Elastoplastic Damage Constitutive Model of Steel-Polypropylene Hybrid Fiber Reinforced Concrete and Its Numerical Implementation
Supervisor:xu li hua
Due to the more superior mechanical performance and more reasonable comprehensive benefits,the steel-polypropylene hybrid fiber reinforced concrete(HFRC)has gained wide attention and prospective application in the vital civil engineering field,such as bridges,roads,tunnels,high-rise building and long-span structures,etc.A mass of civil engineering disaster investigations indicate that the dominant of the failure of concrete structures is the loss of the loading bearing capacity of concrete itself and its attachments.One of the core issues in the prediction of concrete structural performance is the basic mechanical behavior of concrete,namely,constitutive model.Substantial researches find that the plastic strain and damage evolution are the most dominant aspects inducing the non-linearity of concrete materials and structures.Therefore,it is of critical theoretical importance and engineering application values for the non-linear analysis and performance design of concrete structures to establish the accurate and reasonable elastoplastic damage constitutive model.Under the support of Chinese National Natural Science Foundations of "Elasto-plastic damage model of steel-polypropylene hybrid fiber reinforced concrete"(Grant No.51608397)and"Multi-scale constitutive models of steel-polypropylene hybrid fiber-reinforced concrete:From namo-scale to macro-scale"(Grant No.51478367)as well as the bases of previous studies,this paper presents an systematical investigation on the stress-strain relation and damage behavior of HFRC upon the experimental investigation,theoretical analysis and numerical computation.The theoretical elastoplastic damage constitutive model of HFRC is developed.The main work and results are as follows:(1)The tensile cyclic stress-strain behavior of HFRC.Considering the influences of fiber type,volume fraction and aspect ratio,28 groups of 168 cylinderical specimens and 168 prismatical specimens were fabricated and tested to investigate the stress-strain behavior of uniaxial cyclic tension.The failure pattern,total stress-strain curves,plastic strain and performance degradation were analyzed.The results showed that the failure of HFRC can be improved by the hybrid fibers which have a multi-level and step-by-step resistance to the cracking process.The failure modes of plain concrete(PC)and polypropylene fiber reinforced concrete(PFRC)are brittleness,while those of steel fiber reinforced concrete(SFRC)and HFRC are ductility.The cyclic tensile process can be divided into four stages:elastic,micro-crack stable propagation,macro-crack unstable propagation and failure stage.The mechanical properties in terms of peak strength,peak strain,toughness and ductility of concrete under tension increase with increasing steel fiber volume fraction and aspect ratio.The tensile stiffness degradation process of concrete is alleviated by the hybrid fibers,the speed of which is slowed down with an increase of the steel fiber parameters.The effect of polypropylene fiber on the stiffness degradation of concrete is significant.Moreover,insignificant effects of hybrid fiber on the plastic strain accumulation and stress deterioration of concrete are observed.Based on the stiffness degradation process,analytical equations for the tensile damage evolution law of HFRC are established,with the effect of polypropylene fiber neglected.Finally,on the basis of the test data,empirical formulae are developed to generalize the tensile cyclic stress-strain response of HFRC.(2)The compressive cyclic stress-strain behavior of HFRC.The compressive cyclic stress-strain behavior of HFRC was investigated by the cyclic compression tests on 168 prismatical specimens.The effects of fiber type,volume fraction and aspect ratio of hybrid fiber on the failure pattern,total stress-strain curves,plastic strain,stiffness degradation and stress deterioration were analyzed.The results indicated that the failure of HFRC shows obvious ductile behavior.The cyclic compressive process of HFRC involves six stages:elastic,meso-crack propagation,macro-crack propagation,fracture occurring,sustained damage and converging stages.In comparison with plain concrete,the introduction of hybrid fiber has a synergetic effect on improving the mechanical behavior of concrete,e.g.peak strength,post-peak ductility,hysteretic dissipative energy and performance degradation.The hybrid fibers can lead to a remarkable restraint in plastic strain accumulation.The influence of fiber parameters on the compressive stiffness and stress degradations is insignificant.Based on the stiffness degradation process,analytical equations for the compressive damage evolution law of HFRC are established,with the effect of hybrid fiber taken into consideration.Finally,on the basis of the current and previous test data,empirical formulae are developed to generalize the compressive cyclic stress-strain response of HFRC.(3)Damage process and damage mechanism.The failure process of HFRC was monitored by the acoustic emission(AE)technique.Base on the analysis of the AE signal characteristics,the damage process and damage mechanism of HFRC were in-deep analyzed.The results showed that the failure of concrete can be well reflected by the AE technique.At the elastic stage,no AE signals are captured.At the meso-crack propagation stage,only a few AE signals with low AE energy are observed.The AE inducing sources are the occurrence and propagation of initial cracks.When the stress reaches the peak strength,the amount of AE signals with high AE energy has a sudden increase with the appearance of the macro cracks.With an increase in the axial displacement,a decrease in the amount of AE signals is seen,and finally,the concrete is failure.At this interval,the AE inducing sources are the sliding and pullout of steel fibers as well as the split cracks caused by the deformed fibers.The proportion of the shear cracks increases with increasing steel fiber volume fraction and aspect ratio,where the dominant of failure of concrete changes from tensile cracks to shear cracks.The effects of polypropylene fiber on the AE signal parameters are insignificant.(4)Systematical research on the multi-scale hybrid fiber influencing mechanism using SEM,AE and macro observation methods in order to explore the influencing mechanism of enhanhcing effect of hybrid fiber on the damage of concrete.The results found are following:① At the micro-scale,the addition of hybrid fiber increases the micro-pores of concrete matrix,however,decreases the amount of micro-defects at the ITZ between coarse aggregate and matrix and then improves its performance.②At the meso-scale,an increase in the elastic modulus,amount and aspect ratio of fibers increases the loads transferring from the concrete matrix to the fibers through the ITZs,which decreases the damage degree and alleviates the damage speed.In the meanwhile,due to the crack-bridging effects of steel fiber and polypropylene fiber,the amount of shear cracks of concrete increases with increasing fiber volume fraction and then improves the failure behavior of specimens,exhibiting a stronger reinforcing effect.③At the macro-scale,the steel fiber bridges the macro-cracks and undertakes the main tensile stress between cracks.Positive hybrid and synergetic effects of steel fiber and polypropylene fiber are observed,and each of them plays a contribution role.However,in the hybrid system,steel fibers are the dominant role and bridge the cracks after the tensile cracking of concrete.The effect of polypropylene fiber is poor,and they mainly arrest and bridge the micro cracks before the formation of main cracks,and finally enhance the tensile strength of concrete.(5)Elastoplastic damage constitutive model.In order to reflect the basic characteristics of concrete with respect to damage evolution and plastic strain,an elastoplastic damage constitutive model of HFRC is developed using continuum damage mechanics(CDM)and elasto-plasticity based on irreversible thermodynamics,with the coupling effect of plastic strain and damage taken into consideration.A numerical implementation method is introduced,and then a UMAT subroutine of user-defined materials is programmed and implemented on the ABAQUS platform.The model prediction results are compared with test results in literature in terms of uniaxial monotonic and cyclic loadings,biaxial compression and true tri-axial compression.It is evident that the developed constitutive model can well predict the mechanical responses of HFRC materials.Finally,based on the summaries of the present study,some recommendations for the further research are proposed.