Some laboratory hydrofracturing experiments under condition of controlled pressurization rate were conducted using three kinds of materials for two different borehole treatments of the specimen-one is jacketed with silicon rubber membrance at the hole surface (jacketed specimen) and the other is unjacketed (unjacketed specimen). In this work, the following results have been obtained.
1. Based on the statistical analyses, it has become clear that breakdown pressure Pc correlates well with confining pressure Pc-applied to the unjacketed specimen, and that concerning the regression line of Pc-σ∞, both the Pc value at 0 confining pressure and its slope increase in accordance with increasing pressurization rate. The data were further analyzed statistically to yield a trend that the value dPc/dσ_∞ of fitting quadratic curve of Pc-σ_∞ becomes slightly decreasing as the confining pressure σ∞ increases (Table 1).
2. Weak correlation was found between the area of fluid penetration region around the hole and magnitude of breakdown pressure (equivalent to time of pressurization required for failure)(Fig.1).
3. Two types of acoustic emission activity in time were distinguished. In unjacketed specimens, both two types were observed. On the contrary only one type was realized in jacketed specimens (Figs.3, 4, 5).
4. Breakdown pressure of jacketed specimen was found to be fairly higher than that of unjacketed specimen, although such tendency was not recognized concerning the pressure at initial occurence of acoustic emission activity.
5. With an aid of scanning electron microscope, it was observed that growth of induced cracks occurs stably both in jacketed and unjacketed specimens during pressurization process preceding onset of failure (Figs. 6 through 9).

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Laboratory experiments were conducted to examine the validity of the observation equations 1 through 6 in the hydrofracturing stress measurements. Three kinds of rock materials having properties as shown in Table 1 were tested using the apparatus as illustrated in Fig. 2, under the controlled pressurizing rates of 1.0 to 200kg/cm² s for fluid injection, and under the various confining pressure up to 250kg/cm². The results of this study may be summarized as follows:
1. The relationship between the breakdown pressure (P_c) and the confining pressure (σ∞) of these materials was found to be linear as described by Eq. 1, at least in a range of low confining pressure levels (Figs. 7, 8, 9, 10 and 11). However, it was observed in the samples of mortar and plaster that the P_c-σθ curves were bent over a critical confining pressure (Figs. 8 and 9).
2. For permeable materials, it was recognized that the breakdown pressure was liable to be influenced by the flow rate of injection (Fig. 13).
3. Stable crack growth could be achieved, too, for the permeable materials, bymeans of repetition of the testing procedures which consist of first pressurizing to the maximum value then followed by shut-in of the injected fluid and/or by releasing fluid (Figs. 4 and 5). The results of these tests revealed that the breakdown pressures were decreasing and converged to the value defined by Eq. 4, with elapsed pumping cycles. This means that the borehole strenth T_θ in Eq. 1 can be estimated from Eq. 6 (Fig. 10 and 11).
4. It sedms reasonable to define the instantaneous shut-in pressure P'_s used in Eq. 5' as the one at which the pressure-time curve during the shut-in process becomes linear with a constant slope (Table 3).

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As reported earlier, a new-type lead dioxide electrode was developed by the authors. A few of the outstanding charac-t eristics are durability, high resistance to chemical action, and high electric conductivity. The method of preparation consists in electroplating silver on expanded titanium mesh used as a substrate, then applying by electrodeposition a residual-stress-free coating of α-PbO₂, and finally a coating of β-PbO₂.
In the present work, scaling-up experiments were carried out with a view to obtaining technical information necessary for the production of the newly developed electrodes in sizes greater than the commercial electrodes. The results indicate that such electrodes can be produced under conditions determinedfrom the data obtained in the laboratory-scale work.

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