INTRODUCTION It has been known for years that mechanical properties of thin sheet steels, such as yield strength, tensile strength, and ductility, are affected by aging (Chajes et al. 1963, Hertzberg 1989, Yu 1991). The aging effect on mechanical properties can be described as follows. For a virgin steel that is initially loaded in tension beyond its yield strength and into strain hardening state and then unloaded to zero stress, the yield and tensile strengths of the steel increase while the ductility of the steel decreases if the steel is reloaded in tension again after a period of time from the first unloading. Due to diffusion of the carbon and nitrogen in steels and the fact that steel sheets are usually produced in a cold rolling process, in which the sheet steels undergo large plastic deformation, their mechanical properties can be affected by aging. Therefore, the mechanical properties of the thin sheet steels at the time when they are used can be different from the properties at the time when they are produced. The yield and tensile strengths of the sheet steels will increase over time while the ductility of the steels will decrease. In 1992, a research project, sponsored by the American Iron and Steel Institute (AISI), was carried out at the University of Missouri-Rolla to study the effect of strain rate on the mechanical properties of sheet steels (Pan and Yu 1992). In this research, coupons cut from two selected sheet steels (25AK and 50SK) were tested in longitudinal and transverse tension and compression at four different strain rates, namely 0.0001,0.01,0.1, and 1.0 in./in./sec.. The results of the study were reported by Pan and Yu (1992). Later on for another study on the strength of hybrid structural components made of these two steels (Pan and Yu 1995), the mechanical properties of the steels that were obtained in 1992 were used to evaluate the strength of the hybrid members without consideration of the aging effect. In this later study, the hybrid members were tested in bending at the strain rates of 0.0001 and 0.01 in./in./sec.. In order to estimate the aging effect on the mechanical properties of two types of sheet steels used by Pan and Yu (1992), additional coupon tests of the sheet steels (25AK and 50SK) were conducted at the University of Missouri-Rolla in August 1995 and in January 1997. A total of twenty coupons, ten for each steel, were tested at two different strain rates, namely 0.0001 and 0.01 in./in./sec.. Four coupons for each steel were tested at the strain rate of 0.0001 in./in./sec. in August 1995, while three coupons for each steel were tested at the strain rate of 0.0001 in./in./sec. in January 1997 and another three coupons for each steel were tested at the strain rate of 0.01 in./in./sec. at the same time. This report summarizes the results of the twenty coupon tests. In the following discussions, Section 2 presents the test results on the mechanical properties of25AK and 50SK sheet steels. Section 3 compares the mechanical properties of the steels reported by Pan and Yu (1992) with those presented in Section 2 to estimate the aging effect on steels and the computed yield moments. Finally, Section 4 summarizes the findings
