M. Boustie

State-of-the-art laser adhesion test (LASAT)

This paper proposes a state-of-the-art laser adhesion test. It consists of testing material interfaces with laser-driven shock wave. Since the first demonstration in the 1980s by Vossen, many studies and developments have been done. This paper presents recent experiments and developments on the basic physics involved. Results show the ability of the technique to perform a quantitative adhesion test for a wide range of materials and configurations. Edge effect principle and ultra-short shock wave give perspectives for new applications for multi-layer combination of material. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.

A comparative study of three adhesion tests (EN 582, similar to ASTM C633, LASAT (LASer Adhesion Test), and bulge and blister test) performed on plasma sprayed copper deposited on aluminium 2017 substrates

The aim of this study was to compare three adhesion tests carried out on plasma-sprayed copper coatings on aluminium substrates. The first test, the bond pull test, designated EN 582 or ASTM C633, involves a uniaxial static stress and is commonly used in the coating industry. The second test, the LASAT (LASer Adhesion Test), is a recently developed technique based on spallation phenomenon due to laser induced shock waves. In this test, the coating delamination results from spallation at the coating/substrate interface due to uniaxial tensile stress. The last test, the bulge and blister test, involves a quasi-static measurement of the crack propagation energy at the coating/substrate interface. These three techniques have been used to evaluate the influences of different process parameters involved in the coating adhesion such as aluminium surface roughness, substrate pre-heating and plasma spray conditions.

Observation of the shock wave propagation induced by a high-power laser irradiation into an epoxy material

The propagation of laser-induced shock waves in a transparent epoxy sample is investigated by optical shadowgraphy. The shock waves are generated by a focused laser (3?ns pulse duration?1.2 to 3.4?TW?cm?2) producing pressure from 44 to 98.9?GPa. It is observed that the shock wave and the release wave created by the shock reverberation at the rear face are both followed by a dark zone in the pictures. This corresponds to the creation of a tensile zone resulting from the crossing on the loading axis of the release waves coming from the edge of the impact area (2D effects). After the laser shock experiment, the residual stresses in the targets are identified and quantified through a photoelasticimetry analysis of the recovered samples. This work results in a new set of original data which can be directly used to validate numerical models implemented to reproduce the behaviour of epoxy under extreme strain rate loading. The residual stresses observed prove that the high-pressure shocks can modify the pure epoxy properties, which could have an influence on the use made of these materials.

Experimental and numerical investigations of shock and shear wave propagation induced by femtosecond laser irradiation in epoxy resins

In this work, original shock experiments are presented. Laser-induced shock and shear wave propagations have been observed in an epoxy resin, in the case of femtosecond laser irradiation. A specific time-resolved shadowgraphy setup has been developed using the photoelasticimetry principle to enhance the shear wave observation. Shear waves have been observed in epoxy resin after laser irradiation. Their propagation has been quantified in comparison with the main shock propagation. A discussion, hinging on numerical results, is finally given to improve understanding of the phenomenon.

Development of the laser shock wave adhesion test on bonded CFRP composite

Purpose – The purpose of this paper is to develop a laser shock adhesion test (LASAT) and evaluate its ability to reveal various bond qualities of stuck carbon fiber reinforced polymer (CFRP) industrial assemblies. Design/methodology/approach – Four grades of adhesion were prepared by release agent contamination of CFRP prior to assembly. Laser shots were performed at different intensities on these samples. Findings – To characterize and quantify the damage created by the propagation of shock waves in the bonded material, several diagnoses were used (confocal microscopy, ultra-sound inspection and cross-sections microscopy). These three post-mortem techniques are complementary and provide consistent results. Originality/value – The combination of these diagnoses along with the LASAT technique provides relevant information on the bond quality in agreement with GIC values measured by the University of Patras.

Laser Shock Adhesion Test (LASAT) of Electron Beam Physical Vapor Deposited Thermal Barrier Coatings (EB-PVD TBCs)

Damage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock.

Lasers and Thermal Spray

Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Hypervelocity impacts into porous graphite: experiments and simulations

We present experiments and numerical simulations of hypervelocity impacts of 0.5 mm steel spheres into graphite, for velocities ranging between 1100 and 4500 m s−1. Experiments have evidenced that, after a particular striking velocity, depth of penetration no longer increases but decreases. Moreover, the projectile is observed to be trapped below the crater surface. Using numerical simulations, we show how this experimental result can be related to both materials, yield strength. A Johnson–Cook model is developed for the steel projectile, based on the literature data. A simple model is proposed for the graphite yield strength, including a piecewise pressure dependence of the Drucker–Prager form, which coefficients have been chosen to reproduce the projectile penetration depth. Comparisons between experiments and simulations are presented and discussed. The damage properties of both materials are also considered, by using a threshold on the first principal stress as a tensile failure criterion. An additional compressive failure model is also used for graphite when the equivalent strain reaches a maximum value. We show that the experimental crater diameter is directly related to the graphite spall strength. Uncertainties on the target yield stress and failure strength are estimated. This article is part of the themed issue ‘Experimental testing and modelling of brittle materials at high strain rates’.

Use of laser shock waves to test piezochromic coatings for impact detection

Abstract. The design of piezochromic pigments is a promising way to adjust smart painting and therefore to develop a “visual” impact detection coating. This paper deals with the possibility to use laser shock waves to test piezochromic coatings for impact detection. For that purpose, an experimental setup was developed in order to obtain compressive load in the coating thanks to the impedance mismatch between selected materials. An analytical modeling was used to validate the proposed method. The experimental investigation coupled with finite-element modeling on four smart coatings showed that these coatings can reveal impact location by a significant change of color if a relevant pressure threshold is reached. The results presented in this work are promising and demonstrate the ability of the proposed laser shock method for characterizing the pressure thresholds of piezochromic smart paintings. It opens the door for studying future smart paintings with different critical pressure levels, depending on the targeted application.

Enhancement of a dynamic porous model considering compression-release hysteresis behavior: application to graphite

Because of their shock wave attenuation properties, porous materials and foams are increasingly used for various applications such as graphite in the aerospace industry and polyurethane (PU) foams in biomedical engineering. For these two materials, the absence of residual compaction after compression and release cycles limits the efficiency of the usual numerical dynamic porous models such as P-α and POREQST. In this paper, we suggest a simple enhancement of the latter in order to take into account the compression-release hysteresis behavior experimentally observed for the considered materials. The new model, named H-POREQST, was implemented into a Lagrangian hydrocode and tested for simulating plate impact experiments at moderate pressure onto a commercial grade of porous graphite (EDM3). It proved to be in far better agreement with experimental data than the original model which encourages us to pursue numerical tests and developments.