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.