GB1225763A

GB1225763A -

Info

Publication number: GB1225763A
Authority: GB
Priority date: 1967-03-20
Filing date: 1967-03-20
Publication date: 1971-03-24

Abstract

1,225,763. Measuring stress relaxation; measuring stress relaxation electrically. MINISTER OF TECHNOLOGY. June 19, 1968 [March 20, 1967; Jan.16, 1968], Nos.12957/67 and 2268/68. Headings G1N and G1S. An instrument for measuring the stress relaxation of a material comprises means for loading a test piece 4 on the instrument to produce a given deformation of the test piece and means for measuring the reaction of the test piece on the loading means continuously or after a given time. The loading means comprises a rigid anvil 2 fixed to a base 1, a clamp 3 for securing the centre of the specimen to the anvil and loading prongs 9 attached to a ballistic pendulum 7 suspended by four cords 8. When an electromagnet 10 is energized by closing a switch 17, the core 10a attracts an iron armature 11 attached to the pendulum so that the prongs 9 bend the test piece against the anvil. The switch 17 also actuates a timer 16 which, after a pre-set time, de-energizes the electromagnet thereby allowing the pendulum to swing back. The restoring force, the magnitude of which due to the stress relaxation effect decreases as the time for which the test piece is clamped is increased, is measured by any of the following methods: a) A plunger 12 is fixed to the pendulum and has an iron core 12a which moves in a differential transformer connected to a recorder 14. The position of the core when the test piece is clamped and the position of the core corresponding to the maximum amplitude of the swing of the pendulum when the clamping is released are recorded. The measurement of these positions enables the restoring force to be calculated. b) A fine cord 21 passing through a glass guide tube 22 is attached by a bracket 7a to the pendulum and the two previously mentioned positions are measured by noting the change in position of a mark on the cord against a scale. c) A load cell 18 is connected to a recorder 19 and/or an indicating instrument 20 for continuously measuring the reaction of the test piece. The anvil may be enclosed in an oven for tests at various temperatures and the instrument may be used for test pieces in the form of discs, rings or pipes as well as beams. The instrument may be used for testing metals or elastic non-metals and is particularly suitable for testing plastics materials.