Meso-Scale Numerical Investigation Of FRP Strengthened RC Structures Under Blast

An explosion is a rapid release of physical or chemical energy that can cause great damage to reinforced concrete（RC）structures in a short period of time.Fiber reinforced polymer/plastic（FRP）is a high-performance material composed of high-strength reinforced fiber and matrix material in a certain proportion through various complex processes（winding,extrusion or extrusion）,and FRP can increase the structural strength,ductility and reduce debris splashing,increasing the explosion resistance of the structure.Compared with the blast test,the numerical simulation study of FRP reinforced RC components under explosion load is not limited by the test site and test qualification,and can better capture the data that cannot be collected by the test.The bond behavior of FRP-concrete is very important to the effective stress transfer and directly affects the strengthment effect,Therefore,the dynamic bond-slip relation of FRP-concrete interface should be carefully treated in the numerical simulation study.Due to the high strain rate effect,a reliable FRP-concrete dynamic interface bond-slip model has not been proposed for blast analysis.In the absence of a reliable dynamic FRP-concrete bonding-slip model,the debonding process of FRP-concrete interface can be simulated by the fracture failure of concrete elements under the interface based on a suitable concrete constitutive model and assuming that FRP and concrete are ideally bonded,that is,FRP and concrete share nodes.In order to accurately simulate the situation of the interface,a fine finite element mesh（2-5 mm）less than or close to the debonding depth is used.However,when carrying out the numerical simulation study of FRP reinforced RC components under explosion load,the full-scale model using the fine finite element mesh size will cause the finite element calculation quantity to increase by orders of magnitude,so that the calculation cannot be completed.Under the condition of scaled distance Z≥1.2 m/kg^1/3 and span explosion height ratio a≥0.5,it can be approximated that the blast load acts on RC components within a size of 1.0 m by evenly distributed load.Based on the plane stress assumption,an one-way slab can be intercepted and displaced in the z direction along the long side of the unidirectional plate,which can represent the dynamic response of the entire slab.Therefore,based on the explosion load simplification and plane stress hypothesis theory,the 3-D blast model is simplified to a 2-D plane model,and a meso scale element numerical simulation scheme for FRP reinforced RC components is proposed:firstly,the numerical simulation of FRP reinforced RC components at macro scales is carried out.Then,the explosion load obtained under the macro scale mesh is directly applied to the meso-scale mesh structure model in the form of uniform load,which can greatly save the calculation cost.The existing blast test summarized the RC structure type,size,FRP type,explosive equivalent W,explosion distance R,scaled distance Z and span explosion height ratio a,screened out the blast test that meets the explosion load simplification and plane stress assumptions,and used the ANSYS/LS-DYNA software,K&C CONCRETE LOCAL DAMAGE MODEL and related research to obtain the concrete DIF-ε?relationship,considering the material strain rate effect.The reliability verification of the meso-scale element numerical simulation scheme of FRP reinforced RC components was carried out.Firstly,the mesh convergence analysis is carried out based on the screened explosion test.Subsequently,a 20 mm FE mesh was used to carry out numerical simulation research of macro scales,and the time history curves of the explosion pressure read under each working condition were basically consistent with the test,and the maximum error was only 6.67%<10%,which verified the reliability of the blast load.Subsequently,the reliability of the explosion load simplification and plane stress assumption was further verified.Finally,the explosion load obtained under the macro-scale mesh is directly applied to the meso-scale mesh（1 mm）structural model in the form of uniform load,and the simulated structural response is basically consistent with the test,and the maximum error is only 4.2%and less than 5%,which verifies the reliability of the fine finite element numerical simulation scheme of FRP reinforced RC components under explosion load proposed in this study.According to the proposed numerical simulation scheme,the design of the blast test components of modified FRP-concrete bending joints under blast load similar to the interfacial debonding failure mode caused by the middle bending crack（IC-Debonding）is completed.The first mode fracture energy of 34 MPa concrete was calculated and calibrated₁)=143 N/m.When SDF=1.8,the energy dissipated by concrete accounts for 30%of the total fracture energy,and when the SDF reaches 1.95,the energy dissipated by concrete has accounted for 70%of the total fracture energy.Therefore,SDF>1.8 is defined as destruction.The damage failure of concrete meso-scale elements is calibrated by defining the failure of concrete elements（SDF≥1.8）:failure mode I（lower limit of IC-Debonding failure）:the SDF of the concrete unit at2 mm below the interface is greater than or equal to 1.8,and the length of SDF≥1.8of the 2mm concrete unit at the interface is close to 0.5Le≈38±5 mm;Failure mode II（upper limit of IC-Debonding failure）:the SDF≥1.8 of the concrete unit at 5 mm below the interface,and the length of SDF≥1.8 of the 2 mm concrete unit at the interface is close to Le=75±5 mm.According to the proposed numerical simulation scheme,the P-I curve of modified FRP-concrete bending joint in IC-Debonding failure is completed,which is used to guide the blast test load design of modified FRP-concrete bending joint.