Agostino Lanciotti

Fatigue crack propagation in residual stress fields of welded plates

Abstract This paper presents some recent results of a research programme on the evaluation of the fracture mechanics properties, principally the crack propagation rate, of plasma welded aluminium joints, 2219-T851. Crack propagation tests were carried out on large centre cracked tension (CCT) specimens and on small compact tension (CT) specimens. The main difference between the two typs of specimens concerned the residual stresses, which were not present in the CT specimens, due to their reduced dimensions. The residual stresses were measured in cracked and uncracked plates, by means of strain gauges. The results of the crack propagation tests carried out on the two types of specimens were fully comparable in the case of a crack which propagates along the weld bead, in the heat-affected zone, as the residual stress effect was quite low, while the crack propagation rate measured in the CT specimens was lower than that in the CCT specimens in the case of a crack which propagates perpendicularly to the weld bead. Residual stress measurements allowed the evaluation of the residual stress effects on the crack propagation rate and close agreement was found between the results of the two set of tests.

Fatigue Resistance of MAG Welded Steel Elements

Abstract The present paper describes the results of a research programme carried out to assess the fatigue properties of MAG (Metal Active Gas) welded elements, built in Fe510 D1 steel. Tests on both small and large specimens were carried out to evaluate the effects, on fatigue resistance, of the residual stresses produced by the welding process. The joints examined were butt welded and longitudinally welded. Crack propagation tests were also carried out on welded plates. The data available, together with results taken from the literature, allowed us to analyse the fatigue test results on the basis of the methodologies of Fracture Mechanics; by calibrating the initial defects using the results of the tests carried out on small specimens, a good prediction of the fatigue resistance of large specimens was obtained.

Compression behaviour of flat stiffened panels made of composite material

Abstract In certain structural applications, such as stiffened panels loaded in compression, a better exploitation of the properties of composite materials requires the development of design methodologies that allow the component to work in the post-critical range. This paper describes the results of a research activity with such an objective, carried out at the Department of Aerospace Engineering of the University of Pisa in collaboration with Alenia-Transport Aircraft Group. A group of tests has been carried out on 10 flat stiffened panels, made in composite material, and the results have been analysed with two computer codes, providing useful information concerning their capabilities.

Effects of spectrum variations on fatigue crack growth

Abstract The definition of the load spectrum for the full-scale fatigue test of a new aircraft is a very delicate problem: among others, two important aspects are those concerning the maximum load and the minimum load range to be included in the sequence. The present paper describes the results of an experimental programme carried out in support of the certification of the Piaggio P.180 aircraft. The effects on crack propagation of truncating high loads and of omitting small cycles are discussed and compared with similar results in the literature. The test results are compared with analytical predictions, obtained by using a simple closure model. Finally, a number of observations about tensile mode/shear mode transition are made.

Effect of Plasma Cutting on the Fatigue Resistance of Fe510 D1 Steel

The paper describes the results of a research programme, carried out at the Department of Aerospace Engineering of the University of Pisa, for the assessment of the influence of plasma cutting on the physical and mechanical properties of Fe510D1, a low carbon steel widely used in carpentry. The activity started by observing that several industries rework plasma cut edges, particularly in the case of fatigue structures, in spite of the good quality of the plasma cut edges in a fully automatic process. Obviously, reworking is very expensive and time-consuming. Comparative fatigue tests demonstrated that the fatigue resistance of plasma cut specimens in Fe510 steel was fully comparable to that of milled specimens, as the consequence of the beneficial residual stresses which formed in the plasma cut edges.

A method for improving the performance of fatigue machines in variable amplitude loading tests

Abstract A method is described for improving the response of servohydraulic fatigue machines in variable amplitude loading tests, driven by a process computer. The method allows a high working speed to be obtained while the output accuracy is maintained by means of suitable cycle-by-cycle input modification. Test results show a considerable increase in the testing speed without appreciable errors in the applied loads. The method also provides an accurate description of the loads actually applied.

Characterisation of Fatigue and Crack Propagation in Laser Shock Peened Open Hole 7075-T73 Aluminium Specimens

The goal of this research activity is to evaluate the capability of Laser Shock Peening (LSP) technology to improve fatigue life in open-hole aluminium specimens. Thin, dog-bone specimens were LSP treated in direct ablation mode and subsequently tested. The obtained results have not proven the advantage of LSP technology over traditional residual stress insertion techniques around open-holes, such as cold working. Therefore, the focus of the activity was moved towards understanding the causes of the observed fatigue life reduction.

Experimental Tests and Numerical Analyses of Fiber Metal Laminate Panels under Shear Load with Bonded Window Frame

Experimental and numerical activities have been developed to investigate the behavior of Fiber metal laminates (FML) shear loaded panels having a bonded window frame. Both static and fatigue tests have been carried out at the Department of Aerospace Engineering of Pisa on 10 panels manufactured by Alenia Aeronautica, in the context of the DIALFAST (Development of Innovative and Advanced Laminates for Future Aircraft Structures) project, co-funded by the European Commission within the Sixth Framework Programme. In addition to technological problems, the tested specimens represent very critical components, involving fundamental aspects of the structural design of fuselage panels; in particular, the primary interest to exploit the postbuckling behavior of FML components, as commonly applied in conventional aluminum alloy fuselage structures, is associated with the effort of making reinforced holes which do not alter significantly the stress distribution of the main body of the panel. Detailed finite element analyses, performed by means of the ABAQUS V 6.5 package, have been used to investigate the panel response during testing. The numerical results have shown a very good agreement with the experimental data, thus revealing an effective instrument to study the behavior of a postbuckled shear panel, in presence of the neutral hole issue.

Research Activities on Buckling of Composite Structures in Italy

The paper gives an overview of some research activities regarding buckling of composite structures performed in Italy in the last years. In particular, the activities performed at Politecnico di Milano, at Politecnico di Torino and at Universita’ di Pisa are here presented. Some investigations were performed on structures manufactured by Agusta/Westland and by Alenia Aeronautica, while other researches were funded by the Italian Ministry of Research, and other ones were funded by the European Commission within European Projects.

New configurations for the windshield and the canopy for a two‐seater trainer military aircraft

This paper describes the results of a study on new configurations for the windshield and the canopy of the M346, a new military trainer aircraft with two staggered seats, currently in phase of construction at Aermacchi (Venegono Superiore, Italy). The study carried out demonstrated that forward rotation allows a reduction of the weight of the structure, improves the visual field of the pilots and moreover, allows a greater accessibility to the panel instrumentation. Obviously, all the related aspects have been considered in the development of the study, such as the opening/closure, latching/unlatching and locking systems of the canopy, the tightness for pressurisation, tightness for water and dust also when the aircraft is parked, the resistance to bird impact, the forces due to wind when the canopy is opened on the ground and, in the case of emergency, the systems to break the transparent material and to eject the pilots.