Peter Moonen

A continuous–discontinuous approach to simulate fracture processes in quasi-brittle materials

A macroscopic framework for the simulation of failure processes in quasi-brittle materials is proposed. The framework employs the partition of unity (PU) concept and introduces a new cohesive zone model, capturing the transition between the initial continuum state and the final discrete state. The model is generic in a sense that it allows extension of most continuum models to a discontinuous framework in an efficient and robust way, thereby adding the effect of macro-crack formation by the growth and coalescence of micro-defects. Both material failure and interface failure can be studied with this formulation.

Mitigation of surface reflection in PIV measurements

Surface reflections of high-intensity laser light are a common concern when conducting particle image velocimetry (PIV) measurements. Consequences range from a poor signal-to-noise ratio (overexposure) in near-surface areas up to camera sensor damage. The severity depends on the interplay between three factors: surface properties, laser light intensity and relative camera position. In stereoscopic or tomographic PIV setups, material selection is often the only factor which can be adapted. We present a systematic comparative study, involving different materials and surface treatments. Their potential to mitigate surface reflections is quantified against the reference case of a flat black painted wooden surface. The largest reduction of surface reflection intensity is obtained by applying fluorescent paint on wood or by employing electropolished steel. The more widely used flat black painted wood shows poor behavior.

Advancing the visualization of pure water transport in porous materials by fast, talbot interferometry-based multi-contrast x-ray micro-tomography

The spatio-temporal distribution (4D) of water in porous materials plays a fundamental role in many natural and technological processes. The dynamics of water transport is strongly entangled with the material’s pore-scale structure. Understanding their correlation requires imaging simultaneously the 4D water distribution and the porous microstructure. To date, 4D images with high temporal and spatial resolution have been mainly acquired by attenuation-based X-ray micro-tomography, whereby pure water is substituted by saline solutions with high atomic number components to improve image contrast. The use of saline solutions is however not always desirable, as the altered fluid properties may affect the transport process as well or, as it is the case for hydrating cement-based materials, they may modify the chemical reactions and their kinetics. In this study, we aimed at visualizing pure water transport in porous building materials by a new implementation of fast Talbot interferometry-based multi-contrast X-ray micro-tomography at the P07 beamline of the Helmholtz-Zentrum Geesthacht at DESY. We report results from a mortar specimen imaged at three different stages during evaporative drying. We show the possibility of visualizing simultaneously the microstructure and the pore-scale water redistribution by the phase contrast images. In addition, different solid material phases are clearly distinguished in these images. The higher contrast between water and the porous substrate, achievable in the phase contrast images, compared with the attenuation ones, empowers new analysis and allows investigating the correlation between the drying process and the porous microstructure. The approach offers the possibility of studying other chemically inert or reactive water transport processes without any chemical or physical perturbation of the processes themselves.

A hygrothermo-mechanical model for wood: Part B. Parametric studies and application to wood welding

Abstract The correct prediction of the behavior of wood components undergoing environmental loading or industrial process requires that the hygric, thermal and mechanical (HTM) behavior of wood are considered in a coupled manner. A fully coupled poromechanical approach has been used to perform a parametric study on wood HTM behavior, and the results have been validated with neutron imaging measurements on a moist wood specimen exposed to high temperature. Further, HTM behavior of wood during welding has been simulated by the model. For such a simulation, proper material properties are needed, as some of them, for example thermal conductivity, have a significant influence on the local and temporal behavior of the material.

An analytical approach to the characterization of swelling in clay-bearing stone

Clay minerals that swell when exposed to water play an important role in the deterioration of buildings and monuments, as well as in a number of civil engineering problems. A novel method for the in situ determination of the key properties governing these phenomena has been recently proposed by Scherer and Gonzalez [Geol. Soc. Am. Spec. Pap. 390 (2005) p.51]. A comprehensive relation between the experimental observations and the material properties could however not be established up to now. The present study develops an analytical model in which the interplay between geometrical factors and material properties, as well as initial and boundary conditions is accounted for. An extensive parametric study reveals that the discrepancies between the observations and the modelling approach by Scherer and Gonzalez is due to initial and boundary conditions on the one hand and the dispersion of the moisture front on the other hand. Based on the developed model, recommendations for performing in situ tests are formulated and conditions for the applicability of the more simplified modelling strategy by Scherer and Gonzalez are derived. A comparison with experimental data supports these conclusions.

Numerical Modeling of Crystallization-Induced Fracturing in Porous Limestone

This paper presents a computational model coupling heat, water and salt ion transport, salt crystallization, mechanical stresses and fracturing in porous materials, including the damage evolution with time. A continuous-discontinuous framework was developed, accounting for the interaction between discrete fractures and the surrounding porous material. The capabilities of the model are illustrated by a simulation example describing a wet pedestal, saturated with sodium sulfate solution, in which in-pore crystallization is induced by cooling. The combination of crystallization and thermal stresses gives rise to the development of two distinct types of crack formation in a single structure. These results show the potential of the model to predict physical material degradation under complex conditions.