Jérémie Girardot

Recoil pressure and surface temperature in laser drilling

To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process from decades; however several fundamental questions are still unsolved about the physical understanding of the laser matter interaction. The sequential description of the laser drilling process is as following. The irradiated surface is heated by absorption of laser energy. Once the surface temperature is higher than the vaporization one, a vapor flow is normal to the local surface. The recoil pressure generated by the evaporation exerts a force on the melted surface and starts expelling the melt layer out of the hole by the side. A Mach shock disc appears in the flow, the ejected vapor flow is then supersonic. The surface goes deeper in the target. This paper relates to an original way for measuring pressure and surface temperature on target from the observations of the supersonic vapor flow. From these observations and with the Prandtl-Meyer function applied on under-expanded supersonic jet one can correlate the surface temperature, the pressure on the melt layer and the Mach number on the supersonic jet. Results show below 22 MW.cm-2 of absorbed intensity the highest surface temperature reached, is in the range of 5000K, with a Mach number of 5 and a recoil pressure about 150 bars.To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process from decades; however several fundamental questions are still unsolved about the physical understanding of the laser matter interaction. The sequential description of the laser drilling process is as following. The irradiated surface is heated by absorption of laser energy. Once the surface temperature is higher than the vaporization one, a vapor flow is normal to the local surface. The recoil pressure generated by the evaporation exerts a force on the melted surface and starts expelling the melt layer out of the hole by the side. A Mach shock disc appears in the flow, the ejected vapor flow is then supersonic. The surface goes deeper in the target. This paper relates to an original way for measuring pressure and surface temperature on target from the observations of the supersonic vapor flow. From these observations and with the Prandtl-Meyer function applied on under-expanded superson...

Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

The gas turbine combustion chamber of aero-engines requires a thermal barrier coating (TBC) by thermal spraying. Further heat protection is achieved by laser drilling of cooling holes. The residual stresses play an important role in the mechanical behaviour of TBC. It could also affect the TBC response to delamination during laser drilling. In this work, studies of the cracking behaviour after laser drilling and residual stress distribution have been achieved for different bond coats by plasma spray or cold spray. From interface crack length measured pulse-by-pulse after laser percussion drilling at 20° angle, the role of the various bond coats on crack initiation and propagation are investigated. It is shown that the bond coat drastically influences the cracking behaviour. The residual stresses profiles were also determined by the incremental hole-drilling method involving speckle interferometry. An original method was also developed to measure the residual stress profiles around a pre-drilled zone with a laser beam at 90°. The results are discussed to highlight the influence of TBCs interfaces on the resulting residual stresses distribution before laser drilling, and also to investigate the modification around the hole after laser drilling. It is shown that laser drilling could affect the residual stress state.

Reevaluation of the diametral compression test for tablets using the flattened disc geometry

Mechanical strength is an important critical quality attribute for tablets. It is classically measured, in the pharmaceutical field, using the diametral compression test. Nevertheless, due to small contact area between the tablet and the platens, some authors suggested that during the test, the failure could occur in tension away from the center which would invalidate the test and the calculation of the tensile strength. In this study, the flattened disc geometry was used as an alternative to avoid contact problems. The diametral compression on both flattened and standard geometries was first studied using finite element method (FEM) simulation. It was found that, for the flattened geometry, both maximum tensile strain and stress were located at the center of the tablet, which was not the case for the standard geometry. Experimental observations using digital image correlation (DIC) confirmed the numerical results. The experimental tensile strength obtained using both geometries were compared and it was found that the standard geometry always gave lower tensile strength than the flattened geometry. Finally, high-speed video capture of the test made it possible to detect that for the standard geometry the crack initiation was always away from the center of the tablet.

Dynamic energy release rate evaluation of rapid crack propagation in discrete element analysis

A numerical procedure for estimating the critical dynamic energy release rate (

Investigation of delamination mechanisms during a laser drilling on a cobalt-base superalloy

G_{IDc}

Strain Rate Effect on the Compressive Behaviour of Reinforced Cork Agglomerates

GIDc), based on experimental data is proposed. A generation phase simulation is conducted where fracture parameters can be determined using an experimentally measured crack propagation history (position of the crack tip as a function of time). The discrete element method is used to simulate the dynamic fracture by implementing a node release technique at the crack tip. The results are compared with analytical data on the dynamic propagation of a crack in a semi infinite plate. It reveals that the node release technique causes dynamic instabilities that can only be corrected by adding numerical damping on the edges of the crack or in the entire sample. On the other hand, the progressive node release technique, based on an elasto-damage zone model does not generate dynamic instabilities. It is shown that for a linear relaxation scheme and a damage zone length equal to the mean radius of the discrete elements, results comparable to finite element or analytical methods are obtained in plate structure. The present model offers an alternative to the finite element method to simulate self-similar or more complex crack growth. It also gives a first proper analysis of the evaluation of the critical dynamic energy release rate in a lattice-discrete model.

Strain rate influence on mechanical behavior of a single wire entangled material

These works present a numerical alternative to the simulation of the laser drilling process. The use of the finite element method to modeling the hole creation during a laser pulse shows difficulties in front of a moving boundary problem. This moving boundary is induced by a fast phase transformation and also by high thermal gradient. The C-NEM (Constraint Natural Element Method) was tested in order to solve these numerical difficulties and to use the high potential of this original method. The physical interaction of the laser drilling will be reminded and the chosen mathematical model will be specified. A simulation was made with the data for pure iron in order to validate the numerical choice.These works present a numerical alternative to the simulation of the laser drilling process. The use of the finite element method to modeling the hole creation during a laser pulse shows difficulties in front of a moving boundary problem. This moving boundary is induced by a fast phase transformation and also by high thermal gradient. The C-NEM (Constraint Natural Element Method) was tested in order to solve these numerical difficulties and to use the high potential of this original method. The physical interaction of the laser drilling will be reminded and the chosen mathematical model will be specified. A simulation was made with the data for pure iron in order to validate the numerical choice.