Research On Detection Of Rolling Contact Fatigue Crack Based On Eddy Current Pulsed Thermography

When the train travels on the steel rail at high speed,the actual contact area of the wheel rail is only 100 square meter,but it has to bear a load of tens of tons,the friction between the wheel and rail is also strong,it is easy to produce rolling contact fatigue crack(RCF),the initial stage of RCF is just the opening crack on the surface,then they become the microcracks,with a length of about 1 ? 2mm,they gradually develop into diagonally inward cracks,and eventually form rail head transverse cracks or hidden damage.the length of crack already reaches 20 ? 30 mm.The research object of this paper are micro-cracks and hidden damage.Based on eddy current pulse thermography,the thermal characteristics of different coils and different lengths of cracks are studied from simulations and experiments.Different methods and algorithms are used for temperature comparison research.This article mainly carried out two research contents:(1)For the first stage of rolling contact fatigue cracks,microcracks,the difference of microcracks under the excitation of straight wires and spiral wires are compared in the simulation.Line segments of micro-cracks of different lengths were extracted from the temperature responses.Through comparative analysis,the preliminary result is that the extreme value of the temperature of the line segment beside the crack could be used as the basis to judging the length of cracks.Based on the experimental platform built in the previous stage,five tested coupons of U45 Mn was made with cracks length from 1mm to 2.5 mm,and the thermal images are enhanced by algorithms such as normalized difference,principal component analysis etc.The results show that although the characteristics of microcracks are very weak,through differential processing and other algorithms,the characteristics of microcracks have been significantly increased by 0.5d B.(2)When RCF expand to the third stage,the appearance of cracks is likely to be hidden damages before peeling off.Since the oblique crack is included in hidden damage,the research object in Chapter 3 is hidden damage directly.Through researching the form of hidden flaws,its pattern could be simplified to L shape,it approximately equals to the combination of oblique cracks and plane cracks,the differences of L-shaped cracks and Oblique crack/ plane cracks are compared respectively in the simulation and experiment.The cracks of different depths and different angles are also researched,the results show that the large size cracks could be distinguished in the cooling stage,it is initially determined that the depth of the flat plate crack is linearly related to its minimum phase.The experimental results of the crack are also enhanced by algorithm.