Gruppo Italiano Frattura (IGF)
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Study of spatial-time inhomogeneity of serrated plastic flow Al-Mg alloy: using DIC-technique
No full textThe aim of the present paper is the investigation of temporal instabilities and spatial localizationdue to the Lüders behavior, the Portevin–Le Chatelier effect and the shoulder or necking effect during uniaxialtension tests of aluminum-magnesium alloy. This paper presents the brief description of the test procedure andexperimental results of carrying out research by the combined use of a servo-hydraulic biaxial test systemInstron 8850 and a non-contact 3-D digital image correlation measurement system Vic-3D. The digital imagecorrelation is a highly effective computer-vision-based technique, which provides estimation of thedisplacement and strain fields on specimen surface by matching the reference subsets in the undeformed image(before loading) with the target subsets in the deformed images (captured during test). The evolution ofinhomogeneous axial strain and axial strain rate fields has been illustrated for each stage of material’sdeformation. To estimate the kinematics of serrated or jerky flow due to the strain bands propagation, the strainversus time curves and strain diagrams are given here. The experimental results show the recurrence in thestrain distribution leveling along the specimen gauge. The changing between the macroscopic localization of theplastic flow, namely the running of the Lüders and PLC bands and the recovery of strain field homogeneity, hasbeen observed
Pearlitic ductile cast iron: damaging micromechanisms at crack tip
No full textDuctile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainlydepending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grainssize and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defectspresence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steelswith analogous performances are the main DCIs advantages.In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatiguetests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during thetests (“in situ” tests) and identifying different damaging micromechanisms.In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (bymeans of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing theinteraction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules).On the basis of experimental results, and considering loading conditions and damaging micromechanisms, theapplicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferriticDCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role
Numerical experiments in 2D variational fracture
No full textIn the present work we present some results of numerical experiments obtained with a variationalmodel for quasi-static Griffith-type brittle fracture. Essentially the analysis is based on a recent formulation byFrancfort and Marigo the main difference being the fact that we rely on local rather than on globalminimization. Propagation of fracture is obtained by minimizing, in a step by step process, a form of energythat is the sum of bulk and interface terms. To solve the problem numerically we adopt discontinuous finiteelements based on variable meshes and search for the minima of the energy through descent methods. We use asort of mesh dependent relaxation of the interface energy to get out of small energy wells. The relaxationconsists in the adoption of a carefully tailored cohesive type interface energy, tending to the Griffith limit as themesh size tends to zero
Impact loading of a space nuclear powerplant
No full textPreferred formulation of the problem in two space dimensions are described for solving the threefundamental equations of mechanics (conservation of mass, conservation of momentum, and conservation ofenergy). Models of the behavior of materials provide the closure to the three fundamentals equations forapplications to problems in compressible fluid flow and solid mechanics. Models of fracture and damage aredescribed. A caloric model of the equation of state is proposed to describe thermodynamic properties of solidmaterials with the phase transitions. Two-dimensional problems of a high-velocity impact of a space nuclearpropulsion system reactor are solved. High-velocity impact problems of destruction of reactor are solved for thetwo cases:1) at its crash landing on the Earth surface (the impact velocity being up to 400 m/s);2) at its impact (with velocity up to 16 km/s) with the space debris fragments
The variational theory of fracture: diffuse cohesive energy and elastic-plastic rupture
No full textThis communication anticipates some results of a work in progress [1], addressed to explore the efficiency of the diffuse cohesive energy model for describing the phenomena of fracture and yielding. A first local model is partially successful, but fails to reproduce the strain softening regime. A more robust non-local model, obtained by adding an energy term depending on the deformation gradient, describes many typical features of the inelastic response observed in experiments, including strain localization and necking. Fracture occurs as the result of extreme strain localization. The model predicts different fracture modes, brittle and ductile, depending on the analytical form of the cohesive energy function
An experimental study on flexural strength enhancement of concrete by means of small steel fibers
No full textCost effective improvement of the mechanical performances of structural materials is an importantgoal in construction industry. To improve the flexural strength of plain concrete so as to reduce constructioncosts, the addition of fibers to the concrete mixture can be adopted. The addition of small steel fibers withdifferent lengths and proportion have experimentally been analyzed in terms of concrete flexural strengthenhancement. The main objectives of the present study are related to the evaluation of the influence of steelfibers design on the increase of concrete flexural characteristics and on the mode of failure. Two types of beamshave been investigated. The force level, deflection and time to failure of beams have been measured. The shearcrack, flexural crack and intermediate shear-flexural crack have been studied. The steel fiber content controlledcrack morphology. Flexural strength and time to failure of fiber reinforce concrete could be further enhanced if,instead of smooth steel fibers, corrugated fibers were used
A four-parameters model for fatigue crack growth data analysis
No full textA four-parameters model for interpolation of fatigue crack growth data is presented. It has beenvalidated by means of both data produced by the Authors and data collected from Literature. The proposedmodel is an enhanced version of a three-parameters model already discussed in a previous work that has beensuitably modified in order to overcome some drawbacks raised when applied to a quite wider experimental dataset. Results of validation study have also revealed that the new model, besides interpolating accurately crackgrowth data, allows to identify the presence of anomalies in the data sets. For this reason, by a suitable filter tobe chosen depending on the size and number of anomalies, it can be used to remove them and obtain sigmoidalcrack propagation curves smoother than those obtained when the current analysis techniques are used. In theend, possible model parameters correlations are analysed
Distribuzioni di tensione per intagli soggetti a torsione in condizioni elastiche ed elastoplastiche
No full textIl lavoro riporta delle soluzioni analitiche in forma chiusa per le distribuzioni di tensione generate da intagli circonferenziali in componenti assialsimmetrici soggetti a torsione, in condizioni lineari elastiche ed elastoplastiche. Il problema teorico in condizioni lineari elastiche è stato impostato e risolto utilizzando la teoria dei potenziali nel dominio complesso e una serie di opportuni sistemi di riferimento in coordinate curvilinee, evitando l’uso di mappature conformi. Le soluzioni proposte hanno un ampio range di applicabilità, in termini di dimensioni e forma dell’intaglio e di diametro dell’albero. Il problema elastoplastico è stato invece risolto utilizzando la tecnica delle trasformazioni odografiche, al fine di rendere lineari le equazioni nonlineari fondamentali del problema.
Il contributo rappresenta la sintesi di una serie di lavori più ampi a cura degli stessi autori
Strength on cut edge and ground edge glass beams with the failure analysis method
No full textThe aim of this work is the study of the effect of the finishing of the edge of glass when it has astructural function.Experimental investigations carried out for glass specimens are presented. Various series of annealed glass beamwere tested, with cut edge and with ground edge. The glass specimens are tested in four-point bendingperforming flaw detection on the tested specimens after failure, in order to determine glass strength. As a result,bending strength values are obtained for each specimen.Determining some physical parameter as the depth of the flaw and the mirror radius of the fracture, after thefailure of a glass element, it could be possible to calculate the failure strength of that.The experimental results were analyzed with the LEFM theory and the glass strength was analyzed with astatistical study using two-parameter Weibull distribution fitting quite well the failure stress data.The results obtained constitute a validation of the theoretical models and show the influence of the edgeprocessing on the failure strength of the glass. Furthermore, series with different sizes were tested in order toevaluate the size effect
Experimental investigation on possible dependence of plastic zone size on specimen geometry
No full textIn this investigation the extent of the plastic zone size ahead of a crack-tip in single edge notched tension (SENT), compact tension (CT) specimens has been examined experimentally by micro-hardness technique and by elastic-plastic finite element analyses at different applied load levels. The magnitudes of the plastic zone size (PZS), rp ahead of crack-tip in the investigated specimens have been compared using normalized J-integral (J/a?y, where, a-crack length and ?y-yield stress of the material). The results show the dependence of PZS on specimen geometry due to varied in-plane crack-tip constraint. The results also demonstrate that the existing analytical models do not explain the experimental results of PZS satisfactorily