BS 843-7:2010 pdf download – Advanced technical ceramics — Mechanical properties of monolithic ceramics at room temperature Part 7: C-ring tests
6.4 Mechanical testing machine
A mechanical testing machine capable of applying a force to the test jig at a constant displacement rate or at constant loading rate. The testing machine shall be equipped for recording the load applied to the test jig at any point in time. The accuracy and calibration of the testing machine shall be in accordance with EN ISO 7500-1:2004, Grade 1 (1 % of indicated load).
For Method A, the testing machine shall be equipped with hard flat anvils with faces either fixed parallel to within 0,05 mm over the area to be used for testing, or self-aligning during testing. For Method B, the testing machine shall be equipped with a device for permitting two hard steel rollers to be inserted through the ring and supported at each end by yokes connected to the testing machine which permit a tensile force to be applied. Universal joints shall be incorporated into the load train to permit alignment and to minimise stress concentrations towards one face or other of the ring. NOTE Any suitable alternative arrangement may be used in which the load can be applied in compression mode using a lever system containing the loading rollers. Precautions shall be taken to avoid flying fragments from the test-piece during fracture.
6.5 Interface material Thin, compliant materials such as cardboard, metal foil or rubber sheet, to be placed between the test piece and the compression anvils (Method A) or steel rollers (Method B).
7 Test pieces
Test pieces or test material shall be selected in accordance with the guidelines in EN 1006. If the test rings are to be cut from tube stock, select stock which is acceptably round and straight. This standard does not prescribe any particular dimensions for test pieces, but the following size ratios are recommended to avoid problems with alignment or validity of the calculation equations:
a) 0,05 < (wall thickness/outer ring radius) < 0,5; b) 0,2 < (axial length/outer ring radius) < 1,0; c) 1,0 < (axial length/wall thickness) < 4,0. Cut or otherwise machine the side faces of the test pieces to a parallelism of better than 0,015 mm using a diamond saw with or without additional machining by lapping or surface grinding. This process shall introduce a minimum of damage such that failure does not occur from the cut face. NOTE 1 Guidance on machining processes may be found in EN 843-1  and ASTM C1495 . Make an axial cut through the wall of the ring using a diamond saw. For Method A the slot width shall be sufficiently large that the sides of the slot do not close during the test before failure of the test-piece. For thick- walled test-pieces, a slot of 1 mm to 2 mm is adequate, but for thinner walled test-pieces, the slot shall be rather wider. All edges shall be chamfered at 45° to a distance of at least (0,15 ± 0,05) mm or rounded to a radius of at least (0,15 ± 0,05) mm to avoid edge dominated failures. NOTE 2 It is necessary only to make the chamfer over an arc length of at least 90° of the tube wall adjacent to region of maximum tensile stress. NOTE 3 Chamfering may be done by machine or by hand. The size of chamfer may need to be increased above the minimum values above for high-strength materials if it is found by fractography that chamfer dominated failure rather than surface dominated failures result. See  and . At least ten test pieces shall be tested for determination of a mean strength, or at least 30 test pieces if a Weibull statistical analysis in accordance with EN 843-5:2006 is to be performed. 8 Test procedure 8.1 Test-piece dimensions Measure the axial length of the each test piece to the nearest 0,01 mm in several places using the micrometer. Compute the mean result. Measure the outside diameter of each test piece to the nearest 0,02 mm using the vernier callipers, ensuring that several measurements are made along the axial length across the diameter adjacent to that expected to become the mid plane in the test. Compute the mean result. Measure the inside diameter of each test piece to the nearest 0,02 mm using the vernier callipers, ensuring that several measurements are made from both sides of the ring across diameters adjacent to the diameter expected to become the mid plane in the test. Compute the mean result.