求翻译：the yield surface (from experimental determination of stress conditions at plastic yielding) or knowing the plastic potential surface (from experimental determination of the relative magnitudes of plastic strain increments and therefore the direction of the plastic strain increment vector at different stress points) would automatically result in knowing both surfaces. This simplified the mathematical framework for the model of the plastic behavior. Besides, violation of the “normality rule” was considered to have serious implications in terms of stability of the material, and constitutive models based on so-called nonassociated plastic flow rules, for which the plastic potential surface would be different from the yield surface, could result in unstable material behavior.是什么意思？

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