Michael P. Camden

Failure Criteria Development for Dynamic High-Cycle Fatigue of Ceramic Matrix Composites

Acoustic fatigue in aircraft is typically characterized by random, high-frequency excitation that results in fully reversed bending in panels. Because of the expense of testing panels in an acoustic environment, traditional methods have first tested small cantilevered coupons on electrodynamic shakers to determine the strain vs number of cycles to failure behavior. A number of variables have been studied to provide failure criteria for the specimens. Developing criteria are examined for a relatively new class of materials, ceramic matrix composites, which are characterized by having a brittle matrix, which has a stiffness that is a significant fraction of the fiber value. Also discussed are experimental techniques that reduce the scatter associated with such data.

Random amplitude fatigue life of electroformed nickel micro-channel heat exchanger coupons

The use of micro-channel heat exchangers (MCHEX) with coolant flow passage diameters less than 1 mm has been proposed for heat flux, weight, or volume limited environments. This paper presents room temperature, random amplitude, e − N (strain versus number of cycles to failure) curves for MCHEX coupons formed by electroplating nickel on a suitable form. These coupons are unique in two aspects; the microstructure formed by electroplating and the presence of holes as an integral part of the structure. The hole diameters range from approximately 10% to 50% to the specimen thickness. The fatigue life of electroformed nickel can be estimated from constant amplitude data using the formulation presented. The heat exchangers with channels parallel to the coupon direction have a lower fatigue life than the solid material.

A numerical study of the stability of thermohaline convection in a rectangular box containing a porous medium

Abstract The finite element method is used to study double diffusive convection in a rectangular box containing a porous medium. The porous medium is described by means of the Darcy-Brinkman model. The problem solved is the Benard problem in the box. It is found that the stability of the flow is dependent on a combination of thermal Rayleigh number, buoyancy ratio, and Lewis number. This combination for the onset of cellular motion can be written as Ra (1+ N.Le )=4π π 2 . This criterion holds for all combinations of Ra, N, and Le whether the thermal and solutal gradients are aiding or opposing each other. Numerical results are presented in the form of flow, temperature, and concentration fields and average Nusselt and Sherwood numbers.

Natural convection around a cylinder buried in a porous medium - non-Darcian effects

Abstract Natural convection heat transfer around a cylinder embedded in a porous medium is studied numerically using the penalty finite-element method. To model fluid flow inside the porous medium, the Brinkman and Brinkman-Forschheimer equations are used. Numerical results are obtained in the form of streamlines and isotherms. The Rayleigh number values range from 0.04 to 200. Calculated values of average heat transfer rates agree reasonably well with values reported in the literature.

Sigma limiting effects on the response of a ceramic matrix beam

High sigma events were studied in the narrowband random response of a cantilevered beam excited by an electrodynamic shaker. The effects of truncating the input signal to the shaker with a Blackglas beam are discussed. Tests were conducted with the shaker option of the shaker controller sigma limiting turned off, sigma limited to 2.5, and sigma limited to 3.5. Large data files were recorded and analyzed. The results were compared with similar previous tests with an aluminum alloy beam. The sigma values calculated varied considerably with the record lengths. When a fatigue failure was assumed, the cumulative fatigue damage was computed as a function of the sigma value of the response peak probability density function. Very little damage was found for the high sigma peaks compared to the damage that occurred with the many smaller peaks.

Dynamic fatigue of carbon-carbon thermal protection systems

Wright Laboratory has developed high temperature acoustic and high temperature vibration test facilities. The acoustic facility consist of two progressive wave chambers called the Combined Environment Acoustic Chamber (CEAC) and the Sub-Element Acoustic Chamber (SEAC). The CEAC has the capability to test a four-foot by sixfoot specimen in a combined environment of 172 dB sound pressure level (SPL) and 50 Btu/ft-sec impinged heat flux. The SEAC has the capability to test specimens up to 12 inches by 18 inches at 174 dB and 73 Btu/ft-sec impinged heat flux. The high temperature vibration facility consists of a 12,000 pound force shaker, a 20,000 pound force shaker, a quartz lamp configuration for heating and an environmental chamber used to heat specimens while controlling the environment. This paper presents acoustic data on one (1) two-foot by two-foot carbon/carbon (C/C) thermal protection system (TPS) panel. It also presents high cycle fatigue S/N curves developed for C/C wedges at room temperature and 1,000 °F using the thermal/vibration facility.

Sonic fatigue of launch vehicle components

Wright Laboratory has long been a leader in the technologies required for aerospace structures. One of these driving technology areas is that of the dynamic environments of acoustics and vibration to which structures are exposed and required to survive. This paper presents an overview of sonic fatigue of launch vehicle components. An experimental program to develop sonic fatigue design criteria for a proposed thermal protection system is reviewed. Wright Laboratory’s experimental facilities utilized to subject structures to simulated launch vehicle environments which are necessary to generate the experimental data required to provide sonic fatigue design criteria are described.

The properties of the numerical solutions of isothermal q-f turbulence equations using finite elements

Abstract The properties of the numerical solution of turbulence equations are examined using the finite element method. The set of equations used is the q-f turbulence equations, instead of the commonly used k-e equations. It is found that from a purely numerical point of view, the former has several desirable features over the latter in that the coupling between the equations is quite weak, and the signs of diffusivities, sources and sinks are independent of the sign of q and f. An algorithm for solving the q-f equations is presented using Galerkins finite element method. Several numerical examples verifying the robustness and stability of the algorithm are presented.