WO1987000919A1

WO1987000919A1 - Tensile impulse tester

Info

Publication number: WO1987000919A1
Application number: PCT/GB1986/000474
Authority: WO
Inventors: Raymond Bewick Gilmore
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1985-08-06
Filing date: 1986-08-05
Publication date: 1987-02-12
Also published as: GB2188439B; EP0231323A1; GB8519692D0; GB2188439A; GB8708124D0

Abstract

A device, principally intended as an adaptor for universal testing machines enables tensile impulse tests to be performed by such machines on textile specimens and the like. The device utilizes compression springs (17, 18) which are precompressed, latched then released to displace a movable crosshead (10) from a static crosshead (11). Specimen grips (28, 29) on each crosshead are detachable for interchange and ease of mounting the specimen. Pretensioning is provided by screw adjustment of the specimen grips. Piezoelectric transducers (51) give a measure of specimen tension at each end thereof for assessment of shock wave effects.

Description

TENSILE IMPULSE TESTER

This invention is a tensile impulse tester of a construction suitable for the tensile impulse testing of textile materials especially those high strength textile materials used in the aeronautical field for suspension lines and restraint harness etc. The invention may also be useful for performing tensile impulse tests on other additional materials.

Textiles may be defined adequately for the purpose of this specification as any product made from fibre filament or yarn by interlacing. Such textiles as cords ropes ribbons and webbings may be subjected in use to sudden loads and it is consequently of value to secure a clear understanding of their behaviour under these conditions. One area of especial interest concerns the relationship between the properties of the textile and those of the component fibres or tows.

Attempts have been made to simulate the load-time behaviour of real materials under tensile impulse loading from a knowledge of fibre properties which are more readily obtained than those of the textile product- but the weaving or other interlacing process involved in manufacture Introduces variables whose effects cannot be readily predicted. Such behaviour modelling could be facilitated by improved knowledge of product properties.

The testing of textiles under impulse conditions is known. One method known for testing lower strength specimens such as yarns utilises a falling weight or pendulum to provide an axial or transverse impact. Whilst this method is convenient for impulse testing at lower energy levels it is readily adaptable for use at the energy levels, say in the KJ band, required for some aeronautical specimens because of physical size constraints. Another technique which also has similar -limitations uses a driven flywheel which is suddenly connected to a testing mechanism by operation of a clutch. One method which has been used for performing transverse impact tests on high strength textile specimens utilizes a ballistic projectile. This method requires a ballistic firing facility and is consequently not readily adaptable for laboratory use. The standard testing equipments available for testing of metals are not ammenable to use in the performance of tensile impulse tests on high strength textile specimens. Axial testers of screw actuated and servo hydraulic varieties are available but do not provide a sufficient combination of speed and extent of travel for the current purposes. Impulse testing of metals is usually performed under transverse impact conditions using eg a falling weight or pendulum system and a notched specimen, but such testing is not considered to be germane to this application. The metals art also embraces various stored energy systems which have been intended for tensile impulse test usage. It is considered that such of these other stored energy systems as are presently known to the Applicant are not conveniently adaptable to meet the requirements for which the present Invention was devised.

The invention claimed herein is intended to provide a device for conveniently and accurately performing tensile impact tests on materials such as textiles in a device which is compact yet offers high energy levels (say 1 KJ +) and sufficient travel to accommodate strains at break of 30 percent or more. The device is especially intended for use In conjunction with universal testing machines which are readily available from trade sources and are widely held as existing laboratory equipment. The present invention is a tensile impulse tester for materials such as textiles, comprising two crosshead members one of which is unidirectionally displaceable with respect to the other, compression spring means operative to urge apart the two crosshead members, latch means operative to hold the two crosshead members until released at a specific displacement relative to each other at which specific displacement the compression spring means is compressed to a predetermined degree sufficient to provide the impulse required for the test, a specimen attachment on each crosshead member such that thereby a specimen may be affixed between the two crosshead members at least one of the specimen attachments being adjustable with respect to its crosshead member to enable the specimen to be pretensioned, and means for monitoring the results of a test performed thereby. As mentioned already, the tester of the invention has special utility when used in combination with a universal test¬ ing machine. For the purpose of this specification a universal testing machine is defined as any machine incorporating a rigid loadfra e providing a static loadpoint, and a movable driven crosshead. The tester of the invention can be attached to the loadframe and located within this framework in order to utilize the force available at the driven crosshead to compress the compression spring means and thereby allow the device crosβheads to be latched. It is not intended that the invention should be circumscribed by this special utility for the device can be used without a universal testing machine and it is more broadly claimed.

It is desirable that each specimen attachment can be detached from its respective crosshead to enable interchange of such attachments for the purpose of selecting an appropriate attachment for the specimen to be tested. Preferably at least one of the specimen attachments is mounted upon its respective crosshead by a screw-threaded mounting for adjustment purposes. Adjustment is desirable to accommodate the varying spacings resulting from different specimen attachment configurations and also to ensure that the specimen can be pre-tensioned after mounting to avoid wastage of the crosshead travel and its impulse through slack or slippage in the specimen. One or both specimen attachments can be screw mounted in a mounting which has a turnbuckle section for ease of adjustment. One embodiment of such an arrangement is a retractable mounting of three sections with two screwed joints of opposite hand.

The aforesaid means for monitoring the results of a test performed by the tester of the invention can be any means suitable for obtaining the parameters of interest. The load/time history, of the test is of particular interest to the Applicant and this can be derived by incorporating a load transducer eg one of the piezoelectric variety, into the tester. The tester is preferably provided with two such transducers, one for each end of the specimen, in order for assessment of dynamic phenomena within the specimen under test. The tester could include one or more accelerometers in place of or in addition to a load transducer, in^"order to establish a plot of acceleration, velocity, or elongation against time. For use of the tester In quality control duties a simple arrangement adequate to indicate the instant of breakage of a specimen might be sufficient.

It is desirable that the impulse available from the tester can be adjusted in order that the tester can accommodate a wide variety of textile test specimens. It is usual in impulse tests to use an impulse level significantly above the minimum required to break the specimen. Some degree of adjustment may be provided by adjustment of the relative displacement of the two crossheads at their latchable position - this affecting the degree of compression experienced by the compression spring means. However any reduction in impulse level provided by such adjustment is achieved at the expense of reduced movement of the crosshead during the test and this is likely to be a limit¬ ing factor_* As a fine adjustment of impulse level is unlikely to be required for conventional tests adjustment can be provided by interchange of or supplement to the springs comprising the compression spring means.

In one preferred embodiment of the Invention the uni¬ directional relative displacement of the crossheads is ensured by the provision of guide means. This utilizes rigid elongate members affixed to one crosshead (which remains stationary) and these co-operate with bearings affixed to the other crosshead. In such an embodiment the compression spring means can conveniently comprise compression springs encompassing the elongate members and engaging both crossheads. It is convenient also to provide a balanced array of four elongate members each with overfitting compression springs in order to avoid torsional forces in the device. However avoidance of torsion need not be a limiting factor if the device is constructed sufficiently robustly and incorporates a guide means to ensure unidirectional displacement of the crossheads. In this specification the term "compression spring means" has been used to define an arrangement reliant upon compression springs in those instances where it is desired not to be restricted by spring numbers or configuration.

A description of an embodiment of the invention is provided below by reference to the accompanying drawings, of which:

Figure 1 shows a general view of the device; Figure 2 is a longitudinal section through a specimen attachment; and,

Figure 3 is a longitudinal section through a load transducer.

Reference is now made to Figure 1 of the drawings. The tester illustrated has a movable crosshead 10 and a fixed crosshead 11. The latter crosshead is mounted upon a baseplate 12 by which the whole tester can be anchored, eg by attachment to the loadframe of a universal testing machine - not shown. The movable crosshead 10 is constrained by guides 13-16 to conform to a linear path which is perpendicular to both cross- heads. Each of the guides 13-16 comprises a rigid rod 13A-16A mounted upon the fixed crosshead 11 and a linear motion bearing within an accommodating boss (13B-16B) mounted upon the movable crosshead 10. At its free end each of the guide rods 13A-16A has a buffer 32 to limit movement of the movable crosshead 10. These buffers 32 are detachable from the guide rods.

A compression spring means serves to urge the movable crosshead 10 away from the fixed crosshead. This means comprises an assembly of compression springs of size and number chosen to produce a desired impulse level. Each spring fits over one of the guide rods 13A-16A and a concentric assembly of two springs can be fitted on each guide rod. The Figure shows two sets of concentric springs at 17A and B and 18A and B. This is not representative of a realistic configuration but is ^" drawn in this way to avoid obscuration of other detail. In practice a balanced array of four single springs, or of eight springs in four pairs, or possibly a diametric array of two or two pairs will be used. For safety and-also for the prevention of snaking, the compression springs are enclosed within retain- ing tubes (not shown) which are mounted on the fixed crosshead 11. When the springs are used in concentric pairs, springs of opposite hand are used as illustrated to avoid entanglement.

A latch means 24 is attached to the baseplate 12 and this engages with a latchpin 23 attached to the movable crosshead 10. The latchpin 23 Is located on a bridge 22 which is affixed to the movable crosshead 10 by two rods 21. Each rod 21 passes through an accommodating hole in the fixed crosshead 11. The latch means 24 serves to hold the movable crosshead 10 at a fixed displacement in relation to the fixed crosshead 11 against the action of the compression spring means with the compression springs compressed to the degree required to provide the impulse for the test. The latch means 24 provides automatic mechanically initiated engagement with the latchpin 23 and a positive hold until a release signal is given. The latch means 24 used is released upon receipt of an electrical signal by solenoid operation, but hydraulically pneumatically or mechanically operated alternatives could be used. It is essential that the latch means 24 provides an unfettered dis¬ engagement when release is required for safety reasons and also to enable the tests to be consistently performed. A safety handle 25 is provided and this locates within lugs 26 on the latch means. This handle 25 is inserted into the lugs 26, after engagement of the latch means 24 with the latchpin 23, to prevent unintended release of the device by physically restraining the bridge 22.

Upon each of the crossheads 10 and 11 there is provision for mounting a detachable specimen attachment shown at 28 and 29 respectively and between these a textile specimen 27 may be mounted for axial impulse test. Figure 2 illustrates this arrangement in detail with regard to one example of specimen attachment. Each of the crossheads 10 and 11 supports a screwed adjuster (respectively Indicated at 30 and 31) to which a specimen attachment (respectively indicated at 28 and 29)*may be fixed. The specimen attachment illustrated In Figure 2 is one designed to grip a ribbon or webbing textile specimen by a wedging friction grip. This grip attachment comprises a hollow holder 35, a wedge member 36, wedge counters 37 and a mounting member 38. Mounting member 38 provides a detachable connection to screwed adjuster (30, 31) via a pegged plug and socket joint.

A means for adjusting and pretensioning the specimen 27 is provided at each of the specimen attachments 28 and 29. This means comprises a screwed retractable assembly of three sections. The first section is a screwed rod 39 integral with the plug of the plug and socket joint and this rod 39 is screwed into an intermediate section which comprises an adjuster sleeve 31 which has a screw thread on both its inner and outer surfaces. The sleeve 31 is screwed into the third section of the mounting which comprises an internally screwed bush 50. Sleeve 31 incorporates a knurled flange 41 for adjustment of the mounting and a locknut 42.

Figure 3 illustrates the arrangement for measurement prior to and during test, of the tension within the test specimen 27. At each of the crossheads 10 and 11 there is a housing 50 for the specimen attachment adjuster sleeve 31. The arrangement at movable crosshead 10 Is shown. Housing 50 is attached to crosshead 10 by a rod 54 which passes through a hole in the crosshead. Between the housing 50 and the crosshead 10 there is a piezolectric load washer 51. At the opposite side of crosshead 10, rod 54 passes through a cylindrical spacer 52 and a terminal screwed portion of the rod 54 is engaged by a nut 53. Nut 53 is adjusted to compress the load washer 51 so that its datum position is at a suitable point within its operating envelope, this precompression being sufficient to ensure that - the load washer is under compression for the whole of the test. The tension within the specimen 27 reduces the compressive force on the load washer 51 which gives a corresponding change in output level. This output signal is displayed on a storage oscilllscope (the beam sweep of which is triggered by a latch release signal) and the display may be converted to a paper record by a pen recorder.

A test on a textile test specimen 27 is performed using the following procedure. Firstly compression springs are selected to provide a desired impulse level. Any number of springs from 1 to 8 could be utilised but usually a desire to avoid imbalance would dictate that a balanced array of an even number springs is used. The desired spring array is then mounted between crossheads 10 and 11 around the guide rods attached to the latter. At this stage the movable crosshead 10 is supported upon a travelling crosshead 19 of a universal testing machine by an adaptor 33 which engages a flange 20 upon the movable crosshead 10 of the device. The crosshead 1.9 is then lowered by the mechanism of the universal testing machine so as to compress the compression springs slightly and buffers 32 are attached to the guide rods 13-16. The crosshead 19 is then pressed downwards against the resistance of the compression springs by the mechanism of the universal testing machine until the desired degree of compression is secured and automatic engagement of the latch 24 is caused by entry therein of the latchpin 23. At this stage the safety handle 25 is inserted into lugs 26 to prevent inadvertent release of the device. The specimen attachments 28 and 29 are detached at the plug and socket joints by removing the peg (not shown) from hole 40. The grips are then dismantled. A textile specimen in say webbing form is cut to a desired length and a gauge length is marked off within that length. An end of the specimen is inserted through the holder 35 passed around wedge member 36 and the wedge counters 37 are placed around wedge member 36. These may be held in position for reassembly by a circlip, elastic band or the like fitting on a groove. This arrangement Is not illustrated. The textile specimen 27 is adjusted so that each gauge length mark abuts the end of the wedge counters 37, and then the assembly of wedge and wedge counters is located within the holder 35. The mounting member 38 is screwed into the holder 35 and this presses the assembly within the holder 35 to register within a cylindrical enclosure therein. When both ends of the specimen 27 are thus gripped each grip attachment is reconnected to the device at the plug and socket joints by refitting the pegs into holes 40. One or both of the telescoped mounting assemblies is then adjusted to tighten the specimen 27 and provide pretension as required to secure a good friction grip between the wedge 36 and wedge counters 37. This adjustment is obtained by turning adjuster sleeve 31. This sleeve 31 may have a different hand thread on its inner surface to that of its outer surface so that it acts in the manner of a turnbuckle in those applications where rapid adjustment is required. Alternatively where a finer adjustment is required both threads may be of common hand and then adjust¬ ment is provided by a difference in screw pitch between the inner and outer threads. Once correct adjustment is achieved the adjuster sleeve is locked in position by tightening the locknut 41. At this stage the device is set ready for the test to be performed. All external monitoring equipment is connected and switched on. Should a special test environment be desired as could well be required for textile testing, this is provided within an enclosure (not shown) surrounding the device. The travelling crosshead 19 is now cleared from the tester. The safety handle 25 is removed and the test is then performed by triggering the latch 24 to release the latchpin 23. During the test the output of one or each load transducer may be displayed and then recorded. An assessment of shock wave effects can be attempted by comparison of the transducer outputs. The extension characteristics of the specimen 27 may be determined by optical monitoring of the test. This can be secured by monitoring the separation of specimen grips but such a method neglects any slippage at the grips. Preferably the extension is obtained by monitoring gauge marks on the specimen 27 eg with a stroboscope and camera. The motion of crosshead 10 after breakage of the specimen 27 is brought to rest by impact on buffers 32.

A tester made in accordance with the above description has been constructed and used in testing. The tester utilizes inner springs each requiring 2.2 KN for compression and outer springs requiring 3.3 KN. Such a tester is capable of testing very substantial test specimens and is adaptable for a wide range of impulse levels, offering compressed spring energies in excess of 1 KJ.

Claims

1. A tensile impulse tester for materials such as textiles, comprising two crosshead members one of which is unidirectionally displaceable with respect to the other, compression spring means operative to urge apart the two crosshead members, latch means operative to hold the two crosshead members until released at a specific displacement relative to each other at which specific displacement the compression spring means is compressed to a predetermined degree sufficient to provide the impulse required for the test, a specimen attachment on each crosshead member such that thereby a specimen may be affixed between the two crosshead members at least one of the specimen attachments being adjustable with respect to its crosshead member to enable the specimen to be pretensioned, and means for monitoring the results of a test performed thereby.

2. A tensile impulse tester as claimed in claim 1 in which at least one of the specimen attachments is mounted by a screw-threaded mounting to its crosshead member to provide pretensioning adjustment.

3. A tensile impulse tester as claimed in claim 2 in which the screw-threaded mounting includes at least one turnbuckle section.

4. A tensile impulse tester as claimed in any one of the preceding claims in which each specimen attachment is detachable from its crosshead member.

5. A tensile impulse tester as claimed in any one of the preceeding claims in which the means for monitoring the results of the test comprises a load transducer operative to provide a signal indicative of the tension within the test specimen.

6. A tensile impulse tester as claimed in any one of the preceding claims comprising two load transducers one adjacent each specimen attachment to provide a signal indicative of the tension within the textile specimen at the end thereof adjacent that specimen attachment.

7. A tensile impulse tester as claimed in any one of the preceding claims in which the impulse level is adjustable by control of the relative displacement of the crosshead members at their latchable position.

8. A tensile impulse tester as claimed in any one of the preceding claims adapted for adjustment of the impulse level by interchange of or supplement to the springs comprising the compression spring means.

9. A tensile impulse tester as claimed in any one of the preceeding claims incorporating an accεlerometer indicative of the acceleration experienced by the displaceable crosshead.

10. A tensile impulse tester as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

11. A tensile impulse tester as claimed in any one of the preceding claims when used in combination with a universal testing machine, as hereinbefore defined, said universal testing machine being operable to compress the compression spring means by action upon the crosshead members to move the latter to their latchable position.