# Sensitivity study of the pressure load parameters of laser shock on CFRP and its influence on the back-face velocity signal

Bryan Better, Michel Arrigoni, Aboulghit El Malki Alaoui, Serge Guetta, M.

Boustie, Didier Zagouri, Christian Jochum, Gilles Le Morvan, Christine

Espinosa, Frédéric Lachaud

► To cite this version:

Bryan Better, Michel Arrigoni, Aboulghit El Malki Alaoui, Serge Guetta, M. Boustie, et al.. Sensitivity study of the pressure load parameters of laser shock on CFRP and its influence on the back-face velocity signal. 27th DYMAT Technical Meeting, Nov 2023, Colmar, Alsace, France. hal-05291901

HAL Id: hal-05291901

https://hal.science/hal-05291901v1

Submitted on 1 Oct 2025

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# Sensitivity study of the pressure load parameters of laser shock on CFRP and its influence on the back-face velocity signal

Bryan Betterª, Michel Arrigoniᵇ,*, Aboulghit El Malki Alaouiᵇ, Serge Guettaᵉ, Michel Boustieᶜ,ᵈ, Didier Zagouriᵉ, Christian Jochumᵇ, Gilles Le Morvanᵉ, Christine Espinosaᵃ, Frédéric Lachaudᵃ

ᵃ Institut Clément Ader (ICA), Université de Toulouse, CNRS UMR 5312-INSA-ISAE-Mines Albi-UPS, 3 rue Caroline Aigle, Toulouse, 31400 Cedex 4, France

ᵇ Institut de Recherche Dupuy de Lôme (IRDL), 2 rue François Verny, Brest, 29806, France

ᶜ Simchoc, 64 rue Gallieni, Malakoff, 92240, France

ᵈ CNRS Institut Pprime ISAE ENSMA, Futuroscope, Poitiers, 86073 Cedex 9, France

ᵉ NUCLETUDES, 3 avenue du Hoggar, Les Ulis, 91940, France

## Abstract

In this paper, 3D laser shock modelling on a Carbon Fiber Reinforced Plastic composite laminate is conducted to study the impact of different levels of dynamic loadings on the back-face velocity signal and the damage. The source, a 1064nm Nd-YAG laser in nanosecond regime and providing densities of power up to 4 GW/cm-2 in ablation regime, is focused on a thin stationary water confinement. The pressure to be applied on the CFRP is modeled using the ESTHER laser matter interaction code. Time-space diagrams are associated to the computed back-face velocity signal to depict the meaning of the velocity signal. Both experimental and computed signals due to elastic and damaged behavior are reproduced. As the load is critical in modelling accurately the phenomenon, a sensitivity study will then be conducted to quantify the influence of three parameters: the impulsion signal measurement of the photodiode on the laser-matter interaction, the shape of the load area and the irregularities of the pressure spatial distribution on the load area. Due to the orthotropic behavior of plies, differences are observed on a ply but they may be smoothed upon reaching the back-face.

**Keywords:** laser shock, Photonic Doppler Velocimetry (PDV), CFRP, laser matter interaction, numerical modelling

## 1. Introduction

The structural resistance facing with dynamic loadings is a key research field in the industry of transports and defense. In the scope of characterizing the dynamic properties of materials, laser pulses are able to produce high mechanical loads in a very short time span and produce heat in a limited area. Laser shots in ablation regime provide mechanical loadings that are similar to ballistic impacts in pressure and strain rate [1] and induce very few damages to the surface unlike ballistic impacts or free-fall tests making it a promising solution to determine the mechanical properties of materials or assemblies. High energy pulsed lasers are notably able to determine the strength of bonds between two materials using the LASAT (Laser Shock Adhesion Test) method [2]. This method is based on the analysis of back-face velocity signals of the target acquired during the laser shot by laser doppler interferometry. The delamination threshold of the structure can be determined by varying the shock intensity and the analysis of the back-face velocity signal if a plateau velocity is observed after the main velocity peak. However, the back-face velocity signal is built on several contributions of the shock and relaxation waves which are 3D phenomena in essence. In a homogeneous and isotropic material such as metals, if the diameter of the laser shot is more than two to three times the thickness of the target the back-face velocity signal can be considered 1D during the firsts back and forth of the main shock. Nevertheless, sometimes edge effects [3] have to be considered. It is especially true for composite materials because of their anisotropic behavior. Composites laminates are extensively used in the aeronautic industry due to their

<footnote>*Corresponding author</footnote>

<footnote>Email address: michel.arrigoni@ensta-bretagne.fr (Second author)</footnote>
lightness and versatility but few papers study the 3D damage induced by laser pulses on the composite structure and its effect on the back-face velocity signal [4][5]. The damage produced on composite laminates are usually studied using numerical simulations and cross-section analysis by microscopy, which is destructive. The numerical modeling of the load applied to the structure is a critical parameter to reproduce the observed damage on the composite targets and the back-face velocity signal. In the presented study laser induced shock waves in water confined regime are generated on composite samples taken from the aeronautic industry, in order to obtain damages at ultra-high strain rates. The experimental data are recorded with a PDV system. The goal is to assess the importance of assumptions made on the loaded area (temporal pressure profile, spatial distribution and area shape) on the resulting back-face velocity. The finality aims to model laser induced shocks on composites using a 3D finite elements model to study the influence of these loading parameters on the modeling of damage and back-face velocity signal. The standard process includes adding an aluminum layer to the irradiated face of the target to reduce the influence of uncertainties on the experimental parameters as aluminum is the reference material for the laser matter interaction and thus is extensively studied in the literature. Then the maximum pressure applied on the target is determined based on the fluence measured during the test and a pressure signal is scaled to this maximum pressure by using numerical modeling with ESTHER coupled with LS-DYNA. This signal, simplified in its temporal profile or not, is applied to a circular area and the spatial distribution of the pressure is homogeneous with a linear or sine decrease once a radius is reached.

These parameters are the uncertainties of the laser source which could provide an impulsion with several peaks leading to potential peaks in the pressure applied. Another is the shape of the loaded area, the impact on the target may not always be circular and the effects of an oval shaped loading area on the results may be significant due to the anisotropic behavior of the composite. Eventually the spatial distribution of the laser may not be homogeneous leading to the creation of additional relaxation waves which may have an impact on the damage produced in the structure.

## 2. Material and methods

### 2.1. Material

The aeronautic CFRP composite laminate of interest is composed of 13 plies of 127 µm thickness whose stacking sequence is [45/-45/90/45/-45/0/90<sub>0.5</sub>]<sub>s</sub> [6]. To reduce the light scattering of the PDV also called Velocimetry Heterodyne (VH) probe and the scattering of the laser load due to the roughness of the back and front face of the sample, the surface was polished using 600 grain sheets on a rotating polisher. Then to improve the reflectivity of the surface and the quality of the PDV measurement, a thin film of gold was added by electrolysis on the back-face of the target (7mA for 3 minutes twice) corresponding theoretically to a thickness of 500 nm. An aluminum adhesive layer was added on the front face receiving the laser ablation and a low vacuum of 0.3 bar was built to homogenize the bond between the adhesive and the CFRP. Due to this layer, the thermal response of the CFRP structure is not considered.

<figure><img src="image_2.png" /><figcaption>Figure 1 BELENOS Laser test bench, the trajectory of the laser rays is materialized.</figcaption></figure>

### 2.2. Methods

The composite target was exposed to the BELENOS laser facility at Institut de Recherche Dupuy de Lôme (IRDL, UMR 6027 CNRS) hosted by ENSTA Bretagne. BELENOS is a Nd-YAG laser of wavelength 1064nm, has an impulsion of
7,5 ns and is able to deliver a theoretical energy of 3.7 J in an area of a few millimeters [7]. The laser has a frequency of 10Hz but a trigger enables single shot with a thermal stabilization. Each shot was made with a water confinement. The experimental set-up and acquisition chain are shown on Figure 1 and Figure 2.

<figure><img src="image_3.png" /><figcaption>Figure 2 Schematic diagram of the data acquisition and post-processing chain of the back-face velocity.</figcaption></figure>

The numerical modelling was made under the commercial finite element solver LS-Dyna. The geometry and mesh are presented on Figure 3. Each ply has 13 elements through its thickness so that the shock waves are accurately represented based on a mesh refinement study. The oriented mesh was built to represent the directions of failure of the composite, following the method depicted by Chen [8]. Each ply is fixed at its side boundaries. Each composite ply uses the material card MAT_22_COMPOSITE DAMAGE, an orthotropic material which uses Chang-Chang brittle failure criteria. Interlaminar behavior is modeled by tiebreak contacts. Tiebreaks are used instead of cohesive elements because the cohesive elements may hamper the propagation of the waves, especially in compression as discussed by [4]. Damage in the tiebreak is modeled by a linear decrease behavior once a critical distance is reached (Option 6 in LS-Dyna). The material parameters are provided in [6]. The gold and aluminum layers are not modelled.

<figure><img src="image_4.png" /><figcaption>Figure 3 Geometry, oriented mesh, stacking and boundary conditions of the modeled CFRP.</figcaption></figure>

The laser-aluminum interaction is modelled using the ESTHER tool developed by the Commissariat à l'Energie atomique et aux énergies alternatives (CEA). The pressure load temporal evolution to be applied on the composite depends on the fluence, the impulsion signal recorded by photodiode and the material targeted. The ESTHER code solves in 1D geometry Helmholtz equations coupled with hydrodynamics and equation of state (SESAME) with a finite difference method and notably uses optical parameters, thermodynamic and mechanic properties of the materials and parameters of the laser source [7]. The 1D ESTHER simulation is able to provide the pressure load versus time to be applied on the first ply of the CFRP. Two different laser shots are modeled and compared to the experimental results, one in elastic regime and one leading to damage. Damage are expected to be compared to tomographic views showing the position and type of the damages of each ply. The simulated back-face velocity is compared to the one extracted from the PDV signal. To understand the meaning of each peak of the back-face velocity signal, time-space diagrams showing the propagation of the shock and relaxation waves are built using a Matlab code. In order to increase the
robustness of the model, the sensitivity of the results to the prospected loading parameters is evaluated with a parametric study involving different pressure signals, load area shape and spatial distribution of the load as shown in Figure 4. The pressure signals include a variety of observed cases showing one peak or two peaks with the same mid-height impulsion time. The load area shape study involves ellipse with different eccentricity and the spatial distribution will notably study the profile measured by Tahan on the BELENOS laser [5]. Different radial decreases are studied such as sine or linear.

<figure><img src="image_5.png" /><figcaption>Figure 4 Examples of variations of the impulsion signal, load shape and spatial distribution of the pressure that are under study (left) and example of back-face velocity signal difference due to pressure signal (right). Grey: Experimental Data, Black: LS-Dyna simulation using ESTHER 1D loading, Orange: LS-Dyna simulation using modified ESTHER loading to account for loading edge effects. Both simulations use a circular shape loading and a spatial distribution with linear decrease.</figcaption></figure>

### 3. Results and discussion

The first results highlight the influence of the uncertainty of the 3D loading parameters on the physical phenomena simulated such as the release phase following the main shock wave as well as the visible 2D effects on the back-face velocity signal. A modification of the spatial distribution of pressures enables to control the minimum of the back-face velocity while the adaptation of the release of the 1D pressure signal taken from the ESTHER tool enables to influence the mean velocity at long time scale as seen on Figure 4. The shape of the impact area seems to modify the position of some failure area but complementary investigations are needed to confirm this tendency. These results showed that the main back-face velocity peak is not sensible to the uncertainties of the studied loading parameters but that the induced 2D effects change the behavior of CFRP, leading to modifications of the release phase of the back-face velocity signal.

### Acknowledgements

The authors are thankful to the French Agence Nationale de la Recherche (ANR) and Agence Innovation Défense (AID) for funding the SUSTAINED21 project which made it possible to carry out the work that will be presented here. Authors would like to thank Ludovic Lecherboug from CEA DIF for the teaching time on ESTHER he provided.

### References

[1] L. C. Alil & al., Laser Induced Shockwave as Delaminator of Composite Material for Ballistic Protection at High Strain Rate, Soft Target Protection, 2020, Doi: 10.1007/978-94-024-1755-5_2

[2] L. Berthe & al, State-of-the-art laser adhesion test (LASAT), Nondestructive Testing and Evaluation, Volume 26, Issue 3-4, 2011. Doi: 10.1080/10589759.2011.573550.

[3] M. Boustie & al., Study of damage phenomena induced by edge effects into materials under laser driven shocks, Journal of Physics D: Applied Physics 40(22), 2007. doi:10.1088/0022-3727/40/22/036

[4] R. Ecault, Etude expérimentale et numérique du comportement dynamique de composites aéronautiques sous choc laser. Optimisation du test d'adhérence par ondes de choc sur les assemblages composites collés, PhD dissertation, ISAE-ENSMA- Ecole Nationale Supérieure de Mécanique et d'Aérotechnique, 2015. URL: https://theses.hal.science/tel-01242721

[5] E. Gay, Comportement de composites sous choc induit par laser : Développement de l'essai d'adhérence par choc des assemblages de composites collés, PhD dissertation, ENSAM – Ecole nationale supérieure d'arts et métiers, 2011. URL: https://pastel.archives-ouvertes.fr/tel-00667560

[6] A. Bigand, Damage assessment on aircraft composite structure due to lightning constraints, PhD dissertation, Université de Toulouse, 2020. URL: https://www.theses.fr/2020ESAE0027.pdf

[7] G. Tahan, Etude des assemblages collés sous choc – Propriétés mécaniques après choc laser, PhD dissertation, ENSTA Bretagne – Ecole nationale supérieure de techniques avancées Bretagne, 2018. URL: https://tel.archives-ouvertes.fr/tel-03179874

[8] C. Chen, Investigation of environmental effects on impact damage tolerance of composite laminates, PhD dissertation, Université de Toulouse, 2015. URL: https://www.theses.fr/2015ESAE0004