Fatigue specimen obtained by wire electroerosion and associated production process

Abstract

Fatigue Test Specimen Obtained by Wire Electrical Erosion and Associated Production Method. One aspect of the invention relates to a cylindrical test specimen (1) with axis A and diameter D, the latter decreasing progressively between a first diameter (D1) and a second diameter (D2), with the first diameter (D1) being larger than the second diameter (D2). The test specimen (1) comprises two first cylindrical portions (11) of first diameter (D1) at its ends and a second cylindrical portion (12) of second diameter (D2) located in the middle of the test specimen (1). At least one first flat surface (S1) is located between the two first cylindrical portions (11) of first diameter (D1). The first flat surface (S1) and a tangent to a cylindrical surface (121) of the second portion (12) form an angle α greater than 90°. Figure to be published with the abbreviation: Figure 2

Description

Title of the invention: Fatigue test specimen obtained by wire electroerosion and method of obtaining it PARTNER TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of fatigue test specimens used in fatigue tests.

[0002] The present invention relates to a test specimen having particular dimensions linked to a manufacturing process comprising at least one electro-erosion machining step or a shot blasting or sandblasting step. The invention also relates to a method for manufacturing a test specimen. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] To characterize the fatigue resistance of a part, for example the cells of a turbine disk, it is common to use fatigue specimens subjected to mechanical stress during fatigue tests.

[0004] A first known type of test specimen is the smooth cylindrical specimen. However, the cylindrical surfaces of the specimen cannot be obtained by wire cutting.

[0005] A second known type of test specimen is the flat specimen. This specimen can be machined by wire EDM. However, this specimen geometry has sharp or slightly radii, which will potentially generate folds or ridges during a shot peening process.

[0006] Thus, the state of the art does not allow the fatigue resistance of a part obtained by a manufacturing process comprising wire cutting by electro-erosion and a shot blasting or sandblasting step to be characterized. Summary of the invention

[0007] The invention offers a solution to the problems mentioned above, by allowing fatigue tests to be carried out on a specimen having specific characteristics related to its manufacturing process including wire cutting by electro-erosion and a shot blasting or sandblasting step.

[0008] A first aspect of the invention relates to a cylindrical test specimen with axis A and diameter D, the latter decreasing progressively between a first diameter and a second diameter, with the first diameter being larger than the second diameter. The test specimen comprises two cylindrical portions of the first diameter at its ends and a second cylindrical portion of the second diameter located in the middle of the specimen. At least one flat surface is located between the two. The first cylindrical sections of the first diameter, the first flat surface, and a tangent to a cylindrical surface of the second section form an angle α greater than 90°. The specimen is dimensioned to have sufficiently large flat surfaces to be representative of the cell surfaces of a high-pressure turbine disk, particularly in the aeronautical sector. During shot peening, the angle greater than 90° prevents sharp edges and thus avoids the formation of folds at the intersection of the first flat surface and the specimen surface, which could lead to premature crack initiation during fatigue tests.

[0009] Advantageously, the flat surface is located in the middle of the specimen, said flat surface is divided into two equal and symmetrical parts with respect to a plane B located in the middle of the specimen and perpendicular to the axis A.

[0010] Advantageously, the first flat surface is obtained by wire electrical discharge machining (EDM). This will allow the resistance of a specimen having a flat surface obtained by wire EDM to be characterized during fatigue tests.

[0011] Advantageously, the first flat surface is shot-peened by bead blasting or ultrasonic shot peening, or sandblasted. This will allow the resistance of a specimen having a shot-peened or sandblasted flat surface to be characterized during fatigue tests.

[0012] Advantageously, in a second embodiment, a first notch is located in the middle of the first flat surface. This notch makes it possible to create one or more stress concentrations on the specimen, in order to characterize the resistance during fatigue tests of a specimen exhibiting one or more stress concentrations. The stress concentrations will be located at the notch, therefore in the middle of the specimen.

[0013] Advantageously, the first notch has a semi-cylindrical shape in longitudinal section. This shape is determined by simulations so that the stress concentrations at the bottom of the notch are, for example, comparable to those obtained in a honeycomb area of ​​a high-pressure turbine disk.

[0014] Advantageously, the first notch is obtained by wire electro-erosion.

[0015] Advantageously, the specimen comprises a second flat surface located between the first two cylindrical portions of first diameter. The first and second flat surfaces are positioned symmetrically with respect to the axis A of the specimen. This symmetry makes it possible to accurately calculate a useful cross-section and thus determine the deformation or force to be applied for a given stress. Furthermore, the symmetry of the flat surfaces allows for the uniform distribution of stresses over the specimen.

[0016] The invention also relates to a method for manufacturing the test tube comprising: • A step of obtaining at least part of the first flat surface by wire electro-erosion of a cylindrical surface of second diameter.

[0017] Advantageously, the manufacturing process comprises: • A shot blasting or sandblasting step of at least part of the first flat surface.

[0018] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0019] The figures are presented for illustrative purposes only and are in no way limiting of the invention.

[0020] [Fig.1] is an illustration of a manufacturing process for a wire electro-erosion cutting tool;

[0021] [Fig.2] is seen in perspective of a first embodiment of a test tube lindrique according to the invention;

[0022] [Fig.3] is a top view of [Fig.2];

[0023] [Fig.4] is a side view of [Fig.3];

[0024] [Fig.5] is seen in section of [Fig.4] according to section VV.

[0025] [Fig.6] is a perspective view of a second embodiment of a cylindrical test tube having a notch according to the invention;

[0026] [Fig.7] is a top view of [Fig.6];

[0027] [Fig.8] is a side view of [Fig.7];

[0028] [Fig.9] is an enlarged view of [Fig.8];

[0029] [Fig. 10] is a cross-sectional view of [Fig. 8] along section XX DETAILED DESCRIPTION

[0030] Unless otherwise specified, the same element appearing on different figures has a unique reference.

[0031] Figure 1 illustrates a method for manufacturing a wire electrical discharge machining (EDM) part. The cutting process uses a taut, small-diameter electrode wire 21 that unwinds continuously and carries a high-frequency current under a coaxial flow of dielectric fluid 24, for example, deionized water, ensuring optimum lubrication along the electrode wire 21. The material is progressively removed by thermal erosion, itself caused by a succession of sparks between the wire 21 and the part being cut 22. The wire 21 then gradually reproduces the programmed shape, with a groove 23 larger than its diameter, corresponding, for example, to twice the inter-electrode distance. This manufacturing process makes it possible to produce fine cuts, thanks to the small diameter of the wire 21, for example, 0.2 to 0.3 mm.

[0032] Shot blasting is a manufacturing process consisting of projecting an abrasive onto the surface of a part, for example balls.

[0033] The specimen 1 illustrated in figures 2 to 10 can be subjected to fatigue tests, for example oligocyclic or LCF (Low Cycle Fatigue), which consists of imposing a stress that causes cyclic plastic and / or elastic deformation at a frequency between 1 and 10 Hz in the body of the specimen 1. The geometry of the specimen 1 can also be subjected to fatigue tests at a vibration frequency greater than 10 Hz, for example megacyclic or HCF (High Cycle Fatigue).

[0034] By definition, a flat surface is a smooth surface, which has little relief and which has no hollows.

[0035] The invention relates to a cylindrical test specimen 1.

[0036] According to the first embodiment, the cylindrical test specimen 1 illustrated in Figures 2 to 5 comprises: • an axis A; • a diameter D varying from a first diameter DI to a second diameter D2;

[0037] The first diameter DI is greater than the second diameter D2. The diameter D of specimen 1 gradually decreases between the first diameter DI and the second diameter D2.

[0038] The test specimen 1 has two first cylindrical portions 11 of first diameter DI located at the ends of the specimen 1. The first two cylindrical portions 11 of first diameter DI are symmetrical and are long enough to be gripped by jaws during a fatigue test and thus prevent slippage of the part. According to an alternative embodiment, the first portions 11 of first diameter DI have threaded surfaces. Thus, there is no possibility of slippage between the first portions 11 of first diameter DI and the jaws.

[0039] Specimen 1 has a second portion 12 of cylindrical shape and of second diameter D2 located in the middle of specimen 1. The second portion 12 has a length L. Crack initiation during a fatigue test occurs in the second portion 12 which has smaller dimensions than the first cylindrical portions 11. The first cylindrical portions 11 are more massive than the second portion 12 in order to avoid deformation during fatigue tests.

[0040] The surface finish of the second portion 12, for example, has a roughness Ra (arithmetic mean of the absolute values ​​of the heights between the hollows and the bumps of the surface) between 0.1 and 0.3, and preferably 0.2, which is a polished surface finish. A surface with low roughness helps to prevent stress initiation during fatigue tests.

[0041] The test specimen 1 has a first flat surface S1 and a second flat surface S2 extending between the first two cylindrical portions S1 of first diameter D1. The first flat surface S1 has a width less than the second diameter D2. By definition, a flat surface is a smooth surface, which has little relief and which has no hollow. The second symmetrical flat surface S2 has the same dimensions and shape as the first flat surface SI. The first flat surface SI and the second symmetrical flat surface S2 are positioned symmetrically with respect to axis A.

[0042] With reference to Figures 3 and 9, specimen 1 has a plane B located in the middle of specimen 1 and perpendicular to axis A. The first flat surface S1 is divided into two parts of the same size and are symmetrical with respect to plane B. Similarly, the second flat surface S2 is divided into two parts of the same size and are symmetrical with respect to plane B.

[0043] The first flat surface SI can be obtained by wire electrical discharge machining (EDM). The first flat surface SI can also be obtained, for example, by die-sinking, grinding, or milling. The first flat surface SI can also be subjected to shot peening or sandblasting.

[0044] The geometry of the specimen 1 according to the first embodiment thus makes it possible to measure the influence of manufacturing processes such as wire electro-erosion or milling, but also shot blasting or sandblasting on the fatigue properties of a material compared to rectified polished geometries.

[0045] As illustrated in [Fig. 5], which is a cross-sectional view of specimen 1 along section VLVI, the first flat surface SI and the second portion 12 of second diameter D2 form an angle α. Angle α corresponds to the intersection between the first flat surface SI and the tangent of a cylindrical surface 121 of the second portion 12 at the edge of the first flat surface SL. This angle α is greater than 90°. Angle α is present on all edges of the first flat surface SL. Thus, specimen 1 has no sharp edges that could generate folds during a shot peening or sandblasting process. Angle α is also present on all edges of the second flat surface S2.

[0046] Specimen 1 according to the first embodiment can undergo fatigue tests equivalent to reference fatigue tests. The results of the fatigue tests on specimen 1 will be compared to the results of reference fatigue tests in order to determine the influence of the manufacturing processes on the fatigue properties of the material of specimen 1. This analysis thus makes it possible to measure only the influence of the manufacturing process and not the influence of the geometry of specimen 1.

[0047] In the second embodiment, the specimen 1 has the same characteristics as in the first embodiment except that a first notch El, as illustrated in Figures 6 to 10, has been added. The specimen 1 has a first semi-cylindrical notch El located on the second portion 12. The first notch El may also have a semi-spherical shape representing the stress gradient of a highly stressed area on disk cells. The first Notch E1 may also have a V-shape or a square shape forming a flat base at the bottom of the first notch E1. Specimen 1 may include a second notch E2 located on the second portion 12, with the same dimensions and shape as the first notch E1. The first notch E1 and the second notch E2 are positioned symmetrically with respect to axis A.

[0048] The geometry and dimensions of the first notch El are determined by simulations so that the stress concentrations at the bottom of the first notch El are, for example, comparable to those obtained in a cell zone of a high-pressure turbine disk made of a nickel-based alloy. Here, the semi-cylindrical shape of the first notch El is representative of the bottoms of such a cell zone.

[0049] This specimen geometry 1 includes stress concentrations located at the bottom of the notches El and E2.

[0050] This specimen geometry 1 provides additional security against the risk of crack initiation at the edges between the second portion 12 and the first notch EL. Crack initiation will thus occur at the bottom of the first notch El and / or at the bottom of the second notch E2.

[0051] The fatigue tests of specimen 1 according to the second embodiment are carried out under imposed stresses, i.e. a fixed stress without control of the deformation of the center of specimen 1 having a concentration of stresses located at the bottom of the notches E1 and E2. The results of the fatigue tests will be compared with standard specimens in order to determine the influence of the manufacturing process on the service life of the cell of a turbine disk.

[0052] The aim may thus be to determine whether the range of manufacturing by wire cutting by electro-erosion associated with shot blasting does not cause a decrease in the lifespan of the cell of a turbine disc compared to other ranges of manufacturing.

[0053] As illustrated in [Fig.9] which is seen from specimen 1, there is an angle [3 greater than 90° between the surface of the first notch El and the first flat surface SL. The angle [3 is also present between the surface of the second notch E2 and the second flat surface S2.

[0054] As illustrated in [Fig. 10], which is a cross-sectional view of specimen 1 along section XX, there is an angle [3] greater than 90° between the surface of the first notch El and the first flat surface SL. Similarly, there is an angle y greater than 90° between the first notch El and the cylindrical surface 121 of the second portion 12 of second diameter D2. This is to avoid creating folds during a shot peening step of the first notch EL. The angle y is also present between the second notch E2 and the second flat surface S2.

[0055] The first notch E1 and the second notch E2 can be made, for example, by wire or die-sinking electrical discharge machining (EDM), by milling or by grinding. The The first notch El and the second notch E2 can also be shot-blasted or sandblasted under the same conditions as the cells of a turbine disc.

Claims

Demands

1. Cylindrical specimen (1) of axis A and diameter D, the latter decreasing progressively between a first diameter (Dl) and a second diameter (D2) with the first diameter (Dl) greater than the second diameter (D2), the specimen (1) comprising two first cylindrical portions (11) of first diameter (Dl) at its ends and a second cylindrical portion (12) of second diameter (D2) located in the middle of the specimen (1), characterized in that at least one first flat surface (SI) is located between the two first cylindrical portions (11) of first diameter (Dl), the first flat surface (SI) and a tangent of a cylindrical surface (121) of the second portion (12), such that the tangent is orthogonal to a radial direction of a cylinder of axis A, form an angle α at an edge of the first flat surface (SI), greater than 90°.

2. Specimen (1) according to the preceding claim characterized in that the flat surface (SI) is located in the middle of the specimen (1), said flat surface (SI) is divided into two equal and symmetrical parts with respect to a plane B located in the middle of the specimen (1), said plane B is perpendicular to the axis A.

3. Test specimen (1) according to any one of the preceding claims characterized in that the first flat surface (SI) is shot-peened by ball jet or by ultrasonic shot blasting or sandblasted.

4. Specimen (1) according to any one of claims 1 to 3 characterized in that at least one first notch (El) is located in the middle of the first flat surface (SI).

5. Specimen (1) according to claim 4 characterized in that the first notch (El) has a semi-cylindrical shape according to a longitudinal section.

6. Specimen (1) according to claims 4 or 5 characterized in that the first notch (El) is obtained by wire electro-erosion.

7. Specimen (1) according to any one of the preceding claims characterized in that it comprises a second flat surface (S2) located between the first two cylindrical portions (11) of first diameter (Dl), the first flat surface (SI) and the second flat surface (S2) are positioned symmetrically with respect to the axis A of the specimen (1).

8. A method for manufacturing a test specimen (1) according to any one of claims 1 to 7, characterized in that it comprises: - a step of obtaining at least part of the first flat surface (SI) by wire electro-erosion of a cylindrical surface of the specimen (1).

9. Method of manufacturing a test specimen (1) according to any one of claims 1 to 7 characterized in that it comprises: - a shot blasting or sandblasting step of at least a part of the first flat surface (SI).

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