GB2126353A - Impact testing machine - Google Patents

Abstract

An impact testing machine in which a striker 6 is held by a pivotted arm 1 which lies in a horizontal plane and is accelerated to high rotary speed. A specimen 20 is supported on a vertically- movable mounting 17. When the striker has been accelerated to sufficient speed the mounting moves, in the time between one revolution of the striker and the next, to raise the specimen from a position in which it lies below the path of the striker to one in which the striker makes impact with it. A clutch operates to disconnect the pivotted arm from its rotary drive just before impact, so that the momentum of impact is independent of the drive. Means for measuring the speed of the striker at impact and for supporting the specimen on its movable mounting are provided. <IMAGE>

Description

SPECIFICATION Impact testing machine This invention relates to impact testing machines and methods in which material specimens are fractured by the impact of a striker. Measurements are made to determine the values of the kinetic energy of the striker immediately before and after impact, so that the energy required to effect the fracture can be related to the difference between those two values. From the energy required to effect the fracture, much useful information relating to the strength and structure of the specimen may be obtained.

In most known impact testers the striker is either shot at the specimen from a gun, or carried on the end of a pendulum. The gun-type instruments have the advantage that high kinetic energy can easily be imparted to the striker before it hits the specimen: the striker may therefore retain considerable energy even after impact, and the magnitude of the energy levels before and after will be such that the difference between them can be measured easily and with accuracy.A disadvantage of such instruments, however, is that for reasons of safety and accuracy of aim the striker has to be propelled down guiding tubes: glancing impactwith the walls ofthesetubes diminishes the energy of the striker and leads to inconsistency between one test and the next because the pattern of impact between the striker and the tube walls will inevitably vary between tests.

In the other known type of machine, in which the striker is carried on the end of a pendulum arm and makes impact with the specimen as the arm reaches the bottom of its swing, consistency of energy levels in successive tests is easier to achieve because the motion of the pendulum is less subject to variation.

To give the striker energy levels comparable with those that can be given by a gun, however, the pendulum arm must be long and heavy and must be dropped from a considerable height. Such machines therefore tend to be large.

The present invention seeks to provide a relatively compact machine by which the striker can be accelerated to variable but high velocity, of accurate and predetermined value. According to the invention an impact tester comprises a striker held by a pivoted arm, and a motor can be connected to the arm to accelerate it to high rotary speed. There are means to hold a specimen out of the path of the striker until the latter has achieved a desired velocity, and then to move the specimen into position for impact in the time between one passage of the striker and the next.

The arm may be horizontal, may rotate about a vertical axis, and the specimen may be mounted on a table which moves vertically to bring the specimen into the impact position.

A clutch may connect the motor to the arm, so that by disengaging the clutch shortly before impact with the specimen the arm is no longer under drive but is rotating freely when impact occurs. Electrical speed detectors, for instance of photo-electric type, may be located on the table, or alternatively on fixed mountings, at positions to measure the speed of the striker shortly before and shortly after it makes impact with the specimen. Electrical means, for instance of solenoid type, may be provided to drive the table to bring the specimen into the impact position, and a sensor may co-operate both with the clutch and with the table drive so as to energise the drive and disconnect the clutch as soon as the striker passes the impact position one revolution before impact is to take place.

The specimen may be in the form of a block of constant cross-section and appreciable length, and may (as in the known Charpy test) be held at both ends by supports mounted on the table, so that when moved into the impact position by the motion of the table the specimen is shattered by the striker hitting it in the middle, between the two held ends.

Alternatively, as in the known Izod test, the specimen may be mounted in cantilever fashion so that one end is free but the other end is embedded in a support mounted on the table. In another alternative construction, particularly suitable for the testing of material that is to be fed to granulating machinery, in which the raw material is only imperfectly constrained when it is presented to the cutters to be granulated, the specimen may be at the moment of impact lie freely on the horizontal surface of the table with its length lying along a radius relative to the axis of rotation of the arm. The specimen may be steadied by contact with vertical surfaces lying to either side of it - that is to say, to either side of the radius with which it is aligned - but without being positively bonded to either of those surfaces.

The invention is also defined by the claims, the disclosure of which should be read as part of the disclosure of this specification, and will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic elevation of a machine with striker and specimen omitted; Figure 2 is a section on the line ll-ll in Figure 1, including the specimen, and Figure 3 is a plan view taken in the direction of the arrow Ill in Figure 1, but including arm, striker and specimen.

The machine shown in the Figures comprises a horizontal arm 1 mounted on a vertical axle 2 driven through a clutch 3 by a motor 4. All these parts are secured to a robust and generally horizontal base member 5 mounted on adjustable feet 6. At the end of the longer part of arm lisa holder to carry a striker: a counter-weight 9 is mounted on the shorter part of the arm, so as to balance the arm as a whole about axle 2.

At the side remote from where axle 2 is mounted, the surface 10 of base 5 is recessed at 11 to receive a structure, indicated generally by arrow 12, by which a specimen of material to be tested is held and presented to the striker. As shown most fully in Figure 2 structure 12 comprises a high side shoulder 14 and a lower side shoulder 15, both fixed to base 5 by screws 16, and between them a table 17 mounted on the rod 18 of a solenoid unit 19 which fits within a further recess 13. A specimen 20 of the material to be tested, of square section and with its length axis 21 intersecting the axis of axle 2, rests on the upper surface 22 of table 17. Figures 2 and 3 show the rod 18 at one extreme of its travel so that table 17 is in its uppermost position, in which specimen 20 is presented for impact by a striker 8 (Figure 3) carried by holder 7.When in this position, the specimen is supported on one side (the downstream side relative to the direction of rotation 23 of arm 1) by a fixed blade 24 mounted in shoulder 14, and on the other side by a pin 25 carried by table 17. The level of surface 22 is set by the engagement of a step 26 on table 17 with abutments 27, 28 on shoulders 14, 15 respectively. When shaft 18 lies at the other extreme of its stroke within the coils of solenoid 19, surface 22 is retracted so that the exposed tip 29 of specimen 20 lies vertically below and clear of the path of striker 8. In moving between its working and retracted positions, table 17 is guided by pillars 32 which make a sliding fit in circular holes formed in a flange 33 carried by the table.

A pair of photocells shown diagrammaticaily at 35 is mounted on shoulder 14. Striker 8 passes once per revolution through the gap between these photocells, so that their output may be used to indicate the speed of striker 8 at this point of its revolution, that is to say just after it will have made impact with specimen 20 if the latter is in its impact position. A similar pair of photocells 36 is mounted on shoulder 15, and the output of this pair indicates the speed of striker 8 before making impact with the specimen, if presented. By an electric circuit shown in outline only at 37, photocell pairs 35 and 36 are connected to motor 4, clutch 3 and the coiis of solenoid 19, and to counters 38,39 respectively.Of course many other means - in particular those making use of Laser Doppler anemometry - may be employed in place of photocells to measure the speed of the striker.

In operation, table 17 begins in its lowest position with rod 18 fully retracted into the coils of solenoid 19, and a specimen 20 is mounted upon the table. A striker 8 is mounted in holder 7 and circuit 37 is energised to engage clutch 3 and start motor 4 so that arm 1 begins to spin at increasubg angular velocity, indicated by counters 38, 39 each time striker 8 passes through the clearance between photocells 35,36 respectively. Avalue is pre-set into counter 38, corresponding to the speed at which it is desired that striker 8 should make impact with specimen 20. As soon as the output of photocells 35 indicates that the striker has attained that speed, a signal from counter 38 disengages clutch 3 so that arm 1 and striker 8 are now disconnected from motor 4 and rotating under their own momentum.

Simultaneously a signal from the counter energises solenoid 19 so the table 17 rises to its raised position as shown in Figure 2. On its next passage past table 17, therefore, striker 9 makes impact with specimen 20 and fractures it. The readings of counters 38 and 39 for this passage of striker 8 are recorded by conventional means (not shown), whereby a measure of the energy absorbed by the striker due to the impact may be derived from the difference between these readings.

While the known Charpy test would be simulated by holding specimen 20 at both ends by clamps mounted on table 17, and while the known Izod test could be simulated by mounting specimen 20 in cantilever fashion on table 17, with the distal end 40 secured in a cavity formed in the structure of the table, in fact the relatively insecure mounting of the specimen illustrated in Figures 2 and 3 may be particularly appropriate where the test is aimed to predict the behaviour of the specimen material within a granulator, in which items of raw material are not positively or predictably held as they are presented to the cutting knives. As Figure 3 shows best, at the moment of impact by striker 8 the specimen 20 is steadied horizontally by the opposed forces of reaction against pin 25 and the forward corner 41 of fixed blade 24. Any tendency of the specimen to be displaced by the rapid rise of table 17 from its lowered to its raised position may easily be resisted by interposing some mild adhesive between the specimen and the table, or by some other simple expedient.

While the invention has been described with reference to a machine in which the specimen moves from its retracted to its impact position along a line at right angles to the plane in which the striker revolves, the movement might alternatively take place within that plane. Also, while a machine with a horizontal arm rotating about a vertical axis has been described, arms rotating about axes at other orientations are also possible.

Claims (10)

1. An impact testing machine comprising a striker held by a pivotted arm, a motor connected to that arm to accelerate it to high rotary speed, and means to hold a specimen out of the path of the striker until the desired speed has been achieved and then to move the specimen into position for impact by the striker in the time between one revolution of the striker and the next.

2. An impact testing machine according to Claim 1 in which the arm is horizontal and is pivotted about a vertical axis.

3. An impact testing machine according to Claim 2 in which the means to hold the specimen comprise a table movable vertically to bring the specimen into position for impact.

4. An impact testing machine according to Claim 1 including a clutch to connect the motor to the arm, and means to disengage the clutch from the arm shortly before impact between the striker and the specimen so that the arm is no longer under drive but is rotating freely when impact occurs.

5. An impact testing machine according to Claim 1 including electrical speed detectors, for instance of photo-electric type, located at positions to measure the speed of the striker shortly before and shortly after it makes impact with the specimen.

6. An impact testing machine according to Claim 4 including electrical drive means to move the table to bring the specimen into position for impact by the striker, and a sensor co-operating both with the clutch and with the electrical drive means to energise the drive means and to disconnect the clutch once the striker passes the impact position one revolution before impact is to take place.

7. An impact testing machine according to Claim 3 in which the means to hold the specimen comprise the table to support it from underneath and supporting surfaces mounted on the table on either side of the specimen circumferentially relative to the movement of the striker, but without any substantial positive clamp or bond between the specimen and the table or such surfaces.

8. A method of impact-testing a specimen, using a machine according to any of the preceding claims.

9. An impact testing machine according to Claim 1, substantially as described with reference to the accompanying drawings.

10. A method of impact-testing a specimen, according to Claim 8 and substantially as described with reference to the accompanying drawings.