Direct Laser Sintering of Borosilicate Glass

Despite the advantages that selective laser sintering (SLS) offers in terms of material availability, many materials have yet to be explored for feasibility and even fewer are available on a commercial basis. This paper presents initial investigations for one such material, borosilicate glass, which could be of particular interest to filter manufacturers because it presents an attractive alternative to the conventional, time-consuming way of producing filters of various porosity classes. Process results are presented including a determination of the optimal parameter window and the effect of processing parameters on the density and surface quality. The effects of thermal post-processing and the inclusion of an additive are also discussed.Mechanical Engineerin

Clocked Atom Delivery to a Photonic Crystal Waveguide

Experiments and numerical simulations are described that develop quantitative understanding of atomic motion near the surfaces of nanoscopic photonic crystal waveguides (PCWs). Ultracold atoms are delivered from a moving optical lattice into the PCW. Synchronous with the moving lattice, transmission spectra for a guided-mode probe field are recorded as functions of lattice transport time and frequency detuning of the probe beam. By way of measurements such as these, we have been able to validate quantitatively our numerical simulations, which are based upon detailed understanding of atomic trajectories that pass around and through nanoscopic regions of the PCW under the influence of optical and surface forces. The resolution for mapping atomic motion is roughly 50 nm in space and 100 ns in time. By introducing auxiliary guided mode (GM) fields that provide spatially varying AC-Stark shifts, we have, to some degree, begun to control atomic trajectories, such as to enhance the flux into to the central vacuum gap of the PCW at predetermined times and with known AC-Stark shifts. Applications of these capabilities include enabling high fractional filling of optical trap sites within PCWs, calibration of optical fields within PCWs, and utilization of the time-dependent, optically dense atomic medium for novel nonlinear optical experiments

A comparison of single-cycle versus multiple-cycle proof testing strategies

An evaluation of single-cycle and multiple-cycle proof testing (MCPT) strategies for SSME components is described. Data for initial sizes and shapes of actual SSME hardware defects are analyzed statistically. Closed-form estimates of the J-integral for surface flaws are derived with a modified reference stress method. The results of load- and displacement-controlled stable crack growth tests on thin IN-718 plates with deep surface flaws are summarized. A J-resistance curve for the surface-cracked configuration is developed and compared with data from thick compact tension specimens. The potential for further crack growth during large unload/reload cycles is discussed, highlighting conflicting data in the literature. A simple model for ductile crack growth during MCPT based on the J-resistance curve is used to study the potential effects of key variables. The projected changes in the crack size distribution during MCPT depend on the interactions between several key parameters, including the number of proof cycles, the nature of the resistance curve, the initial crack size distribution, the component boundary conditions (load vs. displacement control), and the magnitude of the applied load or displacement. The relative advantages of single-cycle and multiple-cycle proof testing appear to be specific, therefore, to individual component geometry, material, and loading

Non-contact technique for characterizing full-field surface deformation of shape memory polymers at elevated and room temperatures

Abstract Thermally activated shape memory polymers (SMPs) can display modulus changes of approximately three orders of magnitude in transitioning from the high modulus, "glassy" state below the glass transition temperature (Tg) to the low modulus, "rubbery" state above the Tg. In the high temperature region, SMPs can achieve strain levels well above 100%. Their complex behavior includes large modulus changes to as low as ∼1 MPa, extremely high strain levels, and path dependent properties, thus precluding the use of traditional strain gages and low-contact force extensometers. The present study presents a comparison of thermomechanical testing techniques developed to characterize the material behavior of SMPs. Specifically, the performance of strain measurements using contact methods (clip-on extensometers and adhesive strain gages) are compared to non-contact methods (laser extensometer and digital image correlation). An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A series of tensile tests are performed on a commercial SMP (with a Tg of ∼105 °C) at 25 °C and at 130 °C. It is observed that the clip-on extensometer significantly affects the SMP behavior even in the low temperature, high modulus state. Overall, the laser extensometer provides a robust method for controlling the axial strain in the gage section of the samples at moderate strain rates. The digital image correlation allows for full field measurement of both axial and transverse strains of SMPs over a range of temperatures and strain rates

The Importance of Ambient Temperature to Growth and the Induction of Flowering

Plant development is exquisitely sensitive to the environment. Light quantity, quality, and duration (photoperiod) have profound effects on vegetative morphology and flowering time. Recent studies have demonstrated that ambient temperature is a similarly potent stimulus influencing morphology and flowering. In Arabidopsis, ambient temperatures that are high, but not so high as to induce a heat stress response, confer morphological changes that resemble the shade avoidance syndrome. Similarly, these high but not stressful temperatures can accelerate flowering under short day conditions as effectively as exposure to long days. Photoperiodic flowering entails a series of external coincidences, in which environmental cycles of light and dark must coincide with an internal cycle in gene expression established by the endogenous circadian clock. It is evident that a similar model of external coincidence applies to the effects of elevated ambient temperature on both vegetative morphology and the vegetative to reproductive transition. Further study is imperative, because global warming is predicted to have major effects on the performance and distribution of wild species and strong adverse effects on crop yields. It is critical to understand temperature perception and response at a mechanistic level and to integrate this knowledge with our understanding of other environmental responses, including biotic and abiotic stresses, in order to improve crop production sufficiently to sustainably feed an expanding world population