Method for measuring displacement fields in a test specimen

Abstract

A method for measuring displacement fields in a test specimen (10), comprising the following steps: - generating a two-dimensional pattern comprising a plurality of points, - engraving the pattern on a surface (20) of the test specimen (10) using a laser (22), - performing a mechanical test on the test specimen (10) and acquiring images of the pattern using a camera during the test, and - reconstructing the deformation field during the test by analyzing the images acquired by the camera. Figure to be published with the abbreviation: 2

Description

Title of the invention: Method for measuring displacement fields in a test specimen Technical field of the invention

[0001] The invention relates to a method for measuring displacement fields in a test specimen during a mechanical test. Prior art

[0002] It is common practice to perform mechanical tests in the laboratory on test specimens, particularly temperature tests, to determine the deformations of these specimens as a function of the applied conditions and mechanical stresses. The results obtained provide a better understanding of the material's behavior, which allows, for example, more accurate modeling of parts and more efficient dimensioning.

[0003] Traditionally, tracking displacement on the surface of the specimen is carried out using an extensometer comprising two points arranged in contact with said surface of the specimen and allowing the displacements to be measured during the test.

[0004] However, such a device does not allow for precise knowledge of the deformation field in the area between the tips, and only provides an overall view of the specimen's elongation. Furthermore, it is poorly suited to tests to the point of specimen failure, during which contact between the tips and the surface may be lost (slipping), and the tips may be damaged during such tests.

[0005] This method also requires prior calibration of the extensometer, which can prove to be time-consuming.

[0006] Another known method consists of marking the surface of the test specimen with ink applied by felt-tip pen or inkjet, or with paint, allowing for high contrast. The displacements are then tracked with a camera during the mechanical test in order to reconstruct the deformation field by image processing.

[0007] This method also has drawbacks. During temperature tests (with or without the application of temperature gradients) and / or when the material undergoes large deformations, it is not uncommon for the marking to flake off or burn. This then makes displacement measurement impossible.

[0008] The application and drying of the ink or paint also significantly increases the duration of the test, which cannot begin until these steps are completed. This reduces the possibilities of quickly repeating the mechanical test a large number of times. This is often necessary to obtain usable data and reproducible results. Presentation of the invention

[0009] The invention aims to remedy these drawbacks by providing a precise method for measuring the displacement field, in particular the elongation (local or global) in a thermo-mechanically stressed specimen, which allows for rapid setup and monitoring throughout the duration of the test.

[0010] To this end, the invention relates to a method for measuring displacement fields in a test specimen, comprising the following steps:

[0011] - generation of a two-dimensional pattern comprising a plurality of points,

[0012] - engraving the pattern on a surface of the test specimen using a laser,

[0013] - implementation of a mechanical test on the specimen and acquisition of images of the pattern captured by means of a camera during the test, and

[0014] - Reconstruction of the displacement field during or after the test by analysis of the images acquired by the camera.

[0015] Such a method makes it possible to accurately track the displacements within a test specimen during a mechanical test, particularly a high-temperature test. The test setup is quick and precise, exhibits good repeatability, and is adaptable to the specific test conditions and the characteristics of the specimen, such as its size, geometry, or the material from which it is made.

[0016] The points of the two-dimensional pattern can be randomly distributed in a predetermined region.

[0017] Such a pattern allows for very good resolution for local tracking of movements in said predetermined region.

[0018] The pattern may comprise between 4000 and 10000 points, in particular between 5500 and 7000 points.

[0019] The points may have diameters between 50 micrometers and 80 micrometers.

[0020] The predetermined region may have a square or rectangular shape. Each side of the region may have a length between 5 millimeters and 20 millimeters, in particular between 10 millimeters and 15 millimeters.

[0021] The points of the pattern can form at least one series of aligned points, distributed with a constant step over a predetermined region extending over the surface of the test specimen.

[0022] Such a pattern allows for global tracking of displacements within the specimen with precise initial knowledge of the distances separating the points of the pattern. This enables rapid data processing and real-time tracking of displacements during the test.

[0023] The number of points is for example between 4 and 10.

[0024] Each point has a diameter which can be between 0.1 millimeter and 1 millimeter.

[0025] During the engraving of the pattern on the surface of the test specimen, the laser emits a beam forming pulses with durations between 30 nanoseconds and 120 nanoseconds.

[0026] This pulse duration is sufficiently short to avoid damaging the test specimen during engraving and disrupting the measurement.

[0027] The pulse frequency can be between 50 kilohertz and 200 kilohertz.

[0028] The laser beam can travel across the surface at a speed of between 10 millimeters per second and 2000 millimeters per second.

[0029] Such a travel speed makes it possible to engrave the pattern on the surface in a reduced time, in order to limit the time prior to the mechanical test.

[0030] The engraving of the pattern on the test specimen can be carried out by performing several successive steps of traversing the surface with a laser beam to progressively engrave the pattern on the surface.

[0031] Such an engraving method makes it possible to limit the impact of the engraving on the mechanical properties as well as the surface condition of the specimen.

[0032] The number of route steps is for example between 1 and 10.

[0033] During the implementation of the mechanical test, a local temperature of The test tube can exceed 500°C.

[0034] Said temperature is for example measured by means of a thermocouple placed in contact with the test specimen. Brief description of the figures

[0035] [Fig-1] Fig. 1 is a view of a random pattern generated during the setting work of a process according to a first embodiment of the invention,

[0036] [Fig.2] [Fig.2] is a view of an engraving of the motif of [Fig.1] on a test tube,

[0037] [Fig.3] [Fig.3] is a view of the implementation of a mechanical test on the test tube of [Fig.2], and

[0038] [Fig.4] [Fig.4] is a view of a test tube on which a pattern for the implementation of a process according to a second embodiment of the invention. Detailed description of the invention

[0039] A method for measuring displacement fields in a test specimen, according to a first embodiment of the invention, is described with reference to Figures 1 to 3.

[0040] The process includes a first step of generating a two-dimensional pattern. An example of a pattern is shown in [Fig. 1].

[0041] The two-dimensional pattern shown is a speckled pattern comprising a large number of points, for example between 5500 and 7000, randomly distributed within a rectangular window measuring 10 millimeters by 15 millimeters. The two-dimensional pattern is generated digitally using a computer implementing a suitable program.

[0042] The process then includes a step of engraving the two-dimensional pattern on a test specimen 10, shown in [Fig.2].

[0043] The test specimen 10 has an elongated bar shape having two threaded cylindrical ends 12 and a flat middle part 14.

[0044] The threaded ends 12 are adapted to be engaged in two receiving supports of a device for applying a mechanical stress.

[0045] The middle part 14 is defined by two substantially flat opposite faces 16, and includes a central through orifice 18 opening in the faces 16.

[0046] The engraving of the pattern is carried out on a surface 20 of the test specimen 12, which extends over one of the faces 16 around the orifice 18.

[0047] The surface 20 is for example substantially rectangular, 10 millimeters by 15 millimeters, corresponding to the size of the pattern engraved on it.

[0048] The position of the surface 20 is chosen so as to encompass the regions of interest for the mechanical behavior of the specimen 10, in particular with regard to the breakage of the drilled specimen 10.

[0049] In the case of the drilled test specimen 10 shown in Figures 2 and 3, the said regions of interest are those immediately adjacent to the orifice 18, where the rupture is localized.

[0050] In the case, not shown, of a flat, unperforated test specimen, the region of interest extends over the entire flattened face, where the break will take place.

[0051] The engraving of the pattern is carried out by means of a laser source 22 emitting a laser beam 24 towards the surface 20.

[0052] The laser source 22 is configured to emit the beam 22 in a pulsed manner by scanning at least once, so as to engrave the two-dimensional pattern on said surface 20.

[0053] The parameters of the laser source 22, which include for example a peak power, a frequency and duration of pulses, a speed of movement of the beam and a number of scans of the region 20, are configured by means of a control interface of the source 22 and chosen so as to obtain a satisfactory contrast without significantly disturbing the surface condition and therefore the mechanical behavior of the specimen 10.

[0054] The size of each point of the pattern engraved by the laser is, for example, between 50 micrometers and 80 micrometers, which gives good resolution to the measurement of the deformation field.

[0055] The step of engraving the speckled pattern on the test specimen 10 typically takes less than 5 minutes.

[0056] Laser engraving makes it possible to engrave the pattern on a flat or curved surface, or even on a surface with a more complex geometry.

[0057] The process then includes a step of carrying out a mechanical test on the specimen 10, shown in [Fig.3].

[0058] The test specimen 10 is placed in a device 40 suitable for applying mechanical stresses, comprising two supports 42.

[0059] The ends 12 of the test specimen are engaged in said supports 42 and fixed by their respective threads.

[0060] The device 40 is configured to apply a mechanical stress on the specimen 10, for example a tensile stress on the two ends 12, by applying a mechanical force on at least one of the supports 42, which stresses the two ends 12 to move them apart from each other.

[0061] The elongation of the test specimen 10 is monitored by means of a camera 44 positioned and configured to acquire images including the region 20, periodically or continuously.

[0062] The mechanical test is a temperature test, during which the temperature of the specimen 10 is regulated in order to reproduce specific conditions.

[0063] The test specimen 10 is arranged in a heating device 46 which surrounds it radially, and which includes heating elements 48 suitable for providing thermal power to raise the temperature of the test specimen 10.

[0064] The heating device 46 is for example of the lamp oven type, and includes a cooling circuit (not shown).

[0065] The temperature of the test specimen 10 is measured continuously, for example by means of a thermocouple 49 placed against the test specimen 10, connected to a control interface 50. The measured temperature values ​​are advantageously used by the control interface 50 to implement a control loop of the heating device 44, in order to maintain the temperature close to a predetermined setpoint value.

[0066] The temperature of the test specimen 10 is for example maintained during the duration of the test close to a setpoint temperature, for example between -100°C and 900°C, in particular 500°C.

[0067] Alternatively, a temperature gradient is applied to the test specimen 10 during the duration of the test.

[0068] The method further includes a step of reconstituting the deformation field in the specimen 10 during the test, by analyzing the images acquired by the camera 44.

[0069] This reconstruction is carried out by an image processing module 52 connected to the camera, according to an image processing method.

[0070] The reconstruction can be performed continuously during the test, or once the test is completed. It allows the deformation field to be reconstructed and displayed at each point of region 20 throughout the duration of the test.

[0071] According to another embodiment of the invention, shown in [Fig.4], the two-dimensional pattern engraved on the test piece 10 comprises at least a series of points 60 separated by a constant pitch d.

[0072] The test specimen 10 is, in this case, a substantially cylindrical bar, called axisymmetric, with threaded ends 12.

[0073] Each series of points can include between 4 and 10 points, for example.

[0074] The points have diameters between 0.1 millimeter and 1 millimeter.

[0075] The pattern shown in [Fig.4] comprises two series of six points 60 aligned between the ends 12 of the test specimen 10.

[0076] Such a pattern is suitable to allow rapid image processing to measure displacements quickly, especially in real time during mechanical testing.

[0077] Indeed, since the initial spacing of the points is perfectly controlled, the calculation of deformations is faster and more precise, in exchange for a lower spatial resolution. This makes it possible to detect the necking prior to the fracture of the specimen, which is not detectable to the naked eye, and thus to locate the fracture during the test, before it occurs.

[0078] The methods described allow observation of the displacement field and therefore of deformation in a specimen subjected to a mechanical test at ambient temperature, or at high temperature.

[0079] The mechanical test may be a tensile, bi-tensile, compression, shear, bending, torsional, buckling, or other test. The temperature may be kept constant, vary during the test, and / or vary spatially according to temperature gradients applied to the specimen 10. The test may be continued until failure without risk of damage to the material. Alternatively, the test may be a crack propagation test or a fatigue test.

[0080] The pattern engraved on the specimen can be easily adapted to the test conditions to have a suitable deformation field resolution in the regions of interest of the specimen 10, according to its geometry and the type of test.

[0081] The implementation of the process is quick and repeatable, and can be applied to any type of material and to real test specimens or structures of very varied dimensions and geometries.

Claims

Demands

1. Method for measuring displacement fields in a specimen (10), comprising the following steps: - generation of a two-dimensional pattern comprising a plurality of points, - engraving of the pattern on a surface (20) of the specimen (10) by means of a laser (22), - implementation of a mechanical test on the specimen (10) and acquisition of images of the pattern by means of a camera (44) during the test, and - reconstruction of the deformation field during the test by analysis of the images acquired by the camera (44), in which the points of the two-dimensional pattern are randomly distributed in a predetermined region.

2. A method according to the preceding claim, wherein, during the engraving of the pattern on the surface (20) of the test specimen (10), the laser (22) emits a laser beam (24) forming pulses with durations between 30 nanoseconds and 120 nanoseconds.

3. A method according to the preceding claim, wherein the laser beam (24) travels across the surface (20) at a speed between 10 millimeters per second and 2000 millimeters per second.

4. A method according to any one of the preceding claims, wherein the engraving of the pattern on the test specimen (10) is carried out by performing several successive steps of traversing the surface (20) with a laser beam (24) to progressively engrave the pattern on the surface (20).

5. A method according to any one of the preceding claims, wherein, during the implementation of the mechanical test, a local temperature of the specimen (10) exceeds 500°C.

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