【作者】 陈浩；

【导师】 周又和； 雍华东；

【作者基本信息】 兰州大学 ， 力学·固体力学， 2022， 博士

【摘要】 高温超导块体因其具有较高的临界电流密度,且能够产生远高于常规永磁体的磁场,目前已经在医疗、电力、环保、交通等领域有着广泛的应用。然而,块体在实际应用中面临着关键的力学难题与挑战:超导块体在制备过程中会产生裂纹、孔洞以及夹杂,在高场磁化过程中产生的电磁力和热应变的共同作用下,块体内部的微小裂纹会发生扩展,从而直接影响超导块体运行时的稳定性及可靠性。因此,研究高温超导块体中的断裂行为对其工程应用具有非常重要的意义,能够有效揭示影响超导块体失效的关键因素,并为超导块体的制备提供指导。本文主要针对上述关键力学问题开展了高温超导块体断裂行为的数值研究,主要内容如下:首先,基于麦克斯韦方程组并结合有限差分方法,给出了超导块体在场冷磁化过程中的有限差分计算方程,基于差分方程计算了矩形超导块体在场冷磁化过程中的磁场以及电磁力。利用扩展有限单元法建立了含裂纹高温超导块体应力强度因子的计算模型。采用含有体力的互作用积分方法计算了不同临界电流密度、不同尺寸超导块体在电磁力作用下裂纹长度、角度对应力强度因子的影响。此外,采用最大环向应力准则进行了裂纹扩展的模拟。其次,研究了超导块体在脉冲磁化过程中裂纹的动态应力强度因子变化情况。由于脉冲磁化持续时间短,在磁化过程中由于磁场的剧烈变化导致块体内部温升较大,因此需要在磁化过程中考虑块体温度的变化。结合含有热应变和体力的互作用积分方法,建立了在电磁力和热应变共同作用下含裂纹超导块体动态应力强度因子的数值模型。然后,数值模拟了圆形超导块体单次、多次不同峰值磁场组合的脉冲场磁化过程,给出了超导块体中不同裂纹的动态应力强度因子。最后,基于超导块体中裂纹、孔洞对其磁场分布产生的影响,结合遗传算法设计了一种基于磁场探测超导块体内部裂纹的方法。数值结果表明使用磁场探测方法,可以有效地探测到块体内部的边界裂纹和中心斜裂纹。为了提高探测的有效性,根据磁场导出块体内部的电磁力,同时结合扩展有限元计算块体的位移分布,给出了基于位移分布探测裂纹的模型。通过对比基于磁场和位移的探测方法,发现位移探测法能够有效地探测块体内部的水平裂纹。更多还原

【Abstract】 High-temperature superconducting bulks have been widely used in medical treatment,electric power,environmental protection,transportation and other fields owing to their high critical current density and the ability to generate a magnetic field much higher than that of conventional permanent magnets.However,the bulks face key mechanical difficulties and challenges in practical applications.The cracks,holes and inclusions will appear during the fabrication of bulk superconductors,and under the combined action of the electromagnetic force and thermal strain during the high-field magnetization process,the cracks inside the bulk will propagate,which directly affects the stability and reliability of the bulk superconductor in stable operation.Therefore,the study of fracture behavior in HTS bulk is of great significance for its engineering application,which can effectively reveal the key factors affecting the failure of bulk superconductor,and provide guidance for the preparation of bulk superconductor.In this dissertation,a numerical study on the fracture behavior of HTS bulk is carried out aiming at the above-mentioned key mechanical problems.The main contents are as follows:Firstly,based on the Maxwell’s equations solved with the finite difference method,the magnetic field and electromagnetic force of a rectangular bulk superconductor during field-cooling magnetization are calculated.A computational model of stress intensity factor(SIF)for cracked HTS bulk is established by employing extended finite element method(XFEM).The effects of crack length and angle on the stress intensity factor under the action of electromagnetic force are calculated for the bulks with different critical current densities and sizes by the interaction integral method involving body force.In addition,the maximum hoop stress criterion is used to simulate the crack propagation.Secondly,the dynamic stress intensity factors(DSIFs)variation of cracks in bulk superconductor during pulsed field magnetization(PFM)is studied.Due to the short duration of the PFM process and the large temperature rise inside the bulk induced by the drastic change of the magnetic field during the magnetization process,it is necessary to consider the thermal strain during the magnetization process.A numerical model of DSIF of a cracked bulk superconductor under the combined action of electromagnetic force and thermal strain is established by interaction integration method involving thermal strain and body force.Then,the different peak magnetic fields during single and multiple pulsed field magnetization in the circular bulk superconductor,and the DSIFs of different cracks in the bulk superconductor are simulated.Finally,in view of the influence of cracks and holes in the bulk on its internal magnetic field distribution,a method for detecting internal cracks in bulk superconductors based on magnetic field is designed with the genetic algorithm(GA).Numerical results show that edged cracks and central oblique cracks inside the bulk can be effectively detected using the magnetic field detection method.In order to improve the detection effectiveness,the electromagnetic force inside the bulk is derived from the obtained magnetic field.The displacement distribution of the bulk is calculated with the XFEM,therfore a model for crack detection based on the displacement distribution is given.By comparing the detection methods based on the distribution of magnetic field and displacement respectively,it is found that the displacement detection can effectively detect the horizontal cracks inside the bulk.更多还原