K. E. Simmonds

Use of a transient wave propagation code for 3D simulation of cw radiated transducer fields

In order to compute continuous wave (cw) ultrasonic wave fields in complex media, where analytical approaches are extremely difficult, numerical simulations on large computational grids must be employed. The use of a time domain code, normally used for transient wave propagation in heterogeneous media, is used here as a tool to simulate continuous wave fields. As a starting point, the case of the three-dimensional pressure field radiated from a circular aperture in water is computed. These numerical simulations are performed on a massively parallel computer and compared with experiment and known theory. The computations are performed on very large three-dimensional grids that span the near to the far field. As a trial case, a numerical computation of the radiated field from a continuous-wave excited transducer in a baffle is compared with an analytical evaluation using the Rayleigh surface integral and experiment. In addition, results are presented that show the effect of a small defect placed in the beam. To do this, a small cylindrical copper scatterer was placed in the near field, in both the computation and an accompanying experiment. These cases are done in preparation for using the same approach for computing cw fields radiated from a transducer into complex heterogeneous media.

Computational and experimental investigation of the fields generated by a 1-3 piezocomposite transducer.

Large-scale three-dimensional numerical simulations using the finite-difference time domain technique are used to compute the continuous wave fields associated with a composite transducer. The interior of the transducer is made of a periodic array of square rods. This lattice causes elastic wave Bragg diffraction similar to electrons in a periodic lattice. A low frequency mode shape is assumed for the rods. This prescribed motion includes longitudinal and transverse components. It is shown that the transverse motion in the rod gives rise to shear waves causing standing waves (lateral resonances) in the polymer regions. This is also confirmed by experimental results presented here and other independent analytical and experimental work. The full-scale numerical simulation is performed on a large parallel supercomputer and permits modeling of not only the composite transducer but the radiated pressure from near to far field. In addition, cover plates and edge effects are included, unlike analytical treatments. Although only mechanical effects are included, the wave propagation approach captures many essential features.

An Improved System for the Nondestructive Evaluation of Steel

A non-magnetic horizontal load frame has been constructed allowing for x-y motion and rotation about the specimen axis in order to investigate the state of stress in steel components with a SQUID gradiometer. Tensile and cyclic loading can be performed under load or strain control. Data are taken by a Masscomp data acquisition system. Initial tests of this system have been made using a vertically-mounted SQUID gradiometer.

Determination of Elastic Constants of Anisotropic Materials from Oblique Angle Ultrasonic Wave Measurements II: Experimental

There has been a renewed interest in the study of elastic properties of anisotropic materials in recent years, particularly with the wide usage of custom made advanced composites in many aspects of aerospace structures. These composites are heterogeneous and anisotropic materials. Nondestructive evaluation of composites is highly desirable, both for defects characterization and mechanical properties. Ultrasonic methods are especially well suited for determination of the elastic properties of materials.

Modeling and DIC Measurements of Dynamic Compression Tests of a Soft Tissue Simulant

Stereoscopic digital image correlation (DIC) is used to measure the shape evolution of a soft, transparent thermoplastic elastomer subject to a high strain rate compression test performed using a Kolsky bar. Rather than using the usual Kolsky bar wave analysis methods to determine the specimen response, however, the response is instead determined by an inverse method. The test is modeled using finite elements, and the elastomer stiffness giving the best match with the shape and force history data is identified by performing iterative simulations. The advantage of this approach is that force equilibrium in the specimen is not required, and friction effects, which are difficult to eliminate experimentally, can be accounted for. The thermoplastic is modeled as a hyperelastic material, and the identified dynamic compressive (non-linear) stiffness is compared to its quasi-static compressive (non-linear) stiffness to determine rate sensitivity.

Microstructural damage metrics for failure physics

Safe and reliable design and operation of engineered systems are ultimately limited by our understanding of the physical characteristics of material performance and failure. For structural materials, deformation, damage and fracture are important. The assessment of material state at the microstructural scale is a major factor limiting predictions of system capabilities. Quantitative descriptions and metrics of deformation and damage at the microstructural scale require new methods to understand large numbers of interacting features in the complex mesoscale geometry of actual microstructures. In this paper, we discuss the use of experimental and computational techniques to describe material state. These efforts focus on the integrated use of microtomography, image based models, finite element simulation, the percolation characteristics of deformation and cellular automata simulation of morphology. The results of these efforts produce better physical understanding and mathematical descriptions of mesoscale response.

Mapping of Three-Dimensional Radiation Field of Ultrasonic Transducers

Piezoelectric transducers convert radio-frequency (rf) electrical signals into mechanical ultrasonic vibrations and are the key elements in all medical and industrial ultrasound. These are used for ultrasonic imaging, NDE, determination of material property and detection/sizing of flaws[l]. In all such measurements complete knowledge of the radiation source, receiver and associated electronics as well as the field inside the immersion fluid or the material can be useful in the understanding of the material properties. The standard information that is provided for these transducers by the vendors has limited value for imaging purposes. The manufacturers provide data in the form of rf reflection from a small target, such as a ball and its frequency spectra to indicate bandwidth. Advanced precision measurements with piezoelectric transducers will require temporal and spatial distribution of the radiation field in the propagation medium. For example, in the measurement of material properties using Lamb Waves or oblique angle time-of-flight[2] measurements, we measure the phase velocity which is dependent upon the angle of incidence. If the transducer element is misoriented inside the enclosed case, any measurements using the radiation field of such a transducer would require either alignment of the transducer field such that the spatial coordinates of maxima in the amplitude and minima in the phase at all the axial distances from the transducer coincide or else a priori knowledge of three-dimensional (3-D) mapping of fields from the transducer. In the latter case, any perturbation to these fields due to the material property variation can be measured precisely. In this paper, we show a method to map three-dimensional and volumetric radiation fields for piezoelectric transducers.

Blast Response of Protective Armor Concepts Using an Arm Surrogate

Protection and comfort are two key armor requirements to the US warfighter. NRL has supported this effort by developing QuadGard extremity protection [1], and instrumented surrogate torso and brain to assess armor and helmet systems performance [2, 3], among others. Surrogate systems for analyzing personal protection equipment for torso and brain have also been developed by other researchers in US, Australia and Canada and are reported in the literature, but no publications were found on assessment of extremity armors.Copyright

Surrogate Skull-Brain Response to a Pressure Wave

In current US Military operations, warfighters are frequently subjected to blast events, which can lead to traumatic brain injury (TBI). The causes of mild and moderate TBI are not yet well understood by the medical community, and current diagnoses rely on identifying behavioral or physiological symptoms. Characterizing the brain response to various threats should provide a better understanding of possible injury mechanisms, and this knowledge could be applied to equipment design for prevention of TBI.Copyright

Surrogate Arm Modal and Transient Response Computational Analysis

Naval Research Laboratory has been developing measurement devices to study the dynamic response of the human body, commonly known as GelMan technologies in publications. This technology is currently being extended to upper extremity designs (GelMan-Upper Extremity, Figure 1a), consisting of upper arm and forearm with surrogate bones connected by a spherical joint and surrounded by generalized surrogate tissue. Computational low speed localized impact tests on the arm surrogate have been performed and compared to corresponding experiments. The outcome of this analysis can simulate the structural response of the arm, thus providing insight into preventing or mitigating injuries sustained from car accidents, sports and/ or battlefield injuries. A modal analysis and low speed impact transient analysis have also been performed on the arm surrogate constrained at the shoulder using the finite element program ABAQUS (Figure 1b). Linear elastic material properties from open literature are used for each arm component for the analysis using three dimensional, 8-noded hexahedral elements. Modes of vibration below 500 Hertz and strain-based frequency response have been obtained. A transient analysis of the arm is also being performed; von Mises stress contours, displacements and pressures inside the arm and total arm kinematics are extracted. These computational models have been validated with low speed, localized impact experiments using surrogate arm. Impacts of 10 N peak load are applied to upper arm and forearm of the surrogate model for 1 to 3 millisecond duration. Mode shapes of the arm are observed using a high speed camera and strain based frequency response curves are obtained. Experimental data (pressure and displacements) from transient test of the arm is compared to computational analysis. Agreement between the computational and experimental arm models provides a means for more advanced arm designs and loading situations.© 2008 ASME