Rethinking ductility-A study into the size-affected fracture of additively manufactured polymers

Ductility, namely a material's capacity for plastic deformation, is a key property for preventing fracture -driven failure in engineering parts. While some brittle materials are known to exhibit ductility at small scales, the underlying mechanics of such behaviors are not well understood. This work identifies size -affected fracture as a key mechanism for the origin of ductility in two -photon lithography (TPL) fabricated polymers. We conducted microscale single -edge notch bend ( mu SENB) fracture experiments on three distinct specimen sizes and varied the polymer degree of conversion (DC) to be between 17% and 80% by controlling both laser exposure and post -write thermal annealing. For a given specimen size, we find that shifting from low to high DC predictably causes a similar to 3x and similar to 4x increase in strength and bending stiffness, respectively, but the fracture energy correspondingly drops by similar to 6x, from 180 J/m 2 to 30 J/m 2 . Notably, this reduced fracture energy was accompanied by a ductile -to -brittle transition (DBT) in the failure behavior. Using a combination of experiments and finite element analysis, we quantify the fracture yielding zone size ( r p ) in these polymers as a function of DC and demonstrate that ductility emerges when r p approaches the sample width irrespective of the DC. 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.