Through a collaborative effort between the Virginia Commonwealth University and Raytheon, a peridynamic model for sand impact damage has been developed1-3. Model development has focused on simulating impacts of sand particles on ZnS traveling at velocities consistent with aircraft take-off and landing speeds. The model reproduces common features of impact damage including pit and radial cracks, and, under some conditions, lateral cracks. This study focuses on a preliminary validation exercise in which simulation results from the peridynamic model are compared to a limited experimental data set generated by NASA’s recently developed micro-particle gun (MPG). The MPG facility measures the dimensions and incoming and rebound velocities of the impact particles. It also links each particle to a specific impact site and its associated damage. In this validation exercise parameters of the peridynamic model are adjusted to fit the experimentally observed pit diameter, average length of radial cracks and rebound velocities for 4 impacts of 300 μm glass beads on ZnS. Results indicate that a reasonable fit of these impact characteristics can be obtained by suitable adjustment of the peridynamic input parameters, demonstrating that the MPG can be used effectively as a validation tool for impact modeling and that the peridynamic sand impact model described herein possesses not only a qualitative but also a quantitative ability to simulate sand impact events.
The brittle nature of electromagnetic (EM) window and dome materials limits electrical and magnetic performance due to impact of sand particles, hailstones and raindrops. The damage and fracture patterns due to such impacts are well documented with distinct association to the impact type. However, the underlying mechanisms that lead to those patterns are not well understood. Adding to the complexity, multiple layers of coatings with varying thicknesses are applied to the external surfaces of these structures, which affects the extent and nature of the impact damage. A physics -based analysis method that captures correct damage and fracture patterns due to particle impact is well warranted.
In this paper, Peridynamic (PD) Theory is demonstrated as a simulation methodology for fracture analysis of EM windows and domes under particle impact. This theory involves reformulation of classical continuum mechanics in integral form (no spatial derivatives), alleviating the stress singularity problem common to previous fracture analysis approaches. The PD theory enables accurate description of failure events via natural generation and accumulation of defects, cracks, and damage; it can capture complex, 3-D and multiple non-coplanar crack initiation and propagation. The fracture behavior of materials is influenced by an important material parameter, critical stretch, which is specific to PD theory. This study offers a combined experimental-computational method to extract the critical stretch parameter for glass and ceramic materials based on simulations of indentation tests. The critical stretch parameter extracted from indentation simulations is subsequently used for simulations involving sand impact. The predicted damage field is in very good agreement with the experimentally observed fracture patterns.
The mechanical durability of the external electromagnetic window or dome of a sensor often limits the environments in which the sensor or seeker system can be deployed. More durable window and dome materials will allow platforms to fly longer and faster and sustain lower maintenance and replacement costs. Unfortunately, no good models exist for predicting the performance of window and dome materials under harsh erosion environments, especially when the aperture substrates are protected by advanced coating systems.
Recently, Peridynamic (PD) models of sand impact damage have been shown to produce the same phenomenological damage as is observed experimentally in zinc sulfide (ZnS). This paper discusses improvements in the PD impact simulation model which now allow it to simulate coated substrates and non-parallel impact events (where the flat impactor face is no longer parallel to the substrate but tilted by some small impact angle.) Two different substrates are considered, one with the properties of ZnS and another which is twice as strong and stiff as ZnS. Finally, the variation in damage as a function of impact angle is discussed. These modeling results demonstrate the versatility of the peridynamic model of sand impact damage and its potential for identifying trade space and providing design guidance during the development of more durable apertures.
The ability to deploy advanced sensor and seeker systems in harsh environments is often restricted by the mechanical durability of the external electromagnetic window or dome. Mission environments may range from long flights at high speeds through rain, ice, or sand to exposure at slower speeds to debris on runways or from helicopter downwash. While significant progress has been made to characterize, understand, and model rain damage, less is known about modeling damage in windows and domes caused by impacts from solid particles such as stones, pebbles, and sand.
This paper highlights recent progress made to simulate particle impact damage in zinc sulfide (ZnS) using peridynamics (PD). Early versions of the PD model of sand impact damage simulated the sand particle as a rigid disk. Results from these early models indicated that the extent of damage in relation to the size of the impacting particle was significantly larger than the actual damage observed by experimentation. In order to identify possible explanations for this discrepancy, the shape, impact orientation and mechanical properties of the impacting particle were modified to more closely resemble actual sand particle impacts, that is, the particle was made friable (deformable and breakable). The impacting geometries considered include sphere, flat face of a cylinder, cube-face, cube-edge, and cube-corner. Results confirm that modification of the impacting particle’s mechanical properties, shape and impact orientation lead to better agreement between experimental observations and simulation results.
To explore the origin of chuck-related surface nonuniformities in thin films deposited from solutions, real-time observations of evaporative cooling effects during spin-coating of solvents are made using an IR switched-field-effect-transistor (FET) array camera. The evaporative cooling depends strongly on the volatility of the solvent being tested. For acetone (the most volatile solvent tested) the cooling eftect can be as large as 10°C when a silicon wafer is used as the substrate. Comparisons are made between wafer type and position on the substrate and sources of the temperature differences are discussed within the framework of the basic spin-coating paradigm. It is likely that these evaporative cooling effects play an important role in the development of chuck-related surface nonuniformities during spin coating; solvent systems may be selected to help optimize film uniformity. These aspects of solution engineering are also discussed.
Wet chemical processing of ceramics, glasses and inorganic-organic hybrids in the form of films has a large number of both proven and potential optical applications. The present review focuses on progress since 1990 in the areas of ferroelectric films, electrochromic and photochromic films, planar waveguides, and NLO films. Where appropriate, advances are illustrated by results obtained in our laboratories.
Solution chemistry techniques are of interest for the deposition of thin film planar waveguides due to the ease of fabrication, the compositional range available, and the routine synthesis of guides with losses < 0.5 dB ((lambda) equals 632.8 nm). In this work, the wavelength dependence and attenuation of sol-gel waveguides were measured as a function of composition and time in storage, or aging. The compositions investigated in this work were (in mole %) 65SiO2:35TiO2, 50SiO2:50TiO2 and 41.5SiO2:41.5TiO2:17Al2O3 waveguides. Initial losses of the waveguides were < 0.5 dB/cm ((lambda) equals 632.8 nm). The initial wavelength dependence of the guides was also measured. In waveguides where the losses are intrinsically or Rayleigh scatter limited, a dependence of (lambda) 4 is expected. However, the wavelength dependence measured for the sol-gel guides varied with composition. After periods in storage, waveguide attenuation was remeasured, and the guides were found to have deteriorated, with higher losses. The rate of deterioration varied with composition. Possible causes of the deterioration of the guides are presented and discussed.
Optical interference filters were fabricated using multilayer stacks derived from sol-gel SiO2 and SiO2-TiO2 coatings. Laser processing was then used to modify the spectral properties (color) of local regions of these stacks. Changes in color were analyzed with respect to changes in the individual film thicknesses and refractive indices. Design considerations and some basic limitations of laser firing for tuning interference filter colors are also discussed.
Sol-gel derived SiO2-TiO2 and WO3 films were densified with a CO2 laser. Laser fired SiO2-TiO2 films had higher optical loss (6 dB/cm) than similar films fired in a furnace (< 1 dB/cm). For coating shrinkages up to about 40% the increase in refractive index during firing was similar for both laser and furnace fired SiO2-TiO2 coatings. At higher shrinkages both anatase and rutile were observed in the laser fired coatings. Loss measurements of laser fired samples in this and, likely, in previous studies were made in crystalline coatings. It is proposed that this, at least in part, is the cause of the high optical losses typically measured in laser densified silica-titania waveguides. The electrochromic coloring behavior of the WO3 films was examined using proton insertion. The coloring characteristics (< 20 s for 90% coloring) and bleaching characteristics (< 3 s for 90% bleaching) of laser fired WO3 films were comparable to the coloring and bleaching characteristics of high-quality sol-gel derived films fired in a furnace. Optimal coloring was obtained using moderate laser energy densities (approximately 50 - 100 J/cm2). This optimum occurred with less shrinkage than for furnace fired electrochromic films with similar coloring behavior.
A series of sol-gel derived PT-based films, including PT, PZ, PZT, PLT, PLZ and PLZT, was prepared on platinized Si, fused SiO2 and Corning 7059 substrates. These films were fired at 400 - 700 C for 30 mins. The phase assembly and development were dependent on the precursor chemistries, processing and choice of substrates. The presence of Zr impacted significantly on the crystallization behavior, PbO loss and cracking behavior of the films. Crystallization was severely retarded, especially in Zr-containing PZT films when deposited on amorphous substrates compared to crystalline Pt substrates. Amorphous and crystalline PZT films can be utilized for passive and active optical applications. Waveguiding was achieved in an amorphous PZT 53/47 and a crystalline PLT 28 films and gave attenuation losses of 1.0 and 1.4 dB/cm respectively which represent the lowest values reported to date. The optical properties of the films were investigated using ellipsometry, UV-VIS transmission spectroscopy and waveguide loss measurements. Depending on composition and processing conditions, PZT films (2500 A thick) with refractive indices of 1.60 to 2.33 and absorption edges of 2900 - 3100 A can be obtained. It was ascertained that the resulting interfacial reaction layers between the films and substrates affected considerably the optical properties of thinner films (< 2000 A).
Optical interference filters were fabricated using multilayers derived from sol-gel SiO2 and SiO2-TiO2 thin films. Laser processing was then used to modify the spectral properties of local regions of these stacks. The feasibility of using laser processing for selectively changing the optical properties of thin film devices was thus demonstrated. Design considerations and some basic limitations of this technique for tuning interference filter colors are then discussed.
Low loss sol-gel derived polyceram optical waveguides have been prepared for the first time. Polyceram films were obtained by reacting (N-triethoxysilyl propyl) o-polyethylene oxide urethane with silicon and titanium alkoxides. The optical properties of the films were investigated using ellipsometry, UV-VIS transmission spectroscopy and waveguide loss measurements. Refractive index and attenuation loss measurements were carried out as a function of organic/inorganic content, different processing conditions and aging of solutions. Refractive indices as high as 1.685 and attenuation losses as low as 1.4 dB/cm were obtained. In addition, surface morphology, mechanical properties and thermal stability of the polyceram films were studied.
Thickness variations that are associated with the vacuum chuck were observed in wet-chemical-derived dielectric films applied by the spin-coating technique. These thickness variations are controlled by factors such as the thermal properties of the substrate material, the evaporation behavior of the coating solution, and the physical design of the vacuum chuck. Atechnique is described for evaluating the magnitude ofthis effect.
Sol-Gel derived Si02-Ti02 films were densified by scanning a C02 laser beam across the surface. The effects of laser power and scanning rate on the morphology and optical properties of the coatings were studied. At low laser powers a depression, with rough sidewalls and approximately gaussian profile was formed in the coatings. As the laser power was increased the channels became wider and smoother, and flattened out on the bottom. At high laser powers a pair of ridges were formed in the bottom of the channel, apparently due to melting. The refractive index of the laser densified films was higher than for films fired in a furnace. Most laser densified areas supported waveguide modes but exhibited high loss. By processing at powers just below the damage threshold loss was reduced to a measurable value, 12.4 dB/cm for the TM0 mode for one sample. Annealing the films after laser processing also reduced the loss.
Experimental measurement of modal attenuation in solution-derived Si02-Ti02 waveguides is compared to the total waveguide attenuation predicted by surface and volume scattering models. The surface scatter model utilizes measurements of the film index and thickness, and the rms roughness and correlation lengths (measured by Atomic Force Microscopy) of the film and substrate surfaces, to facilitate a realistic comparison of theoretical and experimental waveguide attenuation. Theoretical attenuation, calculated by means of a perturbation technique, in conjunction with a stationary phase method, is used to demonstrate the influence of the most important waveguide and microstructural parameters. The plots provide guidelines for acceptable surface roughness and bulk refractive index fluctuations to fabricate low- loss waveguides. Volume-induced scattering is shown to be the dominant loss mechanism in these waveguides.
The application of solution derived Si02-Ti02 thin films for use in integrated optics has been of considerable interest in recent years. Optical filters,waveguides and chemical sensors5 have been fabricated by a variety of researchers, using several different chemical routes to synthesize the deposition solution.
Sol-gel derived silica, siica-titania, and tantala coatings were covered with a thin metal film and
translated across a Nd:YAG laser beam (1.06 jim). The laser energy was absorbed by the metal film, which
heated the underlying sol-gel coating. This heating densified the sol-gel coatings, thereby increasing the
index ofrefraction of the laser heated region, and forming channel waveguide structures in all three systems.
The channels formed by this technique were etched, to remove the undensified regions, which resulted
in ridged waveguide structures. The structures were also produced by depositing a metal pattern using
photolithographic techniques, and rastering the laser across the entire sample. The refractive indicies of
laser densifled and furnace densified silica coatings were similar. Large differences were observed in the
indicies oflaser and furnace densified coatings for the siica-titania and tantala systems.
Sol-gel methods offer a number of notable advantages for the
synthesis of optical films and coatings. Areas of potential or
actual application of this technology range from single layer and
multilayer antireflection coatings to embossed planar waveguides
and organic-modified oxide materials. The most notable advantages
of these wet chemical nethods will be surveyed, as will progress
achieved to date in a number of the most attractive representative
areas. The technical bases for the success/failure in each case
will be considered. Also to be discussed will be the prospects - in both the near-term and long-term - of future developments in the
sol-gel synthesis of optical films, as well as the principal
technical hurdles which must be overcome in order that such
synthesis methods may achieve more widespread use in the future.
Finally, a comparison will be made between the microstructures and
characteristics of films and coatings deposited using sol-gel
methods with those deposited from the vapor phase. In all cases,
use will be made of recent advances in our laboratory in the
subject area.
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