Simplified Models of Vehicle Impact for Injury Mitigation

Brell, Edward (2005) Simplified Models of Vehicle Impact for Injury Mitigation. PhD thesis, Queensland University of Technology.

Abstract

The following hypothesis is tested by the research:

A single crash test contains information that can be used to predict vehicle response accounting for different crash conditions such as vehicle mass and initial velocity and thus can be used to predict the effect on occupant injury risk for varying occupant positions within a vehicle

It is established that the response of the crumple zone is influential in the level of injury risk. The metric for such response in common use is the NHTSA linear stiffness parameter. This parameter is used to show that stiffness increases with vehicle mass in a demographic setting. However, by comparing vehicle mass trends over 28 years of crash testing with similar trends in stiffness, a mass influence in the stiffness increase is implicated. This influence is supported by the introduction of a mass-independent stiffness metric, called reluctance, which shows a lesser increase in mass-independent stiffness over the 28 years.

The idea that stiffness should increase with vehicle mass runs counter to intuition and is tested by comparing two identical vehicles in crash tests where one of the vehicles carries an extra 555kg. The idea is further tested by simulation using a multiple mass-spring model on

vehicles, varying mass and impact velocity. Using the reluctance stiffness metric it was

concluded that increased vehicle mass decreases stiffness, confirming intuition.

Using the injury risk metric of contact velocity differential between occupant and interior of

the vehicle it is shown that increased vehicle mass reduces injury. This has important implications for the industry where a marginal performer in a compliance crash test needs

only to increase production vehicle mass to reduce injury levels to the statutory injury

reference values. A fleet study presents evidence of increasing average vehicle mass.

The study observes that blunt injury generally commences prior to vehicle rebound and continues well into rebound. Recognizing vehicle rebound to be influential in almost all blunt injury led to analysis of the fleet for improvement to this injury parameter. Using specific energy absorption as criterion, 18 modern cars were compared with 19 cars 15-17 years older at compliance test velocities. No improvement was discerned. Similarly, two baskets of cars (n=41 modern & n=32 older) tested at NCAP speeds separated by nominally 20 years failed to show improvement in rebound velocity. The implications for this study of the rebound findings was to ensure that the model presented was capable of representing injury into the rebound phase of the crash. To assist in this, a rebound formulation to reflect varying initial velocity was determined to be a linear function, studying 7 models of vehicles involved in 20 crashes at nominally 40, 48 & 56 km/h crash speeds.

Occupant position within a vehicle is identified as an important variable in injury determination. Vehicle crash tests require seating positions to be set to mid-track adjustment. This discriminates against vehicles having more "legroom" while appearing to be fair, using seating adjustment as the determinant. An empirical mathematical model is

proposed permitting crash test results to be extended to investigate the effects of varying

occupant positions thus eliminating the legroom anomaly. In addition to the varying occupant position facility, the mathematical model can easily accommodate changes in vehicle mass and varying impact velocity showing fidelity with test data.

The model is used to show that injury risk in the National Fleet has not improved over an

18-year period of crash testing.

Impact and interest:

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ID Code: 15993
Item Type: QUT Thesis (PhD)
Supervisor: Troutbeck, Rodney, Troutbeck, Rodney, Thambiratnam, David, & Thambiratnam, David
Keywords: NHTSA, NCAP, vehicles, velocity, simulation, mass, trends
Divisions: Past > QUT Faculties & Divisions > Faculty of Built Environment and Engineering
Past > Schools > School of Urban Development
Department: Faculty of Built Environment and Engineering
Institution: Queensland University of Technology
Copyright Owner: Copyright Edward Brell
Deposited On: 03 Dec 2008 03:54
Last Modified: 28 Oct 2011 19:41

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