Kristian Haller

Method for monitoring slow dynamics recovery

Slow Dynamics is a specific material property, which for example is connected to the degree of damage. It is therefore of importance to be able to attain proper measurements of it. Usually it has been monitored by acoustic resonance methods which have very high sensitivity as such. However, because the acoustic wave is acting both as conditioner and as probe, the measurement is affecting the result which leads to a mixing of the fast nonlinear response to the excitation and the slow dynamics material recovery. In this article a method is introduced which, for the first time, removes the fast dynamics from the process and allows the behavior of the slow dynamics to be monitored by itself. The new method has the ability to measure at the shortest possible recovery times, and at very small conditioning strains. For the lowest strains the sound speed increases with strain, while at higher strains a linear decreasing dependence is observed. This is the first method and test that has been able to monitor the true material state recovery process.

A self-silenced sound beam

Parametric loudspeakers are transmitting two high power ultrasound frequencies. During propagation through the air, nonlinear interaction creates a narrow sound beam at the difference frequency, similar to a light beam from a torch. In this work is added the physical phenomenon of propagation cancellation, leaving a limited region within which the sound can be heard—a 1 meter long cylinder with diameter 8 cm. It is equivalent to a torch which would only illuminate objects within 1 meter. The concept is demonstrated both in simulation and in experiment.

Slow variations of mechanical and electrical properties of dielectrics and nonlinear phenomena at ultrasonic irradiation

The interconnection between variations of elasticity and dielectric permittivity of mesoscopic solid systems under exposure to ultrasound is experimentally observed. A phenomenological theory generalizing Debye’s approach for polar fluids is developed to explain the measured data. The substitution of acoustic measurements by dielectric ones not only simplifies the procedure, but also offers new possibilities to remotely evaluate the mechanical properties of materials and natural media.

Sound Velocity Dependence on Strain for Damaged Steel

When making acoustic measurements on materials with cracks, there exist two types of behavior that influence the sound velocity monitored through the resonance frequency of the object. One is the materials nonlinearity, and the other is a slow recovery process of the material parameters towards equilibrium called Slow Dynamics. The former is a wave distortion taking place in the presence of the wave while the latter is a slow recovery process that makes the time history of the material state count. For the understanding of the dynamics of these solids it is necessary to be able to separate the effects of nonlinearity and slow dynamics. In this work, this has been accomplished by making measurements on steel at steady‐state through keeping the strain constant. Normal frequency sweeps at different strains are compared to constant strain sweeps. As every material state parameter can induce a slow dynamic response it is important to keep control of humidity and temperature. Measurements performed at differen...

Design Alterations of a High Power Air Transducer

For certain resonant high-power air transducers exists a radiating cone which is in some way connected to a forcing piezoelectric ceramic. This study focuses purely on the cone structure itself, and the form of excitation that gives desired results on the vibration modes of the cone. In the new design three different parameters are changed from existing devices. They are: 1) the change of the round piezoelectric disc center position to a piezoelectric ring shape, while making the cone center fixed, 2) the cutting of the cone into leaves, and 3) the radial thickness decreasing with radius. In simulations, the new design yields considerably higher vibration amplitudes.

Non-contact nonlinear acoustic damage localization in plates. Part 2 : Localized resonance through dynamically trapped modes

This work is the second part of three that presents new tools to be used for damage localization in plates by nonlinear acoustical methods. It introduces an important in-plane localization technique, which is based on the existence of resonant spatially localized wave fields. The wave from the transducer is acting as a dynamic influence on the plate surface, making the waves reflect in a non-ideal way. The non-ideal reflections make the modes underneath the transducer have different resonant frequencies than the modes beside the insonified area. They appear both for contact and non-contact sources. In the nonlinear damage localization application, the trapped mode wave field interacts with another signal at lower frequency. This results in sidebands around the high frequency whose amplitudes are related to the amount of damage underneath the transducer.

Granite rock acoustic measurements of nonlinearity and slow dynamics recovery.

Through monitoring the resonance frequency of a granite bar, the nonlinearity and the slow recovery dynamics have been investigated. Nonlinearity was measured through a constant strain protocol, and slow dynamics was measured through a conditioning and relaxation cycle protocol. It is noteworthy that both tests show the same behavior—an increase in resonance frequency with strain for low amplitudes, which at higher strains turns into the normally expected decrease with strain.

Three Nonlinear NDE Techniques On Three Diverse Objects

Non‐Destructive Evaluation has been carried out on three different test objects, with three different methods based on exhibits of slow dynamics and nonlinear effects. The three diverse objects were cast iron, ceramic semi‐conductors on circuit boards, and rubber. The three approaches were Higher Harmonics detection (HH), Nonlinear Wave Modulation Spectroscopy (NWMS), and Slow Dynamics (SD). For all of the objects the three approaches were tried. The results showed that for each of the objects, a different method worked the best. The cast iron worked best with nonlinear wave modulation, the ceramic semi‐conductors worked well with the higher harmonics detection, while the rubber showed best results with slow dynamics.