Experiment Research On Strain Rate Dependency Of Material Properties Of Lower Limb Cortical Bone Applied For Human Safety Finite Element Models

Finite element method is the main tool to analyze the lower limb bone injury in traffic accidents such as forward collision and vehicle-pedestrian collision,and ac curate definition of lower limb bone material properties is the necessary condition f or this method to obtain reasonable results.However,the dynamic mechanical beha vior of lower limb cortical bone in the collision condition is much different from t hat in the static condition,and there is no suitable test method and test result for t his problem.As a result,the static material parameters of bone are still used in the current finite element simulation,and the results are not accurate.Therefore,the main purpose of this study is to establish a standard test method for dynamic mech anical properties of lower limb bone materials and obtain the strain rate dependence of mechanical behavior of cortical bone materials.At the same time,in order to s olve the problem of lack of human bone samples in the material test and expand t he range of bone material test materials,the effect of drying and preservation meth ods on cortical bone strain rate dependence was also studied.The main research co ntents are as follows:（1）in the full study animal species,age,location,sample production and test method on the biomechanics of bone cortex performance factors have a significant i mpact on the basis of individual differences to maximize the biomechanics of bone cortex ruled out performance and reduce the test error for the principle,using pig f emoral bone substitute,designed the cortical bone sample acquisition process and m achining method.Subsequently,128 cortical bone samples were divided into 15 gro ups according to 5 strain rate ranges between 10^-3 s^-1 and 10³ s^-1（static 2 groups,dynamic 3 groups）and 3 preservation methods（fresh,dry and anticorrosive）.（2）based on the one-dimensional stress wave theory,the method that satisfies the assumption of one-dimensional and linear elasticity in hopkinson bar test is anal yzed,the material and size of hopkinson bar are selected,and the loading system a nd data acquisition system are designed.At the same time,in order to satisfy the c ondition of stress balance and constant strain rate,the pulse shaping effect of each plastic material and its combination was pretested,the shaping effect of plastic met al and soft material was analyzed,and an incident pulse shape design method suita ble for dynamic compression test of cortical bone was proposed.（3）the samples were subjected to static and dynamic compression tests in grou ps to test the mechanical properties of the bones in each group,and the macroscop ic morphology,microscopic surface structure and organic matter mass fraction of th e fragments were recorded.Finally,the relationship between the mechanical properti es of cortical bone materials and their materials and structures was obtained.The results of this paper show that,in terms of the effect of pulse shaping,b oth ends of the sample always maintain stress balance during the whole deformatio n process,and more than 70%of the strain of the sample occurs in the stage of c onstant strain rate.The constant strain rate values of the samples in each group we re in positive proportion to the cross-sectional area of the shaper（R²=0.8157 in the fresh group,R²=0.8508 in the anticorrosion group,and R²=0.8743 in the dry group）,but the linear fitting slope of each group was different.In terms of cortical bone mechanical properties,the strain rate increased from 10^-3 s-1 to 10³ s^-1,and the ulti mate stress of fresh bone increased by 94.5%（R²=0.7113）and the ultimate strain i ncreased by 94.5%（R²=0.7113）and 138%（R²=0.5774）,respectively.The ultimate st rain increased 78%（R²=0.76263）and the ultimate stress increased 67%.The ultimat e strain and stress of dry bone increased by 24%and 1%respectively.There were significant differences in the organic matter mass fraction of the three groups of sa mples（p=0.0048）,but there was no correlation between the organic matter mass fra ction and the mechanical parameters of the materials.In terms of micro-morphology of fracture surface,fresh bone,anticorrosive bone and dry bone were significantly different,and different fracture mechanisms were observed on the surface of static(10^-3 s^-110^-2 s^-1),low-speed dynamic(200 s^-11000 s^-1)and high-speed dynamic(>103 s^-1)fragments.In the macroscopic fracture morphology of samples,fresh bone and anticorrosive bone tended to fracture along the oblique direction,while dry bon e tended to fracture along the axial direction.In terms of test methods,the cortical bone dynamic compression test method e stablished in this paper can effectively maintain the constant strain rate in a large r ange and is easy to be repeated.Bone mechanics performance,consistent with the similar research results at home and abroad,this study fresh cortical bone showed obvious strain rate dependence,ultimate stress and the ultimate strain with the strai n rate increased significantly,therefore in the vehicle collision damage analysis usin g static material parameters,there is a big error may greatly overestimate the occup ants and pedestrians damage values.At the same time,embalmed bone and dried b one are difficult to be associated with fresh bone due to significant differences in material composition and microstructure,so it is not recommended to be used in m aterial mechanics and biomechanical tests.