New ultrasonic methods for measuring deformation and fracture related material states
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I. I N T R O D U C T I O N
THE
characterization of detormation and fracture related material states plays two major roles in materials science. In basic studies of mechanical properties, direct observation of microstructural changes is necessary to test structure-property models and to guide their future development, r'l In the utilization of materials in structures, characterization of degraded material states, coupled with engineering failure models, are necessary for the implementation of various methodologies for prediction of ~erviceability and remaining lifeJ 21 This paper describes recent applications of ultrasonic measurements to these objectives. Two generic modes of deformation are discussed: creep and large scale plastic deformation, as well as fatigue crack propagation. In each of these, ultrasonics holds the promise of providing considerable in situ information during the mechanical tests. Although this information is often less detailed than that obtained by microscopic techniques, the ability to monitor continuously and nondestructively the material state during loading offers many advantages. For example, there is considerable savings in time since the test need not be interrupted. One can also monitor the state of particular samples of a material throughout their entire lives rather than try to infer the evolution of damage from measurements made on many samples, assumed to be identical, which are destroyed after different fractions of their lives. The information so gained can also form the basis for nondestructive test techniques.
R.B. T H O M P S O N , Program Director, Metallurgy and Ceramics Program, and Professor of Materials Science and Engineering, O. B U C K , Senior Scientist, Professor of Materials Science and Engineering, J.F. SMITH, Senior Metallurgist, Professor of Materials Science and Engineering, and W . A . SPITZIG, Senior Metallurgist, are with A m e s Laboratory, Iowa State University, A m e s , IA 50011. This paper is based on a presentation made in the s y m p o s i u m "Nondestructive Evaluation--Predictive and Descriptive Probes of Mechanical Behavior of Metals" presented at the TMS-AIME fall meeting in Cincinnati, OH, October 13, 1987, under the auspices of the T M S Mechanical Metallurgy Committee. METALLURGICAL TRANSACTIONS A
Successful realization of these benefits requires three elements. First, a measurement configuration must be identified whose response is sensitive to the microstructural change of interest. Second, a wave mechanics description of the interaction has been found useful as a basis for the quantitative interpretation of the measurements. Third, suitable deformation theories must be available which relate the microstructural features sensed to the aspect of structural response of interest. In the previously mentioned modes of deformation, these elements have been found to be present, as discussed in the following sections.
II. C R E E P
The evolution of porosity at grain boundaries is an aspect of creep readily sensed by ultrasonic measurements. Un
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