Development of an ultrasonic technique for the measurement of machining induced surface residual stresses
Abstract
A non-destructive measurement method has been developed to detect the presence of the surface residual stresses in a machined workpiece. This method applies the theory of acoustoelasticity to establish the relationship between surface acoustic wave velocity and residual stresses in a near-surface region of a machined part. The application of ultrasonic technique was employed in the generation and reception of surface wave propagation. Rayleigh waves were identified as the means of detection and measurement. The measurement concept was based on the pitch-catch method, in which the generated ultrasonic signals travel through a pre-determined region in the workpiece and being received by a second identical transducer. The measured relative travel times (also known as the flight time) provide residual stress information in surface region over which the ultrasonic signals travel in the form of Rayleigh surface waves, generated of the presence of surface residual stresses. Several design configurations were evaluated to search for a reliable ultrasonic signal reception. The final design configuration adopted the contact configuration, in which the ultrasonic transducer devices were placed directly on the surface of the workpiece through a set of identical acoustic lenses. Repeatable and reliable ultrasonic measurements were achieved with the final measurement design. It was learned that several competing effects such as the workpiece's hardness condition, material prefer orientation (from forming processes), and possible microstructure formation, etc., could have an influence on the measurement of surface wave velocity. Experimental studies were conducted with these influencing parameters isolated to show the presence of stresses in tested samples. A linear relationship between applied stresses and surface wave velocity was established for common material such as aluminum and steel alloys. A finite element model was constructed to provide a verification tool as well as simulation tool for the study of non-uniform stress field in an elastic body.
Degree
Ph.D.
Advisors
Furgason, Purdue University.
Subject Area
Mechanical engineering
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