University of Bath, Materials Research, Department of Mechanical Engineering, Claverton Down, Bath, UK.
University of Bath, Materials Research, Department of Mechanical Engineering, Claverton Down, Bath, UK.
Ultrasonics. 2018 Nov;90:125-143. doi: 10.1016/j.ultras.2018.05.016. Epub 2018 Jun 15.
Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time-consuming, complex and not an easily automated process. A coupled baseline-free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.
线性方法检测到大的刚度变化，而非线性技术则识别小的阻抗不匹配，如微裂纹或闭合分层。通常，非线性超声技术通过测量更高阶谐波来检测材料中的损伤/缺陷。由于幅度和信噪比 (SNR) 较低，这些谐波很难测量：因此需要较大的激励幅度，这可能会进一步使这些方法的可靠性复杂化，因为设备的非线性可能会产生。为了克服这些问题，在特定频率下激励，称为局部缺陷共振 (LDR)，会在损坏区域产生更大的位移。然而，LDR 的估计既耗时又复杂，并且不是一个易于自动化的过程。提出了一种基于无基线的耦合线性和非线性超声成像方法，使用建设性非线性阵列扫描激励和图像减法方法来识别层状材料中的损伤。信号扫描方法使用窄带频率激励来增加 LDR 频率的检测概率。该新的成像方法已在各种复杂复合材料结构中使用激光测振仪测量进行了评估，以评估对飞机工业至关重要的几乎不可见的冲击损伤。结果表明，与传统的线性或非线性超声方法相比，冲击损伤的估计更好，从而提高了飞机检查的可靠性。
