Figure 6 shows that at 25C (77F), 350C (662F) and 450C (842F) the bonded sand behaves in a predominantly elastic manner, with failure occurring in a brittle mode. However, at 180C (356F), and to some extent at 350C (662F), large plas- tic deformations can be observed before the specimen breaks. While the elastic behavior can be expected to be the same under any loading condition,3
the inelastic part of the stress-
strain curves is specific to the present three-point bend test. Figure 6 also indicates that the elastic modulus and the ulti- mate strength and strain (at breaking) vary with temperature in a complex manner. The elastic modulus and strength decrease rapidly with increasing temperature from room temperature to 180C (356F). Then, the elastic modulus and strength increase again with temperature, while the ultimate strain decreases. Between 350C (662F) and 450C (842F), the elastic modulus increases, but the ultimate strength and strain decrease.
the specimens were held at an elevated tem- perature for a long time, such that “steady-state” values of the mechanical properties were obtained and the heating rate played no role. In those studies, the mechanical properties were generally found to be monotonically decreasing with increasing temperature. In the present experiments, the spec- imens are heated at rates that are, to some extent, representa- tive of the heating rates encountered in actual castings (see Introduction), and the measurements are performed instanta- neously once the desired temperature is reached. This causes
These non-monotonic variations of the mechanical proper- ties with temperature, and the presence of inelastic behavior, are a new finding of the present study. As is shown in the following sub-sections for the elastic modulus, they are a direct result of the effect of the heating rate and the amount of time a specimen is held at a given temperature. In all pre- vious studies,4,5
the difference in the observed behavior. Here, it should be kept in mind that the ultimate strength and strain values that can be read from Figure 6 are specific to the present three- point bend test and would be different under pure tension or compression. However, the value of the elastic modulus is independent of the loading method.3
This is the reason why the present study focuses solely on the elastic modulus.
Figure 6. Typical stress-strain curves at four different temperatures. The specimens were heated at a rate of 8°C/min and loaded at the desired test temperature until data in the elastic regime.
Table 2. Sources of Error in an Elastic Modulus Measurement at Room Temperature
Table 3. Errors in the Elastic Modulus for a Room Temperature Test
12
International Journal of Metalcasting/Fall 10
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