Nonlinear Response and Sonic Fatigue of National Aerospace Space Plane Surface Panels

T is becoming increasingly apparent that the development of supersonic/hypersonic vehicles such as the NASP cannot become a reality until the fatigue problem under severe aerodynamic, acoustic, and thermal environment is solved. Furthermore, with an increasing use of light-weight materials such as fiber-reinforced composites, metal matrix composites, and various intermetallic compounds, there is an urgent need for improved analytical methods for nonlinear response and sonic fatigue predictions. The proposed NASP concept involves operations at subsonic, supersonic, and hypersonic speeds. Thus, aerodynamic, acoustic, and thermal loadings should be considered for various phases of flight. In the early stages of the takeoff, the aft surface thermal protection systems will be in the near field of the engine exhaust noise. As the speed of the vehicle increases, the effect of engine exhaust noise (except in the near vicinity of exhaust nozzles) will decrease, and at Mach 1 and higher speeds the acoustic loads are expected to become negligible. However, at supersonic and hypersonic speeds, the fluctuating surface pressures due to convecting turbulent boundary layer will become significant. In addition, local impinging shocks on the structural surface could induce severe dynamic loads. Supersonic/hypersonic vehicles will be subjected to severe surface temperatures exceeding SCOOT.1'4 Structural surface components and control devices can be expected to behave in a highly nonlinear fashion with respect to both geometry and materials.4^8 No reliable analytical procedures have yet been developed which could handle these complexities and give meaningful results that are useful for the design of these vehicles. The thermal surface protection systems of aircraft structures are usually constructed from discretely stiffened panels and stiffened shells. A typical discretely stiffened panel is shown in Fig. 1. Due to anticipated high-surface temperatures of advanced supersonic/hypersonic vehicles, multi-wall and/ or multilayer constructions might need to be utilized for the design of thermal surfaces. High-cycle fatigue failures have occurred in discretely stiffened surface panels with the majority of cracks appearing in the near vicinity of the stiffening element.5'13 Thus, proper dynamic interaction between the panel and various stiffening elements should be taken into account when calculating the nonlinear response of the surface panels. A single bay panel with clamped or simple support boundary conditions might provide reasonable estimates on deformations,14'23 but the predictions of principal stresses of a discretely stiffened panel by a single panel model could be unrealistic. Since fatigue damage is controlled by local stress conditions, accurate stress predictions are essential for fatigue life estimates. A time domain Monte Carlo-type approach has been successfully applied to a variety of problems of a linear and nonlinear nature with complex random inputs.24'33 Utilizing the nonlinear time domain stress response solutions and fatigue information from constant amplitude coupon tests, preliminary fatigue damage models have been constructed. The adverse thermal conditions could result in degradation of strength, stiffness, and fatigue life. In addition, thermal inplane loads could induce buckling and "snap through" type vibrations of surface panels. The time domain procedures could account for these effects in the general formulation of the total response problem. General Features of Time-Domain Method For structural dynamic problems of nonlinear/probabilistic nature where close form or effective approximate solutions are not possible, the time domain Monte Carlo method could provide a feasible approach to construct practical solutions. The recent advent of high-speed digital computers has made this procedure a useful and effective method. To illustrate the basic features of the time domain approach, consider a generalized form of a second-order nonlinear equation