Fracture Resistant Architected Polymers Using Material and Structural Size Effects

<b>Strength and toughness are both highly desirable properties of structural materials, but they are often thought of as mutually exclusive, i.e., it is difficult to increase one without decreasing the other. Much work has gone toward developing materials that are both strong and tough using different composite architectures and material processing techniques, but they generally ignore the role of length scale and structural size-effects on toughness. In this work, we develop nanoarchitected polymeric materials that utilize heterogeneity and size-affected ductility to enhance their toughness without sacrificing strength. We create specimens with layered architectures in a micro-single edge notch bend configuration using two-photon lithography and various post-processing techniques. Gradually reducing the layer thickness (D) resulted in an increased fracture energy and slower, stable crack propagation, a phenomenon that became pronounced as the layer thickness approached or was smaller than the material fracture process zone (D ≦ FPZ). This energetic size effect is quantified using Bazant’s Size Effect Law. The thinnest of these layered structures demonstrated an increase in toughness by 5x from 60 J/m2 to 300 J/m2 as interlayer spacing was increased from 0 to 4 um, a value that is augmented by the creation of heterogeneity along the crack path. Cracks are observed to rapidly propagate in specimens with no interlayer spacing, but then show increasing blunting and deflection in structures with moderately spaced interlayers (~ 3 um). For larger interlayer spacings (&gt;3 um), cracks do not propagate through; rather, deflect along the interlayer even at very large displacements. Notably, these materials do not show an appreciable loss in strength and stiffness up to an intermediate layer spacing despite a ~40% reduction in density. The results of this study not only demonstrate the large degree of tunability in these architectures but also show how to fundamentally utilize size effects to create architected materials with unprecedented properties.</b>