The Effect Of Stiffness Unbalance On Failure Behavior Of Carbon Fiber Reinforced Plastic Drive Shaft Adhesive Joint

Adhesive bonding has many advantages such as no cutting of fibers and lower structural weight,so it has become a vital connection method of carbon fiber reinforced plastic(CFRP)components.However,due to the discontinuity of structures and materials along the bonding location,high localized stress concentration will be caused in the adhesive joint when transmitting load,thus the adhesive joint is usually the weakest part of the structure.As to carbon fiber reinforced plastic drive shaft,the investigation of failure behavior of adhesive joints is directly related to the safety and reliability of drive shaft.The adhesive bonding between carbon fiber shaft tube and metal shaft head is a typical stiffness unbalanced connection form.The dissimilar adherends with unequal stiffness will induce unbalanced load at two ends of the connection,resulting in a remarkable difference in peak stress of both ends,so the failure initiates at one end of the adhesive or adherend.Concerning to the stiffness unbalanced adhesive joint of carbon fiber reinforced plastic drive shaft,the proper design of structural parameters can be conducted to weaken the stiffness unbalance.It can significantly reduce the stress concentration and improve the structural efficiency,therefore,it’s of important significance to strengthen the load transimission capability of carbon fiber reinforced plastic drive shaft.This paper has studied the effect of stiffness unbalance on the failure behavior of the adhesive joint of carbon fiber reinforced plastic drive shaft.Different stiffness unbalanced stateswere obtained by changing the shear modulus ratio and wall thickness ratio between the metal shaft head and carbon fiber shaft tube.The failure behavior considered in this paper mainly included the stress and strain distribution,torque transmission capability,crack initiation and propagation rule and failure mode.Then the structure design method of carbon fiber reinforced plastic drive shaftbased on the adhesive joint stiffness balance was carried out.Firstly,the forced state,failure modes and the corresponding failure criteria of the adhesive joint of carbon fiber reinforced plastic drive shaft were analyzed based on material mechanics.Then the computing method of carbon fiber shaft tube’s torsional stiffness was derived combining material mechanics and the classical lamination theory.It was found that the main parameters influencing the stiffness unbalance of drive shaft were the shear modulus and wall thickness,among which the shear modulus of carbon fiber shaft tube could vary from different lay-up schemes.Secondly,the finite element model of the adhesive joint of carbon fiber reinforced plastic drive shaft was established in ABAQUS.The bilinear cohesive zone model was used to simulate the adhesive layer,the initial damage index QUADSCRT was used to monitor the initial failure and the stiffness degradation index SDEG was used to monitor the failure extension so as to inspect the crack initiation and propagation rule of the adhesive layer.The stress distribution of the two adherends,the initial failure and failure extension of the adhesive layer,the stress and strain distribution of the adhesive layer as well as their change law corresponding to the torque load were mastered under the stiffness unbalanced state,and the evaluation method of the carbon fiber reinforced plastic drive shaft’s torsional strength and stiffness was also mastered.It was concluded that: the peak stress of the carbon fiber shaft tube appeared at the ends of bonding area,whereas the peak stress of the metal shaft head appeared at the free end;the stress and strain distribution,and the initial failure and failure extension of the adhesive layer manifested the characteristics of unbalance-the stress,strain and failure degree at the end of the adherend with lower torsional stiffness was obviously higher than the other end.Through the finite element simulation and typical static torsion tests,the effect of the two critical factors determing the ratio of torsional stiffness between metal shaft head and carbon fiber shaft tube-the shear modulus ratio and wall thickness ratio on the failure behavior of the adhesive joint was investigated.The conclusion could be drawn that: different torsional stiffness ratio led to different failure modes of the drive shaft;when changing the lay-up scheme of the carbon fiber shaft tube,the failure torque of the drive shaft was influenced by the shear modulus ratio,when the shear modulus ratio was close to 1,the maximum failure torque was achieved;the effect of ply orientation on the failure behavior was far greater than that of stacking sequence;when changing the wall thickness ratio,the failure torque of the drive shaft was influenced by the torsional stiffness ratio,when the torsional stiffness ratio increased from 0 to 1,the failure torque increased gradually,nevertheless the increase amplitude of the failure torque would be subtle when the torsional stiffness ratio exceeded 1;the stress distribution and stiffness degradation of the adhesive layer were related to the torsional stiffness ratio,when the torsional stiffness ratio equaled to 1,the stress distribution was the most balanced,the crack initiated and propagated simultaneously at both ends;the effective torsional stiffness of the drive shaft increased with the sum of torsional stiffness of carbon fiber shaft tube and metal shaft head.Eventually,on the basis of the above conclusions,the structure design method of carbon fiber reinforced plastic drive shaft based on the adhesive joint stiffness balance was summarised,which laid the foundation for structure design of carbon fiber reinforced plastic drive shaft under the adhesive joint failure mode.