Failure Behaviour of Carbon-Epoxy Composite in Crack Arrester and Crack Divider Mode

Failure behaviour of a carbon-epoxy composite is studied both in crack arrester (CA) and crack divider(CD) mode by conducting 3-point bend test of un-notched (UN) and single edge pre-crack notched (N) (notch depth varied from 0.5 to 3.3 mm) specimens at various cross head speeds(CHS) such as 0.2, 50, 100, 200 and 500 mm/min. Influence of notch depth (ND) and CHS on the conditional fracture toughness(CFT) is understood using ANOVA. In both the mode, while the CFT values varies linearly with CHS, its values are observed to be lower at short ND, higher at medium ND and again lower at high ND. In both the modes at high ND (~3.3mm) opening mode/fibre breaking (mode I) failure occurs because of high crack tip stress concentration (as analysed from ANSYS 15.0) which cause fibre failure at the tip and corresponding Mode I fracture toughness was observed to be~20 MPa.m0.5. The composite also shows similar flexural strength both in CA mode (641+48) and in CD mode (634+49 MPa). In CD mode N specimen predominantly fails by mode I. While in CD mode the delamination (mode II) is only observed in UN specimens, in CA mode UN or small N specimen shows mode II failure and medium notched specimen shows mixed mode failure. In CA mode, number of delaminated layer is higher for the specimen tested at high CHS. In CA mode during charpy test (strain rate 103/s), the composite shows numerous delamination and absorbed much higher energy(~40J/cm2) than that in CD (~10J/cm2) mode where mode I failure is predominant. During impact test, lowering of test temperature to -40oC has very little effect on the impact energy since it does not significantly affect the mode of failure.