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auto safetyIn crashes between two automobiles, it is generally assumed that the lighter vehicle will suffer more damage and that its occupants will suffer higher levels of injury. However, the outcome of any such crash is dependent on multiple variables - structural dynamics of the vehicles, parameters of the crash and the available safety systems (such as seatbelts, airbags, interior components, etc). The types of the automobiles involved are also critical and the crash of a passenger car with an SUV or a pickup truck (this class of vehicles is referred to as 'light truck vehicles' or LTVs) may have other complicating factors due to the generally higher primary structures of LTVs.

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Magnitude of the problem: For the year 2009, NHTSA data ("Traffic Safety Facts 2009") show that of the total 33,808 trafficrelated fatalities and 2,217,000 injuries, automobile occupants accounted for 24,474 fatalities and 2,011,000 injuries. Further analysis shows that more than 77 percent of these injuries to automobile occupants may be attributed to their vehicle's collision with another motor vehicle in transport.

When the fatality data for occupants of passenger cars only (excluding LTVs) are analyzed for the modes of crash, side impacts are seen to account for more than 24% of all fatalities in passenger cars (counting all crashes for passenger cars and not just the vehicle-to-vehicle impacts).

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Role of Vehicle Structure in Side Impacts: An automobile's side structure can dissipate only a small fraction of the crash energy because there is relatively small deformation space available between the exterior surface of vehicle and the seated occupant. The primary function of the side structure is to provide structural integrity and to minimize the intrusion of the doors and their impact severity with 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: Currently, airbags in use for side impact protection of occupants fall into two categories:

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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). Since they function in tension to reduce or eliminate the head impact with external surfaces, they need to be tethered (i.e. anchored to the car's pillars) appropriately.

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.

Requirements and Regulations for Occupants' Safety in Vehicle-to-Vehicle Crashes: There are no regulations that specifically govern occupant safety in vehicle-to-vehicle crashes (generally referred to as 'vehicle-to-vehicle crash compatibility'). However, there are tests for side impact safety that use moving deformable barriers (MDB).

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1. Side Impact NCAP Test (NHTSA) includes one test which is an 'intersection' type impact to the stationary vehicle by an MDB of approximately 1367 kg moving at 62.2 km/h. The barrier is crabbed (i.e. the wheels on the barrier 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 for each seating position are calculated from the measured data on the ATDs using NHTSA developed formulae. These are then converted into ratings from one star to five stars for the vehicle's safety in side impacts.

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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.

SUMMARY 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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