Designing with Titanium 2

[Zeke04]

I am presently in the process of designing a pressure vessel for sonar equipment to be used in depths up to 6km. After extensive manual calculations and FEA analysis we fabricated parts from the Ti-6Al-4V titanium alloy and experienced several catastrophic failures in testing, which seem to indicate that this titanium alloy reacts as a brittle material in regions of stress concentration. Has anyone had experience with this alloy?

 

[rb1957]

hydrostatic pressure, causing compression in the bulk structure. discontintuities causing local tension fields.

i haven't heard of 6Al4V being brittle, its a fairly commonly used alloy. check properties in Mil Hdbk 5 (or AR MMPDS).
"For maximum toughness, Ti-6Al-4V should be used in the annealed or duplex-annealed conditions" ... in round numbers annealed ftu = 120ksi, heat treated ftu = 160 ksi.
"Its toughness in the annealed condition is adequate at temperatures down to -320F."
"Under certain conditions, titanium, when in contact with cadmium, silver, mercury, or certain of their compounds, may become embrittled.
Refer to MIL-S-5002 and MIL-STD-1568 for restrictions concerning applications with titanium in contact with these metals or their compounds"

is it practical to use interference bushes ?



Quando Omni Flunkus Moritati

 

[Zeke04]

The annealed condition was used in this application, where the heat treated probably would have been a better choice. I do have the MMPDS volume on Titanium and am aware of the properties. In areas of stress concentration this alloy tends to strain harden and react as a brittle material.

 

[rb1957]

i'd've thought the annealled would have been better with your problem.

have you looked (with an SEM) at the fracture surface ? maybe a pre-existing defect initiated the failure ?
how cold does it get (6 km down) ?

have you done an NL FEA ? i wonder how the material can strain in the localised tension fiedl when the bulk structure probably has a pretty damn big compression stress. maybe it's this constraint that makes it look (or behave) brittle ?

i imagine we're dealling with a fairly thick section ?

Quando Omni Flunkus Moritati

 

[Zeke04]

These tests are performed in a test tank, so that low temperature is not an issue. The part that failed most recently was an end cap which is a stepped cylindrical part with an outside flange the OD of the pressure vessel and an stepped diameter that allows for O-ring glands for sealing at the pressure vessel ID. The flange of the end cap is being sheared and the body of the end cap pushed into the tubular pressure vessel. This design is supported by linear static FEA, we do not have nonlinear FEA capabilities. The flange thickness is becoming thicker as aresult.

 

[CoryPad]

Annealed Ti-6-4 should not be brittle. Perhaps you have excessive alpha case, that can produce brittleness.

 

[IRstuff]

"Stepped" sounds like a risky proposition for deep sea. Round has traditionally been the most likely to survive. The bottoms of Coke cans are lessons in designing for high pressure with minimum of material.

TTFN
faq731-376

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