Journal of Aerospace Engineering

The cold expansion technique is often used to introduce beneficial compressive stress at fastener holes, and can be used for remedial work where cracks already exist. In this paper, results are presented showing the effect of preexisting cracks on the residual stress field produced by cold expanding a fastener hole, and on subsequent fatigue crack growth. The effect on the residual stresses was experimentally evaluated in two ways: indirectly, in terms of retained expansion and directly, by measurement of the stresses using the X-ray and neutron diffraction techniques. The retained expansion ratio showed that cold expansion is more sensitive to the existence of precracks at lower levels of applied interference, and the inlet and outlet faces have different behavior. The stress measurements showed that preexisting cracks reduce the compressive residual stresses more on the mandrel inlet face than on the outlet face and in the middle of the specimen. The effect on fatigue crack growth rates was modeled using a linear-elastic fracture mechanics approach. It was found that cold expansion of a hole containing a preexisting crack longer than 1 mm introduces little benefit for subsequent fatigue crack growth behavior.

Communication and solar array inflated structures must be deployed to a very precise geometric configuration in order to meet quality requirements of their application. The focus of this paper is on geometric imperfections associated with inflated structures. To further understand some of the elements, that derive imperfection in a parabolic inflated communication and solar array structures, a computational model is proposed. This computational approach is dictated by the geometric complexity, deformation sensitivity as function of load and boundary conditions, and nonlinear characteristics of inflated structure assemblies. The deformation of a single component depends on the flexibility/stiffness of other components due to their interaction. In order to simulate such deformations of the multicomponent inflated structure, in the present study, the computational model consists of main parabolic shape envelope (reflector and canopy), torus, and catenary’s support and uses geometric nonlinear finite element. Further, tuning of communications and solar arrays is a primary concern in the operation of these systems. To investigate the effects of pressure tuning on geometric imperfection of a parabolic inflated antenna, in this investigation, analyses using uniformly axisymmetric and asymmetric applied load are performed. The analyses assume an initial parabolic shape envelope with a perfect circular edge for the reflector and canopy. Error estimates, which quantify geometric imperfections, are computed. The results show that as the axisymmetric load increases, the surface deviation from the parabolic shape of the envelope also increases. An asymmetric load on the surface of the torus leads to variable tensile forces in the catenary along the circular edge of the envelope, which in turn cause a visible local asymmetrical deformation in the vicinity of the circular edge of the envelope. In general, an asymmetric load causes greater geometric imperfections and should be avoided

Quasi-steady instability initiation studies do not model the actual operation of a compressor in an installed engine. Namely, gas turbine engine compressors are subject to transient operation. To provide needed high-speed centrifugal compressor instability initiation information addressing transient operation, an experimental study is performed to characterize the instability behavior of a high-speed centrifugal compressor during accelerations and decelerations at various throttle settings. Data are acquired from high response dynamic pressure transducers at the inlet and exit of the compressor. Analysis of these data shows that three distinct instabilities occur: Deep surge, mild surge, and rotating stall. The rotating stall is consistent with a nine-lobed stall pattern occurring in or near the diffuser. During accelerations, rotating stall occurs prior to surge initiation. During decelerations, all three instabilities are observed. Rotating stall is observed prior to and following both mild and deep surge.

We consider the effects of increasing or decreasing the security expenditures of possible targets (i.e., the possible victims). The conventional wisdom has been that the higher the security expenditure, the lower the crime rate. We study how the change of security expenditures affects the costs of the targets. The problem is formulated as a game-theoretic setting. Both single-act and multiple-act games will be considered. The results will be compared with the conventional wisdom.

An efficient six-noded triangular element based on refined plate theory is developed for the analysis of sandwich plates with stiff laminated face sheets and it is applied to a free vibration problem in this paper. The plate theory represents parabolic through thickness variation of transverse shear stresses with continuity at the layer interfaces, which introduces discontinuity at these interfaces for the shear strains. It is to be noted that the plate theory requires unknowns at the reference plane only. Moreover, it ensures a shear stress-free condition at the top and bottom surfaces of the plate. Thus, the plate theory has all of the features required for an accurate modeling of laminated sandwich plates. The plate theory suffers from a problem in its finite element implementation since it requires $C^1$ continuity of transverse displacement at the element interfaces. As very few elements based on this plate theory exist and they possess certain disadvantages, an attempt has been made to develop this new element. It has been utilized to study some interesting problems of laminated sandwich plate.