GB1579721A

GB1579721A - Testing load-bearing materials

Info

Publication number: GB1579721A
Application number: GB379776A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1977-05-02
Filing date: 1977-05-02
Publication date: 1980-11-26

Description

-(54) TESTING LOAD-BEARING MATERIALS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation of P.O. Box 236, Kingsgate House, 66/74 Victoria-Street, -London-SWi 6SL, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to testing of load-bearing material, such as concrete, under combined compression in one direction and lateral pressure in-all directions perpendicular to the said one.direction, more particularly, though not exclusively, to creep-testing, especially at elevated temperatures.

According to one aspect of this invention there is provided apparatus for testing a sample of load-bearing material under combined axial compression and lateral pressure, comprising an outer wall capable of withstanding the said lateral pressure and forming an annular pressure chamber around a tubular metallic membrane surrounding a space for containing a sample to be tested, the two ends of the membrane being bonded to respective end portions of the wall, means for filling the pressure chamber with a fluid under pressure to exert lateral pressure on a sample when the sample is within the space surrounded by the membrane, means for applying axial compression to the sample, and means whereby resulting deformations of the sample, in at least one direction, may be measured.Advantageously, means are provided for raising the temperature of the fluid in the pressure chamber and thereby the material under test. The membrane may have a corrugation in each position corresponding to an end of the sample.

According to another aspect of this invention there is provided a test cell for creeptesting a sample of load-bearing material, such as concrete, under compression in one direction while subject to pressure in all directions perpendicular to the-said one direction, comprising a tubular metallic membrane having its two ends bonded to respective ends of an outer wall structure surrounding the membrane and thereby forming pressure chamber around the membrane, and heating means for raising the temperature of fluid in the pressure chamber. Advantageously, a probe assembly is mounted in the outer wall and includes a movable probe which is biassable into contact with the membrane. Preferably, the probe is sealed from the annular space between the wall and the membrane by a metallic bellows.

An embodiment -of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is -a-vertical axial section of a creeptesting pressure cell, Figure 2 shows the region marked-II in Figure 1 on an enlarged scale, Figure 3 shows the region marked III in Figure 1 on an enlarged scale, Figures 4 and 5 are views similar to Figures 1 and 2 of a modified pressure cell in a test frame, Figure 6 shows in axial section a proble of the pressure cell shown in Figures 4 and 5 and Figure 7 shows the hydraulic circuit of creep-testing apparatus incorporating one or more of the cells shown in Figures 4 to 6.

The creep-testing pressure cell shown in Figures 1 to 3 may be used for creep-testing a cylindrical sample of concrete for say six months at 200 "C. under vertical axial compression and lateral hydraulic pressure. The pressure cell comprises an annular pressure chamber 'A' (Figure 1) formed between a tubular aluminium outer wall 1, which is sufficiently strong to withstand the pressure required to be maintained within the pressure chamber 'A', and an aluminium membrane 2 of thickness 0.01" forming the inner wall of the pressure chamber 'A' and surrounding a space 'C' in which a cylindrical sample 'S' is to be tested in place. Each end of the chamber 'A' is closed and sealed by an aluminium ring 3 (Figure 2) secured to the respective end of the membrane 2 by a weld 5, and to the correspond ing end of the wall 1 by a weld 4.Within the pressure chamber 'A', all surfaces except that of the membrane 2 may, if desired, be insulated with thermal insulation such as vermiculite 7.

An electric heating filament 13, coiled around and in close contact with the membrane 2 (Figure 1).extends almost the full length of the pressure chamber 'A' within the latter.

The pressure chamber 'A' is filled with liquid such as non-inflammable oil or silicone fluid by means of two pipes 22 and by means of two assemblies 17 attached to the tubular wall 1, each comprising a collar 18 (Figure 3) welded to the tubular wall 1, each collar having a reduced diameter extemally screw-threaded portion 19, onto which an internally screwthreaded cap 20 is screwed so as to compress sealing washers 21. The associated pipe 22 is screwed into the cap 21 and may include a valve 'V'. Also, wires 23 leading to the heating coil 13, pass through a hole in each of the caps 20, with suitable seals (not shown).

Close to each end of the pressure chamber 'A' the tubular membrane 2 is formed with a stress-relieving annular corrugation 11 (Figures 1 and 2) which allows the membrane 2 to be pressed uniformly over the whole cylindrical surface of the sample 'S' when the pressure chamber 'A' is filled with a liquid under pressure, thus transmitting the full hydrostatic pressure (diminished by any resistance to displacement of the membrane) of the liquid to the sample 'S' over its entire length.

Dial gauges 12 are fitted to the wall 1 in pairs, one at each end of diameters of the membrane 2, each one being screwed into one end of a short steel tube 14, (Figure 3) which in turn is screwed into the wall 1. Deformation of the membrane 2 resulting from dimensional changes in the specimen 'S' is transmitted to each gauge by a rod 15, which extends from the gauge 12 along the centre of the tube 14 through a sealing pad 16 to abut the membrane 2.

Axial compression is applied to the specimen 'S' for example by a conventional compression testing machine or creep-testing by means of platens 10 (Figure 1), and packing material 24, the latter ensuring that compression is applied substantially uniformly to the specimen 'S'.

Holes 25 in the platens 10 and packing material 24 allows measurements of the axial deformation of the specimen 'S' to be made.

A thin sleeve 8 (Figure 2) of small thickness is fused to the inside of the tubular membrane 2 at 9, thus giving some protection to the ends of the membrane 2 from damage, while leaving adequate clearance for introduction of the platens 10 and sample 'S' during assembly.

Any tendency of the sample 'S' to creep under the test conditions may be observed by observing changes in the readings of the dial gauges 12 and/or those applied to the sample through the holes 25.

All components of the apparatus described as being made of aluminium may of course be of steel or any other suitable material.

The membrane 2 is sufficiently thin to give little resistance to displacements which occur when the pressure chamber is filled with a liquid under pressure. Tests may be carried out in order to calibrate the apparatus so as to allow for any resistance afforded by the membrane, and similarly, tests may be per formed in order that the apparatus can be calibrated so as to make allowances for friction between the specimen 'S' and the apparatus.

The modified test call shown in Figures 4 to 6 is, as shown in Figure 4, mounted in a simple but robust test rig comprising a base plate 41 a set of tie-rods 42 and a top plate 43, the tie-rods 42 passing through aligned bores in the base and top plates 41 and 43 and carry ing substantial nuts 44 on their upper and lower screw-threaded end portions.

The sample 's' to be tested is mounted be tween upper and lower platens 45 and 46 with the interposition of suitable load-spreading packing material 'P' such as a high strength mortar in the case of a concrete sample and hard insulating material 'P'. Alternatively, 'P' may be omitted and P then may be hard insu lation such as sindanyo.

The lower platen 46 is supported on a larger diameter spreader 47, again with the inter position of hard insulating material P' there between or P' is omitted and P is a hard insulation material. The spreader 47 rests on and is urged upwards by a flat jacking disc assembly 48 having an internal jacking chamber 49 defined by upper and lower steel sheet discs 50 and 51 which are interconnected at their peripheries by a frusto-toroidal wall 52 which is welded to each of them.Further loadspreading packing material P" is interposed between the discs 50 and 51 and the spreader 47 and base member 41 respectively and a hydraulic connection (not shown) leading into the space 49 enables a suitable hydraulic pressure to be applied within the space 49 to generate the required upwards loading on the sample 'S', bearing in mind the multiplication factor corresponding to the ratio of the horizontal areas of the jacking 49 and the lower platen 46.

A spherical bearing assembly 53 is positioned between the upper platen 45 and the top member 43, again with the interposition of packing material P'. The bearing assembly 43 comprises upper and lower elements 54 and 55 having complementary mating surfaces defined by a portion 56 of a sphere.

As in the case of Figures 1 to 3, the test cell itself (Figures 4 and 5) has a thick outer wall 57 of steel comprising a tubular wall portion 58 and two end rings 59 which are secured to the upper and lower ends of the tubular wall 58 by welds 60. In this embodiment, the outer wall 57 is fonned of steel. A tubular membrane 61 of S321 S12 (B.S. 1449) of 0.01" thickness has its two end regions secured to the inner surfaces of the end rings 59 over regions 62 by a seam-welding or fusion-welding process carried out by Teddington Bellows Limited of Pontordulais, Swansea, SA4 1 RP.

In the region of each end of the sample 'S', the membrane 61 is formed with a corrugation 63 which is semi-circular in cross-section and has a radius of about 0.1".

An inner protective membrane 64, also of stainless steel, extends within each outer end portion of the membrane 61 and has its outer end 65 bent around and fused to an end portion 66 of the membrane 61 which protects beyond the end ring 59. The inner membrane 64 protects the anchored end parts of the membrane 61 during insertion of a sample and also serve to prevent sharp bends in the inner membrane 61 adjacent its anchored part 62 and the transition between the platens 45 and 46, and the sample 'S'.

The space 67 between the outer wall 57 and the membrane 61 is filled with a synthetic silicon-based or other non-inflammable oil through a port 68 which is subsequently closed by a plug 69. An electrical heating tape 70 is wound around the outside of the tubular wall 58 and is connected to a suitable electrical supply through a temperature controlling thermostat (not shown). A jacket 71 of thermally insulating material surrounds the outer wall 57 and its heater tape 70.

A transducer assembly 72 (Figure 4) which is capable of measuring changes in length in the axial and radial directions may be incorporated in the sample 'S' when the latter is cast, as in the case of concrete, together with suitable connecting wire 73 leading to an external recorder or indicator (not shown). Similarly, a thermocouple may also be cast into the concrete.

In many cases, however, it may be preferred not to include such a transducer, in which case changes in the axial length of the sample 'S' may be recorded normally with sufficient accuracy by means of transducers 74 supported from rods 42 (Figure 4) and arranged to indicate changes in the spacing between the upper platen 45 and the spreader 47 while changes in the diameter of the sample 'S' may be measured by one or more probe assemblies 75 which preferably have the form shown in section in Figure 6.

The probe assembly shown in Figure 6 is mounted in an internally screw-threaded bushing 76 which is welded into a counterbore in the outer surface of the mild steel tubular wall 58. A probe rod 77 is mounted for sliding movement in a bellows-retaining plug 78 which is screwed into a bellows-retaining nut 79 which itself is extemally screw-threaded and is screwed into the bushing 76 into sealing contact with a copper sealing washer 80. The plug 78 and nut 79 have complementary frusto-conical surfaces between which is clamped a correspondingly frusto-conical mouth of a stainless steel bellows 81, the opposite end of which is closed at 82.

The probe rod 77 is urged (to the left in Figure 6) into contact with the closed end 82 to urge the latter into contact with the membrane 61 by means of a compression spring 83 mounted between a collar 84 fixed to the rod 77 and a washer 85 bearing against a flange 86 on an adjusting sleeve 87 which is in screwthreaded engagement with the screw-threads in the bushing 76 and can be clamped to the latter by a nut 88.

The spring 83 is only required to provide sufficient loading to ensure that the closed end 82 of the bellows makes proper contact with the membrane 61 (which is held in contact with the sample 'S' by the pressure of the oil in space 67) and in particular to overcome the resilience of the bellows 81 and the force exerted by the oil pressure acting on the closed end. The position of the probe rod 77 may be recorded by a transducer 89. The proper tension in the spring 83 may be determined by screwing in the sleeve 87 until the reading given by a recorder connected to the transducer 89 becomes constant after which the sleeve may be further screwed in through an amount predetermined by experiment as being necessary and thereafter be locked in position by tightening the lock nut 88.

If desired, the axial load acting on the sample may be monitored by a load cell introduced between the jack and the sample.

Figure 7 shows the hydraulic circuit which can be connected to the apparatus shown in Figures 4 to 6 to maintain the necessary hydraulic pressure without constant supervision.

A reservoir 90 for the oil supplies a pump 91 (which may be a hand operated pump) through a valve 92. An outlet valve 93 supplies, through further valves 94 and 95 lines 96 and 97 leading to the jacking space 49 and the pressure cell space 67 respectively, through isolating valves 98 and 99. Each of the lines 96 and 97 has a pressure accumulator 100, 101 connected to it to maintain the required pressure despite slight leaks, and each line 96, 97 has a pressure gauge 102. 103 to monitor the pressures. these gauges including suitable alarm means if required.

As indicated, the lines 96 and 97 may be extended as indicated at 96', 97', to supply further test cells.

It will be noted that in the pressure cells described above, the pressurized oil is contained entirely by metal walls without the need for any synthetic or elastomeric seals which have been found to be unreliable since they are themselves prone to creep-failure under the combination of long term pressure and temperature (e.g. 200 OC.) to which the apparatus is to be subjected.

The elastic properties of the stainless steel forming the membrane 61 are such that it need not be deformed beyond its elastic limit (except perhaps when first used), especially where the outer diameter of the sample 'S' is accurately controlled, for example, by surface grinding of the sample after it has been cast or cut. If desired, the axial compression may be applied to the sample before the radial pressure is applied to reduce the distance through which the membrane must be deformed into full contact with the sample 'S'.

In addition to assessing how the material of the sample 'S' will behave in a projected application, the actual performance of a portion of a structure (for example the pressure vessel of a nuclear reactor) may be reproduced by casting the sample 'S' from the same batch of material as is used in the relevant part of the structure, monitoring the temperature and axial and radial pressure actually occurring in the said portion of the structure and continuously applying these values to the sample in the apparatus of the invention.

An advantage of the apparatuses described above is that, with the pressure chamber filled with a nonqnflammable liquid, both recording and control of a test may safely be automatic, electronic devices such as transducers being employed to measure displacements of the specimen.

Other advantages are that it enables long term axial stress-radial displacement strains to be plotted at various temperatures and for various multi-axial stresses for load-bearing materials over a range making it suitable for testing concrete such as that used in the pressure vessels of nuclear reactors. The invention avoids the need for sample-contacting glands liable to leak at high pressures and temperatures.

WHAT WE CLAIM IS: 1. Apparatus for testing a sample of loadbearing material under combined axial compression and lateral pressure, comprising an outer wall capable of withstanding the said lateral pressure and forming an annular pressure chamber around a tubular metallic membrane surrounding a space for containing a sample to be tested, the two ends of the membrane being bonded to respective end portions of the wall, means for filling the pressure chamber with a fluid under pressure to exert lateral pressure on a sample when the sample is within the space surrounded by the membrane, means for applying axial compression to the sample, and means whereby resulting deformations of the sample, in at least one direction, may be measured.

2. Apparatus according to claim 1 and including heating means for raising the temperature of the fluid in the pressure chamber and thereby the material under test.

3. Apparatus according to claim 1 or 2, wherein the membrane has an outwardly extending corrugation adjacent, but inwards of each of its ends.

4. Apparatus according to any of claims 1 to 3, wherein a protective sleeve is provided within at least one end of the first-mentioned membrane.

5. Apparatus according to any of the preceding claims, wherein the means whereby deformations may be measured comprises a probe rod mounted for sliding movement through the outer wall towards the membrane and sealed to the latter by a deformable metal bellows.

6. Apparatus according to claim 5, wherein the bellows has a closed end receiving an end of the probe rod.

7. Apparatus according to claim 5 or 6 wherein the probe rod is resiliently urged towards the membrane.

8. Apparatus according to any of the preceding claims, wherein the means for applying axial pressure comprises a jack consisting of opposed discs interconnected at their peripheries by a frusto-toroidal wall.

9. Apparatus for testing a sample of loadbearing material substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

10. Apparatus for testing a sample of loadbearing material substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanting drawings.

11. A method of creep-testing a sample of load-bearing material in apparatus according to any of the preceding claims, in which the sample is maintained under axial and radial pressures within the membrane of the apparatus.

12. A method of reproducing the behaviour of a portion of the material of a structure, comprising the steps of measuring the temperature and axial and radial pressures in the said portion and continuously using the obtained values in creep-testing a similar sample by a method according to claim 11.

13. A test cell for creep-testing a sample of load-bearing material under compression in one direction while subject to pressure in all directions perpendicular to the said one direction, comprising a tubular metallic membrane having its two ends bonded to respective ends of an outer wall structure surrounding the membrane and thereby forming a pressure chamber around the membrane, and heating means for raising the temperature of fluid in the pressure chamber

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In addition to assessing how the material of the sample 'S' will behave in a projected application, the actual performance of a portion of a structure (for example the pressure vessel of a nuclear reactor) may be reproduced by casting the sample 'S' from the same batch of material as is used in the relevant part of the structure, monitoring the temperature and axial and radial pressure actually occurring in the said portion of the structure and continuously applying these values to the sample in the apparatus of the invention. An advantage of the apparatuses described above is that, with the pressure chamber filled with a nonqnflammable liquid, both recording and control of a test may safely be automatic, electronic devices such as transducers being employed to measure displacements of the specimen. Other advantages are that it enables long term axial stress-radial displacement strains to be plotted at various temperatures and for various multi-axial stresses for load-bearing materials over a range making it suitable for testing concrete such as that used in the pressure vessels of nuclear reactors. The invention avoids the need for sample-contacting glands liable to leak at high pressures and temperatures. WHAT WE CLAIM IS:

1. Apparatus for testing a sample of loadbearing material under combined axial compression and lateral pressure, comprising an outer wall capable of withstanding the said lateral pressure and forming an annular pressure chamber around a tubular metallic membrane surrounding a space for containing a sample to be tested, the two ends of the membrane being bonded to respective end portions of the wall, means for filling the pressure chamber with a fluid under pressure to exert lateral pressure on a sample when the sample is within the space surrounded by the membrane, means for applying axial compression to the sample, and means whereby resulting deformations of the sample, in at least one direction, may be measured.