Effects of functionally graded adhesive on failures of socket joint of laminated FRP composite tubes

The effects of functionally graded adhesive on failures in socket joint of laminated FRP composite tubes are analyzed in the present research. Various types of failures in the socket joint have been studied using strength of materials and fracture mechanics approach. The critical location in the socket joint has been identified using strength of material approach. Subsequently, the principle of fracture mechanics has been employed with a presumption that a circumferential failure front pre-exist at the critical location in the socket joint. The failure evolution in the functionally graded adhesively bonded socket joint of laminated FRP composite tubes and fracture behaviour of embedded failures have been discussed with respect to the variation of fracture parameters such as the individual modes of strain energy release rates (SERR) along the failure front. The novelty of functionally graded adhesive is introduced with linear function profile in order to improve the failure growth resistance of the socket joint structure. Based on the failure indices, it has been observed that free edges of the coupling region are more vulnerable zones for interfacial failure initiation as failure indices for all the critical surfaces of bond layer are found to be the highest at those locations. Further, the effects of functionally graded adhesive with varied modulus ratios on failure onset are observed for critical surface. It has been observed that the failure onset and its propagation shall be reduced/delayed due to the use of functionally graded adhesive in the joint. Based on the failure growth studies, it has been noticed that interfacial failure propagation is governed by in-plane shearing mode and, other modes are insignificant in propagation. Efforts have been made to enhance the crack/damage growth resistance properties of the bonding material. Series of FE simulations have been carried out for varied interfacial failure length in tubular socket joint with functionally graded adhesive in order to achieve the significant effect of material gradation on SERR. It has been noticed that the effect of material gradation of adhesive on SERR is more predominant for shorter interfacial failure length which leads to delay the failure growth.