GB2439551A

GB2439551A - A constrained layer damping arrangement

Info

Publication number: GB2439551A
Application number: GB0613017A
Authority: GB
Inventors: Brynley Clark
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-06-30
Filing date: 2006-06-30
Publication date: 2008-01-02
Also published as: GB0613017D0

Abstract

A constrained layer damping arrangement comprises a component 2, for example a vane of a gas turbine engine, a viscoelastic layer 4 and a constraining layer 6. The viscoelastic layer 4 comprises two different viscoelastic materials disposed in different regions of the viscoelastic layer 4. The different viscoelastic materials 8, 10 reach their maximum loss factors at different temperatures, so that the damping effectiveness of the combined layer 4 extends over a relatively wide temperature range.

Description

-1-2439551

A CONSTRAINED LAYER DAMPING ARRANGEMENT

This invention relates to a constrained layer damping arrangement and is particularly, although not exclusively, concerned with such an arrangement for damping vibration of aerospace components such as blades and vanes of gas turbine engines.

A conventional constrained layerdamping arrangement comprises a viscoelastic layer disposed between first and second constraining layers. In many cases, one of the constraining layers is the component to be damped and the other is a constraining panel or sheet of, for example, a metal alloy or composite material. The viscoelastic material may be any material which displays elastic behaviour with a relatively low recovery rate after deformation. Some elastomenc materials are suitable, but it is also known to use polymers, such as acrylic polymers, which have the required viscoelastic properties.

The viscoelastic properties of such materials vary significantly with temperature. Such materials exhibit a loss factor (ie the ability to damp vibration) which is at or close to its maximum over a relatively small temperature range. Consequently, the damping ability of conventional constrained layer damping arrangements can become unacceptably low if the temperature of the component to be damped deviates from the optimum temperature for the viscoelastic material being used.

Attempts have been made to increase the temperature range over which the loss factor of the viscoelastic material is adequately high. For example, EP 0540332 discloses a bubble-containing pressure sensitive adhesive as the viscoelastic layer in a constrained layer damping arrangement. The loss factor of the material reaches a maximum at a first temperature, owing to the viscoelastic properties of the pressure sensitive adhesive, and then reaches another maximum at a second temperature, owing to the interaction between the pressure sensitive adhesive and the bubbles. Nevertheless, the bubble-containing pressure sensitive adhesive is, in effect, a single material and so cannot be employed to vary the damping characteristics over the surface of the component in dependence on the magnitude of the strain experienced by the component when it vibrates.

EP 0694709 discloses a tubular component having two coaxial viscoelastic layers disposed one within the other and separated from each other by a constraining layer.

The transition temperatures above which the viscoelastic layers become substantially less stiff may be different for each layer so as to extend the temperature range over which the combined viscoelastic layers provide an adequate loss factor. However, the requirement for two viscoelastic layers, and an additional constraining layer, adds to the bulk and weight of the constrained layer damping arrangement, this being a particular disadvantage in the context of aerospace components.

According to the present invention there is provided a constrained layer damping arrangement comprising a viscoelastic layer disposed between first and second constraining layers, the viscoelastic layer comprising at least two viscoelastic materials having different viscoelastic properties.

In a preferred embodiment, the loss factors of the two materials reach their maxima at different temperatures. For example, the loss factor of one of the materials may reach its maximum at a temperature not greater than 0 C, while that of the other may reach its maximum at a temperature in excess of 20 C.

The viscoelastic materials are preferably disposed in respective different regions between the constraining layers and each extends across the gap between the constraining layers in the regions in which they are present. The viscoelastic materials may be in the form of strips, which extend parallel to each other and side-by-side. The * strips may have equal widths, with the strips of one of the materials alternating with the other. In other embodiments, the strips of one material may have a width different from that of the other. Each two strips of one material may be separated by more than one strip of the other.

One of the constraining layers may be constituted by a component to be damped. The component may be elongate, and, if the viscoelastic layer comprises strips of the viscoelastic materials, the strips may be oriented in the longer direction of the component. Alternatively, the viscoelastic materials may be disposed in an array of regions which alternate with one another in the lengthwise direction of the component.

One of the viscoelastic materials may be disposed in one or more regions of the components subject to maximum strain when the component is vibrated in a first mode at a first temperature at which that viscoelastic material has a loss factor at or close to its maximum.

The component may be an aerospace component, such as a blade or vane of a gas turbine engine.

According to another aspect of the present invention, there is provided a method of damping vibration of a component, the method comprising applying to the component a constrained layer damping arrangement as defined above.

The method may comprise the steps of: (i) determining a region of maximum strain of the component when the component vibrates in a first mode at a first temperature; (ii) selecting a first viscoelastic material having a loss factor which reaches its maximum at or approximately at the first temperature; and (iii) constructing the constrained layer damping arrangement so that the selected first viscoelastic material is disposed on the component at the region of maximum strain in the first mode.

The method may also comprise the further steps of: (iv) determining a region of maximum strain when the component vibrates in a second at the second temperature; (v) selecting a second viscoelastic material having a loss factor which reaches its maximum at or approximately at the second temperature; (vi) constructing the constrained layer damping arrangement so that the selected second viscoelastic material is disposed on the component at the or each region of maximum strain in the second mode.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a constrained layer damping arrangement; Figure 2 is a graph of loss factor against temperature; Figure 3 is a diagrammatic representation of a vane of a gas turbine engine; Figure 4 illustrates a vibration mode of the vane of Figure 3; Figure 5 represents variations in strain over the surface of the vane of Figures 3 and4; Figure 6 represents viscoelastic material applied to the vane of Figures 3 and 4; Figures 7 and 8 represents variations of strain of a vane of a gas turbine engine during vibration in different modes; and Figure 9 represents the disposition of viscoelastic materials on the vane of Figures 7 and 8.

As shown in Figure 1, a vane 2 of a gas turbine engine is provided with a constrained layer damping arrangement comprising a layer 4 of a viscoelastic material, and a constraining layer 6 which may, for example, be made from an aerospace metal alloy or composite material. The vane 2 itself constitutes a further constraining layer. As represented in Figure 1, and in accordance with the present invention, the viscoelastic layer 4 comprises two different viscoelastic materials 8, 10. The materials 8, 10 are disposed in different regions between the vane 2 and the constraining layer 6, each material extending, at its respective region, across the entire gap between the vane 2 and the constraining layer 6.

Figure 2 is a graph showing the loss factor plotted against temperature for two different viscoelastic materials, ISD1 12 and ISD1 13. These two materials can be regarded as the materials 8 and 10 of Figure 1.

It will be appreciated from Figure 2 that the maximum loss factor (ie the ability to damp vibrations) of ISDI 12 reaches a maximum at approximately 40 C, but has a relatively low loss factor at temperatures below 0 C, and in particular has a loss factor below 0.2 at -20 C. Consequently, it will be appreciated that ISDI 12 has a satisfactory loss factor for many purposes at or somewhat above normal ambient temperatures (ie above 20 C), but would perform unsatisfactorily at relatively low temperatures.

By contrast, ISD1 13 has its maximum loss factor just below 0 C, but has a low loss factor at temperatures above 20 C. In practice, many aircraft components that require damping are subjected, in operation, to varying temperatures. Consequently, if a constrained layer damping arrangement is employed, it is necessary for the damping arrangement to provide adequate damping performance over the full range of temperatures experienced by the component in operation. By way of a example, a component such as a vane of a gas turbine engine may be excited at different temperatures, as represented by points A and B in Figure 2. At point A, the temperature is -20 C and at point B it is 50 C. The response to excitation is different at the two temperatures, so that the vibration amplitude at 50 C is higher than that at -20 C. If undamped, both responses are above an acceptable level. However, it will be noted that, while the response at point B lies below the line representing ISDI 12, and so would be adequately damped by a constrained layer damping arrangement comprising ISDI 12 as the sole viscoelastic layer 114, the vibration would not be adequately damped if ISDI 13 were used as the sole material of the viscoelastic layer 4.

Similarly, at point A, the vibration would be adequately damped by using ISDI 13, but would not be adequately damped by using ISDI 12.

However, by applying the two materials ISDI 12 and ISDI 13 in alternating strips, as shown in Figure 1, the net damping effect of the combined materials is represented by the line C in Figure 2. Both points A and B lie on or below this combined line C, so that the overall damping effect would be adequate at both -20 C and 50 C.

Figure 3 represents the vane 2 in a static condition, and Figure 4 is an exaggerated representation of the vane during vibration in the single mode with which Figure 2 is concerned as represented by an arrow X. Figure 5 represents the stress levels in the blade during vibration as shown in Figure 4, with dark regions representing areas of lowest strain, and light regions representing areas of highest strain. Figure 6 represents the alternating strips of materials 8 and 10 (1S0112 and 150113). As shown in Figure 6, the strips extend in the lengthwise direction of the vane 2, but it will be appreciated that the strips could extend in different directions, depending on the vibration mode that it is to be damped. Also, the strips are represented as being of equal width and having parallel sides. However, the strips of the respective materials may have a width ratio other than 1:1, in order, for example, to tailor the line C in Figure 2 to achieve the required loss factor at the temperatures of interest. Also, the strips may be applied so that more than one of the strips 8, 10 of one material are disposed between each two of the other material.

In some circumstances, the mode of vibration of a component may change with temperature. For example, Figure 7 represents a vane vibrating in a first mode at, for example, 40 C, whereas Figure 8 shows the same vane vibrating in a second mode at -20 C. Again, the strain is represented by the shading of the component, with areas of lower strain being darker, and those of higher strain being lighter. It will be appreciated that, at 40 C, the vibration mode causes areas of high strain towards the ends of the vane, while at -20 C (Figure 8) the highest strain is in the central region of the vane, with low strain regions on either side.

To provide optimum damping for a vane which is susceptible to vibration in the two modes represented by Figures 7 and 8, the viscoelastic materials could be applied as represented in Figure 9. Thus, instead of applying lengthwise-directed strips as shown in Figure 6, the different materials ISDII2 and ISDII3 are applied in alternating regions disposed side-by-side in the lengthwise direction of the blade, so as to provide optimum damping, in the regions of greatest strain, in the different vibrating modes.

It will be appreciated that the pattern in which the different viscoelastic materials are applied can be varied from those in shown in Figures 6 and 9, according to the modes of vibration of the component in question. In some circumstances, it may be appropriate to employ more than two different viscoelastic materials in order to achieve the desired damping performance.

Although the invention has been described with reference to a vane of a gas turbine engine, it will be appreciated that the present invention can be applied to other components, whether elsewhere in a gas turbine engine, or in other aircraft environments, for example, in an airframe structure. The invention may also be applied to components other than aerospace components, wherever adequate vibration damping performance is required over a range of temperatures.

Claims

CLAIMS

I A constrained layer damping arrangement comprising a viscoelastic layer disposed between first and second constraining layers, the viscoelastic layer comprising at least two viscoelastic materials having different viscoelastic properties.

2 A constrained layer damping arrangement as claimed in claim 1, in which the temperature at which the viscoelastic materials reach their maximum loss factor is different for each material.

3 A constrained layer damping arrangement as claimed in claim 2, in which the loss factor of one of the viscoelastic materials reaches its maximum at a temperature below 0 C, and the loss factor of the other viscoelastic material reaches its maximum at a temperature in excess of 20 C.

4 A constrained layer damping arrangement as claimed in any one of the preceding claims, in which the viscoelastic materials are disposed in respective different regions of the constraining layers, and each material extends between the constraining layers.

A constrained layer damping arrangement as claimed in claim 4, in which the viscoelastic materials are disposed in alternating strips.

6 A constrained layer damping arrangement as claimed in any one of the preceding claims, in which one of the constraining layers comprises a component to be damped.

7 A constrained layer damping arrangement as claimed in claim 6, in which the component is elongate.

8 A constrained layer damping arrangement as claimed in claim 7, when appendant to claim 5, in which the strips extend in the lengthwise direction of the component.

9 A constrained layer damping arrangement as claimed in claim 7, in which the viscoelastic materials are applied in an alternating array which extends in the lengthwise direction of the component.

A constrained layer damping arrangement as claimed in any one of claims 6 to 9, in which one of the viscoelastic materials is applied to the component in a region of the component which is subject to high strain in a first vibration mode of the component at a first temperature.

11 A constrained layer damping arrangement as claimed in any one of claims 6 to 10, in which the component is an aerospace component.

12 A constrained layer damping arrangement as claimed in claim 11, in which the component is a blade or a vane of a gas turbine engine.

13 A constrained layer damping arrangement substantially as described herein with reference to, and as shown in, Figures 1 and 3 to 6, or Figures 7 to 9 of the accompanying drawings.

14 A method of damping vibration of a component, the method comprising applying to the component a constrained layer damping arrangement as claimed in any one of the preceding claims.

A method as claimed in claim 14, which method comprises the steps: (i) determining regions of maximum strain in the component when the component vibrates in a first mode at a first temperature; (ii) selecting a first viscoelastic material having a toss factor which reaches a maximum at or approximately at the first temperature; and (iii) constructing the constrained layer damping arrangement so that the selected first viscoelastic material is disposed on the component at the region of maximum strain in the first mode.

16 A method as claimed in claim 15, comprising the additional steps: (iv) determining regions of maximum strain of the component when the component vibrates in a second mode at a second temperature; (v) selecting a second viscoelastic material having a loss factor which reaches a maximum at or approximately at the second temperature; (vi) constructing the constrained layer damping arrangement so that the selected second viscoelastic material is disposed on the component at the region of maximum strain in the second mode.

17 A method of damping vibration of a component as claimed in claim 14 and substantially as described herein.