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TTId Calculations and Reference Information

TTId Calculations and Reference Information

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TTId Calculations and Reference Information

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TTI (Thoracic Trauma Index) is the chest injury criterion used in FMVSS 214.

SID must sustain a TTId of 85g or less in the specified moving barrier impact and 90g in 2-door vehicles.

The TTI kernel equation used in the Vehicle Safety Tools is: TTId = 0.5(RIBy + SPINEy)

TTI in Cadavers

TTI has been extensively studied in cadaver tests.  127 whole-body tests have been performed:

•84 tests sponsored by NHTSA (Eppinger et al (1984) and Morgan et al (1986))
•17 tests run at Wayne State University under a CDC funding (Cavanaugh et al (1993))
•26 tests run at the Medical College of Wisconsin and VRTC (Pintar et al (1997))

Equation for TTI in cadavers: TTI = 1.4 x AGE + 0.5(RIBy + SPINEy)(MASS/MASSstd)

Note

This equation is not used in the Vehicle Safety Tools.

TTI includes an age factor.  The higher the age, the greater the risk of injury.

 

Probability of Injury Using TTI

TTI of 145-150 results in 25% probability of AIS 4 or greater based on these studies.

 

SID TTId vs. Cadaver TTI

For a 45-year-old, 165 pound subject, if the TTI kernel is 85g, the TTI is 148.

For a 65-year-old, 165 pound subject, if TTI kernel is 85g, the TTI is 176.

The 45-year-old cadaver subject has about a 25% chance of AIS 4 injury, and the 65-year-old subject has a greater than 75% chance of an AIS 4 injury.

 

Average Spine Acceleration (ASA)

Cavanaugh et al (SAE #933127) formulated ASA.  In a review of 61 cadaver tests, it worked as well as TTI in predicting AIS 4+ injury.  It is obtained by filtering and integrating the T12y acceleration to obtain lateral spine velocity.  ASA15 is the slope between 15% and 85% of peak spine velocity.

In cadavers, ASA is normalized by multiplying AGE/45 x Mass/75kg.

In the Cavanaugh study, an ASA of 30g gave a 25% probability of AIS 4+.

Both ASA and TTI are test-type specific, and were developed in whole-body side impacts.

 

ASA and TTI in SID (Cavanaugh et al, SAE #942225)

In the SID dummy, ASA has advantages over TTI in evaluating injury-mitigating properties of padding.

In 20-mph impacts, SID TTI is well below the 85g-tolerance limit for both soft and stiff padding.  In contrast, ASA greatly exceeds the 30g-tolerance limit with stiff padding.