Investigation of effects of cold forming on mechanical properties

INTRODUCTION All of the divisions of this report consist of either experimental or theoretical investigations of the various aspects of predicting the magnitude and distribution of changes in the mechanical properties (particularly the yield strength) of coldformed members. The prior work performed under this project, was conducted to determine the changes brought about in the mechanical properties of structural sheet steels by (1) simple uniform uni-directional prestrain of varying amounts and (2) simple cold working caused by cold forming sheet into corners as is commonly done in light gage cold formed structural members. Included in this report are: a study of the plastic strains occurring in cold formed corners, a new look at strength versus permanent strain relationships of unidirectionally prestrained flat sheets in terms of true stress and true strain (using data from the First and Second Progress Reports), and an attempt to correlate corner test results for eight different sheet steel materials with yield strength-strain relationships. A brief investigation of the extension of corner plastic strain effects into the adjacent flats for a 10 gage coin press braked corner is also included. The remainder of the report comprises, primarily, an investigation into the full section tensile and compressive behavior of several cold formed structural shapes. Section 3.1.1 of the AISI Specification for the Design of Light Gage Cold-Formed Steel Structural Members, 1962 Edition, permits, for certain types of sections, the utilization of increases in material strengths due to the cold work of forming. Increased allowable stresses are permitted in the case of axially loaded members and in the flanges of flexural members. In order to better understand and to better exploit these increases in axially loaded members, an investigation into the performance of six different cold formed structural shapes was conducted. The members, shown on Figs. 35, 36, and 39, which have been tested are as follows: (1) a 16 gage press braked hat section in two types of material (a) hot rolled semi-killed steel (HRSK16-37.5) and (b) cold reduced killed steel (CRK16-38.3), (2) a 16 gage roll formed track section in HRSK16-37.5 steel, (3) a 10 gage roll formed channel section in HRSK16-37.5 steel (4) a 16 gage press braked channel section in a hot rolled steel, (5) a 16 gage press braked hat section in CRK16-38.3, and (6) a 16 gage press braked lipped angle section in a hot rolled steel. The abbreviations used above are defined in Section II. These sections were fabricated from the same five materials used in the previous phases of the investigation, for which properties are given by the first five items of Table 1. Sections (4) and (5) were fabricated from either HRSK16-37.5 or HRR16-40.5, but no record was kept as to exactly which of these two steels was used. Full section tension tests were conducted on all six types of members shown on Figs. 35 and 36. Full section compression tests with lateral support were conducted on all of the specimen types shown on Fig. 39. Hydrostone was poured around each of these specimens in order to eliminate the possibility of decreased yield strengths due to local buckling. Full section compression tests without lateral support were conducted on the specimen types shown on Fig. 39 (b). In addition, to determine what the effects of cold forming are upon the flat portions of the sections, tension tests of 1/4 wide by 10 long strip specimens were performed for each of the first three types of members, Fig. 37. Similar tests were undertaken on compressive specimens from the flat portions of these sections, Fig. 40. Information from the narrow tensile and compressive strip specimens, as well as from the corner tests, is related herein to that from tensile and compressive full section tests to show that the magnitude and distribution of effects from the cold forming of sections is reliably predictable from data from reasonably simple test procedures