GB2393673A

GB2393673A - Prestressing of components

Info

Publication number: GB2393673A
Application number: GB0223153A
Authority: GB
Inventors: George Durrant; John Richard Webster
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2002-10-05
Filing date: 2002-10-05
Publication date: 2004-04-07
Also published as: GB0223153D0

Abstract

A method of prestressing a component such as a gas turbine engine aerofoil 50 includes the steps of using a die 60 to apply pressure to the component 50 such that regions 62, 64 within the aerofoil 50 undergo plastic deformation and then removing the pressure, leaving residual compressive stresses within those regions.

Description

! 2393673

Prestressing of Components The invention relates to a method of prestressing a component or material and particularly to a method of 5 prestressing a compressor blade or vane of a gas turbine engine. The invention further relates to a prestressed component and particularly to a prestressed compressor blade or vane of a gas turbine engine.

Gas turbine engine components are susceptible to 10 damage caused by foreign object ingestion and general fatigue. Such damage may result in stress concentrations and cracks which limit the components' lives. This is a particular problem in aerofoil leading and trailing edges in compressor blades and vanes. One known solution is to I 15 increase the thickness of the aerofoil section in the leading and trailing edges. However, this adds weight and adversely affects the aerodynamic performance of the blade, reducing the efficiency of the engine.

It has also previously been proposed to introduce 20 regions of residual compressive stress into aerofoils, ideally resulting in the entire crosssection of the leading and trailing edges being under compression. By creating such "through thickness compression" whereby the residual stresses in the edges of the aerofoil are purely 25 compressive, the tendency for cracks to grow is severely reduced. The stress field is equalised out in the less

critical remainder of the blade.

According to the invention there is provided a method of prestressing a region within a component, the method 30 including the steps of: using a die to apply pressure to the component such that a region within the component undergoes plastic deformation; and removing the pressure, leaving residual compressive 35 stresses within the region.

Preferably the pressure is applied to only a part of

the component, the part corresponding to the region to be prestressed. Preferably the component is unrestrained in a first direction transverse to the direction of application of 5 pressure, allowing plastic deformation in the first transverse direction within the region to be prestressed, and inducing elastic stresses in an adjacent region.

Preferably the component is restrained on at least one side in a second direction transverse to the direction of 10 application of the pressure, the region to be prestressed being sandwiched between the restraint and the adjacent region in which elastic stresses are induced.

The method may include the step of initially shaping the component such that the region to be prestressed is 15 larger in at least one dimension than an adjacent region.

The region is preferably larger in a dimension substantially parallel to the direction of application of pressure. The method may include the step of sandwiching the 20 component between at least two dies, the dies being relatively moveable for applying pressure to the component.

The component may be received within a first die, the first die restraining the component in a direction parallel to the direction of application of pressure and in one 25 direction transverse to the direction of application of pressure, and a second die may be movable relative to the first die, to apply the pressure. The method may include the step of providing stop means for limiting the movement of the second die relative to the first.

30 The method may include the step of locating a resilient member between the component and the die, for alleviating variations within the pressure applied to the component. The pressure preferably results from an applied force 35 of between 100 and 1,000 kN.

The component may be an aerofoil for a gas turbine !

engine compressor or turbine and the region(s) to be prestressed may constitute the leading and/or trailing edges of the aerofoil. During the application of pressure the aerofoil may be restrained in a direction parallel to 5 its chord and unrestrained in its radial direction.

According to the invention, there is further provided a component including a region of residual compressive stress produced by a method according to any of the 10 preceding eight paragraphs.

According to the invention, there is further provided an aerofoil for a gas turbine engine, the aerofoil including a region of compressive stress produced by a method as previously described. A region of residual 15 compressure stress may be provided within the leading and/or trailing edges of the aerofoil, on both a suction and a pressure side of the aerofoil. Preferably each region of residual compressive stress extends through the thickness of the aerofoil.

20 Embodiments of the invention will be described for the purpose of illustration only with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic sectional view of a ducted fan gas turbine engine; 25 Figs. 2A and 2B are diagrammatic illustrations of a basic technique according to the invention, for prestressing a component; Figs. 3A to 3C illustrate the technique according to the invention applied to prestress the edge of a strip 30 component; Fig. 4 is a diagrammatic illustration of the technique according to the invention applied to an aerofoil; Fig. 5 is a diagrammatic illustration of a modification of the technique according to the invention; 35 Figs. 6A and 6B are diagrammatic illustrations of a preformed aerofoil for use with the technique of the

invention, before and after the application of pressure; and Fig. 7 is a diagrammatic illustration of the use of pinching dies to give localized plastic deformation, 5 according to the technique of the invention.

With reference to Fig. 1, a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 12, a propulsive fan 14, an intermediate pressure compressor 16, a high pressure 10 compressor 18, combustion equipment 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by 15 the fan 14 to produce two air flows, a first air flow into the intermediate pressure compressor 16 and a second air flow which provides propulsive thrust. The intermediate pressure compressor 16 compresses the air flow directed into it before delivering the air to the high pressure 20 compressor 18 where further compression takes place.

The compressed air exhausted from the high pressure compressor 18 is directed into the combustion equipment 20 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and 25 thereby drive the high, intermediate and low pressure turbines 22, 24 and 26 before being exhausted through the nozzle 28 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 22, 24 and 26 respectively drive the high and intermediate pressure 30 compressors 16 and 18 ad the fan 14 by suitable interconnecting shafts.

The aerofoil sections of the compressor and turbine blades and vanes are susceptible to damage as discussed previously. However, the likelihood of such damage 35 occurring, or if it does occur leading to blade failure due to fatigue effects, may be minimised by surface treatment

of the blades, for example by peering. This imparts to the surface region a residual compressive stress which reduces the effects of the tensile stresses applied to the surface by external loads.

5 In the technique of the invention, the aerofoil sections are treated by using a die to apply pressure to their leading and/or trailing edges, to induce plastic deformation and leave residual compressive stresses within the regions.

10 The basic technique of the invention is illustrated in Figs. 2A and 2B. Referring to Fig. 2A a punch 30 is used to apply a load to a component 32, normally to its surface, in order to cause plastic deformation. The magnitude of the pressure applied will depend upon the precise 15 application, but will typically involve forces of between lOO and l,OOO kN.

The useful component of the plastic deformation is in the direction transverse to the application of the load, this transverse plastic flow being indicated by the arrows 20 A in Fig. 2A. The plastic deformation in this direction under the punch causes elastic deformation in adjacent areas of the component 32. Upon removal of the load, the dimensions of the material in the transverse directions in the plastically deformed area are elastically contracted to 25 a shorter length by the bulk of the material of the component, thus leading to elastic residual compressive stresses (indicated by the arrows B in Fig. 2B) in the transverse directions of the treated area, principally In directions between the centre and the edges of the square.

30 The compressure stresses are balanced by elastic residual tensile stresses (indicated by the arrows C) in the adjacent non treated area.

Referring to Figs. 3A to 3C a useful application of the technique is in the treatment of an edge region 38 of a 35 thin, elongate strip component 40. In this case, the component 40 is received by a bottom die 42 which includes

( 6 a base 44 for supporting the component and an edge 46 for providing lateral restraint at a side edge of the strip.

The strip component 40 will also be restrained on the face opposite to 46, although this is not illustrated. A top S die 48 is used to apply pressure to the edge region 38 of the strip component 40, the base 44 of the bottom die supporting the component and the edge 46 providing restraint in a direction transverse to the direction of application of the pressure. The top die 48 deforms the 1() edge region 38 of the plate 40, causing plastic flow primarily along the length of the strip, in the direction indicated by the arrows in Fig. 3B. The adjacent material which is not under the top die 48 elastically distorts in a direction parallel to the direction of plastic flow. When 15 the load on the top die 48 is removed, the contraction of the elastically distorted material forces the material previously under the top die 48 into compression parallel to the length of the strip resulting in residual elastic compressive stresses indicated by the arrows B. For a 20 relatively thin strip, the induced compressive stresses extend through the entire thickness of the strip. The compressive stresses are balanced by tensile elastic stresses indicated by the arrows C. Fig. 4 illustrates the application of the process of 25 the invention to a compressor areofoil 50. A bottom die 52 is shaped to receive the aerofoil 50, restraining it on its concave side 54 and at its edges 56 and 58. The bottom die 52 thus prevents plastic deformation of the aerofoil 50 in a direction parallel to its chord width. However, the 30 bottom die 52 does not restrain the aerofoil 50 in the radial direction, along its length, into and out of the page in Fig. 4.

A top die 60 is moveable relative to the bottom die in a direction generally perpendicular to the radial and chord 35 dimensions of the aerofoil 50.

Referring also to Fig. 6A, the aerofoil 50 may be

preformed such that its thickness in leading and trailing edge regions 62 and 64 is greater than its thickness in a mid region 65 of the of the aerofoil. Referring to Fig. 4, as the top die 60 is used to apply pressure to the 5 aerofoil, these leading and trailing edge regions 62 and 64 are compressed and caused to deform plastically along the radial length of the aerofoil 50. The bottom die 52 prevents plastic flow taking place parallel to the chord.

Fig. 6B illustrates the aerofoil after the prestressing 10 treatment. The edge regions 62 and 64 have been compressed and have residual radial compressive stresses.

Referring to Fig. 5, compliant layers 66 of resilient material may be provided between the region of the aerofoil to be prestressed and the top and/or bottom dies 52, 60.

15 This provides an even distribution of pressure and alleviates problems associated with uneven surfaces. Such compliant layers may be utilised in any of the embodiments of the invention.

The controlled shape of the preforms as in Fig. 6A may 20 be provided by previous forging operations or by a process such as selective chemical milling, to give deformation in the required regions.

Referring to Fig. 7, pinching dies 70 may be used to give localized plastic deformation of a component 72. The 25 dies 70 may be moved over the component 72 to build up the desired residual stresses, for example along the length of a blade, with the degree of residual stress determined by the deformation load or the number of passes. In this embodiment, the dies may be allowed to rock on flexible 30 mountings in order to overcome local irregularities and reduce the accuracy needed in production of the tooling and component. There is thus provided a method of prestressing a component in which the residual stress pattern may be 35 tailored to the application for maximum benefit. This tailoring may be achieved by a combination of die shape,

( 8 component preform shape, local variations in stiffness of a compliant layer, the number of deformation steps, the local surface texture of the workpiece and/or the compliant layer and the control or limitation of plastic deformation in 5 undesirable directions.

Various modifications may be made to the above described embodiment without departing from the scope of the invention. For example, a mechanical stop may be provided to limit relative movement of the upper and lower 10 dies to control the pressure applied. The method may be used without lateral restraint of the component.

Whilst endeavouring in the forgoing specification to

draw attention to the features of the invention believed to be of particular importance it should be understood that 15 the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

( l CLAIMS

1. A method of prestressing a region within a component, s the method including the steps of: using a die to apply pressure to the component such that a region within the component undergoes plastic deformation) and removing the pressure, leaving residual compressive 10 stresses within the region.

2. A method according to Claim 1 wherein pressure is

applied to only a part of the component, the part corresponding to the region to be prestressed.

3. A method according to Claim 1 or Claim 2, wherein the 15 component is unrestrained in a first direction transverse to the direction of application of pressure, allowing plastic deformation in the first transverse direction within the region to be prestressed, and inducing elastic stresses in an adjacent region.

20 4. A method according to Claim 3, wherein the component is restrained on at least one side in a second direction transverse to the direction of application of the pressure, the region to be prestressed being sandwiched between the restraint and the adjacent region in which elastic stresses 25 are induced.

5. A method according to any preceding claim, the method including the step of initially shaping the component such that the region to be prestressed is larger in at least one dimension than an adjacent region.

30 6. A method according to Claim 5 wherein the region is larger in a dimension substantially parallel to the direction of application of pressure.

7. A method according to any preceding claim, the method including the step of sandwiching the component between at 35 least two dies, the dies being relatively moveable for applying pressure to the component.

( 10 8. A method according to Claim 7, wherein the component is received within a first die, the first die restraining the component in a direction parallel to the direction of application of pressure and in one direction transverse to 5 the direction of application of pressure, and wherein a second die is movable relative to the first die, to apply the pressure.

9. A method according to Claim 8, the method including the step of providing stop means for limiting the movement 10 of the second die relative to the first.

10. A method according to any preceding claim, the method including the step of locating a resilient member between the component and the die, for alleviating variations within the pressure applied to the component.

15 11. A method according to any preceding claim, wherein the component is an aerofoil for a gas turbine engine compressor or turbine and the region(s) to be prestressed constitute the leading and/or trailing edges of the aerofoil. 20 12. A method according to Claim 12, wherein during the application of pressure the aerofoil is restrained in a direction parallel to its chord and is unrestrained in its radial direction.

13. A component including a region of residual compressive 25 stress produced by a method according to any of Claims 1 to 11. 14. An aerofoil for a gas turbine engine, the aerofoil including a region of compressive stress produced by a method according to Claim 12 or Claim 13.

30 15. An aerofoil according to Claim 15, wherein regions of residual compressive stress are provided within the leading and trailing edges of the aerofoil, on both a suction and a pressure side of the aerofoil.

16. An aerofoil according to Claim 15 or Claim 16, wherein 35 the region of residual compressive stress extends through the thickness of the aerofoil.

17. A method substantially as herein described with reference to any of Figs. 2A to 7 of the drawings.

18. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not 5 within the scope of or relating to the same invention as any of the preceding claims. I