Mechanical Anisotropy in Small-Diameter Bar
Very few materials have all their properties behave the same in all orientations of the material. The term for this behavior is anisotropy or anisotropic properties. The properties that are affected and the degree of anisotropy they exhibit, depend on the material family, the alloy, and the processing of the material. This paper will only discuss metals, and limited to those metal specifications adopted for pressure equipment use.
The anisotropic properties of plate have been well recognized explicitly and implicitly for a very long time. At issue, and the specific focus of this paper, is another wrought product form: bars. Bar is a very useful starting material to manufacture pressure parts. The bar can be stocked at the largest feasible diameter and length, and then cost-effectively machined to whatever diameter and length is desired for the part. Due to the anisotropic properties of bar, there is a need for understanding and setting limitations for the use of such material.
Initially, a very limited data set of mechanical properties, clearly showing a significant degree of anisotropy in bar, was the technical basis for the current Code requirements. More recently, more extensive tension testing on small-diameter bar in both carbon steel and austenitic stainless steel was performed. This paper shares the significant results of that testing, some substantial conclusions from the lab work, and some observations on some unexpected behavior.