Rate-independent energy dissipation mechanisms in fiber-matrix material systems

Rate-independent energy dissipation associated with high-strain high-stress behavior of the material is considered. This rate-independent energy dissipation is associated with damage and relative motion inside the fiber-matrix material system. Simple mechanical models for energy dissipation were presented to facilitate the understanding of the phenomena. Three mechanisms for internal energy dissipation in fibermatrix material systems are considered: fiber fracture and failure; frictional sliding at the fibermatrix interface during fiber pull-out: and matrix deformation. Comparison of dissipation energy capabilities through fiber fracture and failure was examined separately for brittle fibers and for ductile wires. Various commercially available brittle fibers were compared in tables and graphs. For brittle fibers, energy dissipation density values as high as 107 J/cm and 45 J/g were found during fiber failure. The ductile wires studied in this paper included stainless steel, aluminum alloys, and superelastic Nitinol. Energy density values as high as 125 J/cm were found for Nitinol, while the other wires had values significantly lower. The energy dissipation mechanism during fiber sliding and fiber pull-out was studied using a micromechanical model. Formulae for the characteristic pull-out length and for the energy dissipation density were derived. Certain simplifying assumptions regarding the correlation between the pull-out length and the characteristic crack spacing were used to derive an upper-bound formula for the energy dissipation density during fiber pull-out in terms of fiber strength and fiber volume fraction. Fiber pull-out energy dissipation density values as high as 555 J/cm were predicted. Concepts for achieving such high energy dissipation through interphase control (fiber coatings, spot-wise adhesion, etc.) are mentioned. Research Professor, Center for Intelligent Material Systems and Structures, member AIAA, ASME, AHS. **Alexander Giacco Professor, Department of Engineering Science and Mechanics, member ASC. Professor and Director, Center for Intelligent Material Systems and Structures, member AIAA, ASME. Experimental results obtained with a Baydur multiorientation fiber-matrix composite are presented at the end of the paper to illustrate the practical applicability of the theoretical concepts. INTRODUCTION One of the main criticisms of high performance fibermatrix material systems is their low impact tolerance and the inability to absorb noticeable amounts of energy before complete failure. This unfavorable behavior has been attributed to the inability of high performance fibers, to undergo "plastic" deformation and to absorb energy before failure. σ