Study of rib fracture mechanisms based on the rib strain profiles in side and forward oblique impact.

Rib fractures constitute a good indication of severity as there are the most frequent type of AIS3+ chest injuries. In 2008, Trosseille et al. showed a promising methodology to exhibit the rib fracture mechanisms, using strain gauges glued on the ribs of Post-Mortem Human Subjects (PMHS) and developing a specific signal analysis. In 2009, they published the results of static airbag tests performed on 50th percentile male PMHS at different distances and angles (pure lateral and 30 degrees forward oblique direction). To complete these already published data, a set of 8 PMHS lateral and oblique impactor tests were performed with the same methodology. The rib cages were instrumented with more than 100 strain gauges on the ribs, cartilage and sternum. A 23.4 kg impactor was propelled at 4.3 or 6.7 m/s. The forces applied onto the PMHS at 4.3 m/s ranged from 1.6 kN to 1.9 kN and the injuries varied from 4 to 13 rib fractures. At 6.7 m/s, the forces applied onto the PMHS ranged from 2.6 kN to 4 kN and the injuries varied from 9 to 16 rib fractures. The results of 24 tests from Trosseille et al. 2008 and 2009 and from the current study were processed in the same way and analyzed together. The time and location of the fractures were determined for each test and a ribcage fracture scenario was defined for each configuration. Strain profile corridors were built for pure lateral and forward oblique impacts, in the case of a rigid impact (impactor) or for an airbag loading. They can be used to assess the human body model biofidelity and the validation of rib fracture mechanisms in these models. Based on these corridors, the effects of the severity, the impact angle and the loading system on rib strain profiles were analyzed and are presented in this paper.

The effect of angle on the chest injury outcome in side loading.

Thoracic injury criteria and injury risk curves in side impact are based on impactor or sled tests, with rigid or padded surfaces while airbags are very common on current cars. Besides, the loading is generally pure lateral while real crashes or regulations can generate oblique loadings. Oblique tests were found in the literature, but no conclusion was drawn with regard to the effect of the direction on the injury outcome. In order to address these two limitations, a series of 17 side airbag tests were performed on Post Mortem Human Subjects (PMHS) at different severities and angles. The subjects were instrumented with accelerometers on the spine and strain gauges on the ribs. They were loaded by an unfolded airbag at different distances in pure lateral or 30 degrees forward. The airbag forces ranged from 1680 N to 6300 N, the injuries being up to 9 separated fractured ribs. This paper provides the test results in terms of physical parameters and injury outcome of the 17 subjects. Geometrical and physical characteristics of the subjects are described as well as the distribution of injuries as a function of test conditions. Then a statistical analysis is presented which gives the effect of the loading angle on the injury outcome. The results of this study provide useful data for the validation of finite element models in terms of injury prediction, as well as for the development of injury risk curves for side impact dummies.

Rib cage strain pattern as a function of chest loading configuration.

Rib fractures are the most frequent types of AIS3+ chest injuries and constitute a good indication of severity. However, the behavior of the rib cage is not well documented, and though chest external measurements are often provided in the literature, the strains of the ribs themselves during a crash remain unknown. In order to address this issue, a test protocol was developed, where the ribs of 8 PMHS were equipped with up to 96 strain gauges. In a first series of 3 tests, the subjects were seated upright and their chests were loaded by a 23.4 kg impactor propelled at 4.3 m/s in 0 degrees (pure frontal), 60 degrees (oblique) and 90 degrees (pure lateral) directions. In a second series of 3 tests, the subjects were loaded by the deployment of an unfolded airbag in the same 3 directions. Finally, a third series of 2 tests was performed with airbags at different distances from the subjects, in a pure lateral direction. This paper presents the results of the tests and an analysis of the strain patterns. The differences between a pure frontal, a pure lateral and an oblique loading are explored. The airbag loading is compared to impactor loading and the severity effect is described. Finally, the time and location of the rib fractures are analyzed as a function of the test configuration.

Assessment of the pubic force as a pelvic injury criterion in side impact.

In the literature, injuries at the ischio or ilio pubic ramus level are reported to occur to approximately (3/4) of the occupants injured at the pelvis during side impact. Assuming that the load going through the pubis was a good indicator of the ramus stress, the pubic force was widely accepted as a protection criterion for pelvic fractures on side impact dummies. However, no data regarding the actual loads going through the pubis is currently available in the literature for Post Mortem Human Subjects (PMHS) in dynamic conditions. The goal of this study was to determine pelvic biofidelity specifications in terms of load path, to evaluate the pertinence of the pubic force as a criterion, and to develop a pelvic injury risk curve as a function of the pubic force. For that purpose, a pubic load cell was developed for PMHS use, and 16 side impact tests were performed on 8 PMHS using boundary conditions similar to impactor tests and sled tests reported in the literature. One kind of impact was applied on one side of a subject and the other kind of impact was then applied on the other side of the same subject, at non injury severities. The ratio between the peak external force and the peak pubic force was calculated for each subject, and a mean ratio was then calculated for each of the test conditions. These ratios were finally used to calculate the pubic forces from the external pelvic forces for 90 PMHS side impact test data available in the literature. Injury risk curves as a function of the pubic force were developed from these data. Two normalized pubic force corridors from the 16 tests are presented, the first one for the impactor tests, the second one for the sled-like tests. The test results show statistically different ratios between the peak external force and the peak pubic force, for the two configurations (an average ratio of 3.3 for impactor tests and 4.6 for sled-like tests). The PMHS injury risk curves based on the external pelvic force were observed to depend on the test conditions (impactor versus sled) while the injury risk curves constructed using the calculated pubic load were not. Therefore, the pubic force is considered as a more pertinent injury criterion for pure lateral impact.