| Composite materials have been widely used in the automotive and aviation fields because of their excellent mechanical properties.With the improvement of the manufacturing process of composite materials,the number of composite structural parts used on car bodies and wings has also increased.Therefore,the strain rate correlation of composites due to the particularity of their structures also needs to be further studied.Under low strain rate conditions,the failure theory and finite element simulation of composite materials are accurate.However,under high strain rate conditions,the mechanical properties of materials are updated in real time according to the numerical strain rates,which reduces the reliability of traditional failure criteria and finite element simulation.In order to improve the accuracy of finite element simulation of composite materials at high strain rates,a constitutive model considering strain rate effect for unidirectional fiber composites and woven fiber composites is proposed in this paper,and the corresponding VUMAT subroutines are written and embedded into ABAQUS for simulation under different working conditions.The main contents of the research include:The damage criterion of composite materials based on the Hashin failure criterion is revised.Different strain rate effect parameters are added to the material parameters of different material directions,which corrects the mechanical properties of the material at high strain rates.For unidirectional fiber composite materials,six material damage criteria are defined,and the material damage uses two different degradation forms: exponential degradation and parameter degradation.For woven fiber composite materials,seven material damage criteria are defined and proposed two different material parameter degradation forms: exponential degradation and parameter degradation.According to national standards,45 unidirectional carbon fiber composite laminates with different angles were produced,and tensile tests under quasi-static and dynamic conditions were carried out.The obtained data were processed and curve fitting was performed.Taking into account the limitations of the test conditions,only unidirectional fiber laminate tensile tests were performed,and the constitutive model of the woven composite laminate was compared with the test results completed by previous researchers.Based on the VUMAT program interface of ABAQUS,according to the proposed unidirectional/woven composite material constitutive model,the corresponding exponential/parameter degradation program was written.Taking into account the different simulation conditions,the programs of the same constitutive model correspond to solide and shell elements,a total of 8 VUMAT programs,and the simulation programs are debugged.A finite element model was established based on the high speed tensile test,and the debugged program was embedded in the ABAQUS software to simulate the corresponding working conditions.The finite element simulation results with and without the strain rate effect were compared with the corresponding test results.The results show that when the element strain of unidirectional fiber composites is below0.0065,the exponential degradation constitutive model can more accurately predict the damage and failure process of the material,and when the element strain is above0.0065,the parameter degradation can more accurately predict the material damage process.When the strain is 0?0.002,the exponential degradation constitutive and the parameter degradation constitutive have little difference,both of which can accurately predict the material damage process.When the strain is 0?0.008,the exponential degradation constitutive model is more accurate.When the strain is above 0.008,the constitutive model of parameter degradation can more accurately predict the damage mechanical properties of the material.The comparison of simulation results and experimental results can fully prove the validity of the damage constitutive model of unidirectional/ woven composites with exponential/parameter degradation.A 16-layer unidirectional fiber composite laminate was established,and four unidirectional composite material programs were used to perform finite element simulations of projectile impact at different impact speeds considering the strain rate.At the same time,the finite element simulation without considering the strain rate under the corresponding working conditions was carried out.The finite element simulation results with and without strain rate are compared.The results show that when the impact velocity is less than 80m/s,the maximum contact force difference between the strain rate and the strain rate is less than 20%.At the same impact velocity,the peak load of the constitutive model with solide element parameters degradation is the most sensitive to the strain rate effect.When the impact velocity is130m/s,the difference between the maximum contact force of the solide element parameter degradation damage constitutive and the strain rate without consideration is the largest,which is 35.10%.A 16-layer unidirectional fiber composite finite element model for high speed tensile simulation was established,and the simulation results were compared without considering the strain rate.The results show that the solide element exponential degradation constitutive model more accurately simulates the energy absorption characteristics of unidirectional fiber composites at high strain rates.Compared with the simulation without considering the strain rate,the percentage of energy absorption is increased by 34.42%.A 16-layer woven composite laminate was established to simulate projectile impact and high speed stretching.Comparing with the simulation results without considering the strain rate,the results show that when the impact velocity is less than80m/s,the maximum contact force difference between the four constitutive models considering the strain rate and the constitutive model without considering the strain rate is less than 10%.Within this speed,a constitutive model that does not consider strain rate can be used to predict the maximum contact force.When the speed is80m/s?130m/s,the difference of the maximum contact force is about 20%.When the speed is greater than 130m/s,the difference in the maximum contact force is greater than 25%.Under the same impact velocity,the solide element exponential degenerate constitutive model is more sensitive to the strain rate effect.When the impact velocity is 130m/s,the difference between the maximum contact force of the damage constitutive with solide element exponential degradation and the no strain rate is the largest,which is 23.47%.A 16-layer woven composite finite element model for high speed tensile simulation was established,and the simulation results were compared without considering the strain rate.The solide element exponential degradation constitutive model in the tensile simulation more accurately simulates the energy absorption characteristics of unidirectional fiber composites at high strain rates.Compared with the simulation results without considering the strain rate,the percentage of energy absorption is increased by 13.35%.Through the lateral comparison between the unidirectional/woven composite programs and the comparison with the simulation results without considering the strain rate.The results show that in the projectile impact simulation,the maximum contact force difference between the shell element and the solide element constitutive model is less than 20% at the same speed,and the stiffness of the shell element finite element model is slightly lower than that of the solide element finite element model of the same constitutive.In the tensile simulation,according to the kinetic energy curve and the fracture condition of the laminate,it can be concluded that the shell element and the body element results are also relatively close.Further verify the effectiveness of each procedure.By comparing the simulation results between the shell/body element,it can be concluded that the shell element considering the strain rate effect is basically consistent with the corresponding constitutive model of the solide element under the impact and tension conditions,which fully proves that the shell element's effectiveness of the application in high strain rate simulation,which provides a certain research foundation for the future application of shell elements in the study of strain rate effects. |
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