Shamachary Sathish

Ultrasonic Linear and Nonlinear Behavior of Fatigued Ti–6Al–4V

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.

Thermo-acoustic fatigue characterization

The nondestructive detection of early fatigue damage states is of high importance for safety in aircraft, automobiles, railways, nuclear energy industries and chemical industries. Titanium alloys commonly used in aerospace for structures and engine components are subject to fatigue damage during service. In the current study fatigue damage progression in a titanium alloy (Ti-6Al-4V) was investigated using thermographic detection of the heat dissipated during short-term mechanical loading. The initial rate of temperature increase induced by the short-term mechanical loading was used to indicate the current microstructural state and presence of prior fatigue damage. Two methods for thermal excitation were investigated (a) high amplitude mechanical loading and (b) small amplitude ultrasonic loading. A formula that describes the temperature enhancement due to heat generation during one loading cycle is derived from high amplitude loading data. A correlation between the temperature increase during short-term ultrasonic loading and accumulated fatigue cycles is used to suggest a methodology for in-field assessment of fatigue condition.

Near-field ultrasonic scattering from surface-breaking cracks

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.

Residual stress distribution on surface-treated Ti-6Al-4V by X-ray diffraction

The x-ray diffraction technique has been used to measure surface residual stress in Ti-6Al-4V samples subjected to shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB). The magnitude, spatial and directional dependence and uniformity of the surface residual stresses have been investigated. The results show that residual stresses due to SP are uniform and independent of direction. LSP has been observed to produce non-uniform residual stress varying from one region to another, and also within a single laser shock. In the case of LPB, residual stresses have uniform spatial distribution but have been observed to be direction-dependent. Various components of the residual stress tensor in the LPB sample have been determined following the Dolle-Hauk method. The results of the residual stress due to three surface treatments are compared, and possible reasons for spatial and directional dependence are discussed.

Nonlinear Laser Ultrasonic Measurements of Localized Fatigue Damage

A nonlinear laser ultrasonic system was developed and used to characterize the fatigue state of a fractured Ti‐6Al‐4V sample with high spatial‐resolution and sensitivity. The measurement system is built around a scanning heterodyne interferometer, which allows detailed displacement field images to be created and visualized for propagating surface and bulk acoustic fields on a material surface. An assessment of the local fatigue damage of the material was made using nonlinear ultrasonic interaction principles, where the local amplitudes of the fundamental and second harmonic displacement fields are monitored simultaneously. This provides a means for evaluating the local acoustical nonlinearity parameter, β, which can be related to the accumulation of fatigue damage in a material. A large increase in β was observed between the unfatigued area (near the grip section) and the heavily fatigued area (gauge section) for a fractured dogbone specimen. The measurements show the potential for spatially‐resolving the...

Focusing of longitudinal ultrasonic waves in air with an aperiodic flat lens.

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.

Development of eddy current microscopy for high resolution electrical conductivity imaging using atomic force microscopy

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.

Understanding and predicting electronic vibration stress using ultrasound excitation, thermal profiling, and neural network modeling

Vibration stress is a major source for failure of electronics components. In this study, we used an ultrasonic/infrared system to generate vertical vibration of a microprocessor on a printed circuit board, in order to study the impact of short burst high load vibration stress on component integrity. The objectives of this research were to (1) understand the impact of vibration on electronic components under ultrasound excitation; (2) model the thermal profile presented under vibration stress; and (3) predict stress level given a thermal profile of an electronic component. Research tasks included: (1) retrofit of current ultrasonic/infrared nondestructive testing system with sensory devices for temperature readings; (2) design of software tool to process images acquired from the ultrasonic/infrared system; (3) developing hypotheses and conducting experiments; and (4) modeling and evaluation of electronic vibration stress levels using a neural network model. Results suggest that (1) an ultrasonic/infrared system can be used to mimic short burst high vibration loads for electronics components; (2) temperature readings for electronic components under vibration stress are consistent and repeatable; (3) as stress load and excitation time increase, temperature differences also increase; (4) components that are subjected to a relatively high pre-stress load, followed by a normal operating load, have a higher heating rate and lower cooling rate. These findings are based on grayscale changes in images captured during experimentation. Discriminating variables and a neural network model were designed to predict stress levels given temperature and/or grayscale readings. Results suggest a 15.3% error when using grayscale change rate and 12.8% error when using average heating rate within the neural network model. Data were obtained from a high stress point (the corner) of the chip.

Quantitative imaging of Rayleigh wave velocity with a scanning acoustic microscope

An acoustic microscope operating with impulse excitation has been used to perform measurements of the Rayleigh wave velocity by measuring the time difference between the direct reflected signal and the Rayleigh wave signal. The accuracy and precision of the methodology have been examined by performing measurements at a single location on an elastically isotropic sample of E6 glass. The accuracy of the Rayleigh wave velocity measurement has been determined to be better than 0.5%. The measured Rayleigh wave velocity of (3035/spl plusmn/5) m/s differs by 0.3% from measurements reported in the literature for a similar sample, using two different techniques. The methodology has been extended to acquire the Rayleigh wave velocity while raster scanning the sample to develop a quantitative velocity image. The background noise in the Rayleigh wave velocity image has been investigated by mapping the velocity on elastically isotropic E6 glass. Possible reasons for background noise in the images is discussed. The methodology has been extended to acquire quantitative Rayleigh wave velocity images on Ti-6Al-4V. The contrast in the images is attributed to the variation of the Rayleigh wave velocity in individual grains or regions. Applicability of the technique to investigate crystallographic texture in materials is discussed.

In-situ monitoring of acoustic linear and nonlinear behavior of titanium alloys during cycling loading

An in-situ technique to measure sound velocity, ultrasonic attenuation and acoustic nonlinear property has been developed for characterization and early detection of fatigue damage in aerospace materials. A previous experiment using the f-2f technique on Ti-6Al-4V dog bone specimen fatigued at different stage of fatigue has shown that the material nonlinearity exhibit large change compared to the other ultrasonic parameter. Real-time monitoring of the nonlinearity may be a future tool to characterize early fatigue damage in the material. For this purpose we have developed a computer software and measurement technique including hardware for the automation of the measurement. New transducer holder and special grips are designed. The automation has allowed us to test the long-term stability of the electronics over a period of time and so proof of the linearity of the system. For the first time, a real-time experiment has been performed on a dog-bone specimen from zero fatigue al the way to the final fracture.