An investigation into the feasibility of internal strain measurement in solids by correlation of ultrasonic images

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This paper investigates the feasibility of an ultrasonic method for measuring internal displacements and strains in engineering components at depths of tens of millimetres. The principle is to use an ultrasonic array to generate images of the speckle pattern produced by the material microstructure before and after the application of load. Under the assumption of constant ultrasonic velocity, a block-matching method is used to find the relative displacement of small portions of the images between the two loading states, and hence the strain. Experiments performed using a 5 MHz ultrasonic array show that good displacement measurement results are obtained from the speckle directly below the array at depths of up to 45 mm. The results demonstrate that the technique can be used to identify the onset of plasticity and non-uniformity in strain across the field of view. However, while the actual values of strain obtained are correct in some directions, they are systematically incorrect in other directions by a factor between five and six. It is shown that this is because the change in ultrasonic velocity owing to load (the acousto-elastic effect) has, in some cases, a more significant effect on ultrasonic propagation time than the change in distance owing to strain.