Titanium Pressure Vessel

[Zeke04]

In the course of designing and testing a PV fabricated from Ti-6Al-4V Titanium, we have experienced several catastrophic failures which seem to indicate that the material is reacting to stress concentration as a brittle material. Has anyone experience with this alloy?

 

[MJCronin]

Where is the source for your material ?

 

[MJCronin]

It is my understanding that Ti-6Al-4V Titanium is equivalent to "Grade 5" titanium (UNS R56400). ..... an aerospace grade.

Most Ti pressure vessels produced today use Grade 2 or Grade 7.

Grade 5 is susceptable to SCC

http://geoheat.oit.edu/bulletin/bull24-4/art6.pdf

Why did you select that particular grade for your application ?

 

[Zeke04]

I am working here as a contractor and the material was picked by the engineer in charge of the program, apparently they have used this material in the past, but didn't try to design to a low FOS as was the attempt here was to reduce size and weight. I now believe that this alloy strain hardened during testing and ultimately reacted as a brittle material, there have been studies that report this.

 

[metengr]

For a pressure vessel you should be using ASME B&PV Code, Section VIII accepted material.

 

[stanweld]

Ti-6Al-4V pressure vessels are common to the aerospace industry. Usually they are designed to a high percentage of their yield strength with prototype pressure tested to failure. You may need to heat treat after cold forming or welding. You do not indicate fracture location/initiaton site, welding process or the service conditions.

 

[Zeke04]

This is a cylindrical PV with a 4.98" OD and a 4.375" ID and separate end caps attached. The end caps are cylindrical with a flat external face and a stepped diameter to mate with the ID of the PV. One of the end caps is shearing at the junction of the flange, see attachment. The part is shearing at the inside fillet.

 

[SnTMan]

Is the flange taking all the trust load?

 

[Zeke04]

The outside of the larger diameter is exposed to the pressure at 6km (8800psi).

 

[SnTMan]

Sorry, "thrust", so that is, yes?

 

[Zeke04]

Yes

 

[SnTMan]

Well aside from all the if this, if that, you probably don't need me to tell you your flange needs to be thicker

Regards,

Mike

 

[Zeke04]

Currently in the process of at least doubling the thickness.

 

[metengr]

Regardless of how common this grade is, I would at least take a step back and visit the issue of using Ti-6Al-4V-ELI versus common grade Ti-6Al-4V. The ELI will have improved toughness and forming versus the common grade.