摘要
大型建筑设施如大厦、桥梁、水坝等的安全与人们的生活息息相关。及时有效地杜绝建筑设施的安全隐患已成为全社会的共同需要。现有的各种混凝土无损检测手段受其单一的信息获取方式限制，往往只能对混凝土结构质量状况的某一方面进行评估，难以实现实时、长期监测。碳纤维法和光纤传感法可以对混凝土结构进行实时、在线地监测。但是利用碳纤维的压敏特性只能监测混凝土结构中有纤维分布的局部应力状况，且碳纤维对静态应力无响应，监测结果不全面。光纤传感法也存在着不少缺点，如光纤频带窄、设备费用昂贵等。
将压电敏感元件植入混凝土中，形成敏感模块，可以在大型建筑设施中扩展成任意分布的压电机敏混凝土系统。利用机敏系统可以感知混凝土结构的静、动态特性。这种结构混凝土监测方法具有实时、主动、长期监测的特点，且成本不高，易于维护，实用性较强。
论文的研究工作围绕着设计可用于大型混凝土结构实时、主动、长期健康监测的压电机敏系统的课题目标展开。首先通过对超声波在混凝土中的传播特性分析，归纳了利用压电机敏系统监测混凝土结构静、动态特性的思路，初步探讨了压电机敏混凝土静态特性监测的数学模型。
为了给压电元件提供多种类型的激励信号，根据直接数字合成原理，设计了利用数据采集卡和计算机实现DDS波形发生的虚拟仪器，并对生成波形的各方面性能进行了理论分析和实验研究，从理论和实验两个方面分析了这种波形发生方式所能取得的效果。
通过对压电元电－声换能的理论分析，得出在混凝土结构不被损毁的前提下，埋入混凝土中压电元的电－声换能特性不随混凝土结构所受静载荷的变化而变化，相应的测试实验验证了理论分析的结果。
将换能器等效为二端口网络，分析了不同声负载下换能器电－声、声－电换能效率的变化特点。分析表明，与空气负载相比，埋入混凝土后压电换能器的电－声换能效率提高、而声－电换能效率降低。仿真实验验证了分析结果，进一步得出换能器电－声－电两次换能的综合效率降低，并针对存在的问题提出了改善措施，改善后的仿真结果表明了这种是措施是有效的。
The security of huge buildings, such as mansions, bridges and dams has a close relationship with people's daily life. Preventing the disaster caused by the collapse of huge buildings from happening is very important. With the limits of information acquisition modes, the existing nondestructive detecting methods for structural concrete can only evaluate certain individual aspects of the structural concrete health. Furthermore, it is difficult to apply these methods to real-time long-term monitoring. The embedded carbon fibers or optical fibers can sense the qualities of concrete structures. However based on the pressure-sensitive effect, embedded carbon fibers can only sense the partial stress closely around. There are several defects of the monitoring method using embedded optical fiber sensors, such as the narrow frequency operation band of the sensors and the expensive cost of the instrument system.
Piezoelectric smart concrete system can be constructed by embedding piezoelectric transducers into concrete structures. It is possible to detect the active and inactive performances of structural concrete with the piezoelectric smart system. This method can realizes active, real-time, long-term monitoring and is noticeable for its characteristics of cheap, practical and great maintainability.
Our researches focus on designing a piezoelectric smart system for active, real-time, remote monitoring the health of huge buildings. Firstly, by analyzing ultrasonic propagation characteristics in concrete, the fundamentals of detecting structure's static and dynamic performances using piezoelectric smart system are summarized. According to the principles of piezoelectric smart concrete structure health monitoring, a mathematic model of monitor system for static performances is discussed.
Then, the virtual waveform generator based on DDS is designed using a data acquisition card and a PC to supply multiform excitation signals to piezoelectric elements in experiment. The theoretical analysis and experimental research demonstrate the performances of the waveform generated by the virtual instrument.
Subsequently, on the basis of theoretical analysis of the piezoelectric vibrator's electro-acoustic transduction embedded in concrete structure, it is concluded that the static force applies no influence on vibrator's transducing behaviors when it has been embedded in an unbroken concrete structure. This conclusion has been proved by
practical experiment.
Finally, the transducer is equivalently expressed as a two-port device. The thesis analyzes the electro-acoustic transducing efficiency and acousto-electric transducing efficiency of the piezoelectric transducers under different acoustic loads. It is found that the electro-acoustic transduction of the transducers embedded into concrete is more efficient than that of the transducers in the air. On the other hand the acousto-electric transduction of the transducers embedded into concrete is less efficient. The result of simulation experiments proves the point. It also indicates that the electro-acoustic- electric transduction of the transducers embedded into concrete is less efficient than that of the transducers in the air. A suggestion is made to improve the transducing efficiency. Simulation experiments in the improved way indicate that the improvement is valid.
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