Figure 7 shows an example of a stress-strain curve that was obtained during a typical test, where a specimen was loaded and unloaded during heating without breaking it. Each load- ing cycle corresponds to a different temperature between room temperature and 500C (932F). At some temperatures, a small amount of inelastic deformation could not be avoided. The total inelastic strain at the conclusion of the test is less than 0.5%. As demonstrated in Figure 6, only the straight portions of the stress-strain curve, during loading, are used to determine the elastic moduli.
Elastic Modulus Variation at an 8°C/min (14.4°F/min) Heating Rate
Numerous tests were performed for an average heating rate of 8C/min (14.4F/min). As indicated in Figure 1b by a dashed line, this heating rate is typical for locations in the mold that are between 2.54 cm (1 in) and 7.62 cm (3 in) away from the mold-metal interface (for a steel cast- ing). Figure 8 shows the elastic modulus measurements, as well as the measured temperatures, as a function of time for a typical test. The center and surface tempera- tures are always within 10C (18F) of each other. This tem- perature non-uniformity of the specimen was still deemed acceptable. The lack of temperature uniformity prevents experiments from being conducted at much higher heat- ing rates. At lower heating rates, the specimens were gen- erally more isothermal. As can be seen from the slope of the temperature curves, the heating rate is not completely constant. The variation in the heating rate was always less than ±1.5C/min (2.7F/min), which again was deemed ac- ceptable. The approximately 80 elastic modulus measure-
ments in Figure 8 provide an almost continuous variation of the elastic modulus with time.
Figure 9 shows the elastic modulus measurements for six dif- ferent specimens that were all heated at a rate of 8±1.4C/min (14.4±2.52F/min). Here, the elastic modulus is plotted directly against the measured temperature. Three of the specimens con- tained black iron oxide and three did not. It can be seen that there is no discernable difference in the elastic modulus be- tween the two types of specimens. This indicates that the ad- dition of black iron oxide has no effect on the elastic modulus. All data fall within a relatively narrow band and were fit to a line using a 13-point moving average. This line is also shown in Figure 9. In addition, Figure 9 shows the average of all room temperature elastic modulus measurements for the six speci- mens. This average is equal to 3,920 MPa, with a standard de- viation of 343 MPa. This relatively large standard deviation is not due to measurement uncertainty (see Table 3). It is a result of unavoidable variations in the preparation of the specimens.
The variation of the elastic modulus with temperature can be best described by following the line in Figure 9. The elastic modulus decreases steeply with increasing temperature from the room temperature value of 3,920 MPa to about 600 MPa at 125C (257F). Note the small knee in the line at about 65C (149F). Between 125C (257F) and 250C (482F), the elas- tic modulus is relatively constant. Above 250C (482F), it increases to 1,200 MPa at 280C (536F) and then decreases again to 900 MPa at 325C (617F). Above 350C (662F), the elastic modulus increases almost linearly with temperature until it reaches 2,400 MPa at 500C (932F). At approximately 500C (932F), the strength of the bonded sand vanishes and the specimens collapse under the weight of the loading head.
Figure 7. Typical stress-strain curve for a single specimen under repeated loading and unloading during heating at an average rate of 8°C/min.
International Journal of Metalcasting/Fall 10
Figure 8. Measured elastic modulus and temperatures as a function of time for a typical test with an average heating rate of 8°C/min.
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