The sub-micron scale energy dissipative deformation mechanisms of Kevlar fibrils

Abstract

In this study, the energies to indent and scratch Kevlar 49 and Kevlar KM2 single fiber surfaces are investigated along with the associated deformation mechanisms of the fiber fibrillar network. For both indentation and scratching, values of energy are determined from the measured forces and displacements under a number of different contact conditions, which include variations in probe size, indentation/scratch depth, and scratch length. These values are correlated to the observed fibrillar deformation mechanisms. The total indentation energy is primarily recoverable for indentations made at low depths and/or by probes that impart low levels of effective strain. Indentations made to greater depths by probes imparting greater effective strains result in additional forms of deformation, which correlate with an increase in the percentage of the total energy that is absorbed and an increase in the total specific energy (energy per volume) of indentation. The energy of a constant depth scratch exceeds the energy of indentation for scratch lengths greater than ~1.5–3 times the indentation depth. The total specific energy of scratching decreases with the onset of microstructure failure, in the form of fibrillation, during scratching. The energies to indent and scratch Kevlar KM2 are generally lower than Kevlar 49, which is likely related to the decreased properties of the Kevlar KM2 shell and/or a difference in the local properties of the two fibers. The fibrillar deformation mechanisms critical to optimizing the specific energies of indentation and scratching, which depend on strain, are identified and discussed.