Monofilament Tensile Performance And Characterization Of Pitch-based Carbon Fibers

Carbon fibers are considered to be high performance materials having great importance in a variety of specialized applications,such as structural materials and functional materials.It is important and practically meaningful to investigate the manufacturing process because it will affect the fiber final properties,such as fiber structure,mechanical properties and thermal properties.At the same time,accurate characterization of fiber properties make it possible to evaluate the process appropriately and efficiently,therefore,it is necessary to characterize fiber properties accurately and reliably.Tensile properties of fibers were analyzed from three different angles:properties test methods,tensile fracture morphology and tensile fracture behavior.In the experiment,fiber cross sections were captured using SEM and were calculated with different methods;the tensile fracture mechanism was examined through examining the fracture morphology with SEM;and tensile fracture behavior were monitored by a high-speed camera,from which fracture mode can be analyzed.Via the three different parts,monofilaments tensile properties of industrial fibers and experimental fibers can be studied thoroughly and systematically.The measurement of single fiber cross sectional dimension can be influenced by several factors,such as the tilted angle between fiber axis and electron beams,the cross sectional shapes.Different measurement methods should be utilized in different situations so that an accurate can be obtained.When the fiber axis is parallel to the electron beams,the direct area measurement method(DAM)is a preferred method,which is suitable for measurement of different cross sectional shapes.Meanwhile,average diameter measurement(ADM)can be accurate if the cross section is a circle or an ellipse with aspect ratio less than 1.5.When the fiber axis is tilted to electron beams,single diameter measurement(SDM)can be immune to the tilted angle and a reliable measurement result can be obtained if the cross section is round.In fact,the closer to a circle the cross section is,the more accurate the single diameter measurement is.On the basis that an accurate property can be obtained,the tensile fracture morphology of mesophase pitch-based fibers with petal-like texture was investigated.The fracture morphology can be mainly classified into two types:planar surface,with a lower average tensile strength,and up-and-down surface,with a higher average strength.Besides,a wave-like surface only exists in the naked tensile specimen,while a surface with petal sunken or protruded only exists in the tensile specimen protected with thin film.This could have something to do with the special graphite crystallite orientation and could be a special morphology in fibers with petal-like texture.To step further,a recoil behavior of single fiber after tensile fracture was observed using a high-speed camera.In different test length,industrial fibers recoil in the same way and recoil behavior was classified in the same standard.According to the recoil form,two different recoil behaviors were identified in SiC fiber and PE fiber:a undulation-like recoil behavior and a linear recoil behavior.Glass fibers in the experiment took on the linear recoil behavior after tensile fracture,while some of them fragmented in the recoil process and the others not.Therefore,recoil behavior of glass fibers was sorted into fragmentation recoil behavior and normal recoil behavior.Refer to mesophase pitch-based carbon fibers from laboratory,when the length of test samples is 20 micrometers,fibers with onion,random and petal-like texture wholly take on a undulation-like recoil behavior after tensile fracture,though the shape of the undulation is not so regular as that of industrial fibers,besides,secondary fracture always happened in the position close to solidified glue;fibers with radial texture take a different behavior,recoil behavior can be sorted into undulation-like recoil behavior and linear recoil behavior.When the length is 10 micrometers,behavior in all of four samples can be classified into fragmentation recoil behavior and normal recoil behavior.Diameters of radial-texture fibers are close to SiC fibers and PE fibers,while quite different with the other mesophase carbon fibers,which could be a factor affecting the recoil behavior in the length of 20 micrometers.