GB2206417A - Thrust interface shear test apparatus - Google Patents

Abstract

A test piece assembly comprising a punch (21) and a die piece (20) each having a single, axially co-operating thread or flange (22,23) configured to a specific thrust interface of interest, is assembled in a purpose-built compression frame (40) ensuring axiality of thrust and then subjected to a measured axial loading. A set of electrical resistance strain gauges (101) attached to a double acting cantilever (60) supported interjacent the two component test pieces is used to measure corresponding variation in axial separation of the two pieces and hence displacement of the thrust interface. <IMAGE>

Description

THRUST INTERFACE SHEAR TEST APPARATUS This invention relates to the measurement of shear stress induced displacements occurring when axial thrust is applied to a thrust face which is cantilevered transversely and symmetrically with respect to the thrust axis, an annular flange for example. In particular but not exclusively, it applies to a means for determining the load/displacement characteristics of the thrust interfaces of a screw-thread or an annular flange sequence disposed to provide axial drive engagement between concentric cylindrical members, one being a drive member and the other being a driven member.

A particular use of such axial drive arrangement occurs in the field of armour piercing projectiles of the type in which a relatively small diameter, dense core is encased in a less dense, discarding sabot of larger diameter suitable for propulsion from a large calibre gun barrel, thereby to impart a high kinetic energy to the dense core. In such arrangements the discarding sabot ana the core are frequently inter-engaged for mutual axial propulsion from the gun barrel by a screw thread or by a series of annular flanges.

Current designs of discarding sabot proJectiles seek to achieve low sabot weights by the use of high strength aluminium alloys and further, by exploiting the high mechanical strength of such alloys so as to reduce selected cross sections of the sabot as far as possible. This approach has led to the use of working stresses which closely approach the limit at which failure will occur.

Occasional in-bore failures have been found to occur with such designs which are thought to have resulted from localisea overstressing due to uneven distribution of load among the multiple thrust interfaces that are disposed along the length of the sabot/ core drive interface.

The present invention seeks to provide a mechanical test for determining the thrust interface, load/displacement performance of a variety of single thread or flange configurations and materialsjat strain rates both quasi-static and of high magnitude comparable with those encountered at a projectile launch, thereby to permit optimisation of design profiles and dimensions. The invention further seeks to provide single thread or flange load displacement data for use in computer models to evaluate the aistribution of load along the length of a sabot/core drive interface aue to acceleration forces at launch.

In accordance with the present invention, a thrust interface shear test apparatus includes: a test piece assembly comprised by a die piece having an axisymmetric first protrusion extending inwaraly towards a thrust axis, and a punch piece axially ana freely insertable into the die piece having an axisymmetric secona protrusion extending outwardly from the thrust axis, which second protrusion is engageable with the first protrusion at a thrust interface transverse to the thrust axis; sensing means attached to the die and punch pieces for measuring relative axial location thereof; and compression means for applying a measured axial load to the test piece assembly.

Preferably the test place assembly includes a close tolerance alignment sleeve insertable interjacent the die and punch pieces at an axial location separate from the engaged first and secona protrusions.

The compression means conveniently comprises the platens of a conventional press and, to ensure axiality of loading, the test apparatus preferably further includes a compression frame insertable between the platens, within which frame the test piece assembly is mounted.

The compression frame may conveniently comprise a piston ana cylinder block, the test piece assembly being freely insertable in the cylinder block, the die piece being engaged with the block and the punch piece being engage able with the piston at a disposition ensuring that the thrust axis of the test pieces is in substantial alignment with the axis of the piston.

A specific embodiment of the invention will now be described by way of example only with reference to the following drawings of which Figure 1 is an axially sectioned detail of a conventional buttress thread interface, Figure 2 is a front view of a test piece assembly, Figure 3 is a part cut-away side view of the test piece asse mbly of Figure 2, and Figure 4 is a part cut-away perspective view of the test piece assembly of Figure 2 mounted in a compression frame.

The buttress thread detail illustrated in Figure 1 is an example of the type of threaa conventionally used for engaging a drive member 1 with a driven member 2, the drive thrust being applied to the drive member 1 in the axial direction indicatea by the arrows so as to transmit thrust to the driven member 2 via a sequence of thrust interfaces 3.

This conventional thread configuration does not provide even distribution of loading throughout the length of the drive interface, i.e. the multiplicity of interfaces 3, which is particularly disadvantageous when the material of one or other of the members is stressed as closely as possible to its failure limit, as can be the case for example with a sabot/core drive interface in which the sabot comprises the drive member 1 and is of a less dense material than the core comprised by the driven member 2.

The test apparatus of the present invention has therefore been devised as a design aid for use in establishing an optimum drive interface configuration. The test apparatus will now be described with reference to Figures 2, 3 and 4.

A test piece assembly illustrated in Figures 2 ana 3 comprises a die piece 20 and a punch piece 21 each respectively machined to have a single annular protrusion 22 and 23 having the profile and dimensions of a thread configuration of interest. The two annular protrusions 22 and 23 are engageable at a thrust interface 24 substantially perpendicular to a thrust axis A. Complete annuli are used in the test pieces in preference to an accurately reproaucea spiral thread-form as the results obtained have been found to have little significant difference and the simple annular form is of course far easier to manufacture. Furthermore, use of a continuous annulus also avoids errors introduced by the unrepresentative discontinuity that inevitably exists with a single turn of a spiral thread.

In a test piece assembly prepared for evaluating a sabot/core interface, the die piece 20 will be representative of the sabot (drive member 1) and the punch piece 21 will be representative of the core (drivencncmber 2). Where the sabot that is to be represented is of split variety, i.e. divided axially into three or more segments so as to allow symirietrical discard from the core upon leaving the gun barrel, the die piece 20 may be similarly segmented (not shown), a circumferential retaining collar (not shown) being provided to maintain integral configuration while under test.

However, pilot tests have shown little significant difference between the performances of a solid die piece and a circumferen anc( Skerefora tially confined, segmented die piece,lthe solid die piece 20 is/to be preferred in view of its simpler manufacture and greater ease of assembly.

The die piece 20 has a lower ena face 25 which is accurately machined to be perpendicular to the thrust axis A and in use freely stands upon a flat-topped plinth 26. The punch piece 21 is provided with a domed compression head 27 to which is attached a pointed knife edge 28 which extends downwardly towards an upwardly extenaing knife edge 29 attached to the die piece 20. Below the protrusion 23, the punch piece 21 extends into a cylindrical location boss 30 having an end face 31 disposed to ensure that when axial thrust is applied to the head 27, the face 31 will engage the plinth 26 as soon as possible after either one of the protrusions 22 and 23 fails.

Precise axial location of the boss 30 within the die portion 20 is achieved by a close tolerance, free fitting alignment sleeve 32 inserted subjacent the protrusion 22 and having sufficient axial separation therefrom to ensure that no direct axial support is provided to the protrusion 22 and that the end face 31 will engage the plinth 26 before a sheared protrusion 22 can engage and damage the sleeve 32.

To ensure axiality of loading, the test piece assembly is mounted in a free standing compression frame 40 which is illustrated in Figure 4. This frame, which is disposed in use between the top and bottom platens 41 and 42 of a hydraulic press (not shown), includes a base cross-bar 43 and a plinth 44, both of which are integral with the plinth 26 and are machined to have top and bottom faces 45 and 46 which are accurately flat and perpendicular to the thrust axis A.

Rigidly attached to the base cross-bar 43 are two vertical cylindrical pillars 47 which are rigidly cross-linked at their upper extremities by an upper cross-bar/cylinder block 48. Vertically slideable within the cylinder block 48 is an outer piston 49 having an upper end flange 50 engagable with the top platen 41 and a lower end face 51 engageable with the compression head 27 of the punch piece 21. An inner piston 52, protrusive through the upper end flange 50 and slideably engaged within the outer piston 49 via an upper and a lower annular protrusion 53, acts to transmit initial thrust applied by the top platen 41 directly to the lower end face 51 of the outer piston 49 so as to prevent barrelling of the outer piston when high loads are applied, and thereby avoid risk of seizure of the outer piston 49 in the cylinder block 48.

In use, a conventional clip gauge comprised by a double acting, captive cantilever 60, having a set of four electrical resistance strain gauges 61 adhesively attached, is sprung into bridged contact between the two knife edges 28 and 29. The gauges are connected as a Wheatstone bridge circuit, the output of which is coupled to a differential amplifier system (not shown) arranged to provide measurement of relative axial displacement of the knife edges 28 and 29 and hence of the thrust interface 24 (Figure 3). Simultaneous measurement of the applied load is obtained from a conventional automatic read-out device (not shown) fitted to the hydraulic press (not shown).

Data obtained from tests performed with this test apparatus enable direct comparisons to be made of a wide range of materials, manufacturing techniques and individual thrust face profiles and further enable the preparation of thrust interface loaa-aisplacement characteristics for use in known computer aidea stress analysis techniques.

Such information is particularly advantageous in the design of axial drive interfaces between dissimilar materials subject to extremely high axial loads. It is also advantageous in the design of threaded connections for highly pressurised containers.

The apparatus can also be adapted for use in impulsive loading tests, thereby enabling measurement at high strain rates such as those that can be encountered at a projectile launch.

Claims (6)

1. A thrust interface shear test apparatus including: a test piece assembly comprised by a die piece having an axisymmetric first protrusion extending inwardly towards a thrust axis, and a punch piece axially and freely insertable into the die piece having an axisymmetric second protrusion extending outwardly from the thrust axis, which second protrusion is engageable with the first protrusion at a thrust interface transverse to the thrust axis; sensing means attached to the die and punch pieces for measuring relative axial location thereof; and compression means for applying. a measured axial load to the test piece assembly.

2. An assembly as claimed in Claim 1 further including an alignment sleeve insertable interjacent the die and punch pieces at an axial location separate fro the engaged first and second protrusions.

3. An assembly as claimed in either one of the preceding Claims further including a compression frame comris,ing a piston and cylinder block, the test piece assembly being freely insertable in the cylinder block, the die piece being engaged with the block and the punch piece being engageable with the piston.

4. An assembly as claimed in Claim 3 wherein the piston is a hollow piston having a closed end engageable with the punch piece and further including an inner piston arranged for transmitting initial applied thrust directly to the closed end.

5. An assembly as claimed in any one of the preceding Claims wherein the sensing means includes a pair of confronting knife edges respectively attached to the die piece and the punch piece, having bridged between them a sprung pair of cantilevers each carrying a pair of electrical resistance strain gauges.

6. A thrust interface shear test apparatus substantially as hereinbefore described with reference to the accompanying Figures 2 to 4.