Effects of Thermal Expansion on Preload of on Composite Cylinders 1

[brady1]

I have graphite cylinder fitted into an aluminum cylinder. The aluminum cylinder is first placed over the graphite cylinder with a 0.05-0.1mm gap, heated uniformly to 400 degrees and stretched longitudinally . Upon cooling the Aluminum cylinder shrinks plastically resulting in a 10 MPA contact pressure at the contacting interface. The dimensions are listed below:

Aluminum Cylinder/Jacket

OD: 60mm
ID: 57mm
CTE: 24.5E-9
E: 71 MPA

Graphite Core

OD: 57mm
ID: 11mm
CTE: 8.9E-6
E 14.5 MPA

How do I determine the effects on the pre-load for a 166 deg uniform temperature change. I reasoned that only some preload will be lost given an initial 10 MPA pre-stress.Note that aluminum expands about 3 times faster than graphite.

 

[unclesyd]

Doesn't the thermal expansion stay the same regardless if the Aluminum has been yielded. The OP posted a stress imposed by his fit as it now stands stands. To get this pressure there has to be some interference at the interface. Since he states that is temperature is the same for both the Carbon and Aluminum at some point he Aluminum will created a gap.
My line of thought was one could get the interference from his stated pressure just for information. By setting up and using the Thermal part he could tell when his interference is approaching 0 or loss of interference

 

[desertfox]

Hi unclesyd

The thermal expansion will stay the same I agree, however when the aluminium is above its yield stress while hot and then cools, will the interference be any greater than my earlier calculations due strains without increase of stress as the material as yielded.
Put a better way during construction of the composite tube the aluminium cylinder is completely yielded before it cools and shrinks onto the solid core, ie it under goes plastic deformation, if you look at the link given by hydtools you will see that the aluminium tube bore is actually larger than the solid core before the composite tube is made.
So that being the case how much more interference is there due to that plastic deformation? I have had a couple of goes even trying to establish strains while the aluminium is hot but I am not 100% sure if I am right.

regards

desertfox

 

[brady1]

From the process, it seems like the material was process annealed. This means that it was cold worked and heat treat to a temperature above it recrystallization temperature. The purpose of annealing the Aluminum while stretching negates strain hardening.

Also, if you read a little about hot working you will learn that hot working in that manner is plastic deformation without strain hardening. Also, notice that they said the hoop stress is equal to the yield strength. If the hoop stress is just equal to the yield, one can assume the new worked aluminum is not plastically deformed in the sense that it will not return to its original length.

 

[desertfox]

Hi brady1

On the bottom of page 20 though of hydtools link, it says that "At the stage of cooling the aluminum pipe shrinks plastically" and goes on to say that the permanent elongation is about 3 times larger than it would be at the yield stress for the material.
Also when a material reaches yield it can under go permanent strain without increase in stress and this is prior to any increase in material strength due to work hardening.

desertfox