Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
  • G01N29/4472—Mathematical theories or simulation
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/04—Analysing solids
  • G01N29/045—Analysing solids by imparting shocks to the workpiece and detecting the vibrations or the acoustic waves caused by the shocks
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/22—Details, e.g. general constructional or apparatus details
  • G01N29/24—Probes
  • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/023—Solids
  • G01N2291/0231—Composite or layered materials

Definitions

• Non-destructive testing method comprising generating a localized and controlled traction state in a multi-material and / or multilayer assembly Technical field
• the present invention relates to the field of multi-material and / or multi-layer assemblies, more specifically to the mechanical quality of the interfaces.
• the present invention relates to a non-destructive testing method comprising generating a localized and controlled traction state in a multi-material and / or multilayer assembly, based on shock generation followed by expansion in the assembly, in order to solicit a chosen interface by a controlled level of traction.
• This solicitation can make it possible to control the quality of the assembly in a non-destructive way.
• US Pat. No. 8,359,924 describes a method of non-destructive control of the adhesion force between two materials (in particular composite and consisting of several layers) assembled by an adhesive so as to constitute an assembly.
• This method is based on the generation of waves that can be generated by laser (shockless multi-pulse), but no indication is given in US 8,359,924 on the interaction regime.
• This patent teaches the use of two successive waves with a time shift, but both waves are applied to the same side of the assembly.
• the nondestructive testing method taught by US Pat. No. 8,359,924 is based on the round-trip time of the wave in each of the layers. Impedance mismatch at the interface is a necessary condition for performing the test, but it is a very restrictive condition that does not allow testing of any type of assembly.
• the impedance (or impedance of shock) of a material is generally defined as the product of the density p by the shock velocity D, the shock velocity being generally expressed as the sum of the speed of sound in the material at atmospheric pressure, and material velocity u corrected by an empirical factor s.
• Impedance mismatch is related to the different nature of two materials in contact, which do not have the same impedance of shock. Depending on the impedance ratio, a portion of the waves will be transmitted from one material to another at the interface between these materials, and the other part will be reflected. Depending on the ratio of impedances also, the reflected wave will be either a shock wave or a wave of relaxation. In the case of a material in contact with air (with almost zero impedance), the reflected wave is an expansion wave (as illustrated in Example 1).
• 8,359,924 presents only in a very general and imprecise way how to create a state of traction in the adhesive (or joint) of the composite, the method taught in this patent being limited to the control of the joint and not the interfaces of the composites constituting the 'assembly. In addition, the fact of neglecting the thickness of the seal does not allow to control the traction created at the joint, or at the interfaces.
• shock generation 1 ' 2 allows control over the amplitude of the stresses induced in the material. If one does not create a shock, as is the case in US Pat. No. 8,359,924, the level of stress generated may not be high enough to allow the threshold of damage of a weak interface to be exceeded. we would try to detect during the generation of traction.
• the applicant has developed a non-destructive testing method using shock waves to generate a localized and controlled traction state in a multi-material assembly and / or multilayer, which overcome these disadvantages.
• the subject of the present invention is a non-destructive testing method comprising the generation of a traction state by initiation and propagation of shock waves in a multi-material and / or multilayer assembly, said assembly comprising two outer faces. opposed and at least one interface parallel to said external faces,
• shock waves are generated, simultaneously or successively, on each of said opposite faces of the assembly (so-called “symmetrical double-shock” principle) so that the state of traction generated is localized and controlled in said assembly.
• the traction state at the biased interface is at a pressure level leading to the mechanical strength of said biased interface without destroying the assembly.
• the external faces are parallel, but in the case of surfaces whose area is less than 1 cm 2 , the effects due to the non-parallelism of the faces are very negligible, as is particularly the case for shock waves generated by laser-matter interaction.
• the amplitude of the traction induced at the desired location is a key point of the invention and allows the quantitative evaluation of the resistance of the tested interface. This is related to the mode of shock generation (for example generated by a laser flux for a laser shock) and is conditioned by the mechanical amplitude of the shock produced on both sides of the system. The characteristics of each component of the assembly play a role in the response of the assembly to the load, and thus influence the stress level.
• the determination of the traction level induced at the interfaces depends essentially on the evolution of the state variables (pressure, density, energy) obtained by the numerical resolution (or analytic for a first-order approach) of the conservation equations of the mechanics of continuous environments (conservation of mass, energy, momentum and fundamental law of dynamics). For this purpose, it is necessary to know for each assembly the following material data:
• the invention then allows non-destructive testing (or NDT control) of the interface leading to the quantization of the resistance that the interface may experience.
• the method according to the invention can be used in different ways, for example to perform a test test or simply to perform a non-destructive test.
• the difference between the two tests lies in whether or not the damage threshold of the tested interface or the tested interfaces has been exceeded.
• non-destructive testing is meant in the sense of the present invention, the quantization of the mechanical strength of an interface of a given assembly.
• the shock parameters time profiles of the shock pressure induced by the generators used
• test of test By test of test, one understands the revelation of a weak interface in a given assembly, compared to a nominal (or said reference assembly). In this case, the shock parameters will be defined to remain sufficiently far from the damage thresholds of the reference assembly, but allow the opening of relatively weaker interfaces.
• the use of the method for this test is particularly interesting in the industrial context, and for assembled structures having to withstand a minimum, maximum or nominal load. This is the case of aeronautical assemblies whose nominal resistance must be verified for all bonding.
• the use of the challenge test allows a quick identification of the weak interfaces in comparison with a reference interface. During the test, good quality assemblies are not damaged, while bad ones are open and can be identified.
• a non-destructive test of the reference assembly must first be carried out in order to quantify its mechanical strength, according to the method. This involves calculating the time offset required for the interface test in the case of this assembly. One can also check the threshold of damage of the interface by successive shots. This characterization is necessary to know the shock parameters to be applied during the test of the test so as not to damage the said reference. The mechanical resistance thus evaluated can be compared with values determined by other means of testing, or that expected from a controlled manufacturing process. In a second step, the shock parameters (in particular the amplitude) are adjusted to allow the assembly proof test. For this purpose, it will be possible for example to reduce by X% the energy of the shocks in order to be sufficiently far from the threshold of damage of the reference.
• the traction generated at the interface will thus make it possible to reveal the presence of a weaker interface than the reference of Y% (Y being a tolerance with respect to the threshold of adhesion of the reference depending on the cases of application, and which can be determined by the numerical simulation, Y being correlated with X).
• the method according to the invention in this test test context then makes it possible to reveal the presence of a weak interface whose resistance is lower than the applied stress level, and to leave the robust interfaces undamaged.
• the shock waves can be generated successively with a time offset At between a shock wave generated on one of the faces and the immediately following wave generated on the opposite face.
• This time offset At can advantageously be calculated so that the state of traction generated in said assembly is located at a desired interface to be solicited.
• the calculation can be carried out analytically or using a finite element software adapted to the fast dynamics as a function of the speed of the shock waves in the different layers and / or the different materials constituting the assembly and in function of the induced pressure state.
• the shock waves can also be generated simultaneously, so that the state of traction induced in the assembly is located and controlled at an interface in the middle of said assembly to equal distance from its walls, as shown in Example 3 (and illustrated in Figure 6).
• the shock waves can be generated by laser / material interaction in the ablation regime, confined or not, or by mechanical impact of the plate type, or by electrical discharge in a liquid medium surrounding the assembly.
• the method according to the invention may be used for non-destructive testing of this interface. If however a weak interface between any two layers is broken, causing a distortion in the propagation of the shock wave, the assembly is not qualified. This is the test test. This distortion is for example visible on the surface velocity measurements (or "free surface speed") performed on one or the other of the faces and allows the identification of the weak interface thus revealed. However, any other diagnosis could be used to control the damage to the interface. If the interface resists this stress in traction, it means that the assembly is qualified because the interface is declared "strong". This level can even be quantified by the use of a numerical model correctly describing the physical phenomenon. This is true, for example, by analyzing the velocity measurements of the surface on one of the two faces, which has a so-called normal profile.
• the method according to the invention thus makes it possible to locate a state of traction controlled at an interface to be solicited to test the mechanical strength in the optics of a non-destructive test.
• the generation of symmetrical and synchronized shocks (with or without a time shift ⁇ ) relative to one another allows the generation and the correct positioning of the traction zone at the interface to be tested.
• Another subject of the present invention is the use of the method according to the invention for performing a test test on a multi-material and / or multilayer assembly as defined above, to determine whether this assembly comprises or not a weak interface does not withstand the applied pressure level by comparison with a reference assembly of which no interface is damaged at this pressure level.
• the course of the test is as described above.
• FIG. 1 represents a schematic view of a multilayer assembly (non-visible layers) in which a traction state has been generated by a single-sided shock (or "isolated shock” represented by a single arrow), in accordance with FIG. a process of the prior art 1 ' 2 ;
• FIG. 2 is a space-time (or xt) diagram of the propagation of waves in the assembly of FIG. 1 subjected to an isolated shock which is generated by a low energy laser impact (not leading to the damage of assembly);
• FIG. 3 is an x-t diagram of the propagation of waves in the assembly of FIG. 1 subjected to an isolated shock which is generated by a laser impact of higher energy, leading to flaking of the assembly;
• FIG. 4 represents a schematic view of a multilayer assembly (visible layers and interface to be stressed) in which a state of traction has been generated by a shock on each of the faces of the assembly (or called "double symmetrical shock”); represented by two arrows in the opposite direction in FIG. 2), in accordance with the method of the invention;
• FIG. 5 is a diagram xt of the propagation of the waves in the assembly of FIG. 4 subjected to a symmetrical double shock, with a location of the state of traction at the interface chosen to be solicited,
• FIG. 6 is a xt diagram (obtained by numerical simulation) of the wave propagation in an assembly of three aluminum layers subjected to a symmetrical double shock in accordance with the method of the invention, with a localization of the state of traction at the first interface located in the middle of the assembly, the impacts on each of the faces of the assembly then being generated simultaneously,
• FIG. 7 is a xt diagram (obtained by numerical simulation) of the wave propagation in an assembly of three aluminum layers subjected to a symmetrical double shock in accordance with the method of the invention, with a localization of the state of traction at the second interface, with an offset ⁇ t of 150 ns between the shock waves generated on each of the faces,
• FIG. 8 is an xt diagram of the wave propagation in a multilayer assembly of composite materials bonded by a glue joint subjected to a symmetrical double shock, with a location of the traction state at the glue interface. composite, with an offset At between the shock waves generated on each of the faces.
• FIGS. 1 to 8 are identical elements shown in FIGS. 1 to 8 are identified by identical reference numerals.
• FIG. 1 to 3 are discussed in more detail in Comparative Example 1.
• Figures 4 and 5 are discussed in more detail in Example 2 according to the invention, while Figures 6 and 7 are discussed in detail. more detailed in Example 3 according to the invention, and FIG. 8 in example 4 according to the invention.
• a multilayer assembly (several layers of the same material, which thus have the same impedance) 1 of total thickness e (shown in FIG. 1, with non-visible layers) is subjected to an impact (or "isolated shock"). , represented by an arrow in FIG. 1) on a single face 22 of the assembly, according to a method of the prior art 1 ' 2 -.
• This expansion wave ODr can then cross the incident expansion wave ODi (also in dashed lines in FIG. 2), initiated at the end of the pressure loading of the front face of the material;
• the level of traction generally does not exceed the threshold of damage of the material: the assembly is not damaged, as illustrated in FIG. 2.
• the threshold for damage to a material is the limit from which the onset of the decohesion of the material is observed.
• the threshold of damage depends on the rate of deformation, itself dependent on the type of loading creating the shocked state.
• the traction level may exceed the damage threshold of the material: in this case, the traction condition thus generated may damage the assembly and a peeling phenomenon is observed at the level of the zone where the state of traction has been created, as illustrated in FIG.
• the reflection of the shock wave OC in the expansion wave ODr at the rear face of the structure makes it possible to locally stress the material in tension, but this configuration does not make it possible to generate a traction state T in a localized and controlled manner in the thickness of the material.
• the position of the traction state T in the assembly 1 remains dependent on the shock parameters.
• each interface passage modifies the shock wave, by a transmission / reflection phenomenon, and therefore the corresponding traction level.
• the pressure delivered must be high to overcome the hydrodynamic damping. Note that in this configuration, it is the crossing of the rear part of the wavefront with the wave resulting from the reflection of the front part on the bottom of the part, which allows to concentrate very high tensile stresses. They are located at a depth directly related to the duration of the loading. However, it is very difficult, if not impossible to have a single source of shock allowing a load of duration that can vary over a wide range of values, ranging from 10 ns to 100 ys for example, and can address a wide range of assembly thicknesses.
• a multilayer assembly 1 (which may also be multimaterial) of total thickness e (shown in FIG. 4 with the layers and the interface to be stressed 201) is subjected to a "double symmetrical shock" (represented by two arrows in the opposite direction). in Figure 2), according to the method of the invention.
• the interface 201 to be solicited delimits two layers 11 and 12.
• This symmetrical double shock is generated in the following way:
• This reflected relaxation wave then propagates in the opposite direction and crosses the incident relaxation wave initiated at the end of the loading;
• a traction wave OT (represented by a succession of letters "e") propagates from layer 2 to layer 1 to interface 201;
• a second shock is generated on the accessible face 22 of the layer 2, after a period of time At following the generation of the first shock on face 21;
• the method according to the invention makes it possible to generate two traction states equal to -PI, one propagating from the layer 11 to the layer 12, and the other of the layer 12 to the layer 11;
• this delay At is computable as a function of the speed of sound in the two layers, and the state of induced pressure; but obviously, if the interface 201 is located exactly in the middle of the assembly equidistant from the faces 21 and 22 solicited by the symmetrical double shock, we will take a zero value for At;
• the delay At to locate a state of traction at an interface 201 that is sought to solicit) an assembly of two layers of the same material one having a thickness of 3 mm and the other having a thickness of 6 mm. If the speed of propagation of the waves in the material is of the order of 3 mm / ys, it will be necessary that the shock generated on the layer 2 is produced 1 ys after that generated on the layer 1, so that the two waves mechanical intersect at the interface 201.
• Example 2 shows schematically and numerically that the method according to the invention makes it possible to create a local and controlled traction zone at the interface 201 between the layer 11 of the layer 12 .
• This example illustrates, by numerical simulation, the efficiency of the method according to the invention on an assembly of three aluminum layers (as illustrated in FIG. 4 but with only three layers). It is given as a time / position diagram, obtained by numerical simulation on LS-DYNA.
• the assembly considered, composed of 3 layers of aluminum, comprises (as illustrated in FIG. 6):
• a third layer of thickness 1 mm.
• each of the shocks being generated by a pressure pulse of the type obtained by laser / material interaction in a regime confined by water. .
• FIG. 8 An assembly of two CFRP-type composites (acronym for a fiber-reinforced polymer) is shown in FIG. 8. This assembly consists of a first 2.5 mm thick composite. thickness, itself composed of several layers or plies, a glue joint 130 ym thick, and finally another composite thickness less thick 1.5 mm thick.
• two successive shocks are generated on each of the faces of the assembly 1 by a pressure pulse of the type obtained by laser / material interaction in a confined regime, the two shocks being separated by a shift of At time.
• the offset of time At at the initiation of the shock generated on the thick composite (of 2.5 mm thickness) in comparison with that produced on the thin composite (1.5 mm thick) makes it possible to generate the traction state "f" at level of the interface j anoint / thin composite.
• the offset of time At is worth here 400 ns.

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• 2013
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