first shot, so thinking they were smart and would save some cash they made the first part out of fiberglass instead of carbon. They didn't think they were so smart anymore after the 12 mandrels, about 6ft long each, were locked in the part. It took one large freezer and a lot of pulling to get them out.)

 

[Hansmeister]

The CTE of fiberglass is about 6.5 ppm/F. Depending on the steel alloy, it is also about 6.5 to 7.0 ppm/F (mild steel).

There is no surprise here about not getting the mandrel to come out.

 

[carbongr]

I used solid aluminium mandrel, polished, lots of high temp liquid release agent and wrapped it with 4 layers UD carbon fibre and one 2x2 (400gsm), cured at 180 C. Next day, after everything was at room temp, the required force we used in order to extract the mandrel was 7.5tons! Quit allot i believe.
Does this sound normal?