Method of fabricating a mechanical part, including a method of predicting the risks of crack initiation in the part in a “fretting-fatigue” situation

• US 9,081,922 B2
• Filed: 06/12/2013
• Issued: 07/14/2015
• Est. Priority Date: 06/28/2012
• Status: Active Grant

• Current Assignee: Airbus Helicopters
• Original Assignee: Airbus Helicopters
• Inventors: Fouvry, Siegfried, Heredia, Stephane, Greco, Eric, Berthel, Bruno
• Primary Examiner(s): COZART, JERMIE E
• Application Number:
  US13/915,895
• Publication Number:
  US 20140000080A1
• Time in Patent Office: 762 Days
• US Class Current: 1/1
• CPC Class Codes: B21D 53/92 other parts for aircraft
• CPC Class Codes: B64C 2001/0081 from metallic materials
• CPC Class Codes: B64F 5/60 Testing or inspecting aircraft components or systems
• CPC Class Codes: G01M 5/0033 by determining damage, crack or wear
• CPC Class Codes: G01N 2203/0062 Crack or flaws
• CPC Class Codes: G01N 2203/0064 Initiation of crack
• CPC Class Codes: G01N 2203/0218 Calculations based on experimental data
• CPC Class Codes: G01N 3/32 by applying repeated or pulsating forces
• CPC Class Codes: G01N 3/56 Investigating resistance to wear or abrasion
• CPC Class Codes: G06F 30/23 using finite element methods [FEM] or finite difference methods [FDM]
• CPC Class Codes: Y10T 29/49771 Quantitative measuring or gauging
• CPC Class Codes: Y10T 29/49776 Pressure, force, or weight determining
• CPC Class Codes: Y10T 29/49863 with prestressing of part

Abstract

A method of fabricating a mechanical part of structure that is defined relative to a predictive search of the risks of crack initiation therein by using a method of calculation by finite elements. A coarse mesh (9) is taken into account, and the individual size of the meshes (20) and a critical distance (d) are defined jointly by an operator. The meshes (20) making up the mesh (9) are defined to be of identical mesh size, with the critical distance (d) being defined as the sum of an integer number of depth dimensions of said meshes (20). If the results of the calculation by finite elements diverge, then a calibration weighting function is advantageously applied that takes account of the size of the meshes (20) by taking account of the critical distance (d).

Claims

1. A method of fabricating a mechanical part, the method comprising an operation of designing said part that is to be fabricated, which operation performs a prediction operation of predicting the risk of crack initiation in the part that is to be fabricated when subjected to fretting-fatigue stresses, said method comprising:

a test operation performed on test pieces made of a material from which the part that is to be fabricated is to be made, during which test operation at least one test piece is subjected during test cycles to fretting-fatigue stresses in order to quantify the cracking-initiation threshold for a given material;

a calculation operation of calculating stresses by finite elements, the operation including a prior step of an operator defining a mesh for a contact zone under stress of the part that is to be fabricated and given the tests that are performed, followed by a step of calculating the fretting-fatigue stresses applied to each of the nodes defined by the meshes making up said mesh and individually having the shape of regular polygons;

a calibration operation of calibration by calculation on the previously-calculated stresses, by applying a calculation criterion calibrated from the data provided by the testing previously performed on test pieces, said calculation criterion being applied to each of the calculated stresses in order to deduce the risks of crack initiation in the part that is to be fabricated, said calculation criterion taking account of a critical distance (d) defined by the operator and extending depthwise into the material from the surface facing towards the outside of the material in said contact zone of the part that is to be fabricated; and

then

a final fabrication operation of fabricating the part that is to be fabricated with structure as previously defined by applying said design operation;

the method being characterized;

in that the individual size of the meshes and said critical distance (d) are defined jointly in correlation by the operator, the meshes together making up said mesh being defined by individual sizes that are identical, the critical distance (d) being defined by summing an integer number of depth dimensions of said meshes of identical individual size; and

if the calculation operation by finite elements diverges, the calibration operation performs a weighting function to correct said divergent nature of the calculation operation.

2. A method according to claim 1, wherein the individual size of the meshes lies in the range 0.05 times the critical distance (d) to the critical distance (d) in full.

3. A method according to claim 1, wherein the calibration operation comprises applying a weighting function based on a coefficient k(l) from which said calculation criterion is defined, said weighting function being such that:

ω

=1+k(l)·

∇

σ

_max

in which;

ω

is said weighting function;

k(l) is a weighting coefficient for weighting the stress calculations and it is determined from results obtained during the test operations; and

∇

σ

_max is the stress gradient in the material.

4. A method according to claim 3, wherein the coefficient k(l) is determined for each of the test configurations, and from said determinations for each of the test configurations, a mean value is deduced for the coefficients k(l), the coefficient k(l) being determined using the following rule:

σ

_max(z)·

(1+k(l)·

∇

σ

_max)=σ

_d

in which;

σ

_max(z) is the stress calculated at depth of the node of a mesh under consideration at the critical distance (z);

(1+k(l)·

∇

σ

_max) is a weighting function for calibration from said data provided by the test, in which ∇

σ

_max is the stress gradient in the material and k(l) is a said correction coefficient of the rule by weighting stress calculations; and

σ

_d is the stress limit that can be withstood by the material.

5. A method according to claim 1, wherein a risk of crack initiation in the part that is to be fabricated is determined by applying the following rule:

6. A method according to claim 1, wherein the method is applied to a part that is to be fabricated from a titanium alloy, and the individual size of the meshes is defined to lie in the range 50 μ

m to 200 μ

m, and the critical distance (d) is defined to lie in the range 50 μ

m to 600 μ

m.

7. A method according to claim 1, wherein the size of a mesh is equal to the critical distance (d), said integer number being equal to 1.

8. A method according to claim 1, wherein said contact zone is of the type defined as a cylinder and plane with a test piece being subjected to stress cycles amounting to about 10⁶ cycles, and with a radius of curvature defining said contact zones lying in the range 20 mm to 80 mm, approximately.