Jan Carmeliet

Nonlinear Elastic Wave Spectroscopy (NEWS) Techniques to Discern Material Damage, Part II: Single-Mode Nonlinear Resonance Acoustic Spectroscopy

Abstract The presence of mesoscopic features and damage in quasi-brittle materials causes significant second-order and nonlinear effects on the acoustic wave propagation characteristics. In order to quantify the influence of such micro-inhomogeneities, a new and promising tool for nondestructive material testing has been developed and applied in the field of damage detection. The technique focuses on the acoustic nonlinear (i.e., amplitude-dependent) response of one of the materials resonance modes when driven at relatively small wave amplitudes. The method is termed single-mode nonlinear resonance acoustic spectroscopy (SIMONRAS). The behavior of damaged materials is manifested by amplitude dependent resonance frequency shifts, harmonic generation, and nonlinear attenuation. We illustrate the method by experiments on artificial slate tiles used in roofing construction. The sensitivity of this method to discern material damage is far greater than that of linear acoustic methods.

Micro-damage diagnostics using nonlinear elastic wave spectroscopy (NEWS)

Nonlinear elastic wave spectroscopy (NEWS) represents a class of powerful tools which explore the dynamic nonlinear stress–strain features in the compliant bond system of a micro-inhomogeneous material and link them to micro-scale damage. Hysteresis and nonlinearity in the constitutive relation (at the micro-strain level) result in acoustic and ultrasonic wave distortion, which gives rise to changes in the resonance frequencies as a function of drive amplitude, generation of accompanying harmonics, nonlinear attenuation, and multiplication of waves of different frequencies. The sensitivity of nonlinear methods to the detection of damage features (cracks, flaws, etc.) is far greater than can be obtained with linear acoustical methods (measures of wavespeed and wave dissipation). We illustrate two recently developed NEWS methods, and compare the results for both techniques on roofing tiles used in building construction.

Quantitative characterization of fracture apertures using microfocus computed tomography

Abstract Microfocus X-ray computed tomography (μCT) was used as a tool to determine the apertures of a fracture in a cylindrical sample of crinoidal limestone. After scanning, artefacts were removed from the images. Phantom objects were used to establish a calibration relationship between real fracture apertures and fracture aperture measurements on the μCT images. The performance of different procedures for quantitative fracture determination was examined. The calibration relationship was then used to determine the fracture apertures in a naturally fractured sample. A comparison of the μCT technique and a microscope technique shows a good agreement between their results.

Diffusion-Convection transport of salt solutions in cracked porous building materials Part 1: Parameters, model description and application to cracks

Diffusion-convection transport through differently oriented cracks is investigated in cell experiments. Testing two perpendicular directions of the cracks proves the existence of rotational convective flow in cell experiments. Despite the saturated condition and the equal water levels at both sides of the sample at the start of the test, the results can not be explained by a concentration dependent diffusion only. Additional convective transport takes place due to a difference of hydrostatic pressure in both cells. Based on these findings a 3D model is developed to simulate the measured transport through isolated cracks. Subsequently, it was possible to simulate transport through cracked ceramic brick, revealing dominant transport through the cracks, largely overpassing transport through the uncracked porous zones.

Quantification of microdamage in slate tiles: Comparison of nonlinear acoustic resonance experiments with visual and x-ray diagnosis

Single Mode Nonlinear Resonant Acoustic Spectroscopy (SIMONRAS) is applied to detect edgecracks in thin slate roofing tiles. The technique focuses on the acoustic nonlinear (i.e. amplitude dependent) response of a particular resonance mode of the material when driven at relatively small wave amplitudes. Undamaged materials are essentially linear in their resonant response. The same material, however, becomes highly nonlinear when cracked. The sensitivity of this method to discern damage due to edgecracking is compared to visual diagnosis of crack density and X-ray images of cracks.