Void interaction, leading to coalescence, is the mechanism leading to ductile failure under intense shearing. Published unit cell model studies have demonstrated that micron-size voids collapse to form micro-cracks while continuous elongation and rotation of the voids thin the intervoid ligaments. At a final stage, the deformation leads to plastic flow localization in the ligament, and the material loses the load-carrying capacity. The micro-mechanism of void collapse, elongation, and rotation has been studied using 2D and 3D unit cell simulations but only within a conventional strain hardening material and, thereby, not accounting for micron scale size effects. However, the severe plastic deformation near the voids implies the development of significant plastic strain gradients, which must be accommodated by geometrically necessary dislocations (GNDs) that strengthens the matrix locally and elevates the stress level. The present research accounts for such gradient strengthening within the matrix in order to investigate the material size effect in ductile shear failure. The work presented leans on the unit cell model approach by Tvergaard (2009), but enables a constitutive length parameter to enter the analysis by substituting the matrix with a Fleck–Willis gradient enhanced material. The results reflect the combined effect of applied load, strain hardening, initial void volume fraction, and microstructure size. The general conclusion is that matrix strengthening, governed by size, delays the loss of load-carrying capacity and leads to less concentrated localization around small void. The results also show that the void collapse, elongation, and rotation mechanism is more sensitive to changes to the applied load, hardening, and initial void volume fraction at small scales.

导致聚结的空隙相互作用是在强烈剪切下导致延性破坏的机制。已发表的晶胞模型研究表明，微米级空隙坍塌形成微裂纹，同时空隙的连续伸长和旋转使空隙间韧带变薄。在最后阶段，变形导致韧带中的塑性流动局部化，材料失去承载能力。已经使用 2D 和 3D 晶胞模拟研究了空隙坍塌、伸长和旋转的微观机制，但仅在常规应变硬化材料内进行研究，因此未考虑微米级尺寸效应。然而，剧烈的塑性变形靠近空隙意味着显着的塑性应变梯度的发展，这必须通过几何必要的位错 (GND) 来适应，这些位错在局部加强基体并提高应力水平。目前的研究考虑了基质内的这种梯度强化，以研究延性剪切破坏中的材料尺寸效应。提出的工作依赖于 Tvergaard (2009) 的晶胞模型方法，但通过用 Fleck-Willis 梯度增强材料代替矩阵，使本构长度参数能够进入分析。结果反映了外加载荷、应变硬化、初始空隙体积分数和微观结构尺寸的综合影响。一般的结论是，受尺寸控制的基体强化，延迟承载能力的损失，并导致小空隙周围的集中度降低。结果还表明，空隙坍塌、伸长和旋转机制对小尺度下施加的载荷、硬化和初始空隙体积分数的变化更敏感。