Study On The Mechanics Behavior Of The Mechanics-electric-magnetic Coupled Composite Materials

Magnetoelectric(ME) material achieves the best ME conversion among known materials, providing it promising application in magnetic sensors, energy harvesters and electronic devices. This dissertation aims at optimizing the performance of ME composite from the viewpoint of mechanics, by providing new design strategy, performance evaluation index and simulation methods.Inspired by the ideal thermal engine model, an ideal ME conversion structure is established, which consists of a magnetostrictive phase, a piezoelectric phase and a rigid level. By theoretical analysis conducted on this imaginary structure, it is found that in general the ME coefficient is determined by two parameters, namely the stress amplification ratio and the volume ratio between two phases. However, typical ME layouts, for example the parallel sandwiched layout and the serial layout, as special cases of the ideal ME structure, have only one adjustable parameter. Therefore typical layouts cannot obtain the theoretical ultimate ME coefficients. An analytical formula is derived for the ME coefficient of the ideal ME structure and by optimizing the variables, it is found that the theoretical ultimate value of the most widely used ME coefficient defined by the electric field is infinite, but the ME coefficient defined by the static electric energy is finite. Therefore we believe that the static electric energy ME coefficient is a more proper index to evaluate the ME performance.An index is provided to evaluate the dynamic ME performance. As the theoretical predicted ME coefficient at the resonance frequency is singular, it is improper to evaluate and optimize the ME performance using ME coefficient in dynamic cases. In fracture mechanics the stress intensity factor is used to treat the stress singularity around a crack tip. Inspired by this idea, the resonance ME intensity factor is provided by analyzing the singularity of the ME coefficient around resonance in this dissertation. Based on this index, a structure optimizing is carried out, indicating that due to the constraint design parameter, the typical parallel sandwiched layout is far from achieving the theoretical ultimate dynamic ME performance. In dynamic problem, to realize various resonance frequency by structural design is another important goal. We suggest the vibration around the buckling state of the structure as a promising way to reduce the resonance frequency and the use of the film/substrate system to apply the compression.The dissertation also investigates several complex factors on ME performance by developing numerical methods. A nonlinear mechanics-electric-magnetic coupling finite element model is established and used to investigate the influence of the interfacial bonding, crack and interfacial curvature on the ME coefficient. Combining theoretical analysis, a dimensionless parameter is provided to evaluate the interfacial stress conversion quantitatively.Finally aiming at evaluating the reliability of the composites, a mechanics-electric-domain switching-thermal coupled nonlinear finite element model is established and developed into commercial software Abaqus. The model is used to verify the mechanism under the thermal-domain switching coupling induced crack propagation when alternative electric field is applied on ferroelectric ceramics.