Kevin Hart, P.E., Ph.D.
Assistant Professor
- Milwaukee WI UNITED STATES
- Allen Bradley Hall of Science: S229
- Mechanical Engineering
Dr. Kevin Hart's research interests lie in additive manufacturing of polymeric materials, as well as advanced, multi-functional materials.
Education, Licensure and Certification
Ph.D.
Aerospace Engineering
University of Illinois at Urbana-Champaign
2016
M.S.
Aerospace Engineering
University of Illinois at Urbana-Champaign
2012
B.S.
Engineering Mechanics and Astronautics
University of Wisconsin-Madison
2010
PE
Licensed Professional Engineer
Wisconsin
Biography
Areas of Expertise
Accomplishments
Aerospace Engineering Alumni Advisory Board Fellowship
2015
American Society of Composite PhD Research Scholarship Grant
2014-15
Social
Patents
Self-Healing Composite Materials and Micro-Vascular Composites For Forming The Materials
US20130189888A1
JF Patrick, KR Hart, BP Krull, NR Sottos, JS Moore, SR White
Method of Making a Self-Healing Composite System
US20150328848A1
JF Patrick, KR Hart, BP Krull, NR Sottos, JS Moore, SR White
Integral printed shell for high strength 3D-printed polymer parts
Provisional patents US 62/885,554 and US 62/885,877
RM Dunn, KR Hart, ED Wetzel
High Strength 3D Printed Polymer Structures and Methods of Formation Patent Application
Publication US20220370206A1
.
Multi-Material Polymer Filament for Three-Dimensional Printing
Patent Application Publication US20220033998A1
.
Research Grants
Composite and Hybrid Materials Branch Seedling Grant
US Army Research Laboratory. Aberdeen Proving Ground, MD.
2017
Scale-up of Dual Material Filament for Field-Use Printed Parts
DoD Harnessing Emerging Research Opportunities to Empower Soldiers (HEROES) Commercialization Development Grant
2022-23
Selected Publications
Evaluation of a Vascularized, Self-Healing Structure Fabricated via Material Extrusion
Journal of Smart materials and StructuresTuricek, J.; Kowal, E.; Holland, K.; Kalchik, D.; Stowe, J.; Hart, K
2022
Material extrusion is a versatile 3D-printing platform for building complex one-off designs. However, the mechanical properties of parts printed using material extrusion are limited by the weak bonding between successive layers of the print, causing premature failure at these critical locations. In this work, an additively manufactured component is crafted which incorporates internal vascular channels capable of autonomously delivering a one-part healing agent to the site of interlaminar damage, when and where it occurs thereby restoring the base structure. The effectiveness of fracture toughness restoration was investigated for various healing times and healing agents. Healing efficiencies of greater than 100% are reported for experimental-type samples using acetone as the healing agent while control specimens using a non-solvent agent demonstrated no recovery. Fractography of damaged surfaces via optical imaging and scanning electron microscopy revealed multiple healing mechanisms that are discussed herein. Lastly, biological analogies and the viability of our design in application are discussed.
Tough, Additively Manufactured Structures Fabricated with Dual-Thermoplastic Filaments
Advanced Engineering MaterialsHart, K.R.; Dunn, R.M.; Wetzel, E.D
2020
Fused filament fabrication (FFF) is the most common additive manufacturing technology, but parts fabricated using FFF lack sufficient mechanical integrity for most engineering applications. Herein, a dual material (DM) filament comprising acrylonitrile butadiene styrene (ABS) with a star-shaped polycarbonate (PC) core is fabricated using a novel thermal draw process. This DM filament is then used as feedstock in a conventional FFF printer to create 3D solid bodies with a composite ABS/PC meso-structure. Subjecting these parts to annealing temperatures intermediate between the glass-transition temperatures of ABS and PC produces a solid body with ductility comparable to injection-molded ABS and fracture toughness values 15x higher than comparable as-printed ABS structures. The PC skeleton of specimens fabricated using the DM filament resists creep and polymer relaxation to maintain accurate part geometry during annealing. This novel DM filament can revolutionize additive manufacturing allowing low-cost printers to produce parts with mechanical properties competitive with injection-molded plastics.
Increased Fracture Toughness of Additively Manufactured Semi-Crystalline Thermoplastics via Thermal Annealing
PolymerHart, K.R.; Dunn, R.M.; Wetzel, E.D.
2020
Polymeric components manufactured via freeform fabrication (FFF) typically have poor inter-laminar toughness resulting from incomplete bonding across layers during production. Here we study the effect of printing and post-processing on the inter-laminar toughness of additively manufactured semi-crystalline (poly-lactide (PLA)) structures. Specimens were subject to post-print thermal annealing to promote inter-laminar bonding, while post-annealing quenching rates were chosen to vary the induced degree of crystallinity in the final structure, as characterized via dynamic scanning calorimetry (DSC). Critical elastic-plastic strain energy release rates (JIc) of annealed samples were evaluated using the single edge notched bend (SENB) geometry and post-testing fractography. The results show that as-printed PLA adopts an amorphous character with good inter-laminar toughness and ductility. Post-print annealing can double the toughness via increased interfacial wetting, but only if the material is quenched rapidly to preserve the amorphous character. In contrast, post-print annealing followed by slow cooling results in a semi-crystalline state (≈25% crystallinity) with low fracture toughness and brittle fracture behavior.
Plateau-Rayleigh Instabilities in Pore Networks of Additively Manufactured Polymers: A Modeling Perspective
Materials LettersHernandez-Rivera, E.; Mock, C.M.; Hart, K.R.
2020
Fused Filament Fabrication (FFF) additive manufacturing (AM) is a popular and widespread polymer AM method that provides rapid part production with relatively low cost prototyping. Polymeric structures made through FFF AM typically have internal pore structures which result from the manufacturing process. A Monte Carlo algorithm is implemented to simulate pore morphology evolution from build-path networks to spheroidal pores in AMed post-annealed structures. The model demonstrated that the evolution of these pore networks phenomenologically follows stages associated with the Plateau-Rayleigh instability and that thermal gradients result in migration of the spherical pores, as was observed experimentally.
Nonisothermal Welding in Fused Filament Fabrication
Additive ManufacturingCoasey, K.; Hart, K.R.; Wetzel, E.D.; Edwards, D.; Mackay, M.E.
2020
Fused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material. Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts.
Improving Fracture Strength of Fused Filament Fabrication Parts via Thermal Annealing in a Printed Support Shell
Progress in Additive ManufacturingDunn, R.M.; Hart, K.R.; Wetzel, E.D.
2019
Polymeric structures fabricated using fused filament fabrication (FFF) have limited use in engineering applications as a result of their poor inter-laminar bonding. In this study, we utilize a dual-material print head to encase a low glass transition temperature (Tg) polymer (acrylonitrile butadiene styrene) within a high-Tg shell (polycarbonate). The resulting structure, if annealed at a temperature between the core and shell polymer Tg values, creates a tough interior with high inter-laminar strength while retaining the as-printed three-dimensional geometry of the part. Fracture toughness of annealed, shelled parts was evaluated using single edge notch bend (SENB) fracture specimens and reached values more than 1800% higher than unannealed specimens. Importantly, the annealed specimens exhibited consistent ductile failure and plastic deformation, unlike the as-printed parts which exhibited brittle inter-laminar fracture. Parts with complex geometries are presented to demonstrate geometric stability during annealing and a practical load bearing application.
Increased Fracture Toughness of Additively Manufactured Amorphous Thermoplastics via Thermal Annealing
PolymerHart, K.R.; Dunn, R.M.; Sietins, J. M.; Mock, C.M.H; Mackay, M.E.; Wetzel, E.D.
2018
Polymeric structures fabricated using Fused Filament Fabrication (FFF) suffer from poor inter-laminar fracture toughness. As a result, these materials exhibit only a fraction of the mechanical performance of those manufactured through more traditional means. Here we show that thermal annealing of confined structures manufactured using the FFF technique dramatically increases their inter-laminar toughness. Single Edge Notch Bend (SENB) fracture specimens made from acrylonitrile-butadienestyrene (ABS) feedstock were isothermally heated in a supporting fixture, post-manufacture, across a range of times and temperatures. Fracture testing was then used to quantify the changes in inter-laminar toughness offered by annealing through measurements of the Mode I critical elastic-plastic strain energy release rate, JIc. Under the most aggressive annealing conditions, the inter-laminar toughness increased by more than 2700% over the non-annealed baseline material. Void migration and aggregation during the annealing process was analyzed using X-ray tomography and provides insight into the toughening mechanisms. Time-scales of reptation and polymer mobility at the interface during annealing are also modeled and agree with fracture experiments.
Recycling Meal-ready-to-eat (MRE) pouches into polymer filament for material extrusion additive manufacturing
Hart, K.R.; Frketic, J.B.; Brown, J.R.Additive Manufacturing
2018
Millions of Meals Ready to Eat (MREs) are consumed annually by soldiers around the world. This results in the generation of thousands of tons of residual polymeric packaging waste, which is either left behind in the environment or otherwise recycled or incinerated at a cost of several millions of U.S. dollars per year.
Advancements in distributed recycling technologies now allow for on-demand reconstitution of traditionally
neglected MRE pouch waste into useful appliances via material extrusion additive manufacturing. In this work,
we demonstrate recycling of MRE pouch materials through a combined compounding, filament extrusion, and
fused filament fabrication (FFF) additive manufacturing protocol. Mechanical properties and barrier properties
of additively manufactured structures were evaluated through tensile testing and water vapor transmission
testing, respectively, and found to be comparable to the native pouch materials. Differential Scanning
Calorimetry and Thermogravimetric Analysis of the extruded filament and printed materials were contrasted
with native pouch materials, showing minimal effects of the manufacturing process on critical thermal transitions in the polymer. Economics and viability of on-demand reconstitution using these proposed methods are briefly discussed and reveal the multiple benefits of this recycling process.
Mechanisms and characterization of impact damage in 2D and 3D woven fiber-reinforced composites
Composites Part A: Applied Science and ManufacturingHart, K.R., Chia, P.X., Sheridan, L.E., Wetzel, E.D., Sottos, N.R., White, S.R.
2017
Low velocity impact damage of 2D and 3D woven glass/epoxy composites with the same areal density and material constituents were examined. Characterization of damage for both plate and beam sample geometries was investigated through the collection of high-resolution cross-sectional images after impact. Load and displacement data collected during impact testing reveals that the threshold load to introduce delamination damage is independent of the fabric architecture and is constant across a range of impact energies. Delamination length and opening of 3D woven composites was less than 2D composites impacted at the same energy as a result of suppression of delamination propagation and opening offered by the Z-tow reinforcement of the 3D fabric architecture. The formation of transverse shear cracks was independent of the fabric architecture.
Comparison of Compression-After-Impact and Flexure-After-Impact protocols for 2D and 3D woven fiber-reinforced composites
Composites Part A: Applied Science and ManufacturingHart, K.R., Chia, P.X., Sheridan, L.E., Wetzel, E.D., Sottos, N.R., White, S.R.
2017
Post-impact mechanical response of 2D and 3D woven glass/epoxy composite plates and beams of equivalent areal density are evaluated using both Compression-After-Impact (CAI) and Flexure-After-Impact (FAI) testing protocols. Residual strength and stiffness for CAI and FAI are compared after normalization of impact energy with respect to specimen volume. Post-impact flexural strength and modulus from FAI testing exhibit larger reductions with respect to impact energy in comparison to CAI results. At the largest impact energies tested, FAI testing yields 70% reduction in flexural strength compared to only 20% reduction (in compressive strength). Architecturally, 3D woven composites retain greater post-impact mechanical performance as a result of the through-thickness Z-tow which suppresses delamination growth and opening during impact.
Fracture behavior of additively manufactured acrylonitrile butadiene styrene (ABS) materials
Engineering Fracture MechanicsHart, K. R., Wetzel, E. D.
2017
The effect of layer orientation on the fracture properties of poly(acrylonitrile-butadienestyrene) (ABS) materials fabricated through the fused filament fabrication (FFF) process was explored. Critical elastic-plastic strain energy release rates of single edge notch bend (SENB) specimens with variable crack-tip/laminae orientations were compared. Results show that the inter-laminar fracture toughness (fracture between layers) is approximately one order of magnitude lower than the cross-laminar toughness (fracture through layers) of similarly manufactured parts. Contrasting brittle and ductile fracture behavior is observed for inter-laminar and cross-laminar crack propagation, respectively, demonstrating that the elastic-plastic response of AM ABS parts is governed by the direction of crack propagation within the laminated structure. Fracture surfaces of failed specimens are examined using scanning electron microscopy to show micro- and macro-scale toughening/embrittling mechanisms. Techniques for designing tougher additively manufactured materials based on biological analogies are discussed.
Repeated healing of delamination damage in vascular composites by pressurized delivery of reactive agents
Composites Science and TechnologyHart, K.R., Lankford, S.M., Freund, I.A., Patrick, J.F., Krull, B.P., Wetzel, E.D., Sottos, N.R., White, S.R.
2017
Recurrent self-healing of fracture damage in fiber-reinforced composites was accomplished by incorporating internal vascular networks for repeated delivery of reactive liquid components to an internal delamination. Double cantilever beam specimens containing embedded microvascular channels were repeatedly fractured and healed by pumping individually sequestered epoxy and amine based healing agents to the fracture plane. The effect of various pumping parameters and component delivery ratios on in situ mixing of the healing agents and the resulting healing efficiency is reported. Confocal Raman spectroscopy was used to quantify the extent of mixing of healing agents within the fracture plane. Using an optimized healing agent delivery scheme, ten cycles of fracture and healing were achieved with, on average, 55% and as high as 95%, recovery of the virgin critical strain energy release rate.
Malleable and Recyclable Poly(urea‐urethane) Thermosets bearing Hindered Urea Bonds
Advanced MaterialsZhang, Y., Ying, H., Hart, K.R., Wu, Y., Hsu, A.J., Coppola, A.M., Kim, T.A., Yang, K., Sottos, N.R., White, S.R., Cheng, J.
2016
Poly(urea‐urethane) thermosets containing the 1‐tert‐butylethylurea (TBEU) structure feature a reversible dissociation/association process of their covalent linkages under mild conditions. Unlike conventional thermosets, TBEU‐based poly(urea‐urethane) thermosets maintain their malleability after curing. Under high temperature (100 °C) and applied pressure (300 kPa), ground TBEU thermoset powder can be remolded to bulk after 20 min.
