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This paper presents the development of a new non-contact acousto-thermal signature (NCATS) nondestructive evaluation technique. The physical basis of the method is the measurement of the efficiency of the material to convert acoustic energy into heat, and a theoretical model has been used to evaluate this. The increase in temperature due to conversion of acoustic energy injected into the material without direct contact was found to depend on the thermal and elastic properties of the material. In addition, it depends on the experimental parameters of the acoustic source power, the distance between sample and acoustic source, and the period of acoustic excitation. Systematic experimental approaches to optimize each of the experimental variables to maximize the observed temperature changes are described. The potential of the NCATS technique to detect microstructural-level changes in materials is demonstrated by evaluating accumulated damage due to plasticity in Ti-6Al-4V and low level thermal damage in polymer matrix composites. The ability of the technique for macroscopic applications in nondestructive evaluation is demonstrated by imaging a crack in an aluminum test sample.

Experimental data on cycles to failure are reported for a SCS-6/TIMETAL 21S [0/90]s composite under isothermal fatigue at 650 and 815°C over a range of frequencies from 0.01 to 200 Hz as well as under in-phase and out-of-phase thermomechanical fatigue from 150 to 650°C. Several models are evaluated for correlating fatigue lives based on computed micromechanical stresses using a life fraction concept that considers both cycle-dependent and time-dependent terms. Fractography is used to illustrate the range of failure modes in this material system as well as to both guide and validate the modeling concepts. Over the range of conditions covered in this investigation, purely time-dependent, purely cycle-dependent, fiber-dominated, and matrix-dominated modes of failure, as well as combinations of these, are observed.

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Interaction of high amplitude acoustic waves with materials produces a small increase in the temperature that can be detected and measured using an IR camera. The changes in temperature as a function of time, due to interaction of high amplitude 20 kHz acoustics, with as received and fatigue damaged polycrystalline Ti-6Al-4V samples are compared. The maximum temperature reached by the sample has been found to increase with increasing fatigue cycles. The role of multiple physical mechanisms, responsible for conversion acoustic energy to heat, like the sample geometry (finite dimension), the microstructure (grain size), and dislocation density are examined. The theoretically evaluated temperature changes are observed to be in reasonable agreement with experimental measurements. The significance of the details of microstructure and dislocation properties needed in theoretical evaluation of temperature changes are used to explain the observed differences between experimental measurements and theoretical calculations.