Micromechanical study on polypropylene-bicomponent fibers to improve mechanical interlocking for application in strain-hardening cement-based composites

Abstract

Polyproplylene (PP) fibers find application in strain-hardening cement-based composite to enable the formation of multiple fine cracks for high energy absorption. Often, however, the mechanical performance of such composites suffers from insufficient fiber-matrix interaction. In the research at hand, bicomponent PP single fibers with rough surfaces for improved mechanical interlocking are produced using an IPF (Leibniz-Institut für Polymerforschung Dresden e. V.), an in-house designed and built, laboratory-scale piston fiber spinning device. The melt-spun fibers consist of a shell component composed of PP and various volume percentages of different inorganic particles of calcium carbonate (CaCO3), aluminum oxide (Al2O3), and of a core component made of the same polymer as in the shell. The bicomponent fibers, with shell diameters between 20 and 45 μm, were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to understand their morphology and to study the fiber surfaces after composite failure. Tensile strength and Young's modulus of the fibers were evaluated using tension tests. Furthermore, the single fiber pullout (SFPO) test was used to investigate the interfacial interaction between fiber and a cement-based matrix. Significant improvements in fiber-matrix bonding were achieved due to the rough surface employed in connection with the particles incorporated in the outer shell. Still further, the fiber's strength, attained using an offline step in the drawing process, contributes to enhanced energy adsorption under dynamic pullout loading. To evaluate the performance of the newly developed bicomponent fibers, they were compared to a self-spun monocomponent PP fiber and a commercial PP fiber. This comparison revealed slip-hardening induced by increasing surface roughness for enhanced mechanical interlocking and plastic polymer deformation in the fiber-matrix contact zone.