Lichun Feng

Specified value based defect depth prediction using pulsed thermography

Several methods have been reported in the literature using pulsed thermography for quantitative measurement of defect depth or sample thickness. In this paper, based on the analysis of a theoretical one-dimensional solution of pulsed thermography, a new method was proposed to first multiply the original temperature decay with square root of the corresponding time to obtain a monotonically increasing function f. A specific time was obtained by setting f equals to a predefined value, the theoretical model shows that the obtained specific time has linear relation with square of defect depth, which was verified with the experimental results of one aluminum and one glass fiber reinforced polymer specimen machined with six flat-bottom wedges as simulated defects. This linearity can be used for defect depth prediction in pulsed thermography.

The application of pulsed thermography in the inspection of wind turbine blades

Wind power is a very promising source of environmentally safe, renewable, and the fast-growing energy source over the past several years. The blades of a wind turbine are considered to be an important component in wind turbine generator. Currently, bigger and more powerful wind blades are being built to increase the swept area of the turbine and extract more energy from the wind. Correspondingly, more capital cost is invested in manufacture and service. In order to reduce damage possibility and extend the wind turbine blades life, there are increasing demands for the inspection of wind turbine blades in the manufacturing factory and on site inspection. The regular inspections of wind turbine blades are done normally by using visual inspection and tapping test. To improve the safety of wind turbine blades, nondestructive testing technique using pulsed thermography is being investigated in this study. This technique utilized an active pulsed heating source that is applied on the outer surface of wind turbine blades, and an infrared camera to monitor the surface temperature distribution controlled by a computer. Reflective pulsed thermography was directly applied on several full scale wind blades, surface and subsurface defects, such as air bubbles, pin holes, edge bonding, etc. were clearly detected. Several specimens were intentionally manufactured to simulate the glue faults between supporting spars and glass fiber reinforced plastic (GFRP) shells with different thickness. Afterwards they were inspected by using pulsed thermography in laboratory. The current test results indicated that pulsed thermography has the potential for the detection of glue faults at least about 15mm thickness GFRP shell. It is shown that pulsed thermography maybe provide a powerful non-contacting technique for the inspection of wind turbine blades as well in the workshop just after the production or in the field that before and after installation of the wind blades and during reparation.

Developing signal processing method for recognizing defects in metal samples based on heat diffusion properties in sonic infrared image sequences

Abstract. In sonic infrared (SonicIR) imaging, heat is generated in defect areas during the sonic pulse; the heat appears bright in SonicIR images as the indication of a defect. However, in practical applications of SonicIR, there are lots of disturbing bright areas in infrared images, such as heat reflection and paint problem. When crack size is small, the generated heat appears not bright enough to be recognizable. Based on heat diffusion properties in the one-dimensional temporal and two-dimensional spatial domain, a method is developed to automatically recognize defect signals from SonicIR image sequences. The algorithm is verified with the SonicIR image sequences of 100 metal plates which may have different thickness, materials, or crack sizes.

Bonding quality evaluation of wind turbine blades by pulsed thermography

The glue defects of the wind turbine blades which are composed of the glass fiber reinforced plastic (GFRP) composite plates make its strength greatly reduced, so security issues could be caused. To improve the safety of wind turbine blades, nondestructive testing technique using pulsed thermography is being investigated in this study. The results of ultrasonic C scan test were compared with the results of thermography. The current results indicated that both methods can successfully detect two gluing situations. However, the inspect specimens need to be putted in the water in the detection process by ultrasonic C scan, and the detection time lasts much longer than pulsed thermography. And in situ applications, the measured wind turbine blades are normally in the size of several tens meter, and also only one side is available for the inspection especially at the tip of blades. Thus, ultrasonic C scan of current experimental setup is not suitable for the applications in the field. Pulsed thermography is not necessary to contact with inspected specimens. The infrared results by pulsed thermography indicate that the shape and size of deficiency glue defects in the specimens show good agreement with the real situation, so it is more suitable for the inspection in the field. The preliminary results in this study indicate that pulse thermography can be used to detect glue faults of GFRP which are not too thick.

Application of ultrasonic infrared thermography on the evaluation of CFRP foam sandwich structure

Carbon Fiber Reinforced Plated (CFRP) with Foam sandwich has the merits of high strength and light weight, which was used in the fields of manufacturing industry, aeronautics and astronautics. The basic principles and characteristics of the ultrasonic infrared thermography is discussed in the paper. The ultrasound is introduced into the sample, and the mechanical vibration weakens and transfers to heat at the place where defect or crack locates. A infrared camera monitors the surface temperature distribution and records this process. A CFRP foam sandwich structure sample with five pre-embedded defects was inspected with ultrasonic infrared thermography. The results is shown and compared with pulsed thermography.

Ultrasonic infrared thermal wave nondestructive evaluation for crack detection of several aerospace materials

The applications of ultrasonic infrared thermal wave nondestructive evaluation for crack detection of several materials, which often used in aviation alloy. For instance, steel and carbon fiber. It is difficult to test cracks interfacial or vertical with structures surface by the traditional nondestructive testing methods. Ultrasonic infrared thermal wave nondestructive testing technology uses high-power and low-frequency ultrasonic as heat source to excite the sample and an infrared video camera as a detector to detect the surface temperature. The ultrasonic emitter launch pulses of ultrasonic into the skin of the sample, which causes the crack interfaces to rub and dissipate energy as heat, and then caused local increase in temperature at one of the specimen surfaces. The infrared camera images the returning thermal wave reflections from subsurface cracks. A computer collects and processes the thermal images according to different properties of samples to get the satisfied effect. In this paper, a steel plate with fatigue crack we designed and a juncture of carbon fiber composite that has been used in a space probe were tested and get satisfying results. The ultrasonic infrared thermal wave nondestructive detection is fast, sensitive for cracks, especially cracks that vertical with structures surface. It is significative for nondestructive testing in manufacture produce and application of aviation, cosmography and optoelectronics.

NDT applications of non-contact thermosonics

Thermosonics or SonicIR has been proven an effective NDT method, in which ultrasonic welding horn is pushed against the tested sample under certain force through a piece of coupling material. Due to the mechanical contact between horn and sample, it may damage the contacted surface, especially for some brittle or fragile samples. In this study, the conventional horn in a small size was replaced by a much bigger horn to avoid the direct contact of horn with the sample. The tested sample could be positioned up to several centimeters away from the bottom or beside of the horn, heat is generated at the defect location under the excitation of ultrasonic field, its heating mechanism is similar with contact thermosonics, the infrared camera could be positioned wherever is convenient to monitor the variation of the surface temperature. The presented experimental results show that the non-contact thermosonics has some potential in NDT application.

Hidden heterogeneous materials recognition in pulsed thermography

Pulsed thermography has been proven to effectively identify fluid ingress in aerospace honeycomb parts while inspecting large areas in a fast manner. Water, hydraulic oil and excess glue between skin and core may have similar appearance in the infrared image sequences, it is useful to detect what kind of ingress it is. In this study, a simple structure was used to simulate the fluid ingress in a honeycomb part, a 20mm thick steel slab was machined four 1.1mm depth and four 2mm depth circular flat-bottom holes with 20mm diameter at the same side. All holes were filled with different materials: water, oil, air and wax to simulate fluid ingress and excess glue. An algorithm was proposed to first find each hole based on fundamental imaging processing technologies, and then it is based on temporal thermal diffusive properties to automatically recognize what kind of fluid ingress it is in each hole. It was verified with the experimental results of different quantities of fluid ingress and several different flash power levels.Pulsed thermography has been proven to effectively identify fluid ingress in aerospace honeycomb parts while inspecting large areas in a fast manner. Water, hydraulic oil and excess glue between skin and core may have similar appearance in the infrared image sequences, it is useful to detect what kind of ingress it is. In this study, a simple structure was used to simulate the fluid ingress in a honeycomb part, a 20mm thick steel slab was machined four 1.1mm depth and four 2mm depth circular flat-bottom holes with 20mm diameter at the same side. All holes were filled with different materials: water, oil, air and wax to simulate fluid ingress and excess glue. An algorithm was proposed to first find each hole based on fundamental imaging processing technologies, and then it is based on temporal thermal diffusive properties to automatically recognize what kind of fluid ingress it is in each hole. It was verified with the experimental results of different quantities of fluid ingress and several different flas...

The effect of flash power on the measurement of thermal effusivity using thermal wave imaging

In aerospace applications, water or oil may ingress in the honeycomb structure, it is important to detect what kind of liquid ingression it is. In this study, a 20mm thick steel plate was milled eight circular holes (four 1.1mm depth and four 2mm depth) at the back side, each hole was filled with different materials: water, oil, air and wax. Thermal wave imaging technology was successfully used in many fields, such as aerospace, automobile, etc. quantitatively and qualitatively, it was used to measure the thermal effusivity of filled materials in this study. A special experimental setup was adopted that the steel sample was horizontally placed on a cover with holes faced above. The bottom surface of the detected sample is heated with a short pulse of light, the sample surface is instantaneously heated to a high temperature and captured by a high speed and high precision infrared camera. The generated heat at front surface propagates to the interior of the sample, and leads to a continuous decrease of the surface temperature. The theoretical model of temperature evolution with time was constructed, and the calculation procedure of embedded material filled in steel holes was deduced based on the theoretical model, and in which the air hole was used as the reference. In thermographic applications, different power supplies, detection distance and infrared camera, etc. may result different signal levels, and noise level may also vary which depends on the usage conditions of infrared camera. In this study, nine different flash power levels, which changed from full scale power level to one ninth linearly, were used to simulate different noise levels. The results of three different filled materials at nine different powers and the corresponding error among different powers were compared. The calculation results indicate that thermal wave imaging is a potential technology to test the thermal effusivity of an unknown material when it is embedded in a known material.

Elimination of reflection induced artifacts in flash thermography

Pulsed thermography is a technique in which pulsed flash energy is applied to the surface and the temperature of the surface is recorded and analysed. Generally the temperature above the defect areas is different from that of the surrounding area. However, when the surface of the specimen is highly reflective, artifact of fixed pattern could be introduced which comes from the reflection of the heated lamp tube. Several methods were used to eliminate the artifact, including spatial filtering, image subtraction and frequency domain filtering. Results show that spatial filtering may be the best method of the fixed pattern artifact elimination.