Effects of Crack Closure on Ultrasonic Transmission
Ultrasonic waves are attenuated as they propagate past the tip of a crack due to the reflection of the energy at the crack face and diffraction at the crack tip. Crack closure modifies the situation since partial transmission can occur at points along the crack face where asperities come in contact. This phenomenon is important in defining the ability to nondestructively detect closed cracks and in developing a more detailed understanding of the closure phenomenon itself. Modified compact tension specimens were used to investigate the effects of partial crack closure on focussed, through-transmission ultrasonic signals. Data obtained from fatigue cracks in 7075-T651 A1 provides evidence for a gradual transition from a fully closed crack condition at the crack tip to an essentially fully open condition at a distance of a few mm from the tip, with additional localized contact along the length of the crack. This interpretation of the data was aided by a two-dimensional, quasi-static model for ultrasonic interaction with a partially contacting interface. The model relates width and separation of asperity contacts to the frequency dependence of the ultrasonic reflection and transmission. These measurements were supplemented by tests in which water infiltrated into the crack opening. The frequency spectra of the ultrasonic transmitted signals for this case were used to estimate the average COD at various points along the crack length.
KeywordsFatigue Crack Plastic Zone Transmission Coefficient Crack Closure Crack Opening Displacement
Unable to display preview. Download preview PDF.
- 2.N. Walker and C. J. Beevers, Fatigue of Engineering Materials and Structures 1, 135 (1979).Google Scholar
- 3.O. Buck and B. J. Skillings, in “Review of Progress in Quantitative NDE”, D. O. Thompson and D. E. Chimenti, eds., Vol. 1 ( Plenum Press ) (1982).Google Scholar
- 5.W. Elber, in “Damage Tolerance in Aircraft Structures”, ASTM STP 486, American Society for Testing and Materials, 1971, p. 230.Google Scholar
- 7.J. Lankford and D. L. Davidson, Advances in Fracture Research, 899, 1981.Google Scholar
- 8.J. M. Baik, L. Hermann, and R. J. Asaro, Mechanics of Fatigue, ASME 33, 1981.Google Scholar
- 9.O. Buck, B. J. Skillings and L. K. Reed, these proceedings.Google Scholar
- 11.O. Buck, J. D. Frandsen, and H. L. Marcus, in “Fatigue Crack Growth Under Spectrum Loads”, ASTM STP 595, American Society for Testing and Materials, 1976, p. 101.Google Scholar
- 12.O. Buck and B. R. Tittmann, “The ultrasonic characterization of fatigue cracks”, in, Advances in Crack Length Measurements, C. J. Beevers, ed., (Engineering Materials Advisory Services (EMAS)), Cradley-Heath, Warley, West Midlands, U.K. (in press).Google Scholar
- 13.H. Tada, P. Paris, and G. Irwin, The Stress Analysis of Cracks Handbook, (Del Research Corporation, St. Louis, 1973 ).Google Scholar