Wright-Patterson Air Force Base

About: Wright-Patterson Air Force Base is a other organization based out in Wright-Patterson AFB, Ohio, United States. It is known for research contribution in the topics: Laser & Microstructure. The organization has 5817 authors who have published 9157 publications receiving 292559 citations. The organization is also known as: Wright-Patterson AFB & FFO.

Simulation-based optimization of laser shock peening process for improved bending fatigue life of Ti–6Al–2Sn–4Zr–2Mo alloy

Abstract: Laser shock peening (LSP) induced residual stresses significantly affect the high cycle fatigue behavior of certain metals and alloys. Residual stress distribution is a function of various laser parameters (energy, laser pulse width, and spot diameter), the geometry, the material and the laser shot sequencing. Considering the wide range of parameters involved in the LSP process, a numerical approach based on 3D nonlinear finite element method has been employed to explore the relation between the processing parameters and the residual stress distribution. This methodology is applied to a thin coupon of Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) alloy, with a view towards establishing conditions for obtaining through-thickness compressive residual stresses and hence improved bending fatigue life. Material response at very high strain rates in the LSP process is effectively represented using the modified Zerilli–Armstrong material model. The numerical approach is verified by comparison with the experimental results. Effects of laser parameters and laser shot sequencing on final residual stress distribution are studied by performing full scale simulations of LSP patches constituting a large number of laser shots. Based on simulation studies, optimal set of parameters is obtained that produces through thickness compression, which leads to a substantial improvement in bending fatigue life. Fatigue testing results support the recommendations made based on simulation results.

Maximum-Likelihood Estimation of the Parameters of a Four-Parameter Generalized Gamma Population from Complete and Censored Samples

Abstract: Consider the four-parameter generalized Gamma population with location parameter c, scale parameter a, shape/power parameter b, and power parameter p (shape parameter d = bp) and probability density function f(x; c, a, b, p) = p(x — c) bp–1 exp {–[(x – c)/a] p }/a bp Γ(b), where a, b, p > 0 and x ≥ c ≥ 0. The likelihood equations for parameter estimation are obtained by equating to zero the first partial derivatives, with respect to each of the four parameters, of the natural logarithm of the likelihood function for a complete or censored sample. The asymptotic variances and covariances of the maximum-likelihood estimators are found by inverting the information matrix, whose components are the limits, as the sample size n → ∞, of the negatives of the expected values of the second partial derivatives of the likelihood function with respect to the parameters. The likelihood equations cannot be solved explicitly, but an iterative procedure for solving them on an electronic computer is described. The results ...

Prior health care utilization as a potential determinant of enrollment in a 21-year prospective study, the Millennium Cohort Study

Abstract: Results obtained from self-reported health data may be biased if those being surveyed respond differently based on health status. This study was conducted to investigate if health, as measured by health care use preceding invitation, influenced response to invitation to a 21-year prospective study, the Millennium Cohort Study. Inpatient and outpatient diagnoses were identified among more than 68,000 people during a one-year period prior to invitation to enroll. Multivariable logistic regression defined how diagnoses were associated with response. Days spent hospitalized or in outpatient care were also compared between responders and nonresponders. Adjusted odds of response to the questionnaire were similar over a diverse range of inpatient and outpatient diagnostic categories during the year prior to enrollment. The number of days hospitalized or accessing outpatient care was very similar between responders and nonresponders. Study findings demonstrate that, although there are some small differences between responders and nonresponders, prior health care use did not affect response to the Millennium Cohort Study, and it is unlikely that future study findings will be biased by differential response due to health status prior to enrollment invitation.