Study On The Mechanical Properties And Constitutive Model Of Brain Tissue For Child Head Protection

Children could be at high risk of brain injury in accidents such as traffic accidents and accidental falls.The constitutive model of brain is very important in the study of the mechanical response of brain.The core objective of this study is to construct a hyperelastic constitutive model and its material subroutine for finite element simulation,which uses a set of parameters to represent the mechanical response of the brain under multi-stress state and multi-rate loading conditions.First of all,as a substitute for the brain material of children,this study completed the experiment in brain specimens from 8-week-old piglets.The materials were sampled from the white matter,gray matter,cerebellum and brainstem.The stress states involved include uniaxial compression,uniaxial tension and simple shear;and the loading strain rates included quasi-static(0.01/s),medium speed(1/s)and high speed(50/s).The engineering stress-strain curves under various loading conditions were obtained by experiments.According to the statistical analysis,it is found that the stress of brain material presents obvious strain rate effect and certain regional differences.Then,the Ogden strain energy density equation and the stress equation of each working condition was derivated.In succession,the deformation tensor,the elongation,the conversion relation of some variables such as the main direction and the strain were analyzed.It is found that the Ogden constitutive model cannot represent the brain mechanical behavior under different stress states using only one set of parameters,therefore,the study put forward an improved Ogden constitutive model which includes an additional hyperelastic function taking the elongation as the independent variable.According to the relationship between the stress and the strain rate,a strain rate function was constructed and was coupled with the aforementioned improved Ogden constitutive model,and then the final improved Ogden constitutive model was established to reflect the effect of both the stress state and the strain rate on the brain mechanical responses using a set of parameters.Furthermore,the UMAT material subroutine of Ogden constitutive model was compiled in LS-DYNA according to the development process and the basic algorithm of hyperelastic constitutive model.According to the simulation of brain specimens' finiteelement models,it is proved that the improved Ogden constitutive model can reflect the mechanical properties of brain specimens under different stress states and strain rates.Finally,the improved Ogden constitutive model was applied to the impact simulation of a brain finite element model of an 8-week-old piglet.Through the analysis of the stress contours,the developed model showed preferable application performance.