
 DOESTD301496
The permissible ductility ratio, µ, is defined as the ratio between
maximum permissible deflection of a structural system and the
deflection at the effective yield for the system (see Section 6.3.3.3).
TimeHistory Analysis Method. A timehistory response analysis method
6.3.3.2
of evaluating the global response or collapse of the target structure uses
the inertial and stiffness characteristics of both the aircraft and the target
structure (including its foundation flexibility). One of the acceptable
methods that has been extensively used is outlined below.
This method, called the force timehistory analysis method, consists of
two major steps. The first step is to determine impact force timehistory
based on the aircraft mass distribution, crushing/buckling characteristics,
impact velocity, aircraft length, and other aircraft structural data, and
conservatively assuming that the target structure is rigid. Impact force
timehistory is determined using the momentum principle, similar to the
method developed by Riera (Reference 3) and modified by Muto et al.
(Reference 9). Accordingly, the impact force, F(t), acting on the rigid
fixed target at time, t, is expressed as
m[x(t)][v(t)]2
F(t)
Pc[x(t)]
(614)
where:
Pc
=
load necessary to crush or buckle the fuselage;
x(t)
=
distance from the nose of the aircraft to the point up to
which crushing has progressed at time, t;
m
=
longitudinal mass per unit length of the uncrushed
aircraft;
v(t)
=
velocity of the uncrushed portion of the aircraft
at time, t;
=
empirical correlation factor (use a value of 0.9 unless
justified otherwise).
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