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An experiment was conducted to study the strain rate sensitivity of yield behavior in an amorphous polymer, poly(methyl methacrylate) (PMMA). Compression, tension, shear, and combined shear-compression tests were conducted at four different loading rates. Experimental results indicate that strain rates had a pronounced effect on the yield behaviors of PMMA. Therefore, a strain rate-dependent yield function was developed, and the influences of hydrostatic pressure and strain rate on yield were analyzed. Based on the detailed discussion of the three material constants of the yield criterion, the influence of hydrostatic pressure on the tension and compression asymmetry was analyzed. A modified Johnson-Cook model was introduced to determine the effect of strain rate hardening on flow stress. The results of a compression test were described well by this mode. The change in the trend of parameter C of the mode (which characterizes the effect of strain rate) was also analyzed under the conditions of quasi-static loading. [ABSTRACT FROM AUTHOR]