Fabien Soulié

From liquid to solid bonding in cohesive granular media

Abstract We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: (1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; (2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; (3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the strengthening of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.

Image processing and geometrical analysis for profile detection during pulsed gas metal arc welding

This article describes an image analysis algorithm used to detect profiles during arc welding processes. The new algorithm is an aggregation of image processing (segmentation, filtering), computational geometry (alpha shape) and graph theory (cycle detection). It allows to extract precise geometrical profile entities, whether open or closed contours, that could be used for the monitoring of the welding process. The algorithm is shown to be really efficient and could be used for real-time monitoring of gas metal arc welding process.

Effect of Capillary and Cemented Bonds on the Strength of Unsaturated Sands

The cohesive interactions between grains play a prevailing role in the mechanical behaviour of unsaturated granular materials such as fine sands. These interactions are generally bonds of various natures that evolve according to the surrounding hygrothermic conditions. We study the case where the liquid present in the material is a water solution saturated with sodium chloride. The bonds are then of capillary type and the cohesive interactions are mainly attractive. In this case, the mechanical strength in an unconfined compression test is relatively low. At low relative humidity, the phase change of water involves a crystallization of salt at the contact points between grains generating thus bonds of solid type. The mechanical strength of the material is thus enhanced. An experimental study of the variation of the mechanical strength during the crystallization of salt allowed us to show two distinct cohesive regimes: capillary and cemented. The transition between these two regimes does not seem to be correlated with the mass of the crystallized salt, but rather with the residual degree of saturation. An analysis of these results is proposed by comparison with numerical simulations based on the discrete element approach.

Etude micromécanique de la cohésion par capillarité dans les milieux granulaires humides

ABSTRACT The capillary cohesion in polydisperse granular media is studied at the local and macroscopic scales. At the local scale, an explicit modeling of capillary cohesion is proposed according to the characteristics of the grain-pair. The modeling of capillary force is validated by experiments performed over pairs of beads. The modeling is then implemented in a 3D numerical code based on a discrete element method, together with a numerical method for the distribution of water in the granular material. Axial compression tests are carried out numerically and experimentally. Successful comparisons between numerical and experimental macroscopic tests approve the approach allowing thus to study the macroscopic behaviour of wet granular materials.

In-situ observation and modelling of solidification and fluid flow on GTAW process

An experimental setup is presented in order to obtain experimental data during solidification of a static weld pool after arc extinction with a GTAW process. Several devices have been set up to extract three kinds of measurements: (i) solidification front velocity (ii) fluid flow velocity at the vicinity of the front (iii) temperature field in the solid part. A high-speed camera is used to film the interface during welding at microscopic level and an infra-red in order to take the temperature field around the weld pool in the solid part. After processing and calibration of the videos, the experimental results are compared to theoritical results founded on an adapted model from the KGT [1] and from the one of Gandin et al. [2]. All the tests are done thin plate of Cu-30wt.%Ni.

A numerical model for transition from liquid to solid bonding in cohesive granular media

The cohesive behaviour of unsaturated granular materials is due to the presence of cohesive bonds between grains. These bonds can have various physico‐chemical characteristics and may evolve with environmental conditions. We study the case of a granular material partially saturated by an aqueous solution. The bonds are thus initially of capillary type and the mechanical strength is weak. At low relative humidity, the phase change of water involves crystallization of the solute at the contact points between grains, generating thus solid bonds. The mechanical strength of the material is then enhanced. An experimental study of the evolution of the mechanical strength during crystallization of the solute shows clearly the transition from capillary regime to cemented regime. This transition is not correlated with the mass of the crystallized solute, but rather with the residual degree of saturation. This behavior is analyzed here in the light of discrete element simulations. We introduce a local cohesion law t...

In situ experimental measurement of temperature field and surface tension during pulsed GMAW

In this paper, non-contact measurements are presented to observe droplet oscillations and to measure surface temperature during Pulsed GMAW. The techniques enable to investigate in situ the behavior of mild steel liquid metal surrounded by Ar + 8%CO2 plasma and then to measure temperature dependence of surface tension. Different temperatures are obtained by changing the process parameters. Surface tension values are a little higher than values found in the literature but the results demonstrate the interest of the developed setup. To conclude, a non-dimensional analysis is carried out, based on experimental results, to investigate the relative importance of the different phenomena acting during the transfer.

In situ observations and measurements during solidification of CuNi weld pools

A two-scale in situ observation set-up has been used to investigate the physical phenomena around the solid/liquid interface in welding. A spot gas tungsten arc welding process is used to observe weld pool solidification phenomenon after arc extinction. The solidification front and the fluid flow in the weld pool have been observed at a microscale by a high speed camera. The whole weld pool and the surrounding base metal have been filmed at a macroscale by an infrared camera to estimate temperature fields around the weld pool and bead shape. A qualitative and quantitative dataset was extracted from these observations that involve useful results for further improvements of the understanding of weld solidification mechanisms and to enhance solidification models.

Capillary Cohesive Local Force: Modelling and Experiment

The capillary cohesion in granular media is studied at the local scale of a doublet of particles by theoretical and experimental ways. An explicit relation is proposed to describe the cohesive force as a function of the characteristics of the doublet. Taking into account contact, capillarity, and rupture aspects, a local cohesive law is established. This law is integrated in a 3D numerical code in order to estimate the influence of the local cohesion on the macroscopic behaviour of granular media.

Micromechanical analysis of water retention phenomenon

We investigate the water distribution and the link between suction and water content in granular media. Firstly, we examine the effect of suction on the shape and the volume of the liquid bridge for different parameters (grain radius, inter‐particle distance, contact angle, surface tension). This local behaviour is then used in a discrete element study of a sample composed of several thousands of grains. We focus our study on the pendular state. The existence of a liquid film around the grains which involves the continuity of the liquid phase is assumed. The water distribution and the water content associated with a given suction are calculated. Then retention curves of the granular media are built. Four different methods are used. The first is based on the local expression of the capillary force coupled with the “gorge method,” the second is based on the Laplace equation, and the third and the fourth are based on the integration of the differential equation that defines the liquid bridge shape. A paramet...