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What is the difference between LT and T directions in MMPDS (formerly MIL-Handbook 5)?

I hand understood that forming metal orients its grains, destroying the isotropic nature of 'pure metal' and that the "Long" direction is the length of the sheet/extrusion/etc, and that there were two transverse directions. There is the "Short Traverse" which would be the thickness (or "depth") of the sheet/extrusion/etc. There is also the "Long Traverse" direction which would be the width of the sheet/extrusion/etc.

But then MMPDS throws in a "T". Is this just for the times when you cannot distinguish between LT and ST (such as in a rod)? I also see datasets where specimen orientations are T and ST, which furthers my confusion. MMPDS itself doesn't seem to offer much further guidance.

Any explanations?

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I believe MMPDS is the new name for Mil-Handbook 5. I have the 1998 version and it defines "T" as `Applied torsional moment; transverse (grain direction); subscript “transverse”. Also see the figure below from Mil-Handbook 5.

In the figure, the "L", "T", and "S" planes are marked clearly. Note that "T" and "L-T" indicate the same plane.

MIL-Handbook 5 1998 definitions

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For Fatigue and Fracture the two letters are separated by a dash. On the picture from Biswajit Banerjee imagine the loading of the part or test sample along the L direction, it gives the first letter L. The crack (or damage) can propagate now perpendicular to the load in S or T direction (laying L-T and Standing L-S samples on the picture) giving the second letter. So L-T means loaded in L direction with crack propagating in T direction.

On the other hand, the MMPDS-07 describes Test Directions as:

  • L-Longitudinal: parallel to the direction of principal metal extension during manufacture of the product.
  • LT-Long Transverse: perpendicular to direction of principal metal extension. In products whose grain structure clearly shows directionality (width-to-thickness ratio greater than two) it is that perpendicular direction parallel to the major grain dimension.
  • ST—Short Transverse: perpendicular to direction of principal metal extension and parallel to minor dimension of grains in products with significant grain directionality. See also ASTM G64-99
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