基于阵元指向性的复合材料曲面结构超声表面契合法研究

采用相控阵超声结合表面契合法(Surface adaptive ultrasonic，SAUL)检测复合材料曲面结构过程中周向缺陷检测分辨力低，给缺陷定量带来困难。为提高SAUL的检测能力，以T700/环氧树脂T形长桁为研究对象，建立弹性各向异性有限元模型，并对照弹性各向同性情况分析其声传播规律，发现弹性各向异性和层间反射共同作用导致肋板处形成明显噪声。在此基础上，基于阵元指向性开展曲面结构成像检测研究，结果表明：引入阵元指向性校正系数对声源脉冲信号幅值进行优化，降低探头频率，可使阵元声场更适应曲面结构，从而减弱两侧肋板反射，提高成像质量。针对周向长度4.5 mm、深1.5 mm的分层缺陷，改进后的SAUL方法对应仿真和试验中周向缺陷长度定量误差较常规SAUL结果分别减小7.4%和13.1%，表明优化阵元指向性可有效改善SAUL周向检测分辨力，提高复合材料曲面结构超声检测缺陷定量水平，具有推广应用价值。

Research of Surface Adaptive Ultrasonic Testing on Defects in Curved Composite Structures Based on Beam Directivity

A weak resolution in the circumferential direction of curved composite structures based on the surface adaptive ultrasonic (SAUL) combined with phased array ultrasonic brings challenges to the quantitative testing of defects. To improve the circumferential resolution of SAUL testing, an elastic anisotropic finite element model based on a T stringer manufactured with T700/epoxy resin by hot compression curing is established. Compared with the elastic isotropic model, the wave propagation behavior is analyzed. It is found that the noise of ribs on both sides is mainly caused the strong interfacial reflection echoes of layers and the anisotropic nature of the material. On this basis, the beam directivity of the array elements is studied. Results show that with the introduction of a correction coefficient of directivity for each element and reduction of the probe frequency, the amplitude of pulse signal is optimized, and the emitted ultrasonic field is more suitable for the curved structure. Accordingly, the reflection of the ribs on both sides is weakened, and the imaging quality is improved. Then a delamination with a 4.5 mm length and 1.5 mm depth in circumference is quantitatively detected. In the simulation and experimental results, the errors of circumferential length are reduced by 7.4% and 13.1%, respectively. The results show that the circumferential resolution of SAUL could be effectively improved by optimizing the directivity of array elements. The method would be valuable and promising to improve the detection quality in curved composite structures.