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Reconsideration of injury criteria for pedestrian subsystem legform test~Problems of rigid legform impactor
Published June 04, 2001 by National Highway Traffic Safety Administration in United States
The legform impactor proposed by EEVC/WG17 is composed of a rigid thigh segment and a rigid lower leg segment. Human bone, however, has flexibility, causing some differences between the EEVC rigid legform impactor and the human leg. This research analyzes the influence of the differences (rigid versus flexible) on the injury criteria. It also reanalyzes the upper tibia acceleration with regard to the fracture index.
The rigid legform impactor cannot simulate bone bending motion, so the injury criteria should consider the legform rigidity. It means the injury criteria need to include the bone bending effect. From several PMHS test results, the shearing displacement becomes 23 mm and 20 degrees for bending angle including the bone bending effect. However, the bone bending effect will change with the loading conditions. Therefore, to establish a certain injury criteria for a rigid legform impactor is impossible. To solve this problem, a flexible legform impactor seems to be needed. If a flexible legform impactor exists, the relationship between rigid bone and flexible bone need not be considered, and the pure ligament tolerance can be applied.
The threshold of upper tibia acceleration as for lower leg fracture (fibula/tibia/tibia+fibula fracture) was found to be 198G for 50% injury risk, but if we focus on the tibia fracture (tibia/tibia+fibula fracture), exclude the fibula- only fracture cases, the threshold becomes 247G.
By the way, to use only the upper tibia acceleration as the leg fracture index is a problem. The relationship between upper tibia acceleration and fracture was obtained from a PMHS test series which were conducted normal bumper height, but if the impact point is changed, the upper tibia acceleration becomes lower but the leg fracture occurs. Therefore, if the acceleration use for the leg fracture index, the accelerometer should be attached proper position. More the worse, the tibia acceleration from the rigid impactor is completely different from human leg because of its rigidity. Therefore, the acceleration from a rigid legform impactor should not be used for the estimation. If the legform impactor is flexible, the acceleration becomes proper, and can be made proper estimation.
Finally, a flexible legform impactor seems to be needed for proper estimation of leg injury.