Establishment Of MSP And Its Application In PZT Ceramics Under Multiple Coupling Field

Modified Small Punch (MSP) test is a method for evaluating mechanical properties of small brittle samples such as ceramics. Combining strengths of punch test and biaxial flexure test, MSP shows a prominent feature:taking small samples that are round or square for tests. Therefore, it not only enables simple preparation and simple test, but also contributes to the accurate test of ceramics on displacement due to the convenience of sample fixing. MSP test now has been widely used in researches of evaluating mechanical properties of many materials at room temperature.Taking the dynamic materials testing machine as the platform, by designing molds and introducing the electric field, acoustic emission and temperature field, this paper shows the accomplishment of the comprehensive test on material strength and fatigue and other properties under different of strength, electricity, heat and coupling conditions. At the meantime, the real-time monitoring of the damage process is achieved and the testing system platform with multi-fields coupling loading is builtin MSP test.Multi-fields coupling MSP test has good potentialities for applications in the strength tests of piezoelectric ceramics. In this paper, the fracture strength of PZT ceramics with different compositions was tested, and the load-displacement curves, which are impossible to be got from four point bending method and other mechanical properties tests, were also obtained. Also the fracture strength of PZT ceramics was evaluated. They show the ability of further extend of the crack-initiation strength from samples with different compositions. The fracture behavior of PZT ceramics were studied under electromechanical coupling field. The DC field and AC field effects on fracture were discussed. Acoustic emission was used to analyze the micro-structure changes in the process of fracture. In the test of mechanical properties of PZT ceramics in electro-mechanical coupling field, it shows that under the condition of electro-mechanical coupling in AC field, with a better piezoelectric property, there will be more impact of the electro-mechanical coupling field on mechanical properties of the ceramic, especially for PZT52/48closed to morphotropic phase. Its mechanical properties decrease sharply in electro-mechanical coupling field. Under the condition of electro-mechanical coupling in DC field, the crack-initiation strength of PZT ceramics show little changes with the negative and positive electric fields while the fracture strength changed a lot with the load of the negative and positive electric fields. In positive electric field, the fracture strength of PZT ceramics increases while in negative electric field it decreases.Along with the increasing demand in practical applications and safety design concerning fatigue and service life, fatigue properties of materials have drawn great attention. In this paper, the dynamic fatigue properties of PZT ceramics were evaluated through the slow crack growth theory. On the basis of different loading rates, materials show different fracture strength, which indirectly contributes to the method of the fatigue property calculating. Moreover, we evaluate the dynamic fatigue performance of materials by calculating the stress corrosion exponent. Through analyzing the curves of MSP strength with time of rapture, the service life of materials can be derived. This method is successfully applied into the evaluation of dynamic mechanical properties of PZT45/55, PZT52/48and PZT55/45. The results show that samples of PZT45/55have the worst resistance of dynamic fatigue under the same condition of firing. Besides, in this paper, the evaluations of the dynamic fatigue of materials in electro-mechanical coupling field have also been achieved. By comparing dynamic fatigue in pure force field and that under electro-mechanical field, it can be perceived that the fatigue performance of materials will sharply decrease in electro-mechanical field. In pure force field,"n", which refers to the stress corrosion exponent, is39while it decreases to18in electro-mechanical field. When evaluating the mechanical properties of piezoelectric materials, electric field should also be taken into consideration to obtain a real evaluation of the mechanical properties of ceramics. By adopting this method when evaluating the dynamic fatigue of ceramics, the performance of resistance to fatigue of materials can be quantitatively analyzed via testing the stress corrosion exponent.Piezoelectric materials work in the electric and force fields which move in circles. After massively periodic loading, ferroelectric ceramics will come up with the force and electrical coupling fatigue, which leads to the deterioration and invalidation of material functions. And the typical characteristic is with the increases of the cycles, the piezoelectric constant decreases and the mechanical properties decrease as well. The method to forecast the fatigue life according the fatigue attenuation theory was established. We also get the exponent of propagation of the cyclic fatigue n was395. According to this, the PZT ceramic can used five years under the maximum strength of79.1MPa. Magnitude, frequency, and wave pattern of the cycle force on cyclic fatigue have been discussed in details via a number of coupling tests on mechanical properties of small samples. In this paper, the changes of PZT ceramics on mechanical properties after the electro-mechanical coupling in DC electric field were also discussed. Results show that after electro-mechanical coupling in negative electric field, the mechanical properties of materials tend to increase while they decrease in the positive electric field. In the meantime, the changes of PZT ceramics on mechanical properties after the electro-mechanical coupling in AC electric field are also investigated. The results show that when there is no difference between the frequencies of AC and force fields, there is no apparent decrease in the properties of materials while the material properties tend to decrease when the frequencies are different.In conclusion, multi-fields coupling MSP test has been proved to be valid, highly-effective and dependable in the evaluation of mechanical properties of ceramics in room temperature and multi-coupling fields.