FROM THE FRANKLIN INSTITUTE LABORATORIES FOR RESEARCH AND DEVELOPMENT NICOL H. SMITH,
E. W. HAMMER
AND R. E. PETERSEN
Certain alloy steels of the stainless variety are important structural materials and are used in the railcar, truck tractor and aircraft industries because of their high strength-weight ratios. While there are a number of varieties of stainless, only type 301 has been considered in this program. Type 301 is of the 17-7 variety, 16.00 to 18.00 per cent chromium and 6.00 to 8.00 per cent nickel. Type 301 stainless steel derives its strength from cold rolling and varies in tensile strength from 90,000 psi. in the annealed condition to 185,000 psi. in the full hard condition. The steel is commercially available in hardnesses of annealed, $, 3 and full hard. Because of cold rolling the material becomes anisotropic (different properties in different direction) in all but the annealed condition. For 301, the material is strongest in the transverse direction in compression and weakest in the longitudinal direction in compression. Thus it is difficult to predict the physical behavior of type 301 stainless by standard methods of analysis. Aware of the increasing demand for this material and of the difficulty in predicting its physical behavior, the American Iron and Steel Institute initiated a research program at The Franklin Institute to prepare a design specification for the structura1 use of type 301 stainless steel. The first part of the program, the preparation of the column curves, has been completed. For a material stressed below the proportional limit, the maximum apparent stress that a column with a stable cross-section can sustain is given by the Euler equation : -
However, for a material with no definite elastic limit (such as stainless steel) the above expression is not valid. Historically, F. Engesser * This note is based upon research conducted by The Franklin Institute Laboratories for Research and Development under the sponsorship of the American Iron and Steel Institute. 257
IJ. F. 1.
proposed that the tangent modulus be used in place of Young’s modulus for columns stressed beyond the proportional limit. Subsequently, Considere, Engesser, and later von Karman suggested that a reduced modulus (a combination of the initial and instantaneousness tangent moduli) be used. Critical column loads computed by the reduced modulus theory are somewhat higher than those computed by the tangent modulus theory. In stainless steel design, it is claimed that the use of tangent modulus column curves for predicting allowable column loads usually gives loads lower than test values. Consequently, some designers have advocated that the reduced modulus be used for stainless steel because it gives higher values, which agree better with test results. Thus, in order to prepare column curves, it was necessary to decide which modulus to use in computing column curves. To resolve this question, an extensive test program was conducted. More than two hundred column specimens of annealed, 2, 4 and full-hard material, in slenderness ratios varying from 15 to 120, were tested. The tests were conducted on 0.025, 0.050, and 0.072 thick, annealed material; on 0.020, 0.045, 0.062, 0.076 thick, 4 hard; on 0.050, 0.025, and 0.075 thick, + hard; and on 0.030, 0.045, 0.050, and 0.076 thick, full-hard material. For 2, & and full-hard material, specimens were made with the grain in both the longitudinal and transverse directions. The specimens consisted of two hat sections shot-welded together to form a closed column. The width of flats was chosen so there The ends of the specimens were ground would be no local instability. parallel to one another within 0.0003 in. The cross section of a typical column specimen is shown in Fig. 1.
Typical column cross section.
The tests were performed on a Baldwin-Tate-Emery Universal Testing Machine with a maximum loading range of 120,000 lb. Hardened steel (Rockwell C60) knife edges were used to obtain pin end conditions. The knife edges were calibrated against solid steel and aluminum rods and showed that pin-end conditions existed within =tl%. See Fig. 2 for picture of test arrangement. The results of the tests disclosed, for all’practical purposes, that the test data for the 2, 3 and full-hard longitudinal specimens agreed with theoretical column curves based upon the reduced modulus in slenderness ratios less than 110; that for i, + and full-hard transverse
INSTITUTE LABORATORIES NOTES
specimens, the test data agreed with the theoretical column curves based on the tangent modulus; and that the agreement of the longitudinal test data with the reduced modulus column curves is due to the increased hardness-and therefore the increased load capacity-of the material in the rounds due to forming rather than to the behavior (A of the column according to the theory of the reduced modulus. “longitudinal” specimen means a specimen made with the grain parallel
with the long axis of the column, and a “transverse” specimen means a specimen with the grain perpendicular to the long axis of the column.) In the slenderness ratios above 110 the reduced modulus column curves are slightly higher than the test data in the 2 and 3 hard material for both transverse and longitudinal grain. Therefore, it is suggested that allowable column curves for +, 4 and full-hard material be prepared based upon the reduced modulus for
the grain in the longitudinal direction and based upon the tangent modulus for the grain in the transverse direction. However, it must be remembered that this use of the reduced modulus is only a “gimmick” to account for the increased load capacity of a column due to forming the round. Typical stress-strain curves for the a, 3 and full-hard material in both grain directions, upon which the design column curves are based, must also be given. The criterion for the use of the column curves will be that the stress-strain curve of the stainless steel to be used must have a slope equal to or greater than the typical stressstrain curves. For annealed material it is suggested that the tangent modulus be used in preparing allowable column curves for slenderness ratios over 70. For lower slenderness ratios, column curves based upon the reduced modulus should be used, although actual loads will be appreciably greater than those predicted.