GB2187293A - A disc strength test apparatus and method - Google Patents

Abstract

Inaccuracies in testing the strength of disc specimens caused by the use of a centrally located local load or by uneven loading due to friction are overcome by supporting a disc specimen (13) in a conventional manner on a ring of ball bearings (14) contacting the disc (13) near to its periphery and loading the opposite face of the disc with the second concentric smaller ring of ball bearings (5). The second ring of ball bearings (5) may be retained on a plug (1) which may be pivotable about the load applying means (11,3) and held substantially perpendicular to the specimen (13) by guide means (18). The plug 1 may be rotated relative to support block 16 as the load is increased. <IMAGE>

Description

SPECIFICATION A material disc strength test apparatus and method The invention relates to a disc strength test apparatus, particularly suitable for brittle materials, such as ceramics, including a disc support means capable of providing at least three support points near to, and within, the periphery of a ceramic disc under test, the support points lying substantially on a circle concentric about the centre of the disc and of greater diameter than a circular loading means which is capable of applying a concentric load to the surface of the disc opposite to that on which the support means can act.

The invention also relates to a disc strength test method for materials comprising the steps of supporting a disc specimen on a ring of at least three supports, and applying a circular load to the disc concentrically within the diameter of the supports and on the opposing face of the disc.

Several types of apparatus for testing of brittle materials such as ceramics are known.

Some are very complicated and require sophisticatd preparatory techniques to be applied to the ceramic material, as in the case of the bursting of internally pressurised discs or uniaxial tensile testing. Others, such as the testing of specially prepared bar specimens are especially susceptable to flaws induced during specimen preparation. The generally adopted methods use a bar specimen of square or rectangular cross-section which is loaded to breaking point in either a three or four point mode. In the three point mode a single central load point moves against two equispaced opposing support points, whereas in the four point mode two load points equispaced about the centre move in unison.In the three point mode only the point opposite the central load point experiences the peak stress, whereas in the four point mode an appreciable length of the specimen is subjected to the peak stress.

More realistic strength estimates are obtained when a greater volume of specimen is subject to peak load in the four point test, because any internal weaknesses are more likely to be subjected to the peak load. The disadvantage of the bar tests is that surface flaws can have a profound effect because the edges of the specimen experience the maximum stress.

Various special preparatory techniques have been tried to overcome this problem, but all require great care, add considerably to the cost and involve quality control problems.

A disc specimen is cheaper and easier to prepare than a bar specimen. This is because testing apparatus for discs can be designed so that the condition of the edges plays less of a determinant role in the results obtained.

The disc surface can be ground and polished to a high quality finish by using microstructural characterisation techniques commonly available in research laboratories.

A method of ceramic disc testing has been used in which the disc is supported on ball bearings near to its periphery and loaded at the centre of the opposite surface by a single ball bearing which is urged towards the disc by a flat steel platten plate. This method is analogous to the three point mode for bar testing as it provides uniaxial loading of the specimen.

In an effort to combine the advantages of the four point mode with those of testing disc specimens, Stanley and Sivill constructed and used an apparatus as in accordance with the prior art portion of Claim 1. This is described in Proc. Brit. Ceram. Soc. 1987,26,97. In that apparatus a circular load was applied in place of the point loading of a single ball bearing.

The load was applied by means of an annular ridge ground onto a single ball bearing. This, however, is an expensive way of providing a circular load apparatus. Also the use of a toroidal, cylindrical or sharp rimmed circle ground onto a ball bearing has now been found to have a more serious disadvantage.

Tests have shown that failure of disc specimens loaded by such circular loading means tends to occur at the loading ring circle, rather than having a random distribution of origin as would be expected. This result has been shown to be due to friction between the ball bearing onto which the circule has been formed and the steel platen plate. This friction prevents the ball bearing from rotating and consequently prevents the circular load from properly aligning itself with the disc specimen.

The lack of alignment causes non-uniform loading and because the consequential excess stress is unquantifiable a serious source of error in strength measurement is unavoidable.

It is also been observed that friction between a solid circular loading ring and the disc specimen hinders elastic flexure of the disc and gives rise to a further source of error in strength evaluation.

The use of a steel ball bearing also makes the apparatus unsuitable for high temperature testing of ceramics, an increasingly frequent requirement.

It has been proposed to replace the single ball bearing by a solid cylinder of 1/16" diameter as in the ASTM standard. This requires the use of a pad of non rigid material to prevent localised loading. This method has however been found to give errors originating from a lack of alignment. This is because even with a very flat disc specimen it is practically impossible to maintain the cylinder in a position relative to the disc that will unsure even load distribution as the loading is increased.

The invention as claimed is intended to remedy these drawbacks. It solves the problem of providing a relatively cheap ceramic disc strength test apparatus and method which is able to provide consistent, reliable strength results and can be adapted for high temperature use. The solution allows the testing of a large volume of each ceramic disc specimen and substantially reduces frictional errors and inaccuracies due to imperfect alignment, thereby overcoming the disadvantages of the prior art.

The provision of a circular array of ball bearings capable of making simultaneous contact with the disc specimen, in a circle about its centre, leads to minimal friction between the load system and the specimen and therefore frictional effects reducing the flexure of the disc are minimised Reduced. friction also lessens errors due to wear because rotation of the ball bearings between tests tends to present a fresh surface for loading the next specimen. Furthermore a circular array of ball bearings is easier to produce than a solid sharp rim, a cylinder or a torus.

The arrangement of the circular array of ball bearings in an annular groove, dimensioned so that each ball bearing is in loose contact with its two neighbouring ball bearings allows the load to be applied at a large enough number of points to give a good approximation to an unbroken circular load.

Forming the annular groove in a plug and mounting the plug so that it is universally pivotal about the load applying means, when utilised in conjuction with the friction reducing array of ball bearings, enable the plug and ball bearings assembly to align adequately with the disc specimen.

Retaining the plug in an alignment means results in further enhancement of the accuracy of alignment between the disc specimen and the array of ball bearings and therefore further increases the evenness of load distribution and the usefulness of the test.

The use of ceramic ball bearings facilitates operation at high temperature to measure strength, when soft interfaclaf inserts cannot be used.

The rotation of the plud during loading ensures rotation of the ball bearings which lowers friction and reduces wear on the ball bearings.

The invention will now be described, by way of example only, and with reference to the accompanying drawings of which: Figure 1 is an axial cross section of a plug; and Figure 2 is a schematic representation of part of a ceramic disc strength test apparatus.

Fig. 1 shows a plug 1 of substantially cylindrical form and arranged with its axis vertical.

In the centre of the upper flat surface of the plug there is formed a hemispherical cavity 2 which is able to retain a steel ball bearing 3 with a diameter the same as that of the cavity 2, in this embodiment 10mm.

The lower surface of the plug 1 is adapted to form an annular groove 4 capable of retaining an inner ring of ball bearings 5 in conjunction with a retaining plate 6 which is affixed to the plug by means of retaining screws 7. In this embodiment of the invention the plug 1 has its lower surface shaped so as to have coaxial cylindrical protuberance 8 with a diameter approximately 12% of the diameter of the plug 1. The protubrance 8 defines the inner circumference of the annular groove 4, a horizontal surface 9 ensures that the ball bearings 5 lie in a plane and the retaining plate 6 defines the outer circumference of the annular groove 4. A small lip 10, formed on the retaining plate 6 retains the ball bearings 5 in the annular groove 4.The dimensions of the ball bearings 5 and the annular groove 4 are chosen to be of suitable dimensions so that each ball bearing is in loose contact with its two neighbouring ball bearings at any one time and the ball bearings 5 are held in a circle. In practice an array of at least six ball bearings has been found to give satisfactory results. A flat metal plate or platen 11 is able to act on the steel ball bearing 3 when a load 12 is applied in the direction indicated.

Fig. 2 shows the plug 1 and a ceramic disc specimen 13 which, in use is supported on a circular outer ring of ball bearings 14 retained in a second annular groove 15 in a support block 16. The diameter of the second annular groove 15 is such that the ball bearings 14 can made simultaneous contact near to the periphery of the disc specimen 13. The diameter of the disc specimen 13 is slightly less than the diameter of the plug 1.

The number of the ball bearings making up the outer ring 14 will need to be varied according to the diameter of the disc specimen 13 to be tested in any one apparatus. For the smallest specimen diameters at the minimum number of three ball bearings may be used and a ring of twelve balls has been found to give good results when used for larger diameter specimens, for example 17mm diameter discs.

The disc specimen 13 is supported laterally by three lugs 17, one on each of three support pillars 18 which rise vertically from and are disposed symmetrically about the support block 16. The support pillars 18 are shaped so as to provide a guide means into which the plug 1 can be inserted and which allows the plug 1 to slide vertically and to rotate axially whilst substantially preventing the axis of the plug 1 from departing from being perpendicular to the supported disc specimen 13.

The lower end of the cylindrical surface of the plug 1 is cut away to form a stub 19 of smaller diameter than that of the plug 1. The diameter of the stub 19 must be small enough to ensure that the plug 1 does not make contact with the lugs 1 7 as it is lowered onto the disc specimen 13 and loaded. At the centre of the support block 16 is provided a cavity 20 which is suffiiciently deep to allow the disc specimen 13 to deflect without making con tact with the support block 16, other than through the outer ring of ball bearings 14.

In use the plug 1 is urged towards the disc specimen 13 supported on the outer ring of ball bearings 14 by transmission of a loading force 12 applied to a flat metal plate 11 which is in contact with the steel ball bearing 3 located in the hemispherical cavity 2 in the centre of the upper surface of the plug 1. The plug 1 is slid vertically within the three support pillars 18 until the inner ring of ball bearings 5 makes even contact with the upper surface of the disc specimen 13. Application of the load 12 then results in deflection and ultimately failure of the disc specimen 13. A measuring instrument (not shown) records the value of the applied load at the instant of failure.

In another embodiment of the invention the plug 1 may be arranged to rotate axially relative to the support block 16 as the load 1 2 is increased.

It has been observed that, although the strength of the disc specimen 1 3 should theoretically be different when using a ring of ball bearings 5 instead of a solid ring as used in the prior art, in practice the difference between results obtained from the two methods is negligible. It is also significant that specimen thickness does not appear to be critical to the test method of the present invention.

Also, since the maximum stress occurs on the side of the specimen remote from the point of application of the load, scratching of the surface by the means of load application is unlikely to be a factor causing failure of the specimen.

Disc testing at room temperature is done as described using steel balls. At high temperatures it is preferable to use ceramic balls. Although the disc tester has been described with reference to the testing of ceramic materials it may be used with other brittle materials. Metal are generally tested in tensile mode, however this flexural technqiue could be applied to metals, particularly brittle ones, where a tensile method is not easily carried out.

Claims (8)

1. A ceramic disc strength test apparatus including a disc support means capable of providing at least three support points near to, and within, the periphery of a ceramic disc under test, the support points lying substantially on a circle concentric about the centre of the ceramic disc and of greater diameter than that of a circular loading means which is capable of applying a concentric load to the surface of the disc opposite to that on which the support points can act, characterised in that the circular loading means includes a circular array of ball bearings (5) which are capable of making simultaneous contact with the disc in a circle about its centre.

2. A ceramic disc strength test apparatus as claimed in claim 1 in which the circular array of ball bearings (5) comprises a pleurality of ball bearings, each being located in an annular groove (4) which is dimensioned so that each ball bearing is able to maintain loose contact with its two neighbouring ball bearings.

3. A ceramic disc strength test apparatus as claimed in claim 1 or claim 2 in which the circular array of ball bearings (5) is retained on a plug (1) which is universally pivotable about a load transmitting means (3,11).

4. A ceramic disc strength test apparatus as claimed in claims in which the plug (1) is cylindrical and is arranged so that, in use, its movements are maintained substantially restricted to an axis perpendicular to the ceramic disc by guide means.

5. A ceramic disc strength test apparatus as claimed in claim 4 in which the guide means comprises at least three support pillars (18) which extend vertically upwards from the disc support means (16).

6. A ceramic disc strength test apparatus as claimed in any preceding claim in which the ball bearings (5) are fabricated from ceramic material.

7. A ceramic disc strength test method comprising the steps of supporting a ceramic disc to be tested (13) on a ring of at least three support points (14) and applying a load (12) to the disc concentrically on the opposite face of the disc, characterised in the that the load is applied by a circular array of ball bearings (5).

8. A method as claimed in claim 7 in which the ball bearing array (5) is caused to rotate as the load (12) is applied.