Jean-François Thimus

Use of ultrasonics to follow crack growth

Cubes made of steel fiber reinforced concrete (SFRC) were progressively sawed, while recording the ultrasonic parameters between the saw-cuts. The waves were analyzed in terms of ultrasonic wave velocity, peak amplitude and wave energy. Different relative positions between transducers and notch were considered. It was found that a relation between the ultrasonic parameters and the notch length could be drawn according to the layout of the transducers: the energy and amplitude drop once the portion of material between both transducers is diminished by sawing, whilst the velocity only changes once this whole portion of material has been cut out. The findings were then used to follow the crack growth of a SFRC beam during a bending test. The propagation of ultrasonic compression waves through the samples was recorded at regular time intervals during the tests. The interpretation of the signals was in this case complicated due to the facts that micro-cracking occurred, that the macro-crack was much thinner than a saw-cut and that it was moreover bridged by fibres. It appeared that in this case both peak amplitude and energy dropped before the onset of a visible macro-crack, because of extensive micro-cracking, while the velocity only changed after considerable crack growth

Micromechanics of the Deformation and Damage of Steel Fiber-reinforced Concrete

This article presents a study of steel fiber-reinforced concrete (SFRC). In its first part, a four-point bending test performed on both plain concrete and SFRC is investigated. The collected nonlinear load—deflection curves are transformed into stress—strain curves with the help of an incremental method, which the authors developed in the nonlinear regime. In the second part of this article, the authors present a micromechanical approach based on Mori—Tanaka/ Voigt mean-field homogenization schemes in order to model the effective nonlinear behavior of the three-phase brittle composite materials. The first phase (concrete matrix) is assumed to obey Jus brittle damage model. The second phase (fibers) is modeled with classical J 2 plasticity, while the third phase represents cavities. Numerical algorithms enable the simulation of SFRC within reasonable CPU time and memory requirements. The homogenization module is interfaced to the finite element package ABAQUS. A two-scale simulation of the bending test is validated against the experimental results.

The properties of coupling agents in improving ultrasonic transmission

Ultrasonic wave propagation is often used in rock mechanics and mining sciences. It is applied in the field for geophysical investigations and tin the laboratory for materiel characterization and non destructive evaluation.

Cracking Behavior of Steel Fiber Reinforced Concrete Revealed by Means of Acoustic Emission and Ultrasonic Wave Propagation

An experimental study on the cracking behavior of steel reinforced concrete (SFRC) is reported. Third-point bend tests were carried out on notched beams. Test setup enabled recording of load bean deflection, and crack mouth opening displacement. Simultaneously, acoustic waves caused by cracking were counted. At regular time intervals, ultrasonic compression waves were induced at one beam end and recorded at the other beam end after propagation through the sample. Both methods reveal the cracking behavior of SFRC. A frequency analysis of the acoustic activity reveals that low-frequency events can be attributed to microcracking, while high-frequency emissions unveil macroscopic happenings such as macrocracks, load transfer from matrix to fibers, and fiber pullout. The ultrasonic waves can be interpreted from their energy or velocity The wave energy is affected by microcracking, while the wave velocity exhibits a change when a macrocrack has developed over a certain beam depth. The conjunction of both methods helps to determine, in an objective manner, the moment when the first macrocrack appears.

Radionuclide transport in clay during climate change

The Dutch national research programme into the feasibility of retrievable storage of radioactive waste (CORA Programme Phase I; CORA: Comite Opslag Radioactief Afval = Committee on Radioactive Waste Disposal) examined the suitability of Tertiary clay deposits for such storage. Long-term isolation – up to 1 million years – of high-level radioactive waste under varying conditions is essential.A key concern is the hydro-mechanical response of the clay deposits in which radioactive waste might possibly be stored, in particular during glacial climate conditions as has happened repeatedly in the Netherlands during the Pleistocene.To evaluate this possibility hydro-mechanical computer simulations and mechanical laboratory experiments have been performed to analyse the effects of glacial loading by a thousand-metre-thick ice sheet on the permeability characteristics, fluid flow rates and the associated migration of radio-nuclides both within and out of Tertiary clays. Glacial loading causes the expulsion of pore water from deeply buried clay deposits into adjoining aquifers.The rates and duration of the consolidation-driven outflow of water from the clay deposit, are very sensitive to the permeability of the clay and the dynamics of the advancing ice sheet.The maximum outflow rate of pore water is 1 mm per year.This rate is approximately three times faster than the flow rate of water in clay prior to ice loading. These preliminary simulation studies also indicate that cyclic loading can result in more rapid migration of radio-nuclides in clays. In clay deposits that are covered by a thick ice sheet, the contribution of dispersed transport relative to the total transport by diffusion amounts to 14%, assuming that there is no absorption of radio-nuclides by the clays and a longitudinal dispersivity of 50 m.

P-wave attenuation in creeping rock and system identification

SummaryThe aim of this paper is to contribute to the evaluation of rock cracking using ultrasonic wave propagation. The rock specimens (siltstone sampled from a coal mine at a depth of 820 meters in Belgium) are subjected to incremental creep. The study of the rock transfer function shows P-wave attenuation in the direction of loading as a function of stress and time during the test: this attenuation becomes more significant at the stage at which the rock dilates. It is obvious from the specimen dimensions that the signal is affected by reflection phenomena. So, we have modelled the output sigrals with a model containing reflections. The simulations obtained from the model and input signal are very conclusive and the model validation is satisfying.