The solution to nonlinear structural dynamics problems with time marching schemes can be very expensive, particularly if the desired time-periodic response takes many cycles to form. Two cost reduction methods, which need not be considered separately, are formulated in this work. The first projects the nonlinear system of equations onto a reduced basis defined by a set of modes computed with proper orthogonal decomposition. The second utilises a monolithic time spectral element method, whereby the system of ordinary differential equations is converted into a single algebraic system of equations. The spectral element method can be formulated such that only the time-periodic response is computed. These techniques are implemented for a planar elastic beam, actuated at its base to emulate a flapping motion. Nonlinear elastic terms are computed with a corotational finite element method, while inertial terms are computed with a standard multibody dynamics formulation. For a variety of actuation frequencies and kinematic motions, results are given in terms of POD modes, reduced order model accuracy, and computational cost, for both the time marching and the monolithic time schemes.

The hypersonic aeroelastic problem of a generic hypersonic vehicle having a lifting-body type fuselage and canted fins is studied using third order piston theory and Euler aerodynamics. Computational aeroelastic response results are used to obtain frequency and damping characteristics, and compared with those from piston theory solutions for a variety of flight conditions. Aeroelastic behavior is studied for the range of 2·5 < M < 28, at altitudes ranging from 10,000ft to 80,000ft. Because of the significant computational resources required, a study on optimal mesh selection was first carried out for use with Euler aerodynamics. The three dimensional flow effects captured using Euler aerodynamics was found to lead to significantly higher flutter boundaries when compared to those based on nonlinear piston theory. The results presented here illustrate some of the more important three dimensional effects that can be encountered in hypersonic aeroelasticity of complex configurations.

Ultrasonic nondestructive evaluation methods are routinely used to detect and size fatigue cracks near fastener holes in aircraft structures as a part of scheduled maintenance. In contrast, statistical crack propagation models provide an estimate of the expected fatigue life assuming a known crack size and future fatigue loadings. Here an integrated approach for in situ diagnosis and prognosis of fastener hole fatigue cracks is proposed and implemented that incorporates both ultrasonic monitoring and crack growth laws. The sensing method is an ultrasonic angle beam technique, and cracks are automatically detected from the ultrasonic response. An extended Kalman filter is applied to combine ultrasonically estimated crack sizes with a crack growth law, effectively using the time history of the ultrasonic results rather than only the most recent measurement. A natural extension of this method is fatigue life prognosis. Results from fatigue tests on 7075-T651 aluminium coupons show improved crack size estimates as compared to those obtained from ultrasonic measurements alone, and also demonstrate the capability of predicting the remaining life. This approach for fatigue crack detection, sizing and prognosis is an example of a general strategy for in situ monitoring of structural damage whereby improved results are achieved from the integration of noisy measurements with imperfect crack growth models.

The long-term performance of aeronautical structures is typically discussed in terms of concepts such as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, and constitutive equations with constant material properties, and constant or cyclically applied load conditions. Loading histories are represented by changing stress (or strain) states, only. But for many situations, especially associated with high performance aircraft, the local state of the material may also change during service, so that the properties used in those equations are functions of time and history of applied conditions. For example, local values of stiffness, strength, and conductivity are altered by material degradation to create ‘property fields’ that replace the global constants, and introduce time and history into the governing equations. The present paper will examine a small set of such problems and offer a construct for using related solutions to estimate future performance based on history of use and current material state, a concept typically called prognosis.

The health monitoring and damage prognosis of aerospace hotspots is important for reducing maintenance costs and increasing in-service capacity of aging aircraft. One of the leading causes of structural failure in aerospace vehicles is fatigue damage. Based on the physical mechanism of damage nucleation and growth, a physics-based multiscale model is considered for fatigue damage assessment in metallic aircraft structures. A guided-wave based sensing approach is utilised to enable effective damage detection in a common structural hotspot: a lug joint. Finite element analysis is carried out with piezoelectric wafers bonded to the host structure and the simulated sensor signals are analysed. A damage classification strategy is developed, which integrates physically motivated time-frequency approaches with advanced stochastic modelling techniques. In particular, a variational Bayesian learning scheme is used to estimate the optimal model complexity automatically from the data, adapting the classifier for real-time use. Classification performance is studied as a function of signal-to-noise ratio and results are reported for the detection of fatigue crack damage in the lug joint. An adaptive hybrid prognosis model is proposed, which estimates the residual useful life of structural hotspots using damage condition information obtained in real-time.

Recent studies show that analytical predictions of crack growth in rotating components can be used in conjunction with displacement measurement techniques to identify critical levels of fatigue damage. However, investigations of this type traditionally have focused on the detection of damage at known flaw locations. This paper addresses the related problem of estimating damage associated with flaws at unknown locations, through the combined use of analytical models and measured vibration signatures. Because the measured data are insufficient to identify a unique solution for the location and severity of fatigue cracks, the function of the analytical model is to bound the extent of damage occurring at life-limiting locations. The prediction of remaining life based on estimates of worst-case fatigue damage and crack locations also is discussed.