Improved Understanding of Damage Precursors Through Local Mechanical Characterization

We report on the use of local mechanical characterization techniques for the understanding of structural damage precursors in various material systems. Instrumented indentation and atomic force microscopy (AFM) were used to characterize local damage in: (1) fatigued metallic beams subject to non-linear vibration, (2) individual polymer and glass microfibers, and (3) additive manufactured thermoplastics. Indentation studies of the fatigued metallic beams showed a compliance effect of up to 40% in relatively highly stressed regions. An approved fiber mounting technique allowed for indentation of unmodified surfaces of single microfibers, while AFM modulus maps of the fibers reveal local regions of relative compliance. Local mechanical tests of 3-D printed acrylonitrile butadiene styrene specimens revealed a variation in properties between printed beads and bead-bead interfaces. The nano-/micro-scale techniques developed in the present study provide a framework for understanding how damage precursors may affect processing-structure-property relationships in present and future structural aerospace materials.