Testing: Fatigue of Titanium Tubing

July 12, 2012

in Engineering, Present - Meaning Today

We’ve running yet another fatigue test series on titanium.  This one will be the longest and most diverse so far – at least in the sense of running everything as an ongoing test.  We test titanium every few years.  We’re running another test series because there’s a lot going on in the titanium market right now – most of it bad:

  • There are only two sources left in the US.  For many years there were three sources; this doesn’t seem like much of a difference but it mattered.  Seven works with both mills.  Material quality from both companies is very good.
  • The aerospace industry wreaks havoc on supply stability; lead-times can bounce from about 6-months to two-years.  That’s no way to run a business – or be run by it.
  • More and more builders are using Asian titanium and, to a lesser extent, European material sources.  Nearly all builders are sourcing from Asia.

Test Parameters – Overview

Parameters we’re testing include:
  • Material sources:  we’re looking at domestic, European, and Asian sources.  We know we will see significant variations – as we did last time we tested.
  • Inside surface finish:  We continue to see this as one of the four largest test variables – and real world usage variables.
  • Contractile strain ratio
  • Outside surface finish
    • Brushed
    • Polished
    • Bead blasted
    • Sand blasted
    • Raw
  • Anodizing
  • Concentricity
  • ASTM standards relative to Seven’s standards
  • Annealed material compared to CWSR.  I’m confident that this test series will show significant degradation for annealed material.  The only reason we’re even testing it is because I know a number of builders use annealed material where bending is required – chain stays, seat stays, down tubes, etc.
  • Stress riser comparisons:
    • Welding style
    • Hole drilling and hole edge finishing

I think this is going to take about three months of total machine time.  We’ve got a lot of pieces ready to go.

Stef A. and Bryan H. are helping run the test program.

Project timeline in reverse:

November 12  |  Fatigue testing of some titanium materials at Seven Cycles continues.  The work is slow and sometimes tedious   But the results are some of the most important work that we can do.  Most results are predictable.  Some are not.  Working to track down some of the chemistry on a few test pieces.

August 1  |  Fatigue testing stress risers.  As part of our latest titanium testing I’ve added some stress riser tests.  Years ago we did some testing that ielded pretty counter-intuitive results.  I’m hoping we can recreate these results for this new generation of material enthusiasts.  I want to get going on this but we can’t begin until the carbon testing round is complete.  Soon.

July 27  |  Added annealed 3-2.5 titanium to the program.  The primary reason for this is that I’m concerned about the extent to which annealed material is making it’s way into bikes.  Annealed 3-2.5 titanium typically has a yield strength of about 70 ksi.  Only about 60% of the strength of cold worked stress relieved – CWSR – 3-3.5 ti.  Annealed material is kind of like building frames from 6061 aluminum or low grade 4130 steel.

July 12, 2012  |  I started on setting parameters for Seven Cycles’ next titanium testing.  We’re looking at a number of test parameters:  surface finish on the ID and OD, concentricity, and – most important – contractile strain ratio.


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