Multiscale Experimental Research On Mechanical Response Under Compound Load Pattern And Deformation Mechanism Of Cortical Bone

Cortical bone is a natural and complicated biological compound material,with excellent mechanical property of bearing load,providing attachment position for other tissue growth and transmitting the load to achieve various complicated movements.However,more and more emergency situations threaten cortical bone material,weaken the service performance of cortical bone,bringing a great threat to people's life,and at the same time increase the social economic burden.Therefore,it is very important to explore the mechanical property of cortical bone material under service load condition and potential deformation mechanism in micro-scale.This research would not only provide important assessment of the service performance of cortical bone,but also provide some theoretical references and directions for the solution design of damaged bone repairing,reason traceability of damaged bone,and bionic bone substitute materials' microstructure design and components proportion.Different load modes result to different cortical bone damage forms,so that the mechanical properties and failure mechanism are different,too.Currently,the researches on mechanical property of cortical bone are mainly focus on single load pattern and at macro-scale,couldn't reveal objectively the mechanical property of cortical bone under the service load condition.As discussed above,the load pattern of cortical bone under emergency is complicated,so that exploring mechanical property of cortical bone under single load pattern couldn't reveal the objective mechanical response under service condition.Therefore,this paper innovatively presents a new research idea of mechanical property of cortical bone material under compound load pattern and multi-scale deformation mechanism.The main research works in this paper are as follows.(1)The current states and progress of mechanical property of cortical bone was reviewed and discussed,including the normal means and techniques.Based on the comprehensive analysis of objective service condition and urgent demand of objective reality,the new idea of mechanical property measurement of cortical bone under compound load pattern and multi-scale research of deformation mechanism was put forward.(2)The biological constitution and the complicated hierarchical structure of cortical bone were introduced in detail.Macro and micro mechanical model were established.The mechanical mode based on compound load was established.Bases on the comprehensive consideration of the objective service condition and the material property,the research route was established,and this research route is mechanical property under compound load on macro-scale,deformation mechanism at micro-scale,including micro-scale bone lamellar structure,micro-scale material components,micro-scale external contact forms,micro-scale reliability assessment of protecting mechanism,and nano-scale deformation mechanism of organic collagen fiber' two phase composition.(3)Macro-scale mechanical property experimental researches were carried out.The single load pattern contains compression load,bending load and torsion load.The compound load pattern contains compression-bending compound load and compression-torsion compound load.In the experiments,to obtain the whole surface strain evolvement law and fracture surface micro-topography of cortical bone specimen,the non-contact digital image correlation method(DIC)and field emission scanning electron microscope(FE-SEM)were used.The mechanical property parameters of cortical bone of elastic modulus,shear modulus,compressive strength,bending strength and torsion strength were obtained by theoretical analysis and calculation.Based on the experiments,it was found that the bending strength of cortical bone increased and the torsion strength and shear modulus all decreased under coupling application of the compression load.The research found the crack initiation,dispersion of propagation path and fracture area all changed under the coupling application of the compression load.At the same time,it was found that the increasing of bending deflection was suppressed at early stage,and then it was promoted at later stage under compression load,according to theoretical analysis.Coupling application of the compression load could result the strain direction conformance in shorter time,so that cortical bone material overcomes the influence of anisotropic faster.At the same time,it was found that the deformation effect on transverse and longitudinal direction was same under compression load and torsion load.Therefore,compression load could promote the effect of torsion load,so that the torsion strength decreased.(4)To obtain the potential micro-scale deformation mechanism of cortical bone material from multi perspective,the micro-scale mechanical property of cortical bone material was explored by nanoindentation experiments.At first,the mechanical property of outer circumferential lamella was explored,and it was found that the elastic modulus and hardness of outer circumferential lamella increased from outer to inner,and present soft to hard property.It was proved that the protecting mechanism of outer circumferential lamella was buffering-supporting function gradient.Then,from cortical bone material level,the coordination mechanism between two components when the water content changed was explored.It was found that the elastic modulus and hardness of cortical bone and deproteinization cortical bone all increased with decreasing of water content.However,the change of mechanical property of deproteinization cortical bone was more drastic.Through the microscopic observation,it was found that shape and size microstructure under the nanoindentation of deproteinization cortical bone were all irregularity.However,the elastic energy and plastic energy of deproteinization cortical bone was all less than cortical bone when water content changed,and the difference value was bigger when water content changed.From micro external contact forms level,the influence of different contact sharpness to the load transmitting and property evolution was explored.Through the contrast test,it was found the blunt contact form has different sharpness mechanical response in different stage,present contact forms from blunt to sharp property.It was found that the sharpness of indenter was related with the plastic energy.After microscopic observation,it was found that the material under the indenter was compressed more serious in blunt contact form,and the material of side surface was compressed more serious in sharp contact form.From the reliability of micro protect mechanism level,based on the periodicity transient partial dehydration of articular cartilage,the micro mechanical property and fracture mechanism of articular cartilage were explored.It was found that the micro particle could take damage to articular cartilage in dehydration condition.Through the microscopic observation,it was found that there was some debris under the effect of micro particle,and the debris adhered to the micro particle and moved with the micro particle.This may be the potential damage reason of articular cartilage.At the same time,it was found there was some microcrack under the surface of articular cartilage,the protection of articular cartilage to cortical bone was achieved by the dynamic balance of produce and repaired.(5)To obtain the essential potential deformation mechanism of cortical bone,through nano-scale simulation experiments,the coordinate deformation mechanism of two components of mineralized collagen fiber molecule and the influence factor were explored.The Hodge-Petruska model of mineralized collagen fiber molecule was established,and the model mainly considered the elastic modulus of the two components and the spacing and length of inorganic component.The research found that shear and compression were the main contributors in the mineralized collagen fiber molecule.The organic component mainly contributed the shear load,and the inorganic component mainly contributed the compression load.There was shear stress concentration phenomenon in end of organic component,and the compression stress concentration phenomenon was produced in the interface between organic component and inorganic component.It was found the elastic modulus of organic component has the most important regulation effect to the elastic strain energy of the whole structure.The change law of apparent elastic modulus of the two components was same.Through contrast,it was found that the sensitivity of the apparent elastic modulus ratio between organic component and inorganic component to parameter change was elastic modulus of organic component,elastic modulus of inorganic component,the length of inorganic component and the spacing of inorganic component.