BS EN 820-4:2009 pdf free download – Advanced technical ceramics一 Thermomechanical properties of mono lithic ceramics Part 4: Determination of flexural creep deformation at elevated temperatures.
The loading assembly comprises a similar pair of freely rolling rods positioned on a loading block. The spacing between these rods shall be between 30 % and 50 % of the spacing of the support rods. The loading block shall be free to articulate relative to the loading column in order to permit alignment of the loading rods on the test piece upper surface.
NOTE I Subject to agreement between parties, other test piece support and loading spans can be employed. This can be particularly advantageous for creep-resistant materials. In addition, in some conditions it is recognized that freely rolling rollers, although preferred, may not be feasible. Such deviations from this method should be reported. The effect of restricted roller rotation may or may not be significant depending on the test material and the testing conditions. There is some evidence to suggest that the surface of glass-phase containing materials, or materials which oxidize to give a viscous glassy surface layer, can have a low coefficient of friction against the roller material at the test temperature, such that over the period of the test any friction becomes negligible. However, this situation cannot always be guaranteed.
The loading block shall be guided appropriately such that the loading rods are positioned mid-way between the support rods, thus centrally loading a test piece when placed on the support rods.
The parts of the loading jig shall be constructed from a ceramic material which is anticipated to be more resistant to deformation than the materials under test. In addition the support and loading rods shall be of a material which does not chemically react with the test piece.
NOTE 2 Suitable materials include high-purity alumina for use with most oxide-based test pieces, or sintered silicon carbide for most non-oxide ceramics.
NOTE 3 Test jig parts manufactured from sintered silicon carbide or other silicon-based non-oxide ceramics develop oxidation films in a short period of time when exposed to temperatures typically above 1 300 °C in an oxygen-containing atmosphere. This can cause prevention of rolling of rollers and impairment of jig function.
6.2 Heating device
A heating device surrounds the loading jig in such a manner as permit access to the jig for the purposes of mounting and demounting test pieces. The heating device shall be capable of maintaining a constant test piece temperature to ± 3 °C over the duration of the test.
The temperature of the test piece shall be recorded using a thermocouple manufactured in accordance with EN 60584-2 allowing the use of reference tables in EN 60584-1 or, alternatively calibrated in a manner traceable to the International temperature scale ITS-gO. The tip of the thermocouple shall be close to but not touching the test piece. It shall previously have been determined that the temperature of the test piece does not vary by more than ± 3 °C over its length when temperature has stabilized for more than 30 mm.
The heating device can incorporate or be incorporated within a vacuum or other appropriate chamber for control of gas atmosphere if appropriate to the determination.
6.3 Loading device
The load shall be applied through a loading column to which the upper part of the jig is connected. The load can be generated by any appropriate means provided that a force constant to within ± I % can be generated at the test piece.
The force applied to the test piece shall be calibrated to an accuracy of 1 % in accordance with
EN ISO 7500-1.
NOTE The mass of the loading system between the load cell and the test piece can be significant and should be taken into account, including any spring force due to the use of bellows systems for protective atmospheres, and any pressure differential between the external atmosphere and inside the test chamber.