Papers by Keyword: Size Effect

Abstract: With the ongoing development of product process, there is a growing demand on micro products. Though the macro-drawing process has been well-developed, the design concepts may not be directly applicable to the micro-drawing due to the size effect occurred in the micro-forming processes. In the present study, experiments were conducted first to establish the stress-strain curves, r-values and work hardening exponents of 304 stainless steel sheets with different grain sizes. The experiment results reveal that the stress-strain and r-value become smaller and the work hardening exponent increases for larger grain sizes. The difference between stress-strain curves in various directions of 0°, 45° and 90°, respectively, is significant when the grain size increases. The stamping of a vibration motor shell of cell phone, which bears a circular cylindrical shape, was also examined in the present study. The finite element simulations were performed to evaluate the formability of the multi-stage drawing process with initial die design. The forming characteristics were identified and an optimum die design was then developed with the use of the finite element analysis. The stamping process with multi-stage tooling design based on the finite element analysis was implemented and the actual stamping experiments were conducted to verify finite element analysis. The experimental results confirm the validity of the modified tooling design and the efficiency of the finite element analysis.

Abstract: Formability in sheet forming processes are usually analyzed by standardized tests, which often requires different test equipment associated with high initial investment cost. The present study purposes a flexible test tooling system for hydraulic bugle test apparatus that allows to evaluate the impact of size effect on the formability of thin metallic sheets. Finite Element Method was used for concept and design of the tooling system and experimental tests were carried out with thin sheets of SUS316L stainless steel to assess the overall performance of the tooling system.

Abstract: Fracture mechanical studies have become a vital aspect of the design of concrete structures. This work aims to analyse/validate the size-effect of structural components in light of principles of Linear Elastic Fracture Mechanics (LEFM). Various models of a quasi-brittle material with different geometries have been analysed for Stress Intensity Factors (SIF) using Abaqus/CAE: a finite element analysis software. The effect of crack-to-depth ratio, span-to-depth ratio and specimen size on SIF values have been studied. Also the variation in SIF values with respect to changing the position of concentrated load application and initial notch on the specimen have been studied. It is found that for a particular notch-to-depth ratio, the SIF increases with a decrease in specimen size. Also, at a constant span-to-depth ratio, SIF increases with increase in the notch-to-depth ratio.

Abstract: The results of the studies presented here are devoted to understanding of microstructure effect on the processes and properties driven by diffusion. The role of various interfaces (intergranular, phase, free surface), as the high-energy defects, is underlined and investigated with special attention. The methodology relevant to analyses of the microstructural processes is first briefly presented. The capability and limitations of classical molecular dynamics, mesoscale Monte Carlo and cellular automaton techniques are described. Two examples of the diffusion driven processes analyzed at various length and time scale are shown: namely, grain growth in nanometallic materials and melting of thin embedded films. The modeling results are also accompanied with experimental studies. Thanks to application of numerical methods, models of relevant processes were proposed, which enabled to provide quantitative relationships between microstructure and the process kinetics. Such relationships can be later used for design of optimized materials for wide range of applications.

Abstract: The size effect on strength of concrete has been studied for a long time from various approaches. In particular, the weakest-link theory remains nowadays the basic tool to interpret statistical size effects, i.e. how the probability of failure under a given stress depends on external size. The main shortcoming of Weibull’s theory is that the activation of fracture from the weakest flaw is assumed to set the final strength, i.e. possible interactions between microcracks and defects during progressive damage are implicitly neglected. The objective of this study is to determine experimentally the influence of “external” (sample) size and “internal” (microstructural) size on compressive strength of concrete. For this purpose, more than 250 uniaxial compression tests were conducted on concrete specimens with two different cylindrical sizes (110×220 mm and 160×320 mm) and prepared from three different compositions (mean aggregate size and proportion). The relationships between compressive strength, dissipated energy up to failure in one hand, and microstructural as well as specimen sizes on the other hand, were analyzed statistically. This demonstrated the failure of the weakest-link approach to describe size effects on compressive strength of concrete.

Abstract: The authors participated in the prediction contest for strength of 4m deep concrete slab strip set forth by University Toronto in Canada. They submitted the best prediction among 66 entries from all over the world. Their solution was achieved with a numerical analysis based on nonlinear constitutive model of concrete using fracture mechanics. The shear strength of beam was significantly affected by its large size. After the results of contest were made public the authors performed a study about mesh sensitivity and element type effects, which resulted in assessment of model uncertainty.

Abstract: Copper bonding wires are frequently used to connect to MEMS devices. Mechanical properties of copper wire are crucial to the reliability of MEMS system. The paper reported a symmetrical bending fatigue test on micron scale copper bonding wires. The test is based on the phenomenon that a micro-cantilever can be set into self-excited vibration between two electrodes under DC voltage. The results demonstrate that the yield strength, ultimate tensile strength and Young's modulus of copper wires with diameter of 20μm are higher than those with a diameter of 30μm and 40μm, which significantly performs size effect. In fatigue test, the number of cycles to failure is 10⁴~10⁷. Under the same stress condition, fatigue strength (N=10⁶) of copper wires (d=20μm, 30μm, 40μm) is 140MPa, 97MPa, 70MPa respectively. The tensile fracture surface is the chisel-shaped peak, and the surface of the fracture appears many spaced strip drawing traces. The fatigue fracture surface is flat. Two cracks almost simultaneously originate from the surface, and the final rupture region is just like a narrow sheet.

Abstract: Hardness measurements can simple, but very useful provide materials mechanical properties. This is atest that is extremely localized in volume and does not affect significantly the structural properties. Theaim of this article is to highlight on influence of the applied load forces to the hardness measurementresults. In engineering practice it is generally the most widely used method of measuring the Vickershardness. This method has a variability of the loading forces in a wide range from nano/micro up tohigh loads in hundreds N. There is a well-known plastic material response, therefore, were taken intoaccount several steels commonly used in technical practice S355 NL+N, 316LVM, 34CrMo-4. Wheninterpreting the results should always point out to the value of the load. In common engineering practiceleads to confusion, or to comparing the hardness of the material obtained at different loads. Thisproblem is quite often occurs at nowadays and needs to be solved. Focus should be not only in termsof results depending on the load, but also from a statistical perspective of scattering measurements atvarious loads. Generally, in the technical practice is discussed independence measurement results ofVickers hardness on applied load, due geometric similarity of indents. When is used different forcesthere is a changed of the hardness results. This phenomenon is known as a size effect. This effect isgenerally related to each mechanical testing of materials. However, engineering practice getting intoconflict with this effect.

Abstract: Microforming process is a promising approach to manufacture microparts for its high productivity, high material usage and good part properties. However, when the part size is scaled down from macro to micro level, the deformation behaviors of materials change and the size effects occur. This makes it difficult to use microforming process in industry. In the last decade, many studies have been conducted with different test methods and materials. In this paper, the main test apparatus and the methods used to study the size effect and the significant results are reported.

Abstract: The size-dependent features concerning the mechanical behavior of the micro/nanoelectronic structures are well known from experiments. They are described by the strain-gradient effect in this paper since the classical elasticity theory fails to capture the size effect of the nanostructures. The electric field-strain gradient coupling is considered in the constitutive equations of the material and the governing equations are derived with the corresponding boundary conditions using the variational principle. The path independent J-integral is derived for fracture mechanics analysis of piezoelectric solids described by the gradient theory.