超声红外热像检测中缺陷发热的瞬态温度场的有限元分析

在超声红外热像技术中，高能量的超声波能使各种材料中的缺陷发热，通过红外热像仪监视样品的温度变化，可以观察到缺陷区在超声激发过程中温度异常升高，从而实现对样品的检测．利用有限元数值计算方法，对脉冲超声波在缺陷(裂纹)处激发的热源引起的瞬态温度场进行分析，计算结果与实验结果很好吻合，从而证明了超声波在缺陷处导致发热是一个平稳的物理过程，且超声红外热像技术对样品的检测是一个无损检测过程．还对材料热学参数与超声激发过程中缺陷中心温度的关系进行了数值分析，为利用红外热像研究材料的各种性质提供了参考依据．

FEM Analysis of Transient Temperature Fields of Samples with Defects during Ultrasonic Pulse Excitation

Recently a new hybrid ultrasonic/infrared technology has been developed, in which a short pulse of ultrasound is used to cause extra-heating of cracks in a variety of materials and make them visible in front of an infrared camera. When a short ultrasonic pulse is excited and infused in a sample, the temperature will increase due to the ultrasonic attenuation. If the sample has surface or subsurface defects, the rise of temperature is more apparent at the defective areas than in other areas. The temperature rise results from frictions inside the crack, so the defective areas are extra-heated selectively and locally by the ultrasound. The heating effect in defective areas can be displayed as a bright feature by an infrared camera. After the beginning of an ultrasonic pulse excitation in the sample, the bright image appears, spreads and approaches the maximum brightness with the continuous heating of ultrasound and the heat diffusion around the crack. Finally, as the ultrasonic excitation stops, the resulting temperature pattern becomes cloudy and weak by the thermal diffusion process. In fact, the process of the acoustic attenuation and the thermal generation is very complex, so analytical modeling and numerical simulations are needed in comprehending the mechanisms of the extra-heating phenomena. In this paper, the transient temperature fields of defective areas of samples induced by ultrasonic pulse are analyzed by using finite element method (FEM) based on a point heat source model. The numerical calculation results are in good agreement with those of experimental observations and measurements. By simulating the temperature fields of different materials, the relationships between the temperature and material thermal parameters are studied, which demonstrate that the temperature is sensitive to the material thermal parameters.