Investigations On CFRP Porosity Using Ultrasonic Testing Based On Random Pores Model

Pore is one of the most common defects in CFRP,and it effects on the performance of CFRP greatly.It is necessary to find a reliable NDT technology for CFRP porosity(P=1%～5%) detection in aerospace engineering.Ultrasonic attenuation method(UAM) is pursued in NDT detection of CFRP porosity,due to its simple theory,easy implementation and less affection by fiber content.Much work has been done in theoretical and experimental investigation,but there's no reliable NDT technology for practical engineering.One important reason is that,theoretical model for describing elastic properties of CFRP with pores has not been established.Therefore,actual relation between porosity and ultrasonic attenuation coefficient(UAC) can not be achieved.The shapes of pores in CFRP are usually simplified to spherical and ellipsoidal in present model.Based on this assumption,analytical solutions of the wave equation are obtained.It is a simple and fast way for calculation of UAC in present model, however a definite deference exist between the experiment and analysis because the assumption deviates from the actual situation of pores in CFRP.To analyze the actual relation between porosity and UAC and reduce the deviation, CFRP random pore model(RPM) is established based on random processes and statistical data of CFRP pores.This model is modified from random medium used in geology.Its constructing parameters are investigated through orthogonal test and optimized by statistical data of CFRP pores,such as autocorrelation lengths a b,their ratio r=a/b,length of division D. Compared with traditional method,RPM describes the characteristics of pores in CFRP accurately,due to the statistical data from the experiment.The model is useful for theoretical research of ultrasonic scatting by pores in CFRP.Numerical calculations of 240 CFRP cross-sections established by RPM are conducted (P=0.03%～4.96%).Then theoretical curve between UAC and porosity is obtained.Experiment tests of epoxy based CFRP are conducted(P=0.03%～4.96%).Then experimental curve between UAC and Porosity is obtained.The results that UAC increased with CFRP porosity,and that the relation between UAC and CFRP porosity behaves linear and conic relationship before and after P=0.5% respectively,are showed in numerical calculations and experiment tests.