1. NUMERIC-EXPERIMENTAL ANALYSIS OF THE IMPACT IN
PANELS OF ALUMINIUM
Elza Marisa P. de F. Chagas, Denilson Laudares Rodrigues, Ernani Sales Palma
Program of Pos-Graduate in Mechanical Engineering of the Pontificia Catholic University of Minas Gerais
Belo Horizonte, Brazil,
Email: {quissala,denilsonlr,palma}@pucminas.br
João Manuel R. S. Tavares
Faculty of Engineering of University of Porto
Porto, Portugal,
Email: tavares@fe.up.pt
SYNOPSIS
In this paper, an attempt is made to evaluate reliability of failure critical structural elements
subjected to loading conditions characteristic for power equipment. An estimated lifetime is
one of the most important parameters defining equipment reliability. The paper shows
complexity of the lifetime evaluation in such conditions. Practical solutions made in the
investigation are based on the experimental data, obtained in low-cycle fatigue strength and
fracture mechanics tests. The lifetime is considered both at the stage of crack initiation and at
the stage of crack propagation. Safety factors related to both of these stages are investigated
and recommendations related to lifetime calculation are formulated.
INTRODUCTION
Metallic panels are part of several structures presents in our daily one, such as the
automobiles, skull of ships and fuselage and wings of airplanes. They are projected to provide
us larger mobility, comfort and safety. The motivation in studying the behavior of thin plates,
when subject to the impact of small objects, it proceeds of the need of the engineering of
controlling possible damages caused in those structures, which can be catastrophic. Usually
disposed in several layers, panels are also used in space aircraft, covering and protecting its
structures. These thin metallic plates foils suffer the action of a great variety of agents, from
drops of rains (Adler, 1999), hail even meteorites (Johnson and Holzapfel, 2003) and
fragments (Palmieri et al., 2001). The damages caused by a space vehicle can be classified in
degradation of the surface, penetration and plasm discharge (Landgraf et al., 2004). The
degradation of the surface is caused mainly by small objects that collide with the fuselage of
the airplane. Regardless of they be in the order of micrometers, they are capable to alter the
thermal and optic properties of the layer of more external protection. The penetration is
characterized by the drilling of the layers of protection of the fuselage, reaching pressurized
cameras of the airship and could cause from a simple leak to a disastrous damages. In the
plasm discharge, ions are created locally by the interaction produced by the collision,
generating high conductivity zones. This can interfere in the electronic mechanisms of
mensuration and control of the airship. Researches have been intensified in this area, mainly
in the study of damages that happen during the flight, when fragments come off the rotor of
the airship’s turbine (Sciuva et al., 2003). These fragments need to be retired for the camera
of the rotor, because they should not perforate the turbine completely and to reach other
Paper Ref: S1507_P0360
3rd
International Conference on Integrity, Reliability and Failure, Porto/Portugal, 20-24 July 2009
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2. structures, as compartments of fuel and the fuselage that protects the passengers. A failure of
this type happened with the flight 1288 of the Delta Air Lines in 1996, Florida - USA
(National Transportation Safety Board, 1998). During take off of the airship, a failure in the
front part of the turbine liberated fragment that crossed the fuselage, hurting four passengers
seriously, taking two the death. Accidents as these did with that the aviation agencies, such as
the Federal Aviation Administration agency (FAA), they modified projects and production
processes and maintenance of the components of the airships. This need makes ways for
amplification of researches in the impact areas and perforation of materials.
A second source of damages in the aviation is the impact of objects found in the track, such as
small pieces or pieces of tires left by other airships. Those fragments are thrown by the own
tires of the airship. This failure happened with the Concord of Air France in June of 2000,
resulting with the 113 people's death. According to the report of the organ official French of
investigation of accidents in the civil aviation (Bureau d’Enquêtes et d’Analyses, 2001),
metallic fragments left in the track were thrown by one of the tires against the left wing,
breaking one of the tanks of fuel and causing a fire that implied in the fall of the airship. In
August of 2007, a Boeing 737-800 of China Airlines exploded after the landing due to
penetration of a screw in the wall of the tank of fuel inside one of the wings (MSNBC -
Associated Press, 2007). In Brazil, failures of high gravity happen due to the impact of birds.
In April of 2003, a Boeing 737 of Varig collided with a vulture (O Popular, 2003).
In this context, dynamic interaction present on impact of steel spheres against aluminum
panels is studied through a numerical model of experimental tests, whish reproduce common
collisions of small debris against airplane fuselages. The molding of the material behavior is
treated then as a decisive factor for the correct numeric representation of the available
experimental tests in the specialized literature. The change proposal in the conventional
method of dynamic characterization decreases the rub effects, responsible for mistakes in the
experiment molding. This work emphasizes the panels-material visco-plastic behaviour
modeling, which leads to a modification of the material dynamic characterization method.
Additionally, influence of possible errors in some numerical simulations parameters, related
to contact, structural damping and material behaviour is discussed by sensitivity numerical
analyses. Impact tests were used to obtain the ballistic velocity of the panels. Also, the final
maximum permanent deformation at low velocities was measured. The dynamic
characterization method is explored by comparing the numerical simulation the impact test
present in the literature.
RESULTS
The numeric representation of the impact tests was accomplished in finite elements, with the
objective of establishing a model representative to the real case. It was looked for to evaluate
the influence of the parameters of the material in the determination of the ballistic speed and
of the permanent maximum displacement without fracturing the blank. The used program of
finite elements was LS-DYNA together with HYOERMESH and HYPERVIEW. The impact
of a sphere of steel against foils of two leagues of aluminum was numerically studied, seeking
the representation of the impact of small fragments against the airplane’s fuselage. Whereof
from the studies of dynamic characterization, a modification was proposed in the used
conventional methodology that was shown promising. Numeric predictions for the permanent
maximum displacement of the blanks, front to the impact the low speed, as for the ballistic
speeds, in tests of total perforation, were accomplished through the program LS-DYNA.
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3. Many group of data of impact tests picked in the average literature allowed the observation
and the understanding of the several present phenomenons in the interaction between the
sphere and the blank. Permanent displacement values, ballistic speeds, ricochet, elastic
restitution, method of absorption of energy, manners of propagation of trines and fracture
manners are some factors that could be observed (Figure 1).
Fig. 1. Elements of peel mesh - penetration of the sphere.
They were not considered, in the numeric analysis of the impact tests, the effects caused by
the adiabatic heating (Poteet and Blosser). In the tests where there is perforation, which would
be the more critics with relationship to the dependence of the temperature, the material fails in
traction; in this case, it happens a deformation of rupture of the insufficient order from 10% to
20%, to generate considerable heat.
CONCLUSIONS
The predicted maximum impact point displacement, for no perforation cases, and the ballistic
velocity, for total perforation tests, present a good numerical-experimental agreement, bearing
in mind the observations about the various used parameters. This result is coherent, because
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4. the tested materials possess little influence of the deformation rate. Besides, the dependence
of the results of the simulations with the mesh, mainly element size, it is aimed as critical
factor in the cases of determination of ballistic speed. The obtaining of the final configuration
of the blanks for the simulations was a stage executed success when using peel elements and
solids, even so to a computational cost extremely loud if compared to the case of axi-
symmetrical elements.
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