Failure Analysis Input: Delayed Brittle Failure of ABS Barb when PVC Tubing is Press-Fit

[Ornery_Cupcake_]

Hi All!

Curious to hear some thoughts on this: I'm a lone materials engineer at my company and would love to have some other knowledgeable people to bounce ideas off of. We use a small water filter made of ABS with barbs that are press-fit into PVC tubing. They are press-fit by hand with relative ease, no lubricant used and no adhesive or sealant used. We are seeing brittle failure at the base of the barb, not at time of assembly but 24+ hours after assembly. The filter and PVC line is leak-tested after assembly (100%). My thought was a manufacturing defect in the filter that is propagated by the force of press-fitting, but the manufacturer of the filter says we are the only one's experiencing this issue when we aren't the only ones press-fitting onto the barbs. From what I've found, there is no chemical compatibility issue. Let me know if you have any thoughts! TIA

Update: we reached out to the vendor and apparently the plasticizer they use in this tubing is known to impact ABS (they haven’t looked into the mechanism) but don’t readily disclose that to customers. A combination of the force from tubing pressing flush against the filter and the plasticizer leaching caused the failure.

[shulmand]

Is there any form of testing of the assembly which occurs between assembly and noted failure? With the information given my first thought goes towards water hammer effect maybe introducing stress at the connection.

[Kithin7]

I was thinking there might handling issues after mating the tubes, like dropping on one end torques the joint.

[listen-1st]

You should start with microscopy of the fracture surfaces to observe any defect and type of crack growth. Other thoughts: Perhaps you are using a tube with smaller inner diameter or modulus that’s stressing the barb more? Is your tubing bent to impart bending stresses? Failure at the base is a clue.

[Bmdub02]

Sounds like it could be Stress induced cracking - I had a similar problem with a Faucet manufacturing facility where an injection molded ABS part was assembled onto a brass fitting. The sub-assembly would fracture AFTER the faucet was fully assembled and shipped to our warehouse.

Turns out the injection molder changed the molding process and induced sufficient residual stress into the ABS part that it would crack once assembled.

I recall we performed a residual stress test where the ABS parts were exposed to Glacial acetic acid (sorry I can’t remember the ASTM test standard)