GB2368644A

GB2368644A - Tester for applying tensile force to a fixing

Info

Publication number: GB2368644A
Application number: GB0015310A
Authority: GB
Inventors: Alan Thomas Anderson; Christopher Barry Hirst
Original assignee: CLAN PRODUCTS
Current assignee: CLAN PRODUCTS
Priority date: 2000-06-23
Filing date: 2000-06-23
Publication date: 2002-05-08
Also published as: GB0015310D0

Abstract

A portable tester (1) for use in the <I>in situ</I> tensile testing of a fixing (34) secured in a wall, floor of the like (35). The tester (1) incorporates means (6), such as rotatable handles, for applying a tensile load to a load cell (2) and the load cell (2) transmits the applied tensile load to the fixing (34) and provides an indication of the applied tensile load. The tester may include a second load cell (3) to provide an indication of the load-induced displacement of the fixing (34). Both the tensile load and the displacement may be measured by means of strain gauges. The tensile force is applied through a load cell (2), which is formed as a screw threaded shaft. Strain gauge (16) then indicates the applied force. Displacement can be measured by a second load cell formed as a metal plate (3) with strain gauges (not shown) located behind. Spring (28) engages the plate and allows displacement to be measured. The tester may be battery operated and can be provided with a suitable bridge circuit and amplifier for the signals from the strain gauge.

Description

TESTER

The present invention relates to a tester particularly, but not exclusively, for in situ, pull or tension testing a member secured to a body such as a reinforcing wall tie, a cavity wall tie, roofing screw, structural anchor or the like.

For many building constructions it is necessary to measure the tensile force which can be withstood by a fixing whilst in situ, i. e. mounted in a body such as a wall or floor, in order to ascertain whether the fixing complies with given safety standards, in so far as its load bearing capacity is concerned.

A known pull tester progressively applies a tensile force to the fixing using a hand-operated hydraulic ram arrangement, the applied force being measured by a pressure gauge calibrated to give readings of force. The displacement of the fixing under the applied load is displayed on a separate displacement gauge in the form of a screw member projecting from the ram which moves with the cam, within a sealed slot.

However, this tester is relatively expensive due to the expressive pressure gauge which is also prone to damage, oil leakage and seizure of metallic parts. The pressure gauge is also difficult and expensive to calibrate.

A further problem identified is that, with known testers of the above kind, it is difficult to obtain reliable and accurate load v. displacement

readings for the member, as the displacement readings must normally be taken manually by the user from a displacement gauge whilst load application is momentarily halted. This arrangement is not ideal as to comply with certain testing standards, the load application must be continuous and uninterrupted. Thus, whilst in some circumstances it is desirable to obtain accurate readings for load v. displacement as the load applied is progressively increased, this is impossible using a device of the kind described above.

The object of the invention is to provide an improved tester which is low cost and which can be used for accurate and convenient in situ tensile testing of a member secured in a body, such a wall fixing.

According to a first aspect of the present invention there is provided a portable tester for use in the in situ tensile testing of a member secured in a body, wherein the tester incorporates means for applying a tensile load to a load cell, and the load cell transmits said applied tensile load to the member and provides an indication of the applied tensile load.

The term load cell as used herein, is intended to mean a body of elastic material, having a property/properties which varies/vary predictably according to the load applied to the body such that measurement of the said property or properties provides an indication of the characteristics of the load applied e. g. the amount of load applied and/or the strain induced by said applied load.

Most preferably, the body will be a metal as this material has properties, which vary predictably under high loads whilst, providing the elastic limit of the particular metal is not exceeded, such property changes are not permanent.

According to a second aspect the invention provides a method of in site tensile testing a member secured to a body wherein the tester incorporates means for applying a tensile load to a load cell, and the load cell transmits the applied tensile load to the member and provides an indication of the applied tensile load.

With this arrangement, in situ tensile testing of member e. g. a fixing, secured in a body such as wall/floor fixing can be accurately and conveniently effected with a tester which is also relatively low cost.

The tester may incorporate a second load cell to provide an indication of the displacement of the loaded member.

Accordingly, in a third aspect of the present invention there is provided a portable tester for use in the in situ tensile testing of a member secured in a body, wherein the tester incorporates means for applying a tensile load to a load cell, and the load cell transmits said applied tensile load to the member and the load cell provides an indication of the applied load, and further including a displacement indicating device to provide a corresponding indication of the load-induced displacement of the loaded member.

The displacement indicating device may be a second load cell. Preferably, this device is configured relative to the said first load cell so as to provide a related indications of the load applied and the corresponding induced displacement. For example there may be first and second load cells which are mechanically and/or electronically coupled together.

Preferably, the tester is operable to continuously apply a variable e. g. progressively increasing and/or decreasing load and the load cell and displacement indicating device are configured relative to each other so as to provide an indication of the varying load applied and an indication of the corresponding varying displacement.

With this arrangement, load v displacement measurements can be measured accurately and conveniently by the tester, without repeated stop/starting of the load application.

It may also be desirable to obtain member displacement readings. Accordingly, In a fourth aspect of the present invention there is provided a portable tester for use in the in situ tensile testing of a member secured in a body, wherein the tester incorporates means for applying a tensile load to the member, and a displacement indicating device provides an indication of the load-induced displacement of the member.

The displacement indicator device may be a load cell.

The load cell may take any suitable form and may comprise an elongate shaft or a portion of an elongate shaft. However, where the load cell is to provide an indication of the applied load, preferably this is configured such that the force is applied axially of the shaft.

Preferably, the load cell providing load indications, includes a portion interengageable with a corresponding portion of the means for applying the load and may include a threaded portion for inter-engagement with a corresponding threaded portion of the load application means such that rotation of the load application member results in the application of an axial force on the load cell. Preferably this load cell also incorporates means for releasably securing the load cell to the fixing.

Preferably, the load cell incorporates means for providing an indication of the strain induced by the applied load which may comprise of one or more strain gauges which may be attached to a portion of the shaft.

The or each strain gauge may take any suitable form and may incorporate a Wheatstone bridge for measuring the resultant change in electrical resistance. Alternatively, other stress or strain measuring means could be used, such as a pressure transducer, piezoelectric measuring devices, linear variable differential transformers (LV. D. T's).

Where the tester is as according to the third aspect of the invention, the load cell may be coupled to the displacement indicating device so that the displacement indicating device is operable to provide an indication of the

displacement of the load cell, and therefore the member, under the applied load. In one embodiment, the device is a second load cell and displacement of the first load cell is detected by the second load cell which gives an indication of the displacement of the first load cell.

The corresponding movement in the second load cell may be indicated using a strain gauge arrangement.

The second load cell may take any suitable form and may be in the form of a flexible member which may bend in response and in correspondence to displacement of the first load cell, wherein such bending can provide an indication of the displacement. The second load cell may be biassed eg using a spring means to bend in correspondence with the displacement of the first load cell.

The means for applying the load may take any suitable form and may incorporate a handle or handles so as to allow manual rotation of the load application means.

Preferably the tester incorporates processing means for processing the above mentioned indications so as to provide a required measurement.

Thus, resistance readings taken from the strain measuring means may be converted to provide force, displacements or other readings as required.

The tester may incorporate display means for displaying data which may include for example said indications and or measurements obtained by the processing of such indications.

Preferably the tester incorporates connection means e. g. terminals for coupling the tester to a data processing device e. g. computer. With this arrangement the results of tests can be downloaded and evaluated further, eg using dedicated stress analysis software packages.

A further problem identified is that a member may be secured in a body with a certain amount of allowable positional adjustment or'play'and initially on application of a force, the fixing will shift to a position where there is no further play. Readings taken whilst the fixings shift are not representative and normally have to be identified from the test results and then be disregarded.

To this end, the tester may include means for zeroing (e. g. using a zeroing potentiometer) any of the displayed readings. With this provision for ad hoc re-calibration, initial positional adjustment of the fixing under load can be easily accounted for.

The tester may be mains or battery operated, and may be rechargeable using an integral or removable battery unit.

The tester is preferably housed in a protective housing unit which may be oil and/or waterproof and provides for electrical and shock insulation of the internal components.

Preferably the housing is formed from a durable flexible material e. g. rubber, synthetic rubber, plastics The invention will now be described further by way of example only

and with reference to the accompanying drawings in which : Fig. 1 is a part sectional side view of a tester according to the invention, the tester shown in a resting (unloaded) state; Fig. 2 is a diagrammatic plan view of the tester of Fig. 1; Fig. 3 is a part sectional side view of the tester in Fig. 1, the tester shown in a testing (loaded) position; Figs. 4-8 are diagrammatic views of the component parts of the load cell frame.

Fig. 9 is a schematic diagram of the electric circuitry of the tester of fig 1; Fig. 10a is a plan view of the second load cell as used in the tester of Fig. 1 and Fig. 1 Ob is a side view of the second load cell of Fig. 1 osa.

Referring to the drawings, a tester 1 is shown the tester being for the in situ tensile testing of a member 34 (which could be a wall reinforcing wall tie, cavity wall tie, roofing screw, structural anchor) secured to a body 35 e. g. a floor, wall ceiling or the like.

The tester 1 comprises a first load cell 2 and a second load cell 3 both mounted in a frame 4 having a box construction (the components of which are shown more clearly in figures 4-8).

The first load cell interengages with load application means 6 by means of a threaded portion which engages with a corresponding threaded

end portion 2c of the load cell 2. The load application also incorporates a handle 7 to allow for manual rotation of the load application means.

The frame is housed in a moulded rubber housing 8. Also within the housing is a power supply comprising are-chargeable 9 volt (PP3) battery 10 with charging terminals 32 and mains supply terminal 33 (for mains power supply), processor means 12, display means in the form of an LCD display 14 and an array of user operable buttons coupled to the processor means 12, power supply.

The first load cell 2 comprises an elongate metal cylindrical shaft 2 which has one threaded end portion 2c (thread not shown) and an opposite end portion 2b configured for secure but releasable connection with the fixing. (This end portion may e. g. have a threaded portion).

The first load cell is received by the frame 4, so as to allow only axial movement of the cell within relative to the frame 4. The frame has a return spring 28 located between a wider portion of the shaft and a second load cell (described below).

A portion of the shaft at 2a has a reduced cross-section to provide an active section of the load cell and bonded to this section with epoxy adhesive are two multi-element foil strain gauges 16 (shown schematically) in a full Wheatstone bridge configuration (i. e. containing four active strain gauges and no dummy resistors providing double sensitivity with full temperature compensation) and sealed with weatherproofing lacquer.

The whole of the Wheatstone bridge (of the gauges) is attached to the shaft section to help with temperature compensation strategy. The strain gauges are therefore selected to be of sufficient size to be easily attached whilst being large enough to ensure high levels of heat dissipation so avoiding self heating and its inherent errors.

The multi-element grid of each gauge is selected so that the active gauge in each half of the bridge is aligned along the axis of the applied force with the other (dummy) gauge being aligned perpendicular to this. This allows for use of the full bridge which covers only a small area of the metal giving enhanced temperature compensation characteristics.

The second load cell as shown more clearly in Figs. 10a and 10 b, comprises a flexible rectangular metal plate with side flanges 22 and 24 to distance the cell from the back plate and a central circular aperture 26 dimensioned for receipt of the elongate shaft 2.

The second load cell is located within the frame as shown in Figs. 1 and 3 between the return spring 28 and the frame end plate 29 (see Fig. 5) and the elongate shaft of the first load cell received in the aperture 26. The plate is flexible in the direction of the longitudinal axis of the first cell and bears on strain gauges (not shown) in a Wheatstone bridge configuration in a similar manner as described above. The strain gauge arrangement is disposed to provide an indication of the strain in the plate due to bending in this direction (which, as will be made clear from the description below, will

be used to measure the axial displacement of the second cell and therefore the fixing member).

The strain gauges of the first and second load cells are connected via wires to the processing means 12 incorporating electronic processing units which are schematically shown in Fig. 9.

In practice a strain gauge bridge provides a low output and therefore the processing means 12 incorporates a signal amplifier to provide sufficient signal gain, so that the full scale bridge output matches the input scale of the following stage which is an analogue to digital converter (ADC). The amplifier must have a differential input as the signal is developed across the bridge and not to ground. Also the amplifier has a high input impedance, so as not to load the bridge, good stability, linearity and very importantly, a high common mode rejection ratio (CMRR) which is an important parameter in differential amplifiers. Ideally such amplifiers respond only to the difference between the two input terminals and reject the pickup and ground voltages that appear in phase on both signal lines. A suitable bridge amplifier may be the Burr Brown INA118.

Importantly also, both inputs must be 1. 1V above ground for linear operation, explaining the presence of the tail resistor in the low side of the bridge. The keeps the input within the common mode range of the amplifier and serves to limit the bridge current even further. A balance potentiometer across the bridge output is used to offset any zero load voltage present.

The ADC preferably is a low power device (as with all electrical components of the tester) and interfaces directly with the LCD 14 without the need for further components. The ADC has decimal point drivers to allow the display to be in kN, differential inputs and an on-chip voltage reference.

The on chip reference provides a voltage to 2.8V less than the positive supply (nominally 9V). The reference is tied to the negative signal input of the ADC and the Vref pin on the bridge amplifier. This means the output of the amplifier is not referenced to ground but to the reference voltage.

Because the ADC input is differential it does not notice that the inputs are floating above ground. Therefore the bridge amplifier can act as an ideal differential amplifier. Also as the battery terminal voltage drops, the reference voltage drops with it and the whole system is immune to this until the end of life voltage is reached. If a whole circuit has a current consumption of approximately 9.12 mA, a Duracell Procell PP3 would give approximately 55 hours use.

The processing units are operable to convert the strain gauge readings (the electrical resistance) to readings of applied force (in the case of the first load cell) and displacement (in the case of the second load cell.

User operable buttons 30 are also provided in connection with the electrical circuitry so as to allow the user to activate the tester, zero the displacement and/or force readings if necessary.

This is necessary as many members are mounted with a certain amount of unintentional but unavoidable freedom of movement or'play'and an initial application of force will produce a certain amount of positional adjustment and any force/displacement readings taken at this point would not be representative and should be disregarded. The zero button allows for re-calibration of the readings to take account of this.

The device functions as follows: The end portion 2b of the load cell 2 is connected to the exposed end of the member 34 (shown only in figure 1) and the locking plate 40 then prevents lateral shift of the member 34. The clamping face 42 of the tester abuts the body surface and the abutting surfaces are sufficiently distanced from the member so that the reaction forces do not interfere with the test.

The handle 7 is turned, and the screw action of the threaded portion of the load application means with the threaded end portion of the load cell 2 produces a pulling or tensile force on the load cell 2 with is transmitted to the member 34. The strain induced into the load cell 2 is then detected by the strain gauge arrangement and the signal from this bridge is processed by the processing means 12 to display the force necessary to induce such a strain.

The processing is based on Hookes law for elastic deformation: E = ole where: a = stress = Force/Area and e = strain (indicated by the strain

gauge) and E is the Modulus of Elasticity, a mechanical constant for a given metal.

The tester preferably incorporates an anti-torsion device to prevent and twisting of the load cell 2 under load. This could be a simple screw located transversely of the load cell connecting the cell to the frame, the screw located to allow longitudinal movement of the cell 2. Alternatively the locking plate may function to lock the member to be tested and prevent any twisting of the member about the longitudinal axis.

The frame 4 also functions the allow transmission of the applied load along the shaft 2 with producing any bending moments in the shaft.

Twisting and bending of the shaft would produce non axial forces and resultant strains which would interfere with the test.

The second load cell 3 functions by bending under the compression force of the spring 28 which is compressed as the first load cell 2 is displaced under the applied load. The bending introduces a strain in the second load cell 3 and this provides a linear indication of the displacement of the first load cell 2 which is then processed by the processing means 12 to provide a displacement reading corresponding to the load induced displacement of the member.

The load is applied continuously and progressively increased and at the same time, both load and corresponding load-induced displacement readings are taken to conveniently provide accurate load v. displacement

data, which is so valuable for the structural analysis of many constructions incorporating e. g. wall fixings and the like.

Readings and calculated load/displacement (stress/strain) values are displayed by the LCD display 14 (which is coupled to the processing unit this can display).

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.

Claims

CLAIMS.

1. A portable tester for use in the in situ tensile testing of a member secured in a body, wherein the tester incorporates means for applying a tensile load to a load cell, and the load cell transmits said applied tensile load to the member and provides an indication of the applied tensile load.

2. A portable tester as claimed in claim 1, wherein the tester further comprises a displacement indicating device to provide a corresponding indication of the load-induced displacement of the said member.

3. A portable tester as claimed in claim 2, wherein the load cell and displacement indicating device are mechanically coupled together.

4. A portable tester as claimed in claim 2 or claim 3, wherein the load cell and displacement indicating device are electronically coupled together.

5. A portable tester as claimed in any of claims 2 to 4, wherein the tester is operable to continuously apply a variable load and the load cell and displacement indicating device are configured relative to each other so as to provide an indication of the varying load applied and an indication of the corresponding varying displacement.

6. A portable tester as claimed in any preceding claim, wherein the load cell incorporates means for providing an indication of the strain induced by the applied load.

7. A portable tester as claimed in any of claims 2 to 6, wherein the displacement indicating device is a second load cell.

8. A portable tester substantially as described herein with reference to the accompanying drawings.

9. A portable tester for use in the in situ tensile testing of a member secured in a body, wherein the tester incorporates means for applying a tensile load to the member, and a displacement indicating device provides an indication of the load-induced displacement of the member.

10. A portable tester as claimed in claim 10, wherein the displacement indicating device is a load cell.