Skull fracture can be a complex process involving various types of bone microstructure. Finite element analysis of the microscopic architecture in the bone allows for a controlled evaluation of the stress wave interactions, micro-crack growth, propagation and eventual coalescence of trabecular fracture. In this paper, the microstructure and mechanics of small-volume sections of a 6-month-old Gottingen Minipig skull were analyzed. MicroCT scans were used to generate finite element models. Various computational methods were investigated for modeling the intricacies contained within the porous microstructure of the trabecular bone. Pores were explicitly meshed in one method, whereas in the second, a mesh was created from a microCT image-informed mapping algorithm that mapped the trabecular porosity from an image stack to a solid volume mesh of the model. From here, all models were subject to uniaxial compression simulations. The output of the simulations allowed for a detailed understanding of the failure mechanics of the skull structure and allowed for comparison between the methods. Fracture typically occurs in the weakest areas where the bone is highly porous and forms a fracture surface throughout the material, which causes the bone to collapse upon itself.
The development of a multi-axial failure criterion for trabecular skull bone has many clinical and biological implications. This failure criterion would allow for modeling of bone under daily loading scenarios that typically are multi-axial in nature. Some yield criteria have been developed to evaluate the failure of trabecular bone, but there is a little consensus among them. To help gain deeper understanding of multi-axial failure response of trabecular skull bone, we developed 30 microstructural finite element models of porous porcine skull bone and subjected them to multi-axial displacement loading simulations that spanned three-dimensional (3D) stress and strain space. High-resolution microcomputed tomography (microCT) scans of porcine trabecular bone were obtained and used to develop the meshes used for finite element simulations. In total, 376 unique multi-axial loading cases were simulated for each of the 30 microstructure models. Then, results from the total of 11,280 simulations (approximately 135,360 central processing unit-hours) were used to develop a mathematical expression, which describes the average three-dimensional yield surface in strain space. Our results indicate that the yield strain of porcine trabecular bone under multi-axial loading is nearly isotropic and despite a spread of yielding points between the 30 different microstructures, no significant relationship between the yield strain and bone volume fraction is observed. The proposed yield equation has simple format and it can be implemented into a macroscopic model for the prediction of failure of whole bones.
Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1D-strains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated.
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