GB2324876A

GB2324876A - Bending apparatus

Info

Publication number: GB2324876A
Application number: GB9708709A
Authority: GB
Inventors: Gert Roebben; Marc Steen; Endre Nagy; Victor Harrison
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1997-04-29
Filing date: 1997-04-29
Publication date: 1998-11-04
Also published as: NO314522B1; EP0979397A1; GB9708709D0; WO1998049534A1; NO994897L; JP2001522459A; CA2286799A1; NO994897D0

Abstract

The present invention relates to an apparatus for the mechanical testing of materials and, in particular, to a fully reversed multi-point bending jig suitable for the mechanical testing of stiff and brittle engineering materials. The apparatus comprises:- (i) at least four upper loading pins 11a,10a etc. wherein each upper loading pin opposes a corresponding lower loading pin 10b,11b etc. resulting in at least two pairs of inner loading pins and at least two pairs of outer loading pins for clamping a test specimen 5; (ii) an external frame 50 fixed relative to the outer loading pins; (iii) an internal frame 40 fixed relative to the inner loading pins; and (iv) an articulated load train (note fig.2) for transmitting a loading force to the loading pins, the load train comprising an inner array linking the inner loading pins to the internal frame and an outer array linking the outer loading pins to the external frame.

Description

Bendinq Apparatus The present invention relates to an apparatus for the mechanical testing of materials and, in particular, to a fully reversed multi-point bending jig suitable for the mechanical testing of stiff and brittle engineering materials.

Mechanical testing is essential for obtaining information regarding a material's physical characteristics, more specifically how a particular material responds to a particular stress distribution.

Mechanical tests at elevated temperatures are valuable in the assessment and development of components for high temperature applications, such as turbine blades and automobile engine parts. It is known that the behaviour of engineering materials at elevated temperatures is characterised by a severely timedependent plasticity.

A common mechanical test is the uniaxial test in which the stress distribution remains homogeneous in sections perpendicular to the loading axis. This enables a direct correlation between applied parameters and the material's response. For stiff and brittle engineering materials, such as ceramics, fibre reinforced ceramics, intermetallic alloys and glasses, a special highly aligned clamping system and relatively large accurately machined specimens are required to conduct effective and reliable tests.

This is because the uniaxial load needs to be applied to the specimen without introducing secondary parasitic bending stresses which adversely affect the homogeneity of the stress distribution. The cost of conducting uniaxial tests on stiff and brittle materials is accordingly very expensive.

An alternative to uniaxial testing is to conduct the test in a bending configuration, for example fourpoint bending, and most tests on ceramics are performed by this method. Although favoured from the point of experimental simplicity, the conventional bending test suffers in that the known initial stress distribution changes in an indeterminate way as soon as non-elastic strain accumulates. This is because flexural loading results in a part of the specimen experiencing tensile stresses, which is compensated by compressive stresses in the remaining part of the specimen. In most cases, a material's response to tension and compression is different and the stress distribution consequently changes in an unpredictable manner. For stiff and brittle materials this is especially the case at high temperatures. For constant or monotonic loading, the initial linear stress distribution over the thickness of the specimen therefore changes with time, and the plane in which no normal stress is experienced (the neutral plane) shifts away from its initial position in the centre of the specimen towards the surface under compression, where deformation and crack growth are slower. This migration of the neutral plane cannot be monitored during the test and precludes the establishment of a relationship between the applied load and the mechanical response of the specimen.

Under static or monotonic loading the uniaxial technique is the only valid alternative which can overcome the disadvantages of the conventional bending test. However, under reversed fatigue loading conditions, another method can be used, since when complete load reversal is imposed during cyclic fatigue (load ratio R = -1), the difference between the material's behaviour under tension and compression is suppressed. Such periodically reversed loading consequently leads to a stationary position of the neutral plane and a stress distribution which does nol change from cycle to cycle. The common four-point bending apparatus does not allow reversal of the applied load and the specimen can only be bent in one direction resulting in load ratios between 0 (loading - unloading - reloading, etc.) and 1 (constant load).

This range can, however, be extended by using a fully reversed cyclic four-point bending apparatus (A.-P.

Nikkilä, T.A. Mäntylä, Cyclic fatigue of silicon nitrides, Ceram. Eng. Sci. Proc., 10, pp. 646-656 (1989)). This apparatus contains four additional loading pins situated opposite the original four loading pins and enables a fully reversed load to be applied to the specimen by alternating the loading force between the two sets of four pins. However, thE steel construction and clamping system are fit for use only at ambient temperatures.

We have now developed a bending apparatus which addresses many of the problems associated with the prior art. Accordingly, the present invention provides a bending apparatus for the mechanical testing of materials, the apparatus comprising: (i) at least four upper loading pins, wherein each upper loading pin opposes a corresponding lower loading pin resulting in at least two pairs of inner loading pins and at least two pairs of outer loading pins for clamping a test specimen; (ii) an external frame fixed relative to the outer loading pins; (iii) an internal frame fixed relative to the inner loading pins; and (iv) an articulated load train for transmitting a loading force to the loading pins, the load train comprising an inner array linking the inner loading pins to the internal frame and an outer array linking the outer loading pins to the external frame.

In use, the bending apparatus according to the present invention is connected to a mechanical testing machine which has means for applying a reversible loading force to one of the said frames, which frame is moveable relative to the other frame in a direction substantially parallel to the direction of the loading force. The mechanical testing machine also has means for measuring the applied loading force.

Articulation of the load train facilitates alignment of the specimen in use and ensures axial loading.

For mechanical testing at elevated temperatures, the apparatus further comprises heating means in which a test specimen is heated in a heating zone, for example an electric furnace, an inductive furnace or a bulb furnace. The temperature to which the specimen may be heated is primarily determined by the thermal stability of the specimen and components of the apparatus, but up to about 15000 is typically possible. In this case, a thermal barrier is advantageously provided to facilitate isolation of the (heated) specimen and adjacent components from other parts of the apparatus. The thermal barrier preferably comprises one or more ZrO2 rings.

The inner array and/or the outer array preferably comprise(s) an upper section above the upper loading pins and a lower section below the lower loading pins, each section comprising two or more articulated parts.

Articulation of the two or more parts of each section is preferably effected by the provision of a ball joint or a roller joint between each of the said parts.

In a preferred embodiment of the present invention, each of the upper and lower sections of the inner array comprises a first generally cylindrical part articulated at a ball joint with a second generally cylindrical part, the first part being adjacent to the inner loading pins and the second part being adjacent to the inner frame. In this case, when the bending apparatus is connected to a mechanical testing machine, both the first and second cylindrical parts have a major axis which is substantially parallel to and coincident with the direction of the loading force.

Each of the upper and lower sections of the outer array preferably comprises a first tubular part, which surrounds the first generally cylindrical part, articulated at a roller joint with a second tubular part, which surrounds the second generally cylindrical part, the first tubular part being adjacent to the outer loading pins and the second tubular part being adjacent to the outer frame. Again, when the bending apparatus is connected to a mechanical testing machine, both the first and second tubular parts have a major axis which is substantially parallel to and coincident with the direction of the loading force.

The roller joint in the upper section of the outer array preferably comprises first and second rollers disposed between the first and second tubular parts, the rollers having a common axis of rotation which is substantially perpendicular to the major axes of the first and second tubular parts and being separated by a distance substantially equal to the diameter of the first and second tubular parts. The roller joint in the lower section of the outer array preferably comprises third and fourth rollers disposed between the first and second tubular parts, the rollers having a common axis of rotation substantially perpendicular to the said major axes and also the axis of rotation of the first and second rollers and being separated by a distance substantially equal to the diameter of the first and second tubular parts.

Accordingly, the outer array can articulate in two perpendicular directions and the provision of the roller joints and ball joints act as a passive alignment device to ensure axial loading of the test specimen.

In use, it will be appreciated that the axes of rotation of the first and second rollers and third and four rollers will be substantially perpendicular to the direction of the loading force.

For high temperature testing, the loading pins and the inner and outer arrays (including ball and roller joints) are formed from a ceramic material, such as SiC, Al203, Si3N4 or a mixture of two or more thereof. In a preferred embodiment, the loading pins, the inner and outer arrays and the roller joint are formed from SiC and the ball joint is formed from Si3N4.

As stated above, the bending apparatus can be connected to a testing machine having means for applying a reversible loading force and means for measuring the applied loading force. The means for applying the reversible loading force typically comprises a cyclically reversible piston or ram which is attached to one of the said frames of the bending apparatus. The means for measuring the applied loading force typically comprises a load-cell which is connected to the other frame, which does not move during testing.

The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional illustration of the bending apparatus of the present invention connected to a testing machine; and Figure 2 is a cross-sectional illustration of the upper section of Figure 1.

Referring to Figure 1, a bending apparatus (connected to a testing machine) 1 is shown in which a test specimen 5 is clamped by four upper loading pins lla, 10a, lOc and 11c sited on the upper surface 6 of specimen 5 and four opposing lower loading pins llb, lOb, lOd and 11d sited on the lower surface 7 of specimen 5. The loading pins are arranged to provide two pairs of inner loading pins 10a and b and lOc and d (nearest the centre of the length of the specimen) and two pairs of outer loading pins 11a and b and 11c and d (nearest the ends of the length of the specimen). The loading pins 10a-d and 11a-d are preferably formed from silicon carbide and have sizes and shapes tailored to avoid excessively high contact stresses which could result in failure of the specimen 5 or parts of the apparatus 1. As shown, the loading pins 10a-d and 11a-d have a cylindrical shape (end faces shown in figures) and the inner loading pins 10a and b and lOc and d have a greater diameter than the outer loading pins 11a and b and 11c and d.

An articulated load train is provided for transmitting a loading force to the loading pins 10a-d and lla-d and consists of upper 20 and lower 29 cylindrical sections forming an inner array and upper 30 and lower 39 tubular sections forming an outer array (for clarity, the manner in which the load train is articulated is not shown in Figure 1, but is described in detail below with reference to Figure 2).

The main (vertical) axes of sections 20, 29, 30 and 39 are substantially coincident and substantially perpendicular to the longitudinal axis of the specimen 5. The sections 20, 29, 30 and 39 are preferably formed from silicon carbide.

The upper 20 and lower 29 sections forming the inner array are clamped together by an inner steel frame 40 comprising an upper flange 41 connected to a lower flange 42 by two or more connecting rods, such as that shown at 43. Likewise, the upper 30 and lower 39 sections forming the outer array are clamped together by an outer steel frame 50 comprising an upper flange 51 connected to a lower flange 52 by two or more connecting rods such as those shown at 53 and 54. As shown, the inner frame 40 is connected at lower flange 42 to a piston 60, which provides a cyclically reversible loading force in directions (as shown by the arrows) substantially perpendicular to the longitudinal axis of the specimen 5. The inner frame 40 is moveable in the direction of the loading force with respect to the outer frame 50, which remains fixed during testing. A load-cell 70 is connected to the upper flange 51 of outer frame 50 to measure the load applied to the specimen 5. It will be understood that this arrangement may be reversed so that the outer frame 50 is connected to the mobile part of the apparatus, i.e. the piston 60, and the inner frame 40 remains fixed during testing.

A heating zone 80 is shown generally by the dotted line in Figure 1 and this enables mechanical testing to be performed at elevated temperatures.

The upper and lower sections of the inner and outer arrays are substantially symmetrical and for clarity the upper section will now be further described with reference to Figure 2. Upper section 20 of the inner array comprises a first generally cylindrical part 21 and a second generally cylindrical part 22. Both parts 21 and 22 have the same or a similar diameter and are formed from SiC. A Si3N4 sphere 23 is located in suitably sized hemi-spherical recesses 24 and 25 formed in the adjoining ends of parts 21 and 22, respectively, resulting in a ball joint which allows articulation of the parts 21 and 22. The inner loading pins 10a and c are located in semi-cylindrical recesses (not shown) formed in a lower surface 26 of the part 21. Both the first 21 and second 22 parts have a major (vertical) axis which is substantially parallel to and coincident with the direction of the loading force.

The upper section 30 of the outer array consists of a first tubular part 31, which is concentrically arranged around the first generally cylindrical part 21, and a second tubular part 32, which is concentrically arranged around the second generally cylindrical part 22. The first tubular part 31 takes up the reaction forces at the outer loading pins 11a and c, which are located in semi-cylindrical recesses (not shown) in a lower surface 36 thereof. The first 31 and second parts 32 also have a major (vertical) axis which is substantially parallel to and coincident with the direction of the loading force.

First 33 and second 34 SiC rollers are disposed between the first 31 and second 32 tubular parts to provide a roller joint, hence allowing the first 31 and second 32 tubular parts to articulate. Rollers 33 and 34 are located on a diameter of the first 31 and second 32 tubular parts and have a common axis of rotation, which is substantially perpendicular to the direction of the loading force and substantially parallel to the long axis of the specimen 5. The roller joint in the lower section 39 of the outer array (not shown) comprises a similar arrangement, but the two rollers have a common axis of rotation substantially perpendicular to both the direction of the loading force and the axis of rotation of the first 33 and second 34 rollers, i.e. the rollers in the lower section are rotated through approximately 900 in a plane perpendicular to the direction of the loading force. The outer array can therefore articulate in two perpendicular directions. Together with the Si3N4 balls, these two sets of SiC rollers act as a passive alignment device and ensure that loading of the test specimen is substantially axial.

In the apparatus illustrated in Figure 2, a first ZrO2 heat-barrier ring 90 is provided between the upper surface 27 of the second cylindrical part 22 and a lower surface 92 of a first Al203 extension tube 91 which extends up to and is in contact with the inner frame 40. Similarly, a second ZrO2 heat-barrier ring 93 is provided between the upper surface 37 of the second tubular part 32 and a lower surface 95 of a second A1203 extension tube 94 which extends up to and is in contact with the outer frame 50. The second extension tube 94 is arranged concentrically around the first extension tube 91. Flanges 40 and 50 have a cylindrical recess (not shown) in their respective lower surfaces of slightly larger diameter than that of the first 91 and second 94 extension tubes respectively. The first 91 and second 94 extension tubes are accordingly clamped in place. The low thermal conductivity of ZrO2 compared with silicon carbide reduces heat losses along the loading axis.

The alumina extension tubes 91 and 94 allow dissipation of the heat escaping from the heating zone 80 (not shown in Figure 2) by conduction through the ceramic into the surrounding air, and limit the temperature of the steel frames 40 and 50 to an acceptable level. Consequently, the first 90 and second 93 heat-barrier rings together with the first 91 and second 94 extension tubes facilitate the isolation of the (heated) specimen 5 and adjacent components from the remaining parts of the apparatus.

Referring back to Figure 1, the relative movement of the inner array with respect to the outer array upon movement of the piston 60 results in a four-point bending load being applied to the specimen 5. The inner pins 10a-d load the specimen 5 in a central region, whereas the outer pins 11a-d support the specimen adjacent to its ends and take up the reaction forces. By applying an alternating load, fully reversed four-point flexural loading of the specimen 5 is achieved, resulting in a stress distribution which changes sign each half cycle but which does not change between cycles. Also the neutral plane does not migrate from its original position at the centre of the specimen 5, and the stress distribution is substantially homogeneous between the inner loading pins 10a and b and lOc and d.

The present invention provides for flexural fatigue loading with a negative load ratio and a spatially constant stress distribution between the inner loading pins at high temperatures. It significantly broadens the test field covered by the common four point-bending apparatus, enabling the generation of reliable high temperature fatigue data which can be used for the assessment and development of engineering materials. Accommodation of thermal dimensional changes in the test specimen and the apparatus during testing at elevated temperatures are also provided.

Claims

CLAIMS:

1. A bending apparatus for the mechanical testing of materials, the apparatus comprising: (i) at least four upper loading pins, wherein each upper loading pin opposes a corresponding lower loading pin resulting in at least two pairs of inner loading pins and at least two pairs of outer loading pins for clamping a test specimen; (ii) an external frame fixed relative to the outer loading pins; (iii) an internal frame fixed relative to the inner loading pins; and (iv) an articulated load train for transmitting a loading force to the loading pins, the load train comprising an inner array linking the inner loading pins to the internal frame and an outer array linking the outer loading pins to the external frame.

2. An apparatus as claimed in claim 1, further comprising heating means to heat a test specimen.

3. An apparatus as claimed in claim 2, wherein a thermal barrier is provided to facilitate isolation of the heated test specimen and adjacent components from other parts of the apparatus.

4. An apparatus as claimed in any one of the preceding claims, wherein the inner array and/or the outer array comprise(s) an upper section above the upper loading pins and a lower section below the lower loading pins, each section comprising two or more articulated parts.

5. An apparatus as claimed in claim 4, wherein articulation of the two or more parts of each section is effected by the provision of a ball joint between each of the said parts.

6. An apparatus as claimed in claim 4, wherein articulation of the two or more parts of each section is effected by the provision of a roller joint between each of the said parts.

7. An apparatus as claimed in claim 4, wherein each of the upper and lower sections of the inner array comprises a first generally cylindrical part articulated at a ball joint with a second generally cylindrical part, the first part being adjacent to the inner loading pins and the second part being adjacent to the inner frame.

8. An apparatus as claimed in claim 7, wherein each of the upper and lower sections of the outer array comprises a first tubular part, which surrounds the first generally cylindrical part, articulated at a roller joint with a second tubular part, which surrounds the second generally cylindrical part, the first tubular part being adjacent to the outer loading pins and the second tubular part being adjacent to the outer frame.

9. An apparatus as claimed in claim 8, wherein the roller joint in the upper section of the outer array comprises first and second rollers disposed between the first and second tubular parts, the rollers having a common axis of rotation substantially perpendicular to the major axes of the first and second tubular parts and being separated by a distance substantially equal to the diameter of the first and second tubular parts, and wherein the roller joint in the lower section of the outer array comprises third and fourth rollers disposed between the first and second tubular parts, the rollers having a common axis of rotation substantially perpendicular to the said major axes and also the axis of rotation of the first and second rollers and being separated by a distance substantially equal to the diameter of the first and second tubular parts.

10. An apparatus as claimed in any one of the preceding claims, wherein the loading pins and the inner and outer arrays are formed from a ceramic material.

11. An apparatus as claimed in any one of claims 5 and 7 to 10, wherein the ball joint is formed from a ceramic material.

12. An apparatus as claimed in any one of claims 6 and 8 to 11, wherein the roller joint is formed from a ceramic material.

13. An apparatus as claimed in any one of claims 10 to 12, wherein the ceramic material is SiC, Al203, Si3N4 or a mixture of two or more thereof.

14. An apparatus as claimed in any one of the preceding claims further comprising means for applying a reversible loading force to one of the said frames, which frame is moveable relative to the other frame in a direction substantially parallel to the direction of the loading force.

15. An apparatus as claimed in claim 14 further comprising means for measuring the applied loading force.

16. An apparatus substantially as herein described with reference to or as illustrated in Figures 1 and 2 of the accompanying drawings.