The change in ultrasonic nonlinear property of a titanium alloy subjected to cyclic loading has been studied, with an objective to develop a new characterization methodology for quantifying the level of damage in the material undergoing fatigue. In order to determine the degree of nonlinearity, the ultrasonic second harmonic generation technique has been used. The second harmonic signal was monitored during the fatigue process, and a substantial increase in the second harmonic amplitude (180% increase in nonlinear factor) was observed. This indicates that the second harmonic signal is very sensitive to the microstructural changes in the material caused by fatigue.

The near-field scattering of ultrasonic Rayleigh waves from surface-breaking cracks has been studied using scanning heterodyne interferometry. Distinct two-dimensional, localized displacement patterns were observed in the near field of the scattering sites, which provide an effective tool for detecting and characterizing the defects. The observed patterns showed a dramatic increase (2×–4×) in the ultrasonic displacement levels near the crack faces, allowing the cracks to be easily distinguished from background levels. A simple explanation for the increased near-field displacement amplitudes is presented that is based on wave propagation and free-boundary reflection arguments.

A B S T R A C TThe effects of shot-peening intensity on fretting fatigue crack-initiation behaviour of titanium alloy, Ti-6Al-4V, were investigated. Three intensities, 4A, 7A and 10A with 100% surface coverage, were employed. The contact geometry involved a cylinder-on-flat configuration. Residual stress and improvement in fretting fatigue life were directly related to shot-peening intensity. The magnitude of compensatory tensile stress and its location away from the contact surface increased with increasing intensity. The relaxation of residual stress occurred during fretting fatigue which increased with increasing the number of cycles. An analysis using a critical plane-based fatigue crack-initiation model showed that stress relaxation during the fretting fatigue affects life and location of crack initiation. Greater relaxation of the residual stress caused greater reduction of fatigue life and shifted the location of crack initiation from inside towards the contact surface. Modified shear stress range (MSSR) parameter was able to predict fretting fatigue crack-initiation location, which agreed with the experimental counterparts. Also, the computed parameter showed an appropriate trend with the experimental observations of the measured fretting fatigue life based on the shot-peening intensity. a = half-width of a contact zone m = coefficient N = number of cycles N f = fatigue life s 11 = normalized longitudinal normal stress s 22 = normalized transverse normal stress s 12 = normalized shear stress S 11 = longitudinal normal stresses S 22 = transverse normal stress S 12 = shear stress S 12 max = shear stresses due to the maximum applied axial force S 12 min = shear stresses due to the minimum applied axial force S b max = maximum applied force of fatigue cycle S b min = minimum applied forces of fatigue cycle Correspondence: S. Mall.

Modeling and experimental results of an ultrasonic aperiodic flat lens for use in air are presented. Predictive modeling of the lens is performed using a hybrid genetic-greedy algorithm constrained to a linear structure. The optimized design parameters are used to fabricate a lens. A method combining a fiber-disk arrangement and scanning laser vibrometer measurements is developed to characterize the acoustic field distribution generated by the lens. The focal spot size is determined to be 0.88 of the incident wavelength of 80-90 kHz at a distance of 2.5 mm from the lens. Theoretically computed field distributions, optimized frequency of operation, and spatial resolution focal length are compared with experimental measurements. The differences between experimental measurements and the theoretical computations are analyzed. The theoretical calculation of the focal spot diameter is 1.7 mm which is 48% of the experimental measurement at a frequency of 80-90 kHz. This work illustrates the capabilities of a hybrid algorithm approach to design of flat acoustic lenses to operate in air with a resolution of greater than the incident wavelength and the challenges of characterizing acoustic field distribution in air.

The directional dependence (angular dispersion) of surface acoustic phonons propagating on {100}, {110}, and {111} oriented planes of nickel single crystals has been measured by Brillouin scattering and continuous wave scanning acoustic microscope. All the elastic constants C11, C12, C44 are obtained from the angular dispersion of each plane separately. These are compared with the elastic constants determined with conventional ultrasonic measurements on the same specimens. The difference in the sampling depth or the approximate depth of propagation of the acoustic waves into the specimen in each technique is used to characterize the extent of the polish-induced damage zone in the sample.

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We present a high resolution electrical conductivity imaging technique based on the principles of eddy current and atomic force microscopy (AFM). An electromagnetic coil is used to generate eddy currents in an electrically conducting material. The eddy currents generated in the conducting sample are detected and measured with a magnetic tip attached to a flexible cantilever of an AFM. The eddy current generation and its interaction with the magnetic tip cantilever are theoretically modeled using monopole approximation. The model is used to estimate the eddy current force between the magnetic tip and the electrically conducting sample. The theoretical model is also used to choose a magnetic tip-cantilever system with appropriate magnetic field and spring constant to facilitate the design of a high resolution electrical conductivity imaging system. The force between the tip and the sample due to eddy currents is measured as a function of the separation distance and compared to the model in a single crystal copper. Images of electrical conductivity variations in a polycrystalline dual phase titanium alloy (Ti-6Al-4V) sample are obtained by scanning the magnetic tip-cantilever held at a standoff distance from the sample surface. The contrast in the image is explained based on the electrical conductivity and eddy current force between the magnetic tip and the sample. The spatial resolution of the eddy current imaging system is determined by imaging carbon nanofibers in a polymer matrix. The advantages, limitations, and applications of the technique are discussed.