Gruppo Italiano Frattura (IGF)
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A study of the stored energy in titanium under deformation and failure using infrared data
No full textThe work is devoted to the experimental study of heat dissipation caused by plastic deformationand failure processes taking place in a titanium alloy Ti-4.2Al-1.6Mn. To investigate the spatial and timeevolution of temperature, a set of experiments has been carried out on plane titanium smooth specimens andspecimens with pre-grown centered fatigue cracks. The original mathematical algorithm for experimental dataprocessing has been applied to obtain the rate of heat dissipation generated by plastic deformation and storedenergy. It is shown that the stored energy is accumulated in titanium specimens undergoing fatigue tests, and atthe time of damage to fracture transition it is equal to zero
Fracture toughness and fatigue resistance of quenched and tempered steels with microstructures deriving from a slant quench. Consequences on technical standards
No full textMechanical components fabbricated with quenched and tempered steels, exhibiting mixed microstructures as derived from slant quench conditions, are frequently encountered in the industrial practice, owing to a tendency to employ quite low alloy steels or due to quite large sections. The low notch strength of mixed microstructure steel samples was already emphasized in the 1950s; yet, it has never been investigated again. Also, technical standards have not addressed the risk deriving from the use of steel components with mixed microstructures. When pearlite and ferrite are present alongside tempered martensite and bainite, the fracture toughness of steel pieces diminishes to very dangerous levels. Results of an experimental program on the fracture toughness of plastic mould steels are reported, singling out microstructure mixtures with too a low toughness. In addition, the fatigue crack propagation rate is adversely affected by inhomogeneous metallographic structures. It is inferred that experimental results and ensuing considerations should be taken into account when formulating technical norms
Numerical modelling of intergranular fracture in polycrystalline materials and grain size effects
No full textIn this paper, the phenomenon of intergranular fracture in polycrystalline materials is investigated using a nonlinear fracture mechanics approach. The nonlocal cohesive zone model (CZM) for finite thickness interfaces recently proposed by the present authors is used to describe the phenomenon of grain boundary separation. From the modelling point of view, considering the dependency of the grain boundary thickness on the grain size observed in polycrystals, a distribution of interface thicknesses is obtained. Since the shape and the parameters of the nonlocal CZM depend on the interface thickness, a distribution of interface fracture energies is obtained as a consequence of the randomness of the material microstructure. Using these data, fracture mechanics simulations are performed and the homogenized stress-strain curves of 2D representative volume elements (RVEs) are computed. Failure is the result of a diffuse microcrack pattern leading to a main macroscopic crack after coalescence, in good agreement with the experimental observation. Finally, testing microstructures characterized by different average grain sizes, the computed peak stresses are found to be dependent on the grain size, in agreement with the trend expected according to the Hall-Petch law
Study of overload effects in bainitic steel by synchrotron X-ray diffraction
No full textThis work presents an in-situ characterisation of crack-tip strain fields following an overload bymeans of synchrotron X-ray diffraction. The study is made on very fine grained bainitic steel, thus allowing avery high resolution so that small changes occurring around the crack-tip were captured along the crack plane atthe mid-thickness of the specimen. We have followed the crack as it grew through the overload location. Oncethe crack-tip has progressed past the overload event there is strong evidence that the crack faces contact in theregion of the overload event (though not in the immediate vicinity of the current locations of the crack tip) atKmin even when the crack has travelled 1mm beyond the overload location. It was also found that at Kmax thepeak tensile strain ahead of the crack-tip decreases soon after the overload is applied and then graduallyrecovers as the crack grows past the compressive region created by the overload
Are the Paris’ law parameters dependent on each other?
No full textThe question about the existence of a correlation between the parameters C and m of the
Paris’ law is re-examined in this paper. According to dimensional analysis and incomplete self-similarity
concepts applied to the linear range of fatigue crack growth, a power-law asymptotic representation relating
the parameter C to m and to the governing variables of the fatigue phenomenon is derived. Then, from the
observation that the Griffith-Irwin instability must coincide with the Paris’ instability at the onset of rapid
crack growth, the exponents entering this correlation are determined. A fair good agreement is found between
the proposed correlation and the experimental data concerning Aluminium, Titanium and steel alloys
Fatigue behaviour of titanium dental endosseous implants
No full textIn this work two different titanium dental implants are analyzed in order to evaluate their mechanical strength. An ad-hoc designed experimental apparatus is prepared to test against fatigue these implants in a way that approximates as much as possible the actual stresses occurring during mastication motion. The results of these endurance tests are summarized in the form of Wohler-type diagrams showing the duration of a specific implant for different applied loads. These plots show a fatigue limit below which the implants could resist indefinitely. Other aspects of this research concern the influence of a potentially corrosive medium and the analysis of the deformation and failure of the specimens. During fatigue cycling, the titanium implants do not seem to be affected by a more aggressive environment, such as a saline solution. The analysis of the broken specimen allowed the crack initiation sites and the type of fracture propagation to be investigated in depth. In all the considered implants fatigue cracks were seen to initiate preferentially from sites in which the tensile stress concentration is the highest. The results of a finite element analysis performed on one of the specimens is in good agreement with the failure mode observed after the tests. The SEM fracture surface analysis shows a clear similarity between the fracture mode of the tested implants and of the actual implants broken after a certain operating period
Elementi di interfaccia per l’analisi di strutture murarie
No full textThe present paper deals with the modelling of the mechanical behaviour of masonry elements regarded as heterogeneous systems, made of mortar, bricks and interfaces. Thus, the adopted computational strategy consists in modelling the brick units, the mortar joints and the interfaces responsible for the mortarbrick decohesion mechanisms; to this end, a special interface model combining damage and friction is proposed. A numerical procedure, based on the backward Euler time-integration scheme, is introduced; the time step is solved adopting a displacement driven predictor-corrector scheme. Some numerical applications are performed in order to assess the performances of the proposed model and algorithm in reproducing the nonlinear response of masonry material due to damage localization. Finally, a masonry arch model is studied, comparing the numerical results with experimental ones; it is show the ability of the proposed model to simulate the global behaviour of the arch structure in term of ultimate load and collapse mechanism
L'infragilimento da idrogeno. Un caso di studio In una barra di comandi di un velivolo dell'aviazione generale.
No full textLa degradazione delle caratteristiche meccaniche di un materiale di impiego aeronautico ha pesanti ricadute sulla sicurezza del volo. Tale degradazione si può manifestare sia per l'effetto combinato tra l'ambiente nel quale il velivolo opera e le sollecitazioni cui è sottoposto sia che a causa di anomalie avvenute in fase produttiva che, dopo un periodo di incubazione, manifestano il loro pericoloso effetto: è questo il caso dell'infragilimento da idrogeno che, nel presente articolo che esamina sia i punti critici del processo di produzione che possono indudo che la morfologia delle superfici di rottora che ad esso si possono ricondurre. Viene infme presentato un caso di studio che ha interessato un velivolo dell'aviazione generale nel quale è stato individuato l'infragilimento da idrogeno di un componente in acciaio AISI 4130 è stata individuata quale causa primaria dell'incidente
Failure of steam line causes determined by NDT testing in power and heating plants
No full textThis paper examines leakage and damages of steam and provides an overview of NDT testing inorder to determine the cause of steam lines failure in power plants and heating plants. This approach may beapplied to similar structures and its application in preventive maintenance would help extend the life of steampipes
Evaluation of seismic mitigation of embankment mode
No full textConducting experiment on embankment model by shaking table could be an accurate method to evaluate the behavior of embankment or any structures under seismic loading. In this research work, in order to assess the function of seismic force and accurate placement of dense zone in the embankment model, the results of three experiments have been considered. To evaluate the reaction of the embankment model, it was measured the stress in the system and photographs were taken. The results of three experiments indicated that suitable arrangement of dense zone is the main factor at the play in embankment stability, and in predicting the possibility of embankment behavior