Phenolic inserts

[MikeWaggoner at UW]

Why do people use phenolic instead of aluminum when making inserts in tubs? I know it has a lower density, but it seems to have a lower modulus and higher CTE; both of those things make me favor aluminum. I'd also think phenolic being a brittle thermoset plastic would lead to undesirable failure modes and problems with overloading (aluminum overloading leads to bending but with remaining strength, while the phenolic would crack). Anyone out there have a justification?

-Mike Waggoner
UW

[MikeWaggoner at UW]

Why do people use phenolic instead of aluminum when making inserts in tubs? I know it has a lower density, but it seems to have a lower modulus and higher CTE; both of those things make me favor aluminum. I'd also think phenolic being a brittle thermoset plastic would lead to undesirable failure modes and problems with overloading (aluminum overloading leads to bending but with remaining strength, while the phenolic would crack). Anyone out there have a justification?

-Mike Waggoner
UW

[Kevin Hayward]

Mike,

Your problem is quite an easy, and inexpensive one to test. Well easy if you have the equipment to heavily load test pieces.

Kev

Time waster
UWA Motorsport

[Big Bird]

I'll start with a disclaimer - I'm no expert in composites.

I'd imagine that phenolic is used because it is (or can be) more compatible with the resin matrix. Maybe two polymers bonding is better than a polymer to a metal.

The strength of the insert is limited by the strength of the insert/matrix joint. From memory, the modulus of composite resins is around the 2-10MPA range, so little point having inserts of much higher modulus if the glue holding them in isn't up to it. The strength of the composite itself is higher due to the fibres, but unless you can invent a way to attach the fibres directly to the insert they don't really contribute to the joint.

Also, I'd guess that CTE of phenolic is closer to the CTE of the composite than that of aluminium.

A bit of guesswork here, and it has been a while since I really thought about this stuff so maybe I'll have changed my mind by tomorrow!

Cheers,

Geoff Pearson
RMIT FSAE 2003

Design it. Build it. Write it off two weeks before the event.

[danW]

This may not be the only reason but here is on possibility...

Depending on the resin, there may be a galvanic corrosion issue with aluminum.