1,721,011 research outputs found
Influence of specimen size on the mechanical properties of microlattices obtained by selective laser melting
No full textA detailed study of compression tests on lattice structures obtained by selective laser melting with AlSi7Mg powder is presented here. Two different cell topologies have been investigated: the body-centered cubic cell and the face centered cubic cell or 3D Warren structure. Specimens of different volume have been printed in order to investigate the effect of the size on the mechanical response and properties of the structure. Particular attention has been paid to the definition of the test procedure and the analysis of the data to properly characterize the microlattice. No remarkable effect of the specimen size has been found in terms of elastic modulus and yielding stress. On the contrary, the maximum stress and the failure mechanism are influenced by the size of the specimen; for the body-centered cubic cell, a detailed analysis has been performed through digital image correlation of the failure. Test results have been compared with the results of an elasto-plastic simulation performed on a single cell of lattice with periodic boundary conditions, showing a good prediction in terms of elastic modulus and yielding stress
Cyclic response of 3D printed metamaterials with soft cellular architecture: The interplay between as-built defects, material and geometric non-linearity
Full text linkThe paper investigates the cyclic response of soft cellular materials undergoing repeated local instabilities. Our focus is mainly on the coupling between material non-linearities, geometric non-linearity as well as defects induced by 3D printing. Two paradigmatic lattices (triangular and hexagonal), each with its own distinct deformation mode and defect sensitivity, are examined, and the emergence of as-built material and geometric defects in the form of microporosity, strut thickness reduction, and nodal dispersion is studied via computed tomography and optical analyses. Experiments are carried out on the base material and lattice specimens for given cycling strains and cycle ratios. Numerical models are developed to understand the individual role of the main constitutive aspects of the base material, e.g. damage, creep, and visco-elasticity, as well as to assess the role of defects in each architecture. The results show that the activation of local buckling combined with the engagement of material non-linearities has multiple outcomes. It leads to local storage of inelastic strain, which in turn perturbs the lattice geometry after the second cycle and severely impacts the subsequent response, e.g. softening; it reduces the tangent modulus at zero strain; and it also decreases the maximum and minimum cyclic stresses. The detriment is further fueled by geometric deviations caused by 3D printing. Furthermore, a theoretical model is presented to obtain stress bound estimates of the stabilized response, hence offering guidelines for the design of 3D printed soft metamaterials under cycling loading. The paper concludes with a systematic discussion on the coupled role of non-linearities (material and geometry) and defects, and on the accuracy of the numerical and theoretical models herein presented
Response of an aluminium Schwarz triply periodic minimal surface lattice structure under constant amplitude and random fatigue
Full text linkThis paper presents an investigation about fatigue behaviour of an aluminium triply periodic minimal surface lattice structures, printed with Selective Laser Melting. Aim of the paper is to experimentally characterize constant and variable amplitude fatigue strength and to assess if current methodologies for predicting random fatigue strength of solid materials can be extended also to lattice structures, in a homogenized setting. The investigation is complemented by a detailed analysis of samples fracture surface, corroborated by numerical analyses, and a comprehensive discussion on the evolution of the damage observed in the experiments
A comparison of DIC-based techniques to measure crack closure in LCF
No full textCrack closure is one of the most important phenomenon for the comprehension of fatigue behavior of metallic alloys. The effect of mean stress, overloads and variable amplitude loadings can be predicted modeling the crack closure in terms of opening and closing stress levels. Experimentally, the characterization of the crack closure in high and low cycle fatigue has gained large attention in the last fifty years. In fact, the proper detection of the opening and closing levels enhances the definition of a proper crack driving force and its modeling. This work primarily focuses on the measurement of the opening and closing stresses for cracks propagating in low cycle fatigue conditions. High-resolution full-field digital image correlation technique was adopted to track the crack profiles during cyclic loading and different approaches were adopted to analyze the displacement fields extracted with the virtual extensometers. The closure measurements were performed for three metallic alloys, different strain ranges (from elastic to dominant-plastic behaviors) and strain ratio to enlarge the field of applicability of the techniques presented
A critical plane approach for LCF evaluation of gas turbine disks and rotors
No full textIn order to safely increase gas turbine efficiency without issues of early damage and failures, component life evaluation must neither be too conservative nor too optimistic. The method used for designing the parts is supposed to be as accurate as possible, so that the subsequent application of the appropriate statistically defined safety factors will not excessively reduce the component life estimation. In order to achieve such important targets, all phenomena that can be detrimental for fatigue damage, such as multiaxiality and non-proportionality, need to be properly addressed by the Low Cycle Fatigue (LCF) assessment method. In particular, high temperature gas turbine rotating parts are characterized by a superposition of thermal and centrifugal stresses, which act in the same location but along different directions (i.e. multiaxiality) in different moments of the start-up/base-load/shut-down cycle (i.e. non-proportionality). In this framework, critical plane approaches are the most appropriate methods for an accurate and reliable low cycle fatigue life estimation. These methods search the most critical plane where the greatest damage will be accumulated by defining a damage parameter that is calculated for all planes. Among several methods developed in the academic community and considered in this work, the Fatemi-Socie method was identified as the most effective for the studied materials and components. The Fatemi-Socie damage parameter is a combination of in-plane shear strain and normal stress, which accounts for the fact that fatigue damage (of the materials object of the study) is developed along shear strain with an important contribution of tensile normal stress, which is responsible for crack opening and propagation. An extensive testing campaign was performed to identify the most appropriate approach for the studied materials and to accurately fit all the model parameters. The testing campaign therefore included high temperature LCF tests and several types of multiaxial tests (tension-torsion, notched specimens, etc.). Since the search for the critical plane can be demanding from the point of view of computing resources, an in-house software was developed with the scope of reducing the calculation time consistently with the speed required by the industrial design loops. This paper will cover the background and the assumptions behind the development of a complete industrial workflow for the evaluation of LCF life of rotors and disks, prior to its systematic application for a significant number of Ansaldo components
Analysis of strain and stress concentrations in micro-lattice structures manufactured by SLM
No full textPurpose: Additive manufacturing (AM) enables the production of lightweight parts with complex shapes and small dimensions. Recent improvements in AM techniques have allowed a significant growth of AM for industrial applications. In particular, AM is suitable for the production of materials shaped in lattice, which are very attractive for their lightweight design and their multi-functional properties. AM parts are often characterised by geometrical imperfections, residual porosity, high surface roughness which typically lead to stress/strain localisations and decreasing the resistance of the structure. This paper aims to focus on the study of the effects of geometrical irregularities and stress concentrations derived from them. Design/methodology/approach: In this paper, several technique were combined: 3D tomography, experimental tests, digital image correlation and finite elements (FE) models based on both the as-designed and the as-manufactured geometries of lattice materials. The Digital Image Correlation technique allowed to measure local deformations in the specimen during the experimental test. The micro-computed tomography allowed to reconstruct the as-manufactured geometries of the specimens, from which the geometrical quality of the micro-structure is evaluated to run FE analyses. Findings: Experimental and numerical results were compared by means of a stress concentration factor. This factor was calculated in three different specimens obtained from three-different printing processes to compare and understand their mechanical properties. Considering the as-designed geometry, it is not possible to model geometrical imperfections, and a FE model based on an as-manufactured geometry is needed. The results show that the mechanical properties of the printed samples are directly related to the statistical distribution of the stress concentration factor. Originality/value: In this work, several techniques were combined to study the mechanical behaviour of lattice micro-structures. Lattice materials obtained by different selective laser melting printing parameters show different mechanical behaviours. A stress concentration factor can be assumed as a measure of the quality of these mechanical properties
Crack-closure simulations of Ni-based super-alloy polycrystal, a comparison between experiments and crystal plasticity
No full textFatigue crack growth in polycrystalline specimens was investigated considering a Ni-based superalloy, Haynes 230. The analyses were carried out through a comparison between experiments and numerical simulations. Crack closure experiments were conducted with digital image correlations: the crack closure was estimated by measuring the relative displacements of crack flanks. The experimental results were compared with crystal plasticity finite element simulations of randomly generated polycrystalline structures, in which the crack growth was simulated through a node release technique. The simulations provided a good estimation of the experimental results in terms of crack opening levels for different loading conditions
Multiaxial static strength of a 3D printed metallic lattice structure exhibiting brittle behavior
Full text linkThis paper focuses on numerical the prediction of multiaxial static strength of lattice structures. We analyze a body-centered cubic cell printed with Selective Laser Melting in AlSi10Mg aluminum alloy. Parent material is experimentally characterized, and the Gurson-Tveergard-Needleman (GTN) damage model is calibrated to predict failure in numerical simulations. The GTN model is used to predict failure of the lattice structures exhibiting brittle localized fracture, and it is validated through static tests. The results of experimental tension/compression monotonic tests on lattice samples are compared with the results of numerical simulations performed on as-built geometry reconstructed by X-ray computed tomography, showing a good correlation. Combining the damage model with computational micromechanics, multiaxial loading conditions are simulated to investigate the effective multiaxial strength of the lattice material. Yielding and failure loci are found by fitting a batch of points obtained by some multiaxial loading simulations. A formulation based on the criterion proposed by Tsai and Wu (1971) for anisotropic materials provides a good description of yielding and failure behavior under multiaxial load. Results are discussed, with a specific focus on the effect of as-built defects on multiaxial strength, by comparing the resistance domains of as-manufactured and as-designed lattices
Strain localizations in notches for a coarse-grained Ni-based superalloy: Simulations and experiments
Full text linkAlloys used for turbine blades have to safely sustain severe thermomechanical loadings during service such as, for example, centrifugal loadings, creep and high temperature gradients. For these applications, cast Ni-based superalloys characterized by a coarse-grained microstructure are widely adopted. This microstructure dictates a strong anisotropic mechanical behaviour and, concurrently, a large scatter in the fatigue properties is observed. In this work, Crystal Plasticity Finite Element (CPFE) simulations and strain measurements performed by means of Digital Image Correlations (DIC) were adopted to study the variability introduced by the coarse-grained microstructure. In particular, the CPFE simulations were calibrated and used to simulate the effect of the grain cluster orientations in proximity to notches, which reproduce the cooling air ducts of the turbine blades. The numerical simulations were experimentally validated by the DIC measurements. This study aims to predict the statistical variability of the strain concentration factors and support component design
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