Process And Experimental Study On Vibration-assisted Low Pressure Processing Of Carbon Fiber Reinforced Plastic Composite Components

Carbon fiber reinforced plastic（CFRP）composites have a lot of excellent mechanical properties such as high-specific strength and modulus,powerful design-ability of mechanical properties and the potential of manufacturing of large-scale structures,which enable them to displace conventional materials in the aerospace sector as an ideal material for structural application.Currently,the autoclave process is the major manufacturing process for composite structures.However,the hot-air circulation mode of autoclave and the pressure transfer path between component and mould will result in the non-uniform distribution of thermal force field,which induces curing defects such as void and cure-residual stress.Thus,the accurate integration of precision and performance of composite parts is difficult to achieve.Although the scientific community has conducted many expansive researches to regulate the distribution of temperature during the composite structure processing,the non-uniform distribution of curing pressure brought from the complex structure characteristic is still rarely performed.Therefore,in order to solve the strong dependence on the high and uniform curing pressure during the manufacturing process of large composite parts,a new method of vibration-assisted vacuum processing（VAVP）is proposed from the mechanism of gas-fluid two phase dynamic equilibrium under vibration action to reduce the dependence of forming qualities on curing pressure.The inhibition mechanism of void under the vibration-thermal force field,the process optimization of VAVP of composite laminates and related scientific problems are systematically carried out,which provides the theoretical and technical support for the high quality manufacturing technology of large composite aircraft structures.The main research work are as follows:1.The basic motion equations of void under vibration process were established by combining the classical nucleation theory,bubble dynamics and constitutive equation of power law fluid.The effects of vibration-material parameters on reduction laws of void in terms of collapsing,escape and interfusion motions were comprehensively studied.The numerical simulation shows that when the vibration field in the frequency range of 10～2000 Hz takes along with enough energy that provides acoustic pressure coefficient between 0.5～1.5 in the resin matrix,the void（₀R≥500μm）tends to implosion.At the same time,the existential small voids（5μm≤₀R≤500μm）only respond with small oscillations around their equilibrium radius,but the motion of surrounding resin near voids will further enhance these voids removal through vacuum system from the interior of compsites prior to curing reaction.Moreover,the adjacent voids that the effect of vibration unable to influence the collapsing and reduction in the liminted vibration cycles have a strong tendency to combine or coalesce during the vibration process,which realizes the quick collapsing of reconstructed voids.In addition,when the resin viscosity is less than 1 Pa·s,the effective collapsing and escape range of different sizes voids are basically consistent,which indicates that the vibration can be applied in the temperature range of relatively low viscosity during the curing process to reduce the void content.Besides,in order to satisfy the vibration conditions of reduction requirements of void,a high performance repetitive shock machine was developed to realize the reasonbale match between acoustic pressure coefficient and frequency.When platen vibration signals caused by pneumatic vibrators under centralized control system provide vibration acceleration between 5～20 g,the energy proportion of wide frequency range is the highest,which can provide the effective acoustic pressure coefficient within the composite.2.Based on the high performance repetitive shock machine,the effects of different process parameters including vibration acceleration,heating rate and temperature platform on the voids were comprehensively studied.The compaction behavior of fibers and the evolution law of strain/stress during VAVP were explored.In this paper,the aeronautical T800/X850 polymer matrix composite was used.The orthogonal and single factor experiments showe that the viscosity begins to below 1 Pa·s when the temperature stage gets to 80℃,and thereafter,the vibration acceleration influence on void content is beyond heating rate and first dwell time.When the vibration acceleration gets to 10 g,void content within composite laminate is found less than 0.5%,and the morphological characteristics of voids in the plies are circular voids with minor diamete and elliptical voids with minor aspect ratio.By using the method consisted by the facile thermogravimetric analysis based methodology and a cure monitoring approach consisted by SEM micro-characterization technology and quenching method,comparing the COMSOL thermal-flow-solid multi-field coupled model,the compaction behavior of fibers under vibration process was carried out.It is found that applying vibration into curing cycle shows that the positive and negative pressure part of vibration enable the compaction behavior of fibers in the bidirectional way,which decreases the distance and dispersion between fibers.The fiber volume fraction value increases to 67.79%for the composites produced by VAVP,which is similar to 68.93%of fiber volume fraction within composite laminates cured under 0.6 MPa autoclave process.Meanwhile,the evolution of strain and stress during curing process and cure-residual stress of composite laminates were systematically studied by the on-line monitoring consisted by fiber Bragg grating sensor and thermocouple sensor.When the vibration is introduced into curing process,the mismatch of thermal expansion coefficients between carbon fibers and resin is decreased,and the inhibition effect of carbon fibers on thermal expansion,cold shrinkage and curing shrinkage of resin in different directions is improved.Therefore,in comparison with autoclave process,the cure-residual stress of composite laminates gets a reduction of 25.4%.3.The effect of VAVP on the performance of composite interface was studied,and the mechanical damage evolution of composite interface under the statical and dynamical action was revealed.The interlaminar shear strength of composite laminate produced by VAVP was characterized by short-beam three point bending test coupled with DIC method.It is found that applying vibration into the curing process of composite laminates production reduces the stress concentration points caused by voids.And vibration process plays a key role in impeding crack energy dissipation mechanisms such as fiber pull-out and sliding,and interface debonding.The composite laminates with excellent interlaminar shear property of 97.12 MPa,which shows a slight reduction of 1.82%compared with the 0.6 MPa autoclave process.The testing technology and the characterization technology of 5μm fiber of micro interface were overcome by developing the single fiber push-out test system of nanoindentation instrument.It is found that the better impregnation of resin matrix into fiber bundles and the remarkable reduction of the micro-defects around the fiber-resin interface can help composite laminates produced by VAVP obtain a significant interfacial shear strength of 82.18 MPa,which shows a slight reduction of 2.15%compared with the 0.6 MPa autoclave process.A new method to calculate the micro interface fracture toughness was proposed by combining the fiber push-out test and theoretical model of bilinear cohesion.Results show that the strong adhesion between the fiber and matrix treated by the vibration leads to a marked increase of interface fracture toughness to56.4 J/m²,which healps the composite laminates obtain a significant resistance to initial micro crack growth.Besides,when the composite laminates produced by VAVP suffer dynamic action during the drop hammer impact test,microcracks and delamination failure within composite laminates after the low velocity impact are limited.Therefore,the low velocity impact property and residual mechanical property of composite interface produced by VAVP show the similar performance level of composite interface produced by autoclave process with 0.6 MPa curing pressure.4.The T-shaped stiffened structure is used as typical primary structure in the aviation industry,which is selected as the research target to reveal the engineering application feasibility of VAVP.The test results show that the void content of key sections of T-joint produced by VAVP can be reduced to less than 1%,which reaches the requirements of main load-bearing structure on aviation.And the reduction mechanism of void content under vibration process is also suit for complex stiffened structures with the variable thickness,variable cross-section and triangular region.Besides,the application of vibration during the curing process can improve the fluidity of resin matrix,which enables the resin to flow towards the complex region where manufacturing defects such as resin starvation and resin-rich areas are occurred easily under the vacuum bag pressure.The ultimate pull-off load shows a remarkable improvement of 210%,which can reach the similar quality of composite T-joint manufactured by autoclave process with 0.6 MPa high curing pressure.Through the method combined by theoretical modeling,experimental research and component verification,this study focus on the key theoretical and technical problems of the vibration-assisted vacuum processing of polymer matrix composite laminates,and it provides an innovative technical route for the high quality manufacturing of large complex composite primary structures.