Title: Rethinking Ductility -- A Study Into the Size-Affected Fracture of Polymers Show Chinese title

Title: 重新思考延展性 -- 聚合物尺寸影响断裂的研究

Abstract: Ductility quantifies a material's capacity for plastic deformation, and it is a key property for preventing fracture driven failure in engineering parts. While some brittle materials exhibit improved ductility at small scales, the processes underlying this phenomenon are not well understood. This work establishes a mechanism for the origin of ductility via an investigation of size-affected fracture processes and polymer degree of conversion (DC) in two-photon lithography (TPL) fabricated materials. Microscale single edge notch bend ($\mu$SENB) specimens were written with widths from 8 to 26 $\mu$m and with different laser powers and post-write thermal annealing to control the DC between 17\% and 80\%. We find that shifting from low to high DC predictably causes a $\sim$3x and $\sim$4x increase in strength and bending stiffness, respectively, but that there is a corresponding $\sim$6x decrease in fracture energy from 180 $J/m^2$ to 30 $J/m^2$. Notably, this reduced fracture energy is accompanied by a ductile-to-brittle transition (DBT) in the failure behavior. Using finite element analysis, we demonstrate that the DBT occurs when the fracture yielding zone size ($r_p$) approaches the sample width, corresponding with a known fracture size-affected transition from flaw-based to strength-based failure. This finding provides a crucial insight that ductility is a size-induced property that occurs when features are reduced below a characteristic fracture length scale and that strength, stiffness, and toughness alone are insufficient predictors of ductility. Show Chinese abstract

Abstract: 延性量化材料的塑性变形能力，是防止工程零件由断裂驱动失效的关键特性。 虽然一些脆性材料在小尺度下表现出改进的延性，但这一现象背后的机制尚不明确。 本研究通过调查尺寸影响的断裂过程和双光子光刻（TPL）制备材料的聚合物转化度（DC），建立了延性的起源机制。 微尺度单边缺口弯曲（$\mu$SENB）试样被制造，宽度从8到26$\mu$m，并使用不同的激光功率和写入后热退火来控制DC在17%到80%之间。 我们发现，从低DC向高DC转变可预测地导致强度和弯曲刚度分别增加$\sim$3倍和$\sim$4倍，但断裂能则相应减少$\sim$6倍，从180$J/m^2$降至30$J/m^2$。 值得注意的是，这种降低的断裂能伴随着失效行为中的延性-脆性转变（DBT）。 使用有限元分析，我们证明当断裂塑性区尺寸（$r_p$）接近样品宽度时会发生DBT，这对应于从基于缺陷的破坏到基于强度的破坏的已知破坏尺寸相关转变。 这一发现提供了重要的见解，即延展性是一种尺寸诱导的特性，当特征尺寸降低到特征断裂尺度以下时发生，并且仅靠强度、刚度和韧性无法充分预测延展性。