Investigator: Ewa Drescher-Krasicka

Technical Description:

Ultrasonic Computer Tomography of Stress is based on comparison of the amplitudes of polarized modes propagating through stressed and unstressed areas in the sample and its interference. Although the theoretical solutions for longitudinal and leaky modes do not exist, such an effort is in progress in cooperation with J. R. Willis from the Department of Mathematics and Theoretical Physics in Cambridge. The first approach to this problem is to apply a surface leaky wave propagating partially in solid material and partially in liquid also known as an interface wave and scan the same areas with the different wave length (different frequencies). It is known that the penetration depth of a surface-leaky wave in solid is on the order of one wave-length. By changing the scanning frequencies one can observe different images of the distribution of stress averaged over the thickness equal to the wave length. Even without the existing solution one can experimentally calibrate the grey scale of image for the layer with known distribution of stress across the thickness and attribute certain shades of color to certain ranges of stress values. The surface leaky wave monitors the sum of the two principal stresses close to the surface if the principal stresses are acting along the surface. At present, the scanning acoustic imaging can not assess the values of residual stresses for an arbitrary orientation of the principal stresses acting in a sample.

Technical Objectives:

• Attempt a development of tomography based on the accumulated effects of the perturbations, with the object providing quantitative assessment of stress in a sample when this varies across the thickness (experimental approach-four point bending- cooperation with the Mechanical Engineering Department of Stanford University).

• For assumed (known) distribution of stresses, calibrate the intensity of images as a function of stress experimentally for the materials tested (aluminum, glass, inconel).

Anticipated Outcome:

• Experimental data of the distribution of stress in the plane of scan obtained by surface waves and comparison with the analytical or FEM model solutions for the few chosen problems (four point bending, diametrally compressed disk, uniaxial compression). The penetration depth of the acoustic waves defines the thickness of the "slice" of the material where the average value of stress is monitored.

Accomplishments for FY 1995:

• Obtained qualitative distribution of residual stresses in the sections of thick cylindrical pipe containing a weld, comparison with the DuPont experimental data from drill-hole technique.

• Obtained distribution of residual stresses in the spot-welded steel samples-solution for Ford Motor.

Impacts and Technical Highlights:

• NIST proved feasibility of ultrasonic technique of assessing the distribution of stress across the thickness of the sample.

• "Ultrasonic Computer Tomography of Stress," Patent Application filed May 1994. (pending).

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Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister