Research On The Deformation Mechanism Of The Fabric In The Preforming Of Resin Transfer Molding Process

Resin transfer molding(RTM)is an important process of composite manufacturing,and it has received an extensive concern from the vehicle industry.The RTM process includes two stages of fabric prefroming and resin injection.In the fabric preforming,due to the inherent viscoelastic behavior of the fabric,the target fiber volume fraction is difficult to avhieve when the fabric is compacted in the thickness direction.Besides,the fabric deforms into the three-dimensional shapes,and the deformation mechanism is complicated so that the unreasonable process parameters easily induce defects.Therefore,the fabric preforms obtained in the preforming is significant to the target mechanical behavior of the composites.However,for the fabrics commonly used in the vehicle body,the viscoelastic behavior considering the effects of binder content and temperature is not clear.The suppression mechanism of pre-tension on the wrinkle defect induced by the excessive shear deformation has not been reported.The detect generation mechanism,the detect suppression strategy and the correspongding model in the double curved forming have not been reported.Therefore,this thesis focuses on the preforming behavior of the fabrics to explore the deformation mechanism.Detailed contents are listed as follows:(1)The effects of temperature and binder content on the compaction viscoelastic behavior of the fabrics.The temperature and binder were applied to improve the permenant deformation of fabric in the preforming,achieving the preforms with a high fiber volume fraction.Two compaction experiments,including compression-relaxation-recovery and compression-creep-recovery,were conducted for the fabrics.The effect of fiber volume expansion induced by elevated temperature,and the increasing viscous force among fibers induced by the adding binder on the viscoelastic behavior were revealed.The methods to obtain the optimal process parameters of two compaction experiments were established.A theoretical model that can accurately describe viscoelastic behavior considering the effects of temperature and binder content was proposed.The mechanism that the elevated temperature and adding binder content respectively improve and reduce the compression stiffness was clarified.(2)The influence of pre-tension on the shear deformation and the wrinkle defect suppression for the fabricsTo investigate the effect of pre-tension on the shear deformation,and to reveal the suppression mechanism of pre-tension on wrinkle defect.A novel picture frame fixture with an adjustable pre-tension was designed,and the effect of pre-tension on the shear behavior of the woven and non crimp fabrics was explored.The influence of pre-tension on the variation of yarn width was revealed.It is disclosed that the increase of pre-tension reduces the variation of yarn width,resulting in the suppression of the wrinkle defect.The effect of cycle test on the shear behavior f or fabric is explored.The results show that the effective shear responses of positive and negative shear deformation should be characterized in the cycle 1 and cycle 2,respectively.Moreover,the mechanism that the pre-tension suppresses the wrinkle defect and delay the triggering shear angle of the wrinkle defect was revealed.(3)The double curved forming behavior and defect suppression of fabrics.The double curved hemispherical forming experiments were conducted for the monolayered and multilayered woven and non crimp fabrics.The forming force and defects of the monolayered and multi-layered fabrics were investigated,revealing the effect mechanism of interaction between layers on the forming results.In addition,the forming force rate,relative distance between layers under different layer orientations were analyzed,revealing the effect mechanism of layer orientations on the defect generation.Then,the optimal methods of bland holder weight and specimen size were proposed to suppress the defects and improve the quality of fabric preforms.(4)Analytical model and experiment of yarn sliding and shear deformation for the fabric in hemispherical forming.The analytical model of hemispherical forming was further established for the fabric to predict the yarn sliding in different positions,and the shear angle distribution of draped region.Besides,the validity of the model was verified by the hemispherical forming experiments.Both the analytical model and experiment revealed that the slight shear deformation is generated in the tensile direction of the hemispherical forming.Besides,the effects of process and material parameters on the yarn slippage,shear deformation and draping behavior were revealed by the analytical model.The results showed that the yarn sliding and draping degree are increased with the reduction of the corner radius of die and the gap between the punch and die.With the growth of yarn spacing,the shear angle with the same position is enlarged.