Recent progress in quantitative ultrasound (QUS) has shown increasing interest toward measuring long bones by ultrasonic guided waves. This technology is widely used in the field of nondestructive testing and evaluation of different waveguide structures. Cortical bone provides such an elastic waveguide and its ability to sustain loading and resist fractures is known to be related to its mechanical properties at different length scales. Because guided waves could yield diverse characterizations of the bone's mechanical properties at the macroscopic level, the method of guided waves has a strong potential over the standardized bone densitometry as a tool for bone assessment. Despite this, development of guided wave methods is challenging, e.g., due to interferences and rnultiparametric inversion problems. This paper discusses the promises and challenges related to bone characterization by ultrasonic guided waves.

Ultrasonic guided waves in bone

Classical techniques based on bone imaging allow visual examination or provide quantitative parameters like bone mineral density, which is a mass per unit of surface. Unfortunately, these techniques are generally expensive. A new method of bone characterization that uses ultrasound techniques is presented in this article. This method is able to evaluate mechanical properties like Young's modulus E and cortical bone thickness with low-frequency transducers and does not require the use of a coupling medium. Some results of Young's modulus measurements are presented and compared to the literature values and the pulse echo method.

Development of a new ultrasonic technique for bone and biomaterials in vitro characterization.

Semi-analytical formulation for the guided waves-based reconstruction of elastic moduli

Broadband Lamb wave measurements in aluminium and carbon/glass fibre reinforced composite materials using non-contacting laser generation and detection

Surface wave analysis is usually applied as a 1D tool to estimate V S profiles. Here we
evaluate the potential of surface wave analysis for the case of lateral variations. Lateral
variations can be characterized by exploiting the data redundancy of the ground
roll contained in multifold seismic data. First, an automatic processing procedure is
applied that allows stacking dispersion curves obtained from different records and
which retrieves experimental uncertainties. This is carried out by sliding a window
along a seismic line to obtain an ensemble of dispersion curves associated to a series
of spatial coordinates. Then, a laterally constrained inversion algorithm is adopted to
handle 2D effects, although a 1D model has been assumed for the forward problem
solution. We have conducted different tests on three synthetic data sets to evaluate the
effects of the processing parameters and of the constraints on the inversion results.
The same procedure, applied to the synthetic data, was then tested on a field case. Both
the synthetic and field data show that the proposed approach allows smooth lateral
variations to be properly retrieved and that the introduction of lateral constraints
improves the final result compared to individual inversions.

Retrieving lateral variations from surface wave dispersion curves

A nondestructive technique is described for the measurement of elastic constants of isotropic plates using ultrasonic Rayleigh-Lamb waves. The experimental method employs continuous harmonic waves and a pair of variable-angle contact transducers in pitch-catch mode. The phase velocity of the R-L waves at a particular frequency is determined from the phase shift over a measured path length. This simple experimental technique can measure phase velocity over the range 1–10 mm/µs with an error of less than 0.5% over a frequency range of 50 kHz-2 MHz. Individual symmetric and antisymmetric modes can be generated through the selection of transducer angle and frequency. Young's modulus and Poisson's ratio for the material are calculated from measurements of frequency and phase velocity by a nonlinear least squares solution to the dispersion equations. The sensitivity of the nonlinear least squares function to the measurement region of the dispersion curve is investigated. It was found that estimations of material properties are more accurate and less sensitive to small experimental errors when only selected frequencies and R-L modes are used in the least squares calculation. This technique is demonstrated with several isotropic materials and with both thick (6 mm) and thin (0.8 mm) plates. Values for elastic constants determined by the contact transducer Lamb wave technique compare favorably with values measured using the pulse-echo-overlap method. The uncertainty in measurements of Young's modulus and Poisson's ratio was less than 1% and 2%, respectively. The technique has advantages over more traditional methods for measuring elastic properties when it is desirable to use wavelengths greater than the plate thickness, when properties may vary with frequency, or when it is necessary to measure in-plane elastic properties of thin plate structures.

W. P. Rogers

Papers

Elastic property measurement using Rayleigh-Lamb waves