The use of a lock-in amplifier to apply digital image correlation to cyclically loaded components

• A method for processing DIC strain data obtained under cyclic loading is described.
• Expensive high speed cameras are not used to capture data to map the load cycle.
• A lock-in amplifier is used to obtain the amplitude of the strains from DIC.
• Images are captured during a fatigue test without interrupting the test.
• Strain maps are obtained at loading rates greater than the camera frame rate.

An approach for processing strain data obtained from Digital Image Correlation (DIC) that can be used directly on components subject to cyclic loading, such as in fatigue testing, is described. A key challenge addressed herein is obtaining an accurate and precise surface strain map without recourse to expensive high speed cameras to capture data to map the load cycle. The basis of the approach is the application of a lock-in amplifier to the strain data obtained from images taken from low-cost low-frame rate cameras. The technique enables images to be captured throughout a fatigue test and hence evaluate the effect of any damage on the strain field, without interrupting the test. The methodology is assessed using an aluminium disc in diametric compression. It is demonstrated that the approach enables accurate strain maps to be obtained using a range of loading frequencies that are greater than the camera frame rate. A realistic example application of the technique on a fatigue test with an evolving crack in a T-shaped specimen is presented.