Development and field performance of indy race car head impact padding.

The close-fitting cockpit of the modern Indy car single seat race car has the potential to provide a high level of head and neck impact protection in rear and side impacts. Crash investigation has shown that a wide variety of materials have been used as the padding for these cockpits and, as a result, produced varying outcomes in crashes. Additionally, these pads have not always been positioned for optimal performance. The purpose of this study was to investigate the head impact performance of a variety of energy-absorbing padding materials under impact conditions typical of Indy car rear impacts and to identify superior materials and methods of improving their performance as race car head pads. An extensive series of tests with the helmeted Hybrid III test dummy head and neck on an impact mini-sled was conducted to explore head padding concepts. Following this, a performance specification for a simplified impact test using a rigid headform that simulates the helmeted head was developed and recommendations for performance levels of head padding based on biomechanical data on helmeted head impacts were made. In 1997, during the time that the head pad research was being performed, the Indy Racing League introduced a new chassis specification for their cars. There were a number of rear- and side-impact crashes during that season that resulted in seven severe head injuries. Examples of the head padding in those cars were included in the experimental study. The results of the head pad research were used to specify new padding materials that met the new biomechanical criteria. The placement of the head pads was also changed for better location of the padding. These changes instituted in 1998 have reduced the number of head injuries in crashes similar to or more severe than those of 1997 and have resulted in only occasional moderate head injuries (concussions) in the 1998 and 1999 seasons.

Finite element model study of head impact based on Hybrid III head acceleration: the effects of rotational and translational acceleration.

The translational and rotational components of acceleration measured at the center of gravity of a Hybrid III dummy head were used to investigate their individual and combined effects on a two-dimensional finite element model of the human brain. Each component of acceleration generated distinct patterns of deformation. Although translational acceleration is related to pressure and rotational acceleration has a dominant effect on shear deformations, complete acceleration (combination of translation and rotation) yielded the highest values in all stresses and produced a maximum shear stress at the top of the brain.

Development of tissue level brain injury criteria by finite element analysis.

A three-dimensional finite element model of the direct cortical impact experiment was built and a preliminary validation against mechanical response was completed. The motion of the impactor was enforced in the model by applying the same acceleration history as that of the experimental impactor. A nonlinear contact surface algorithm was used for impactor-brain interface with the ABAQUS general purpose finite element program. The resulting motion of the impactor and the contacting node in the brain model confirmed that the impactor moved realistically and contacted the brain surface. The pressure generated in the model compared favorably with that measured by a pressure transducer in the experiment. The pattern of high shear deformation generated at the impact site in the model was similar to the pattern of contusion hemorrhage seen in the experiment. The pressure generated at the impact site propagated to the skull-brain boundary, especially, at the posterior margin of the cerebellum. Analysis of experimental data using a biomechanically validated finite element model will enable determination of tissue-level injury criteria for application in human brain models to predict head injury potential in contact, noncontact, or side impact situations.

Fracture mechanics of bone.

This paper reviews the progress that has been made in applying the principles of fracture mechanics to the topic of fracture of long bones. Prediction of loading conditions which result in the propagation of fractures in bones has been of interest to the field of trauma biomechanics and orthopedics for over one hundred years. Independent verifications, by various investigators, of bone fracture mechanics parameters are reviewed and investigations of the effects of bone density and specimen thickness on the critical fracture mechanics parameters and of other factors such as critical crack length and plastic zone size in bovine femoral bone, and the effects of crack velocity on fracture mechanics parameters in bovine tibial bone are discussed. It took over ten years for the techniques of bone fracture mechanics to be applied to human compact bone, due primarily to geometric constraints from the smaller size of human bones. That work will be reviewed along with other continuing work to define the orientation dependence of the fracture mechanics parameters in bone and to refine the experimental techniques needed to overcome the geometric constraints of specimen size. A discussion is included of work still needed to determine fracture mechanics parameters for transverse and longitudinal crack propagation in human bone and to establish the effects of age on those parameters. Finally, a discussion will be given of how this knowledge needs to be extended to allow prediction of whole bone fracture from external loading to aid in the design of protective systems.

Injury biomechanics research: an essential element in the prevention of trauma.

The central aspects of injury biomechanics research are defined and research approaches described. These aspects include the identification and definition of impact injury mechanisms, the quantification of biomechanical response to impact, the determination of impact tolerance levels, and the development and use of injury assessment devices and techniques for evaluating injury prevention systems. The current status of knowledge and technology is then reviewed for the head, cervical spine, thorax, abdomen, and lower extremity. Important gaps are identified, and research priorities emphasizing functional impairment are proposed.

Mechanical behavior of fetal dura mater under large deformation biaxial tension.

The mechanical behavior of fetal dura mater was investigated by means of a biaxial tension test designed to simulate the constraints imposed on the membrane by the cranial bones. The experimental results are compared with the theoretical results obtained by using two published strain energy functions: one defined by Mooney and Rivlin (MR) and the other by Skalak, Tozeren, Zarda and Chien (STZC). The latter constitutive relations fit the experimental results consistently well. The STZC stiffness values from this series of tests are compared with those from membrane inflation tests performed previously and reported elsewhere by the authors.

Mechanical behavior of fetal dura mater under large axisymmetric inflation.

The nonlinear mechanical behavior of fetal dura mater was tested experimentally and compared to two published nonlinear material strain energy functions, the Mooney-Rivlin and the Skalak, Tozeren, Zarda, and Chien (STZC). The STZC constitutive relations best fit the behavior of the dura mater and were used to describe quantitatively its stiffness. Runge-Kutta numerical procedures were used to fit the theoretical data to the experimental results. The material's stiffness was positively correlated with fetal weight (r = 0.67, p less than 0.05). These results are discussed and directions for future research indicated.

Failure properties of passive human aortic tissue. II--Biaxial tension tests.

Descending mid-thoracic aortas were obtained from 16 autopsies and biaxial inflation tests performed on the tissue at dynamic (approximately 20 s-1) and quasi-static (approximately 0.01 s-1) strain rates. A bubble inflation technique was developed for this purpose. Extension histories of the specimens were recorded photographically and values of ultimate stresses and extension ratios in biaxial stretch have been calculated. Under conditions of uniform biaxial stretch the tissue consistently failed in a direction perpendicular to the long axis of the aorta and pressure values at failure were greater by a factor of two in the dynamic tests than those in the quasi-static tests.

Failure properties of passive human aortic tissue. I--uniaxial tension tests.

Samples of descending mid-thoracic aortas were obtained from human autopsies and experiments were performed to determine the effect of strain rate and direction of loading on the failure properties of the tissue in uniaxial tension. The tests were performed at quasi-static strain rates in the range 0.01 s-1-0.07 s-1 and dynamic tests in the range 80 s-1-100 s-1. Ultimate stress and extension ratio values have been calculated for specimens tested in the longitudinal and transverse directions of the aorta.

Cervical fractures and fracture-dislocations sustained without head impact.

Because of its flexibility and structure, the cervical spine is disposed to various mechanisms of injury: although not so common as injuries caused by head impacts, cervical fractures and/or fracture-dislocations have been reported without direct impact to the head. Some cervical injuries reported have been sustained by wearers of lap and shoulder belts in auto accidents; however, we do not consider belt use a potential hazard because ample evidence has accrued in the medical and engineering literature to document general injury and fatality reduction by use of seatbelts. We believe that in many instances occupants would be more seriously injured or killed were belts not worn. The present paper reviews reports of cervical injuries without head impact found in the literature and case histories of such injuries from the Highway Safety Research Institute of The University of Michigan, as well as experimental studies in animals, cadavers, and volunteer subjects.

Impact trauma of the human temporal bone.

A cooperative study between the Department of Otorhinolaryngology and the Highway Safety Research institute of the University of Michigan was designed to study temporal bone fracture produced in cadavers subjected to realistic automotive impact situations. Utilizing sled and piston impact configurations frontal and parietal impacts were noted to produce ipsilateral and contralateral fractures of nine temporal bones in seven cadavers. The impact velocities varied between 18.1 and 25.0 mph. Using standard otologic microsurgical techniques, the temporal bones were dissected and numerous gross and microscopic injuries to middle and inner ear structures were found. The authors conclude that extensive comminuted fracture of the human temporal bone is seen with realistic crash situations of low velocity, and that lateral impact which produces a longitudinal fracture with a posterior fossa comminution is associated with disruption of the cochlea and facial nerve, as well as of middle ear structures. The classical transverse fracture of extensive skull trauma lies medial to these structures and does not involve the otologic contents of the human temporal bone. Associated brain and skull injuries are also described.

Static and dynamic impact trauma of the human larynx.

The response of fresh human larynges to static and dynamic compressive loading has been determined for 24 specimens. Mean static force values producing thyroid and cricoid cartilage fractures were 15.8 and 20.8 kg, respectively, and the similarity of this experimental injury to a mild clinical laryngeal fracture syndrome is discussed. Dynamic fracture loading, at velocities up to 11 mph, caused cartilage fractures at forces averaging 30% more, and comparison with the static data is made. Interaction of the thyroid and cricoid cartilages to impact force are analyzed in reference to airway protection. The 50% compressive strain level, at which structural collapse is imminent, averaged 55 kg. The significance of these previously unreported low force levels producing fracture is discussed with reference to automotive design.