Aerospace
Materials and Structures
Probabilistic design methodology The use of the probabilistic
approach in the fatigue design is evaluated against the deterministic
approaches, safe life and damage tolerance, generally now applied.
To this end, it is carried out a computer code - PISA - by means
of which the risk assessment is performed. The code is based
on the simulation of the damaging process starting from an Equivalent
Initial Flaw Size until to catastrophic failure; the Monte Carlo
method is employed to handle the statistical variables. The
inspection program and the repair are also taken into account.
G. Cavallini, A. Lanciotti, L. Lazzeri - A Probabilistic
Approach to Aircraft Structures Risk Assessment, "Fatigue
in new and ageing aircraft", [Proc. 19th ICAF Symposium,
1997, Edinburgh, pp. 421-440], EMAS Publ., ISBN/ISSN: 0-947817-97-2
G. Cavallini, R. Galatolo, G. Cattaneo - An experimental and
numerical analysis of multi-site damaged butt-joints - ICAF
'99 - 12-16 July, 1999 Seattle USA, 20th ICAF Symposium.
Short cracks characterization The knowledge of the short
cracks and propagation is an essential item for the fatigue
design of aircraft structural component as well durability
evaluation and damage tolerant requirement. The riveted joints
are significant components on this restect, in particular
for the Multi Site Damage phenomenon.
So the capability of the Equivalent Initial Flaw Size approach
and some numerical codes for the short crack growth is assessed
by means of experimental data obtained from a test program
on riveted lap-joints subjected to constant and variable amplitude
fatigue loading. Also the effect of the corrosive environment
and riveting process is evaluated.
G. Cavallini, R. Galatolo, R. Lazzeri - Short crack growth
in the lap joints: experimental and numerical results, [Proc.
21th ICAF Symposium, 2001, Toulouse].
Compressive behaviour of composite stiffened panels Research
activity were carried out to development and set-up design
methodologies for stiffened structures in composite materials,
working in the post-critical range. The post-critical design
allows much greater structural efficiency, undoubtedly an
advantage over the solution that is resistant to buckling
up to failure load.
An experimental program was performed by testing ten 700x1000
mm stiffened panels. The panel were representative of different
structural configurations; three configuration were designed
to fail in the post critical range while a configuration was
designed to obtain coincidence between global instability
and failure.
During the compressive tests the out of plate displacements
of the specimens were measured by means of Moiré interferometry.
Tests demonstrated that the post critical configuration is
lighter and less sensitive to impact damage The post critical
behaviour of the panels was quite well predicted by using
two computer codes, PANDA2 and MARC.
M.Chiarelli, A. Lanciotti, L. Lazzeri - Compression Behaviour
of Flat Stiffened Panels Made of Composite Material.
Composite Structures, Vol 36, 1996, pp. 161-169.
Welded Structures Welding is a joining process cheaper and
faster than riveting but the mechanical properties of welded
structure often do not respect the aerospace requirements
relevant to fatigue, fracture and corrosion resistance.
The quality of welded joints increased in the last years;
at the same time new processes were developed, such us electron
beam, laser and friction stir welding. The main problems connected
with welded structures, such as residual stresses, fatigue
resistance, crack propagation and stress corrosion resistance
have been the topics of several research programmes carried
out at the Department.
A. Lanciotti, L. Lazzeri, S. Ottaviano - Durability, Damage
Tolerance and Environmental Assisted Crack Propagation Charactheristics
of a TIG Welded Titanium Alloy, 22th International Council
of the Aeronautical Sciences Congress, Harrogate, United Kingdom,
28 August – 1 September 2000.
M. Chiarelli, A. Lanciotti, M. Sacchi - Fatigue Resistance
of MAG Welded Steel Elements. International Journal
of Fatigue, Vol. 21, n. 10, pp. 1099-1110, 1999.
M. Chiarelli, A. Lanciotti, M. Sacchi - Effect of Plasma
Cutting on the Fatigue Resistance of FE510 D1 Steel.
Journal of Engineering Materials and Technology, Transactions
of the ASME, Vol. 122, n.1, January 2000, pp. 141-145.
E. Ciompi, A. Lanciotti - Susceptibility to Stress Corrosion
of 7050-T7351 Electron Beam Welded Specimens. Engineering
Fracture Mechanics, n. 62, 1999; pp. 463-476.
A. Lanciotti, A. Belmondo - Mechanical Properties of Al
2219 VPPA Weldments. 48th International Astronautical
Congress, Turin, 6-10 October 1997. Paper IAF-97-1.4.07.
Fatigue resistance of rivet joints in metal and composite
materials A research activity was carried out at the Department
in cooperation with Costruzioni Aeronautiche G. AGUSTA, aimed
at the characterisation of the fatigue behaviour of composite
joints. The activity was performed as a support to the certification
of the tail of the new helicopter EH 101.
Two types of specimens were defined taking the move from the
design of the structure; they mainly differed in the thickness
of the laminates, 2 e 4 mm, and were representative of critical
areas. The specimens were impact damaged with an energy level
capable to induce a barely visible indentation, as required
by the Certification Authority. Fatigue tests were carried
out both in dry and wet conditions; the stiffness of the specimens
was monitored during the fatigue tests.
The comparison between the results obtained shows the considerable
effect of impact damage and of humidity absorption on the
fatigue resistance of composite joints.
Notwithstanding its relatively rough aspects, riveting is
still a very popular joining method in the aviation industry,
even if there are still aspects that need to be investigated.
Very recently, the ageing aircraft theme has highlighted the
actuality and importance of such a joining technique. A research
was carried out at the Department to study the role of the
riveting technological parameters on the fatigue behaviour
of metallic lap joints. For this purpose, the attention has
been focused on lap joints in 2024-T42, thickness 1.2 mm,
where rivets which differed for the head, the material, the
heat treatment and the squeeze force were installed, in order
to assess separately the influence of each parameter. Contemporarily,
a numerical analysis has been carried out; particular attention
has been paid to study the influence of the specimen width,
keeping also geometrical non-linearity into account, to compare
the stress distributions in realistic joints with those obtained
in typical laboratory coupons.
A. Lanciotti, L. Lazzeri, M. Raggi - Fatigue Behaviour of
Mechanically Fastened Joints in Composites Materials.
Composite Structures, Vol.33, pp. 87-94, 1995.
M. Chiarelli, A. Lanciotti, L. Lazzeri - Fatigue behaviour
of metallic lap joints: a study on the effect of riveting
and of specimen geometry. International Committee Aeronautical
Fatigue, 20th Symposium, Bellevue, Washington, USA, 14-16
July 1999.
Delamination growth in composite materials Delamination is
a type of defect that may promote failure of composite materials
components. A great interest is devoted to the study of the
mechanisms of growth and to the validation of theories and
hypotheses about their behaviour. Current theories are based
on the strain energy release rate as the fundamental parameter
that governs the phenomenon, according to its partition in
the three different basic Fracture Mechanics modes. A long
term research activity is in progress for the evaluation of
the static and fatigue behaviour of components containing
delaminations; experiments and analyses are carried out on
components, and basic material characterization is performed
for the measurement of critical fracture parameters, under
Mixed Mode conditions.
L. Candiani, U. Mariani, M. D’Alessandro Caprice, L.
Lazzeri - An evaluation of damage tolerance characteristics
of sandwich composite structures, in ‘Fatigue
in new and ageing aircraft’, Proceedings of the 19th
ICAF Symposium, Edinburgh, EMAS Publ., pp. 287-305
L. Lazzeri, U. Mariani - Damage tolerance characteristics
of composite sandwich panels, in RTO-MP-24 ‘Application
of damage tolerance principles for improved airworthiness
of rotorcraft’, Proceedings of AVT/RTO Specialists’
Meeting, Corfu, April 1999.
Fatigue crack propagation under spectrum loading Models for
the prediction of fatigue crack growth in various materials,
under spectrum loading, are continuously updated and improved.
The performance of a model may be more or less accurate according
to the type of material or type of spectrum. It is therefore
important to validate predictions against many types of load
sequences: aircraft flight-by-flight, blocks, helicopter spectra.
Experimental data are generated for different stress levels
and with different omission levels.
L. Lazzeri, A. Pieracci, A. Salvetti - An evaluation
of fatigue crack growth prediction methods use in aircraft
design, in ‘Estimation, enhancement and control
of aircraft fatigue performance’, Proceedings of the
18th ICA Symposium, Melbourne, EMAS Publ., pp. 615-645, 1995.
L. Lazzeri, A. Salvetti - An experimental evaluation
of fatigue crack growth prediction models, in Proceedings
of the 1996 ASIP Conference, Vol. I, pp. 477-508.
Fibre Metal Laminate structures This advanced class of materials
are evaluated with great interest by the aircraft community,
due to their inherent excellent damage tolerance characteristics.
A research has been carried out, within the ADPRIMAS project
funded by the European Union, for the evaluation of the behaviour
of riveted and bonded joints, representative of solutions
or fuselage longitudinal splices, and for the assessment of
the residual strength of stiffened panels, representative
of solutions for a fuselage side panel.
G. Cavallini, R. Galatolo, L. Lazzeri - Fatigue problems
in the design of fuselage panels in Glare, 21st ICAF Symposium, Toulouse, June 2001.
L. Lazzeri - Fatigue behaviour of riveted Glare joints,
Fatigue and Fracture of Engineering Materials and Structures,
Vol. 24, No. 9, September 2001, pp. 579-589.
High Temperature Fatigue Crack Growth in Engine Disk Materials Engine disk nickel-based superalloys, such as IN718, Waspaloy
and Astroloy, and the titanium alloy Ti-6Al-4V were evaluated
in terms of fatigue crack growth rate under both constant
and variable amplitude loading. The test temperature ranged
from 450 °C to 650 °C. The d.c. Potential Drop technique
was used to measure the crack length on the specimens during
the tests. This technique proved to be very reliable and accurate
even at high temperature. The same technique was also used
to monitor a crack growing in a cyclic spinning disk. In this
case, the Potential Drop calibration curves were determined
for the particular engine disk examined. A slip-ring was used
to acquire the signals from the rotating disk. A demonstration
test, carried out on a IN718 disk in a safety container, by
means of a spinning machine, successfully concluded the research
programme.
E. Campo, A. Frediani, R. Galatolo, G. Pasquero - Turbine
disks: Lifing against defects and materials development,
AGARD Report 790, Impact of Materials Defects on Engine Structures
Integrity, Advisory Group for Aerospace Research and Development,
Neuilly-sur-Seine (France), April 1993, pp. 10.1-10.4
R. Galatolo, A. Frediani - Full-Scale crack propagation
test on spinning turbine disks using the potential drop technique,
Estimation, enhancement and control of aircraft fatigue performance,
Proceedings of the 18th ICAF Symposium, Melbourne, 1995, Vol.
II, pp. 717-730.
Effect of Multiple Site Damage on Ageing Aircraft Aircraft
structural components having many fatigue critical sites,
such as riveted joint panels, are particularly prone to the
Multiple Site Damage (MSD) phenomenon. In this case, the deterministic
approach generally used in the fatigue life evaluation is
doubtfully applicable. To study this topic, a huge test campaign
was carried out on riveted Butt-joint and Lap-joint panels,
made of the aluminium alloy 2024-T42. The laboratory tests
were stopped after prescribed number of cycles, then the joints
were opened by applying a tensile load above the residual
strength, in order to measure the fatigue crack distribution.
The experimental data was used to set up a model based on
a probabilistic approach. The static residual strength of
cracked joints was also evaluated, showing a resistance reduction
when MSD is present together with a long crack.
G. Cavallini, R. Galatolo, G. Cattaneo - An experimental
and numerical analysis of multi-site damaged butt-joints -
in Structural integrity for the next millenium, ICAF '99 -
12-16 July, 1999 Seattle USA, 20th ICAF Symposium.
G. Cavallini, A. Lanciotti, L. Lazzeri – Analisi teorica
e sperimentale della mutua interazione tra cricche e intagli–
Atti XV Congresso AIDAA, pp. 1225-1233, Torino, 1999.
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