The Study On Characterizing Material Constitutive Of Thoracic Aorta Based On Uniaxial Tensile Test And Its Forensic Application

Background and ObjectiveThe aorta,the largest blood vessel in the body,is the main artery of the systemic circulation.Once aorta rupture occurs,the mortality rate is as high as 80%-94.4%,which is one of the important causes of death in forensic practice.Aortic ruptures can be caused by disease,trauma,or medical practices.How to distinguish between pathologic,traumatic or medical aortic rupture is one of the difficulties in forensic identification,especially in the identifying the degree of participation of trauma when multiple trauma or trauma and disease coexist.The biomechanical analysis method of injury based on the finite element can be used to study the mechanism of aortic injuries,and one of the key research bases is the material property data of the aorta,which directly affects the accuracy of simulation results.Uniaxial tensile test is the simplest and most widely used test to characterize the mechanical properties of aorta.However,there is no unified method and procedure for aortic uniaxial tensile test,and the results of aortic uniaxial tensile test reported in the literature are often inconsistent or even contradictory.In the first part of this study,the optimal strain measurement method and sample processing scheme of aortic uniaxial tensile test were determined.Secondly,on the basis of the optimized experimental scheme,uniaxial tensile tests were carried out on the segments and age groups of human thoracic aorta to characterize the mechanical properties and material constitutive of different segments of the aorta in different age groups.Then,the finite element model of aorta uniaxial tensile test was established and verified.Finally,a case of aortic rupture in a pedestrian after being hit by two vehicles was reconstructed and the mechanism of aortic injury was analyzed.Materials and MethodsPart 1.The establishment of experimental methodIn this part,porcine aorta was used as the research object.The samples were processed with custom-designed cutters,clamps and molds,and circumferential and axial uniaxial tensile tests were performed on rectangular and dog-bone-shaped samples of different widths using the Instron 8874.The width of rectangular specimens is 6mm,8mm and 10mm,and the width of the narrow part in the middle of dog-bone-shaped samples is 2mm,4mm and 6mm,respectively.Divide those with the same width and direction into a group and name them as RC6,RC8,RC10,RL6,RL8,RL10,DC2,DC4,DC6,DL2,DL4 and DL6 respectively.R,D,C and L represent rectangular,dog-boneshaped,circumferential and axial directions respectively.Numbers represent the width of the samples.Both the Instron and DIC(Digital Image Correlation)were used to measure the strain for rectangular specimens,and DIC was used to measure the strain for dog-bone-shaped specimens.Part 2:Characterizing mechanical properties and material constitutive of adult thoracic aortaThoracic aortas were obtained from 50 fresh(death within 24 hours)corpses without thoracic aortic disease from Academy of Forensic Science,aged from 27 to 86 years old.They were divided into six groups according to age.The thoracic aorta was divided into proximal and distal segments.The customed 4mm cutter was used to punched a circumferential and an axial dog-bone-shaped specimen in each segment,and the aortic ostia and calcification were avoided.Instron 8874 and DIC were used to carried out uniaxial tensile test on each sample.Part 3:Establishment and verification of finite element model of aorta uniaxial tensile testEstablishment and verification of finite element model of uniaxial tensile test of an axial specimen of aorta were carried out.HyperMesh was used to establish the geometrical model of the specimen and to partition the mesh.The material type 181 in the material library of LS-DYNA was selected,and the engineering stress-engineering strain curve and failure strain of the specimen were defined.The boundary conditions and loading conditions were set up the same as those in the uniaxial tensile test,and then calculated in the LS-DYNA Solver.The post-processing animation was viewed using HyperView,and the post-processing data was extracted using HyperGraph.Part 4:Case applicationThe aorta injury mechanism of a pedestrian hit by two vehicles was analyzed.The mechanical properties of the aorta were obtained by uniaxial tensile test of axial dogbone-shaped specimen near the rupture site of the aorta.The initial velocity of vehicle was determined by the reconstruction of accident process by MADYMO(Mathematical Dynamic Model),which was used as the boundary condition of finite element simulation.The cause of aortic rupture was determined by simulation of two collision(rolling)processes using THUMS(Total Human Model for Safety)in combination with the aortic material model and parameters of assignment method established and validated in Part 3.ResultsPart 1.Only 11.11%of the rectangular specimens failed in the middle part of the samples,and 88.89%of them failed in the vicinity of the clamps.In contrast,midsample failure occurred in 91.67%of the dog-bone-shaped specimens,whereas 8.33%of them failed in the vicinity of the clamps.The elastic modulus of elastic fiber of RC6 was greater than that of RC8 and RC10,when the strain was measured by Instron.The elastic modulus of elastic fiber of DC2 was greater than DC4 and DC6,the circumferential tensile failure stress increases with the decrease of the width of the dogbone-shaped samples.The failure stress of DL6 was greater than that of RL6,when the strain was measured by DIC.Part 2.Four samples of each thoracic aorta obtained ideal stress-strain curves.All parameter fitting regressions of the selected mathematical model were convergent and the best fitting parameters of each sample were obtained.Elastic modulus of collagen fiber,failure stress and strain showed a decreasing trend with age,while elastic modulus of elastic fiber showed an increasing trend with age.Elastic modulus of collagen fiber,failure stress and strain of circumferential tensile were all greater than those of axial tensile.There was no statistical difference in model parameters and physiological modulus between proximal and distal segments.The failure stress and strain of the male group were greater than those of the female group in the proximal circumferential tensile,distal circumferential tensile and distal axial tensile.Finally,the Fung-type hyperelastic constitutive equations of different segments in different age groups were fitted.Part 3.In the finite element simulation,the sample models at different time points were consistent with the displacement and size changes of the aorta sample in the experiment,and the failure positions were also highly similar.The true stress-strain curve and failure values converted from the simulation results were highly consistent with the experimental results.The finite element model and parameters of assignment method could characterize the kinematic and mechanical responses of aorta under uniaxial tensile test conditions.Part 4.The mechanical parameters of the deceased aorta were obtained by uniaxial tensile test.The speed of the vehicle was determined to be 60km/h by reconstruction using MADYMO combined with field investigation.The simulation results show that the collision between vehicle A and the pedestrian caused the fracture of the fourth thoracic vertebrae and the rupture of the aorta,and the collision between vehicle B and the pedestrian caused the fractures of rib cartilages of the pedestrian,but the rupture of the aorta could not be caused.Vehicle A collision was the cause of the pedestrian’s death from aortic rupture.Conclusion1.The geometry,size and strain measurement method of the sample affected the results of uniaxial tensile test of aorta.Using the grip displacement as a direct measurement of the elongation of the samples was not suitable for aortic uniaxial tensile test,while DIC could be used for aortic uniaxial tensile test.Rectangular specimens were not appropriate for uniaxial tensile test of aortic stretching to rupture.The failure stress of circumferential stretching of aorta was affected by the size of specimens,and the size effect was in accordance with Weibull theory.The dog-bone-shaped specimen with a size of 24mm×4mm in the narrow part was the optimal choice for aortic uniaxial tensile test.2.The model parameters,failure stress,strain,elastic modulus of elastic fiber and elastic modulus of collagen fiber in different segments of thoracic aorta of 50 adult were obtained by circumferential and axial uniaxial tensile tests.The differences between groups and gender were statistically analyzed.The Fung hyperelastic constitutive equations of different segments of thoracic aorta in different age groups were fitted.3.The kinematics and biomechanical responses of aorta under uniaxial tensile test could be characterized by the finite element simulation analysis using material model 181 coupled with the engineering stress-engineering strain curve and failure strain of the specimen being defined.The material model,parameters and the failure definition form could be used to study the mechanism of aorta injury in accident reconstruction.4.Based on aorta uniaxial tensile test,MADYMO and finite element simulation,a case of pedestrian death from aortic rupture after being hit by two vehicles was reconstructed and the injury mechanism was analyzed to determine the vehicle causing the aortic rupture.