Delamination behavior of quasi-isotropic graphite epoxy laminates subjected to tension and torsion loads

Sixteen and thirty-two ply quasi-isotropic laminates fabricated from AS4/3501-6 were subjected to pure tension, simultaneous tension and torsion, and torsion fatigue. Layups tested were (45 sub n/-45 sub n/O sub n/90 sub n) sub s, with n = 2 or 4. A torsion damage pattern consisting of a localized matrix crack and delaminations was characterized, and the measured torsional stiffnesses were compared with calculated values. It was found that a combination of tension and torsion led to failure at smaller loads than either type of deformation acting alone. Further work is required to determine the exact form of the failure criterion

Novel Low-Temperature Poss-Containing Siloxane Elastomers

One route to increased aircraft performance is through the use of flexible, shape-changeable aerodynamics effectors. However, state of the art materials are not flexible or durable enough over the required broad temperature range. Mixed siloxanes were crosslinked by polyhedral oligomeric silsesquioxanes (POSS) producing novel materials that remained flexible and elastic from -55 to 94 C. POSS molecules were chemically modified to generate homogeneous distributions within the siloxane matrix. High resolution scanning electron microscope (HRSEM) images indicated homogenous POSS distribution up to 0.8 wt %. Above the solubility limit, POSS aggregates could be seen both macroscopically and via SEM (approx.60-120 nm). Tensile tests were performed to determine Young s modulus, tensile strength, and elongation at break over the range of temperatures associated with transonic aircraft use (-55 to 94 C; -65 to 200 F). The siloxane materials developed here maintained flexibility at -55 C, where previous candidate materials failed. At room temperature, films could be elongated up to 250 % before rupturing. At -55 and 94 C, however, films could be elongated up to 400 % and 125 %, respectively

Interlaminar fracture toughness of thermoplastic composites

Edge delamination tension and double cantilever beam tests were used to characterize the interlaminar fracture toughness of continuous graphite-fiber composites made from experimental thermoplastic polyimides and a model thermoplastic. Residual thermal stresses, known to be significant in materials processed at high temperatures, were included in the edge delamination calculations. In the model thermoplastic system (polycarbonate matrix), surface properties of the graphite fiber were shown to be significant. Critical strain energy release rates for two different fibers having similar nominal tensile properties differed by 30 to 60 percent. The reason for the difference is not clear. Interlaminar toughness values for the thermoplastic polyimide composites (LARC-TPI and polyimidesulfone) were 3 to 4 in-lb/sq in. Scanning electron micrographs of the EDT fracture surfaces suggest poor fiber/matrix bonding. Residual thermal stresses account for up to 32 percent of the strain energy release in composites made from these high-temperature resins

Molecular Dynamics Simulations of Adhesion at Epoxy Interfaces

The effect of moisture on adhesives used in aerospace applications can be modeled with chemically specific techniques such as molecular dynamics simulation. In the present study, the surface energy and work of adhesion are calculated for epoxy surfaces and interfaces, respectively, by using molecular dynamics simulation. Modifications are made to current theory to calculate the work of adhesion at the epoxy-epoxy interface with and without water. Quantitative agreement with experimental values is obtained for the surface energy and work of adhesion at the interface without water. The work of adhesion agrees qualitatively with the experimental values for the interface with water: the magnitude is reduced 15% with respect to the value for the interface without water. A variation of 26% in the magnitude is observed depending on the water configuration at a concentration of 1.6 wt%. The methods and modifications to the method that are employed to obtain these values are expected to be applicable for other epoxy adhesives to determine the effects of moisture uptake on their work of adhesion

Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf

The discovery of the first methane brown dwarf provides a framework for describing the important advances in both fundamental physics and astrophysics that are due to the study of companions of stars. I present a few highlights of the history of this subject along with details of the discovery of the brown dwarf Gliese 229B. The nature of companions of stars is discussed with an attempt to avoid biases induced by anthropocentric nomenclature. With the newer types of remote reconnaissance of nearby stars and their systems of companions, an exciting and perhaps even more profound set of contributions to science is within reach in the near future. This includes an exploration of the diversity of planets in the universe and perhaps soon the first solid evidence for biological activity outside our Solar System.Comment: 31 pages, 13 figure

Hyperpolarizability and operational magic wavelength in an optical lattice clock

Optical clocks benefit from tight atomic confinement enabling extended interrogation times as well as Doppler- and recoil-free operation. However, these benefits come at the cost of frequency shifts that, if not properly controlled, may degrade clock accuracy. Numerous theoretical studies have predicted optical lattice clock frequency shifts that scale nonlinearly with trap depth. To experimentally observe and constrain these shifts in an $^{171}$Yb optical lattice clock, we construct a lattice enhancement cavity that exaggerates the light shifts. We observe an atomic temperature that is proportional to the optical trap depth, fundamentally altering the scaling of trap-induced light shifts and simplifying their parametrization. We identify an "operational" magic wavelength where frequency shifts are insensitive to changes in trap depth. These measurements and scaling analysis constitute an essential systematic characterization for clock operation at the $10^{-18}$ level and beyond.Comment: 5 + 2 pages, 3 figures, added supplementa

The Sensitivity of US Wildfire Occurrence to Pre-Season Soil Moisture Conditions Across Ecosystems

It is generally accepted that year-to-year variability in moisture conditions and drought are linked with increased wildfire occurrence. However, quantifying the sensitivity of wildfire to surface moisture state at seasonal lead-times has been challenging due to the absence of a long soil moisture record with the appropriate coverage and spatial resolution for continental-scale analysis. Here we apply model simulations of surface soil moisture that numerically assimilate observations from NASA's Gravity Recovery and Climate Experiment (GRACE) mission with the US Forest Service"TM"s historical Fire-Occurrence Database over the contiguous United States. We quantify the relationships between pre-fire-season soil moisture and subsequent-year wildfire occurrence by land-cover type and produce annual probable wildfire occurrence and burned area maps at 0.25-degree resolution. Cross-validated results generally indicate a higher occurrence of smaller fires when months preceding fire season are wet, while larger fires are more frequent when soils are dry. This result is consistent with the concept of increased fuel accumulation under wet conditions in the pre-season. These results demonstrate the fundamental strength of the relationship between soil moisture and fire activity at long lead-times and are indicative of that relationship's utility for the future development of national-scale predictive capability

Molecular Modeling for Calculation of Mechanical Properties of Epoxies with Moisture Ingress

Atomistic models of epoxy structures were built in order to assess the effect of crosslink degree, moisture content and temperature on the calculated properties of a typical representative generic epoxy. Each atomistic model had approximately 7000 atoms and was contained within a periodic boundary condition cell with edge lengths of about 4 nm. Four atomistic models were built with a range of crosslink degree and moisture content. Each of these structures was simulated at three temperatures: 300 K, 350 K, and 400 K. Elastic constants were calculated for these structures by monitoring the stress tensor as a function of applied strain deformations to the periodic boundary conditions. The mechanical properties showed reasonably consistent behavior with respect to these parameters. The moduli decreased with decreasing crosslink degree with increasing temperature. The moduli generally decreased with increasing moisture content, although this effect was not as consistent as that seen for temperature and crosslink degree

Jackknifing in Non-Linear Regression

1 online resource (PDF, 18 pages