1,721,063 research outputs found
Microstructural, multilevel simulation of notch effect in ferritic ductile cast iron under low cycle fatigue
Full text linkTriaxiality of stress affects damage and failure of ductile metals. In mechanical components, triaxiality increases in the proximity of a notch, or, at the microstructural level, due to inclusions or voids. In this work, the effect of triaxiality on the LCF of ductile cast iron is investigated by a multilevel approach, homogenizing the response of a microstructural model which feeds a notched specimen. Moving from micro- to macro-scale, results indicate that triaxiality shorten the fatigue life. Thus, the notch effect on fatigue life of cast iron can be explained in terms of the combined effects of microstructure and applied triaxiality
Hybrid joints: Evaluation of the increase of the mechanical performances with respect to simple joints
No full textHybrid joints allow to bring together the advantages of bonded joints (i.e. lightweight design, cost reduction and higher energy absorption in case of impact loading) with the confidence, the high specific strength and well know production process of traditional mechanical joining technologies like welding, riveting and clinching. In this way, higher performances in terms of strength, stiffness and energy absorption are achieved with respect to simple adhesive, welded or fastened joints, while costs can be reduced with respect to welding or fastening and the manufacturing process is facilitated with respect to adhesive bonding. Many works deal with the static [1-4] and fatigue [5, 6] characterization of hybrid joints, and they point out higher mechanical properties in comparison with simple joints. The reason is found in a synergistic effect of the joining techniques [7] and in a more favourable stress distribution [8, 9]. In this work an extensive experimental campaign was carried out in order to compare the strength of weld-bonded, clinch-bonded and rivet-bonded joints with that of the related non-hybrid joints, evaluating also the influence of geometrical and environmental factors. The experimental analysis was conducted using the Design of Experiments (DoE) methodology, taking the maximum load (F max), the stiffness (K) and the energy absorption prior to the failure (E n) as objectives
Microstructure-based RVE modeling of the failure behavior and LCF resistance of ductile cast iron
Full text linkIn this work the failure behavior of ductile cast iron microstructure subjected to tensile and low-cycle fatigue loadings is simulated by a 3-D, FE Reference Volume Element approach. A fully ferritic matrix is considered as representative of the low-hardness, high-ductility material class of nodular cast irons. Plastic flow potential rule, ductile and low cycle fatigue damage models are implemented at the micro-scale for the matrix constituent in conjunction with nonlinear cyclic hardening laws, and periodic boundary conditions are imposed over the RVE at the meso-scale. Different values of triaxiality are imposed. Numerical results confirm experimental findings of the behavior at the meso-scale and correctly predict the LCF lifetime, driving the interpretation of inner strain distribution, voids interaction and triaxiality effects on failure mechanisms
Assessment of load ratio effect on fatigue crack growth using partial crack closure
No full textAccording to a damage-tolerant approach, fatigue life becomes virtually infinite if the ΔK due to service loads is lower than the threshold for fatigue crack growth (FCG). For this motivation, a proper evaluation of ΔKth is very important. On the other hand, for a given material this parameter appears to be strongly influenced by material microstructure and R=Kmin/Kmax ratio. At high propagation velocity this influence is explained by classical closure concept introduced by Elber, but this approach does not always work at threshold. Recently, new methods to account for microstructure and R-ratio effects at low propagation rates were proposed. The aim of this work is to evaluate the effectiveness of those methods testing very different materials: (i) a case hardened steel and (ii) an aluminium matrix particulate composite
Erratum to: A procedure for the simulation of fatigue crack growth in adhesively bonded joints based on the cohesive zone model and different mixed-mode propagation criteria
No full textMicromechanical modeling of the effect of stress triaxiality on the strain to failure of ductile cast iron
No full textThis paper presents modeling of the tensile and failure behavior of different nodular cast iron microstructures under variable stress triaxialities. The model is based on a Representative Volume Element (RVE) approach, which reproduces periodic stochastic distributions of nodules within a 3-D cell. Three cast iron matrices, from fully ferritic to fully pearlitic, are considered as representing the various nodular cast iron classes in relation to strength and ductility. Ductile damage and shear damage models are used for ferrite and pearlite, respectively. Several values of stress triaxiality are applied within the RVE. It is found that numerical results support experimental findings in relation to the local strain distribution and damage initiation and accumulation, reproducing macroscopic tensile responses with good approximation. As highlighted by experiments, triaxiality is found to have a strong effect on material ductility and void volume fraction growth
Influence of high frequency on the fatigue life of metallic single lap joints
Full text linkPolymer adhesives are known to exhibit time dependent mechanical behaviour, that becomes more and more evident approaching the glass transition temperature, Tg. When subjected to fatigue, bonded joint lifetime can be therefore affected by the loading frequency and amplitude. According to the literature, the influence of the frequency on the fatigue behavior manifests as a rise of the temperature in the adhesive due to the hysteretic heating, which is related in turn to Tg and to the viscoelastic properties of the adhesive. In this paper, cyclic loading was applied to single lap shear joints until failure at frequencies in the range of 70-90 Hz, that are little explored by the existing literature. The aim is twofold: on one side, to investigate the possibility to speed up fatigue tests in comparison to tests performed in the range of 7-9 Hz; on the other side, to explore the possibility to generate stress-life data in a range of number of cycles between 106 and 108, therefore two order of magnitude higher then usual, without exceeding with the time required for the tests. The joints were made with both aluminum and stainless-steel substrates bonded with a structural epoxy adhesive, to manufacture a Single Lap Joint (SLJ). Two loading ratios R (respectively 0.1 and 0.4, defined as the minimum over the maximum force of the fatigue cycle) and different load ranges were applied. Digital Image Correlation (DIC) technique was used to assess the presence of creep strains. A preliminary investigation consisting in a Dynamic Mechanical Analysis (DMA) of bulk adhesive for the range of frequency of interest, and in the measurement of the temperature of the adhesive layer during some selected fatigue tests was carried out. No significant changes of the viscoelastic properties of the adhesive were found for the frequency of interest, and, at the same time, temperature mea-surements revealed that the temperature increased by a few degrees, remaining in any case far from the glass transition temperature. The test results showed that only small effect due to the application of a high frequency cyclic loading on the fatigue life apparently occurs for tests carried out at the lowest load ratio, in particular when the applied loads were relatively low, and the number of cycles at failure relatively high. On the opposite, the results of tests carried out at the highest load ratio are affected by the loading frequency, and it has been related to the presence of significant creep deformation within the adhesive layer
Optimal design of shape memory alloy composite under deflection constraint
Full text linkShape-adaptive or morphing capability in both aerospace structures and wind turbine blade design is regarded as significant to increase aerodynamic performance and simplify mechanisms by reducing the number of moving parts. The underlying bistable behavior of asymmetric cross-ply composites makes them a suitable candidate for morphing applications. To date, various theoretical and experiential studies have been carried out to understand and predict the bistable behavior of asymmetric laminates and especially the curvature obtained in their stable configurations. However, when the bi-stable composite plate is integrated with shape memory alloy wires to control the curvature and to snap from a stable configuration to the other (shape memory alloy composite, SMAC), the identification of the design parameters, namely laminate edge length, ply thickness and ply orientation, is not straightforward. The aim of this article is to present the formulation of an optimization problem for the parameters of an asymmetric composite laminate integrated with pre-stressed shape memory alloys (SMA) wires under bi-stability and a minimum deflection requirement. Wires are modeled as an additional ply placed at the mid-plane of the composite host plate. The optimization problem is solved numerically in MATLAB and optimal design variables are then used to model the SMAC in ABAQUSTM. Finite element results are compared against numerical results for validation
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