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Schau K.A.,Florida Atlantic University | Gaonkar G.H.,Florida Atlantic University | Polsky S.,Naval Air Warfare Center Aircraft Division
Annual Forum Proceedings - AHS International | Year: 2014

Helicopter downwash impact on ship airwake is addressed from a three-pronged approach: 1.) Predictions of one-point statistics of autospectrum and two-point statistics of cross-spectrum and coherence from a Computational Fluid Dynamics database of flow velocity predictions; 2.) Development of a mathematical framework for extracting interpretive autospectral models in closed form from a database; 3.) Simulation through white-noise-driven filters for the extracted models. The framework begins with an earlier-exercised perturbation series expansion of the autocorrelation for all three velocity components, and the first term of the series has a form of the von Karman longitudinal or lateral correlation function. After transformation into equivalent series of autospectrum, the perturbation coefficients are evaluated by satisfying the theoretical constraints and fitting a curve on a set of selected autospectral data points generated from the database. These constraints ensure that the extracted models and the autospectral data points have the same time scale, intensity, energy, and asymptotic decay according to the Kolmogorov -5/3 law. The framework represents a sensible combination of series expansion, exploitation of a database, and theoretical constraints to provide a foothold on airwake-downwash phenomenon for engineering analysis. The framework's strengths and weaknesses and its advancement over the earlier series-expansion schemes are also included. Downwash increases airwake energy by one order of magnitude, and all of this airwake-downwash energy is concentrated within the bandwidth (0 < (Hz) < 1.6) that affects flight mechanics. Copyright © 2014 by the American Helicopter Society International, Inc. All rights reserved.

Bark L.W.,Naval Air Warfare Center Aircraft Division
Annual Forum Proceedings - AHS International | Year: 2014

A full-scale crash test of a USMC CH-46 helicopter airframe was conducted at NASA-Langley Research Center. One of the internal experiments was an assessment of mobile aircrew restraint concepts. Two Hybrid III Pedestrian Anthropomorphic Test Devices (ATD's) were positioned in a standing position, just aft of the crew door. On the left side, a traditional gunner's belt was employed. On the right side, the Mobile Aircrew Restraint System (MARS) was employed with the Aircrew Endurance vest. The motivation behind this experiment was based on several mishap-based injuries of mobile aircrew that were using traditional gunner's belts. However, correlation of presumed injury causes with equipment deficiencies was difficult because of a near total void of mobile aircrew restraint testing data in a system-level environment. For the condition tested, the measured results for the two ATD's indicated a dramatic reduction in injury probability when employing the MARS. In contrast, the ATD equipped with the gunner's belt experienced accelerations and forces that could generally be considered lethal. © 2014 by the American Helicopter Society International, Inc. All rights reserved.

Mikheevskiy S.,University of Waterloo | Glinka G.,University of Waterloo | Lee E.,Naval Air Warfare Center Aircraft Division
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2013

The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic-plastic stresses-strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress-strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith-Watson-Topper (SWT) fatigue damage parameter, D = σmaxΔε/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, ΔK p K max 1-p . The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth model. The method can be also used to predict fatigue crack growth under constant amplitude and spectrum loading in various environmental conditions such as vacuum, air, and corrosive environment providing that appropriate limited constant amplitude fatigue crack growth data obtained in the same environment are available. The proposed methodology is equally suitable for fatigue analysis of smooth, notched, and cracked components. © 2012 The Minerals, Metals & Materials Society and ASM International.

Stanley C.L.,Naval Air Warfare Center Aircraft Division | Sheeny J.B.,Naval Aviation Enterprise
The World's Leading Conference on Laser Safety, ILSC 2011 - International Laser Safety Conference | Year: 2011

There are many misconceptions about how to identify, select, and use laser eye protection (LEP). For instance a common misperception is that the color of the eye protection should be the same as the laser, that is if the laser beam is green then so should the LEP. Unfortunately, it is not quite that simple, the color of the LEP does not correspond to the color of the beam. To help alleviate some of these falsehoods this paper discusses a simple approach of how to choose the appropriate LEP. Whether using LEP indoor with the lights on or off, or outside on a range during the day or at night, it is important that the LEP have the correct optical density (OD) at the specified wavelength(s), with sufficient transmittance to be usable, as well as physically and spectrally compatible for your specific needs.

Lee E.U.,Naval Air Warfare Center Aircraft Division | Goswami R.,U.S. Navy | Goswami R.,SAIC | Jones M.,Naval Air Warfare Center Aircraft Division | Vasudevan A.K.,U.S. Navy
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011

The influence of cold work and aging on the environment-assisted cracking (EAC) behavior and mechanical properties of Custom 465 stainless steel (SS) was studied. Four sets of specimens were made and tested. All specimens were initially solution annealed, rapidly cooled, and refrigerated (SAR condition). The first specimen set was steel in the SAR condition. The second specimen set was aged to the H1000 condition. The third specimen set was 60 pct cold worked, and the fourth specimen set was 60 pct cold worked and aged at temperatures ranging from 755 K to 825 K (482 °C to 552 °C) for 4 hours in air. The specimens were subsequently subjected to EAC and mechanical testing. The EAC testing was conducted, using the rising step load (RSL) technique, in aqueous solutions of NaCl of pH 7.3 with concentrations ranging from 0.0035 to 3.5 pct at room temperature. The microstructure, dislocation substructure, and crack paths, resulting from the cold work, aging, or subsequent EAC testing, were examined by optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The aging of the cold-worked specimens induced carbide precipitation within the martensite lath, but not at the lath or packet boundaries. In the aged specimens, as aging temperature rose, the threshold stress intensity for EAC (KIEAC), elongation, and fracture toughness increased, but the strength and hardness decreased. The KIEAC also decreased with increasing yield strength and NaCl concentration. In the SAR and H1000 specimens, the EAC propagated along the prior austenite grain boundary, while in the cold-worked and cold-worked and aged specimens, the EAC propagated along the martensite lath, and its packet and prior austenite grain boundaries. The controlling mechanism for the observed EAC was identified to be hydrogen embrittlement. © 2010 The Minerals, Metals & Materials Society and ASM International.

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