【正文】 an incipient stage during fabrication and in service. The requirements for nondestructive evaluation are driven by the need for low cost methods and instruments with great reliability, sensitivity, user friendliness and high operational speed as well for applicability to increasingly plex materials and structures.In recent years, the thermography technology has been successfully applied to many kinds of materials, such as metals, ceramics, polymers, posites, thin films, coatings and even biological specimens. There are mainly two kinds of detection technology, . Ultrasonic Thermography and Pulsed Thermography. The difference between them is the excitation source, namely, one is ultrasonic and the other is flashlight.In this paper, the two NDT technologies are conducted to detect the defect of aircraft posites. As mentioned above, the NDT of posites is still a relative new research area, especially in the field of quantitative assessment. A new quantitative assessment method is introduced to pute the defect area and perimeter of the specimens on the basis of processing of the thermal images. The experimental results show that the method can assess the defected area size as well as perimeter size. What’s more, the length precision of the defect is estimated accurately.II. FUNDAMENTAL PRINCIPLEIn thermal wave imaging, a beam of energy, usually an ultrasonic or a flashlight beam, is focused and scanned across the surface of a specimen. As the beam scans across the specimen it is absorbed at or near the surface, and periodic surface heating results at the beam modulation frequency. This periodic heating is the source of thermal waves, which propagate from the heated region. The thermal waves are diffusive waves similar to eddy current waves, evanescent waves, and other critically damped phenomena that travel only one to two wavelengths before their intensity bees negligibly small. Nevertheless, within their range, the thermal waves interact with thermal features in a manner that is mathematically similar to the scattering and reflection processes of conventional propagating waves. Thus any features on or beneath the surface of the specimen that are within the range of these thermal waves and that have thermal characteristics different from their surroundings will reflect and scatter the waves and bee detectable. These thermal features can be defined as those regions of an otherwise homogeneous material that exhibit variations, relative to their surroundings, in either the thermal conductivity, the volume specific heat, and in some instance the thermal expansion coefficient. Variations in these thermal parameters arise, most monly, from variations in the local lattice structure of the material and, for metals and other good electrical conductors, from variations in the local carrier concentration.As soon as the excitation power is excited into the specimen, the infrared camera controlled by the puter begins to collect the thermal information of the surface of specimen. The information obtained by the camera will be sent to the puter, and then the sequence of the thermal images will be shown visualized after analyzed and processed by the special software, so that the size and position of the defects in the subsurface of the structure can be identified.III. EXPERIMENT SETUP Sketch map of experimental setupThe typical infrared thermal wave NDT system consists of an excitation device, an IR camera and a puter etc. The sketch map of experimental setup is shown in . Two different excitation sources are employed for the NDT of a posite specimen. One is ultrasonic excitation device, and the other is flashlight excitation. The source of ultrasonic excitation is a Branson ultrasonic welding generator with model and exciting frequency of 20 KHz. The model of infrared camera is JADE III, which has an array detector size of 320240 elements, and has a temperature resolution better than degree Kelvin. The collecting frequency and time can be adjusted, and its work wave range is um to um. While high energy flashlights are used in the flashlight excitation whose pulse time and width can be set respectively. The original digital information should be analyzed and processed by the special software of IR camera.IV. EXPERIMENT AND RESULTS Composite Specimen Outline of Composite Specimen 1The carbon fibre posite of the aircraft is used to evaluate the performance of the thermal inspection technique. Specimen 1(180mm180mm) has a sizeknown label (upper right corner) and a defect area (lower left corner) as shown in . The label () is used to test the accuracy of the method of calculation and calibrate the real parameters of defects. The defect with maximum width and height mm respectively is generated by a falling heavy ball. Fig. 3 displays another triangle hollow specimen. The triangle is 95mm in base length and 655mm in width, what’s more, the height of the specimen is 55mm and the thickness is . A small yellow circular label whose diameter is is adhered on the surface of the specimen, which is shown in a). A manmade defect formed by striking maximum in length and mm in width is on the reverse side, as is listed in b). The defect cannot be identified by naked eyes from the right side.a) Right sideb) Reverse side Outline of Composite Specimen 2 Experimental ResultsTwo experiments with different excitation sources are carried out. For the ultrasonic thermography technology, a bench clamps is used to fix the specimen while the ultrasonic is being excited in. The distance between the IR camera and specimen is about 40cm. Then turn on the puter and ultrasonic device, adjust the position of IR camera so that the specimen has a suitable position in the field of view, and adjust the focus of IR camera. The ultrasonic weld time is set to , and the force we used is 44N. The IR camera uses the frame frequency of 50Hz to collect thermal information in 8s. The