NDT of joined surfaces using digital time-integrated shearography with multiple-frequency sweep

Abstract

This paper first describes the basic principles of digital time-integrated shearography in which speckle patterns of laser-illuminated object surfaces subjected to single excitation frequency are recorded over a finite time period. With image subtraction, the resulting phase-change manifests as a visible fringe pattern similar to that in conventional time-average shearography, except that dark areas denote nodal points. When this method is used for evaluating the soundness of joining, the test structure must be excited at or near resonant frequency; this requirement inhibits practical usage for field tests. To alleviate this concern, a new technique that uses a multiple-frequency sweep in digital shearography is proposed. Depending on whether or not phase-shifting by π radians is used during testing, improperly joined surfaces manifest as either a white patch (when phase-shifting is not used) or a dark patch (when phase-shifting is used) on the synthesized intensity map. This map also serves as a diagnostic map for rapid flaw detection. Thus, the user not only can conduct tests with little training provided, but also can very quickly interpret the test results from this map and report on defective areas in the joints. When need arises, the information recorded during testing also enables quantitative analysis of vibration displacements at any point on the test structure.