Biomedical Engineering Reference
In-Depth Information
tests of shock isolation systems and, in particular, for sled tests of safety
devices for crashing vehicles. Typically, the test sled equipment consists of
a high-speed sled with a mock-up (buck) of the vehicle interior, an occu-
pant seat, a restraint system (e.g., seat belt), and a dummy as the occupant.
The buck with seat belts is rigidly attached to the sled. The sled, dummy,
and seat belts are equipped with sensors that measure accelerations, dis-
placements, strains, forces, and other characteristics of the response of the
system. The sled is accelerated to a prescribed velocity and then is sub-
jected to a deceleration pulse simulating the impact. The various measured
responses can be used to form injury criteria that can be compared with
established tolerance levels. The impact deceleration pulses occurring in
different runs of the test differ from one another because the conditions of
the experiment cannot be reproduced with absolute accuracy. If the sensitiv-
ity of the responses to the variations in the time history of the deceleration
pulse is high, the results of the tests can show a large spread, which does
not enable a reliable assessment to be made of how the system being tested
corresponds to the standard.
Normally, test standards specify a “corridor” or an “envelope” within
which the deceleration pulses must lie. A typical corridor is shown in
Fig. 3.12. The time history of the deceleration pulse must lie in the shaded
area bounded by the curves
v
−
(t)
and
v
+
(t)
. The numerical parameters of
the corridor illustrated are typical of a U.S. standard for sled tests of an
automobile child seat with the restraint systems. For the test results to be
reliable, the ratio of the maximum and minimum values of the response
measures over the corridor of allowed disturbances (worst-to-best ratio)
should be reasonably close to unity. To calculate this ratio, it is necessary
to solve the best and worst disturbance problems. If the upper bound for the
worst-to-best ratio is prescribed, one can verify whether a given standard
provides reliable results of testing. If this ratio is unacceptably high, the
30
v
+
(
t
)
20
10
v
(
t
)
0
0
25
50
75
100
125
Time (ms)
FIGURE 3.12
Typical corridor for deceleration pulses allowed in impact tests.
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