Civil Engineering Reference
In-Depth Information
Fig. 14.2
Comparison of geometric dimensions in a vertical section: Phantom-Airbus A320-
Boeing 747
To calculate the time-dependent contact force during the impact between two
bodies, two different approaches are available, both of which have their roots in
Newton's second law of motion (force
acceleration) in addition to
respecting the conservation of energy and momentum. Riera developed a compar-
atively simple method for the process of an aircraft impact in the late 1960s [
1
],
which yields adequately applicable results. It is still used today amongst complex
numerical simulations for crash situations and has been verified in numerous
experiments. The so-called Riera-Model is a one dimensional mathematical
model in the form of a thin walled tube with specific mass and bursting load
distributions. The load on the rigid target body is therefore dependent on the
mass
M
and the deformation speed
v
of the impacting body. As a matter of
principle, the impact force is calculated as the change of momentum
M·v
in the
region of the impact zone:
¼
mass
M
∂
v
ðÞ¼
∂
∂
v
∂
M
∂
Ft
t
Mv
ðÞ¼
t
þ
t
∂
This simple relationship used for the impact of an aircraft shows the influence of
different effects. The first term on the right is determined by the change in mass of
the projectile within a fixed control volume at the impact point. The second term
corresponds to the force in the impact zone that is transmitted to the rest of the
projectile, which is assumed to be rigid, and must be equal to the crushing load of
the structure in front of the impact zone for reasons of equilibrium. For high impact
speeds (
150 m/s), the first term and therefore the mass distribution of the aircraft
is the dominant factor in determining the impact load-time function, whereas for
lower speeds the bursting load has a significant influence. It is important to note that
the mass distribution of an airplane is usually very well known, but that the bursting
load distribution of an aircraft fuselage can only be stated within a wide range. This
shows that the Riera-Model calculates the load function for high speeds relatively
reliably, however only with larger uncertainties for smaller speeds.
Different conditions apply to the impact of a large commercial plane than to that
of a military aircraft. Taking into consideration the geometric relationships the
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