Inspection And Recognition On The Defects Of Laminates Manufactured By Liquid Composites Molding With Tailored Fiber Placement

As the structure of the aircraft the compositewindow frame manufactured by liquid composites molding(LCM)with tailored fiber placement(TFP)process should haveenoughstructural strength.But the compositewith defects will affect the mechanical properties,thereby itwillthreaten the safety of the aviation.The TFP/LCM process technology is developed in recent years,the defects inthe TFP liquid forming composite are more complicated and difficult to test.There are few domestic and foreign researches on the defects of composite manufactured by the TFP/LCM process,which affects its further use.Therefore,it is of great significance totest and recognise the defects of TFP/LCM composites.In this paper,the carbon fiber preform is manufactured by the TFP technology,andthe composite is manufactured by the resin transfer molding process(RTM).The TFP/LCM compositeswithdelaminationdefect and different porosityaremanufacturedby adjusting RTM process parameters.The use of ultrasonic non-destructive testing,metallographic microscope and image analysis to systematically study the delamination and pore defects,clarify the pore morphology of the TFP/LCM composites,and establish the relationship between the ultrasonic attenuation coefficientap and porosity P,,.The specific research results are as follows:The pore size changes with the porosity.When the porosity is in the range of 0 to 2.5%,the range of the maximum diameter(Dmax)and minimum diameter(Dmin)of the pores is roughly 0 to 1700 ?m and 0 to 350?m,respectively.When the porosity is lower(1.19%?Pv<2.5%),the Dmaxand Dminare concentrated in 75-200?m and 25-100?m,respectively;when the porosity is higher(1.19%-P,,<2.5%),theDmax and Dminare concentrated in 100-400?m and 50?200?m,respectively.the pore size is increasing with the increase of porosity,and the Dmax and Dmin both take the form of a lognormal distribution.The stitching yarn affects the distribution of pore position,shape and direction.When the porosity is lower,the pores are mainly located in the stitching yarn,the pore shape is more regular,the proportion of elliptical and elongated pores is higher,and a large number of stitching yarns are perpendicular to the fiber layer so that the proportion of pores are perpendicular to the fiber layerdirectionis higher than that of the poresareparallel tothefiber layerdirection;when the porosity is higher,the proportion of pores outside the stitching yarn is significantly higher than that of poresinsidethe stitching yarn,and the increase in porosity mainly comes from theincreaseof the number of pores outside the stitchingyarn and the size of the pores increase,Meanwhile,irregularly shaped pores increase,and the pores are mainly distributed along the direction of the parallel fiber layers.The reflection method ultrasonic non-destructive testing can effectively identify the delamination and pore defects of the TFP/LCM composite,but when the porosity is higher(Pv?1.76%),the ultrasonic attenuation is large,so the probe with a lower frequency should be selected;Themultiple continuous echoes appear close to the surface echo of the TFP/LCM composite arenotthe internal defect signals.Through regression analysis,the relationship between the porosity Pv of the TFP/LCMcomposite and the ultrasonic attenuation coefficient ap is established and obtained the corresponding threshold,that is,the porosity is 1.76%as the limit,ap and Pv have a double slope polyline relationship,that is,when the porosity is between 0.09%-1.76%,ap=-0.0004+0.98Pv;when the porosity is between 1.76%-2.49%.ap=-4.47+3.66Pv.