# 激光超声在木材无损检测中的仿真研究Simulation of Laser Ultrasound in Wood Nondestructive Testing

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By the numerical simulation, the mechanism of laser ultrasound and propagation characteristics in the wood and porous structure of wood and special effect on the resonance absorption and scattering of ultrasonic signal have been investigated. Based on the analysis of the inner cavity surface defects and the influence of ultrasonic signal, the numerical simulation results show that the existence of defects in ultrasonic obvious reflection, transmission and diffraction phenomenon, can identify the existence of the inner cavity and surface defects of wood. This provides a numerical and theoretical basis for the realization of wood defect nondestructive testing by laser ultrasound technology, contributes to enrich the theory of laser ultrasonic and, more importantly, provides a new way for the nondestructive testing of wood and wood materials.

1. 引言

$f\left(t\right)=\frac{2}{\sigma \sqrt{\text{2π}}}\mathrm{exp}\left[-\frac{{\left(t-{t}_{0}\right)}^{2}}{2{\sigma }^{2}}{\omega }_{c}^{2}\right]\mathrm{sin}\left({\omega }_{c}t\right)$ (1)

$\begin{array}{c}F\left(\omega \right)=\mathrm{exp}\left\{-\frac{{\sigma }^{2}}{2{\omega }_{c}^{2}}{\left(\omega -{\omega }_{c}\right)}^{2}\mathrm{exp}\left[-i\left(\omega -{\omega }_{c}\right)\right]{t}_{0}\right\}\\ \text{\hspace{0.17em}}\text{\hspace{0.17em}}-\mathrm{exp}\left\{-\frac{{\sigma }^{2}}{2{\omega }_{c}^{2}}{\left(\omega +{\omega }_{c}\right)}^{2}\mathrm{exp}\left[-i\left(\omega +{\omega }_{c}\right)\right]{t}_{0}\right\}\end{array}$ (2)

$\frac{1}{\rho {c}^{2}}\frac{{\partial }^{2}{p}_{t}}{{\partial }^{2}{t}^{2}}+\nabla \cdot \left(-\frac{1}{\rho }\left(\nabla {p}_{t}-{q}_{d}\right)\right)={Q}_{m}$ (3)

$\frac{1}{\rho {c}^{2}}\frac{{\partial }^{2}{p}_{t}}{{\partial }^{2}{t}^{2}}$ 为声压随着时间的变化项， $\nabla \cdot \left(-\frac{1}{\rho }\left(\nabla {p}_{t}-{q}_{d}\right)\right)$ 为声传导引起的变化， ${Q}_{m}$ 为声源项， ${p}_{t}$

2. 模型的建立

2.1. 激光在木材中的超声激发机制

Figure 1. (a) The time domain wave distribution of an equivalent force source of laser; (b) Spectrum diagram of an equivalent force source of laser; (c) Solid wood panel model

2.2. 木材模型建立

Figure 2. (a) The stress distribution at ${t}_{1}=8.5×{10}^{-5}\text{\hspace{0.17em}}\text{s}$ ; (b) The stress distribution at ${t}_{2}=9.0×{10}^{-5}\text{\hspace{0.17em}}\text{s}$ ; (c) The displacement distribution at ${t}_{1}=8.5×{10}^{-5}\text{\hspace{0.17em}}\text{s}$ ; (d) The displacement distribution at ${t}_{2}=9.0×{10}^{-5}\text{\hspace{0.17em}}\text{s}$

2.3. 多孔性对超声信号的影响

3. 缺陷的数值仿真与分析

3.1. 木块缺陷模型

Figure 3. Effect of porous wood panel on ultrasonic propagation. (a) The ultrasonic propagation of no porouswood panel at t = 8 us; (b) The ultrasonic propagation of no porouswood panel at t = 8 us; (c) The spectrum diagram of no porouswood panel at opposite receiving points; (d)The spectrum diagram of porouswood panel at opposite receiving points

1.5 cm，宽1.5 mm，深度1.5 mm。

3.2. 激光激发超声波测内部缺陷

Figure 4. Propagation of ultrasonic in the longitudinal section (a) t = 7.4 us; (b) t = 9.2 us; (c) t = 13.4 us

Figure 5. Propagation of ultrasonic in the longitudinal section (a) t = 7.4 us; (b) t = 9.2 us; (c) t = 13.4 us

3.3. 激光激发超声波测木材表面缺陷

Figure 6. Propagation of acoustic in the board surface (a) t = 3.4 us; (b) t = 5.9 us; (c) t= 6.75 us

Figure 7. Acoustic Spectrum diagram of the receptor (a) Surface defects located between the laser source and the receptor; (b) No surface defects

Figure 8. Distribution of sound pressure in three-dimensional solid wood (a) = 3 us; (b) = 9 us

Figure 9. Propagation ultrasonic in longitudinal section of solid wood (a) 9 us; (b) 15 us; (c) 18 us

Figure 10. Propagation of ultrasonic in surface of the solid wood (a) 3 us; (b) 6 us; (c) 12 us

4. 结论

NOTES

*通讯作者。

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