In a crash between two vehicles, injuries may occur to the occupants of one or of both vehicles if the speeds are sufficiently high. In addition, the likelihood and the severities of injuries depend on multiple factors, related to the crash parameters (impact speed, direction, location of impact, etc.), the types of vehicles involved, deformations of door and side structure and restraint system usage, etc. Thus, the task of analyzing a vehicle-tovehicle crash for assessing the performance of various systems in the vehicles, is highly complex and requires that all relevant parameters be taken into consideration. This article discusses the dynamics of vehicle-to-vehicle crashes and automobile designs for safety in such crashes.
Scope of the Problem
The fatalities associated with the directions of impact in vehicles can be evaluated from the FARS database and are shown here for one year. The percentages shown are the distributions of fatalities for occupants of passenger cars (excluding light-truck-vehicles such as SUVs, pickup trucks and vans). The numbers in these charts include all crashes for passenger cars (and not just the vehicle-to-vehicle impacts) and side impacts are seen to account for more than 24% of all fatalities in passenger cars during one year (2009 data shown).
U.S. Laws Governing Occupants' Safety in Vehicle-to-Vehicle Crashes:
There are no U.S. regulations that specifically mandate vehicle-to-vehicle crash tests. However, there are tests for side impact safety using moving deformable barriers (MDB) against a stationary automobile. These tests are conducted separately by both the National Highway Traffic Safety Association as well as the Insurance Institute for Highway Safety. Each agency uses different MDB and test protocols and assesses the safety for the occupants in the test vehicle. Each uses different parameters as measures of safety and they both publish their 'ratings' for each vehicle. In addition, tests are also conducted to simulate the vehicle sliding laterally into a pole.
|1. Side Impact NCAP Test (NHTSA) includes a test using an MDB of approximately 1367 kg moving at 62.2 km/h. The MDB is crabbed (i.e. the wheels on its base are turned) to simulate the case where the struck vehicle is moving in a forward direction. The driver is represented by a fiftieth percentile male ATD and the rear passenger by a fifth percentile female ATD. Estimates of 'relative risk' of injury are published as ratings from one star to five stars for the vehicle's safety in side impacts.
|2. IIHS Side Impact Test: This test consists of a 1500 kg MDB impacting a stationary test vehicle perpendicularly at 50 km/h. Ratings are based both on the vehicle deformations and on the measured responses of the front driver and the rear passenger ATDs. These data for the test vehicle are then compared with IIHS-defined 'corridors' for the four categories of ratings (Good-Acceptable-Marginal-Poor) and based on this, IIHS publishes the ratings for the overall vehicle as well as separately for the structure and for the protection of the occupant's body segments. In addition, a 'Head Protection' rating is published based on the observed performance of curtain airbags in these tests. The MDB in the IIHS test is heavier than NCAP MDB and may be thought of as simulating an impact by a light-truck vehicle (LTV) into the test vehicle.
Vehicle Design Factors in Side Impacts:
In the example shown here, the incoming vehicle's impact is on the side structure components (e.g. the rocker, the A- and B-pillars, door etc.) of the struck car. Relatively small differences in the impact configuration (such as higher bumper, higher front structure on the striking car, etc.) can lead to an entirely different sequence of events.
|Role of Vehicle's Side Structure: The space available between the door and the vehicle occupant is quite small, signifying very little crush space for dissipating the crash-generated kinetic energy. Thus, the principal function of the struck vehicle's side structure is to maintain the door's structural integrity with the side pillars and thereby reduce the impact velocity of the door against the occupants. The figure here illustrates the dynamics of the vehicles when the impact is lateral and the striking vehicle is moving with velocity V at the moment of impact. Post impact, the striking vehicle will decelerate and the struck vehicle (initially stationary for illustration) will accelerate in the direction of impact. Also, the door of the struck vehicle deforms during the impact and, with respect to the CG of struck vehicle, it will appear to intrude inwards.
An example of the structural deformation of a struck vehicle is shown from a side crash test conducted by NHTSA.
The deforming interior surface of the front door impacts the driver's pelvis and shoulder (since no side impact airbags were present) as the available clearance is taken up by the deformation. Further intrusion of the door then causes outward rotation of the driver's head and may lead to its being impacted by the oncoming vehicle.
Airbags for Side Impact Protection: Presently, the airbags in use for occupant protection in side impacts fall into two categories:
1. Pelvis & Thorax airbags are mounted in the seat or in the door and when deployed, provide protection for the occupants' pelvis and thorax regions. Their principal function is to distribute the impact load over a larger area of the occupants' torso, eliminating any 'hard contacts' with parts of the door. They also function to reduce the rate of the occupant's acceleration (and thus the impact severity) by deflating at a controlled rate after loading by the occupant.
2. Curtain airbags are usually mounted in the roof rail area of the automobile. When deployed, they cover the window openings and reduce lateral excursion of the occupants' heads through the nearest opening (the tempered glass in the side windows usually breaks away during the initial part of the impact).
Sensors for Side Impact Airbag Deployment: Both of the above types of airbags need to be deployed between the occupant's body and the impacting objects (such as the door). Therefore, sensors for detecting these crashes (and for deploying airbags if needed) must be engineered such that the airbags are in place prior to the instant of occupant's impact.
|Several essential requirements determine the location and the properties of the sensors for deployment of side airbags (pelvis/thorax and curtain). The sensors should be able to detect crashes reliably and within the available time span. They also must provide sufficient discrimination between deployment conditions and non-deployment crashes. In many vehicles, such sensors are located in a location close to likely impact (such as in the door, or in the B-pillar) as well as in the vehicle interior (near the driver's seat). In the example cited here, the side impact sensors are located in two places - in the B-pillar bottom and under the driver seat - and initiated the deployment of airbags at around 5 milliseconds after t=0.
The above material describes some aspects of vehicle-to-vehicle side impacts and of the factors governing the safety of the vehicles' occupants. The analysis of such crashes requires that the properties of each vehicle as well as the dynamic interactions between the structures of the two vehicles be comprehended in addition to the analysis of each of the vehicles and its crashworthiness. A longer version of this article may be obtained by contacting the author.
Dr. Mukul Verma, is a well-known expert in Automobile Safety and Crashworthiness, Vehicle Structures, Product Design, and Statistical Analyses of Traffic Trends and Regulations . He has worked in many engineering and management positions at a major automobile manufacturer including assignments in R&D, vehicle design, analysis and testing and engineering program management.
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