Session: 12-28-01: Functional Soft Composites - Design, Mechanics, and Manufacturing

Paper Number: 149333

149333 - Utilizing Mechanophores to Quantify Stress in Polymer Composites 

Polymer matrix composites (PMCs) combine the toughness of thermoset polymer with a stiff reinforcement phase which produces a material that is both stronger and tougher than either material individually. This strengthening requires a transfer of stresses from the matrix into the reinforcement phase. Due to non-uniform straining during mechanical deformation, significant stress concentrations develop within the matrix near the interface. To produce better composite designs and understand failure mechanisms, developing methods to measure this stress concentration is crucial. One recent approach to experimentally observe and quantify the magnitude of these stress concentrations are mechanophores. Mechanophores are a type of stimuli responsive molecules which have a labile bond in the structure, allowing for a change in color, fluorescence, or catalytic abilities after breaking. In this study, we utilize nitro spiropyran (SPN), a common mechanophore that is relatively easy to synthesize as a probe to highlight stress concentrations in loaded polymer composites. First, we establish a protocol to calibrate the fluorescence intensity of the mechanophore with the expected maximum principal stress values calculated through finite element analysis (FEA). Next, we apply this calibration to several composite geometries ranging from variously shaped reinforcing particles to varying materials and diameters of reinforcing fibers. Additionally, the interactions of the stress fields that arise in the matrix between nearby particles in a loaded composite are evaluated. While FEA can work well for simple geometries, it becomes computationally expensive as the composite geometry becomes complicated. By embedding SPN into a polydimethylsiloxane (PDMS) matrix, we obtain force responsive polymer networks that enable direct visualization of the location and magnitude of stress localizations in mechanically loaded systems. Using this SPN PDMS, we fabricated model PMCs using silane functionalized spherical silica particles. Samples were loaded in uniaxial tension over a laser scanning confocal microscope and in situ fluorescence images were obtained. The system was modeled using FEA in a simple 2D model. By correlating fluorescence intensity data from our micrographs to the maximum principal stress values calculated by FEA, we created a calibration curve that allows us to measure the stresses in a PMC based only on fluorescence intensity. This system was validated, and the versatility of this technique was demonstrated by testing PMCs with differing geometries and complexities which are difficult to model in FEA. Our methodology can be used to study stress concentrations in high complexity composites and help to predict material properties and optimize new composite designs.

Presenting Author: Chelsea Davis University of Delaware

Presenting Author Biography: Dr. Chelsea Davis joined the faculty of the University of Delaware as an associate professor in the Departments of Mechanical Engineering and Materials Science and Engineering in 2023. Previously, she worked in the School of Materials Engineering at Purdue University as an assistant professor where she established the Illuminating Interfacial Mechanics Lab which focuses on the development of micromechanical characterization tools to investigate the interfacial and surface properties of soft materials. Dr. Davis obtained a B.S. in Textile Engineering from North Carolina State University in 2005 and an M.S. and Ph.D. in Polymer Science and Engineering from the University of Massachusetts Amherst in 2007 and 2012, respectively. Dr. Davis was a Michelin Postdoctoral Research Fellow at the ESPCI in Paris (2012-2013) and then an NRC Postdoctoral Fellow in the Polymers and Complex Fluids Group at the National Institute of Standards and Technology (2013-2016). Dr. Davis’s work has been recognized through the NSF CAREER Award and the Adhesion Society Early Career Scientist Award.

Authors:

Jared Gohl Purdue University
Nazmul Haque Purdue University
Chelsea Davis University of Delaware