The existence of an indentation size effect (ISE) in the measured hardness in poly (methyl methacrylate) is investigated by performing nanoindentation experiments. The continuous stiffness measurement technique was employed to measure the hardness and the elastic modulus of the sample as a function of the indentation depth at different strain rates. Results establish the existence of a strain rate dependent ISE in the glassy polymer as hardness increases with decreasing the indentation depth. Employing the well-known shear transformation (ST) mediated flow theory and considering the statistical nature of the formation and distribution of the flow units, a rate dependent ISE model for glassy polymers is developed. This model is based on the possibility of the occurrence of discrete shear transformation sites within the deformed volume under the indenter which is controlled by the indentation depth and geometry. Results obtained from the proposed model show good agreement with the experimental values. The details considered in the model development in terms of the variations of indentation strain rate and elastic modulus with depth as well as validity and limitations of the model are discussed.

通过进行纳米压痕实验研究了在聚甲基丙烯酸甲酯中测得的硬度中压痕尺寸效应（ISE）的存在。采用连续刚度测量技术来测量样品的硬度和弹性模量，该硬度和弹性模量是在不同应变率下压痕深度的函数。结果表明，随着硬度随着压痕深度的减小而增加，玻璃状聚合物中存在应变率相关的ISE。利用众所周知的剪切转换（ST）介导的流动理论，并考虑流动单元的形成和分布的统计性质，开发了速率依赖的ISE模型，用于玻璃状聚合物。该模型基于在压头下的变形体积内出现离散剪切转变点的可能性，该压头由压痕深度和几何形状控制。从提出的模型获得的结果显示出与实验值良好的一致性。讨论了模型开发中考虑到的压痕应变率和弹性模量随深度的变化以及模型的有效性和局限性的细节。