The thorax comprises of a number of vital life supporting organs, heart, lungs, liver, spleen and kidneys protected by the rib cage. The rib cage consists of a set of thin half circular bones with flexible attachments to the sternum at the front and spinal vertebrae at the back. The ribs are all interlinked with muscles, which allow the thoracic cavity to be expanded and contracted to assist breathing. There are two major injury mechanisms to the thorax.
Due to the different injury mechanisms, different injury criteria have been proposed for the rib fracturing and internal organ injury.
Also both frontal and lateral thorax impacts must be considered as both these occur to occupants in rail vehicles. Although they have the same injury criterion, as they have the same injury mechanisms, the chest has different dimensions longitudinal to lateral so the criterion would have different tolerance levels.
The large amount of biomechanical impact testing on animals and cadavers has lead to a whole range of different injury criteria and tolerance levels for both front and lateral impacts on the thorax. These are presented shown in Appendix 1, Table 25 based on a review by J. M. Cavanaugh (Ref. 2.1 Pages 385 / 386)
In terms of the acceleration injury criterion adopted in the automotive industry a tolerance level of cumulative 60g for 3 msec is used. This was introduced when the only thorax parameter measured on the HII ATD was spine acceleration and is not the optimum criterion for either rib fracture or internal organ injury. The tolerance level of 60g was selected from the results of sled tests, using volunteers when subjected to constant accelerations of 45g reported no injuries. At the time no other parameters were measured.
However acceleration does not correlate with either of the two main thorax injuries, rib fracture and injury to internal organs. The ribs themselves have considerable elastic compliance allowing a relatively large compression of the rib before fractures. Rib fractures are therefore directly related to both force and chest compression as both are linked to the biomechanical stiffness of the thorax. Chest compression, instead of force, has been adopted as the better criterion for predicting rib fracture, as it is potentially easier to measure.
The Rib Fracture Criterion can be applied to all direct impacts to the chest. The tolerance levels proposed are based on blunt object impacts, although there is a low probability of a direct chest impact, blunt objects are most frequent in the rail vehicle interior. There are different tolerance levels for frontal and lateral impacts due to the relative overall rib cage strength related to geometry being different.
Crash Test Dummy – Both the HIII frontal and EuroSID side impact dummies measure rib cage compression. Care should be taken using the HIII compression measurement as an arm attached to a rotary potentiometer measures the overall sternum compression, and is not accurate at measuring localised compressions particularly at the bottom of the rib cage.
Computer Simulations - Both MADYMO and dynamic finite element models reproduce the dynamic stiffness of the crash test dummy thorax area and produce chest compression and rib fracture.
As mentioned in the injury mechanisms, injury to the internal organs, heart, lungs, liver, spleen and blood vessels is better linked to the rate of intrusion and not the chest compression. As can be seen in the Appendix 1, Table 25 there have been many criteria with different tolerance levels applied to predict injury levels. Frontal impact acceleration based criterion has been adopted in FMVSS214 as acceleration was the best parameter to measure in the USSID Crash Test Dummy. However, most researchers agree that a compression and rate dependent criterion is the most applicable. Therefore the velocity/compression based, or viscous criterion, has been adopted as the best indicator of injury level. The advantage of this criterion is that it accommodates impacts of different velocities, 3 to 30 m/s, and mass or energy as well as being used in both frontal and lateral impacts.
The viscous criterion, or V*C, is calculated as the product of the relative velocity of the chest wall to the spine and the relative displacement or compression of the chest wall to the spine in both frontal and lateral impacts using the formula below.
V*C(t) = ( V(t) x C(t))
The tolerance levels applying to both frontal and lateral impacts are used in conjunction with the chest compression criterion, which predicts rib fractures.
The Thorax Internal Organ Injury Criterion is applied to all chest direct impacts irrespective of the object shape. The criterion is to be used in conjunction with the Rib Fracture Criterion.
Crash test dummies – V*C is calculated from the chest deflections measured in both the HIII and EuroSID dummy. Reduction in the accuracy of the data can result from the differentiation of the deflection to produce the velocity. As with the Rib Fracture Criterion deflections measured on the HIII dummy should be treated carefully as only on overall central sternum, and not localised, deflections are measured.
Computer Simulations - As the V*C is calculated from chest deflections, and both MADYMO and dynamic finite element models reproduce the dynamic stiffness of the crash test dummy thorax area, V*C can be readily predicted. Improved models with better biofidelic thorax dynamic characteristics, would produce even better injury predictions.
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Dr. A.R. Payne |
S. Patel |
© MIRA 2001 |
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Project 427519 |
Version 1.1 |