If you want to input A, Iss, etc. directly, you must use a resultant beam formulation, i.e., ELFORM=2. With such a formulation, stresses are not calculated because the shape of the cross-section is unknown. You’ll only get forces and moments. ELFORM=2 is compatible with only a few material types. See the material table at the beginning of the *MAT section of the Users Manual.
ELFORM=1 is an integrated beam formulation. With an integrated formulation, the shape of the cross-section is defined in the input and so stresses can be computed at the beam integration points. The parameter CST on Card 1 of *SECTION_BEAM indicates whether the section is circular or rectangular. You must give cross-section dimensions on Card 2 of *SECTION_BEAM.
For circular sections, you give outside and inside diameters at the two ends. (For a solid circular section, the inside diameter is zero.) For solid rectangular sections, you give the cross-section width and breadth at the two ends.
For hollow rectangular tubes, it’s a bit trickier as you must also use *INTEGRATION_BEAM which is referenced by setting QR/IRID in *SECTION_BEAM to -IRID, where IRID is given in *INTEGRATION_BEAM. In *INTEGRATION_BEAM, you can leave NIP and RA blank and set ICST to 5. On Card 2 of *INTEGRATION_BEAM, give the 4 values W, TF, D, and TW which are shown in the figure in the Users Manual. If QR/IRID in *SECTION_BEAM is negative, it follows that *INTEGRATION_BEAM defines the location of the integration points. If QR/IRID is positive and CST is zero (rectangular section), refer to the Figure 5.3 on p. 5.11 of the 2006 Theory Manual. If the integration rule is 2×2, 3×3, or 4×4 Gaussian, the locations of the integration points shown in Figure 5.3 are in accordance with the columns labeled as 2 point, 3 point, and 4 point, resp., in the table under *SECTION_SHELL in the Users Manual. Integration points for a circular cross-section are positioned sequentially in the circumferential direction of the cross-section, all at the same distance from the cross-section center. For example, for 3×3 Gauss quadrature, the nine integration points in the cross-section are 40 degrees apart with the first integration point on a ray 20 degrees off the local s-axis (toward the t-axis). The radial position of the integration points for a solid circular cross section is at r = 0.707 * the outside radius.
You can get axial strain at beam integration points by setting BEAMIP (*DATABASE_EXTENT_BINARY) to the number of beam integration points in your LS-DYNA input deck. Then, after running the model, read D3PLOT into LS-PrePost and click History > Int.Pt. > Etype: Beams > (click on any beam element) > Axial Strain > Plot.
Also, plastic strain at beam integration points is written to elout (see *DATABASE_HISTORY_BEAM and *DATABASE_ELOUT).