A simple four-point bend creep testing apparatus for brittle ceramic materials

Abstract

A knowledge of the creep behaviour of ceramic materials is important in computing the lifetime at high temperatures. Very often creep tests are performed as bending tests rather than tensile creep, because it is relatively difficult to perform a satisfactory tensile creep test on brittle materials. The other common method is compression creep testing. Davies and Sinha Ray [1] built apparatus for carrying out constant-load creep tests in tension on polycrystalline ceramics at temperatures up to 1750 °C. A special type of specimen grip-universal joint assembly was used to hold the specimen accurately on the tensile axis. The strain was measured using two cathetometers, which focused on fine platinum wire tied to the specimen ends. Also, a dial gauge and linear-displacement transducer were used for the same purpose. It is reported that these three methods of measuring strain give results that are in good agreement with each other. Many investigators such as Morrell [2], Kossowsky et al. [3], Lange et al. [4], Govila [5] and Wakai et al. [6] have used tensile creep tests to a great extent. In tensile creep tests the cost of test fixtures and specimen preparation are two of the limiting factors. Carrol and Wiederhorn [7] developed an inexpensive technique for measuring the tensile creep and a simple method used to produce test specimens. Flags with platinum wire were attached to measure the creep deformation. Carroll et al. [8] later measured the tensile creep deformation, using a laser extensometer to monitor the relative displacement of flags that were attached to the gauge section of the specimen. Kandil and Dyson [9] discussed in detail the development of tensile creep facilities for ceramics. Hulse and Copley [10] developed three apparatuses for obtaining stress-strain and creep data in compression at elevated temperatures. Many types of compressive creep testing equipment have been described in the literature [11-13]. Some are rather complex and bulky in design. Smith and Moore [14] made a relatively simple and inexpensive compression creep testing apparatus which can be used at temperatures up to 1550 °C. A linear variable differential transducer was used to measure the strain. Recently, Carroll and Weiderhorn [7] reported a compressive creep testing equipment which consists of a screw-driven testing machine which applies the load to the specimen through a series of rounded pedestals and loading blocks. In general, the compressive creep test is relatively simple and has been frequently used to characterize the creep