This study investigates the effect of stress triaxiality on the failure mechanisms of an-isotropic perfect and imperfect planar FCC (Face Centred Cubic) truss lattice metamaterials. Three types of imperfection have been considered in the numerical modelling, namely, distorted struts, missing struts, and strut diameter variation. In order to maintain constant stress triaxiality during the simulations, a novel numerical framework was developed to overcome computational difficulties within the existing numerical approaches beyond elastic region. Three modes of microscopic localization were observed in perfect and imperfect lattices before failure: crushing band, shear band and void coalescence. A clear separation exists between the three modes of localization depending upon the type and level of defects, as well as the stress triaxiality. Under compressive loading, all lattices fail owing to crushing band; the distorted lattices are prone to shear band localization with increase in distortion, whereas missing lattices majorly fail due to void coalescence at high missing struts defect. Strut diameter variation, within the range of the strut diameters selected, shows no significant influence on the macroscopic mechanical response and strain localization. This work may open the door for predicting failure mechanisms of imperfect lattices under variety of loading conditions.

本研究调查应力三轴性对各向同性完美和不完美平面 FCC（面心立方）桁架晶格超材料失效机制的影响。在数值建模中考虑了三种类型的缺陷，即扭曲的支柱、缺失的支柱和支柱直径变化。为了在模拟过程中保持恒定的应力三轴性，开发了一种新的数值框架来克服现有数值方法中超出弹性区域的计算困难。破坏前在完美和不完美的晶格中观察到三种微观定位模式：破碎带、剪切带和空隙聚结。根据缺陷的类型和水平以及应力三轴性，在三种局部化模式之间存在明显的分离。在压缩载荷下，所有格子都因破碎带而失效；随着畸变的增加，扭曲的晶格容易出现剪切带定位，而缺失的晶格主要是由于高缺失支柱缺陷处的空隙聚结而失效。在选定的支柱直径范围内，支柱直径变化对宏观力学响应和应变定位没有显着影响。这项工作可能为预测各种加载条件下不完美晶格的失效机制打开大门。显示对宏观力学响应和应变定位没有显着影响。这项工作可能为预测各种加载条件下不完美晶格的失效机制打开大门。显示对宏观力学响应和应变定位没有显着影响。这项工作可能为预测各种加载条件下不完美晶格的失效机制打开大门。