Papers by Keyword: Size Effect

Abstract: As components with proportional feature and tooling sizes are miniaturized, strain gradients through the cross-section increase. This causes strain gradient hardening as the density of geometrically necessary dislocations increases. This will lead to higher required forces in the process than expected. In this paper, an analytical model to predict the dislocation density increases, and thus strain gradient hardening, during microbending is presented. These results match previous research in terms of the feature size where modest and significant strain gradient hardening was observed.

Abstract: This paper presents results of fatigue tests performed on standard specimens and minispecimens taken from two EN 1.4301 acid-resistant steel plates of different thickness. The tests were required for determination of the state of vibrating machines made from the same material. The results confirmed that the proposed testing method can be used for experimental tests.

Abstract: In this study, a scale-dependent model is employed to investigate the size effects of copper on the behavior of the crack-tip. This model includes the homogeneous and non-homogeneous strain hardening based on the wavelet interpretation of size effect. Introducing additional micro/nano structural considerations together with decreasing grain size, different size effects can be obtained. As the size dependency is not taken into account in conventional plasticity, an enhanced theory which is related to the strain gradient introduces a length scale will give more realistic representations of state variables near the crack-tip. Accordingly, the contribution of geometrically necessary dislocations (GNDs) activity on strengthening and stress concentration factor is identified in the crack-tip. Finally, the affected zone which is dominated by presence of GNDs is identified

Abstract: In the meso-scale machining, feed rate, grain size and tool edge radius are in the same order of magnitude, and cutting process is often carried out in the grain interior and grain boundary. In this paper the meso-cutting process of hot-rolled AISI1045 steel is studied and its metallographic microstructure is analyzed for the establishment of multiphase models which incorporate the effect of ferrite and pearlite grains. In order to discover the applicability of multiphase models to the simulation of meso-cutting, three contrast simulation models including multiphase model with rounded-edge cutting insert (model I), multiphase model with sharp edge cutting insert (model II) and equivalent homogeneous material model with rounded-edge cutting insert (model III) are built up for the meso-orthogonal cutting processes of hot-rolled AISI1045. By comparison with the experiments in terms of chip morphology, cutting force and specific cutting force, the most suitable model is identified. Then the stress distiribution is analyzed. And it is found that multiphase model with tool edge radius can give a more accurate prediction of the global variables and reveal more about these important local variables distribution.

Abstract: Micro manufacturing has received good attention globally in terms of its manufacturing methods and processes. One of the most popular micro manufacturing processes is micro forming. Although there were efforts made to realize micro extrusion for industrial application, the technology itself was seen as being insufficiently mature and unlike conventional methods, there is no in-depth knowledge. It has become essential to develop a proper understanding which in turn could be used to develop dedicated processes for the manufacturing of metallic micro components. In this work, an attempt has been made to realize this special application of metal forming. A novel experimental setup consisting of forward extrusion assembly and a loading set-up has been developed to obtain the force-displacement response. The effects of minaturization on micro components and the material behavior during forward extrusion are investigated. As per industrial requirement and application of micro part in micro manufacturing process, grain size is an important factor. In this context the effect of grain size is also studied in this work. The realization of such a productive forward extrusion assembly poses significant advantages when compared to the conventional manufacturing technologies in the production of micro parts.

Abstract: The effect of different inclusion contents on the VHCF strength of H13 tool steels is presented. Two different H13 tool steels were investigated: the Uddeholm Orvar^® 2 Micronized obtained by conventional casting, and the Uddeholm Orvar^® Supreme obtained by electroslag remelting (ESR). Ultrasonic tests were performed on Gaussian specimens (risk volume about 2300 mm3) up to 1010 cycles or up to failure and fracture surfaces were investigated with SEM in order to analyze the inclusions from which VHCF crack nucleated. Experimental results show that the VHCF strength estimated by using the Murakami’s model of the H13 Uddeholm Orvar^® Supreme steel is about 15% larger than that of the H13 Uddeholm Orvar^® 2 Micronized steel.

Abstract: In the present paper, the characteristic and the application of ultrasonic fatigue testing technology is illuminated. The main problems i.e. the size effect, the thermal effect and frequency effect due to the high frequency are discussed. The results show that: 1. As there is a size effect, a uniform specimen size should be adopted in the very-high cycle fatigue standard and for special designed specimen the designed size should be noted along with the fatigue test results; 2.the heat generation attributes mainly to the low yield strength and the high applied stress, as a result, ultrasonic fatigue testing technology can be mainly applied to the ultra-high cycle fatigue test of high-strength steel; 3.the frequency effect is related to the crystal structure of metallic materials, however, ultrasonic fatigue testing technology can be applied to conduct the comparison of the fatigue properties of the same steel grade before and after the smelting process.

Abstract: This work focuses on the application of a higher-order gradient-dependent plasticity-damage model for microstructural modeling of dual-phase (DP) steels. Damage evolution is governed by the evolution of a nonlocal plasticity measure which is a function of the local equivalent plastic strain rate and its corresponding first-order gradient. Different RVEs of DP microstructures are virtually generated and simulated in order to predict the macroscopic mechanical response. Size effects and additional hardening due to evolution of geometrically necessary dislocations are predicted.

Abstract: In this study, a planar spring lattice model is used to study the evolution of damage variable d_L in disordered media. An elastoplastic softening damage constitutive law is implemented which introduces a cohesive length scale in addition to the disorder-induced one. The cohesive length scale affects the macroscopic response of the lattice with the limiting cases of perfectly brittle and perfectly plastic responses. The cohesive length scale is shown to affect the strength-size scaling such that the strength increases with increasing cohesive length scale for a given size. The formation and interaction of the microcracks is easily captured by the inherent discrete nature of the model and governs the evolution of d_L . The proposed method provides a way to extract a mesoscale dependent damage evolution rule that is linked directly to the microstructural disorder.

Abstract: A novel computational model is presented for the representation of polycrystalline grain structures and crystal plasticity finite element model are used to investigate size effect due to grain orientation and number of grains through the thickness, in term of activity of slip system, roll force and the scatter involved. It is predicted that increasing the number of grains through the thickness has detrimental effects on mechanical performance. It is clear from the current study that foil thickness significantly influences the deformation heterogeneity which leads to non-uniform distributions of strain and the inhomogeneous slip systems active in the cold rolled samples. It is found that the decrease of roll force and slip systems activities in surface grains are caused by the increasing of free surface grain effect when the thickness is decreased.