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2018 Research Highlight: Determination of Rate Dependent Mode II Traction Separation Laws for Composite Interphases

Determination of Rate Dependent Mode II Traction Separation Laws for Composite Interphases

CMEDE Researchers
Professor John W. Gillespie, Jr.  
University of Delaware
Dr. Daniel J. O’Brien
U.S. Army Research Laboratory
Dr. Sandeep Tamrakar
Ford Motor Company

Lightweight composite materials offer superior specific stiffness, strength and energy dissipation during impact events. Under dynamic loading, energy is dissipated through various strain rate dependent micromechanical mechanisms such as fiber breakage, interfacial debonding and frictional sliding and localized matrix plasticity and cracking. The interaction between fiber and matrix at the interface determines the overall energy absorption capability of the composite material. Our Materials by Design strategy requires accurate characterization of rate dependent properties of the fiber, matrix and composite interphases to validate our lower length scale molecular dynamic models as well as serve as input into our higher length scale computational models to predict continuum properties. In this study, our focus is on the methodology to characterize rate dependent Mode II Traction laws for composite interphases.

Our experimental studies use a model composite material comprised of a single S-2 glass fiber (10 micron) coated with 3-glycidoxypropyl) trimethoxy silane coupling agent and epoxy film former sizing. The matrix is a DER 353 epoxy with a PACM-20 curing agent. During processing the interphase between the fiber and matrix forms through diffusion and reaction with a thickness in the range of 10-100nm.

The study involved development of novel experimental methods. A micromechanical test method uses a microdroplet test specimen. A typical epoxy droplet size is 100 microns. The average interfacial shear strength (IFSS) can be measured at loading rates spanning six decades of magnitude. At higher loading rates, a tensile Hopkinson bar has been designed that can load the interface in the range of 1-10 m/s. Crack initiation at the interface was studied by modifying the fiber surface through the deposition of an electrically percolating carbon nanotube network using electrophoretic deposition method. Experimental results and post-failure inspection of the fiber matrix interface showed that the test method is effective in measuring high rate interface properties of composites.

A methodology using Finite Element (FE) modeling of the experiments is established to uniquely determine rate dependent Mode II cohesive traction laws of the composite interfaces. To accurately model the microdroplet experiments,  rate dependent resin properties were measured over a wide range of strain rates (0.001/s to 12,000/s) and strain range up to 70%. For DER 353 epoxy resin, yield stress increased significantly with applied strain rate and exhibited a bi-linear dependency. Thermal softening was observed under high strain rates at large strains due to adiabatic heating. A constitutive model was developed and used in the modeling of experiments. The simulation allows partitioning of energy absorbing mechanisms (interface and resin plasticity) and prediction of cohesive zone sizes for all loading rates. Incorporating strain rate dependent resin plasticity ensures energy absorption during interface softening is accurate. A minimum of three droplet sizes ranging from 75 to 125 micron are tested at each loading rate and used to validate the rate dependent traction law parameters.