4.1 This classification involves alphabetical ratings intended only to provide a qualitative guide for materials selection. The ratings are based primarily on the results of standard corrosion tests.

4.2 Interpretations of the SCC ratings in terms of typical problem areas including service experience are given in Table 1. Practical experience has shown that SCC problems with aluminum alloys generally have involved situations where the direction and magnitude of the tensile stresses resulting from manufacturing or use, or both, of the material were not recognized.

(A) The sum of all stresses including those from service loads (applied), heat treatment, straightening, forming, and so forth.

4.3 A list of the SCC ratings for the heat-treatable aluminum alloy products is given in Table 2. Revisions to the table will be required as new materials become available and additional test results are accumulated.

(A) The ratings apply to standard mill products in the types of tempers indicated, including stress-relieved tempers, and could be invalidated in some cases by application of nonstandard thermal treatments or mechanical deformation at room temperature by the user.(B) Test direction refers to orientation of the stressing direction relative to the directional grain structure typical of wrought materials, which in the case of extrusions and forgings may not be predictable from the geometrical cross section of the product.

L - Longitudinal: parallel to 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.

(C) Sections with width-to-thickness ratio equal to or less than two, for which there is no distinction between LT and ST.(D) Rating not established because the product is not offered commercially.(E) Rating is one class lower for thicker sections: extrusions, 25 mm (1 in.) and over; plate and forgings 40 mm (1.5 in.) and over.

4.4 These alphabetical ratings are not suitable for direct use in mathematical models for material selection, but numerical weights and confidence factors can be devised on the basis of experience and judgment of the materials engineer.5