| Hot press molding is an efficient manufacturing technology with short processing cycle time and suitable for mass production.As one of the classic WFTRPs(woven fabric reinforced thermoplastics),PW-CFRPP(plain woven carbon fiber reinforced polypropylene)exhibit the advantages in recyclability,easy storage,high toughness,good forming properties and excellent mechanical properties,and can be used to obtain lightweight body parts of specified shapes quickly through hot press molding.Although PW-CFRPP have the potential to be widely used in automotive bodies,the research on hot press molding manufacturing process and structural crashworthiness design of WFTRPs has just started,and the related forming mechanism is not fully clear,and there still lacks of effective forming defects evaluation indicators,reliable foming constitutive model and efficient forming optimization algorithm.Besides,the influences of typical process parameters and environmental factors on fundamental mechanical properties of PW-CFRPP material are not clear,lack of failure criteria and models for failure prediction of PW-CFRPP structures.There still lacks efficient optimization methods considering multiple materials,multiple loading condition and multi-objectives in the design progress.Most important of all,the structural crashworthiness design has not yet considered the influence of process effects,and the coupling model of forming-performance and the optimization design are not yet established.This study aims to conduct related research on above-mentioned key fundamental scientific issues that restrict broad applications of WFTRPs,including the optimization design of hot press molding process,the influences of process parameters and environmental factors on the mechanical properties of PW-CFRPP,the establishment of new failure criterion of PW-CFRPP,the optimization design of multiple material hybrid crashworthy structure,the establishment of process-performance coupled model and the optimization design based on the coupled model.The research on the key scientific issues provides a scientific guidance method for further promoting wider applications of WFRTPs in automotive industries.This paper mainly develops and completes the following four aspects of research:(1)Efficient and global multi-objective optimization design of formability of fabric of PW-CFF(plain woven carbon fiber fabric): firstly,fundamental mechanical parameters of PW-CFF were obtained through both uniaxial and off-axis tensile tests to provide reliable parameters support further FEMs(finite element models).Then,the influence of BHF(blank holder force)and stacking orientation on the formability of PW-CFF were explored through press molding experiments.The non-orthogonal forming constitutive model and FEM were developed and validated based on the principle of frame invariance.Based on the validated FEM,the forming mechanism of PW-CFF was deeply revealed and typical forming defect indicators were defined.The multi-objective optimization design of formability of PW-CFF was carried out by using EGO(efficient global optimization)algorithm,which provided the foundation for the establishment of process-performance model and the integrated optimization design of coupling model.(2)Clarifying influences of typical process parameters and environmental factors on mechanical properties of PW-CFRPP,and establishing a new failure criterion for PW-CFRPP: firstly,PW-CFRPP laminates were prepared by adjusting three different BHFs and three different temperature preservation times,respectively,and different mechanical responses are obtained through uniaxial tension and off-axis tension experiments,and tensile results together with failure morphologies of specimens were analyzed.Then,the uniaxial and off-axis tensile specimens were prepared under the same process conditions,and the experiments were carried out under four different environmental temperatures and three different loading velocities,respectively,and tensile indicators as well as failure morphologies were analyzed.Furthermore,a new type of PW-CFRPP bulging specimen design method was proposed by improving the traditional metal fomring method,and a novel failure criterion based on FLC(forming limit curve)was established by combining the equivalent fiber strain and the ratio of minor strain to major strain,then the reliability of novel FLC failure criterion was verified in FEA by comparing with the maximum stress/strain criteria.The study of the above physical properties and failure criteria laid the foundation for the follow-up optimization design of crashworthiness performance of the multiple material hybrid crashworthy structures and the establishment of the process-performance coupled model.(3)Multiple variables,multi-objectives and multiple loading conditions optimization design on LVI(low velocity impact)resistance of carbon fiber reinforced polypropylene/glass fiber reinforced polypropylene(CFRPP/GFRPP)hybrid laminates: firstly,single CFRPP and GFRPP laminates for low velocity impact tests with three different thicknesses were prepared by hot molding process,and impact features under of these laminates were investigated under three different impact energies by drop weight tests.Three different deformation modes and LVI resistance characteristics of all specimens are identified.Then,the LVI constitutive models and FEMs of two single material laminates are developed and validated based on continuum mechanics and new FLC failure criterion,and the validated FEMs were used to further reveal failures of inter-laminar and inter-laminar materials.Based on results of parametric analyses,Taguchi design method,gray analysis method and mean analysis method are combined to carry out the discrete multiple variables,multi-objective and multiple loading conditions optimization design of CFRPP/GFRPP hybrid structures,which provides the fundaments for the follow-up establishment of process-performance coupled model and integrated optimize design.(4)Clarifying the influence of molding process on structural performance,and establishing process-performance coupled FEM,and further carrying out integrated optimization design based on the coupled FEM: firstly,single hat-shaped PW-CFRPP thin-walled specimens with different fiber angles,stacking orientations and wall thicknesses were prepared by hot molding experiments,and contour map of fiber angle variations of each layer was drawn by X-ray CT(X-ray computed tomography)technology,and the influence of different process on the fiber angle variations of non-orthogonal specimens was analyzed.Then,uniaxial tensile and off-axis tensile tests were carried out to research into effects of various fiber yarn angles and stacking orientations on tensile responses of the PW-CFRPP laminates;meanwhile,three-point bending experiments for single hat-shaped PW-CFRPP specimens were carried out,and influences of fiber angle variations,stacking orientations and wall thicknesses on the bending responses were summarized,and the influence of forming process on structural performance was clarified.Based on the research foundation of(1),(2)and(3),the process-performance coupled constitutive model and related numerical model were developed and validated.Finally,combining Kriging surrogate model and genetic algorithm,the integrated optimization design of forming-bending coupled model for PW-CFRPP thin-walled structure is studied based on the coupled model. |
PDF Full Text Request