BS ISO 28704:2011 pdf download – Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for cyclic bending fatigue of porous ceramics at room temperature
5 Test piece
5.1 Test piece size
The shape of a test piece shall be a beam of uniform rectangular cross-section. Its standard dimensions shall be 70 mm ± 0,5 mm in total length, 8,0 mm ± 0,1 mm in width, and 6,0 mm ± 0,1 mm in thickness. The parallelism between the upper and lower surfaces of the test piece shall be better than 0,02 mm, as specifed in ISO 1101. The longer edges shall be rounded or chamfered by an amount of 0,1 mm to 0,3 mm, as given in Figure 4. This chamfering shall be carried out with caution so as not to introduce defects resulting in a fracture initiation site. The chamfering is not necessary when the grain size of material is larger than 0,1 mm. Before carrying out the test, the width and thickness of the test piece shall be measured to the nearest 0,1 mm by using micrometer callipers (4.4).
If, for any reason, different test piece dimensions have to be used, they shall be stated in the test report.
5.2 Test piece preparation The surface shall be fnished by grinding with a wheel of grain size 20 µm or smaller. The contamination by wax, if any, should be removed by organic solution or by heating at 773 K (500 °C) for 1 h to evaporate wax.
6 Test method
6.1 Waveform of loading stress The waveform shall be sinusoidal and the ratio of minimum to maximum stress R = 0,1. The frequency shall be selected by agreement between parties, with 20 Hz being recommended. For other cases, the waveform of the loading stress shall be stated in the test report. A series of tests shall be carried out using the same stress waveform.
6.2 Loading method
6.2.1 The distances between internal support points and external support points shall be measured by a calliper before the tests. 6.2.2 The test piece shall be centrally positioned in the test fxture.
6.2.3 The number of cycles from the start of loading to the end of adjusting to a specifc cyclic loading stress shall be as few as possible. In this case, the stress during initial adjusting shall not exceed the specifc maximum stress.
6.2.4 The test shall, as a rule, be carried out without a pause on the same test piece from start to end. However, when the test is interrupted for any reason, the number of cycles to the interruption and the duration of interruption shall be recorded.
6.2.5 The fatigue test shall be carried out on at least three test pieces at each of at least three maximum stress levels. 6.3 Number of cycles for interruption Unless otherwise specifed, if the test piece is not fractured by 10 7 loading cycles, the test may be halted, and the result deemed to be a “run-out” and indicated on the plot of results with a right-pointing arrow.
6.4 Reuse of test piece Do not reuse a test piece which has been used once.
6.5 Recommended test procedure in fatigue test When the maximum stress and the number of test pieces in a fatigue test are not previously agreed upon between the purchaser and the supplier, the test may be carried out according to the following procedure.
6.5.1 Measurement of fexural strength In order to obtain the standard value of the maximum stress in a fatigue test, measure the fexural strength of a test piece prior to the fatigue test. Use the test piece of the same shape as that of a fatigue test piece for measurement of fexural strength, and obtain it by the same loading method as that of the fatigue test. Allow the speed of a cross-head in the measurement of fexural strength to be 0,5 mm/min. Calculate the fexural strength from the measured maximum forces of respective test pieces.
6.5.2 Stress level in fatigue test Determine the maximum stress in a fatigue test by referring to the arithmetic mean value of the fexural strength obtained by the measurement in 6.5.1. Start the fatigue test from a condition wherein the maximum stress is high, and preferably measure the life of at least three test pieces by the same stress. When all the test pieces tested at the same stress are unfailed at the selected number of cycles for test termination, select a higher stress level for the next group of test pieces. Do not carry out tests at a lower stress level. It is preferred that the results of a test programme should contain at least three valid failures at each of at least three maximum stress levels.