GB2587397A

GB2587397A - Split-Hopkinson pressure bar device

Info

Publication number: GB2587397A
Application number: GB1913938.5A
Authority: GB
Inventors: Alexander Stuart Macdougall Duncan; Reed Julian
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2019-09-27
Filing date: 2019-09-27
Publication date: 2021-03-31
Also published as: GB201913938D0

Abstract

A split-Hopkinson pressure bar device includes first and second loading bars 42, 43 between which is a sample 41. A first loading system 44 such as an axial force system for applying a tension or compression force is attached to the first loading bar 42 by a first clamp 46. A second loading system 45 may be a torsional force system, and is clamped to the second loading bar 43 by a second clamp 47. A clamp release mechanism 48 simultaneously releases the first and second clamps 46, 47 such that first and second waves are applied to the specimen 41. The forces applied by the loading systems 44, 45 may be a static tension, compression or torque. The clamps 46, 47 may include fuses 49, 50 which fracture to release the clamps 46, 47. There may be an impact load transfer frame 52 for transferring an impulse from an impact, e.g. from a dropped missile 55, to the fuses 49, 50.

Description

SPLIT-HOPKINSON PRESSURE BAR DEVICE

Field of the disclosure

The present disclosure relates to a split-Hopkinson pressure bar device, in particular for measuring loading on a test specimen, and a method of measuring loading on a test specimen.

Background

Components of, for example, gas turbine engines undergo extreme mechanical conditions in use, including experiencing both shear stresses and tensile or compressive stresses. To obtain data on the mechanical response of materials under high strain rate deformation, tests can be conducted in uni-axial tension, uni-axial compression and in pure shear (i.e. torsion).

Known arrangements for carrying out tests as discussed above allows testing in uniaxial stress in tension, in compression and in pure shear. However, impact loading in engineering applications typically involves more complex stress states. One such example is the loading present in the target in a ballistic impact (e.g. armour plating, rotor containment casings). In these applications, a complex combination of shear and tensile loading is present in the target.

The present invention aims to at least partly address these problems.

Summary

According to a first aspect, there is provided a split-Hopkinson pressure bar device for measuring loading on a test specimen, the device comprising first and second loading bars, between which the test specimen can be arranged, a first loading system for applying a first load to the first loading bar, a second loading system for applying a second load to the second loading bar, a first clamp located between the first loading system and the test specimen, and configured to hold the first loading bar against the first load, a second clamp located between the second loading system and the test specimen, and configured to hold the second loading bar against the second load, and a clamp release mechanism arranged to release the first and second clamps simultaneously to thereby apply a first wave and a second wave to the test specimen. This may allow the specimen to be tested at high strain rate under combined loading from the first and second loads, and the response to such loading to be better understood.

The first loading system may be an axial force system for applying a static axial force to the first loading bar. The second loading system may be a torsion system for applying a static torque to the second loading bar. The first clamp may be configured to hold the first loading bar in tension or compression. The second clamp may be configured to hold the second loading bar in torsion. The first wave may be an axial wave. The second wave may be a torsion wave. This may allow the specimen to be tested at high strain rate with a combination of axial loading and torsional loading.

The axial force may be a tension force. The axial wave may be a tensile wave.

The axial force may be a compression force. The axial wave may be a compression wave.

The first and second clamps may be secured to hold the respective first and second loading bars by means of first and second fuses for controlling release of the clamps.

The clamp release mechanism may be arranged to interact with the fuses to release the first and second clamps. This may provide improved control over the release times of the clamps, and allow the load to be applied to the specimen quickly (i.e. at high strain rate).

The clamp release mechanism may be arranged to fracture the first and second fuses to release the first and second clamps.

At least one of the first and second fuses may comprise at least one notch to provide a fracture point. This may reduce the force required to fracture the fuses.

The force applied to hold the first and second bars by the first and second clamp may be less than the fracture load of the fuses, and may preferably be less than 85% of the fracture load of the fuses. This may reduce the force required to fracture the fuses.

The clamp release mechanism may comprise an impact load transfer frame arranged to transfer an impulse to the first and second fuses to thereby fracture the fuses.

The impact load transfer frame may comprise at least one protrusion for contacting the first and second fuses.

The first and second protrusions may be for providing a line contact with the first and second fuses.

The first and second protrusions may be for providing at least one point contact with the first and second fuses.

The first and second clamps may be located 80mm -150mm from the respective ends of the first and second loading bars closest to the test specimen.

According to a second aspect, there is provided an apparatus comprising a split-Hopkinson as described above and a missile. The clamp release mechanism may comprise an impact load transfer frame arranged to transfer an impulse received from the missile to the first and second fuses to thereby fracture the fuses The missile may allow reliable fracture of the fuses.

The missile may have substantially the same impedance as the impact load transfer frame.

This may avoid undesired vibrations being transferred to the frame, and to the specimen.

The missile may have a hemispherical end with a radius of greater than 2 metres. This may provide an improved contact between the missile and the transfer frame.

The impact velocity of the missile may be 2 m/s to 20 m/s.

The missile may be a weight dropped under gravity.

The apparatus may further comprise a gas gun. The missile may be propelled from the gas 30 gun.

According to a third aspect, there is provided a method of measuring torsional loading combined with axial loading on a test specimen, the method comprising arranging a specimen between first and second loading bars, applying a first load to the first loading bar, applying a second load to the second loading bar, holding the first loading bar against the first load using a first clamp located between the first load and the test specimen, holding the second loading bar against the second load using a second clamp located between the second load and the test specimen, and releasing the first and second clamps simultaneously to thereby apply a first wave and a second wave to the test specimen. This may allow the specimen to be tested at high strain rate under combined loading from the first and second loads, and the response to such loading to be better understood.

The first load may be a static axial force. The second load may be a static torque. The first loading bar may be held in tension or compression. The second loading bar may be held in torsion. The first wave may be an axial wave. The second wave may be a torsion wave.

The method may further comprise securing the first and second clamps to hold the respective first and second loading bars by means of first and second fuses for controlling release of the clamps, and fracturing the first and second fuses to release the first and second clamps.

The method may further comprise selecting the location of the first and second clamps to thereby control the timing of the arrival of the axial wave and torsion wave at the test specimen. This may allow different loading patterns to be investigated.

The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein.

Brief description of the drawings

Embodiments will now be described by way of example only, with reference to the Figures, in which: Figure 1 is a schematic diagram of a known split-Hopkinson bar apparatus for compression loading; Figure 2 is a schematic diagram of a known split-Hopkinson bar apparatus for tension loading; Figure 3 is a schematic diagram of a known split-Hopkinson bar apparatus for torsion loading; Figure 4 is a schematic diagram of a split-Hopkinson bar apparatus according to the present disclosure; Figure 5 is a close-up schematic diagram of the clamp release mechanism according to the

present disclosure;

Figure 6 is a close-up of an arrangement of fuse and protrusion according to the present disclosure; and Figure 7 is a close-up of another arrangement of fuse and protrusion according to the present disclosure.

Detailed description

Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art Components of, for example, a gas turbine engine, undergo extreme mechanical conditions in use, including experiencing both shear stresses and tensile or compressive stresses. As such, material selection for such components must be considered carefully. Whilst existing material testing systems allow samples to be subjected to pure uniaxial stress or pure shear, they do not allow for the samples to be subjected to both simultaneously. The ability to conduct combined high strain rate testing in tension and torsion (shear) would be advantageous to understanding these phenomena better.

In known arrangements, to obtain data on the mechanical response of materials under high strain rate deformation, tests can be conducted in uni-axial tension, uni-axial compression and in pure shear. A material model can then built which assumes a yield surface (such as Von Mises or Tresca) and failure surface (such as Mohr-Coulomb).

At strain rates between approximately 100 s-1 and 10,000 s-1, a known apparatus for measuring this response is the split-Hopkinson pressure bar. To use this device, a specimen is sandwiched between two slender rods or loading bars, each of which is instrumented with strain gauges. A stress wave (compression, tension or torsion) is introduced into one of the rods (sometimes referred to as the input rod or input bar), which transmits it to the specimen.

This causes the specimen to deform. The change in impedance between the rods and specimen causes some of the wave to be reflected, and the rest to be transmitted into the second rod (also referred to as the output rod or output bar). By measuring these three stress waves, it is possible to calculate the force and displacement at the two rod-specimen interfaces, and thus infer the stress-strain response of the specimen.

Figures 1, 2 and 3 show schematic diagrams of known split-Hopkinson bar apparatuses for compression, tension and torsion loading.

In known compression systems, as shown in Figure 1, the stress-wave can be introduced by means of a striker bar 155, impacted into the input bar 151 by, for example, a gas gun.

In known tension systems, either energy storage (e.g. by holding an input bar in tension) or impact devices can be used to create the stress wave. Figure 2 shows an example of an impact system in which a projectile 155 acts a striker against an anvil 158 at the end of a loading bar 151a.

In known torsion systems, as shown in Figure 3, the stress-wave can be introduced by the rapid release of torque stored in the input bar 151, remote from the specimen 153. The torque can be introduced by a torque pump 157, for example, and stored by means of clamp 156 holding the input bar 151 until the torque is to be released. Impact devices can also be used to provide the torque.

Figure 4 illustrates a split-Hopkinson bar arrangement according to the present disclosure that enables a high strain rate application of combined axial loading or torsional loading on a test specimen 41.

The split-Hopkinson device according to the present disclosure includes a first loading bar 42 and a second loading bar 43. In use, a test specimen 41 is arranged between the first loading bar 42 and the second loading bar 43, such that it is secured between one end of the first loading bar 42 and one end of the second loading bar 43. The test specimen 41 may be of any suitable form, such as, for example, a thin walled tube or a dumbbell shape. The test specimen 41 may be secured to the first and second loading bars by any suitable means.

A first loading system, which may be an axial force system 44, is provided for applying a static axial force to the first loading bar 42. The axial force may be a tension force or a compression force. The axial force system 44 may be arranged to apply the axial force at one end of the first loading bar 42, which may be the end opposite to where the test specimen 41 is secured.

A second loading system, which may be a torsional force system 45, is provided, for applying a static torque (i.e. a torsional force) to the second loading bar 43. The torsional force system 44 may be arranged to apply the torsional force at one end of the second loading bar 43, which may be the end opposite to where the test specimen 41 is secured.

In the arrangements described below, the first loading system is an axial force system 44, and the second loading system is a torsional force system. That is, the system is for measuring torsional loading combined with axial loading. However, it will be appreciated that arrangements are possible in which the first and second loading systems are both axial force systems (i.e. arranged to apply a static axial force to the first and second respective loading bars), or are both torsional force systems (i.e. arranged to apply a static torque to the first and second respective loading bars). In arrangements in which the first and second loading systems are both axial force systems, one loading system may provide a tension force and the other may provide a compression force, or both loading systems may provide the same type of force (i.e. tension or compression). In arrangements in which the first and second loading systems are both torsional force systems, the directions of the torsional forces may be the same or opposite to each other.

A first clamp 46 is provided and secured to the first loading bar 42, at a location between the axial force system 44 and the test specimen 41. The clamping force applied by the first clamp 46 is such that, when the axial force system 44 applies an axial force to the first loading bar, the axial force is not transferred to the test specimen 41. In other words, the clamping force of the first clamp 46 is sufficient to hold the first loading bar 42 against the axial force applied by the axial force system 44.

Likewise, a second clamp 47 is provided and secured to the second loading bar 43, at a location between the torsional force system 45 and the test specimen 41. The clamping force applied by the second clamp 47 is such that, when the torsional force system 45 applies a torque to the second loading bar, the torque force is not transferred to the test specimen 41. In other words, the clamping force of the second clamp 47 is sufficient to hold the second loading bar 43 against the torsional force applied by the torsional force system 45.

The first loading bar 42 and the second loading bar 43 can be constructed, for example, from a titanium alloy, high strength steel or phosphor bronze. However, other materials (such as ceramics) may be used depending on the specimen type and temperature of the specimen.

As shown in Figure 4, there is also provided a clamp release mechanism 48, which is arranged to release the first and second clamps simultaneously. When the first clamp is released, an axial wave travels along the first loading bar 42, and is applied to the test specimen 41. Likewise, when the second clamp 47 is released, a torsional wave travels along the second loading bar 43, and is applied to the test specimen 41. Such an arrangement may allow a combination of axial (i.e. compression or tension) loading and torsional loading to be applied to the specimen at very high strain rates. Thus, the response to such combined loading of the specimen can be investigated.

It will be understood that, although the clamp release mechanism 48 is arranged to release the first and second simultaneously, the respective axial and torsional waves may not necessarily arrive at the specimen 41 at the same time. The distance between the clamps and the specimen can be chosen so as to control the timing of arrival of the two waves at the specimen.

For example, the location of the clamps along the bars may be chosen such that the torsional wave and axial wave arrive at the specimen at the same time, or may be chosen such that the waves arrive at the specimen at different times. To coordinate the relative timings of the arrival of the axial wave at the specimen 41 with the arrival of the torsional wave (whether that is so the waves arrive substantially simultaneously, or at a predetermined temporal spacing), it should be appreciated that torsional waves may travel more slowly than axial waves (circa 3km/s versus 5km/s respectively in titanium).

Based on the desired timing of arrival of the waves at the specimen and the speed of travel of the waves through the loading bars, the distances of the clamps from the specimen can be chosen accordingly. In some arrangements, the clamps may be located 50mm-200mm from the closest end of the bar to the test specimen (i.e. the end at which the test specimen is secured), and preferably 80mm -150mm from the closest end of the bar to the test specimen.

Also shown in Figures 4 and 5 are a first fuse 49 and a second fuse 50. The fuses 49, 50 may be, for example frangible or breakable members, such as bolts. It will also be understood that a fuse is not limited to being a breakable member or other breakable component, and may be, for example, a releasable arrangement using a removable pin.

In one arrangement, one or both of the fuses may include one or more notches 51, which provide a fracture point (i.e. a weakened point which allows the fuse to be broken using less force). The notch may extend around all or part of the fuse.

In use, the first clamp 46 is secured by means of a first fuse 49, and the second clamp 47 is secured by means of a second fuse 50. That is, when the clamps hold the bars, they are secured, or held shut, by a fuse. Thus, when the clamps are used to apply a clamping force to the bars, the clamping force is carried by the fuses.

The clamp release mechanism 48 is arranged to interact with the respective first and second fuses 49, 50, to release the respective first and second clamps, 46, 47.

In one arrangement, the first and second fuses 49, 50 may be arranged such that they are fractured, or broken, by the clamp release mechanism 48 in order to release their respective clamps. When the fuses include one or more notches, the clamp release mechanism may interact with a notch on the fuse in order to break the fuse.

As shown in Figure 5, a clamping load 57 may be applied to the clamps 46, 47 in order to secure the clamp to the first and second loading bars 42, 44. Such a clamping load may be applied by, for example, a hydraulic ram. This, in conjunction with the fuse 49, 50 may secure the clamping of the loading bars by the clamps by providing a clamping force. The clamping force (or load), and thus the force applied to the bars by the clamps, is less than the fracture load of the fuses, so that the fuse can withstand the clamping load without breaking. Preferably, the clamping load on the clamp is less than 85% of the fracture load of the fuses, and more preferably approximately 80% of the fracture load of the fuse. This may reduce the force needed to break or fracture the fuse, because a large proportion of the fracture load is already applied to the fuse before the clamp release mechanism 48 interacts with the fuse to break it.

As shown in Figure 5, the clamp release mechanism 48 may include an impact load transfer frame 52 arranged to transfer the impulse caused by an impact to the first and second fuses, thereby fracturing or breaking the fuses. That is, an external impact load is provided, and the impact load transfer frame 52 applies this load to the fuses.

The impact load transfer frame 52 may be designed such that it has a high stiffness, and thus a high natural frequency. This may avoid undesired vibrations at the parts of the impact transfer frame which contact the fuses.

The impact load transfer frame 52 may include at least one protrusion for contacting the first and second fuses to break the fuses. In the arrangement shown in Figure 4, first and second protrusions 53,54 are provided for contacting respective first and second fuses 49,50. That is, the first protrusion 53 interacts with the first fuse 49 and the second protrusion 54 interacts with the second fuse 50. In arrangements where the fuses comprise a notch 51, the protrusions may be arranged to contact the notch to provide improved breakage of the fuse. However, it will be understood that other arrangements are possible, such as a single elongate protrusion arranged to contact both the first and second fuses 49,50.

In some arrangements, the protrusions may be arranged such that, when they are in contact with the fuses, they have one or more point contacts with the fuse. In arrangements where the fuse includes a notch, the point contact may be located at or adjacent the notch. As shown in Figure 6, the angles of the sides of the protrusion may be chosen so as to provide two point contacts with the fuse. In an alternative arrangement (not shown), the angles of the sides of the protrusion may be chosen to provide a single point contact with the fuse.

Alternatively, as shown in Figure 7, the protrusions may have a line (or edge) contact with the fuse, and, where present, with the notch located on the fuse. The angles of the sides of the protrusion may be chosen to match the angles of the sides of the notch This may provide a larger line contact area.

In the arrangements shown in Figures 6 and 7, the protrusion has a pointed end. However, the protrusion may also have a rounded end, or a blunt end.

As shown in Figures 4 and 5, a missile 55 may be used to apply an impulse to the impact load transfer frame 52. The missile may be, for example, a weight dropped under gravity, or may be propelled from a gas gun. The impact velocity of the missile 55 can be chosen in accordance with the required impulse, but is typically in the range of 2 m/s -20 m/s. It will be understood that the missile 55 may be considered to be part of the pressure bar device (and may be considered as part of the clamp release mechanism), or may be considered as a separate element.

The missile 55 may be designed such that the characteristics of the impact between the missile and the impact load transfer frame are optimised. The missile may have a hemispherical end with a radius of greater than 2 metres, and preferably greater than 3 metres. The missile may also be designed so that it has substantially the same impedance as the impact transfer frame.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

CLAIMS1 A split-Hopkinson pressure bar device for measuring loading on a test specimen (41), the device comprising: first and second (42, 43) loading bars, between which the test specimen (41) can be arranged; a first loading system (44) for applying a first load to the first loading bar; a second loading system (45) for applying a second load to the second loading bar; a first clamp (46) located between the first loading system and the test specimen, and configured to hold the first loading bar against the first load; a second clamp (47) located between the second loading system and the test specimen, and configured to hold the second loading bar against the second load; and a clamp release mechanism (48) arranged to release the first and second clamps simultaneously to thereby apply a first wave and a second wave to the test specimen.
2. The split-Hopkinson pressure bar device according to claim 1, wherein: the first loading system is an axial force system (44) for applying a static axial force to the first loading bar; the second loading system is a torsion system (45) for applying a static torque to the second loading bar; the first clamp (46) is configured to hold the first loading bar in tension or compression; the second clamp (47) is configured to hold the second loading bar in torsion; the first wave is an axial wave; and the second wave is a torsion wave.
3. The split-Hopkinson pressure bar device according to claim 2, wherein the axial force is a tension force, and the axial wave is a tensile wave.
4. The split-Hopkinson pressure bar device according to claim 2, wherein the axial force is a compression force, and the axial wave is a compression wave.
5. The split-Hopkinson pressure bar device according to any preceding claim, wherein: the first and second clamps (46, 47) are secured to hold the respective first and second loading bars by means of first and second fuses (49, 50) for controlling release of the clamps; and the clamp release mechanism is arranged to interact with the fuses to release the first and second clamps.
6 The split-Hopkinson pressure bar device according to claim 5, wherein the clamp release mechanism is arranged to fracture the first and second fuses (49, 50) to release the first and second clamps.
7. The split-Hopkinson pressure bar device according to claim 5 or 6, wherein at least one of the first and second fuses comprises at least one notch (51) to provide a fracture point.
8 The split-Hopkinson pressure bar device according to claim 6 or 7, wherein the force applied to hold the first and second bars by the first and second clamp is less than the fracture load of the fuses, preferably less than 85% of the fracture load of the fuses.
9 The split-Hopkinson pressure bar device according to any one of claims 6-8, wherein the clamp release mechanism comprises an impact load transfer frame (52) arranged to transfer an impulse to the first and second fuses to thereby fracture the fuses.
10. The split-Hopkinson pressure bar device according to claim 9, wherein the impact load transfer frame (52) comprises at least one protrusion (53, 54) for contacting the first and second fuses.
11. The split-Hopkinson pressure bar device according to claim 10, wherein the first and second protrusions (53, 54) are for providing a line contact with the first and second fuses.
12. The split-Hopkinson pressure bar device according to claim 10, wherein the first and second protrusions are for providing at least one point contact with the first and second fuses.
13. The split-Hopkinson pressure bar device according to any preceding claim, wherein the first and second clamps are located 80mm -150mm from the respective ends of the first and second loading bars closest to the test specimen.
14 An apparatus comprising the split-Hopkinson pressure bar device according to any preceding claim and a missile (55); wherein the clamp release mechanism comprises an impact load transfer frame arranged to transfer an impulse received from the missile to the first and second fuses to thereby fracture the fuses.
15. The apparatus according to claim 14, wherein the missile has substantially the same impedance as the impact load transfer frame.
16. The apparatus according to claim 14 or 15, wherein the missile has a hemispherical end with a radius of greater than 2 metres.
17. The apparatus according to any one of claims 14-16, wherein the impact velocity of the missile is 2 m/s to 20 m/s.
18. The apparatus according to any one of claims 14-17, wherein the missile is a weight dropped under gravity.
19. The apparatus according to any one of claims 14-18, further comprising a gas gun, wherein the missile is propelled from the gas gun.
A method of measuring torsional loading combined with axial loading on a test specimen, the method comprising: arranging a specimen between first and second loading bars; applying a first load to the first loading bar; applying a second load to the second loading bar holding the first loading bar against the first load using a first clamp located between the first load and the test specimen; holding the second loading bar against the second load using a second clamp located between the second load and the test specimen; and releasing the first and second clamps simultaneously to thereby apply a first wave and a second wave to the test specimen.
21. The method according to claim 20, wherein the first load is a static axial force, the second load is a static torque, the first loading bar is held in tension or compression, the second loading bar is held in torsion, the first wave is an axial wave and the second wave is a torsion wave.
22. The method according to claim 20 or 21, further comprising: securing the first and second clamps to hold the respective first and second loading bars by means of first and second fuses (49, 50) for controlling release of the clamps; and fracturing the first and second fuses (49, 50) to release the first and second clamps.
23 The method according to claim 20, 21 or 22, further comprising selecting the location of the first and second clamps to thereby control the timing of the arrival of the axial wave and torsion wave at the test specimen.