| Carbon fiber reinforced polymer(CFRP)is a material composed of fiber reinforced phase and resin matrix phase.Because of its light weight,high strength,strong designability and other characteristics,it has broad prospects in the application of auto body.At present,the main materials used in car body are steel,aluminum and other metal materials.When using a CFRP structure,the joining process with metal materials needs to be considered.As a new type of joining process,hole-clinching has advantages of fast forming speed and CFRP laminates do not deform during the procedure.It only needs to form self-locking by relying on the deformation of metal materials after the hole is rrefabricated on the surface of CFRP laminates,which can effectively join the metallic materials and non-metallic materials.The body structure may be subjected to collision,impact and other loads in the service process,so it is necessary to study the mechanical properties of the joints under quasi-static and dynamic loading.In this dissertation,the damage evolution and failure modes of the hole-clinched joints with CFRP laminates and 6061 aluminum alloy under quasi-static and dynamic tensile shear loading are studied.The main research contents of this dissertation are summarized as follows:The mechanical properties of CFRP with epoxy resin were investigated based on strain rate effect.The dynamic mechanical properties of CFRP laminates were obtained.The tensile tests under quasi-static state,0.05 m/s,1 m/s,5 m/s and 10 m/s were carried out on CFRP laminates with 0° layers,90° layers and ±45° layers,respectivel.The mechanical properties of CFRP laminates at 0.001 /s,1 /s,20 /s,100 /s and 200 /s were obtained.It was found that the strain rate during loading affects the mechanical properties of CFRP laminates in fiber direction,matrix direction and off-axis tension.According to the results,a prediction function between the strain rate and the mechanical properties of CFRP was established for the subsequent research on the dynamic mechanical properties of hole-clinched joints.Macro/mesoscale study was carried out on the damage and failure of CFRP laminates considering fiber rotation.In this dissertation,CFRP laminates with fewer than 8 layers are defined as low layer specimens,and CFRP laminates with more than or equal to 8 layers are defined as high layer specimens.The off-axis tensile properties of CFRP laminates with high and low number of ±45° layers were studied.Through macroscopic tests on laminates with different numbers of layers,it was found that the fiber rotation strengthening phenomenon appears in the off-axis tensile stress-strain curves of the joints with high number of ±45° layers.The origin of fiber rotation strengthening phenomenon in the process of off-axis tests was investigated by means of meso unit cell finite element simulation.The stress-strain function relationship of fiber rotation strengthening stage was fitted as the basis for studying the mechanical properties and failure behavior of CFRP laminates with high number of layers.Strain rate-dependent CFRP constitutive model was established based on damage mechanics.Based on the orthotropic constitutive model,a strain rate modified orthotropic stiffness matrix was established.The plastic hardening criterion and fiber rotation hardening function were introduced to fit all parameters related to strain rate.Therefore,the corresponding failure criterion and damage evolution law of strain rate correction were established,and the constitutive model was validated by simulations.Study was carried out on tensile shear damage and failure of hole-clinched joints with CFRP laminates in quasi-static conditions.A clinching die and a punch for CFRP and aluminum alloy hole-clinching were designed,and the joining process of different materials was realized.The quasi-static tensile shear tests of the hole-clinched joints with CFRP laminates and aluminum alloy were carried out,and the effects of different layer angles and number of layers on the static tensile shear properties of the joints were studied.The finite element simulation model of tensile shear conditions was established and validated by simulation.The deformation process of aluminum alloy,the failure modes and damage evolution of CFRP laminates during tensile shear procedure were studied.Study was carried out on tensile shear damage and failure of hole-clinched joints with CFRP laminates in dynamic conditions.Tensile shear tests and finite element simulation were carried out on the hole-clinched joints of CFRP laminates and aluminum alloy under dynamic loading at 1 m/s,5 m/s and 10 m/s,respectively.The influence of loading speed on dynamic tensile shear properties and damage evolution of joints were studied.It was found that the loading speed changes the failure mode and damage distribution of the joints,and the effect on the joint with ±45° layers is more remarkable than that of the joint with other layer angles.In general,the failure displacement decreases and the peak load of the joint increases with the increasing loading speed.In conclusion,this dissertation begins from the aspects of strain rate test and simulation of CFRP laminates,the combination of meso and macro investigation to reveal the fiber rotation mechanism of CFRP,the introduction of plastic hardening criteria to establish the strain rate-related damage constitutive model of CFRP,to the tests and simulations of quasistatic/dynamic tensile shear properties of hole-clinched joints,mastering the damage evolution and failure rules of materials and joints and the study for the damage constitutive model of CFRP material and the joint failure modes,which can provide scientific basis for guiding the design of CFRP and metal hybrid structures,expand the application range of CFRP and aluminum alloy joining process and provides guidance for lightweight structure design. |
