T. Oshima* and R.D. Kriz **
* Professor, Kitami Institute of Technology, Japan
** Associate Professor, Director, Laboratory for Scienitific Visual Analysis, Virginia Polytechnic Institute and State University, USA
Our goal in this study is to investigate how a SAM can be used to evaluate elastic property gradients that exist near a fiber-matrix interface. Unlike images from a reflecting microscope, a SAM image can highlight differences in elastic properties. For example look at this SAM image of a [0/90] glass/epoxy fiber- reinforced composite laminate [Briggs, 85]. Closer examination of this image reveals a gray region ("interphase") surrounding each fiber in the 90 degreee ply which could be the result of an elastic property gradient. Superior fracture properties of fiber- reinforced composites are the result of various crack arrest mechanisms that are known to exist near this fiber-matrix interface. Recent research suggests that the mechanics of fiber-interface deformation and failure are controlled by these eastic property gradients.
These gradients can not be measured if they exist in a region smaller than the SAM apperature (spot size) on the 90 degree surface. In such cases our attention changes to the 0 degree ply where we observe gradients near fibers that are well below the surface. Unlike the gradients in the 90 degree ply, gradients below and parallel to the surface can be measured by accurately controlling the wavelength to within fractions of the gradient dimensions (a dispersion problem). From a full field simulation below the 0 degree ply surface it is possible to observe and interpret the physical origin of the reflected wave. Unexpectedy we also observe the creation of a guided wave.
A special interactive software tool was developed to visually compare the influence of different interphase structures on the creation of these reflected and guided waves. Results suggest that a more accurate measure of interphase gradients is possible by scanning and measuring the guided mode shape on the surface adjacent to the transmission [Kriz-Oshima-Nomachi, 92]. Play the SAM animation and observe how two different interphase structures affect the developement of guided modes along the fibers. Note the animation also shows how these guided modes are created by reflections from the top and bottom fiber interphase reflections.
References:
Briggs A. , An Introduction to Scanning Acoustic Microscopy, Oxford University Press, University of Oxford, England, pg. 30, 1985.
Kriz R.D. , Oshima T. , and Nomachi S.G. , "Scanning Acoustic Microscope Simulation for Determining Interphase Structure," International Union of Theoretical and Applied Mechanics Symposium: Local Mechanics Concepts for Composite Material Systems, Springer-Verlag, pp. 395-412, 1992.
