Kevin N. Long

Photo-origami—Bending and folding polymers with light

Photo-origami uses the dynamic control of the molecular architecture of a polymer by a combination of mechanical and non-contact optical stimuli to design and program spatially and temporally variable mechanical and optical fields into a material. The fields are essentially actuators, embedded in the material at molecular resolution, designed to enable controllable, sequenced, macroscopic bending and folding to create three-dimensional material structures. Here, we demonstrate, through a combination of theory, simulation-based design, synthesis, and experiment, the operative phenomena and capabilities of photo-origami that highlight its potential as a powerful, and potentially manufacturable, approach to create three-dimensional material structures.

Light-induced stress relief to improve flaw tolerance in network polymers

We demonstrate the ability to use photoactivated stress relaxation to improve flaw tolerance in network polymers. Unlike most self-healing polymers, which effectively close flaws by locally introducing healing agents (such as uncured resins), here light is used to relax elevated stresses around a flaw before it reaches a critical state, which reduces the threat that the flaw poses to the structural integrity of the material. In this study, we fabricate specimens with well-defined flaws and uniaxially stretch them to failure. By irradiating the specimens with UV light (365 nm) before failure, the nominal strain at failure is increased by 70% and the corresponding nominal stress is increased by 30% compared with nonirradiated specimens. To better understand the phenomena that occur at the multiaxial stress state at the flaw, we model the photomechanics using a recently developed finite element approach that accurately describes the light propagation, photochemistry, radical-induced network evolution, and th...

Agile software development in an earned value world: a survival guide

Agile methodologies are current best practice in software development. They are favored for, among other reasons, preventing premature optimization by taking a somewhat short-term focus, and allowing frequent replans/reprioritizations of upcoming development work based on recent results and current backlog. At the same time, funding agencies prescribe earned value management accounting for large projects which, these days, inevitably include substantial software components. Earned Value approaches emphasize a more comprehensive and typically longer-range plan, and tend to characterize frequent replans and reprioritizations as indicative of problems. Here we describe the planning, execution and reporting framework used by the LSST Data Management team, that navigates these opposite tensions.

Using Sampling Moiré to Extract Displacement Information from X-Ray Images of Molten Salt Batteries

Full-field axial deformation within molten-salt batteries was measured using x-ray imaging with a sampling moire technique. This method worked for in situ testing of the batteries because of the inherent grid pattern of the battery layers when imaged with x-rays. High-speed x-ray imaging acquired movies of the layer deformation during battery activation. Numerical validation of the technique, as implemented in this paper, was done using synthetic and numerically shifted images. Typical results of a battery are shown for one test. Ongoing work on validation and more test results are in progress.

Thermomechanical indentation of shape memory polymers.

Shape memory polymers (SMPs) are receiving increasing attention because of their ability to store a temporary shape for a prescribed period of time, and then when subjected to an environmental stimulus, recover an original programmed shape. They are attractive candidates for a wide range of applications in microsystems, biomedical devices, deployable aerospace structures, and morphing structures. In this paper we investigate the thermomechanical behavior of shape memory polymers due to instrumented indentation, a loading/deformation scenario that represents complex multiaxial deformation. The SMP sample is indented using a spherical indenter at a temperature T1 (>Tg). The temperature is then lowered to T2 (g) while the indenter is kept in place. After removal of the indenter at T2, an indentation impression exists. Shape memory is then activated by increasing the temperature to T1 (>Tg); during free recovery the indentation impression disappears and the surface of the SMP recovers to its original profile. A recently-developed three-dimensional finite deformation constitutive model for the thermomechanical behavior of SMPs is then used with the finite element method to simulate this process. Measurement and simulation results are compared for cases of free and constrained recovery and good agreement is obtained, suggesting the appropriateness of the simulation approach for complex multiaxial loading/deformations that are likely to occur in applications.

A multiscale study of damage in elastomeric syntactic foams

Damage mechanisms in elastomeric syntactic foams filled with glass microballoons (GMB) and resulting effects on the macroscale elastic constants have been investigated. Direct numerical simulations of the material microstructure, composite theory analyses, and uniaxial compression tests across a range of filler volume fractions were conducted. The room temperature and elastic behavior of composites with undamaged, fully debonded, and fully crushed GMBs were investigated for syntactic foams with a polydimethylsiloxane matrix. Good agreement was obtained between numerical studies, composite theory, and experiments. Debonding was studied via finite element models due to the difficulty of isolating this damage mechanism experimentally. The predictions indicate that the bulk modulus is insensitive to the state of debonding at low-GMB-volume fractions but is dramatically reduced if GMBs are crushed. The shear behavior is affected by both debonding and crush damage mechanisms. The acute sensitivity of the bulk modulus to crushed GMBs is further studied in simulations in which only a fraction of GMBs are crushed. We find that the composite bulk modulus drops severely even when just a small fraction of GMBs are crushed. Various material parameters such as GMB wall thickness, volume fraction, and minimum balloon spacing are also investigated, and they show that the results presented here are general and apply to a wide range of microstructure and GMB filler properties.

Constitutive Model for Photo-Mechanical Behaviors of Photo-Induced Shape Memory Polymers

Light-activated polymers are an exciting class of materials that respond mechanically when irradiated at particular wavelengths. Recent demonstrations include two novel polymers developed by Scott et al (2006) and Lendlein et al (2005). In these polymers, photochemistry alters the microstructure of the cross-linked polymer network, which is further translated as light-induced deformation and when properly used light-induced shape memory effect. In this work, we develop a model framework to simulate the photomechanical response of light-activated polymer systems. This framework breaks down the observed macroscopic photomechanical phenomenon into four coupled sets of underlying physics, which occur throughout the material during irradiation and mechanical deformation. In the context of this framework, a basic photomechanical phenomenon involves simultaneously modeling photophysics, photochemistry, chemomechanical coupling, and mechanical behavior. Furthermore, network alteration are accounted for through the parallel decomposition of the cross-linked network into two components, an original network and a photochemically altered network, which allows to capture the observed photomechanical behaviors demonstrated in these materials. One of the principal strengths of this model framework is its generality as it can be applied to light activated polymer systems with fundamentally different of photophysics, photochemistry, and chemomechanical behaviors simply by choosing different field equations for the four sets of physics specific to a material system.

Photomechanics of Light-Activated Shape Memory Polymers

Photomechanical shape memory polymers are an exciting class of materials that are able to store a temporary shape and recover their original shape when stimulated by light. In this work we develop a model to simulate the photomechanical behavior of light-activated shape memory polymers. To the best of our knowledge this is the first theoretical model developed to describe this exciting class of active materials. Our model incorporates the interplay among four aspects of the underlying physical phenomena: light propagation, photo-chemistry, chemical-mechanical coupling, and mechanical response. The model framework is applied to a recently developed photo-induced shape memory polymer system [1, 2]. We describe a suite of experiments used to guide the modeling efforts, calibrate the model parameters, and then validate model predictions. Regarding the latter, we measure and then simulate the photo-induced bending behavior of shape memory polymer samples; model predictions are in good agreement with measurements. We use the model to then explore the effect of important photomechanical parameters (applied strain magnitude, irradiation time and intensity, and photoabsorber concentration) on material response with a view toward the design of novel actuator materials and structures.Copyright