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The concept of a dual stiffness sensor that measures material degradation at the micromechanics scale and introduces damage variables to reflect average material degradation at a macromechanics scale is developed. The idea of a mechanical multifunctional dual stiffness sensor for in situ real-time stiffness and energy density measurements is motivated by Eshelby’s equivalent inclusion theorem. A complete analytical derivation is presented and the use of a dual stiffness/energy sensor to diagnose damage and to determine the remaining life of structures is evaluated. A numerical verification using finite element analysis is presented, and the results indicate that the sensor can be used for diagnostics and prognostics of structures. A distributed array of sensors is used to determine the onset and location of damage. Experimental results presented for uniaxial experimental tests under monotonic quasi-static loading conditions indicate that the concept can easily be implemented. For a sensor pair of known geometry and material properties, the Eshelby tensors are independent of the host: A methodology was thus developed to detect material properties of different hosts and was used to predict accurately elastic material properties of diverse materials.

The vibratory response of a structure can be represented by the superposition of traveling waves propagating along a complex structural network. Wave dynamics generated at natural boundary conditions and subsequently reflected at geometric boundary conditions can lead to pole-zero characteristics of conventional reverberated transfer functions. However, by implementing model-based wave virtual controllers at each structural element, a dereverberated transfer function can be obtained from the measured reverberated transfer function response. Because the dereverberated transfer function represents the direct path of energy transmission across a structure, it can be used to infer damage in a structure by tracking how variations in local element properties affect the propagation of incident energy through the structure. A methodology is presented for obtaining the dereverberated transfer function response for four types of structural elements, including symmetric and asymmetric discrete spring mass elements and spectral rod and beam finite elements for continuous structures. The dereverberated transfer function response is obtained by attaching virtual controllers at the terminals of each structural element. A phase damage index method is proposed based on the relative phase propagation error from element to element to quantify the type and amount of damage. Analytical results on several simulated examples confirm that the dereverberated response can be used as a method for locating and quantifying damage in structures.

In this work, the detection and identification of damage in a helicopter rotor blade is considered. An eigenstructure assignment technique is developed using measured modal test data and a finite element model of the blade to detect and characterize the extent of damage in the system. Previous eigenstructure assignment techniques have not attempted to consider the centrifugal and gyroscopic forces found in rotating systems. Additional elemental stiffness matrix terms are generated for the inboard elements due to the centrifugal force caused by the mass of the outboard elements. This force creates an enhanced sensitivity of the eigenstructure to mass changes thereby leading to an improved damage detection capability. Feasibility of the methodology is demonstrated analytically on a finite element model of the TH-55A main rotor blade.

Summary An experiment to study the interactive effect of large, threedimensional outer eddies on small streamwise vortices was conducted. The TBL near-wall structure was emulated by G¨ streamwise vortices. Outer-region, three-dimensional eddies were shed from the pitch oscillation of an airfoil with sawtooth trailing edge. The three-dimensional eddies included skewed leading and trailing edges and a wallward motion similar to insweeps observedinTBLs.Theinteractioninvolvede uctuationofthenear-wall structure as the outer eddy approached. Violent mixingof e uid and breakdownofthe near-wall structure ensuedas high-speed e uidassociatedwiththe outer eddy arrived.Thissuggeststhat theinsweep associated with the outer region helps trigger the bursting of the near-wall structure in bounded turbulent shear e ows.