BS EN 820-4-2009 pdf download.Advanced technical ceramics — Thermomechanical properties of monolithic ceramics Part 4: Determination of flexural creep deformation at elevated temperatures.
The test is intended to evaluate the deformation of a test piece under nominally constant force as a function of time at elevated temperatures. In particular it can be used for materials comparison, or for determining the temperature at which creep deformation becomes significant for a prospective engineering use.
Dunng the course of such a test, the test piece can fracture. This can be due either to a subcritical crack growth process unrelated to the mechanism of creep (stress rupture), or to the accumulation of creep damage leading to acceleration of creep rate and the linking of damage to form cracks (creep rupture). In some circumstances it is not possible to distinguish the mechanism of failure. In either case, the test piece lifetime under the imposed temperature and stress conditions can be an important aspect of a materials performance.
The analysis given in this European Standard (see 8.6) produces purely nominal data, assuming that the actual maximum nominal stress in the test piece is linearly proportional to the test force applied and is constant during the test. Moreover, an additional assumption of linear dependence of strain on stress is made for some deflection measurement methods. Furthermore, It does not give engineering creep data equivalent to separate pure tensile or compressive conditions. In many cases, the creep rate dependence is to the maximum stress, and can differ In tension and compression. Typically, the true maximum stress in the test piece Is less than that calculated using Equation 1 because of faster relaxation at higher stress levels, and the true surface strain rate can be greater than a linear prediction in certain geometrical arrangements for determining the deformation, particularly If this is done using the relative displacement of the loading system. The Bibliography contains references to more detailed theoretical analyses of flexural creep accounting for such non-hnearities.
5 Principle
The method involves supporting a bar test piece on two supports near its ends, heating it to the required elevated temperature which is maintained constant, applying a force to two loading points spaced symmetrically between the support points, and recording the deflection of the test bar with time
The deflection of the test piece is measured indirectly and continuously or at appropriate time intervals during the test using the displacement of the loading system (see e.g. Figure la), or by using contacting extensometer rods at given positions on the test piece (see e.g. Figures lb to le). The indirect measurement of deflection (Figure 1 a) is converted into a nominal maximum surface strain In the test piece assuming a linear relationship between stress and accumulated strain. Similar assumptions are involved in analysing deflections between support points and the span centre (Figure lb and ic). When employing displacement measurement between the loading points and the span centre (Figures id and le), the analysis assumes uniform curvature of the test piece, a linear relationship between strain and distance from the neutral axis, and equal behaviour in tension and compression. The slope of the strain/time curve can be converted to a creep strain rate.
6 Apparatus
6.1 Creep test loading jig
The test jig is essentially a four-point bend flexural test jig similar to that described in EN 820-1 for flexural strength testing at elevated temperatures. It comprises a pair of parallel 5 mm diameter support rods positioned 40 mm apart on a refractory supporting structure. These rods shall be free to roll to eliminate friction effects. In contrast to the articulating requirement in EN 820-1, articulation is not required provided that the rods are accurately parallel in the horizontal plane to within 0.001 mm per mm length of rod.
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.BS EN 820-4-2009 pdf download.