Papers by Keyword: Four-Point Bending

Abstract: A rheological model based on the new formulation of fractional calculus with variable order is developed to study the viscoelastic behavior of wood. The model, which uses only two rheological elements, exhibits an enhanced memory effect compared to constant-order fractional derivative models, demonstrated by a satisfactory fit to the experimental deformations observed in four-point bending tests on \textit{Pericopsis elata} samples. By determining the parameters of the linear function of the fractional order used, a physical significance emerges that explains the changes occurring within the material during the tests. This type of model, therefore, provides wood engineers with additional information about the behavior of the material under stress.

Abstract: This paper presents the results of a study that aimed to analyze the flexural behavior of self-compacting rubberized steel-reinforced concrete. A four-point bending test was performed on three reinforced beams made with conventional concrete and three similar beams made using the same concrete mixture with a 10% volumetric substitution of natural aggregates with rubber particles. The results showed a statistically significant decrease (about 24%) in the cracking load for the rubberized concrete beams, which is attributed to the reduced indirect tensile strength and modulus of the rubberized concrete. However, no statistically significant difference was observed between the control and rubberized concrete beams in terms of ultimate load and maximum deflection Additionally, the estimated adhesion strength, based on the average measured crack spacing, was also statistically similar between the tested beams. Existing equations derived from reinforced concrete beam theory were deemed suitable for rubberized concrete, since the estimation trends for these equations were similar for both types of concrete. Therefore, the main conclusion of this study is that the presence of rubber particles, at a 10% volumetric substitution, did not affect the flexural behavior particularly the quality of adhesion between the reinforcing bars and the surrounding concrete of steel-reinforced beams.

Abstract: In this study, we investigated the adhesion strength of SiO₂/SiN/TiW/Cu film stacks on silicon by the use of cross-sectional nanoindentation (CSN) technique. The delamination occurred along the SiN/TiW interface as determined by means of SEM and EDX analysis. The critical energy release rate was determined as a quantitative measure of the adhesion strength by application of analytical models as well as Finite Element Method (FEM). Comparative measurements on samples of the same layer composition using the well-established four-point bending (4PB) technique were performed to validate the results of the CSN measurements. FEM was performed to calculate the loading conditions and stress distribution in the samples. The calculations also allowed separating the contribution of plastic and elastic energy in the metallization layers during delamination testing and thereby estimating the value of the interfacial adhesion energy. The experimental results show the good applicability of both the 4PB and CSN method for determining quantitative values of the fracture toughness of thin-film interfaces found in microelectronic components and indicate a good agreement between the two methods.

Abstract: Single Shockley stacking faults (SSFs) expand from basal plane dislocations (BPDs) under forward current operation of 4H-SiC bipolar devices, giving rise to a reliability deterioration mode called “bipolar degradation”. Several groups have proposed models for the expansion of SSFs, in which the SSFs expand when electron-hole pair recombination takes place at BPDs. Maeda proposed a formulation of SSF expansion that includes stacking fault energy. However, the mechanisms by which mechanical stress affects the expansion of SSFs are unclear. In this paper, we evaluated the “expansion threshold current” of bar-shaped SSFs in a mechanical stress field using a p-i-n diode fabricated on 4H-SiC. To confirm the effect of mechanical stress on the threshold current for bar-shaped SSF expansion, a SiC-p-i-n diode was evaluated by the four-point bending method. Experimental results show that the threshold current of SSFs decreases or increases by more than 100 A/cm² depending on the direction of the applied stress of SSFs. This result indicates that mechanical stress is an important factor for SiC bipolar device design.

Abstract: The paper is about a research focused on a development of thin slabs made from white UHPC. These slabs are reinforced by scattered PVA fibres and textile glass reinforcement in form of 2D net. The aim of the experiment is the development of tin slabs that can be used as facade or roof panels. The course of the experimental tests of these slabs and its correspondence with the numeric analysis made with ATHENA software is in the paper. Three slab specimens with size of 750 x 125 x 15 mm were tested in four-point bending. The white UHPC matrix was reinforced by the scattered PVA fibres and the 2D textile glass reinforcement with mesh 20 x 20 mm. The used 2D reinforcement was coated against alkali conditions, so called AR - Glass with 2400 TEX fineness. The reinforcement was placed and fixed near the lower surface of the slabs. The course of the tests was continually monitored by an automatic measuring unit where especially the acting force, the deformation in the middle of the span, the deformation in the supports and the progressive formation and development of cracks were detected. The numeric model was created in 3D surroundings of Gid 2011 program and then converted into ATENA software.

Abstract: Properties of laminated glass comprising two or more sheets of glass are greatly affected by the composite action of these panes which is influenced by material properties of used interlayer. Generally plastic foil or cast resin is used as the interlayer in lamination process. Laminated glass has been experimentally investigated in the laboratory condition in a four-point bending test on several kinds of interlayers in combination with variable thickness of the annealed glass. This paper establishes the so-called effective bending thickness according to standard method and presents comparisons with experimentally determined values.

Abstract: This article will examine the effect of the knots distribution on strength of glued laminated timber beams. A finite element model was made to simulate glued laminated timber beams with defects (knots) to simulate and predict behavior in areas containing knots while bending. This model was then compared with results from static bending test of glued laminated timber beam. The initial position of the rupture was then compared with FEM model [6], so the influence of knots on strength could be determined.

Abstract: The paper reports on two specific research activities devoted on the one hand to detailed finite element simulations of glued timber beams with emphases on random nature of the modulus of elasticity and, on the other hand, to destructive testing of a set four glued timber beams having structural dimensions. While the stochastic simulations were founded on the knowledge of probability density distributions of the modulus of elasticity acquired from non-destructive testing of local moduli from 3600 regularly spaced indentation measurements, the destructive testing of beams in four-point bending aimed at providing information to be used a-posteriori in improving the original predictions in the framework of Bayesian statistics. In this regard the non-destructive testing together with stochastic simulations of beam deflections is assumed to provide prior information to be updated in the second Bayesian step. This, however, will be discussed elsewhere so that only necessary prerequisites are mentioned here.

Abstract: Two common problems of mechanical strain relaxation(MSR) residual stress measurement methods are investigated in this work:(1) assumption of stress uniformity and (2) the effect of plasticity at relaxation. A new MSR technique, designed specifically for highly non-uniformin-plane residual stress fields, is applied in this work to measure the residual stress field resulted from pure bending of an Al7075 alloy.The method involves introducing a straight cut across the whole part in a single increment, and collecting full field displacement fields from the side surface. Application of a 2D high resolution digital image correlation (DIC) method proved successful in this work.The reconstructed residual stress agrees well with that predicted by FE modelling. It is shown that the direction of the propagation of the slit has a major influence on plastic flow during relaxation.The major conclusion from this work is that it is possible to substantially reduce, or completely eliminate, plastic flow on relaxation by careful planning of the slit orientation and the cutting schedule.

Abstract: Fracture behavior of hard nanocrystalline coatings decisively influences the lifetime and performance of coated tools. In this work, residual stresses in as-deposited and annealed CrN coatings deposited at 350 °C using bias voltages of −40 V and −120 V were evaluated using synchrotron X-ray diffraction coupled with four-point bending. The stress development during the bending experiments was used to analyse fracture properties of the coatings. The results indicate that an annealing at 550 °C does not deteriorate the fracture behavior of the coatings prepared using −40 V bias. In the case of −120 V bias coatings, the residual stress relaxation after the thermal treatment is accompanied by a fracture strain decrease and a fracture stress increase. The as-deposited and annealed CrN coatings deposited using −120 V bias exhibit significantly large fracture strains in comparison with −40 V samples. Finally the results document that the fracture stress may not be the only relevant parameter when comparing different coating systems. Also the strain at fracture can be considered as significant indicator of the coating fracture response. Methodologically, the results indicate that in-situ X-ray diffraction coupled with four point bending can be effectively used to evaluate macroscopic fracture behaviour of hard coatings.