Hillman and Bakis awarded Multidisciplinary Research Seed Grant by College
03/21/2018
UNIVERSITY PARK, Pa. – Computational mechanics is a powerful tool for accelerating the development of new structural materials by allowing the properties of the material to be predicted before the material even exists. It can predict virtually all metrics of interest, but it is only reliable if properly validated.
Effective numerical methods for actually simulating the three-dimensional fracture process of composites remain elusive, but a pair of Penn State researchers have received a Multidisciplinary Research Seed Grant from the College of Engineering to develop and experimentally validate a novel computational model that could simulate and predict complex fracture behavior of heterogeneous materials.
“Our goal is to combine two completely different particle methods to simulate three-dimensional fracture in a composite material under a single, robust physics-based framework where the advantages of each can be leveraged,” said Michael Hillman, principal investigator (PI) and L. Robert and Mary L. Kimball Assistant Professor of Civil and Environmental Engineering. “The unified method will be validated against experimental measurements of stiffness, fracture toughness and fracture surface roughness. If successful, this new model could serve as the foundation for future efforts aimed at manufacturing more complicated multiscale composites with improved stiffness and strength for numerous applications.”
For the project, Hillman will focus on one particular material, a nanosilica (NS)-filled epoxy. Epoxy is increasingly being used for manufacturing high-strength fiber reinforced (FRP) polymer composites for aerospace and civil infrastructure applications because of its high strength and elastic modulus (stiffness), and the addition of NS fillers has been recently shown to further increase their modulus and resistance to sudden fracture.
Approximately 60 percent of the volume of FRP composites consists of relatively stiff fibers, such as carbon or glass, which carry most of the load. The epoxy matrix retains the overall shape of the structure and supports the fibers so they may carry compression and shear loads. Since fiber composites are usually made in laminar arrangements, the resistance of the laminate to delamination during impact loading and the buckling resistance of the fibers under axial compression loading is controlled by fracture toughness and modulus of the polymer matrix.
Preliminary evaluation of the modulus and fracture behavior of epoxy with NS fillers has been conducted in the Penn State Composites Materials Lab under the supervision of Charles Bakis, co-PI and Distinguished Professor of Engineering Science and Mechanics. The results have shown that modulus and fracture toughness both increase with NS fillers, and the augmentation of fracture toughness is proportional to surface roughness induced by the NS particle pullout and crack deflection.
“Normally, when a crack grows through epoxy, it has no obstacles and it grows on a very smooth plane, similar to how glass breaks,” said Bakis. “However, with the addition of nanosilica to the epoxy, the crack is very rough and uneven because the nanosilica makes it more difficult for the crack to work its way through the material, thus, increasing the fracture toughness.”
Using the experimental data provided by Bakis, Hillman intends to reproduce the metrics of modulus, fracture toughness and fracture surface roughness to provide better understanding of the effects of fracture toughness of the interface between particles and the epoxy, the cause of crack surface deflection, the relationship between crack deflection and surface roughness and the extent to which fracture toughness increases based on the concentration of NS particles.
Hillman’s proposed computational method will potentially be able to answer the questions regarding the toughening mechanisms behind NS fillers much more quickly versus an experimental trial-and-error approach which would rely on large numbers of specimens and material parameters.
“Even though we want to simulate fracture in a smart, effective way and understand what’s driving the fracture behavior in this particular application, in looking at the bigger picture, we want to be able to simulate complex fractures for other materials that can be used for broader applications,” said Hillman.
Bakis added, “Future generations of FRP composites with improved stiffness and toughness will open the door to further weight reduction in composite structures used in applications where weight is expensive in terms of energy efficiency, payload and range, such as ultra-high efficiency automobiles and aircraft.”
Established in 2014, the Multidisciplinary Research Seed Grant program supports research that will increase the competitiveness of faculty in attracting high-impact multidisciplinary and center-level research funding from the state and federal government, industry or foundations.
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Michael Hillman, L. Robert and Mary L. Kimball Assistant Professor of Civil and Environmental Engineering
Charles E. Bakis, Distinguished Professor of Engineering Science and Mechanics
Epoxy fracture specimen with a starter notch (on the bottom), in the 3-point bend test fixture. The specimen is shaped and tested according to ASTM test standard D5045.
