完备表征CFRP单层刚度系数的超声体波/Lamb波方法研究

Material stiffness properties characterization for carbon fiber reinforced polymer (CFRP) is highly required during the process of structural designing, quality assessment and accumulated fatigue damage evaluation. Literature indicates that not all of the nine independent stiffness coefficients can be determined reliably, and the evaluated parameters are commonly the approximate ones- equivalent single layer stiffness coefficients. This project will thus focus on ultrasonic bulk /Lamb wave techniques dedicated to identifying all the nine single-layer stiffness coefficients, with three following contents: 1) establishing a finite element model for Lamb waves in CFRP under multi-modes/ high frequency condition, and proposing a theoretical ultrasonic scheme for reconstructing all the nine stiffness parameters, with promoted accuracy and efficiency; 2) exploring a ultrasonic bulk wave approach to determine the stacking sequence of CFRP laminates in order to reconstruct its single-layer stiffness properties later, revealing the coupled relations between the transducer/ material inhomogeneity and the bulk wave characteristics, and designing the signal processing strategy for obtaining the stacking sequence accurately; 3) conducting intensive study on Lamb wave techniques for evaluating single-layer stiffness coefficients in CFRP laminates, deducing the forward problem model, proposing an experimental laser ultrasonic methodology to analyze the guided wave properties under multi-modes/ high frequency domain, and verifying the reliability and feasibility of bulk/Lamb wave methods on determining all the single-layer stiffness parameters for CFRP. This project is of significant academic and application value since it contributes to consummating the characterization methods of CFRP material properties, and thus accelerating the development of both CFRP industries and ultrasonic techniques.

CFRP刚度性能评估涉及到其结构设计、质量检验和疲劳累积损伤表征等诸多应用层面。目前CFRP层合板刚度表征结果仅部分有效，且识别对象多为等效单层参数。因此，面向完备识别CFRP单层刚度系数开展超声体波/Lamb波方法研究。研究内容包括：1) 建立多模态/大频厚积下的导波有限元模型，提出CFRP刚度表征的超声波理论方法，兼顾刚度识别完备性与高效性；2) 为表征结构单层刚度系数，预先研究CFRP铺层顺序检测的超声体波方法，揭示探头/材料不均匀性对体波的作用机理，开发铺层顺序识别的先进信号处理方法；3) 深入研究CFRP单层刚度识别的超声Lamb波方法，建立正问题理论模型，提出多模态/大频厚积下导波频散特性分析的激光超声方法，验证超声波方法在完备表征CFRP单层刚度属性方面的普适性。项目有利于完善CFRP材料性能表征方法，促进CFRP领域和超声波检测技术的共同发展，具有重要的学术和工程价值。

本项目旨在探讨完备识别CFRP层合板单层刚度属性的超声体波与导波方法，核心内容包括：完备识别CFRP刚度属性的正问题模型、灵敏度分析方法以及求逆算法研究；CFRP铺层顺序识别的超声体波方法研究；CFRP层合板单层刚度属性识别的非接触式激光超声实验方法研究。项目聚焦CFRP刚度属性识别的完备可解性以及铺层顺序识别的体波物理机制等关键科学问题，1）提出了基于Floquet周期性边界条件的特征频率法以及半解析有限元方法的正问题理论模型，高效准确地揭示了多模态/大频厚积下的导波传播特性；以导波波结构特征为切入点，建立了刚度-波速的核理论，从模态、频率以及传播角度等维度多方位挖掘多独立刚度与导波波速间的灵敏度关系，确保了求逆过程的完备可解性；优化了基于遗传算法的逆问题求解策略，嵌入了并行运算与刚度矩阵正定的非线性控制单元，提升了求逆方法的效率与稳定性；创新地提出以标准偏差误差作为衡量识别结果可靠性的评定因子，并通过数值方法验证了正问题模型、灵敏度核方法以及逆问题算法的可行性。2）从材料微观组分角度，揭示了超声体波方法检测CFRP铺层顺序的物理机制，探究了超声波在界面以及材料内部不均匀组分处的反射行为，揭示了探头参数对铺层顺序检测可靠性的影响；面向复杂的三维体波信号，建立基于空间傅里叶变换的纤维方向识别方法；建立双面融合的体波扫描方法，识别了CFRP上下表面铺层的纤维方向、夯实了中间层的识别结果并大幅度提高了可检测铺层数量。3）基于非接触式激光超声测量系统，实现了多模态/大频厚积下导波传播特性的实验分析，最终完成了CFRP层合板中单层刚度属性的完备识别。项目研究内容和成果丰富了超声体波与导波在CFRP中传播特性的理论与实验分析方法、变革了刚度-波速的灵敏度理论、拓宽了材料表征的范畴、丰富了超声体波与导波在CFRP中的应用模式，可为CFRP设计、使用与维护过程提供更全面细致的重要数据，为CFRP广泛应用以及超声波方法的推广添砖加瓦。