Development
of
shock
test
specifications
117
The
adjustable parameters
are the
amplitude
and
possibly
the
number
of
cycles.
The
number
of
frequencies
is
selected
so
that
the
point
of
intersection
of the
spectra
of two
adjacent signals
is not
lower
by
more than
3 dB
than
the
amplitude
of
the
peak
of the
spectrum (plotted
for a
damping equal
to
0.05). Like
the
slowly
swept sine, this method does
not
make
it
possible
to
excite
all
resonances
simultaneously.
We
will
see in
Chapter
9 how
this waveform
can be
used
to
constitute
a
complex
drive
signal restoring
the
whole
of the
spectrum.
4.5. Interest behind simulation
of
shocks
on a
shaker using
a
shock spectrum
The
data
of a
shock specification
for a
response spectrum
has
several advantages:
- the
response spectrum should
be
more easily exploitable
for
dimensioning
of
the
structure than
the
signal x(t) itself;
-
this spectrum
can
result directly
from
measurements
of the
real environment
and
does
not
require,
at the
design
stage
to
proceed
to an
often
delicate equivalence
with
a
signal
of
simple shape;
- the
spectrum
can be
treated
in a
statistical
way if one has
several measurements
of
the
same phenomenon,
it can be the
envelope
of
several
different
transitory events
and
can be
increased
by a
uncertainty coefficient;
-the reference most commonly allowed
to
judge quality
of the
shock simulation
is
comparison
of the
response spectra
of the
specification with
the
shock carried out.
In
a
complementary way, when
the
shock
tests
can be
carried
out
using
a
shaker,
one
can
have direct control
from a
response spectrum:
- The
search
for a
simple
form
shock
of a
given spectrum compatible with
the
usual
test
facilities
is not
always
a
simple operation, according
to the
shape
of the
reference
spectrum resulting
from
measurements
of the
real environment.
- The
shapes
of the
specified
spectra
can be
very varied, contrary
to
those
of the
spectra
of the
usual shocks (half-sine, triangles,
rectangles
etc) carried
out on the
shock machines.
One can
therefore improve
the
quality
of
simulation
and
reproduce
shocks
difficult
to
simulate with
the
usual means
(case
of the
pyroshocks
for
example)
[GAL
73] and
[ROT 72].
-
Taking into account
the
oscillatory nature
of the
elementary signals used,
the
positive
and
negative spectra
are
very close, which makes
a
reversal
of the
test
item
[PAI
64]
useless.