Research On Nondestructive Testing Of Defects In The Insulation Layer Of EWIS Wires In Civil Aircraft Using Infrared Thermal Imaging

Civil aircraft electrical wiring interconnection system(EWIS)refers to a system composed of all wires and wiring devices used to transmit electrical energy on a civil aircraft.EWIS wires in civil aircraft is comparable to the blood vessels and neural networks in the human body.Failures in EWIS wires will cause major flight accidentInsulation layers of wires may be broken due to cabin vibration,harsh environment and irregular wire installation.If defects in the insulation layers of wires are not detected in time,flight safety is not guaranteed.Consequently,the testing of defects in the insulation layers of EWIS wires is of immense significance.The existing methods for testing defects in the insulation layers of EWIS wires have major limitations.Infrared testing has unique advantages of being efficient,convenient,accurate,reliable,and easy for automation.However,few studies have been dedicated to infrared testing of EWIS wire insulation defects.This thesis analyzes the influence of defects on the temperature distribution on wire surface and the regulation of the influence varying with time under different types of thermal excitation.The thesis also improves and optimizes the thermal excitation devices and processes the infrared thermal images to improve the recognition of defects.The thesis studies the testing of wires under each of the four types of thermal excitation(heating with halogen lamp,heating with flashlight,blowing with heat gun,and energizing the wire)through numerical simulation.A three-dimensional finite element model of wire testing is established.Through the numerical simulation,the influence of defects on the temperature distribution on wire surface and the regulation of the influence varying with time under different types of thermal excitation are analyzed.Furthermore,the influence of defect depth,energizing current intensity,and the angle of hot-air heating on the temperature distribution on wire surface are analyzed.Finally,the influence of single or double defects and the distance between double defects on the temperature distribution on wire surface is studied.Experimental study is carried out on wire testing under each of the four types of thermal excitation,and the corresponding experimental platforms are built.The experimental results show that under heating by halogen lamp,defects have similar effects on the temperatures on the surfaces of the two types of wires with different insulating materials.That is,the temperature at defect is lower than that outside of the defect.If there is a defect on the reverse side of the wire,its location can also be identified.Under the action of heat gun,by adjusting the wind speed and air-vent temperature,it is found that a heat gun with low wind velocity and low air-outlet temperature is good enough for defect detection.The efficiency of testing is very high with pulse thermal excitation.What’s more,the defect indication is more obvious with higher power of the flashlight.Under energizing the wire,it is found that when increasing the energizing time and current intensity,the difference between the temperatures at defect and outside of the defect is increased;correspondingly,the defect indication is more obvious.Moreover,the defect indication appears more quickly and is more noticeable if the abrasion defect in the insulation layer is more serious.It is also found that the appearance of crack indication is slower than that of the indication of abrasion defect and the indication of longitudinal crack is more apparent than that of transverse crack.The thermal excitation devices are improved.For the halogen lamp,the light is focused on the wire,and the experimental results show that the concentration of light makes the defect indication more obvious.Under the action of heat gun,the influence of blowing method on testing results is studied.The experimental results show that the point blowing,the line blowing and the surface blowing are suitable for the testing of single wire,side-by-side multi-wires and large-area testing,respectively.The thesis proposes two image enhancement processing technologies based on wires stripping from background and background temperature removal,respectively to improve the recognition of defects in the infrared thermal images through image enhancement.Then,the temperatures on wire surface in the processed image are extracted and the temperature curve is drawn.The relationship between the length of the defect and the number of pixels in the region of temperature change in the curve is obtained.