Nonlinear Ultrasonic Measurement of the Remaining Life of Engineering Structures

无损检测（NDT）的最大挑战之一是测量结构的剩余寿命。到目前为止，大多数无损检测方法的目的是使用线性超声检测和修复结构寿命结束时出现的相对较大的缺陷。最近，非线性超声的潜在用途已在文献中被证明对早期形成的缺陷如微裂纹敏感。这是因为随着材料老化或出现缺陷，它们变得越来越非线性。不同的技术已经发展，即二次谐波产生，波混合，和扩散场方法。然而，与线性超声相比，来自非线性的信号相对较小，因此可以容易地被噪声或其他非线性所掩盖。因此，这些技术尚未得到充分利用，因为它们没有达到对材料非线性的积累进行成像所需的灵敏度。该项目旨在开发所需的建模工具，以充分了解测量方案，利用非线性技术，了解其局限性，并可能提取（或成像）材料非线性。我一直在利用的技术是波混合技术。在这种方法中，两个主要波（蓝色和绿色）相互作用，并在某些特定条件下，它们将产生一个额外的波场，称为共振波（红色）。该波可以与相互作用点处的材料非线性相关，因此具有空间局部化的潜力。到目前为止，已经成功地开发了一个二维数值模型，并验证了非线性波混合的理论。它已被用于材料内非线性的1D映射，表明必须将校正因子包括到将谐振波的振幅与材料非线性相关的文献中的方程中。下一步是探索阵列用于波混合的潜力。

One of the grand challenges in non-destructive testing (NDT) is the measurement of the remaining life of a structure. Up to now, most NDT methods aims to detect and characterise relatively large defects which occur at the end of the life of a structure using linear ultrasound. Recently, potential use of non-linear ultrasound has been demonstrated in the literature to be sensitive to early formation of defects such as microcracks. This occurs because as material ages or become defective, they become increasingly non-linear. Different techniques have been developed, namely second harmonic generation, wave mixing, and diffuse field method. However, signals from non-linearity are relatively small compared to linear ultrasound, and hence can be easily masked by noise or other non-linearities. As a result, these techniques have not been fully exploited yet, due to them not having reached the required sensitivity to image the build-up of material non-linearity. The project aims to develop the required modelling tool to fully understand the measurement scenario to exploit non-linear techniques, understand its limitations and potentially extract (or image) material non-linearity. The technique I have been exploiting is the wave mixing technique. In this method, two primary waves (blue and green) are made to interact, and under some specific conditions, they will produce an additional wavefield, termed as the resonant wave (red). This wave can be related to the material non-linearity at the point of interaction, hence having the potential of spatial localisation. Until now, a 2D numerical model has been successfully developed, and validated against theory for non-linear wave mixing. It has been used for the 1D mapping of non-linearity within material, showing that a correction factor has to be included to the equation from the literature relating the amplitude of the resonant wave to material non-linearity. The next step is to explore the potential of arrays for wave mixing.