Documentation:FIB book/problems/Inflatable Seatbelt

Inflatable seatbelt system is used by many car manufacturing companies. This system was introduced in 1970's. You can see one example of this system in figure 1. The device consists of an inflatable shoulder portion and a normal lap belt. The shoulder portion is a tubular belt with an inflatable bag inside it. As soon as the inflation process starts, the bag tears the belt open (figure 2 is a GIF demonstrating how this system works. Click on it and watch it. It is recommended to watch this GIF during the questions, too).

Figure 1. Inflatable Seatbelt System

Figure 2. How the inflatable seatbelt works

Question 1

Considering the six maxims generally applied to restraint design, inflatable seatbelts try to improve which two maxims over the conventional ones? Explain your reasoning.

SOLUTION (expand to show)

Maxim number 4 - distribute forces over greatest possible areas: The width of the inflatable belt will increase after the inflation. This means that the contact surface between the belt and the torso increases. Maxim number 3 - Minimize the rotation or translation at occupant joints, deformation of anatomic structures and rate of deformation of structures: The interaction of the chin with the inflated shoulder belt can reduce the rotation of the head.

Question 2

What injuries are less likely to occur when inflatable seatbelts are used rather than conventional 3-point seatbelts?Why?

SOLUTION (expand to show)
Some injuries that are less likely to occur include thoracic injuries (especially the number of rib fractures), seatbelt signs and abrasions under the path of the shoulder portion, sternum fracture, clavicle fractures, and cervical spine injuries associated with seatbelts.

Question 3

In one study, researchers compared "3-point seatbelts with a load limiter and a buckle potentiometer" with "inflatable seatbelts". Cadavers were restrained with either inflatable seatbelts or 3-point seatbelts. Then, sled tests were performed to measure biomechanical parameters and identify injuries caused by the restraint system.

A part of the study focused on chest deflection. Figure 3 shows chest deflection in three of the cadavers. Match each cadaver with its type of restrain, inflatable or 3-point?

Note: Chest compression results in a positive deflection.

Figure 3. Chest deflection curves for three of the experimented cadavers.

SOLUTION (expand to show)

When using inflatable seatbelts, we expect to see a reduction in compressive chest deflection (positive deflection). Therefore, the blue curve is for the cadaver restrained by the 3-point seatbelt (maximum deflection: 37.8895 mm); the red and yellow curves are for cadavers restrained by inflatable seatbelts (maximum chest deflection: 11.8 (Red) and 4.4 (Yellow)). It is important that the student realizes the compressive deflection is of interest (positive values).

Question 4

These inflatable airbags are usually aimed to lower the injury risk in children. Generally, what are your concerns when using these belts? This is an open question. There are many answers possible. Three concerns are enough to mention.

SOLUTION (expand to show)

The concerns are: Does the airbag get filled with hot gas? How much time does it take for the airbag to be deflated? Or for how many milliseconds it stays filled? If the belt is not used, what happens to the passenger when it inflates? Some child seats are locked by belts. Can we use inflatable belts with those seats? As they should incorporate the airbag, these belts may be wider and thicker than normal ones. If the belt does not activate, do the injuries increase or new ones are introduced?