Multi-scale computational analysis of the nano-indentation and nano-scratch testing of Kevlar® 49 single fibers

Abstract

To carry out virtual nano-indentation and nano-scratch Kevlar® 49 single-fiber tests, a multi-scale computational framework has been developed and employed. Such tests are generally conducted to determine fiber local properties, as well as to provide some insight into the interaction of hard nano-particles with the fibers. The Kevlar® fabric-based soft armor is infused with these nano-particles for improved ballistic resistance, and tip geometry of the nano-indentation/-scratch probes is selected to match nano-particle size and geometry. Due to the fact that Kevlar® 49 fibers (typical diameter 12 µm) are effectively assemblies of parallel fibrils (typical diameter 100–300 nm), while atomic bond length in Kevlar® fibers is of the order of 0.2 nm, a continuum-level finite-element framework has been developed. However, to more accurately account for some of the key aspects of the fiber-material constitutive behavior, e.g. inter-fibril cohesion, the continuum-level computational analysis has been supplemented with atomic-level molecular-statics/-dynamics calculations. In good agreement with their experimental counterparts, the results obtained revealed that the extent of participation of different fibril-deformation modes (e.g. transverse compression, inter-fibril shear, axial tension, axial tensile fracture, fibrillation, axial compression, buckling and pile-up formation ahead of the nano-scratch probe, etc.) is a function of the indentation/scratch depth. Also, a relatively good agreement was obtained between the computed and experimentally measured nano-indentation forces/energies for both shallow and deep indentations, and for the nano-scratch forces/energies, but only for shorter scratch lengths. At longer scratch lengths, the “short-fiber” effects cause the computation/experiment agreement to worsen.