Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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Determination of Stress Intensity Factors and J integral based on Digital Image Correlation
Get PDFDigital image correlation (DIC) is a technique in experimental mechanics to acquire full-field measurement data of displacements and deformations from the surface of specimens or components. Especially for the investigations of cracks it provides additional benefits. The actually present deformation field in the vicinity of the crack tip can be obtained which directly reflects for example crack closure effects or plasticity. Against this background the paper summarizes a procedure to compute the J integral and the stress intensity factors KI and KII based on DIC data. For this purpose the J and interaction integral are computed as line and domain integrals. Through experiments it is shown that the domain integral is less affected by scatter of the DIC data. Furthermore, the specific domain, facet sizes and facet distances slightly influence the results
Steady plastic wave fronts and scale universality of strain localization in metals and ceramics
Get PDFMechanisms of structural relaxation are linked with the metastability of nonequilibrium potential of solid with defects and the generation of collective modes of defects responsible for the plastic strain and damage localization. It is shown that spatial-temporal dynamics of collective modes (auto-solitary and blow-up dissipative structures) provide the anomalous relaxation ability of nonlinear system “solid with defects” in the conditions of the specific type of criticality – structural-scaling transition. These modes have the nature of self-similar solutions of evolution equations for damage parameter (defect-induced strain) and represent the “universality class” providing the four power law for a steady plastic front, splitting of an elastoplastic shock wave front, and elastic precursor decay kinetics. Wide-range constitutive equations reflecting the linkage between defect-induced mechanisms and structural relaxation are used in the numerical simulation for shock wave loading of metals and ceramics in the comparison with experiments
Corrosion’s impact on wire rope strand response – Comparison with a theoretical predictive model
Get PDFIn this work, we investigate the behavior of wire ropes subject to corrosion damage. The method is based on accelerated corrosion. Decreasing ultimate force and a loss in rigidity was observed from the static tensile tests made on the corroded strands. Otherwise, a predictive model was developed to evaluate the residual ultimate force at a time t in function of the initial diameter, the residual diameter at time t and the ultimate force of the original strand. Thus, experimental and predictive residual force curves are drawn in function of the strand’s fraction of life. From the good correspondence obtained, the proposed method can be applied by engineers to assess the possibility of maintaining structures using wire ropes in service. Indeed, when the applied force on the wire rope approaches its residual bearing force determined by a measure of the residual diameter, the wire rope removal is mandatory
Estimation of tensile mechanical parameters of existing masonry through the analysis of the collapse of Volterra’s urban walls
Get PDFIn this work, numerical analyses on the Volterra’s ancient walls are performed to understand the causes of a collapse occurred after an extreme rainfall (2014) and to identify the mechanical parameters that most influenced it. The stretch of collapsed wall is modelled with finite elements by distinguishing the material composing the multi-leaf wall. Non-linear static analyses are carried out with the DIANA software considering different levels of hydraulic head up to reach the collapse of the structure. The impact on the failure modes of the involved parameters, such as tensile strength, fracture energy and shear strength, are discussed. Tensile strength is found as the most relevant parameter in the failure mode. Finally, a 3-dim model is presented, and the results show a very good agreement with the failure mode actually occurred in terms of both failure modes and displacement fields. The safety evaluation is performed in the as-built and the retrofitted configuration, consisting in the wall restrained by tie-rods. The results show a significant increasing of the safety level of the retrofitted configuration showing a considerable improvement not only in terms of limit equilibrium but also in terms of structural capacity of the wall
Evaluation on the fatigue behavior of sand-blasted AlSi10Mg obtained by DMLS
Get PDFIn the present paper, fatigue tests were performed on sand-blasted AlSi10Mg samples produced by Direct Metal Laser Sintering (DMLS). The effect of sand-blasting on surface properties was evaluated by roughness and residual stresses measurements, together with morphological analysis, in comparison with as-built condition. An evident improvement of surface finishing was observed after sand-blasting, which also leaded to the presence of compressive residual stress on the external surface of samples, as revealed by XRD2 measurements. Furthermore, defects analysis allowed the identification of a uniform distribution of porosities in the cross section in terms of number of defects, while larger porosities seem more abundant close to the surface. It was found that the tested material exhibits good fatigue resistance, supporting the positive role of sand-blasting as a simple post-processing treatment. Superficial defects are the preferential crack initiation sites, as demonstrated by SEM analysis of fracture surfaces
Characterization of stress-strain state in gas turbine engine compressor disc taking into account damage accumulation
Get PDFThe stress-strain state analysis of the gas turbine engine compressor disk was carried out taking into account damage accumulation under various loading conditions in the temperature range.
Characterization of the constraint effects was performed using the local stress triaxiality h, TZ–factor and In-factors for the various crack sizes and different operation conditions.
The distributions of elastic, plastic and creep stress intensity factors were determined by numerical calculation. The results were obtained for several crack front positions, temperature values and disk rotation angular velocity.
The differences in the behavior of elastic and nonlinear fracture resistance parameters depending on loading conditions, crack geometry and temperature are demonstrated. The nonlinear stress intensity factors are preferred as fracture resistance parameters of materials and structures
Stress intensity factor for small embedded cracks in weldments
Get PDFIn the present work, the stress intensity factor (SIF) of embedded small cracks placed at the weld toe is calculated by means of two procedures based on the Oore-Burns integral. In the first approach, the defect is considered as a circular disk and the SIF is evaluated by means of the Oore-Burns weight function. By taking advantage of a suitable change of variable, the singularity of the weight function on the crack border can be removed. In this way, the numerical evaluation of the SIF is possible without the use of specific integration algorithms, although the nominal stress field becomes singular when the crack approaches a V-sharpe notch. As an example, the obtained equations are applied to a defect located in the neighbourhood of a weld toe with an opening angle of 135 degrees under mode I loading. Subsequently, for a crack similar to a star domain with a border expressed by means of the Fourier series, the SIF is given by means of an explicit equation based on the Oore-Burns weight function
Experimental investigation of fatigue crack initiation from notch for 2024 T351 Al-alloy
No full textIn this investigation, fatigue criterion was established to predict crack-initiation at the tip of a notch in aged hardening aluminum alloy. This criterion was used to predict the residual lifetime in aeronautical structures using concept of local stress at notch and subjected to constant amplitude loading characterized by mean stress. Charpy V-notch specimens were taken from sheet plate and the notch radius accurately machined at value of 0.2mm. The local stresses were determined numerical and validated analytically. In experimental investigation, the V-notched specimens were loaded in four point bending fatigue with a load ratio R=0.1 and variation in amplitude loading
Analytical and numerical analysis on the collapse modes of least-thickness circular masonry arches at decreasing friction
Get PDFDeparting from pioneering Heyman modern rational investigations on the purely-rotational collapse mode of least-thickness circular masonry arches, the hypothesis that joint friction shall be high enough to prevent inter-block sliding is released. The influence of a reducing Coulomb friction coefficient on the collapse modes of the arch is explicitly inspected, both analytically and numerically, by tracing the appearance of purely-rotational, mixed sliding-rotational and purely-sliding modes. A classical doubly built-in, symmetric, complete semi-circular arch, with radial joints, under self-weight is specifically considered, for a main illustration. The characteristic values of the friction coefficient that limit the ranges associated to each collapse mode are first analytically derived and then numerically identified, with self-consistent outcomes. Explicit analytical representations are provided to estimate the geometric parameters that define the limit equilibrium states of the arch, specifically the minimum thickness to radius ratio, at reducing friction. These formulas, starting from the analysis of classical Heymanian instance of purely-rotational collapse, make new explicit reference to the mixed sliding-rotational collapse mode, arising within a narrow range of limited friction coefficients (or friction angles). The obtained results are consistently compared to existing numerical ones from the competent literature
NURBS-based kinematic limit analysis of FRP-reinforced masonry walls with out-of-plane loading
Get PDFA three-dimensional (3D) general upper-bound limit analysis procedure for the determination of the collapse load of out-of-plane loaded masonry walls with Fiber Reinforced Polymer (FRP) reinforcement strips is presented. The geometry of a given FRP reinforced masonry wall of arbitrary shape is represented by its Non-Uniform Rational B-Spline (NURBS) description in the three-dimensional Euclidean space. The NURBS parameter space is partitioned by means of a number of possible fracture lines and the original reinforced wall geometry is subdivided into an initial set of rigid elements, accordingly. An upper-bound limit analysis formulation, accounting for the main characteristics of both masonry material and FRP reinforcement by means of homogenization techniques, is deduced. Internal dissipation is allowed along element edges only and the effect of vertical loads and membrane stresses is considered as well. Numerical experiments show that a good estimate of the load bearing capacity is obtained provided that the initial arrangement of yield lines is adjusted by means of a suitable Genetic Algorithm (GA)