LASER SHOCK METHOD

Abstract

A method for focusing acoustic waves on an interface between two layers of a multilayer structure includes providing laser means able to emit a laser beam toward an exterior surface of the multilayer structure to produce a longitudinal wave from the centre of a laser-beam spot projected onto the exterior surface and a transverse wave from the periphery of said spot, determining the distance between said interface and the exterior surface, determining one or more propagation velocities of each of the longitudinal wave and the transverse wave in the one or more layers passed through by said waves to reach said interface, and determining a radius of the laser-beam spot depending on the one or more propagation velocities and on said distance so that the time taken by the longitudinal wave to reach said interface is equal to three times the time taken by the transverse wave to reach said interface.

Claims

1-8. (canceled)

9. A method of focusing acoustic waves at an interface between two layers of a multilayer structure, the method comprising: emitting a laser beam in the direction of an external surface of the multilayer structure, the laser beam defining a spot at the external surface, the spot having a center and a periphery, the emission of the laser beam thereby producing a longitudinal wave propagating from the center of the spot and a transverse wave propagating from the periphery of the spot; determining a distance between said interface and the external surface; determining one or more propagation velocities V.sub.L of the longitudinal wave and one or more propagation velocities V.sub.T of the transverse wave to reach said interface from the layer(s) crossed by said longitudinal and transverse waves; and determining a radius r of the spot of the laser beam as a function of the propagation velocities and of said distance so that a travel time of the longitudinal wave to reach said interface and to be superimposed with the transverse wave is equal to three times a travel time of the transverse wave to reach said interface and to be superimposed with the longitudinal wave.

10. The method according to claim 9, wherein the distance corresponds to a thickness e of a first layer of the multilayer structure, the first layer carrying the first external surface.

11. The method according to claim 10, wherein the radius of the spot of the laser beam is determined by the following formula: $r=e\sqrt{\frac{9V_T^2-V_L^2}{V_L^2}}$ wherein r is the radius of the spot of the laser beam, e is the thickness of the first layer, V.sub.L is the propagation velocity of the longitudinal wave of the first layer, and V.sub.T is the propagation velocity of the transverse wave of the first layer.

12. The method according to claim 9, wherein the laser beam has an intensity having a substantially square profile.

13. The method according to claim 10, wherein the first layer is a layer comprising a metal, for example titanium, the multilayer structure further comprising a second layer adjacent to the first layer, the second layer comprising a composite material.

14. The method according to claim 13, wherein the second layer has a thickness that is larger than the thickness e of the first layer.

15. The method according to claim 13, wherein the first layer and the second layer are bonded.

16. A device including a processing circuit configured for performing a method of focusing acoustic waves at an interface between two layers of a multilayer structure, the method comprising: emitting a laser beam in the direction of an external surface of the multilayer structure, the laser beam defining a spot at the external surface, the spot having a center and a periphery, the emission of the laser beam thereby producing a longitudinal wave propagating from the center of the spot and a transverse wave propagating from the periphery of the spot; determining a distance between said interface and the external surface; determining one or more propagation velocities V.sub.L of the longitudinal wave and one or more propagation velocities V.sub.T of the transverse wave to reach said interface from the layer(s) crossed by said longitudinal and transverse waves; and determining a radius r of the spot of the laser beam as a function of the propagation velocities and of said distance so that a travel time of the longitudinal wave to reach said interface and to be superimposed with the transverse wave is equal to three times a travel time of the transverse wave to reach said interface and to be superimposed with the longitudinal wave.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0033] FIG. 1, described above, shows an exposure diagram of a multilayer structure to a laser beam according to the prior art.

[0034] FIG. 2, described above, shows an exposure diagram of a multilayer structure with two laser beams on two opposite faces of the structure according to the prior art.

[0035] FIG. 3, described above, shows an exposure diagram of a multilayer structure with two laser beams on the same face of the structure according to the prior art.

[0036] FIG. 4 shows an exposure diagram of a multilayer structure according to an example of the method of the present document.

[0037] FIG. 5 shows a block diagram of an example of the method of the present document.

[0038] FIG. 6 shows a schematic block diagram of a processing circuit for the implementation of an example of the method according to the present description.

DETAILED DESCRIPTION OF THE INVENTION

[0039] Referring to FIGS. 4 and 5, the method 200 allows focusing acoustic waves of a laser beam 102 at a given thickness e in a multilayer structure 104 in order to produce a tensile force at said thickness e.

[0040] The method 200 comprises a step 202 of providing laser means to emit the beam 102.

[0041] The method comprises a step 204 of providing the desired thickness e. This thickness e corresponds to an interface 106 between a first layer 108 and a second layer 110 of the multilayer structure 104. The first layer 108 and the second layer 110 are bonded together at the interface 106 by a glue joint.

[0042] The thickness of the second layer 110 is larger than the thickness of the first layer 108, in particular the thickness of the second layer 110 may be considered as being semi-infinite from in acoustics terms.

[0043] The first layer 108 is made of a first material having a first impedance Z1 and the second layer 110 is made of a second material different from the first material and having a second impedance Z2.

[0044] In particular, the first layer 108 is a layer made of a metal, for example titanium, and the second layer 110 is a layer made of a composite material.

[0045] The laser beam 102 has an intensity having a substantially square profile.

[0046] When the laser beam 102 is projected on an external surface 112 of the multilayer structure 104, a longitudinal wave 114 originating from a central portion of a projection spot of the laser beam 102 on the external surface 112 and a transverse wave 116 originating from a peripheral portion of the spot propagate in the structure 104 according to the direction 120. The peripheral portion surrounds the central portion.

[0047] The longitudinal wave 114 has a compressed head shown in solid line and followed overtime according to the axis 118 by an expanded tail shown in broken line. The longitudinal wave 114 undergoes a phase change from expansion to compression and vice versa at each interface encountered between different materials, namely the interface 106 and the external surface 112. The transverse wave 116 also has a compressed head shown in continuous line and followed over time according to the axis 118 by an expanded tail shown in broken line.

[0048] The transverse wave 116 propagates in the multilayer structure 104 at a propagation velocity V.sub.T in the first layer 108 lower than the propagation velocity V.sub.L of the longitudinal wave 114 in the first layer 108.

[0049] The method 200 also comprises a step 206 of providing the propagation velocity V.sub.T of the transverse wave 116 and the propagation velocity V.sub.L of the longitudinal wave 114.

[0050] To create a tensile peak at the interface 106, the laser beam 102 is configured so that the longitudinal wave 114 is superimposed with the transverse wave 116 at the interface 106.

[0051] For this purpose, the method 200 comprises a step 208 of determining the radius r of the laser beam 102 according to the thickness e and the propagation velocities V.sub.T and V.sub.L. The radius r of the laser beam 102 is determined so that the travel time of the longitudinal wave 114 to reach the interface 106 is equal to three times the travel time of the transverse wave 116 to reach the interface 106.

[0052] The radius is determined by the following formula:

[00002]$\begin{array}{cc}r=e\sqrt{\frac{9V_T^2-V_L^2}{V_L^2}}& \left[\mathrm{math}2\right]\end{array}$

[0053] The radius r thus determined allows improving the tensile force at the interface 106 in a simple and inexpensive manner.

[0054] The method 200 may be used in a method for verifying the quality of bonding between the two layers 108 and 110. For example, method 200 may thus be used to focus the laser beam 102 at the bonded interface 106 to generate a given tensile force. If the bonded interface 106 withstands the tension generated by the longitudinal wave 114 and the transverse wave 116, the glue joint at this interface 106 is considered to be resistant.

[0055] The method 200 may be used to detach the two layers 108 and 110 by applying a suitable tensile force.

[0056] FIG. 6 illustrates, according to a particular embodiment of the invention, a device 1000 for implementing the acoustic wave focusing method.

[0057] The device 1000 comprises a storage space 1002, for example a memory MEM, and a processing unit 1004 equipped for example with a processor PROC. For example, the storage space 1002 is a non-volatile memory (ROM or Flash, for example), and may form a recording medium, this recording medium may further comprise a computer program.

[0058] The device 1000 further comprises a communication module enabling said device to connect to a network to exchange data with other devices. For example, the communication module may be a Wifi or Ethernet network interface, or a Bluetooth communication module.

[0059] The communication module of the device 1000 comprises a data reception module 1006, for example an IN receiver, and a data emission module 1008, for example an OUT emitter.

[0060] The module 1006 is configured to receive the distance between the interface where the laser beam will be focused and the external surface of the structure, propagation velocities of the longitudinal and transverse waves in the structure. The module 1008 is configured to return back a ray of the laser beam.

[0061] The storage space 1002, which may be secure, is configured to record and store any data read by the module 1006, processed by the unit 1004 and/or sent by the module 1008.

[0062] The processing unit 1004, which may be controlled by a program, is configured to implement the method 200 for focusing acoustic waves as described with reference to FIG. 5.

[0063] Upon initialization, the instructions of a program driving the processing unit 1004, are for example loaded into a random-access memory (RAM, for example) not shown that the device 1000 comprises, before being executed by the processor of the processing unit 1004.