J. RA[KA, M. OBERTHOR: DYNAMIC RESPONSE OF A MATERIAL DURING A HIGH-SPEED IMPACT
113–115
DYNAMIC RESPONSE OF A MATERIAL DURING A HIGH-SPEED
IMPACT
DINAMI^NI ODGOVOR MATERIALA NA ZELO HITRE TRKE
(UDARCE)
Jan Ra{ka, Martin Oberthor
*
Czech Aerospace Research Centre (VZLÚ), Beranových 130, 199 05 Praha – Letòany, Czech Republic
Prejem rokopisa – received: 2019-07-15; sprejem za objavo – accepted for publication: 2019-11-04
doi:10.17222/mit.2019.158
The material of an aircraft structure must resist a high-speed impact – an impact of a bird or hail at the flight speed (a bird strike
or hail strike). The proof of this resistance can be provided in the experimental or numerical way. The recent development of
numerical methods takes advantage of the numerical way. However, the necessity to verify the used numerical models creates
new challenges for the experiments. The goal is to measure and record a great deal of data during the very short time of a
high-speed impact. The use of high-speed load cells makes it possible to record the time-dependent force response of an impact
event. However, the major difficulty of this approach is the existence of parasitic resonances. The experimental stand, the
support of the studied specimen, exhibits an eigen dynamic response and in consequence, the final measured dynamic response
is coupled with the stand-structure one. The objective of a stand design is therefore to minimize these parasitic resonances,
occurring due to the stand-structure dynamics. In this way, special equipment for the experimental research of the dynamic
response during a high-speed impact is designed and realized. The stand is to support the specimen during the impact of a
projectile (a bird or hail) ejected from a pneumatic gun. The stand design enables an impact at any angle (from 0° to 90°),
without changing the contact point. The dynamic-response measurement is realized by means of load cells. The equipment is
designed and optimized by means of FEM numerical simulations with the aim to minimize the parasitic resonances of the stand
structure.
Keywords: high-speed impact, bird strike, hail strike, impact projectile, parasitic resonances, virtual-load-cell record
Strukture iz materialov za letala morajo prenesti trke razli~nih predmetov, ki z veliko hitrostjo udarjajo vanje. To so lahko udarci
ptic, to~e ali drugih delcev med potovanjem letala z veliko hitrostjo. Odpornost proti tak{nim udarcem lahko dolo~imo
eksperimentalno ali numeri~no oz. z ra~unalni{kimi simulacijami, ki imajo z razvojem novih metod mnoge prednosti. Vendar pa
je numeri~ne metode oz. modele potrebno eksperimentalno verificirati, kar predstavlja dolo~en izziv. Cilj je merjenje in
zapisovanje velikega {tevila podatkov v zelo kratkem ~asu, zelo hitrega udarca. Uporaba celic za merjenje hitrih obremenitev
omogo~a zapisovanje odgovora ~asovno odvisnih sil med trki. Vendar je pri tem glavna te`ava tega pristopa nastanek {kodljivih
(parazitnih) resonanc. Izdelano eksperimentalno stojalo je bilo podpora za {tudiranje primerov z lastnimi vrednostmi
dinami~nega odgovora in posledi~no kon~nimi izmerjenimi dinami~nimi odgovori v povezavi s strukturo stojala. Cilj
oblikovanja stojala je zato zmanj{anje tak{nih {kodljivih resonanc zaradi dinamike strukture stojala. Na ta na~in so avtorji
oblikovali in izdelali posebno opremo za eksperimentalne raziskave dinami~nega odgovora med udarjanjem zelo hitrih
projektilov. Stojalo je prena{alo udarce razli~nih projektilov, ptic ali to~e, izstreljenih s pnevmatskim topom. Oblika stojala je
omogo~ala izvajanje udarcev pod kakr{nimkoli kotom (od 0° do 90°), ne da bi pri{lo do spremembe udarne to~ke. Meritve
dinami~nega odgovora so izvajali z merilnimi celicami za zelo hitre obremenitve. Opremo so oblikovali in optimizirali glede na
potrebe numeri~nih simulacij, izvedenimi z metodami kon~nih elementov (FEM) in s ciljem minimiziranja {kodljivih resonanc
strukture eksperimentalnega stojala.
Klju~ne besede: udarci z veliko hitrostjo, udarci ptic, to~e in projektilov, {kodljive resonance, bele`enje dejanskih obremenitev s
senzorji (celicami) za zelo hitre obremenitve
1 INTRODUCTION
Airworthiness standards require the resistance of
aircraft structures to a high-speed impact – an impact of
a bird or hail at the flight speed (a bird strike or hail
strike).
1,2
The proof of this resistance can be provided in
the experimental or numerical way. The recent develop-
ment of numerical methods, especially the explicit
solution of the finite-element method, takes advantage of
the numerical way.
5
However, the necessity to verify the
used numerical models – particularly the model of a
material damage and the constitutive model of an impact
body (projectile) – creates new challenges for the experi-
ments. The goal is to measure and record a great deal of
data during a very short time of a high-speed impact.
This is technically demanding, but necessary for a nume-
rical-model verification.
4
The use of high-speed load cells makes it possible to
record the time-dependent force response of an impact
event. However, the major difficulty of this approach is
the existence of parasitic resonances. The experimental
stand, the support of a studied specimen, exhibits an
eigen dynamic response and in consequence, the final
measured dynamic response is coupled with the
stand-structure one. The objective of the stand design is
therefore to minimize these parasitic resonances, occurr-
ing due to the stand-structure dynamics. For this reason,
at the first iteration, the equipment was optimized by
means of FEM numerical simulations.
Materiali in tehnologije / Materials and technology 54 (2020) 1, 113–115 113
UDK 533.6:629.3.015:666.9-16 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(1)113(2020)
*Corresponding author's e-mail:
raska@vzlu.cz (Jan Ra{ka)

2 SPECIMENS
The experimental stand was designed for large flat
panels, convenient for advanced developmental experi-
ments. The disadvantage of a standard square (or
rectangle) specimen is the risk of non-regular flaws
(Figure 1).
4
The specimen is typically over-loaded at the centers
of the edges, causing the risk of cracks and disruptions,
while damages of a specific form can appear in the
corners. To avoid these types of flaws, we should use a
round specimen as the optimum form is the circle. For
the technological reasons, an octahedron was adopted
(Figure 2).
3 EXPERIMENTAL-STAND CONCEPTION
Three basic objectives were set when designing the
equipment. At first, the stand was designed to support a
specimen during an impact of a projectile (a bird or hail)
ejected from a pneumatic gun. Secondly, the equipment
was adapted to the dynamic-response measurement and
recording by means of load cells.
At last, the stand design enabled an impact at any
angle (from 0° to 90°), without changing the first contact
point. This means that in all cases, the gun axis points to
the center of the specimen. At any impact angle, the first
contact of a bird or hail with a specimen occurs at its
center.
For this reason, the stand structure is divided into two
parts. The movable part is connected with the fixed one
by means of pivots placed in the plane of the specimen
(Figures 3 and 4). In consequence, the gun axis always
points to the same point, to the center of the specimen.
8
The impact angle is controlled, in principle, with a
telescopic rod. As demonstrated on Figure 3, if the teles-
copic rod is made long, the impact angle decreases. By
contrast, a short telescopic rod allows a large impact
angle. On Figure 4, a perpendicular impact is demon-
strated. The real, detailed design is a little different (see
the right parts of Figures 3 and 4).
9
4 NUMERICAL SIMULATIONS AND
OPTIMISATION
The major difficulty of a time-dependent force-res-
ponse measurement during an impact event, by means of
high-speed load cells, is the existence of parasitic reson-
ances.
7
The experimental stand exhibits an eigen dyna-
mic response and in consequence, the final measured
dynamic response is coupled with the stand-structure
one. For this reason, the minimization of these parasitic
resonances was the priority objective.
For the first iteration of the detailed design, FEM
numerical simulations were applied. The finite-element
model was loaded with the time-dependent force func-
tion, simulating the contact interaction between the
projectile (bird or hail) and the specimen. The number of
appropriate force functions was deduced in the analytical
way (with some arbitrary assumptions), in the empirical
way (experimental results) and with numerical simu-
lations (an explicit FEM solution, a liquid-like bird
model).
1
The time-dependent force function, applied to the
complete structure model (a specimen, the stand struc-
ture and the load-cell model) causes a dynamic response.
J. RA[KA, M. OBERTHOR: DYNAMIC RESPONSE OF A MATERIAL DURING A HIGH-SPEED IMPACT
114 Materiali in tehnologije / Materials and technology 54 (2020) 1, 113–115
Figure 3: Experimental-stand configuration for a low-angle impact
7,8
Figure 4: Experimental-stand configuration for a perpendicular
impact
7,8
Figure 2: Optimized specimen form
Figure 1: Typical non-regular flaws of a standard square specimen

The dynamic behavior is analyzed by means of a numer-
ical FE simulation. The essential result is the time-
dependent virtual-load-cell record – the computed
internal force of the FE elements – simulating the load
cells. In an ideal case, the stand-structure parasitic reson-
ances are completely negligible and in consequence, the
virtual-load-cell record corresponds to the force applied
via equilibrium conditions. In the real case, the
deviations from the equilibrium condition are incurred
due to the parasitic-resonance influence. The goal of the
optimization is to minimize, as much as possible, the
parasitic resonances for all the applied force functions.
3
The example demonstrated on Figure 5 shows the
case of the force applied to the specimen plane (the force
due to the friction between the projectile and the speci-
men in the case of a small-angle impact). Theoretically,
only one load cell is loaded. According to the equilib-
rium condition, the load of this load cell is equal to the
applied force.
As it is observed on Figure 5, the applied force (the
black solid line) is not exactly equal to the virtual-load-
cell record. The applied force function has a trapezoid
form; after a short impact pulse, the applied force is zero.
The virtual-load-cell record oscillates around the applied
force function. This deviations are relatively important;
however, the oscillations at only one dominant frequency
are easily eliminated with filtration. Virtual records of
the other load cells are very close to the theoretical
value, very close to zero.
5 CONCLUSIONS
Special experimental equipment for researching
dynamic response during a high-speed impact was
designed, developed and realized. The challenge was to
measure and record a great deal of data during the very
short time of a high-speed impact, necessary for a com-
plex-numerical-model verification.
The stand was destined to support the specimen
during an impact of a projectile (a bird or hail) ejected
from a pneumatic gun. The specimen was defined as a
large flat panel with a special octahedron form to avoid
non-regular flaws. The stand design enabled an impact at
any angle (from 0° to 90°), without changing the contact
point. Irrespective of the impact angle, the gun axis
always points to the same point, to the center of the spe-
cimen.
The dynamic-response measurement was realized by
means of load cells. For the first iteration of the detailed
design, the equipment was designed and optimized by
means of FEM numerical simulations with the aim to
minimize the parasitic resonances of the stand structure.
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J. RA[KA, M. OBERTHOR: DYNAMIC RESPONSE OF A MATERIAL DURING A HIGH-SPEED IMPACT
Materiali in tehnologije / Materials and technology 54 (2020) 1, 113–115 115
Figure 5: Example of virtual-load-cell records for a specimen-plane applied force
7