GB2472193A

GB2472193A - Rotor blade test specimen

Info

Publication number: GB2472193A
Application number: GB0912967A
Authority: GB
Inventors: Paul James Mason
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-07-27
Filing date: 2009-07-27
Publication date: 2011-02-02
Also published as: GB0912967D0

Abstract

A test specimen 50 for representing a blade component in multiple strength tests comprises a longitudinal axis x and a generally elongate shank portion 52 and first and second end portions 54, 56 at opposed ends of the shank. The first end portion is wider than the shank for engagement with a retaining portion of a test rig and the second end portion is wider than the shank for engagement with a clamp of a test rig, and wherein the second end portion 56 is stronger than the first end portion 54 such that when the specimen is tested to failure it will fail at a location closer to the first end portion than to the second end portion. The specimen may have dovetail or fir-tree roots and is used in testing turbine rotor blades.

Description

TEST SPECIMEN

The present invention relates to the testing of materials, and is concerned particularly with a specimen for use in test.

Gas turbine engine fan blades, compressor blades and turbine blades are subjected in use to a combination of low-cycle fatigue (LCF) and high-cycle fatigue (HCF) stresses during the engine's operation. These LCF and HCF stresses can have a detrimental effect on the integrity of the blades. The LCF stresses result largely from the centripetal forces experienced by the blades as they rotate about the engine axis. The HCF stresses result from aerodynamic and other vibration excitation of the blades during use.

Dovetail roots are commonly used to attach aero-engine rotor-blades to a rotor disc. They have the advantages of simplicity of manufacture, ease of assembly and high load carrying capability.

In order to design fan blades, compressor blades and turbine blades that are resistant to fatigue, a good understanding is required of the combination of the steady and alternating stresses to which a blade may be subjected in operation.

To enable the testing of a composite dovetail root design for a fan blade, a specimen which is representative of the blade at a particular chord-wise location is used.

Both quasi-static testing, i.e. the application of a continuous load which steadily increases until failure occurs, and fatigue testing, i.e. cycling between loads, of specimens is typically carried out using a simple dovetail-and-shank specimen in which the specimen is restrained by the dovetail root in a disc feature which is representative of the fan disc, whilst load is applied by clamping the shank region over a large enough area to transfer the load to the specimen by shear.

During operation of an aero engine, the rotor-blade assembly is subject to a complex loading system comprising centripetal load, gas load and vibration. Rotation of the fan assembly results in a large load on the dovetail root, due to centripetal acceleration as the blade tries to pull out of the retention slot. Hoop stresses are generated in the disc rim as the disc grows under the influence of its own mass and that of the attached blades. In addition, as the fan blade compresses the incoming air the pressure differential across the blade causes the aerofoil to bend, imparting an additional bending load into the dovetail root. The dovetail root experiences a stress concentration in the region where it is held by the rotor disc. The exact location of this so-called "edge-of-bedding" stress can be altered by the effect of the centripetal force acting so as to pull the blade out from the disc, thereby dilating the disc and causing the blade to move outwards. This is known as "contact slip".

Additionally blade mechanical resonances and aerodynamic forcing impose a vibratory load to the dovetail root.

These processes in combination with the applied loads can lead to the initiation of fatigue cracking at the dovetail edge of contact. The underlying component stress field due to centripetal load, gas bending and vibration may then propagate these cracks resulting in blade or disk failure.

When testing, the intention is to replicate the contact slip and edge-of-bedding stresses seen in fan blade dovetails.

As well as testing cyclic fatigue and ultimate strength separately, there is a need to be able to perform ultimate strength testing on specimens that have already been subjected to fatigue tests, in order to determine the effect of the fatigue test on the residual strength of the specimen.

A number of different test rigs can be used for testing specimens of this kind. One previously considered test rig for testing titanium, dovetail-root fan blade specimens is described in our US Patent Number 7 204 152 and depicted in Fig 1 of the accompanying drawings.

Referring to Fig 1, a test machine shown generally at has a load frame 12. Pillars 14 support a first hydraulic actuator 16, a first load cell 18 and a pair of grips 20. Further structure 22 supports second hydraulic actuators 24, second load cells 26 and a pair of loading bars 28. Each loading bar 28 has a hole 30 near its end, into which a pin (not shown) can be fitted.

A fixture 38 is securely held between the grips 20 of the testing machine 10. The fixture 38 has two recesses 39 or retaining portions, for retaining the roots of test specimens, which match the profile of the disc slots whose behaviour is to be reproduced.

Two titanium test specimens 36 each have, at one end, a mounting feature 37 matching the root profile of the blade whose behaviour is to be reproduced and, at the other end, a suitable hole permitting direct pin-attachment of the test specimens 36 to the loading bars 28 in a conventional manner.

In operation, the first actuator 16 applies a load to the fixture 38. The applied load is measured by the first load cell 18. This loading represents the hoop stress in the disc, in the real component. The second actuators 24 apply loads to the test specimens 36. These loads are measured by the second load cells 26. These loads represent the centripetal loading of the rotor-blades in the real component.

The second actuators 24 are mounted on a floating carriage supported on springs 32 (only one of which is shown) . These support the weight of the carriage, while permitting limited vertical movement. This permits the second actuators 24 to apply their loads without introducing undesired bending, even though the fixture 38 will move up and down slightly during the test as the load applied by the first actuator 16 decreases and increases.

The actuators 16 and 24 are controlled by a computer 48 which receives information from the load cells 18 and 26 via controllers 46 and sends controlling signals 50 to the actuators.

Attached to each test specimen 36 is an HCF shaker 40.

Each HCF shaker 40 is attached to, but vibrationally isolated from, the load frame 12 by a spring 42. The load applied by the HCF shaker 40 represents the mechanical and aerodynamic vibrations in the blade in the real component.

Figure 2 shows in side and top views a previously considered composite test specimen 236 for use in quasi-static or fatigue testing. The specimen 236 comprises a dovetail root end 236a and a shank, or clamp end 236b. In use, the dovetail root end 236a is mounted in a recess, similar to the recess 39 shown in Figure 1. Because pin loading of composite test specimens is generally not practicable (because their low through-thickness strength tends to cause debonding and failure under such loading) the shank 236b of the test specimen is clamped so that a load can be applied in the direction B (as by the actuator 24 in Figure 1) . Because this load must be transferred to the specimen by shear, the shank 236b must be clamped over a relatively large area 236c, shown shaded in Figure 2.

However, the testing of a composite test specimen 236 on a rig such as is described above presents a number of problems. Firstly, due to the configuration of the test rig, and in particular the limited space allowed to accommodate the test specimens, a limitation is placed upon the length of the shank of the specimen and therefore on the shear area which is being tested. There is also a limited capability for applying load in the transverse direction A (as by the HCF shakers 40 in Figure 1), which prevents the use of heavy clamps to apply the load to the shank. The loading can prove difficult due to the large area 236c needed for clamping conflicting with the desired location to apply the load. Furthermore, the composite specimen will be subject to excessive wear at the location where it is clamped by the shaker unit 40 of the rig.

Embodiments of the present invention aim to provide a test specimen which at least partly addresses some of the aforementioned problems.

The present invention is defined in the attached independent claim to which reference should now be made.

Further, preferred features may be found in the sub-claims appended thereto.

According to one aspect of the invention there is provided a test specimen for representing a component in multiple strength tests, the specimen having a longitudinal axis and comprising a generally elongate shank portion and first and second end portions at opposed ends of the shank, the first end portion being wider than the shank for engagement with a retaining portion of a test rig and the second end portion being wider than the shank for engagement with a clamp of a test rig, and wherein the second end portion is stronger than the first end portion such that when the specimen is tested to failure it will fail at a location closer to the first end portion than to the second end portion.

The first and second end portions are preferably of generally dovetail shape, each having flank surfaces which diverge from the longitudinal axis.

Preferably, in the second end portion the maximum angle of divergence of the flank surface from the longitudinal axis is less than that that of the flank surface of the first end portion.

In a preferred arrangement, at least the second end portion has a pair of substantially parallel surfaces of generally dovetail shape. The first end portion may also have a pair of substantially parallel surfaces of generally dovetail shape, and the substantially parallel surfaces of the second end portion may have a greater surface area than those of the first end portion.

The parallel surfaces of the second end portion may be arranged in use to receive bonded load-application plates for clamping to a test rig.

Preferably the shank has opposed surfaces each of which has a stress distribution pad mounted thereon, which pads are of a material that is less stiff than that of the shank.

The stress distribution pads may be bonded to upper and lower surfaces of the shank and may be arranged to distribute stress applied to the specimen by a test rig actuator in use.

In a preferred arrangement the stress distribution pads are made of glass material.

The shank and end portions may be formed integrally of a composite material.

Preferably the specimen is suitable for use in multiple strength tests from a group consisting of: high cycle fatigue test (HCF), low cycle fatigue test (LCF), tensile strength test, tensile fatigue test, transverse fatigue test and residual strength test.

A preferred embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows schematically a previously considered test rig for use in testing specimens; Figure 2 shows respectively side and top views of a previously considered test specimen; Figure 3 shows a test specimen according to a preferred embodiment of the present invention; and Figure 4 shows the specimen of Figure 3 with load-application plates fixed thereto.

Turning to Figure 3, this shows generally at 50 a specimen for use in fatigue testing of dovetail root designs for fan blades, typically for use in aero-engines.

The specimen comprises a generally elongate shank 52 and first and second end portions 54 and 56. The generally cuboid or plank-shaped shank 52 is formed integrally with the end portions 54 and 56, which are of a dovetail, or broadly trapezoidal prism shape with, in the example shown, upper and lower corners removed. Upper and lower flank surfaces 54a and 54b of end portion 54 and 56a and 56b of end portion 56 are in fact not planar, but rather curve divergently away from the major -i.e. longitudinal axis X-X' of the specimen.

The first end portion, 54 (the so called feature end) is shaped so as to fit within a retaining portion in the form of a complementary shaped slot in a testing machine, such as is described above with reference to Figure 1, which slot is designed to replicate the rotor-disc of the engine. The second end portion, 56 (known as a clamp end) is designed to create a large surface area for bonding of the loading plates (60 in Figure 4), as will be explained presently. The specimen 50 is also suitable for testing in an alternative test rig (not shown) in which both end portions are retained in dovetail shaped recesses and the specimen is pulled apart until it fails, in an ultimate strength test.

The second end portion 56 has been designed to be stronger, in this embodiment of the order of twenty per cent stronger, than the feature end portion 54, in order to ensure that in an ultimate strength failure test the sample will fail at the desired location -i.e. at or towards the feature end portion 54. This has been achieved by reducing the angle between the longitudinal axis X-X' of the specimen and the upper and lower contact flank surfaces 56a and 56b as compared with the angle between the corresponding flank surfaces 54a and 54b of the feature end 54 and the longitudinal axis X-X' . In addition the flank lengths, and therefore areas, have been increased. The effect of this is to reduce the edge of bedding peak stress which occurs at the end of the contact region between the rotor-disc and the clamped dovetail end 56 of the specimen, and also reduce the compressive stress over the contact area.

Glass pads 58, which are less stiff than the composite material of the rest of the specimen, are bonded to upper and lower surfaces of the shank 52 with the purpose of redistributing the stress imparted by a test actuator, such as a shaker unit (40 in Figure 1), and lessening the resultant wear caused by the application of the transverse fully-reversed loading. A low friction coating (not shown) can be applied to the surface of the glass pads to further limit wear.

Ensuring that the failure occurs at a desired location is essential in order for the efficient use of test recording equipment such as high speed and thermal cameras which are conventionally positioned to record the failure of the specimen.

The intention is that the specimen is suitable for use in multiple test conditions including tensile tests to failure, LCF, HCF and combined LCF and HCF (bi-axial) Turning to Figure 4, this shows the specimen of Figure 3 together with bonded titanium load application plates 60 which are arranged to carry an applied load. The titanium plates each have a hole for receiving a pin (not shown) to attach the specimen to load bars such as are shown at 28 in the example rig of Figure 1. This allows the composite specimen to be clamped in a way that would damage the specimen if it was directly attached. The double ended or "dog-bone" shape of the specimen 50 provides for a large contact area at the clamp (load) end which allows the specimen and clamp mass to be kept to an acceptable level and thus allows the application of the transverse and fully reversed loading at the desired high frequencies. In this embodiment, the length and area of the second end portion 56 are larger than the length and area of the first end portion 54, in order to reduce the compressive stresses arising from the clamping.

Advantages of this design include a compact size which permits use with existing test rigs such as the example described in relation to Figure 1, and also to other kinds of test rigs in which a dovetail clamping or similar is used at both ends of the specimen. Fully reversed transverse loads can be applied to the specimen whilst providing a resistance to de-lamination and wear caused by transverse loading, due to the glass pads, and guaranteeing failure in the correct location, due to the clamp end flank angle and the size of the contact area. Furthermore, a low combined specimen and clamp mass allows the application of fully reversed transverse loading at high frequencies without the need for large actuators, due to the bonded plates. The suitability of the specimen for multiple tests removes the need for a plurality of alternative specimens which would require additional moulds and ply lay-up definitions.

Since the specimen can be used in different kinds of rigs it is possible to perform multiple tests on the same specimen. For example, a cyclical fatigue test can be performed first, followed by a residual strength test. The results of this can then be compared with those for a specimen which has undergone only an ultimate strength test, in order to study the effects of cyclical fatigue on residual component strength.

Whereas the above described embodiment is of dovetail root design, other designs, such as fir-tree root designs could also be used without departing from the scope of the invention as defined in the claims.

Furthermore, specimens according to the invention can be used in the testing of rotor blades for applications other than gas turbine engines.

Claims

CLAIMS1. A test specimen (50) for representing a component in multiple strength tests, the specimen having a longitudinal axis and comprising a generally elongate shank portion (52) and first and second end portions (54, 56) at opposed ends of the shank, the first end portion being wider than the shank for engagement with a retaining portion of a test rig and the second end portion being wider than the shank for engagement with a clamp of a test rig, and wherein the second end portion (56) is stronger than the first end portion (54) such that when the specimen is tested to failure it will fail at a location closer to the first end portion than to the second end portion.
2. A test specimen according to Claim 1, wherein the first and second end portions are of generally dovetail shape, each having flank surfaces (54a, 54b, 56a, 56b) which diverge from the longitudinal axis.
3. A test specimen according to Claim 2, wherein in the second end portion the maximum angle of divergence of the flank surface from the longitudinal axis is less than that that of the flank surface of the first end portion.
4. A test specimen according to Claim 2 or Claim 3, wherein each of the end portions has a pair of substantially parallel surfaces of generally dovetail shape and the substantially parallel surfaces of the second end portion have a greater surface area than those of the first end portion.
5. A test specimen according to Claim 4, wherein the parallel surfaces of the second end portion are arranged in use to receive bonded load-application plates (60) for clamping to a test rig.
6. A test specimen according to any of Claims 1 to 4, wherein the shank has opposed surfaces each of which has a stress distribution pad (58) mounted thereon, which pads are of a material that is less stiff than that of the shank.
7. A test specimen according to Claim 6, wherein the stress distribution pads (58) are bonded to upper and lower surfaces of the shank and are arranged to distribute stress applied to the specimen by a test rig actuator in use.
8. A specimen according to any of Claims 5-7, wherein the stress distribution pads (58) are made of glass material.
9. A test specimen according to any of Claims 1 to 8, wherein the shank and end portions are formed integrally of a composite material.
10. A test specimen according to any of Claims 1 to 9 wherein the specimen is suitable for use in multiple strength tests from a group consisting of: high cycle fatigue test (HCF), low cycle fatigue test (LCF), tensile strength test, tensile fatigue test, transverse fatigue test and residual strength test.