Research On Structural Damage Detection Of CFRP Composite Plates Using Vibro-thermography

Carbon fiber reinforced polymer(CFRP)is an advanced composite material that has been widely used in recent years in aerospace,military,and automotive engineering fields.Due to imperfect manufacturing process and harsh service environments,defects such as delamination,debonding and matrix cracks are easily generated inside composite components.These defects lead to degradation of material and structure performance,especially closed delamination,which seriously reduces material performance and structural integrity,so it is important to carry out research on rapid and effective detection techniques for such defects.vibro-thermography is an efficient,fast and reliable detection technique,which can selectively heat the defects and is especially effective in detecting closed defects.In this thesis,the detection of delamination of CFRP composite plates by vibro-thermography was investigated by numerical simulation and experimental methods,and the heat generation mechanism and influencing factors of delamination defects were analyzed.In terms of theoretical analysis,this thesis deduces the heat generation and heat transfer mechanisms of defects and the principles related to image enhancement algorithms under the action of ultrasonic based on the theory of ultrasonic propagation within CFRP composite plates,which provides a theoretical basis for the detection of delamination in CFRP composite plates by vibro-thermography.In terms of numerical simulation,detection models with different wave propagation distances,delamination sizes,and distribution depths were established using the finite element analysis method.The calculation results show that heat is generated by delamination under the action of ultrasonic and the generated heat is transferred to the surface of the CFRP composite plate.According to the contact theory,it is proved that frictional heat generation is the main cause of heat generation from delamination,and the effects of wave propagation distance,delamination size,and delamination distribution depth on heat generation from delamination are discussed.The temperature on the surface of the CFRP composite plate corresponding to the delamination reaches a maximum value rapidly at the beginning of heat transfer,and then the temperature starts to decrease and stabilizes.The closer the delamination is to the ultrasonic source,the shallower the distribution and the larger the size of the delamination,the more heat is generated by the delamination.For the experiments,an vibro-thermography system was built to detect the prefabricated delaminations within the CFRP composite plate.The thermal image captured by the infrared camera was processed by background reduction and then image enhancement was performed by three algorithms: thermal signal reconstruction(TSR),principal component analysis(PCA),and fast Fourier transform(FFT).The PCA algorithm was selected as the optimal image processing scheme using signal-to-noise ratio and temperature contrast parameters as criteria,and the delamination in the thermal image was quantified using the Otsu algorithm.The results show that the thermal image processed by the three algorithms can detect the delamination with minimum diameter of 5mm and depth of 1.5mm,and the detection error of the delamination is minimum 0.6% and maximum not more than 28%.The experimental and simulation results show that the vibro-thermography can provide a fast,reliable and effective means for delamination detection of critical CFRP components in service.