Turbine engine blade including structural reinforcement adhesively bonded using an adhesive bond of increased toughness

Abstract

A turbine machine blade has an aerodynamic surface that is made of organic matrix composite material reinforced by fibers and metal structural reinforcement that is adhesively bonded by an epoxy adhesive bond on the leading edge, which is of matching shape, and that presents along its entire height a section that is substantially V-shaped with a base extended by two lateral flanks of respective profiles that become thinner at free ends going towards the trailing edge. In order to increase the toughness of the epoxy adhesive bond in the event of the epoxy adhesive bond cracking, the epoxy adhesive bond includes a reinforcing sheet of elastomeric polymer enabling the reinforcing sheet to be torn into two portions, the elastomeric polymer having the following properties at 23° C.: Young's modulus E≈10 MPa; stress at rupture σ.sub.r>10 MPa; strain at rupture ε.sub.r>80%.

Claims

1. A turbine machine blade having an aerodynamic surface extending along a first direction between a leading edge and a trailing edge, and along a second direction that is substantially perpendicular to said first direction between a blade root and a blade tip, said aerodynamic surface being made of organic matrix composite material reinforced by fibers, the blade also including metal structural reinforcement that is adhesively bonded by an epoxy adhesive bond on said leading edge, which is of matching shape, and that presents along its entire height a section that is substantially V-shaped with a base extended by two lateral flanks of respective profiles that become thinner at free ends going towards said trailing edge, wherein, in order to increase the toughness of the epoxy adhesive bond in the event of the epoxy adhesive bond cracking, said epoxy adhesive bond includes a reinforcing sheet consisting of elastomeric polymer enabling the reinforcing sheet to be torn into two portions, said elastomeric polymer having the following properties at 23° C.: Young's modulus E≈10 MPa; stress at rupture σ.sub.r>10 MPa; strain at rupture ε.sub.r>80%, wherein said elastomeric polymer reinforcing sheet is constituted by latticework made up of a plane array of square and/or rectangular meshes or of circular meshes.

2. The blade according to claim 1, wherein said elastomeric polymer reinforcing sheet has a cracking rate per unit area γ of about 0.02 so as to double the critical energy release rate G.sub.C of said adhesive bond as defined by the following formula:
G.sub.C=α∫.sub.epoxyσ(δ)dδ+γ∫.sub.elastomerσ(δ)dδ where α is the cracking rate per unit area in the epoxy resin and δ represents the opening of the lips of the crack and σ represents the associated cohesion stress.

3. The blade according to claim 1, wherein said elastomeric polymer reinforcing sheet is completely embedded in said adhesive bond.

4. The blade according to claim 1, wherein said elastomeric polymer reinforcing sheet is integrated on an outside face of said adhesive bond.

5. The blade according to claim 1, constituting a turbine engine fan blade.

6. A turbine engine including at least one blade according to claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and benefits of the present invention appear from the following description given with reference to the accompanying drawings which an embodiment having no limiting character, and in which:

(2) FIG. 1 shows a side view of a turbine engine fan blade;

(3) FIG. 2 is a fragmentary section of the FIG. 1 blade showing metal structural reinforcement for the leading edge adhesively bonded on the composite material blade;

(4) FIG. 2A shows a detail of the adhesive bond between the structural reinforcement and the composite material blade;

(5) FIG. 3 shows crack propagation in a known adhesive bond including a nylon support;

(6) FIG. 4 shows a theoretical model for crack propagation from the free edge of healthy material;

(7) FIG. 5 shows crack propagation in an adhesively bonded structure in accordance with an embodiment of the invention;

(8) FIGS. 6A and 6B show two examples of reinforcing sheets that are to be incorporated in the adhesively bonded structure of FIG. 5; and

(9) FIGS. 7A and 7B show two examples of a method enabling the reinforcing sheets of FIG. 6A or 6B to be put into place in an adhesive bond in accordance with embodiments of the invention.

DETAILED DESCRIPTION

(10) FIG. 4 is a graphical illustration of a theoretical model enabling the present invention to be achieved.

(11) Specifically, the technical solution proposed by the inventors for reducing cracking in the adhesive bond between metal protection and the leading edge of a composite material fan blade is to increase the toughness of the polymerized adhesive bond. Specifically, toughness is the ability of a material to withstand crack initiation or propagation. It may be quantified by the critical energy release rate G.sub.C that represents the energy needed to propagate a crack of unit area through a healthy material C from its free edge A and via its intermediate cohesive zone B. This rate is expressed as a function of the opening δ between the lips of the crack and of the associated cohesion stress σ:
G.sub.C=∫σ(δ)dδ

(12) Consequently, it is desirable to act on the cohesion relationship σ(δ) that controls the opening of a crack in order to increase the critical energy release rate G.sub.C and thus the toughness of a material. This is the principle used by the present invention.

(13) In the prior art adhesive bond including a nylon support, G.sub.C may be expressed as the sum of the energy dissipated by the crack propagating in the epoxy plus the energy dissipated by the crack propagating at the nylon/epoxy interface:
G.sub.C=α∫.sub.epoxyσ(δ)dδ+β∫.sub.nylon/epoxyσ(δ)dδ
where α is the cracking rate per unit area in epoxy and β is the cracking rate per unit area at the nylon/epoxy interface. This scenario has been validated by analyzing the appearance of ruptures, which show that the nylon support is systematically uncovered after the crack has propagated, as explained in the introduction. This indicates the presence of low adhesion between nylon and epoxy, thereby facilitating crack propagation along that interface. It may also be observed that the person skilled in the art takes advantage of this feature by conventionally using nylon tear fabrics for texturing the surfaces of epoxy matrix composite materials. The energy dissipated during this propagation stage is relatively low since it does not involve high stress nor does it involve large strain in the material. For a nylon support, it can thus be estimated that the energy dissipated by cracking in the epoxy resin contributes to the majority of the critical energy release rate G.sub.C of the adhesive bond:
G.sub.C=α∫.sub.epoxyσ(δ)dδ

(14) The proposed technical solution thus consists in replacing the nylon support with an elastomer reinforcing sheet (properties at 23° C.: Young's modulus E≈10 MPa; stress at failure σ.sub.r>10 MPa; strain at failure ε.sub.r>80%) so as to replace a low energy propagation stage at the epoxy/nylon interface with a higher energy propagation stage in the elastomer element. An example of such an elastomer element is known under the reference 23HP90 from the supplier ITC.

(15) Specifically, the low Young's modulus of elastomer causes it to deform during opening of the lips of the crack. The deformation of the elastomer may become very large and involves non-negligible stresses as it advances towards rupture of the reinforcing sheet. Under such circumstances, the elastomer reinforcing sheet 44 tears into two separate portions 44A and 44B and creates “bridges” between the lips of the crack, as shown in FIG. 5. The energy dissipated by the process of deforming and cracking the elastomer reinforcing sheet contributes greatly to increasing the critical energy release rate G.sub.C of the adhesive bond:
G.sub.C=α∫.sub.epoxyσ(δ)dδ+γ∫.sub.elastomerσ(δ)dδ with: ∫.sub.elastomerσ(δ)dδ≈10,000 joules per square meter (J/m.sup.2) (for natural rubber)>>∫.sub.epoxyσ(δ)dδ≈200 J/m.sup.2 (for a standard epoxy)
Where α is the cracking rate per unit area in the epoxy resin and γ is the cracking rate per unit area in the elastomer reinforcing sheet. By way of example, a beneficial value for γ is 0.02, thereby doubling G.sub.C and retaining all of the benefit of epoxies in terms of stiffness.

(16) The reinforcing sheet may present various shapes, with two examples being shown in plan view in FIGS. 6A and 6B. FIG. 6A shows a latticework made up of an array of square and/or rectangular meshes, and FIG. 6B shows latticework made up of a plane array having circular meshes, like chain-mail.

(17) The reinforcing sheet 44 may be incorporated in the adhesive bond 40 using two distinct methods, that may be referred to as single impregnation or double impregnation. In the single impregnation method, shown in FIG. 7A, the elastomer reinforcing sheet 44 is integrated solely at the surface on an outside face of the raw epoxy adhesive bond, whereas in the double impregnation method, as shown in FIG. 7B, the elastomer reinforcing sheet is deposited between two films 40A and 40B of raw epoxy adhesive and, after polymerization, it is completely “embedded” in the adhesive bond. In known manner, this polymerization is obtained in specific conventional tooling, e.g. at a temperature of 180° C. and a pressure of 3 bars for 60 minutes.

(18) Thus, an embodiment of the present invention, which consists in integrating an elastomer reinforcing sheet in a film of raw adhesive in order to increase the toughness of the bond after polymerization, and as a replacement for a nylon support having the sole function of calibrating the thickness of the bond, makes it possible, in comparison with the state of the art, to benefit in particular from the following advantages: controlling the distribution and the position of the elastomer in the film of adhesive, in particular by working on the shape of the reinforcing sheet; working with any film of raw adhesive, including a film of adhesive that already contains elastomer fillers; continuing to perform a thickness calibration function for the polymerized adhesive bond; and limiting uptake of moisture (problematic for aging) which is a known drawback of a nylon support.

(19) Although the above description is illustrated by means of a turbine engine fan blade, it should be observed that the invention is also applicable to making metal structural reinforcement for reinforcing a leading edge of any type of turbine engine blade, whether for terrestrial or aviation use, and in particular a helicopter turboshaft engine or an airplane turbojet, and also for propellers such as unducted contrarotating double fan propellers.