Dynamic response of two dimensional photoelastic models of human head

Three photoelastic models were constructed to represent cross sections of the human head along the following three sagittal planes: the mid sagittal plane, a left lateral sagittal plane through the superior orbital fissure, the foramen ovale and the jugular foramen; and a right lateral sagittal plane through the optic foramen and foramen lacerum. Four photoelastic models were constructed to represent the cross sections of the human head along the following four coronal planes: a plane through the jugular foramen, a plane through the foramen lacerum and foramen ovale, a plane through the posterior clinoid process, and a plane through the superior orbital fissure in the small wing of the sphenoid bone. The photoelastic models were constructed from a high modulus epoxy resin to represent the skull and a low modulus urethane rubber or gelatin compound to represent the neural tissue. The cross sectional configuration of a typical skull was determined by sectioning anatomically correct plastic models of the adult human skull. The cross section geometry was duplicated in the epoxy skulls of the photoelastic models. Techniques of high speed photography and transmitted light photoelasticity were used to record and analyze the shearing stresses which occurred in the simulated neural tissue as a result of impact. The impacts to the models were of short duration, (five to ten milliseconds). The acceleration of the “skull” was detected and recorded with piezoelectric accelerometers and associated instrumentation. The dynamic material fringe values and dynamic stress-stress curves were determined for the low modulus materials which were used to simulate the neural tissue. When the strain rate increased from static to 45 in./in./sec, the dynamic modulus of elasticity ranged from 30 to 250 psi for the urethane rubber and from 10 to 40 psi for the gelatin. The photoelastic material fringe values were also rate dependent. The material stress fringe value increased with increasing strain rate and the material strain fringe value decreased with increasing strain rate. The effect of attachments between the “brain” and the “skull” of the photoelastic models was investigated. Attachments were established at the location of the sutures and along the base of the cranial vault. In all regions where there were no attachments the interface between the “brain” and the “skull” was lubricated with either silicone grease or mineral oil. The response of the models with these boundary restraints was determined. The response of the models in which the material representing the neural tissue was attached only to the base of the “skull” was also determined. The change in boundary restraints between the “brain” and the “skull” did not noticeably affect the impact force and acceleration response of the model. The shearing stress which resulted in the “neural tissue” was, however, significantly changed by a change in the boundary restraints. The maximum shearing stresses on the periphery of the “brain” were greatly affected by the roughness of the inner surface of the skull