A Material Point Method for Elastoplasticity with Ductile Fracture and Frictional Contact

Simulating physical materials with dynamic movements to photo-realistic resolution has always been one of the most crucial and challenging topics in Computer Graphics. This dissertation considers large-strain elastoplasticity theory applied to the low-to-medium stiffness regime, with topological changes and codimensional objects incorporated. We introduce improvements to the Material Point Method (MPM) for two particular objectives, simulating fracturing ductile materials and incorporation of MPM and Lagrangian Finite Element Method (FEM).Our first contribution, simulating ductile fracture, utilizes traditional particle-based MPM [SSC13, SCS94] as well as the Lagrangian energy formulation of [JSS15] which uses a tetrahedron mesh, rather than particle-based estimation of the deformation gradient and potential energy. We model failure and fracture via elastoplasticity with damage. The material is elastic until its deformation exceeds a Rankine or von Mises yield condition. At that point, we use a softening model that shrinks the yield surface until it reaches the damage thresh- old. Once damaged, the material Lam ́e coefficients are modified to represent failed material. This approach to simulating ductile fracture with MPM is successful, as MPM naturally captures the topological changes coming from the fracture. However, rendering the crack surfaces can be challenging. We design a novel visualization technique dedicated to rendering the material’s boundary and its intersection with the evolving crack surfaces. Our approach uses a simple and efficient element splitting strategy for tetrahedron meshes to create crack surfaces. It employs an extrapolation technique based on the MPM simulation. For traditional particle-based MPM, we use an initial Delaunay tetrahedralization to connect randomly sampled MPM particles. Our visualization technique is a post-process and can run after the MPM simulation for efficiency. We demonstrate our method with several challenging simulations of ductile failure with considerable and persistent self-contact and applications with thermomechanical models for baking and cooking.Our second contribution, hybrid MPM–Lagrangian-FEM, aims to simulate elastic objects like hair, rubber, and soft tissues. It utilizes a Lagrangian mesh for internal force computation and a Eulerian grid for self-collision, as well as coupling with external materials. While recent MPM techniques allow for natural simulation of hyperelastic materials represented with Lagrangian meshes, they utilize an updated Lagrangian discretization and use the Eulerian grid degrees of freedom to take variations of the potential energy. It often coarsens the degrees of freedom of the Lagrangian mesh and can lead to artifacts. We develop a hybrid approach that retains Lagrangian degrees of freedom while still allowing for natural coupling with other materials simulated with traditional MPM, e.g., sand, snow, etc. Furthermore, while recent MPM advances allow for resolution of frictional contact with codimensional simulation of hyperelasticity, they do not generalize to the case of volumetric materials. We show that our hybrid approach resolves these issues. We demonstrate the efficacy of our technique with examples that involve elastic soft tissues coupled with kinematic skeletons, extreme deformation, and coupling with various elastoplastic materials. Our approach also naturally allows for two-way rigid body coupling

Infrared Study of Recluse Spider Silk

We want to gain insight into the composition and structure of spider silk to discover the origin of its extremely high mechanical properties. We are especially interested in the organization of the crystalline ß-sheets that are expected to contribute to the high strength of the silk from the recluse spider, Loxosceles laeta. The recluse spider produces a thin, ribbon-like silk, which has a unique geometry amongst arachnids. We measure the silk\u27s optical properties, particularly the infrared-active vibrations. Broadband infrared transmission spectra were collected in the spectral range between 600 cm−1 and 4000 cm−1, with light polarized parallel and perpendicular to the long axis of the silk. The infrared vibrational modes are fit with Lorentzian and Voigt functions. The vibrational modes are assigned to specific structures and electronic bonds in the silk. We find that at least one-fourth of the recluse spider silk consists of crystalline ß-sheets

Benefits of Continuously Spaced Energies and Scanned Beams for Electron Bolus Conformal Therapy for Left-Side Post-Mastectomy Chest Wall

Purpose: This study assessed the benefits to the heart and lung of using scanned electron beams and continuous energy spacing (ΔR90=0.1 cm) for left-side post-mastectomy radiotherapy (PMRT) patients previously treated with volumetric modulated arc therapy (VMAT). Such beams offer a sharper distal falloff (R90-10) than do currently available scattered, discrete energy beams, which increases sparing of healthy organs distal to the target. Methods: Seven left-side PMRT patients previously treated with VMAT at the Mary Bird Perkins Cancer Center were planned in this study. The patients were divided into two sets; Patient Sets 1 (three patients) and 2 (four patients) used one and two fields, respectively, to plan chest wall irradiation. Four and five intensity modulated bolus electron conformal therapy (IM-BECT) plans, respectively, were created per patient using combinations of scattered/scanned and discrete/continuous energy beam data. For Patient Set 2 the inferior edge of the upper field was feathered to match the penumbra of the superior edge of the lower field. Dose distributions and dose volume histograms (DVHs) were used to evaluate plan quality, to calculate physical dose metrics for the target, heart, and lung, and to calculate the biological metrics, normal tissue complication probability (NTCP), and secondary cancer complication probability (SCCP) for the heart and lung. Results: Scanned and/or continuous energy electron beams showed patient-dependent, sometimes significant reductions in both physical and biological dose metrics for the heart and lung. For patient CW1, as compared to scattered, discrete energy beams, scanned, continuous energy beams reduced V22.5Gy from 10.4% to 2.3%, V30Gy from 3.1% to 0.1%, Dmean from 8.3 Gy to 4.3 Gy, and NTCP from 0.4% to 0.1% for the heart. The comparison reduced V20Gy from 17.2% to 10.7% and SCCPlin from 14.4% to 8.7% for the lungs. Such reductions tended to increase as R90 increased. Compared to VMAT, IM-BECT plans with scanned beams on average reduced V22.5Gy from 9.4% to 3.4%, Dmean from 9.3 Gy to 4.4 Gy, and NTCP from 1.0% to 0.2% for heart and Dmean from 8.8 Gy to 7.1 Gy, NTCP from 2.8% to 0.2%, and SCCPlin from 15.1% to 10.2% for lung. Conclusion: Scanned and/or continuous energy electron beam treatment plans showed reduced physical and biological dose metrics for heart and lung compared to scattered, discrete energy beams. These improvements were patient dependent, although patients requiring higher energy beam(s) tended to show the greatest benefits

Self-Assurance and Literature

Recognizing our faults and failures is no voluntary task. We are cautious, almost reluctant, to do so as those shortcomings cast a shadow over the ideal lives we would like to have. Our inability to confront our problems leads us to follow the lives of characters in books and stories whose flaws are apparent to us – characters who struggle valiantly against or fall miserably to the challenges they face. From the epic Beowulf, where the god-like hero Beowulf fights glorious battles, to Chaucer’s The Canterbury Tales, where common folk embark on a pilgrimage, we are fascinated by the exposure of the characters’ flaws and vulnerabilities. This disclosure, shared by both stories of humans with unimaginable abilities and smarts and those of characters that are as ordinary as ourselves, allows us to indulge in self-assurance in the midst of our lives’ own difficulties. Though we initially read stories of these seemingly different personalities for their characteristic qualities, our underlying motive is our desire for self-assurance in the midst of our lives’ own difficulties

Infrared and Ultraviolet Observations of VIRGOHI 21 and NGC 4254's Outer Disk

We present the results of Spitzer and Galex observations of gas/dust and star formation activities in the extreme outer disk of Virgo galaxy NGC 4254 and its surrounding regions. These observations were motivated in part by the potential existence of a “dark galaxy” in the vicinity. In the intergalactic VIRGOHI 21 region where the free-floating HI gas is found, neither UV nor mid-IR shows corresponding emission, thus providing stringent upper limits on the stellar mass and star formation rate in these clouds.On the other hand, we find clearly discernible excess ultraviolet emission in parts of the extended disk of NGC 4254, which is yet unseen in the optical and infrared. These UV emission appears different from the so-called “XUV disks” of other nearby galaxies in both their distribution pattern and physical origin, which we suggest is directly related to the gas concentration of VIRGOHI 21

Ring Current Proton Decay Timescales Derived from Van Allen Probe Observations

The Earth's ring current is highly dynamic and is strongly influenced by the solar wind. The ring current alters the planet's magnetic field, defining geomagnetic storms. In this study, we investigate the decay timescales of ring current protons using observations from the Van Allen Probes. Since proton fluxes typically exhibit exponential decay after big storms, the decay time scales are calculated by performing linear regression on the logarithm of the fluxes. We found that in the central region of the ring current, proton decay timescales generally increase with increasing energies and increasing L-shells. The ~10s keV proton decay timescales are about a few days, while the ~100 keV proton decay time scale is about ~10 days, and protons of 269 keV have decay timescales up to ~118 days. These findings provide valuable insights into the ring current dynamics and can contribute to the development of more accurate ring current models.Comment: 8 pages, 7 figure

Ellis Raincoat

Designed for spring, Rainflower draws inspiration from the styles of ancient China: loose fits, layering, and pleats, with decorative as well as functional ties, reminiscent of the past. The collection was designed with the intention of layering as a key component, so pieces could be added or removed for versatility as the weather changes. In creating the collection, a theme of nostalgia and never growing up was a key influence, with a storybook aesthetic being the style