Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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Effect of cast part size on the microstructure and mechanical properties of a bainitic High-Carbon and High-Silicon Cast Steel
Get PDFThis study aims at assessing the impact of casting size on the bainitic transformation, resulting microstructures, and tensile properties of a high-carbon, high-silicon steel austempered at different temperatures. The casting size was analyzed by using Y blocks of two different thicknesses. The microsegregation, a common occurrence in cast parts, leads to different bainitic transformation rates at the microscopic scale. Specifically, interdendritic areas with higher alloying contents exhibit a slower transformation, resulting in a lower degree of transformation and a higher amount of blocky austenite. Despite differences in solidification structure and distribution of alloying elements, samples obtained from the thinner and thicker Y blocks yield comparable transformation times and mechanical properties, leading to enhanced uniformity in the mechanical behavior of the entire component. However, it is essential to ensure that the bainitic transformation is completed to minimize the detrimental effects of microsegregation in these cast steel components.
The presence of very fine microstructures results in ultra-high strength with low ductility cast steel
Revisiting classical concepts of Linear Elastic Fracture Mechanics - Part II: Stretching finite strips weakened by single edge parabolically-shaped notches
Get PDFThis is the second part of a short three-paper series, aiming to revisit some classical concepts of Linear Elastic Fracture Mechanics. Being the intermediate step of the analysis between infinite domains (discussed in Part-I) and finite bodies (that will be discussed analytically in the third part of the series), the present part offers an alternative theoretical approach for the confrontation of problems dealing with both infinite and finite bodies with geometrical discontinuities. The method is here applied to a stretched, single-edge notched strip. Assuming that the strip is made of a linearly elastic and isotropic material, the complex potentials technique is used. The solution is achieved by extending Mushkelishvili’s procedure, for the confrontation of the problem of an infinite perforated plane. Closed form, full-field formulae are obtained for the stresses all over the notched strip. Using these formulae, the stress concentration factor at the base (tip) of the notch is quantified and studied in terms of the geometrical features of the notch and its dimensions relatively to the respective ones of the strip. The stress distributions plotted along characteristic loci, resemble closely, from a qualitative point of view, the respective ones provided by well-established analytical solutions. Preliminary numerical analyses in progress provide results in very good agreement with those of the present analysis
A method for rapid estimation of residual stresses in metal samples produced by additive manufacturing
Get PDFThe mechanical methods for measuring residual stresses typically rely on so-called destructive techniques where some stress components can be determined based on part deflection after material removal (cutting, etching, drilling, etc.). While these methods don't provide a comprehensive representation of residual stresses within the entire part, they can be readily applied in most manufacturing labs. In this study, we propose an efficient method for determining residual stress within additively manufactured cylindrical samples of stainless steel. The method is based on the assumption of a relation between the axial component of residual stress (normal to cross-section) and the cylinder radius. The general form of this relation is proposed based on data from numerical simulations using linear, parabolic or piecewise approximations. The parameters for the proposed relation are defined using equilibrium equations for total force and moment. The proposed method relies on an experiment with a mechanical cut along the cylinder. Consequently, the deflection of the cylinder halves after the cut allows for obtaining the equivalent bending moment
Effectiveness of Partial Wrapping of Stainless-Steel Wire Mesh on Compression Behavior of Concrete Cylinders
Get PDFPartial confinement can provide sufficient reinforcement to enhance the compressive strength and ductility of concrete with lesser confining material. This paper presents the results of an axial compression test conducted on eighteen plain concrete cylinders of 150 mm diameter and 300 mm height partially confined with a Stainless-Steel Wire Mesh (SSWM) strip of different widths. The study included two specimens without wrapping, two fully wrapped specimens, and others wrapped with two SSWM strips of varying widths at both the ends of concrete cylinder. The strain on SSWM up to failure is measured to understand the effectiveness of lateral confining pressure on the behaviour of concrete cylinders. The peak axial compressive strength and corresponding strain of unconfined and SSWM-confined concrete cylinders are compared. The result shows a significant increase in peak confined compressive stress as compared to an unconfined concrete cylinder. However, the confinement efficiency is reduced when the height of the unconfined region exceeds the diameter of the cylinder, and significant strain localisation is detected within the unwrapped region. Based on experimental investigation, a confinement coefficient is suggested for a partial wrapping of SSWM on the concrete cylinder
A Numerical Study on Predicting Bond-Slip Relationship of Reinforced Concrete using Surface Based Cohesive Behavior
Get PDFOverall structural integrity and load transfer between concrete and reinforcement enabling composite action relies on the bond between concrete and reinforcement. Bond strength determination of reinforced concrete is an essential task that a pullout test can determine. Experimental pullout behaviour can be affected by various parameters, such as concrete and steel strength, boundary conditions, etc. Therefore, a parametric study on pullout tests is time-consuming. In addition, measurements of internal stress-strain components and damages of constitutive materials are also difficult in experimental endeavours. In this context, finite element (FE) models should be developed to perform a parametric study with cost and time-saving. This study proposes a finite element modeling strategy of reinforced concrete under pullout force by using the expected failure mechanism to predict bond-slip behaviour in ABAQUS. In FE models, surface-to-surface cohesive interaction behaviour was used to assign interaction between reinforcement and concrete. The proposed modelling strategy was validated with available experimental data from four reference specimens, having all possible failures (i.e., pullout, splitting, and splitting-pullout) under pullout loading. The finite element analysis showed that the proposed FE modeling strategy performed well in predicting bond-slip behaviour in elastic regions. Additionally, maximum bond stresses were predicted satisfactorily, except for the splitting failure pattern, using the proposed FE modelling strategy
Enhancing the flexural performance of lightweight concrete slabs with CFRP Sheets: an experimental analysis
Get PDFThe flexural behavior of lightweight concrete two-way slabs is investigated in this work, with a focus on the strengthening or repairing method of externally attaching carbon fiber-reinforced polymer (CFRP) sheets. Five 1000 mm by 1000 mm by 120 mm reinforced lightweight concrete slab slabs were used in the experiment. Tested one specimen with no strengthening and another with CFRP sheet strengthening and repaired the rest with a single layer of CFRP at damage ratios of 50%, 60%, or 70% of the ultimate load, consciously making each slab crack under bending loads while keeping the exact measurements. As to the experiment findings, the ultimate load capacity increased by 30.3% at the strengthened specimen, 17.7% at the 50% damage level, 12.6% at the 60% damage level, and 10.9% at the 70% damage level. As degradation increases, so does the carrying capacity of LWC slabs. The amount of damage LWC slabs sustain influences their stiffness and flexibility. Effectively repairing the sample, CFRP sheets raised the reinforced concrete slabs’ failure stress and stopped the fractures from growing. Reinforced concrete slab failure was increased, and CFRP sheet repairs of the specimens successfully stopped crack propagation
Numerical study of the influence of the parameters of statistical distribution of the structural elements’ ultimate strength on deformable bodies’ fracture processes
Get PDFThis work is dedicated to the numerical study of the influence of the parameters of statistical distribution of the structural elements’ ultimate strength on fracture processes of deformable bodies with stress concentrators. The loading diagrams, the macro level behavior and the damaging process kinetics have been studied using various probability distribution laws and their standard deviations. The main types of the damage accumulation process have been identified. The approach basing on the analysis of the solutions of the boundary value problems within the elasticity theory has been proposed in order to predict the fracture process kinetics, the corresponding parameters have been introduced. Advantages, limitations and potential practical use of the proposed methodology have been discussed. It has been concluded that the statistical distribution of the structural elements’ strength properties should be taken into account in the numerical modeling of the fracture processes to ensure the reliability and safety of critical structures
Using the wavelet transform to process data from experimental studies of the discontinuous plastic deformation effect
Get PDFVast number of theoretical and experimental works has been devoted to the study of discontinuous plastic deformation (the Portevin-Le Chatelier effect), which manifests itself for most widely used alloys in certain ranges of both temperatures and strain rates. Due to the statistical nature of this phenomenon, difficulties arise in processing, qualitative analysis and quantitative comparison of test results and calculations. Using of statistical methods for these purposes makes it possible to get only some averaged characteristics of the obtained data (usually - moments of the first and second orders in amplitudes and frequencies). A possible alternative for processing of experimental and theoretical results of this effect research is wavelet analysis using.
The results of experimental studies of the Portevin–Le Chatelier effect, realized during the deformation of thin-walled tubular specimens made of aluminum alloy AMg6M at certain strain rates at room temperature are presented. Diagrams of deformation under uniaxial tension, shear, proportional and disproportionate loading of specimens were obtained. The inhomogeneity of strain fields and their rates is shown, illustrating the manifestation of the Portevin – Le Chatelier effect under conditions of complex loading of thin-walled tubular specimens made of AMg6M alloy. A brief overview of existing methods and means of non-destructive testing is presented that make it possible to non-contactly record the spatial heterogeneity of plastic yielding. Some possibilities of using the wavelet transform to process certain types of non-monotonic stress-strain diagrams obtained for the specimens made of the aluminum alloy in question are discussed. Using wavelet analysis, a compact presentation of data from field experiments in the form of amplitude-frequency characteristics was obtained. The scalograms analysis of specimen loading diagrams was carried out
Methodology to minimize the dynamic response of tall buildings under wind load controlled through semi-active magneto-rheological dampers
Get PDFThis paper proposes a methodology to evaluate and optimize the dynamic response of tall buildings under wind loading, controlled through semi-active Magneto-Rheological dampers (MR dampers). For this, a tall building, modeled as a 2D frame, is taken as case study and three structural control configurations are proposed. The original structural configuration of the building is the first configuration analyzed, called Uncontrolled Original (C1). In the second configuration, called Uncontrolled Optimized (C2), the fundamental frequency of the building is optimized via PSO algorithm as a function of its mass. Then, in the third configuration, Controlled Optimized (C3), a set of MR dampers with behavior formulated via the modified Bouc-Wen rheological model and controlled through the Linear Quadratic Regulator associated with the Clipped Optimal control strategy (LQR-CO) is applied to the structure. Finally, the dynamic response of the three scenarios under wind action is analyzed and compared to performance criteria established in the literature. The results demonstrate that the C3 configuration is the only one able of satisfying all the established performance criteria, proving that the proposed methodology y that combines structural optimization with MR dampers is a powerful tool for vibration control
Effect of Hybrid Nano Particle Reinforcements on Fractographic, Mechanical and Wear Behavior of Al6061 Alloy Composites Developed by Ultrasonic Assisted Stir Casting Technique
Get PDFThe purpose of this study is to investigate the influence of hybrid nanoparticle reinforcement with Al2O3 and ZrO2 on the mechanical and wear parameters of Al6061 alloy-based Nano composites. MMC’s (Metal Matrix Composite) specimens were produced using an ultrasonic-assisted stir casting method. Al6061 with varying weight percentages of Al2O3 (0.5%, 0.75%, 1% and 1.25%) and ZrO2 (0.5%, 0.75%, 1% and 1.25%) were used as reinforcement. The fabricated samples were made to undergo various tests as per ASTM standards to evaluate tensile strength, hardness and wear properties. Scanning Electron Microscopy was used to analyze the microstructure of the produced nano composite to ascertain the distribution of Al2O3 and ZrO2 nano particles and to analyze the fractured and wear characteristics. The results indicate that there is increase in tensile behavior for the composition of Al6061matrix alloy reinforced with 1wt. % Al2O3 and 1wt. % ZrO2 hybrid reinforcement. The maximum hardness achieved was 90.9 Hv with 1% ZrO2 and 1.25% Al2O3, representing a significant improvement over pure aluminum. The prepared specimens were subjected to a wear test utilizing a pin-on-disc machine and results reveal that highest wear resistance was obtained for the hybrid reinforcement of 1wt.% Al2O3 and 1wt.% ZrO2 with Al6061 alloy matrix