Dissimilar materials, thermal expansion, and cracking
Dissimilar materials, thermal expansion, and cracking
(OP)
Hi everybody,
I am new at this, and I'm not sure where to find documentation on it, so I figured I'd ask the community.
I have 2 materials (epoxy and steel) undergoing thermal cycling. I'm getting cracking between the two. Is there a rule of thumb to determine conditions when cracking won't likely occur? I am considering putting a 3rd piece of material between the steel and the epoxy with a thermal expansion coefficient bigger than steal, but smaller than the epoxy. I'd still have to use a bonding agent between the steel and the new material though, so this gets a little tricky.
Other notes:
If possible, I'd prefer not to blindly trial and error this. Even minimum quantity epoxy costs a fortune.
I might be able to get a different epoxy with different thermal expansion coefficient, but I don't believe it will be drastically different enough to solve my problems.
Reference material may be more useful to me than anything.
Thanks for your help!
I am new at this, and I'm not sure where to find documentation on it, so I figured I'd ask the community.
I have 2 materials (epoxy and steel) undergoing thermal cycling. I'm getting cracking between the two. Is there a rule of thumb to determine conditions when cracking won't likely occur? I am considering putting a 3rd piece of material between the steel and the epoxy with a thermal expansion coefficient bigger than steal, but smaller than the epoxy. I'd still have to use a bonding agent between the steel and the new material though, so this gets a little tricky.
Other notes:
If possible, I'd prefer not to blindly trial and error this. Even minimum quantity epoxy costs a fortune.
I might be able to get a different epoxy with different thermal expansion coefficient, but I don't believe it will be drastically different enough to solve my problems.
Reference material may be more useful to me than anything.
Thanks for your help!





RE: Dissimilar materials, thermal expansion, and cracking
Are you sure the above is not a delamination problem caused by poor surface preparation?
RE: Dissimilar materials, thermal expansion, and cracking
Good question. I did use cleaner on the steel, but I didn't rough the surface or anything. I'll have to ask what prep they recommend for steel/aluminum. Regardless, I know I am going to have cracking in some areas. I need some way to prove this isn't likely to happen in one critical area of the prototype. If I can ensure there won't be cracking there, I may be a made man.
Man this is a really hard problem :(
RE: Dissimilar materials, thermal expansion, and cracking
i guess you know the thermal expansion co-efficients for both the steel and the epoxy, so you know the difference in the expansion. over the width of the bond, the steel is trying to stretch the epoxy. this'll break either the epoxy (in tension) or the bond (in shear).
RE: Dissimilar materials, thermal expansion, and cracking
Have a look at this link:-
http://www.scribd.com/doc/43333468/Thermal-Stress
RE: Dissimilar materials, thermal expansion, and cracking
I've filled a steel cup with epoxy. So I believe both materials experience thermal expansion over the same length. Would that not suggest that they epoxy expanded more because the CTE is larger? Maybe I'm thinking about this incorrectly. I would hope the epoxy datasheet is correct, but who knows.
RE: Dissimilar materials, thermal expansion, and cracking
according to that document, I need to determine the max allowable strain. The manufacturer may have that, or I could probably determine it on my own. As long as I use the same prep techniques, and as long as I ensure that thermal strain + elastic strain is less than the allowable strain, I should be in good shape.
Thanks Desertfox. That makes perfect sense. Thanks for the document. Very helpful!
RE: Dissimilar materials, thermal expansion, and cracking
Is the failure a crack of the material, or a bond failure?
Thermal cycling --> not everything is at the same temperature at the same time --> expansion is not uniform.
Geometry is important, not only material properties.
Stress > strength --> cracks or bond failure
RE: Dissimilar materials, thermal expansion, and cracking
This is a bond failure. I'm doing some R&D on potting of electrical stator cores, so no, the expansion will not be uniform, but full thermal simulation with thermal expansion integration is a bit beyond the project scope at the moment. I may try to break the material later. I'll have to see what the bosses say.
What do you mean by geometry importance? I have my own ideas on the importance of geometry, but I'd like to hear what you have to say. Perhaps I can get a new perspective on my design. My geometry is very complex, but there's really only 1 area of critical bond importance which will make or break this research effort. Perhaps I could make some geometry changes to that region and make my life easier.
RE: Dissimilar materials, thermal expansion, and cracking
RE: Dissimilar materials, thermal expansion, and cracking
is that really what you'd do in your final design ? i'd've thought that most of the bore would be filled with some metal and the epoxy would be only a thin-ish band. but then i don't know much about "mounting electrical stator cores".
is the epoxy intended as an electrically inert isolator ?
i the epoxy also doing a structural job ? (ie are you asking alot of the epoxy ?)
RE: Dissimilar materials, thermal expansion, and cracking
I prefer not to comment on those specifics of what we're doing, but I appreciate your willingness to help.
RE: Dissimilar materials, thermal expansion, and cracking
Regards,
Mike
RE: Dissimilar materials, thermal expansion, and cracking
RE: Dissimilar materials, thermal expansion, and cracking
If I understand your situation correctly, you are getting cracks on the edges of the part where the epoxy is separating from the steel.
It really sounds to me like you have an issue with surface prep.
When I go to bond anything, whether with epoxy, braze alloy or anything else I assume that the surface of the material has a skin of some sort. This can be an oxidized layer in titanium or stainless steel, free carbon or oxides on carbide and always incidental shop oils and greases.
Steel is often treated so that it will not rust. We see this with steel tool bodiess all the time. They are sprayed with some sort of a rust protecting that really screws up any sort of a bonding unless you remove it. I do not like solvents for this application. Instead I greatly prefer something that actually saponifies the protectant such as a strong caustic solution.
You might spray your steel part with oven cleaner and see if that helps with the bonding.
A rough and surface will give you greater surface area and thus increase the bonding strength of the total bond.
When brazing carbide to steel, if the parts are over 1 inch in any dimension, we use a tri-metal brazing sandwich which is braze alloy - copper - braze alloy. The copper in Niels to a dead soft condition in helps compensate for the differences in thermal expansion.
Were I you, I would see if I could find a good 3M rep and ask him to come out.
Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Dissimilar materials, thermal expansion, and cracking
Fairly typical surface prep recommendations
http://www.devcon.com/UserFiles/File/Surface_Prep....
tensile strength vs bond line thickness
http://www.specialchem4adhesives.com/home/editoria...
RE: Dissimilar materials, thermal expansion, and cracking
For oil, grease etc. I like a strong caustic that saponifies rather than a solvent.
Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Dissimilar materials, thermal expansion, and cracking
How thick is your bond line?
Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Dissimilar materials, thermal expansion, and cracking
Tomwalz, what exactly do you mean by bond line? Thickness of the epoxy section or bond area?
Just for an update, I am remaking my samples today. I should have results sometime next week. The plan is to try a roughed bond surface. If that's not fruitful, then I'll look into more drastic changes.