6.6.1 By using an instrumented manikin, assumptions
regarding the behavior of the seat/occupant coupling can be avoided.
There still remains a potential problem when it comes to interpreting
the data collected from the dummy, however. There are no clearly defined
criteria for injury risk as a function of parameters measured in the
dummy. This is due to a number of factors. Firstly, there is no agreed
level of injury that should serve as a threshold for tolerance. Injury
is a spectrum extending from the trivial to the fatal. Secondly, there
are wide differences in the tolerance of individuals to the same insult.
The description of a person as having "weak bones" is not altogether
a case of uninformed lay labelling. Ignoring the well recognized but
rather rare cases of bone disease causing brittleness, there is a
wide variation in the structural strength of normal adult healthy
bone. This variation is in the region of three to one. Following is
a discussion of generally accepted criteria for evaluating data collected
from a Hybrid III human surrogate.
6.6.2
Head Injury Criteria. The bony
skeleton of the head may be fractured by blunt or sharp trauma. Generally
speaking, however, it is not so much the existence of 'a fracture
which is important, rather the impairment of consciousness which accompanies
the fracture in the immediate aftermath of the injury. This impairment
of consciousness can hinder, or prevent, the next and may be vital
actions necessary to survive.
6.6.3 Impairment of consciousness can be caused
with or without a fracture to the skull. The mechanisms involved are
not well understood but there is a relationship between the amount
of energy imparted to the brain during an impact, the rate of transfer
of the energy and the result in terms of impairment of consciousness.
This concept was incorporated by Gadd (SAE,1986) into a weighted impulse
criterion for establishing a severity index (SI) which is:
In Equation 6.6, a(t) is the resultant acceleration time-history
(in G's) at the center of gravity of the head and T is the duration
of the acceleration impulse (in seconds). Gadd proposed a tolerance
value of 1000 as the threshold of concussion from frontal impact.
6.6.4 The National Highway Transport Safety Administration
(NHT5A) defined a new criterion based on the SI. This criterion is
called the Head Injury Criterion and is defined by NHTSA to be:
Where t1 and t2 are the initial and final times (in
seconds) of the interval during which the HIC attains a maximum value.
HIC replaced 81 in later versions of the Federal Motor Vehicle Safety
Standard (FVMSS 208) with a value of 1000 specified as the concussion
tolerance level (SAE 1986).
6.6.5 The HIC index can be easily calculated from
a dummy head impact using the data from a triaxial accelerometer located
at the center of gravity of the head. HIC values of 1000 are associated
with a concussion hazard and therefore represent a threshold value
which should be avoided. It is interesting to note, however, that
in a study by Hogson and Thomas (SAE 1986) it was concluded that the
HIe interval (t2 - t1) must be less than 15
ms in duration in order to pose a concussion hazard even if the HIC
exceeds 1000.
6.6. 6
The Ambulatory System. The
importance of the ambulatory system cannot be underestimated in any
survival situation. Here, the forces in the lower limbs of the Hybrid
III dummy can be measured during an impact. The critical bending moment
for the adult femur is 248 N-m. Studies on ejection seats, where the
magnitude of the acceleration vector in the Z direction was 10 G have
produced bending moments of 170 N-m. This is fairly safe margin for
healthy adults in a free-fall lifeboat, however, one must consider
the injured survivor. These forces in a already fractures lower limb
could have fatal consequences as it is well recognised that severe
shock can ensue from an inadequately treated femoral fracture.
6.6. 7
The Neck. The neck is a complex
structure and has a wide range of injury mechanisms. The range of
movement in the neck is considerable. It ranges from simple flexion
(forward bending) and extension (rearward bending) to complex combinations
of flexion, lateral flexion and rotation. The strength of the human
neck is not the same in all directions but the atlanto occipital junction
(between the top of the neck and the base of the skull) can tolerate
88 N-m in flexion without causing injury (Mertz, 1971).
6.6.8 During a recent trial involving a free-fall
lifeboat launched from 30 meters, bending moments of 11 N-m were recorded
in the neck of a Hybrid III dummy. These records were made on an inadequately
restrained dummy in order to assess the risk of neck injury in the
event of a precipitate departure. Because of the free-fall height
and the lack of restraint, this case represents most of the worst
conditions that can be envisaged. It is noteworthy that the bending-moments
recorded in the dummy neck during normal launches with this same lifeboat
were approximately 3.5 N-m. This experimental evidence illustrates
the importance of having a suitable harness. It also allows the operator/regulator
to evaluate the risks involved in not having the harness properly
adjusted.