Influence of interfacial properties and inhomogeneity on formation of microdamage in bone
Microdamage accumulation at the nanoscopic level of bone affects the overall mechanical behavior of the bone. This makes it necessary to study the mechanisms through which microdamage accumulation can take place at the nanoscopic level. Experiments on bone's different hierarchy are difficult because of the small sizes of these hierarchical structures. Prevention of bone fractures is greatly enhanced with the help of predictive computational tools and hence used to evaluate the effects of microdamage in bone. There are two main types of microdamage that can form in the bone; linear cracks and diffuse damage. The bone nanostructure consists of mineral platelets embedded in soft protein called collagen and can be treated as a composite material.
In this study, a two-dimensional probabilistic finite element model of the bone nanostructure was developed to evaluate the likely formation of the microdamage in the nanostructure due to changes in material properties of the nanostructure. The influence of the microdamage formation due to the collagen-mineral interface strength and also the effects of inhomogeneity were studied. To study interfacial strength effects, cohesive elements using bilinear traction separation laws were used to simulate the behavior of the interface (by way of interfacial debonding) between the collegen-mineral layers. Random field theory was used to assign spatially correlated random variables in order to assign inhomogeneous material properties to the bone. Correlation lengths were used to control the level of inhomogeneity in the model.
The analysis showed that the type of microdamage was significantly influenced by the strength of the mineral-collagen interface. Probabilistic failure analyses indicated that strong interfaces resulted in limited interfacial debonding and narrow stress concentrations around an initial defect in the mineral-collagen composite, thereby suggesting that the likely location of failure was in same plane of the initial defect. This led to the conclusion that the likely type of microdamage formation was linear cracks. With weaker interfaces, interfacial debonding was significant and wider stress concentrations were formed around the initial defect. The probabilistic analysis determined that with weaker strengths and significant interfacial debonding, the next likely location of failure was scattered away from the initial defect. This led to the conclusion that the likely type of microdamage formation would be diffuse damage.
The influence on microdamage formation due to the inhomogeneity of the bone material was also evaluated and it was determined that the type of microdamage formation was not influenced by the inhomogeneity. Differences in correlation lengths did not influence the type of microdamage formation. It was determined that the interfacial strength was a greater influence on the microdamage formation than the material inhomogeneity.