RESEARCH
Soft Functional Materials
A fundamental interest of our research group is the interaction between electromagnetic fields and soft field-responsive matter.
Dielectric elastomers. Dielectric elastomers are a promising class of soft electroactive materials that have attracted significant attention because of their extreme voltage-induced mechanical deformations, fast response times, high specific energy, and strong electro-mechanical conversion efficiency. Our current work in this space is focused on investigating and fundamentally understanding interesting new nonlinear dynamic features in dielectric elastomer membranes, including multiple stable solution branches and jump phenomena in the nonlinear frequency response. Recent results have uncovered large-amplitude nonlinear dynamic responses near resonances not reported in prior analyses, and explored entire branches of nonlinear dynamic response that were missed in prior investigations.
Soft piezoelectrics. The most common materials with intrinsic piezoelectricity are stiff, brittle ceramics such as lead zirconate titanate and barium titanate, which are inherently confined to small-deformation applications. To expand the performance space of piezoelectric materials, soft piezoelectric composites (SPCs) consisting of an ultra-stretchable, rubbery matrix interspersed with carbon nanotubes and micron-sized piezoelectric filler particles have been developed. Although SPCs show great promise for next-generation energy harvesting applications, they remain an emerging and relatively unexplored class of materials. Our current work in this space is focused on holistically exploring SPCs across the full mechanics spectrum, from composite fabrication and electro-mechanical property characterization to large-strain constitutive modeling and thin-membrane dynamics.
Soft multiferroics. Soft multiferroic magnetoelectric materials that couple ferroelectricity and ferromagnetism are an emerging class of stretchable smart materials. The ability to electrically control their magnetic response, or magnetically control their electrical response, make soft multiferroics compelling candidates for disruptive technologies ranging from multi-modal energy harvesters and electrically tunable microwave devices to soft robots and stretchable electronics. Our current research in this area is focused on developing a continuum-scale framework for modeling this class of materials, with an emphasis on thermodynamically consistent finite-strain constitutive models and representative free energy functions that appropriately characterize fully nonlinear magneto-electro-elastic coupling.
Collaborators. Dr. Chris Cooley, Dept. of Mechanical Engineering, Oakland University. Dr. Paul Kladitis, Multifunctional Structures and Materials Group, University of Dayton Research Institute.
Soft Additively Manufactured Materials
DLP elastomers. Digital light processing (DLP) additive manufacturing (AM) is a recent development in 3D printing where full layers of photo-curable polymers are irradiated and cured with projected ultraviolet (UV) light to create a three-dimensional part layer-by-layer. Recent breakthroughs in polymer chemistry have led to a growing number of ultra-stretchable UV-curable elastomeric materials, some with self-healing capabilities. Coupled with the practical manufacturing advantages of DLP AM, these novel UV-curable elastomers are compelling candidates for numerous exciting applications, ranging from regenerative medicine (e.g., vascular grafts and tissue scaffolds) to soft robotics. Our current experimental work in this space is focused on developing a fundamental understanding of the mechanical behavior (i.e., deformation and fracture) of emerging UV-curable elastomeric materials over a broad range of loading conditions. Our current modeling work in this space is focused on fundamentally understanding the mechanics and dynamics of prototype pneumatically-actuated soft robotic actuators, with an emphasis on dynamically-induced (i.e., inertia-driven) snap-through instabilities.
FDM elastomers. Soft, ultra-stretchable thermoplastic elastomers have recently became available for use with desktop, fused deposition modeling (FDM) printers. However, the effects of additive manufacturing process parameters on final mechanical properties are presently not well-known for this class of materials, making predictive modeling and product design difficult. Our current work in this space is focused on a design of experiments investigation of a commercial elastomeric material that the manufacturer claims to have up to 580% strain at fracture. Within this investigation, three factors -- extrusion temperature, layer height, and printer model -- have been selected as the independent variables. Mechanical properties will be extracted as dependent variables based on quasi-static uniaxial tension testing, and primary statistical results will be assessed based on an analysis of variance.
Collaborators. Dr. Chris Cooley, Dept. of Mechanical Engineering, Oakland University. Dr. Tim Osborn and Ms. Ally Cox, Additive Manufacturing Technology Development Group, University of Dayton Research Institute. Dr. Chris Crouse, Polymer and Responsive Materials Team, Air Force Research Laboratory. Dr. Tim Reissman, EMPOWER Lab, University of Dayton.
Ductile Fracture of Aerospace Metals
Generally, continuum-scale ductile fracture models are calibrated using standard mechanical tests, e.g., notched axisymmetric (round), plane stress (thin), and plane strain (thick) specimens subjected to tensile loading. However, these standard tests are only able to capture a limited window of stress states, leaving potentially important regions of the fracture loci of aerospace metals unpopulated with experimental data. For instance, recent investigations have revealed (a) the potentially unanticipated importance of the positive triaxiality (compressive) region of Lode-triaxiality stress space and (b) the need to experimentally revisit previous interpretations of the “cut-off” value of the triaxiality. To address this compelling research opportunity, we are exploring novel concepts for new experiments that investigate ductile fracture at positive (compressive) triaxialities. These novel experiments, currently being developed using "virtual" or simulation-aided design, are predicated on a common theme: superposing hydrostatic pressure on conventional "plane stress" mechanical tests to achieve unprecedented and previously inaccessible states of stress at fracture.
Collaborators. Dr. Amos Gilat and Dr. Jeremy Seidt, Dynamic Mechanics of Materials Lab, The Ohio State University.
Plastic Deformation and Ductile Fracture of Additively Manufactured Ti-6Al-4V
At present, the impact physics, structural integrity, and ductile fracture of additively manufactured metals remain poorly understood relative to wrought or as-cast metals, particularly under loading conditions representative of domestic and foreign object damage (DOD/FOD). This, in turn, has hindered the development of predictive computational tools for the simulation-aided design, optimization, validation, virtual testing, and certification of additively manufactured components and structures for DOD/FOD impact survival. The overarching goal of this project is to develop a predictive modeling tool to facilitate simulation-aided design, optimization, analysis, and certification of additively manufactured Ti-6Al-4V titanium alloy structures.
Plastic deformation. The plastic deformation of laser powder bed fusion (LPBF) additively manufactured Ti-6Al-4V is being experimentally characterized under various loading conditions, strain rates, and build orientations. A new, efficient finite element model updating approach for obtaining the post-necking hardening behavior is being developed and benchmarked. The resulting test data will be used to calibrate various parameterized and/or tabulated constitutive models, starting with rate-independent J2 flow theory and successively building in complexity to rate-dependent anisotropic plasticity with tension-compression-shear yield asymmetry.
Ductile fracture. Modeling and simulating the impact physics of laser powder bed fusion (LPBF) additively manufactured Ti-6Al-4V requires a robust ductile fracture model capable of predicting multiple potential failure modes, which are intimately related to the impact conditions and subsequent state of stress at impact. Many modern continuum-scale (phenomenological) ductile fracture models (for metals) regard the equivalent plastic strain at fracture as a function of the state of stress (stress triaxiality and Lode parameter), whose three-dimensional representation is coined the failure locus (or failure surface). The stress-state-dependent ductile fracture of LPBF additively manufactured Ti-6Al-4V is currently being experimentally characterized using notched axisymmetric (round) specimens in uniaxial tension and thin-walled tube specimens in pure torsion. The resulting test data will be used to calibrate modern ductile fracture models for LPBF Ti-6Al-4V, both tabulated (e.g., *MAT_224 in LS-DYNA) and parameterized (e.g., Mohr-Coulomb and Hosford-Coulomb).
Collaborators. Dr. Luke Sheridan and Dr. Dino Celli, Turbine Engine Fatigue Facility, Air Force Research Laboratory (AFRL/RQTI).
Sponsors
We thank the following sponsors for supporting our research:
