Study On Mechanical Properties Of Cancellous Bone Under Overloading

Bone is the frame of human body with the functions of support,movement and protection.Cancellous bone,as an important part of bone,is a kind of porous,inhomogeneous,anisotropic and viscoelastic structure.It plays a very important role in the load transmission and energy absorption.Its mechanical properties is different from that of the cortical bone.At present,the most studies of bone mechanics is in the field of cortical bone.In recent years,an increasing number of researches of cancellous bone are reported.However,the mechanical properties of cancellous bone under overloading,such as the viscoelastic properties of cancellous bone and the ratcheting effect,need to be further studied.To sum up,it is of great significance to study the mechanical properties of cancellous bone under overloading.The material used in this study was the fresh cancellous bone taken from the knee joint of 8-month-old pig.In this paper,the cancellous bone of porcine femur was studied as the research object.Creep properties and stress rate dependent properties of cancellous bone under uniaxial compression were investigated by the experiment and theoretical model prediction.It is found that the creep strain and creep compliance increase rapidly at first and then increase slowly as the creep time goes by.With increase of compressive stress the creep strain increases,while the creep compliance decreases.Stress rate has a great influence on the compression behavior of cancellous bone.Its Young's modulus increases with increase of stress rate,which shows that cancellous bone is a kind of rate-dependent nonlinear viscoelastic material.The creep model and nonlinear viscoelastic constitutive model of cancellous bone were proposed in term of the nonlinear viscoelastic characteristics of cancellous bone.They were used to predict its creep property and rate-dependent compressive property.The results show that there are good agreements between the experimental data and predictions.The ratcheting tests under different loading conditions were conducted and the influences of stress amplitude,stress rate,mean stress and peak time on ratcheting behaviors of cancellous bone were studied.It is found that both the ratcheting strain and ratcheting strain rate increase with increase of stress amplitude or with decrease of stress rate for the ratcheting compression tests.The ratcheting strain of cancellous bone without marrow is smaller than that of normal cancellous bone under the same conditions.The ratcheting strain in different depth is basically coincident in terms of the microscopic images.We can find that the mean stress,stress amplitude and the peak time affect the ratcheting behavior of bone under cyclic tension-compression loading.The increase of mean stress,stress amplitude and peak stress time will accelerate the accumulation of ratcheting strain.Meanwhile the high-frequency ratcheting fatigue experiments were conducted under cyclic tension-compression loading and it is noted that the fatigue life is greatly reduced with increase of stress amplitude.Microscopic observation shows that fatigue crack initiation and propagation will eventually lead to fracture of cancellous bone sample.