Laser ultrasound (LU) is a contactless and couplant-free remote non-destructive (NDE) technique, which uses lasers for ultrasonic generation and detection rather than conventional piezoelectric transducers. For a transducer, an important characteristic is the directivity, the angle-dependent amplitude of the ultrasonic waves generated in the material. In the non-destructive thermoelastic regime, LU source has been widely modelled as a surface force dipole. However, the directivity of LU in more complex material, where there is an increasing demand for NDE, such as carbon fibre reinforced plastic (CFRP), is yet to be understood. In the current paper, a finite element (FE) modelling methodology to obtain the directivity of LU in complex material is presented. The method is applied to a conductive isotropic material (aluminium, Al) for validation against an existing analytical solution and then applied to a heterogeneous anisotropic material (carbon-fibre reinforced plastic, CFRP). To get the directivity of a specific wave mode, the signal for that mode needs to be resolved in time from other modes at all angles. This is challenging for shear (S) waves in a small model domain due to the head wave, so a technique for suppressing the head wave is shown. The multi-physics model solves for thermal expansion, which models the laser source as a surface heat flux for the Al case, and a buried heat source for the CFRP case, according to where the energy is deposited in the material. The same ultrasound generation pattern can be obtained by using a suitable pure elastodynamic loading, which is shown to be a surface force dipole as per the validation case for Al, and a buried quadrupole for the CFRP case. The modelled directivities are scaled and fitted to experimental measurements using maximum likelihood, and the goodness of fit is discussed. For the Al case, the S wave is preferred over the longitudinal (L) wave for inspection due to greater signal amplitude. For the CFRP case, the quasi-longitudinal (qL) wave in CFRP shows a maximum amplitude directly below the source, and has a greater amplitude than the quasi-shear (qS) wave, suggesting a better choice for inspection.

中文翻译：

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用于确定热弹性激光超声方向性的有限元建模策略

激光超声 (LU) 是一种非接触式、无耦合剂的远程无损 (NDE) 技术，它使用激光而不是传统的压电换能器来产生和检测超声波。对于换能器来说，一个重要的特性是方向性，即在材料中产生的超声波的与角度相关的幅度。在非破坏性热弹性领域，LU 源已被广泛建模为表面力偶极子。然而，LU 在更复杂的材料（例如碳纤维增强塑料 (CFRP)）中的方向性仍有待了解，这些材料对 NDE 的需求不断增加。在本文中，提出了一种获得复杂材料中 LU 方向性的有限元 (FE) 建模方法。该方法应用于导电各向同性材料（铝，Al），根据现有分析解决方案进行验证，然后应用于异质各向异性材料（碳纤维增强塑料，CFRP）。为了获得特定波模式的方向性，需要从各个角度及时解析该模式的信号与其他模式的信号。由于头波，这对于小模型域中的剪切 (S) 波来说是一个挑战，因此显示了一种抑制头波的技术。多物理场模型求解热膨胀问题，根据能量沉积在材料中的位置，将激光源建模为 Al 情况下的表面热通量，以及 CFRP 情况下的埋藏热源。通过使用合适的纯弹性动力负载可以获得相同的超声波生成模式，根据 Al 的验证案例，该负载显示为表面力偶极子，而对于 CFRP 案例，该负载显示为埋入式四极子。使用最大似然对建模的方向性进行缩放并拟合到实验测量，并讨论拟合优度。对于 Al 情况，由于信号幅度较大，因此 S 波优于纵波 (L) 进行检查。对于CFRP情况，CFRP中的准纵向（qL）波在源正下方显示出最大振幅，并且具有比准剪切（qS）波更大的振幅，这表明是更好的检查选择。