Ronald A. Kline

Wave propagation in fiber reinforced composites for oblique incidence

on an analysis of the reflected wave amplitudes. This procedure works well for identifying gross composite defects. However, in many cases, one is concerned with more subtle defects (e.g., porosity, local variation in fiber orientation, segregation of reinforcing fibers, etc.); defects which are difficult ot identify with conventional data analysis procedures. Since defects such as these will principally affect the local moduli, ultrasonic velocity measurements are quite useful in analyzing these types of problems [1]. Further, since fiber reinforced composites are anisotropic materials, one would ideally like to examine directional

Plate Wave Propagation in Transversely Isotropic Materials

In this work, the modes of plate wave propagation in a fiber reinforced com posite (assumed to be transversely isotropic) were investigated. The waves were modeled as harmonic plane waves propagating in the plane of an infinite plate whose bounding sur faces were assumed to be stress free. A numerical analysis procedure was developed to calculate the dispersion relationships for plate waves propagating in arbitrary directions in the plate. Particle displacements and stress distributions were calculated for several im portant propagation modes. Possible applications to nondestructive testing of composites are discussed.

Nondestructive Evaluation of Changes in Mechanical Properties in Carbon-Carbon Composites during Processing:

In this work, changes in the mechanical properties of carbon-carbon composites during processing are studied using nondestructive characterization techniques. Ultrasonics and radiography are used to measure the local stiffnesses in carbon-carbon panels after each stage in the processing cycle. The degree of material inhomogeneity (as measured by the standard deviation in the local moduli) was monitored throughout the fabrication process. The nondestructively measured stiffnesses were compared with direct mechanical measurements to assess the viability of the technique.

A finite difference approach to acoustic tomography in anisotropic media

In this paper, a finite difference formulation for acoustic wave propagation is used as the basis for tomographic reconstruction. This approach offers some interesting advantages over traditional, ray based methods; particularly for anisotropic media. Since this approach provides information on the full acoustic field (not individual rays), it offers a conventional way to incorporate beam skew and ray bending phenomena directly into the problem formulation. Here, we present a tomographic reconstruction algorithm which is adapted from the algebraic reconstruction technique (ART) to take full advantage of the finite difference formulation of the problem. Results are presented to illustrate the utility of this approach.

Ultrasonic Characterization of Composite Microstructure

In this work, nondestructive methods for the quantitative evaluation of composite microstructure are described. Parameters of interest in this regard might include fiber volume fraction, porosity, degree of cure (thermoset resins) and crystallinity (thermoplastic resins). These methods rely upon the high degree of accuracy which can be obtained with ultrasonic measurements of acoustic velocities; hence the mechanical properties of the material, and well established composite micromechanics relationships between elastic properties and composite microstructure. This results in sets of nonlinear equations which can be solved for the unknown microstructural parameters.

Residual stress analysis using multiparameter tomographic reconstruction

In this work, the potential application of acoustic tomography to determine the distribution of residual stresses is discussed. Multiparameter reconstruction techniques are presented for both 2-D and 3-D residual stress states along with results from synthetic data. The effect of measurement errors on the accuracy of the reconstruction is also presented.

Wave Propagation in a Composite with Wavy Reinforcing Fibers

In the manufacture of composite structures the alignment of the reinforcing fibers can be distorted into wavy patterns due to uneven curing and shrinkage of the resin. Such wavy distortions will reduce both the strength and the stiffness of the composite structure. Yet if the fibers are intentionally formed into wavy patterns in an elastomer matrix a composite material with a stiffness that responds to changing loads and strains is produced. Such materials can be applied to uses were great flexibility is required within a limited range but high stiffness is required outside the range. This is the requirement for athletic knee braces and aircraft arrestor nets to name only two applications. Wave propagation techniques can be used to study the effects of fiber waviness on the stiffnesses of composite materials.

Perturbation and Finite Difference Solutions for Wave Propagation in Composites with Periodic Stiffness Variations

Wave propagation in composite materials with discrete changes in properties has been extensively studied and is well understood. In contrast, wave propagation in composites with smooth continuous periodic stiffness variations has only begun to be studied [1]. Use of direct analysis techniques for wave propagation in a composite material with varying stiffness has lead to mathematical contradictions and has indicated the need for a different approach [2]. The present study investigated wave propagation in a composite with smooth continuous periodic stiffness variations using perturbation techniques and a model simulation with a refined finite difference method.

Parallel Processing Technique for Acoustic Tomograghy of Multilayers

Computerized tomography (CT) is one of the most efficient tools for the nondestructive inspection of a material specimen. Depending on the nature of the application (e.g. in medicine, mechanics or geophysics) and of the particular requirements of the situation, different types of radiation (waves) may be used, thus giving different information about the material properties. For example in diagnostic medicine, x-ray transmission CT gives an attenuation map, which allows different tissues to be distinguished. In the case of x-rays, this type of wave passes through an entire material structure in an almost straight line.

Acoustic ray tracing with the connection machine

Acoustic rays do not travel in straight raypaths in nonhomogeneous media. This causes difficulty in tomographic image reconstruction. A method for acoustic ray tracing in nonhomogeneous media has been developed using the parallel processing capabilities of the Connection Machine. This method of ray tracing is intended to facilitate a parallel processing approach to tomographic image reconstruction.