Induction Thermography Of Carbon Fiber Reinforced Plastics

Carbon fiber reinforced polymers which is abbreviated by term of CFRP are widely used in aerospace,automobile,sport products because of superior property of its high strength,low density.The application of carbon fiber reinforced plastics has brought a new challenge to nondestructive testing and evaluation NDT&E.However previous NDT research work in the field didn't handle all of details on many types of CFRP.The research could be divided into three steps regarding the complexity.Firstly,one millimeter thick plate of non-woven uni-directional CFRP(abbreviated for UD CFRP)was studied by induction thermography.It was found that close to the short edge of rectangular coil there was minimum line of heating in fiber direction surrounded by heated area.The minima area far from being heated could lead to dead inspection zone when performing NDT&E.In supplement experiments,potential confusing factors,like specific set-up between specimen and induction coil,nature imperfection of specimen and coil geometry uniqueness were excluded successively by specimen rotating,testing set-up changing,numerical simulating and attempt with diverse coil.Secondly,more complicated bi-axial six-layer UD stacks was studied.Phase image derived by lock-in calculation showed clearer multi-layer fiber structures.By function fitter,relationship between amplitude signal and modulation frequency suggested that induction heating of CFRP was to be volume heating rather than surface heating.Thirdly,inspection on more complicated multi-layer woven CFRP with different impact damage levels was issued respectively by induction thermography,flash thermography and computed x-ray tomography.Results showed that x-ray tomography was the best approach to reach highest spatial resolution and internal probing.However,for a fast inspection,phase image of induction thermography offered better defect contrast indicator for NDT on CFRP than tomography and LED thermography when impact energy is higher than 3 J.In addition,simulation on internal eddy current behavior during induction thermography revealed that although orthogonal volume of current component with respect to fiber direction was less than one percent of current component along fiber direction,it was the dominate contributor for material heating.Till then,heating mechanism for anisotropy CFRP was partially determined.This research evaluated inspection effectiveness on various complex-level CFRP material,which made it clear that UD CFRP is able to be induction heated which is contrary to previous literature.The found and interpreted of minima heating area is complementary work for previous induction thermography checking on UD CFRP.Also,performance of induction thermography testing on impact damage detection was verified.All the work could be utilized to build a more developed approach for performing NDT on CFRP material.