WO1997047956A1

WO1997047956A1 - Measurement of bundle strength of fibres

Info

Publication number: WO1997047956A1
Application number: PCT/AU1997/000359
Authority: WO
Inventors: Shouren Yang; Neil Blenman; Mark De Ravin
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1996-06-07
Filing date: 1997-06-06
Publication date: 1997-12-18
Also published as: AUPO036196A0

Abstract

This invention concerns an apparatus for the measurement of the bundle strength of fibres, and in a further aspect a method for measuring the bundle strength of fibres. The apparatus and method have application to testing the fibre strength of fibre bundles, and in particular but not exclusively to the measurement of wool fibre tensile strength. The apparatus comprises four clamps (9, 12, 16, 17) arranged along a common axis and operable independently of each other. One of the clamps, the 'fixed' clamp (9), is positioned at one end of the apparatus, and is movable along the common axis relative to the remainder of the apparatus. The 'fixed' clamp (9) and the next adjacent 'moving' clamp (12) may be used to secure fibre bundles during measurement of the bundle strength. The remaining two clamps are used exclusively for preparation of sample bundles. A 'combing' clamp (17) at the other end of the apparatus is used for combing fibres, and a 'pre-tensioning' clamp (16) between the 'combing' clamp and the 'moving' clamp is used for pre-tensioning the fibres.

Description

"MEASUREMENT OF BUNDLE STRENGTH OF FIBRES" Technical Field

This invention concerns an apparatus for the measurement of the bundle strength of fibres, and in a further aspect a method for measuring the bundle strength of fibres. The apparatus and method have application to testing the fibre strength of fibre bundles, and in particular but not exclusively to the measurement of wool fibre tensile strength. Background Art

Fibre strength (tenacity) has been determined either for single fibres or fibre bundles. The single fibre test method is extremely time-consuming and not practical in a commercial mill.

A method of bundle strength measurement has been available since the 1950's and is detailed in an IWTO test method. It involves numerous stages of cutting, combing, mounting and transferring of clamps. The techniques of tightening the jaws and applying pre-tension are complex and time consuming.

In summary this method is slow, has low accuracy, and has operator dependent problems which lead to wide variations in results between different laboratories, and poor repeatability. As a result, the measurement of tensile strength of wool fibres in top is still not widely practised in the industry. Unlike the cotton industry, fibre strength is not included in wool top specifications.

There is also a lack of recognition for the degree of variation in wool fibre tensile strength, and its importance to yarn strength and spinning performance.

With a gauge length of 3.2 mm, variations in bundle tenacity from 10.6cN/tex to I5cN/tex for South African wools and from ll.lcN/tex to 14.6cN/tex for US Standard tops have been reported. A recent study at CSIRO has shown that there is a large variation in wool fibre strength. Differences of around 76% in fibre bundle tenacity and breaking extension between the extremes were observed.

Such large variations in wool fibre bundle tensile strength can have a significant influence on yarn strength and spinning performance. It has been observed that yarn strength varies in proportion to fibre bundle strength and that a 5.3% loss in bundle tenacity, due to dying damage, doubled the spinning end's downtime. Disclosure of the Invention

In a first aspect, the present invention provides an apparatus for the measurement of the bundle strength of fibres, comprising: four clamps arranged along a common axis and operable independently of each other. One of the clamps, the "fixed" clamp, is positioned at one end of the apparatus, and is movable along the common axis relative to the remainder of the apparatus. The fixed clamp and the next adjacent clamp may be used to secure fibre bundles during measurement of the bundle strength. The remaining two clamps are used exclusively for preparation of sample bundles. This arrangement provides the significant advantage that it allows a sample to be prepared for testing and to be tested within the same apparatus, and without any interference with the sample between the preparation and testing phases.

A further advantage arises because the apparatus is able to test bundles of larger size than the test specified by the IWTO. This in turn allows the balance used to weigh the sample after testing to be less sensitive, and in consequence significantly less expensive.

The apparatus lends itself to computer control. This in turn removes the propensity for operator dependent problems, and improves the repeatability of the measurements.

The clamps generally comprise a movable jaw which opens and closes against a block or platform. The jaw in each case may be coated with polvurethane rubber of required hardness, and the surface of the block or platform is typically metallic with a smooth finish. The first pair of clamps, the "fixed" clamp and the "moving" clamp mav be pneumatically operated, and hold the fibre bundle securely during the testing phase. The other pair are spring loaded, and may be independently adjusted to apply appropriate clamping force. For instance, the "combing" clamp at the opposite end of the apparatus from the "fixed" clamp may be adjusted to securely hold fibre bundles while they are combed. The next clamp, the "pre-tension" clamp, may have its clamping pressure adjusted lo allow some fibre slippage, so that all the fibres in a bundle are pre-tensioned to the same degree.

The moving clamp, pre-tension clamp, and combing clamp may all comprise jaws which close against surfaces of a common platform, whereas the "fixed" clamp may close against a surface of a separate block. The platform and block may be mounted on a mechanism which allows axial movement of the fixed clamp relative to the remainder of the apparatus.

The apparatus is, in use at least, mounted into a tensile tester. The block of the fixed clamp may be adapted for connection to the load cell, and the platform may be adapted for connection to the base.

In a second aspect, the invention provides a method for the measurement of the bundle strength of fibres, comprising the following steps:

(a) clamping a fibre bundle at one end: (b) combing the fibre bundle in the direction away from the clamped end to remove fibre ends from a test zone;

(c) clamping the fibre bundle at the other end;

(d) combing the fibre bundles in the opposite direction, again to remove fibre ends from a test zone; (e) clamping the fibre bundle at a first test zone end and at a second location between the ends: then

(f) pre-tensioning the fibre bundle clamped between the first end and the second location;

(g) clamping the pre-tensioned fibre bundle at a third location intermediate the first zone end and the second location to define the test zone between the first test zone end and the third location: and (h) driving the first end and the third location apart from each other until fibre bundle breakage occurs.

From the force required to break the fibre bundle (peak breaking force) a measurement of the bundle strength can be made.

The fibre bundle may be weighed, preferably after step (h). and the weight may be taken into account along with any other measurements, to calculate the strength.

The operations of the method, including the weighing may be under computer control

In a preferred embodiment, immediately before step (h). the fibre ends protruding from the outside edges of the clamps at the first end and the third location are trimmed.

It is preferred that the measuring method of the present invention is conducted in the measuring apparatus of the present invention. In this case, the fibre bundle is initially clamped, step (a), in the combing clamp for the first combing, and then clamped, step (c). in the fixed clamp for the second combing. Pre-tensioning, step (f), takes place with the fibre bundle clamped between the fixed clamp and the pre-tensioned clamp, and testing, step (h), takes place between the fixed and moving clamps. Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a pictorial view of an apparatus embodying the first aspect of the present invention; Figures 2(a) to 2(h) are schematic diagrams illustrating the sequence of steps of a method embodying the second aspect of the present invention:

Figures 3(a).(b),(c) and (d) are histograms of the co-efficients of variation (CV) of bundle tenacity and breaking extension, 3(a) and 3(c) are results obtained from the conventional method and 3(b) and 3(d) are results obtained from a method embodying the present invention.

Figure 4(a) and (b) are graphs showing the observed tenacity against the calculated tenacity without and with bundle tenacity taken into account, respectively.

Best Modes of Carrying out the Invention Referring first to figure 1, the measuring apparatus 1 comprises four clamps indicated generally in the regions 2,3.4 and 5. arranged along a common axis. Two rods 6 and 7 run the length of the device, parallel to the common axis, and constrain relative movement between the clamps to the axial direction. The "fixed" clamp 2 comprises a jaw block 0 onto which a clamping jaw 9 opens and closes. To the left of jaw block 8 is a fitting 10 which in use is mounted to the load cell of a tensile tester (not shown).

Proceeding along the assembly, "moving" clamp 3 comprises a jaw block 11 onto which a clamping jaw 12 opens and closes. Clamping jaw 12 is identical to clamping jaw 9.

At the other end of the device is another block 13 having a fitting 14 which in use is mounted to the base of a tensile tester.

Between block 13 and block 11 is the "pre-tension" clamp 4 and the "combing" clamp 5, these comprise a single block 15 on which two identical clamping jaws 16 and 17 are mounted. Blocks 11. 15 and 13 are locked to the rods 6 and 7 whereas block 8 is able to ride along those rods.

The upper surfaces of the blocks provide a smooth metal horizontal surface. Each of the clamping jaws 9. 12, 16 and 17 have a polyurethane rubber pad 18.19.20 and 21 respectively, of specified hardness. The rubber hardness is chosen as a compromise between hardness for wear and distortion resistance, and softness to accommodate a range of bundle sizes without slippage.

The clamping jaws 9 and 12 are for clamping a fibre bundle during tensile test, and they are pneumatically operated to ensure sufficient force is supplied for that purpose.

The remaining jaws 16 and 17 are used for sample preparation, and these jaws are spring loaded with four small coil springs providing clamping force. They are independently adjusted to apply appropriate clamping force. The combing jaw 21 is adjusted to clamp a fibre bundle securely during combing. However, the pre-tension jaw 16 is adjusted to allow some fibre slippage between that jaw and the upper surface of block 15. so that all fibres in a particular bundle are pre-tensioned to the same degree. Jaws 16 and 17 are forced down by use of a lever (not shown). In use. all the clamping jaws are opened and the upper surfaces of blocks 11.15 and 13 provide a horizontal platform spaced apart from the coplanar horizontal surface of block 8. Blocks 8 and 11 are moved relative to each other by a distance determined by a spacer (not shown) to establish the gauge length for the test. After squaring the wool top. a bundle of wool fibres are taken up from the top using a grip with leather lined jaws, and the bundle is laid onto the horizontal surface.

The combing jaw 17 is then clamped down onto platform 22, as shown in figvire 2(a), and the bundles are combed from right to left using a parallelising comb to remove loose fibres: as shown in figure 2(b). The fixed jaw 9 is then clamped down and the combing jaw 17 released. Combing then takes place from left to right: as shown in figure 2(c). The double combing action ensures that no fibre ends will be between the fixed jaw 9 and the combing jaw 17: this ensures the same pre-tensioning is applied to all fibres involved in the breaking test. Pre-tensioning jaw 16 is then clamped down, and pre-tensioning is applied by moving blocks 8 and 13 apart: as shown in figures 2(d) and 2(e). The moving jaw 12 is then closed to define a test zone between jaws 9 and 12. Fibres projecting beyond the outer edges of jaws 9 and 12 are trimmed with a sharp razor blade, and then the entire apparatus is mounted onto the load cell of a tensile tester. The tensile tester drives blocks 8 and 11 apart until the fibre bundle breaks. The force applied is monitored, and the peak force is recorded.

The tensile tester is operated under control of a computer, and the test data is transmitted down a serial communications line from the tensile tester to the computer. After the fibre bundle is broken the apparatus is removed from the tensile tester and the jaws opened. The broken fibres are then weighed on a digital balance also under the control of the computer, and the weight measurement is also recorded by the computer. From these measurements the strength of the bundle is determined. Experimental Results

The bundle weight specified is lOOO± lOO tex. which corresponds to approximately 2300 fibres in the bundle cross-section for 20μm wool. The bundle weight influences the results. Therefore it is important to control the bundle weight to within a specified range. Generally, the narrower the range, the smaller the testing error.

Fibre bundle strength was measured on twenty six top samples. The testing speed was 20 mm/min and the gauge length was 3.2 mm. Measurements were repeated for two of the twenty six samples because the coefficients of variation of bundle tenacity were slightly higher than 3%. The average of the coefficients of variation of bundle tenacity is 1.9%. For bundle breaking extension, however, the coefficients are higher with an average of 4.1%.

The distributions of the coefficient of variation of bundle tenacity and bundle extension are plotted in figures 3(a)-(d) together with tire results from a previous study, where the bundle tensile strength of thirty one wool tops was measured on an Instron with Pressley jaws at a gauge length of 10 mm (7). Comparison of these figures show a significant improvement in the accuracy of bundle tenacity with the improved method. No clear improvement in breaking extension is observed. Direct comparison between the two is not strictly possible because of the different gauge length adopted in the two studies. However, the accuracy of the tenacity would have been expected to improve only marginally using the shorter gauge and the accuracy of the breaking extension should have decreased considerably.

Eight wool tops were retested under identical conditions to study the reproducibility of bundle tenacity with the new bundle tester. The mean difference between repeated measurements was 0.56% with a maximum of

1.26%. The method appears to give good reproducibility for bundle tenacity measurement.

Theory shows that yarn tenacity varies in proportion to fibre strength. Therefore a precise measurement of fibre strength is important to accurately predict yarn strength. In the following we give an example to demonstrate the improvement in yarn strength prediction with the bundle tenacity measurement.

A spinning trial has been carried out at CSIRO. Twenty one wool tops covering a range of fibre diameter and fibre length were processed into rovings and then spun into yarns with a range of counts giving thirty five and forty five fibres in the yarn cross-section. All yarns had a metric twist factor α = 90. Fibre bundle tenacity was measured as detailed above.

Linear multiple regression was performed to analyse the influence of various factors on yarn tensile properties. It has been found that for yarns with the same twist, variations in yarn tenacity are mainly determined by three factors: the ratio of mean fibre diameter to mean fibre length D/H, fibre bundle tenacity and yarn evenness. This is in agreement with yarn helical structure theory. Regression equations for yarn tenacity were derived with and without fibre bundle tenacity involved. It has been found that about 80% of the variations in yarn tenacity can be accounted for with only D/H and the predicted yarn tenacity involved, However, the percentage accounted for value increases to about 95% when fibre bundle tenacity is taken into account.

Yarn tenacity was calculated using the derived multiple regression equations. The calculated yarn tenacity against the observed are plotted in

Figures 4(a) and (b). Comparison of these two figures shows that the accuracy of vain tenacity can be significantly improved when fibre bundle tenacity is taken into account.

It should be pointed out that in order to derive more general yarn tenacity prediction formulae a proper yarn strength prediction model, which takes into account the yam helical structure and the effect of predicted yarn evenness, should be used rather than just the multiple regression method. This is because multiple regression results may depend on the population involved in a study and on mill specific processing conditions.

The trial results have shown that the present method is quicker than the conventional one because all of the sample preparation and most of the testing procedure are carried out on the jaws and associated transfer device, it is estimated that with further improvement the time for performing a bundle tensile test could be reduced by half compared with the conventional method. Because of the simplified testing procedure, it is also easier for a new operator to achieve the necessary skills.

The technique for applying pre-tension in the improved method is easier and superior to the conventional method. This should reduce the error associated with the fibre crimp related uneven or insufficient pre¬ tension problems. The double combing action removes all fibre ends between the jaws, which is also critical to the pre-tensioning of all fibres in the bundle.

The bundle weight specified in the IWTO standard is 350±50 lex. Due to the large capacity of the tensile test unit (25Kg) and the larger size and higher pressure of the jaws, it becomes possible to use a relatively large bundle weight for the tensile test. The bundle weight adopted in the new method is lOOO±lOO tex, which makes it impossible to reduce the requirement for the precision of a balance from 0.02 mg to 0.1 mg. This will bring the cost for a balance down from around A$20,000 to A$3.000. Furthermore, the new jaws are much more durable than the conventional Pressley jaws.

Whilst the test procedure is simplified, the consistency and precision of the test results are better than the conventional method. The preliminary trial results have indicated that it is not difficult for an operator to achieve the precision of 3% for coefficient of variation required by the IWTO standard with 10 tests per lot. In view of the larger specimen size, which involves about 2000 fibres in each test, it is possible to reduce the number of tests required by the existing method to achieve a reasonable measure of fibre bundle strength.

All testing and reporting of the results are computer-controlled which makes the bundle test quicker, easier and more accurate. Although the invention has been described with reference to particular embodiments, it should be appreciated that it may be embodied in many other forms. For instance, all the clamping jaws and associated devices may be built into a tensile testing machine to form an integral unit. All sample preparation and testing will then be performed on the same machine and it will no longer be necessaiy to take the apparatus onto and off of the tensile tester. In addition there may be separate jaw blocks for each clamp, and more than the fixed clamp may be moveable with respect to the others. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS :-

1. An apparatus for the measurement of the bundle strength of fibres, comprising: four clamps arranged along a common axis and operable independently of each other: wherein one of the clamps, the "fixed" clamp, is positioned at one end of the apparatus, and is movable along the common axis relative to the remainder of the apparatus.

2. An apparatus for the measurement of the bundle strength of fibres according to claim 1, wherein the fixed clamp and the next adjacent clamp, the "moving" clamp, are used to secure fibre bundles during measurement of the bundle strength.

3. An apparatus for the measurement of the bundle strength of fibres according to claim 2. wherein the remaining two clamps are used exclusively for preparation of sample bundles.

4. An apparatus for the measurement of the bundle strength of fibres according to any preceding claim, wherein the apparatus is controlled by computer.

5. An apparatus for the measurement of the bundle strength of fibres according to any preceding claim, wherein the clamps comprise a movable jaw which opens and closes against a block or platform.

6. An apparatus for the measurement of the bundle strength of fibres according to claim 5. wherein the jaw in each case is coated with polyurethane rubber of required hardness, and the surface of the block or platform is metallic with a smooth finish.

7. An apparatus for the measurement of the bundle strength of fibres according to any preceding claim, wherein the first pair of clamps, the

"fixed" clamp and the "moving" clamp are pneumatically operated, and hold the fibre bundle securely during the testing phase.

8. An apparatus for the measurement of the bundle strength of fibres according to claim 7. wherein the other pair of clamps are spring loaded, and are independently adjusted to apply appropriate clamping force.

9. An apparatus for the measurement of the bundle strength of fibres according to claim 8, wherein the "combing" clamp at the opposite end of the apparatus from the "fixed" clamp is adjusted to securely hold fibre bundles while they are combed.

10. An apparatus for the measurement of the bundle strength of fibres according to claim 8. wherein the next clamp, the "pre-tension" clamp, has its clamping pressure adjusted to allow some fibre slippage, so that all the fibres in a bundle are pre-tensioned to the same degree.

11. An apparatus for the measurement of the bundle strength of fibres according to any preceding claim, wherein the "moving" clamp, "pre-tension" clamp, and "combing" clamp all comprise jaws which close against the surface of a common platform, whereas the "fixed" clamp closes against a surface of a separate block.

12. An apparatus for the measurement of the bundle strength of fibres according to claim 11 wherein, the platform and block are mounted on a mechanism which allows axial movement of the fixed clamp relative to the remainder of the apparatus.

13. A tensile tester comprising a base and a load cell, and having the "fixed" clamp of an apparatus according to any preceding claim connected to the load cell, and the remainder of the apparatus connected to the base.

14. A method for the measurement of the bundle strength of fibres, comprising the following steps:

(a) clamping a fibre bundle at one end;

(b) combing the fibre bundle in the direction away Irom the clamped end to remove fibre ends from a test zone; (c) clamping the fibre bundle at the other end;

(d) combing the fibre bundles in the opposite direction, again to remove fibre ends from a test zone;

(e) clamping the fibre bundle at a first test zone end and at a second location between the ends: then (f) pre-tensioning the fibre bundle clamped between the first end and the second location; (g) clamping the pre-tensioned fibre bundle at a third location intermediate the first zone end and the second location to define the test zone between the first test zone end and the third location; (h) driving the first end and the third location apart from each other until fibre bundle breakage occurs, and measuring the force required to break the fibre bundle.

15. A method for the measurement of the bundle strength of fibres according to claim 14, wherein the fibre bundle is weighed after step (h) and the weight is taken into account to calculate the strength.

16. A method for the measurement of the bundle strength of fibres according to claims 14 or 15, wherein the steps of the method are under computer control.

17. A method for the measurement of the bundle strength of fibres according to claim 14, wherein immediately before step (h), the fibre ends protruding from the outside edges of the clamps at the first end and the third location are trimmed.

18. A method for the measurement of the bundle strength of fibres according to any one of claims 14 to 17 wherein, the measuring method is conducted in the measuring apparatus of any one of claims 1 to 12.