Electromagnetic Thermography Nondestructive Evaluation: Physics-based Modeling and Pattern Mining

Bin Gao, Wai Lok Woo, Gui Yun Tian
2016 Scientific Reports  
Electromagnetic mechanism of Joule heating and thermal conduction on conductive material characterization broadens their scope for implementation in real thermography based Nondestructive testing and evaluation (NDT&E) systems by imparting sensitivity, conformability and allowing fast and imaging detection, which is necessary for efficiency. The issue of automatic material evaluation has not been fully addressed by researchers and it marks a crucial first step to analyzing the structural health
more » ... of the material, which in turn sheds light on understanding the production of the defects mechanisms. In this study, we bridge the gap between the physics world and mathematical modeling world. We generate physics-mathematical modeling and mining route in the spatial-, time-, frequency-, and sparse-pattern domains. This is a significant step towards realizing the deeper insight in electromagnetic thermography (EMT) and automatic defect identification. This renders the EMT a promising candidate for the highly efficient and yet flexible NDT&E. Since 1960s, thermal testing has been successfully explored in NDT&E applications 1,2 to measure the surface temperature variations in response to induced energy. The energy generates a temperature contrast at material discontinuities that can be detected by an infrared (IR) camera. It was the development of the infrared camera in the late 1970s which made it possible to directly detect the temperature contrast over large inspection areas. The IR cameras detect radiation in the IR range of the electromagnetic spectrum and generate images of IR or thermal emission called thermograms, allowing very sensitive non-contact temperature measurement. Thermal testing is also used for defect characterization and material property evaluation and inspection since it is completely noncontact and offers for the rapid inspection over a large area within a short time. It can be used in a wide range of areas 3 , such as in agriculture, civil engineering and architecture, diagnosing electrical equipments, automotive industry, medicine and biology, manufacturing industry, food quality control and protection of historic heritage. Thermal testing is generally divided into two main streams: passive infrared thermography (PIT) and active infrared thermography (AIT). Passive Thermography (PIT) is defined as measuring the temperature differences between the target materials and the surroundings under different ambient temperature conditions. AIT 4 was developed to provide more accurate information by considering the amount of thermal radiation and heat transfer. The common thermal stimulation techniques in AIT 5 are: pulsed thermography (e.g., flash thermography), step heating (long pulse), lock-in thermography, and vibrothermography (e.g., ultrasonic IR thermography). Traditional thermal radiation heating is the earliest and the simplest direct technique in AIT. The method employs a lighting source or radiative source to heat the surface of the test object. Flaws or suspicious response can be captured, according to the slow heat transfer, by an infrared (IR) camera inspecting system. Electromagnetic thermography (EMT) 6-15 which combines eddy current (EC), magnetic and thermography, and involves the application for a short period of a high current electromagnetic pulse to the conductive material under inspection. In comparison with other thermography NDT&E techniques, the heat in EMT is not limited to the sample surface, rather it can reach a certain depth, which governed by the skin depth of eddy current. Furthermore, EMT focuses the heat on the defect due to friction or eddy current distortion, and subsequently increase the temperature contrast between the defective region and defect-free areas. From point of
doi:10.1038/srep25480 pmid:27158061 pmcid:PMC4860578 fatcat:xjojaexghnhvnpdfw57jom2aku