(647h) Structural Statistical Mechanics of High-Rate Impact Transport in Kevlar®

Improved high-rate impact materials are needed to help protect the lives of those who serve our communities and our nation. This study provides the first characterization of the nanoscale structural mechanics within a multi-layer Kevlar® fabric system in response to high-rate impact. Our novel methodology based on FIB-notch atomic force microscopy and multi-dimensional statistical mechanics reveals how the chemical nanostructure of these high-strength polymer fibers determines their local mechanical properties as well as their ultimate performance during high-rate impact events. The high-rate impact event affects the nanoscale structure described in terms of crystalline pleat lengths, inter-pleat angles and fibril widths. The high-rate impact event results in significant extensional and compressive deformations throughout the system, in stark contrast to state-of-the-art models that predict only extensional deformations. Better understanding of how layered Kevlar® systems respond to high-rate impact will help guide the design of improved high-rate impact resistant armor systems.

Individual Kevlar® KM2+ fibers were resected from a standard multi-layered pack of woven fabric used to measure performance during high-rate projectile impacts. Fibers were collected from 5 distinct regions. Control fibers were collected from woven regions that were not affected by the high-rate impact event. Broken fibers were collected at the first impacted layer (IL) and the stop layer (SL) where the projectile was stopped and became lodged. Unbroken fibers were collected in two regions in the first layer beneath the projectile that was unbroken by the high-rate impact, the unbroken layer (UL)- directly underneath the projectile and underneath the location of the IL and SL broken fibers. To make internal morphology accessible to high resolution atomic force microscopy, a focused ion beam (FIB) was used to mill two notches perpendicular to the fiber so the fiber can be pulled apart. This action cleaves the highly crystalline fibers in a plane parallel to the fiber axis without altering the fiber internal morphology. Internal morphology was quantified using multi-channel amplitude modulation and frequency modulation of a high-resolution atomic force microscope (AFM) tip. Analyses of the AFM maps in topological height, storage modulus, and loss modulus channels enable nanoscale measurements of crystalline fibril widths, crystal pleat misorientation angles, and crystal pleat lengths. Multi-dimensional statistical distributions of these nanoscale features are fingerprints of the mechanics that occurred during high-rate impact. Comparisons among all sampled layers for each feature measurement distributions were performed using statistical tests, showing differences between morphology distributions that are statistically significant. These morphological changes were then related to specific forces and deformation mechanisms using controlled fiber deformation experiments. Discontinuities of the multichannel AFM signal maps indicate changes in roughness and defects relative to perfect, single crystals. The count of these discontinuities is used to compute the defect density enabling the quantification of changes in defectivity among all sampled layers.

The high-rate impact event results in significant extensional and compressive deformations throughout the system. Statistically significant changes to internal morphology, relative to the control, are present in each layer analyzed. The observed changes in morphology indicate a transition from tensile to a mixture of tensile and compressive forces as the projectile interacts with layers further from the first layer impacted. Defectivity analysis shows that only the SL, the layer where the projectile was found lodged after high-rate impact, has increased defectivity relative to the control. This increased defectivity indicates the SL absorbed more energy density than the other layers analyzed. These are surprising results and in stark contrast to state-of-the-art models that predict only extensional deformations. The observed deformation mechanisms indicate that improved performance of layered high-rate impact armor fabrics can be achieved by altering layer properties and enhancing compressive strength.