PSI - Issue 21

Kadir Bilisik et al. / Procedia Structural Integrity 21 (2019) 146–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

147

2

1. Introduction Carbon fiber preform and composites have been used in space and aero vehicle industries as well as defense sectors as structural parts due to their extraordinary thermo-mechanic, electro-magnetic and damage resistance properties Kamiya et al. (2000). However, traditional carbon/matrix structures exhibited inferior delamination and damage tolerance resistances. Hence, binder fiber in fabric architecture were introduced by using the traditional textile technologies such as stitching Tong et al. (2002), Bilisik and Yolacan (2014), Bilisik and Yolacan (2012), carpet and velvet as well as flocking. Other innovative techniques were considered as three dimensional weaving Bilisik (2010), Bilisik et al. (2013), three dimensional braiding Bilisik and Sahbaz (2012), Bilisik (2011) and z-pin anchoring Pingkarawat and Mouritz (2014). In recent past, nano materials as a form of nanoparticle, nanotubes and rods as well as nanofibers were used to make nanofibrous composites. Nano materials were added in the resin by using several techniques as mixing, or ultrasonication, spraying and transfer-printing before composite process Garcia et al. (2008). The nano materials were placed onto two dimensional fiber substrate or uniaxial/biaxial prepreg as nano/resin mixture or sprayed forms. They can also be attached or grafted to the substrates Khan and Kim (2011). Typically, the in-plane properties of the 3D woven composites have low due to the binder fiber and total fiber volume fraction in which higher degree of interlacements at three fiber sets effected preform fiber volume during fabric formation. Z-pin in the preform was not continuous due to lack of loop section. The nano materials in the fibrous composite were also discrete form and they were not continuous in the fabric architecture. T he resin properties influenced the toughening mechanism of mode˗II interlaminar shear fracture Kuwata and Hogg (2011). It was obtained that fiber surface treatment in laminated composite enhanced the interlaminar strength and the mode‒II shear fracture Madhukar and Drazal (1992). But, the interfacial modification was not effective compare to the toughened resin. The matrix and fiber/resin interface mostly influenced the interlaminar fracture behavior of composites Deng and Ye (1999). Several surface modification techniques which were fiber surface roughening and plasma treatments were employed to improve the adhesion between matrix and filaments Li et al. (2015). Nonetheless, it was reported that the high modulus fibres was damaged by these surface modifications Wu and Cheng (2006). One of the studies demonstrated that 3D orthogonal carbon/epoxy composite demonstrated extraordinarily better fracture toughness performance compared to the classical laminated composites Guenon et al. (1989). The stitching yarn in the through-the- thickness of composite improved the mode‒II toughness due to using the high areal stitching density Herwan et al. (2014). Mouritz identified that the fracture resistance was improved due to tortoise crack path which was generated by stitched fiber Mouritz (2004). It was discovered that interlaminar toughness of stitched carbon structure was greater than the neat sample Jain et al. (1998). The crack growth in stitched structure was impeded via bridging and arresting. The fractured surface had stitched fiber/resin debonding and intra/interlayer opening Tan et al. (2012). It was demonstrated that the G II was enhanced by accumulative effect of the na nofibers/z‒pins in the composite Ravindran et al. (2019). It was obtained that the high z-pin density composite demonstrated improved mode‒II toughness Huang and Waas (2014). The z-pin length/diameter ratio and density were the critical parameters Partridge and Cartie (2005). Recently, it was reported that the mode‒II toughness of the nanostitched carbon/epoxy nanocomposite showed several fold increase compared to the pristine Bilisik et al. (2019 a; DOI: 10.1177/0021998319857462). It was found that nanoprepreg carbon/epoxy composite exhibited threefold increase for mode‒II and average two fold increase for mode‒I as compared to the control sample Garcia et al. (2008). The reason could be the complex accumulative interaction of resin/nanotubes/filaments. These interactions were probably bridging, pull-out, friction, and stick-slip. Another research study reported that the nanocomposite exhibited one and half fold increase on fracture toughness (mode‒II) compared to the base sample Falzon et al. (2013). The cr ack propagation and plastic deformation in the crack tip region caused nonlinear mode‒II failure Carlsson et al. (1986). The failure in the plastic damaged zone of the mode‒II crack front was micro tearing and minor fiber bridging Hashemi et al. (1990). On the other hand, the carack growth and coalescence of micro tensile cracks on the mode‒II crack tip was identified Xia and Hutchinson (1994). Therefore, the purpose of this research was to investigate the mode‒II interlaminar fracture toughness of nanostitched carbon/epoxy composite structures via the end notched flexure. Grafting CNT on the filament bundle surfaces led to critical enhancement in flexural and mode‒II properties but it did not affect the tensile properties Khan et al. (2018). It was reported that the carbon/epoxy nanocomposite exhibited 27% increase in the G II because of addition of fluorine functional group to the carbon nanotubes (f-CNTs)