Abstract
In order to replicate the fracture behavior of the intact human skull under impact it becomes necessary to develop a material having the mechanical properties of cranial bone. The most important properties to replicate in a surrogate human skull were found to be the fracture toughness and tensile strength of the cranial tables as well as the bending strength of the three-layer (inner table-diplöe-outer table) architecture of the human skull. The materials selected to represent the surrogate cranial tables consisted of two different epoxy resins systems with random milled glass fiber to enhance the strength and stiffness and the materials to represent the surrogate diplöe consisted of three low density foams. Forty-one three-point bending fracture toughness tests were performed on nine material combinations. The materials that best represented the fracture toughness of cranial tables were then selected and formed into tensile samples and tested. These materials were then used with the two surrogate diplöe foam materials to create the three-layer surrogate cranial bone samples for three-point bending tests. Drop tower tests were performed on flat samples created from these materials and the fracture patterns were very similar to the linear fractures in pendulum impacts of intact human skulls, previously reported in the literature. The surrogate cranial tables had the quasi-static fracture toughness and tensile strength of 2.5 MPa√ m and 53 ± 4.9 MPa, respectively, while the same properties of human compact bone were 3.1 ± 1.8 MPa√ m and 68 ± 18 MPa, respectively. The cranial surrogate had a quasi-static bending strength of 68 ± 5.7 MPa, while that of cranial bone was 82 ± 26 MPa. This material/design is currently being used to construct spherical shell samples for drop tower and ballistic tests.
Highlights
FRACTURE TOUGHNESS AND TENSILE TESTING OF SURROGATE CRANIAL TABLE MATERIALS Fracture toughness tests were performed on 41 combinations of materials and Figure 5 is a plot of the fracture toughness versus weight percent fiber
If EPON 862 were used in molding the cranial tables of the actual cranial surrogates, the inability to mold reproducible cranial table materials would result in surrogate skulls that would not adequately represent fracture properties of human skulls
Based on the ability to adequately mix the materials to achieve a reproducible value of fracture toughness for the cranial table materials, especially for the varying thickness, curved surrogate skull geometry, the milled glass fiber content will be limited to 30–40 weight percent
Summary
A number of studies have been performed using post-mortem human surrogate (PMHS) specimen to study the effect of blunt impact on pressures and displacements in the brain (Hardy et al, 1997, 2001), determine skull failure thresholds and characterize skull fracture patterns (Gurdjian et al, 1949; Melvin et al, 1969; Hodgson et al, 1970; Schneider and Nahum, 1972; Sarron et al, 2004; Hart, 2005; Delye et al, 2007; Verschueren et al, 2007; Raymond et al, 2009). Due to the complexity of these PMHS experiments along with the inherent likelihood of significant specimen variability, it would be prudent to have a skull surrogate which can represent the stress, vibration, and fracture characteristics of human cranial bone in a repeatable manner. This surrogate could be used to assess the likelihood of skull fracture during impact and influence the design of mitigation techniques. Areas away from the impact point that have experienced convex curvature can exceed the tensile strength on the outside of the skull and fracture. The fracture toughness and tensile strength of the www.frontiersin.org
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