热波成像技术及其在无损检测中的应用

     自十九世纪80年代光声效应发现以来，随着光声效应研究与应用的深入开展，作为光声效应的中间过程的热波现象研究也引起很大重视。最初的热波成像技术是光声（或电子声）热波显微镜成像技术研究及应用。由于热波能透入样品的表层内一个波长的深度，因此能检测到光学显微镜或电子显微镜所不能检测到的结构或缺陷。
  上世纪80年代以来，热波成像技术、机理及应用的研究广泛而迅速地开展起来，并取得显著成功。同时，随着高科技的发展和无损检测技术的广泛应用，将热波成像技术与其他先进的无损检测技术相结合，发展新型热波成像技术成为研究的热点。至今已成功发展两类热波无损检测技术，并已在先进工业、航空航天和国防技术等领域发挥重要应用。

1.光热红外热波成像技术：
     利用强度周期性调制（或脉冲）的强光源照射试件，试件吸收光能而激发热波，利用红外热像仪检测试件表面的温度分布。如果试件表层内出现缺陷或结构异常，表层温度就呈现不均匀分布，因此红外热像仪上出现不均匀图像。由于强光源照射的范围（面积）和红外照相机的视场都可以比较大，由此可迅速、方便地检测试件的缺陷、裂纹或其他不均匀结构问题。这种热波成像检测系统可实现无损、非接触式检测，可在较远距离对大型设备或有毒、有害物质的容器进行检测，已得到广泛应用。

2.超声红外热波成像技术：
     结合并发挥超声检测和红外热像的优点，利用强度周期性调制（或脉冲）的强超声作用于试件，类似地在试件中激发热波，并由红外热像仪检测表层温度分布情况。由于强超声作用在试件中产生非线性振动现象，使缺陷或不均匀结构区域产生比较强的热波，与结构完整部分的温差增大，因而红外热像仪能更灵敏地检测缺陷等结构不均匀区域。本技术已在许多领域对多种材质的设备或元器件进行成功的检测。

Thermal wave imaging techniques and their applications in NDE/T
Zhang Shu-yi
Institute of Acoustics, Nanjing University, Nanjing 210093, China
Since photoacoustic (PA) effects had been discovered in 1880, the studies and applications
of PA effects have been developed widely. Meanwhile, the study of thermal wave phenome
na as an intermediate process of the PA effects has also attracted a great attention. The
initial study and application of thermal wave imaging were electron-acoustic thermal wave microscopy and PA microscopy due to the thermal wave can penetrate
to subsurface of the sample, which can be used to detect structures or defects in subsurface
that cannot be detected by optical or electronic microscopies.
With the development of the advanced science and technology and the important and wide
applications of NDE/T, several mixed imaging techniques composed of thermal wave and infr
ared detections, called thermal wave imaging techniques, are developed fast and applied
widely in industries, aeronautic and astronautic engineering and militaries, etc. The thermal
wave imaging techniques are mainly classified into two categories as follows:

1.  Photothermal infrared imaging technique:
As intensity-modulated optical beams are used to illuminate a sample, the thermal waves are excited in
the sample. If there are defects existed in the surface or subsurface of the sample, the areas
around the defects are more heated, then the temperature increase in the areas are more
than the other parts. An infrared camera is used to display the inhomogeneous temperature
distribution. Since the optical sources and infrared camera have large field of view, the
detections are more fast and convenient. In addition, the technique is noncontact and
nondestructive, so it is very effective NDE/T for remote detections, especially for huge
constructions and/or instruments with poisonous and harmful environments.

2.  Ultrasonic infrared imaging technique:
Using intensive ultrasonic beam instead of optical beams infuses into a sample, the thermal
waves are also excited in the sample. The infrared camera is similarly used to display the
temperature distribution, which can easily detect the existence of defects in subsurface
layer of the sample. The technique combines the advantages of ultrasonic and infrared
detections and is very sensitive, effective and convenient..