Sikander Anwer
Department in Textile Engineering
University of Management & Technology, Lahore, Pakistan
Email: 111811016@umt.edu.pk
History of Airbags:
In the event of a frontal collision, airbags protect the upper part of the body, neck, and head of an occupant. The risk of severe head and chest injuries is significantly lower when a car is equipped with an airbag. The airbag specified for automobile use traces its origins to air-filled bladders as early as 1941. John W. Hetrick, an industrial engineer and member of the United States Navy, designed the original safety cushion commonly referred to as an airbag. This invention included the idea of an airbag to protect passengers in an automobile in the event of an accident. It was designed based on his experiences with compressed air from torpedoes during his service in the navy, as well as a need to provide protection for his family in their automobile during accidents. Hetrick worked with the major American automobile corporations at the time, but they chose not to invest in it. Between 1972 and 1975 cars with airbags were first produced in the United States. In 1990, an American law was passed that required vehicles to either have an airbag or a safety belt automatically closing at the driver’s side. Today airbags are standard equipment in every modern vehicle.
Fig: Airbags in car
In 1967, a breakthrough occurred in the development of airbag crash sensors when Allen K. Breed invented a mechanically-based ball-in-tube component for crash detection, an electromechanical sensor with a steel ball attached to a tube by a magnet that would inflate an airbag under a 30 milli-second window. Sodium azide instead of compressed air was also used for the first time during inflation. Breed Corporation then marketed this innovation first to Chrysler.
Definition of Airbag: An airbag is an elastic bag or cushion like makeup which inflates and deflates quickly at some stage in certain types of car accidents. It is a safety device aimed at preventing or minimizing injury to passengers when such an accident occurs. An airbag system is composed of four parts: a gas generator, a case, the airbag, and a cover. Additional elements are necessary to attach the airbag in the car.
The airbag is an inflatable rubber envelope designed to protect automobile occupants from serious injury in the case of collision. The device is fitted inside a vehicle on the steering wheel, dashboards and even on door panels. The airbag blows into a soft pillow during a car crash, minimizing the impact to the driver / passenger to prevent injury.
Types of Airbags:
Frontal Airbags
Shaped airbag
Side Airbag
Side Torso Airbag
Curtain airbag
Knee Airbag
Rear Curtain Airbag
Center Airbag
Working Principle of Airbags in Car:
Airbags in car are designed as a crucial safety feature to protect passengers during a collision. The airbag system basically consists of sensors, electrical control, supervision, ignition release, and the airbag module. The airbag module itself consists of a gas generator with electrical igniter, airbag, and cover. The gas generator contains sodium acid as a solid matter propellant that is ignited during the impact. The airbag is released and inflated in case of a severe frontal collision. A piezoelectric transmitter triggers the release.
Fig: Working principle of airbags in car
The design is conceptually simple; a central “Airbag control unit” (ACU) (a specific type of ECU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors, wheel speed sensors, gyro scopes, brake pressure sensors, and seat occupancy sensors. When the requisite ‘threshold’ has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag’s volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant’s body, compared to a seat belt alone. The entire process is precisely timed and designed to work within milliseconds.
Sensors Signals Working:
The signals from the various sensors are fed into the Airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and “curtain” airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.
Reaction Sequence:
Inside the airbag is a gas generator containing a mixture of NaNO3, KNO3, and SiO2 . The signal from the deceleration sensor ignites the gas generator mixture by an electrical impulse when head-on collision, creating the high temperature conditions necessary for sodium asides to decompose at 300˚C . This causes a relatively slow kind of detonation (Deflagration) that liberates a pre-calculated volume of N2 gas through series of chemical reaction, which fills the air bag.
(1) 2NaN3 → 2Na + 3N2 (g)
The first reaction is the decomposition of NaN3 under high temperature conditions using an electric impulse. This impulse generates to 300°C temperatures required for the decomposition of the NaN3 which produces Na metal and N2 gas. Since Na metal is highly reactive, the KNO3 and SiO2 react and remove it, in turn producing more N2 gas.
(2) 10Na + 2KNO3 → K2O + 5Na2O + N2 (g)
The second reaction shows just that. The reason that KNO3 is used rather than something like NaNO3 is because it is less hygroscopic. It is very important that the materials used in this reaction are not hygroscopic because absorbed moisture can de-sensitize the system and cause the reaction to fail.
The final reaction is used to eliminate the K2O and Na2O produced in the previous reactions because the first-period metal oxides are highly reactive. These products react with SiO2 to produce a silicate glass which is a harmless and stable compound
Airbags Production Process:
The airbags are manufactured by a complete process that has been mentioned in a flow chart. It is a typical manufacturing line of airbag.
Fig: Production flow chart of airbags
Properties of Airbags: The airbag is a safety product; therefore, the physical properties are essential. The airbag has to be temperature resistant. Cleaning fastness is unimportant for an airbag. Other properties are:
High Tensile strength
Good heat stability
High Tear strength
Low Air permeability
Free of knots, splices, spots and broken ends.
Good Heat capacity
Good Folding behavior
Better Energy absorption
Good Coating adhesion
Functionality at extreme hot and cold conditions
Package ability
Reduced skin abrasion (softness)
Fabric Construction for Airbags:
Mostly used raw material for the airbag fabric is nylon 66 yarns in the deniers ranging from 420 to 840. The side impact airbags used 1880 D nylon-6.6. These fabrics are generally woven with the constriction of:
840 X 840 D, 98 X 98 /dm plain weave, 60” width.
420 X 420 D, 193 X 193 /dm plain weave, 60” width
Fabrics for Airbags:
There are generally two different concepts for the production of airbags: coated fabrics and uncoated fabrics. With coated fabrics, the damping properties of the bag are controlled by the dimensions of the openings for the air release. This principle is also used, albeit infrequently, with uncoated materials of low air permeability. The essential parameter is the air permeability of the bag, which also needs to sustain aging. Coating materials are polychloroprene and silicone.
For the manufacture of bags from uncoated fabrics of defined air permeability, wovens for filters are used that fulfill special tolerances with respect to air permeability and aging stability. Because of its economic efficiency, uncoated fabrics are becoming more and more popular compared to coated materials.
Woven fabrics for airbags are produced mostly in plain weave with projectile loom and rapier loom. The most important parameters for weaving are thread density and air permeability of the fabric.
Finishing for Airbag Fabrics: In textile finishing, gray goods for airbags are usually washed and dried afterwards. In the washing process all spinning and weaving preparations have to be removed carefully. A spraying of these substances during the explosive inflation of the airbag can be avoided because under high temperatures this could lead to a deflagration. At present, airbag fabrics are often equipped with a silicone finish. To achieve a defined air permeability of the airbag fabric, a shrinking process can be triggered during the washing process by adjusting the temperature. During the drying process of the fabric a simultaneous fixing of the fabric takes place to confer the necessary dimensional stability. Airbag fabrics are neither finished chemically nor treated with a calender.
Required Tests for Airbag: The importance of airbags in automotive industry specially in car is increasing as they play crucial role in saving lives in the incident of a collision. Different tests such as static deployment and full vehicle chamber deployment tests can be employed to evaluate the performance of the airbags. After analysis of the airbags and the computer simulation tests, the performance of a complete safety restraint system can be tested in the airbag laboratories.
The testing of airbags for functional safety in car is performed by special testing programs. Complete adult or child manikins are used for the airbag testing in several laboratories.
The testing consists of
A fall test,
A mechanical shock test at –35°C, +20°C, and +85°C,
Vibration stress with the influence of temperature,
A test with change of climate,
A temperature shock test,
A dust test,
A salt fog test,
A sun simulation and UV influence,
Airbag testing (tensile stress of the fabric and the seams, venting property of the fabric, volumetric content of the airbag),
A can plate test at –35°C and +85°C (testing of gas generator in a closed pressure tank), and
An inflation test at –35°C, +20°C, and +85°C.
Conclusion:
Airbags are essential safety devices in car that use sensors and a rapid inflation mechanism to protect passengers in accident. Generally airbags reduce the force of impact on the body, particularly the head, neck, and chest, which significantly lowers the risk of serious injury. This article provides an in-depth analysis of airbags in car, evolution of airbags, properties, types, working principles, required fabrics and testing.
References:
Textile Technology: An Introduction, Second Edition by Thomas Gries, Dieter Veit, and Burkhard Wulfhorst
Textiles in Automotive Engineering by Mike Hardcastle and Walter Fung
Manikins for Textile Evaluation Edited by Rajkishore Nayak and Rajiv Padhye
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
