Dynamic Fracture Mechanics Of Ferroelectric Solids

Ferroelectric materials have been widely used in modern technologies because the piezoelectricity can convert mechanical energy into electrical energy and vice versa. Typical examples illustrating the use of these materials include transducers, sensors, delay lines, filters and actuators as well as sensing and actuating elements embedded in smart structures. The low fracture toughness of the widely used ferroelectric and piezoelectric materials in current technological applications raises the big concern about their durability and safety. In this work, main contributions to the dynamic fracture mechanics of ferroelectric solids are summarized below:Firstly, a cracked ferroelectric solid under single mechanical impact or electric pulse is reserached.(1) Based on the experimental data of the ferroelectric hysteresis loop under different electric loading frequency and temperature, the Johnson-Cook-type domain switching critera considering the frequency and temperature effects was proposed.(2) Using a modified split Hopkinson pressure bar method with a PVDF-based gauge, both static and dynamic fracture toughness for different poling directions were obtained. A rate dependent stress induced domain switching model was futher put forward to explain the phenomena that the anisotropy of dynamic fracture toughness is not obvious.(3) A compact crack in ferroelectric ceramics was loaded under four different electric pulse frequencies, and the frequency dependent electric field induced domain switching model was established and successfully used to explain the test results.Secondly, crack instability of ferroelectric solids under alternative electric loading was studied.(1) The coefficients of the domain switching work converted into heating were defined and measured.(2) For the first time we showed experimentally self-heating induced crack instability of ferroelectric solids under alternative electric loading. We developed a fracture phase diagram to show the safe Operating Area curve as a function of both the frequency and the amplitude of an applied electric field. This phase diagram could be directly applied to engineering safety assessment of ferroelectric devices. We theoretically analyzed the behind mechanisms accounting for heating at a crack tip in ferroelectric solid, and determined the geometry of reverse domain switching zone under electric loadings.(3) A rapid, efficient and nondestructive method called active electric pulsed thermography(AEPT) was proposed to detect the surface and sub-surface defects in the piezoelectric/ferroelectric ceramics devices based on the domain switching-induced heat principle.Finally, crack propagating models in the ferrelectric material were investgated.(1) Mode-I transient response of a semi-infinite conducting crack propagating in a piezoelectric material with hexagonal symmetry under normal impact loading was investigated. The integral transform methods together with the Wiener-Hopf technique were used to solve the mixed boundary value problem under consideration. The dynamic stress intensity factor and dynamic electric displacement intensity factor as well as their universal functions were obtained in a closed form. It is noted that electromechanical coupling coefficient has an important influence on the dynamic fracture characteristics.(2) A moving polarization saturation(PS) model was proposed to study the plane problem of a Yoffe-type crack moving with constant velocity in ferroelectric materials considering electric saturation. Based on the extended Stroh formalism, the model was solved using complex function method. The closed-form expressions for the electroelastic fields were obtained in a concise way. It predicts that crack propagation may be promoted by a positive applied electrical field and inhibited by the negative one. For high crack velocity, 0.28 times minimum body wave speed, the hoop stress is maximal for an angle θ≠0.