Study On VIMP Manufacturing And Performance For Thick-section Carbon Fiber Composites

Vacuum infusion molding process (VIMP) is an integral manufacturing techniquefor large-scale composite structures. The basic research related to the VIMPmanufacturing process for large-size thick-section carbon fiber composite structures issystematically carried out in this paper. The research focuses on carbon-fiber/resininfiltration mechanism, rheological behavior and curing kinetics behavior of resinsystem, compaction response of multilayer fabric preform, in-plane (x-y plane) andout-plane (z direction) permeability characteristics of multilayer fabric preform, thethermochemistry effects and thermal residual stress in thick-section composite duringcuring, as well as the scale effects on mechanical properties of the thick-section carbonfiber composites. The main work includes:To improve carbon-fiber/resin interfacial adhesion, epoxy resin is modified byincorporation of amino-silane coupling agent. Based on the determined optimalsilane-coupling-agent and its content, an epoxy resin modified by silane coupling agentis chosen as the matrix resin for thick-section carbon fiber composite manufacturedintegrally by VIMP.Isothermal and non-isothermal rheological behavior of the epoxy resin isinvestigated by viscosity experiments. The results show that the low-viscosityrheological properties and low-viscosity maintain time of the epoxy resin is appropriatefor VIMP manufacturing of thick-section carbon fiber composites. In order to improvethe prediction accuracy of a single viscosity model, an empirical combined viscositymodel is proposed to predict the rheological behavior of the epoxy resin in a wideviscosity range. The results show that the viscosity predicted by the combined model isin good agreement with the experimental data in the viscosity range of0~5000mPa s.DSC analysis is employed to determine the optimal pre-curing, curing andpost-curing temperatures of the epoxy resin. Based on the DSC analysis, the curingkinetics model is established to predict the curing degree of the epoxy resin. The resultsshow that the dynamic and isothermal curing behavior of the epoxy resin can beexpressed by autocatalytic model and modified autocatalytic model, respectively. Themaximum curing degree increases linearly with the isothermal curing temperatures.Compaction response behavior of multilayer fabric preform under vacuum pressureis systematically investigated. Numerical simulation is used to analyze the compactionstress variation of the fiber preform and the effects of thickness variation on liquidpressure, permeability as well as fiber volume fraction of the fabric preform during resininfiltration process. Effects of material configurations and processing parameters on thecompaction response behavior of the dry fabric preform are experimentally investigated. The results show that a hysteresis phenomenon can be obviously observed in thecompaction and relaxation process of the fabric preform. The relationship between fibervolume fraction, thickness and vacuum compaction stress, compaction time as well ascyclic times can be expressed by two or three-parameters power law models for thefabric preform. The fiber volume fraction and relaxation factor of the fabric preformincreases with the increasing thickness (layers). Fabric form, fiber types, the way oflayup and hybrid modes have remarkable effects on the compaction response behaviorof the fabric preform. Effects of VIMP processing parameters on the compactionbehavior of wet fabric preform, the thickness and fiber volume fraction of the compositestructures are investigated by experiments. The results show that the thickness variationof the fabric preform follows different compaction response models during pre-filling,filling and post-filling process in VIMP. Thickness of the composite structuremanufactured by VIMP decreases gradually along direction from the injection gate tothe vent, while the fiber volume fraction increases along the direction.Flowing experiments are conducted to investigate the in-plane (x-y plane)permeability characteristics of the multilayer fabric preform, including the effects ofdistribution medium on the in-plane permeability and the resin flowing behavior, thevariation rule of in-plane permeability as thickness increasing and thickness effects. Theresults show that the leakage model and Lead-lag effect can be used to describe theaccelerating mechanism of distribution medium on the resin filling flow velocity. Theaccelerating function of distribution medium reduces gradually with the increasingthickness (layers) of the fabric preform. The in-plane permeability of the fabric preformdecreases with the increasing thickness (layers) following two-parameter power lawmodel. It indicates that the accelerating function of the distribution medium and thein-plane permeability of the multilayer fabric preform would be significantly affectedby the thickness of the fabric preform.An innovative measuring methods is proposed to investigate the out-plane (zdirection) permeability characteristics of the multilayer fabric prefrom, including theeffects of puncture stitching on the out-plane permeability and the variation rule ofout-plane permeability as thickness increasing. The results show that the out-planepermeability of the fabric preform is1~2orders of magnitude less than that of thein-plane permeability. The out-plane permeability of the fabric preform decreases withthe increasing thickness (layers) following three-parameter power law model. Theout-plane permeability of the fabric preform can be significantly improved by puncturestitching. The stitched factor increases with the increasing thickness (layers) followingthree-parameter power law model. Flow coupling behavior of the in-plane (x-y plane)and the out-plane (z direction) flow is investigated by using Lead-lag effect. The resultsshow that the effect of the flow coupling behavior increases with the increasingthickness (layers). Considering single-sided asymmetric heating condition during curing process ofthe thick-section carbon fiber composite manufactured by VIMP, distribution oftemperatures and curing degree as well as the effect of thickness variation areinvestigated by experimental and finite element methods. The results show thattemperature overshoot is observed clearly in the temperature variation history of thethick-section composite during curing process. Along thickness direction, thetemperature and curing degree of the composite are of gradient distributions. Internalmaximum temperature and curing-degree difference of the composite is up to30℃and10%, respectively. The temperature overshoot, maximum peak temperature, temperaturegradient and curing degree gradient increase linearly with the thickness and volume ofthe composite. Effects of thickness variation on the thermal residual stress of thethick-section carbon fiber composite are investigated by experimental and finite elementmethods. The results show that the transverse residual thermal-strain is significantlygreater than the longitudinal residual thermal-strain of the thick-section carbon fibercomposite. And the thermal residual stress of the thick-section composite increases withthe increasing thickness.Size effects of static mechanical properties of the thick-section carbon fibercomposite are investigated by experiments and finite element simulation. The resultsshow that flexural strength, short-beam shear strength and compression strength of thecomposite decrease with the increasing thickness following logarithmic model, whilethe flexural and compression modulus basically remain a constant as thicknessincreasing. The size effects of flexural strength, short-beam strength and compressionstrength of the thick-section composite can be expressed by Weibull probability model.The size effects of flexural and short-beam shear strength depend on not only thenumber of the “weakest link” but also the three-dimensional stress states of thespecimen. While the size effects of compression strength only depends on the numberand distribution of the “weakest link”.