Interest in using the higher harmonic generation of ultrasonic guided wave modes for nondestructive evaluation continues to grow tremendously as the understanding of nonlinear guided wave propagation has enabled further analysis. The combination of the attractive properties of guided waves with the attractive properties of higher harmonic generation provides a very unique potential for characterization of incipient damage, particularly in plate and shell structures. Guided waves can propagate relatively long distances, provide access to hidden structural components, have various displacement polarizations, and provide many opportunities for mode conversions due to their multimode character. Moreover, higher harmonic generation is sensitive to changing aspects of the microstructures such as to the dislocation density, precipitates, inclusions, and voids. We review the recent advances in the theory of nonlinear guided waves, as well as the numerical simulations and experiments that demonstrate their utility.
The need for micro-mechanics based understanding leading to meso-scale models for understanding relation between microstructure and ultrasonic higher harmonic generation is emphasized. Three important aspects of material behavior, namely tension-compression asymmetry, shear-normal coupling and deformation induced anisotropy that are relevant to ultrasonic higher harmonic generation are identified. Of these, the role of tension-compression asymmetry in micro-scale material behavior on ultrasonic higher harmonic generation is investigated in detail. It is found that the tension-compression asymmetry is directly related to ultrasonic even harmonic generation and an energy based measure is defined to quantify the asymmetry. Using this energy based measure, a homogenization based approach is employed to quantify the acoustic nonlinearity in material with micro-voids and the findings are discussed.
The generation of cumulative second harmonic ultrasonic guided wave modes is analyzed with respect to their
applications for nondestructive evaluation (NDE) and structural health monitoring (SHM). Due to the multimodal nature
of guided waves, the selection of a primary wave mode that will generate a cumulative second harmonic is a critical first
step for NDE and SHM applications. Thus, the nonlinear boundary value problem that must be solved by perturbation
analysis is summarized and the results are tabulated for steel plates and circular cylindrical shells (pipes). The analysis
includes shear-horizontal and Lamb waves in plates and axisymmetric torsional and longitudinal waves in pipes.
Nonlinear finite element analyses that include kinematic and material nonlinearities are conducted for plate and pipe
geometries. An excitation is applied to both plate and pipe samples by a simulated interdigitated transducer; SH1 for the
plate and T(0,2) for the pipe. In each case a second harmonic mode having an orthogonal polarization is generated; S1
for the plate and L(0,4) for the pipe. In both cases the second harmonic grows linearly with propagation distance, and
therefore is cumulative. A third example simulation is presented that demonstrates mode mixing in a pipe. T(0,3) and
L(0,2) primary modes traveling in opposite directions intersect and generate significant harmonics at the sum and
difference frequencies. Mode mixing provides a great opportunity to expand the potential of harmonic generation for
NDE and SHM.
Piezoelectric fiber composite (PFC) transducers can be used to transmit and receive ultrasonic guided waves for
structural health monitoring. Comb-type surface mounted PFC transducer strips are used to excite planar Lamb waves
that interact with cracks and corrosion in a plate. Both finite element simulations and experiments examine the
wave/defect interaction within a square domain that could be bounded by four strip transducers. In addition to
transmitted, reflected, and scattered wave energy, beam spreading is investigated. Boundary conditions are applied in the
finite element simulation to eliminate artificial end wall reflections. The experiments use a Doppler laser vibrometer to
visualize wave propagation. Parallel PFC strips at wavelength spacing comprise an actuator that generates a wave field
that is imaged by the scanning vibrometer. The results of the finite element simulation are qualitatively confirmed by the
The durability of concrete pavement to freeze/thaw cycles is mainly dependent on the air void system. Air entraining
admixtures are used to provide a beneficial air void system. Large entrapped air voids reduce strength and make it
insufficient to simply characterize the porosity by the air content; void spacing and specific surface parameters are also
important. Ability to perform quality assurance testing - nondestructive evaluation - of concrete pavement soon after
placement is highly desirable. Thus, laboratory experiments have been conducted to investigate Rayleigh surface waves
for characterization of porosity in fresh concrete. A mediator mounted onto a Plexiglas wedge is used to introduce waves
from an ultrasonic transducer onto the surface of the concrete. The challenges are that fresh concrete is highly
attenuative and that the material properties evolve as the concrete sets. Rayleigh wave speed is shown to be sensitive to
porosity by simple micromechanical modeling, and results are presented for normal concrete with large aggregate, sieved concrete, and mortar. Wave speeds are significantly less (10-22% depending on time after placement) for concrete with approximately 5% porosity relative to concrete with no air entrainment admixture.
Continuous strip transducers are investigated to generate ultrasonic guided waves for structural health monitoring of
plate and shell structures. A theoretically driven approach, based on the application of wave mechanics principles, will
be used to research and design a network of strip transducers. The initial transducer concept is that the strips will
function like comb transducers and generate planar Lamb waves that travel normal to the longitudinal axis of the strip.
Fibrous piezoelectric composites are considered for the comb elements, expanding the design space of these elements to
include fiber orientation and volume fraction in addition to size, configuration, and location of the electrodes. This paper
presents results from wave propagation studies of continuous strip actuators. As intended, the strip transducer excites
Lamb waves that are reasonably planar with little interaction with the energy that propagates in the direction of the strip.
Micromechanical modeling results for overall material properties of piezoelectric fiber composites are presented and
demonstrate the wide design space for these types of devices.
The ultrasonic guided wave phased array technique offers an efficient means to interrogate damages in plate-like
structures. When applying this technique to multilayer composite plates, however, the anisotropic behavior of the
composite materials leads to significant influences on the beam steering performances of the phased arrays. This paper
investigates the beam steering performances of guided wave phased arrays for multilayer composite plates in terms of
phased array directivity profiles under influences of anisotropy. Angular dependences of guided wave amplitudes and
phase variations in composite plates obtained through a Green's function based method are implemented into directivity
profile calculations to account for the influences of anisotropy in a quantitative way. Guided wave phased array
experiments are carried out to validate the directivity profile calculations.