Page 303 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

280 BIOMECHANICS OF THE HUMAN BODY

Risk of Injury. Exposures to vibration magnitudes between the threshold for perception and that
for health effects commonly occur in daily life. Near-daily exposures to values of a WAS(8) and VDV
in excess of those estimated to result in 5 to 10% injury in Table 11.2 occur in numerous occupations
(involving some 8 million persons in the United States) and lead to the symptoms described in
Sec. 11.1.2.
The risk of injury from multiple shocks and impacts is estimated by employing (Morrison et al.,
1997): (1) biodynamic models, to predict the motion at the spine from that at the seat; (2) a dose-
recovery model for repeated shocks, to accumulate the exposure experienced into a dose; and (3) an
injury risk model based on the cumulative probability of fatigue failure. A neural network model is
employed for motion in the Z-direction, and simple, one-dimensional biodynamic models for the
X and Y directions (see Sec. 11.3.1). The acceleration dose is constructed, separately, from the
output of the biodynamic models using Eq. (11.3), with F(a (t)) representing the peak acceler-
W
ation of the shock or impact and m = r = 6, and includes the potential for recovery. The combined
acceleration dose is converted into an equivalent static compressive stress, which is then inter-
preted for potential fatigue failure of the vertebral end plates, taking into account the reducing
strength of vertebrae with age (ISO 2631-5, 2004). The restricted range of applicability of the
2
neural network model (to peak accelerations of from 10 to 40 m/s ) is of little practical conse-
quence, as smaller accelerations may be evaluated by the VDV and larger accelerations are unlikely
to be tolerated.
Survivable Shocks and Impacts. Estimates for survivable exposures to single impacts and shocks
are given for both headward acceleration and spineward deceleration in Table 11.2. For headward
acceleration, experience with nonfatal ejections from military aircraft suggests that a DRI of 18 is
associated with a 5 to 10 percent probability of spinal injury among healthy, young males who are
restrained in their seats. For spineward deceleration, the estimate is based on the head injury criterion
(HIC), which was developed from the severity index (see Sec. 11.1.1). The metric is defined as:

⎧ ⎡ 1 ⎤ . 25 ⎫
⎪
⎪
t
()dt
HIC = ⎨ (t 2 − ) ⎢ ∫ t 2 at ⎥ ⎬ (11.13)
1
t
⎩ ⎪ ⎣ (t 2 − ) t 1 ⎦ ⎭ ⎪ max
⎪
1
where t and t are the times between which the HIC attains its maximum value and a(t) is measured
1 2
at the location of the center of gravity of the head. The HIC is applied to instrumented crash test
dummies; its ability to rank order, by magnitude, the severity of injuries in humans has been ques-
tioned. The time interval (t − t ) is commonly chosen to be 36 ms, a value prescribed by the
2 1
NHTSA, though a shorter interval (e.g., 15 ms) has been suggested as more appropriate (SAE J885,
1986). The value listed in Table 11.2 is the maximum allowable by the NHTSA in frontal collisions.
The assessment is related to the Wayne State concussion curve (see Sec. 11.3.3) and is applicable to
head injury in vehicle and aircraft crash. Survivable injuries to the neck and chest, and fracture
limits for the pelvis, patella, and femur in frontal and side impacts have also been proposed for manikins
(Mertz, 2002b).
11.4.2 Protection against Whole-Body Vibration
Vibration Isolation. Excessive whole-body vibration is most commonly encountered in trans-
portation systems, where it predominantly affects seated persons. In consequence, an effective
remedial measure is to reduce the vertical component of vibration transmitted through seats (and,
where applicable, vehicle suspension systems), by means of low-pass mechanical filters. The trans-
missibility of a vibration-isolated seat or vehicle suspension (neglecting tire flexure) can be modeled
in the vertical direction by the mechanical system shown in Fig. 11.6. For a vibration-isolated seat,
the spring and damper represent mechanical elements supporting the seat pan and person, which are
represented by a single mass m. For a vehicle suspension, the mechanical elements support the
passenger compartment. With this model, it is evident that the vibration transmitted to the body will