Gruppo Italiano Frattura (IGF)
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The failure criterion based on hydrogen distribution ahead of the fatigue crack tip
No full textThe hydrogen effect on the fracture toughness and fatigue crack growth behaviour in themartensitic high strength steel is investigated. The secondary ion mass spectrometry method has beenemployed to analyse the distribution of hydrogen concentration in the zone of the crack tip and at its edges.Changes in hydrogen concentration are observed in the vicinity of the propagating crack tip and at a remotesite. The hydrogen peak H C is reduced and moves away from the fatigue crack tip with the increase of themaximum stress intensity factor max K . The concept of damage evolution is used to explain fatigue crackpropagation in connection with the hydrogen redistribution ahead of the crack tip. The physical failure criterionbased on the hydrogen peak in the vicinity of the fatigue crack tip and the maximum stress intensity factor hasbeen proposed. The criterion reflects changes in the hydrogen peak which resulted from the hydrogenredistribution due to the increase of the maximum stress intensity factor as the crack length increases underfatigue loading
Densità di energia di deformazione locale e resistenza a fatica di giunti saldati di geometria complessa
No full textA recent criterion based on the local strain energy density (SED) averaged over a given control
volume is applied to well-documented experimental data taken from the literature, all related to steel
welded joints of complex geometry. This small size volume embraces the weld root or the weld toe, both regions
being modelled as sharp (zero notch radius) V-notches with different opening angles.
The SED is evaluated from three-dimensional finite element models by using a circular sector with a radius
equal to 0.28 mm. The data expressed in terms of the local energy fall in a scatter band recently reported in
the literature, based on about 650 experimental data related to fillet welded joints made of structural steel
with failures occurring at the weld toe or at the weld root
Carbon nanotubes cement composites
No full textThe present paper reviews the current state of the art of carbon nanotubes cement-based composites and the possible applications. The influence of carbon nanotubes additions onto cement paste mechanical and electrical properties are discussed in detail. Though promising, several challenges have still to be solved before the introduction of these new materials into the public sphere through civil infrastructures
Coating by the Cold Spray Process: a state of the art
No full textA brief description of cold spray coating process is presented. This paper intends to review some the previous works which are mostly about the influences of the cold spray parameters, mostly the surface of the substrate, on the deposition efficiency (DE). Almost all the important parameters, with more focus on the roughness of the substrate, on increasing the DE are briefly studied; this review also includes a description of application of cold spray and of some important effect of this method on substrate properties. On this basis, some possible development in this field of research are drawn and discussed
Esistenza del modo O in piastre tridimensionali con fori circolari ed ellittici soggette a modo II
No full textOn the mode O existence in three-dimensional plates weakened by circular and elliptical holes under mode II loading It is known for long time that in a cracked plate subjected to anti-symmetric plane loading, the Poisson’s effect leads to the generation of a coupled out-of-plane singular mode (Mode O). Recent theoretical and numerical analyses have shown that this effect is present also in plates weakened by pointed V-notches and might play a role in failure initiation phenomena in plates under mode II loading particularly in the presence of large opening angle. Since the degree of singularity characterising the mode O is greater than that of mode II, the mutual incidence of the two modes depends on the component size.
Dealing with circular and elliptic holes, the presence or not of the out-of-plane mode has never been discussed in the literature. The main aim of this work is to demonstrate the existence of a three-dimensional effect tied to mode O. On the basis of a large bulk of numerical results the following conclusions can be drawn:
- An out-of-plane mode is present in plates weakened by U-notches and subjected to Mode II loading, as previously demonstrated for plates weakened by cracks and pointed V-notches; its intensity is not negligible also for finite values of the notch radius.
- Out-of-plane mode results in a shear stress distribution symmetric with respect to the mid-plane. The intensity of this stress component strongly depends on the Poisson’s ratio. It increases as increases.
- The strain energy density averaged on a given control volume which should, in principle, be material dependent is a parameter suitable for the natural of capture all three dimensional effects occurring through the plate thickness. It is surely a powerful tool in practical cases where the role played by three-dimensionality is not negligible, or is not known in detail
Cyclic bend tests for the reliability evaluation of printed circuit boards under dynamic loads
No full textThe reliability of printed circuit boards under dynamic loads is a key issue in the handheld electronic products industry. In order to predict the performance of the boards in their application lifetime, different tests were developed. The current industry-wide standard testing method is a board level drop test. In this test, the boards are dropped under defined conditions until a failure in the board is detected. The main failure driver is a flexural oscillation of the board due to the impact event. As this test method has a number of drawbacks, an alternative test method was evaluated in this study. A board level cyclic bend test was used and the results of both tests were compared. A very good correlation between the methods could be observed, supporting the suitability of the board level cyclic bend test for the determination of the drop test performance. The advantages of the alternative test method were shorter testing times, better adaptability and test simulations at lower computing time. In future analysis, test simulations will be used to generate Wöhler curves related to the local stresses
Application of thermoelastic stress analysis for the experimental evaluation of the effective stress intensity factor
No full textIn recent years, the advent of staring array detectors has made Thermoelastic Stress Analysis (TSA)a technique with considerable potential for fatigue and fracture mechanics applications. The technique is noncontactingand provides full field stress maps from the surface of cyclically loaded components. In addition, thetechnique appears to have a great potential in the evaluation of the effective stress intensity factor range duringfatigue since fracture mechanics parameters are derived directly from the temperature changes in the vicinity ofthe crack tip rather than from remote data. In the current work TSA is presented as a novel methodology formeasuring the effective stress intensity factor from the analysis of thermoelastic images. ?K values inferredusing TSA have been employed to estimate an equivalent opening/closing load at different R-ratios in a crackedaluminium 2024 CT specimen. Results have been compared with those obtained using the strain-offsettechnique showing a good level of agreement
Case studies in numerical simulation of crack trajectories in brittle materials
No full textStatistical Fracture Mechanics, formalism of few natural ideas is applied to simulation of cracktrajectories in brittle material. The “diffusion approximation” of the crack diffusion model represents cracktrajectories as a one-dimensional Wiener process with advantage of well-developed mathematical formalismand simplicity of creating computer generated realizations (fractal dimension d = 1.5). However, the most ofreported d values are in the range 1.1-1.3. As a result, fractional integration of Wiener processes is applied forlowering d and to generate computer simulated trajectories. Case studies on numerical simulation ofexperimentally observed crack trajectories in sandstone (discs tested in indirect tensile strength test) andconcrete (compact tension specimens tested in the quasi-static splitting tensile test) are presented here
Rilievo in tempo reale di difetti superficiali su corpi in movimento a velocità elevata con ultrasuoni senza contatto
No full textIl rilievo di difetti superficiali durante le ispezioni periodiche è importante poiché in genere le sollecitazioni sono maggiori in superficie e possono accelerare la crescita delle discontinuità. I difetti superficiali possono essere rilevati utilizzando alcuni metodi tradizionali di controllo non distruttivo, quali correnti indotte, liquidi penetranti, polveri magnetiche ed ultrasuoni. Alcuni tra questi metodi possono essere usati solo in condizioni statiche; gli altri presentano delle limitazioni per l’ispezione dinamica. I progressi recenti nel campo dei sensori ultrasonori senza contatto ci hanno permesso di sviluppare un sistema semplice per l’ispezione in tempo reale di corpi in movimento a velocità elevata. Nel lavoro viene presentata la possibilità di usare, col sistema sviluppato, due metodologie, basate sugli ultrasuoni generati e ricevuti senza contatto con la struttura, per rilevare difetti superficiali su corpi in movimento a 100 km/h, così da poter effettuare l’ispezione anche in servizio. Una metodologia, basata sull’uso di laser e trasduttori senza contatto, utilizza i vantaggi delle onde superficiali generate con il laser; l’analisi viene fatta sull’onda riflessa, creata dall’interazione dell’onda superficiale con il difetto. Lo spessore superficiale ispezionato è selezionabile dalla lunghezza d’onda dell’onda superficiale generata. L’altra metodologia, basata sull’uso di trasduttori senza contatto, sfrutta gli svantaggi della trasmissione degli ultrasuoni all’interfaccia aria/metallo; l’analisi viene fatta sulla diffrazione dell’onda riflessa dalla superficie. L’esecuzione delle ispezioni risulta semplice con entrambe le tecniche. I risultati sperimentali indicano una buona efficienza delle due metodologie proposte per il rilievo, in tempo reale, di difetti superficiali su corpi in movimento ad alta velocità
Hydrogen Re-Embrittlement of Aerospace grade High Strength Steels
No full textHydrogen Re-Embrittlement on anodically coated high strength steels is a relevant risk for aerospace structures due to the possibility of hydrogen uptake during the operative life of the components. AISI 4340 and Maraging 250 unnotched tensile specimens were subjected to SSRT in order to evaluate the influence of test environment on time to failure. Fracture surfaces were examined by SEM analysis to evaluate the degree of embrittlement and to correlate it with hydrogen diffusivity of the tested steels