Gruppo Italiano Frattura (IGF)
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Combining experimental and numerical analysis to estimate stress fields along the surface crack front
No full textCombining experimental and computational method for determination of the singular and the nonsingularstress terms along the front of the 3D surface crack is proposed. Evaluation of the terms is based oncomprehensive comparison between deformation responses (for measurement points on the surface) obtainedexperimentally and from numerical solutions of the corresponding boundary problem of solid mechanics. Theproposed approach allows carrying out an adequate and a comprehensive assessment of stress fields in thevicinity of the surface crack front
High temperature initiation and propagation of cracks in 12%Cr-steel turbine disks
No full textThis work aims to study the crack propagation in 12%Cr steel for turbine disks. Creep CrackGrowth (CCG) tests on CT specimens have been performed to define the proper fracture mechanics whichdescribes the initiation of the crack propagation and the crack growth behaviour for the material at hightemperature. Results have been used to study the occurrence of crack initiation on a turbine disk at the extremeworking temperature and stress level experienced during service, and validate the use of C* integral incorrelating creep growth rate on the disk component, in case C* is numerically calculated through FEM analysisor calculated by the use of reference stress concept
On numerical integration for effective stress assessment at notches
No full textThis paper presents a numerical method for non-local stress assessment by means of a general FEtool and the local stress field. Unlike usual calculations by means of a numerical PDE solver, a more generalnumerical integration is used. Different solutions are compared theoretically and numerically by evaluating theresults obtained by two different FEM commercial software. The application of the non-local tension field isapplied to the strength assessment of notches, welded joints and cracks
Numerical study of fracture arrest on snow cover
No full textUnder the hypothesis of a perfectly brittle phenomenon, avalanche triggering can be investigated numerically by means of Linear Elastic Fracture Mechanics (LEFM). Since, however, the real phenomenon is intrinsically dynamical, another aspect to investigate is represented by dynamic fracture propagation. In this paper, we model dynamic crack propagation into a dry snow slab and we investigate the possibility to arrest the crack propagation through the presence of weak zones distributed along the extension of the snow slope. Assuming that the weak layer is almost collapsed, we simulate the efficiency of artificial voids in the slab to arrest fracture propagation, into the framework of Dynamical Fracture Mechanics. We put forward here a new philosophy for the use of artificial discontinuities (void) into the snowpack able to perform as crack arresters distributed along the snow slope area: the target is to split a large avalanche slab into smaller slabs, causing small avalanches to propagate with less catastrophic effects
Ductile cast irons: microstructure influence on fatigue crack propagation resistance
No full textMicrostructure influence on fatigue crack propagation resistance in five different ductile cast irons(DCI) was investigated. Four ferrite/pearlite volume fractions were considered, performing fatigue crackpropagation tests according to ASTM E647 standard (R equals to 0.1, 0.5 and 0.75, respectively). Results werecompared with an austempered DCI. Damaging micromechanisms were investigated according to the followingprocedures:- “traditional” Scanning Electron Microscope (SEM) fracture surfaces analysis;- SEM fracture surface analysis with 3D quantitative analysis;- SEM longitudinal crack profile analysis- Light Optical Microscope (LOM) transversal crack profile analysis
The interface between metallurgy and mechanics in material performance
No full textThis paper considers an important topic, and one that is often poorly understood or misinterpreted, but which is a determining factor in many aspects of the service performance of metals (and other materials). Engineering components and structures must, of necessity, provide a bridge between the macroscopic, homogeneous and generally continuum aspects of applied load and displacement, and the microscopic, heterogeneous and often non-continuum reality of material structure and behaviour. This bridge can take the form of a genuine interface between material and environment, e.g. at a surface, or can be a virtual one where the differing philosophies of design have to be merged. The interface has particular importance in circumstances where environmental influences have a key role in determining performance characteristics (e.g. creep, environmentally-assisted cracking, or corrosion), where performance is dominated by fatigue or fracture, where welding is used to join components, or where tribology plays a role. The paper focuses on the problems associated with cracking and uses case study examples drawn from engineering practice to illustrate the role of metallurgical factors in mechanical performance of materials
Characterisation of crack tip fields under non-uniform fatigue loading
No full textThe paper analyses previously reported work, which uses digital image correlation to measurefatigue crack closure. As well as determining crack opening loads, the information on crack shape may be usedto estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. Anumber of specimens were subjected to single overload cycles, which produced a significant retardation in crackgrowth rate. The method previously applied to the analysis of constant amplitude loading is here used toanalyse the single overload case. The stress intensity factor history is found to be very different in the two casesand the consequences of this observation for analysis of fatigue crack propagation are discussed
Research of the effectiveness of mechanical testing methods with analysis of features of destructions and temperature effects
No full textIn this paper is carried out the comparative analysis of effectiveness of test methods ofdetermination of stiffness and strength properties of highly filled unidirectional fiberglass (Direct "E" roving 0.7- orthophthalic polyester resin 0.3) via tensile testing along the reinforcement and three-point bending testing atseveral bases. The necessity of deviation from standard procedures is substantiated. Deformation and failurefeatures of the material under quasi-static loading, as well as at low and high temperatures, are shown
L’applicazione della diffrattometria dei raggi X per l’analisi del cedimento dei componenti meccanici
No full textX-ray diffraction is a well-known experimental technique for measuring residual stresses inmetallic materials. If XRD is applied to the fracture surface of a broken part it becomes a fractographicaltechnique, that is to say that it is possible to relate the results of the measures to the loading condition thatlead a component to fail. In this paper, after an introduction about the technique, XRD fractography is appliedto a fatigue failed diesel engine crankshaft. It was possible to determine the load that lead the crankshaftto fail and to evidence some original aspects about the application of this technique to real machineparts
Development of a formalism of movable cellular automaton method for numerical modeling of fracture of heterogeneous elastic-plastic materials
No full textA general approach to realization of models of elasticity, plasticity and fracture of heterogeneousmaterials within the framework of particle-based numerical methods is proposed in the paper. It is based onbuilding many-body forces of particle interaction, which provide response of particle ensemble correctlyconforming to the response (including elastic-plastic behavior and fracture) of simulated solids. Implementationof proposed approach within particle-based methods is demonstrated by the example of the movable cellularautomaton (MCA) method, which integrates the possibilities of particle-based discrete element method (DEM)and cellular automaton methods. Emergent advantages of the developed approach to formulation of manybodyinteraction are discussed. Main of them are its applicability to various realizations of the concept ofdiscrete elements and a possibility to realize various rheological models (including elastic-plastic or visco-elasticplastic)and models of fracture to study deformation and fracture of solid-phase materials and media.Capabilities of particle-based modeling of heterogeneous solids are demonstrated by the problem of simulationof deformation and fracture of particle-reinforced metal-ceramic composites