Less-sudden impact

March 7, 2002
Smart fibers "stretch" the definition of safety belts.

Securus safety belts undergoing frontal impact tests.

For average-sized individuals, Securus safety belts behave much like conventional models with constant force retractors (CFRs). However, Securus belts yield at a lower force than where CFRs engage which lowers g force for smaller adults and children. Likewise, Securus belts better restrain larger adults with a proportionately higher force.

Automobile occupants who wear safety belts are about half as likely to be killed or sustain serious injury in a collision than those who don't, says the National Center for Statistics and Analysis. Though safety belts work well, automotive engineers are developing energy-management systems such as pretensioners that lower crash forces on belted occupants.

Another, perhaps simpler, approach comes from Honeywell Performance Fibers, Morristown, N.J.: energy-absorbing safety belts. Conventional safety belts made from polyethylene terephthalate (PET) fibers tend to be relatively unyielding and, as such, transfer considerable force to an occupant's shoulders, chest, and pelvis. For example, such belts can exert about 2,000 lb at the torso in a 35-mph frontal crash.

In contrast, Honeywell's Securus block copolymer polyethylene terephthalate – polycaprolactone (PET - PCL), builds belts with a three-step stress response. At stress levels to about 1 gram/denier, a high initial modulus lets the seat-belt retractor "lockup" to hold passengers in place. Here, Securus belts behave much like conventional seat belts.

But as stress rises from 1.0 to 1.5 gm/denier, modulus drops precipitously. The elasticity lets an occupant move forward in a controlled way to help minimize overall force. Honeywell says the fiber absorbs three times the energy of standard polyester in this regime and lowers force 30% on average over the entire energy range.

Finally, exceeding 1.5 grams/denier boosts modulus to restrain passengers until crash energy dissipates. Using the same hypothetical 35-mph collision, passengers wearing belts made of the Securus fibers would experience about 1,200-lb peak force at the torso, or 40% less.

Some experimental prototypes combine relatively stiff as well as elastic fibers in the same belt or include a webbing section that elongates on impact. These approaches have yet to win significant marketplace acceptance, says Honeywell.

Probably the most widely used force limiters are so-called constant force retractors (CFRs). Here, impact loads exceeding a preset level (typically 1,000 lb) engage a ratchettype device. This allows some belt movement or payout at the attach point and reduces loads on passengers.

Securus seat belts essentially mimic CFR's load-leveling action, though the belts may be more effective for a wider range of passengers. CFRs are not able to discriminate between occupant size and weight and apply a constant force most appropriate for a 50th percentile (175-lb) person. Contrast this with

Securus safety belts. Initial sled tests with fifth percentile test dummies (represents children and small adults) show the Securus seat belts begin to yield or cushion at a lower force than CFRs. Consequently, this lowers gs to the head and chest which helps reduce the probability of injury.

Probably one of the biggest advantages of smart safety belts, however, is that they require no CFRs or mods to existing hardware. This saves both weight and space, important metrics for automotive engineers attempting to maximize interior space in smaller, more efficient cars.

How Securus seat belts are made
Securus fibers are formed in a special reactive twin-screw extrusion process that combines polyethylene terephthalate (PET) and epsilon-polycaprolactone (PCL) in the presence of a stannous (tin) octoate catalyst. The catalyst encourages the reaction but isn't consumed in the process. In effect, the PET chain is extended with epsilon-caprolactone monomer to create the polycaprolactone block. Additional processing gives the fibers the desired physical properties.

Filaments cool rapidly as they exit the extruder and are taken up by various mechanical devices and formed to a specified denier/decitex (linear density). They are then wound into yarn and the yarn is woven into belts.

Tenacity — A fiber's resistance to breakage for a given denier/decitex or linear density. Modulus — Short for modulus of elasticity, it is a measure of fiber stretchiness. Materials elongate at a certain rate based on applied force.

Elongation at break — How far a fiber stretches before it breaks as a percentage of the original length.

Thanks to Monique Levy, Chris Phillips, and Monte Nagy of Honeywell Performance Fibers for providing information for this article.

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