Comparative Study On Analysis And Detection Of Three Kinds Of Carbon Fiber Composite Porosity

Porosity is a kind of defect of carbon fiber composite,the content,shape and distribution of porosity can affect the properties of the composites.Using three grades of carbon fiber composite laminates(T700/QY9611,T700/5429,B8-250) as the research object,the ultrasonic C scanning system is used to examine defects in the composite,the attenuation coefficient of the normal area is recorded.Then,the porosity is calibrated by metallography,the shape,size and distribution of pores in the composites laminates are observed by optical microscope and field emission scanning electron microscope.The porosity is calibrated by the method of nano focus computed tomography(micro-CT),each pore size parameters(diameter,area,volume,sphericity and so on) are tallied after color,log,filtering and threshold segmentation of the original scanned image.Three-dimensional reconstruction images of composites can be obtained using VG Studio software.The mathematical relationship between porosity and ultrasonic attenuation coefficient is derived,The variation law of the second-order nonlinear coefficient with the porosity is pointed out.The accuracy and influence factors of porosity by metallography,micro-CT and the method of ultrasonic attenuation are analysised comparatively,and the effect of composites' thickness and layers on porosity are studied.Based on metallography,the porosity of T700/QY9611,T700/5429,B8-250 carbon fiber composites are 0.62%,1.64% and 2.89%,respectively.Opitical microscope and SEM observation found that the large pore is oval,mostly distributed in layer resin.Small pore mostly distributed in the fiber bundle randomly,circular cross section.Based on micro-CT,the porosity of T700/QY9611,T700/5429,B8-250 carbon fiber composites are 1.13%,1.88% and 2.49%,respectively.The results show that the small pore is circular,the large pore shape is strip,this is consistent with the observation of metallography and SEM.The composites porosity is higher,the ultrasonic attenuation coefficient and second-order nonlinear coefficient is greater.There is a critical porosity,the relationship between the ultrasonic attenuation coefficient and the porosity is different around it.Error analysis on the relationship between ultrasonic attenuation coefficient shows that when the porosity is low,the porosity relative error between the attenuation formula and metallographic method is large.As the porosity increases,the error becomes small.Metallography makes porosity result more accurate.In the engineering technical field,ultrasonic attenuation model based on metallography results can be used to calibrate composites' porosity.Under the same process conditions,the thickness of the composite is greater,the porosity is higher.The porosity of the composite laminates with cross ply laminates is higher than the unidirectional laminates.