在经典塑性理论中不包含长度尺度，因此无法解释实验中的尺寸效应现象；而工程中迫切需要解决的微米和亚微米量级的设计、制造问题；以及为韧性材料中的解理断裂现象提供一种合理的解释，都需要发展应变梯度理论。本文针对基于细观机制的应变梯度塑性理论进行了一系列研究，主要取得了以下成果：1. 推导了MSG塑性流动理论的本构方程，其中考虑了摩擦应力的影响。2. 发展MSG流动理论的有限元方法，并模拟了微压痕实验和静止裂纹问题。MSG流动和形变理论都与微压痕实验符合得很好；压痕硬度平方和深度倒数之间近似成线性关系。MSG理论预测的裂尖应力明显高于HRR场；其应力奇异性不仅高于HRR场，而且还达到或超过弹性场-1/2奇异性；幂硬化指数不影响应力奇异性；MSG塑性理论的主控区很小，且对外加应力强度因子不敏感。3. 利用MSG流动理论研究定常扩展的平面应变I型裂纹。计算表明，在裂尖附近应变梯度明显的区域内，裂尖分离应力可达到相当高的水平。MSG理论控制区内奇异性很高，甚至超过弹性场奇异性，且幂硬化指数不影响奇异性。将MSG流动理论和EPZ模型相结合，分析定常扩展裂纹的断裂韧性表明，断裂韧性与界面最大分离应力和材料特征尺度相关。因而MSG塑性理论可以为韧性材料的解理断裂现象提供一种有效的解释。4. 推广MSG塑性形变理论，考虑了Taylor模型中摩擦应力的影响。纯弯、扭转和孔洞长大问题的研究表明，应变梯度和摩擦应力之间的相互作用很弱。分析bcc材料钨的微压痕实验得知：摩擦应力显著提高压痕硬度；压痕硬度平方与深度倒数之间的线性关系仍近似成立，但直线的斜率受摩擦应力影响很大。5. 研究了韧性材料孔洞长大问题中的应变梯度效应。对理想刚塑性和幂硬化材料，得到孔洞长大率和远场外加静水应力的关系。发现应变梯度效应可以降低微孔洞的长大速率。

Recent experiments at the micron scale have repeatedly shown that metallic materials display significant size effect. Classical plasticity theories do not possess internal material lengths and therefore cannot explain the observed size dependence of material behavior. However, there is an impending need to deal with design and manufacturing issues at the level of microns and submicrons. Moreover, to explain the cleavage fracture in ductile materials also needs the development of strain gradient theory. In this paper, some problems of the mechanism-based strain gradient plasticity are researched. The following objectives have been achieved:1. The constitutive relations of MSG flow theory are given, with consideration of the effect of friction stress.2. For the flow theory of MSG plasticity, we developed a finite element formulation which is then used to study the problems of micro-indentation and fracture of static crack. It is established that the flow theory of MSG plasticity as well as deformation theory, agrees very well with the micro-indentation experiments. There is an approximate linear relation between the square of indentation hardness and the inverse of indent depth. The stress level in vicinity of crack tip in MSG plasticity is significantly higher than that in classical plasticity (HRR field). The stress singularity of MSG plasticity is not only larger than HRR field, but also equals to or exceeds the square-root singularity of elastic field. And that singularity is independent of the plastic hardening exponent. The dominance zone size of near-tip field in MSG plasticity is very small and insensitive to the level of the remote applied stress intensity.3. Mode I crack under steady-state growth and plane strain is analyzed employing MSG flow theory. The results show that during crack growth, the normal separation stress will achieve considerably high value within a sensitive zone of strain gradient near the crack tip. The crack tip stress singularity within the dominance zone of MSG plasticity is much stronger, even than elasticity singularity. And the singularity is independent of the plastic hardening exponent. Moreover, the steady-state fracture toughness for the solid characterized by MSG theory is analyzed by adopting the embedded process zone model (EPZ model). The result shows that the steady-state fracture toughness is strongly dependent on the material parameter of separation strength and the internal material length. Therefore, MSG theory may provide an effective approach to explain the cleavage fracture in ductile materials.4. MSG deformation theory is generalized in order to account for the effect of friction stress in the Taylor dislocation model. The effect of friction stress is investigated via three examples of pure bending, torsion and void growth. It is established that the interaction between the strain gradient effect and the friction stress is weak. The hardness increase due to friction stress is nearly independent of the strain gradient effect. The linear relation between the square of micro-indentation hardness and reciprocal of indentation depth still hold approximately for bcc tungsten, but the slope of straight line is strongly influenced by the friction stress.5. Strain gradient effect of microscopic voids growth in ductile solids is investigated. The relation between the void growth rate and remote applied hydrostatic stress is obtained for both rigid-plastic and power hardening material. It is established that the strain gradient effect may decrease the microscopic void growth rate.