Preload Relaxation And Failure Mechanism Of CFRP Composite Interference-fit Joints Under Thermal-mechanical Coupling Effects

Carbon fiber reinforced polymer(CFRP)composites are increasingly utilized in airframe structures due to their light weight,high strength,good corrosion resistance and design flexibility.Commonly the composite components are assembled by mechanically fastened joints,among which the interference fit and bolt clamping technologies have been proved to benefit the performance of composite joints and improve joint efficiency without redesigning the joint structures.But it should be noted that the composite joints are structures which contain multiple materials with dissimilar physical properties.The joint-holes undermine the integrity of fibers and cause potential weak spots in composites.While in service,the composite joints are frequently subjected to external excitations such as vibration,impacting,thermal cycling and dry-wet alternation.Under these dynamic loading fretting with small amplitude occurs in the mating interfaces and causes damage to the joint-holes.Meanwhile,the hygrothermal environments not only degrade the composites but also alter the contacting properties between mating components,resulting in the preload relaxation of joint structures.The combined effects of them complicate the mechanical response and failure mechanism of composite interferencefit joints,which limits the safe and sound application of such joint structures.The present research is involved in the preload relaxation and damage evolution of composite interference-fit joints under thermal-mechanical coupling effects,including the fretting behaviors of interface between CFRP and coated titanium alloy under service conditions,the preload and relaxation mechanism of composite interference-fit joints under thermal effects,the mechanical response of single-lap bolted CFRP composite interference-fit joints and their modeling of damage evolution and failure mechanism under hygrothermal environments.The main contents and findings are summarized as follows:(1)The fretting behaviors and tribological mechanism of interface between carbon fiber reinforced polymer(CFRP)composites and titanium alloy in composite interference-fit joints under service condition were investigated using a ball-on-flat configuration.The effects of fiber orientation,ambient temperature,surface treatment and interface condition were evaluated.The coefficients of friction(COFs),3D surface morphologies and microstructures of worn scars were presented to characterize corresponding fretting mechanisms.An obvious tribological anisotropy was observed in the CFRP specimens where the longitudinal surface possessed much smaller COFs and less wear damage than the normal one known as brush-like surface.The wear damage and anisotropic properties can be eliminated completely by surface treatment of molybdenum sulfide.Compared with dry service condition,a lubricant film was generated on the CFRP tribological surface in water or oil condition to protect the interface from abrasive wear.But it should be noted that it took much longer time to reach friction equilibrium on the brush-like surface than the normal surface.(2)The analysis and prediction model of stress relaxation on clamp-up load in composite interference-fit joints was developed to describe the preload loss phenomenon under thermal effects.During bolt inserting and tightening process in the experiments,the clamping forces on the both ends of the fasteners were measured to evaluate the load balanced by the friction behavior between bolt shanks and composite joint-holes.After that the preload relaxation of composite joints under various levels of interference-fit size and temperature were monitored for 240 hours.Based on these experimental data,the analysis and prediction model of preload and stress relaxation was established on the assumption that the instantaneous response of bolt and composites in joints were regarded as linear elasticity,and that the delay response of composites in joints were characterized with viscoelasticity.The research reveled that the increase of temperature would accelerate the preload relaxation in composite joints remarkably,while the interference fit in the joints contributed to slowing down the relaxation process even in thermal cycling environments when compared with composite clearance-fit joints.(3)The mechanical response of single-lap bolted composite interference-fit joints in their entire life span was systematically studied via experimental tests.Particular focus was given to interface behavior,bearing response,strain distribution and out-of-plane deformation under varying multiple parameters including interference-fit percentage,tightening torque,stacking sequence and temperature.The 3D digital image correlation system was used to characterize the development of strain concentration and deformation.Microscopy studies on coupled interface and bearing plane were conducted to understand the damage mechanism.It was found that the bolt-inserting could act as cold expansion which facilitated forming tightly coupled interface.The response of the joints was divided into elastic linear phase,damaged nonlinear phase and post-failure phase.The linear phase of stress-strain behavior was prolonged by tightening torque which inhibited delamination growth in bearing plane,induced friction force to balance external load and postponed bolt-to-hole bearing action.The increase of temperature gradually undermined the performance of composite joints and switched the failure mode from material crushing to ply buckling.(4)The bearing behavior and damage evolution of single-lap bolted composite interference-fit joints under mechanical-thermal coupling effects were modeled with proposed numerical methods.Shear nonlinearity constitutive relations for composites were defined with the Ramberg-Osgood equation.The anisotropic damage model established on continuum damage mechanics incorporated extended 3D failure criteria and exponential damage evolution rule to describe damage initiation and material stiffness degradation,respectively.Numerical simulations of composite joints based on the proposed model under varying interference-fit sizes,bolt preload levels,composite layups and temperatures were conducted and validated by experimental tests.Results showed that the strain contours and hyperechoic strips detected in bearing specimens were accurately characterized by damage patterns in joint models.The fiber breakage and matrix crushing in micro-morphology on the bearing plane were well captured in numerical model,suggesting the robust ability of the proposed model in application.