• Transactions of Materials Processing
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• v.19 no.1
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• pp.11-16
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• 2010

The influence of pre-strain on low cycle fatigue behavior of Fe-18Mn-0.05Al-0.6C TWIP steel was studied by conducting axial strain-controlled tests. As-received plates were deformed by rolling with reduction ratios of 10 and 30%, respectively. A triangular waveform with a constant frequency of 1 Hz was employed for low cycle fatigue test at the total strain amplitudes in the range of ${\pm}0.4\;{\sim}\;{\pm}0.6$ pct. The results showed that low-cycle fatigue life was strongly dependent on the amount of pre-strain as well as the strain amplitude. Increasing the amount of prestrain, the number of reversals to failure was significantly decreased at high strain amplitudes, but the effect was negligible at low strain amplitudes. A new model for predicting fatigue life of pre-strained body has been suggested by adding ${\Delta}E_{pre-strain}$ to the energy-based fatigue damage parameter. Also, high-cycle fatigue lives predicted using the low-cycle fatigue data well agreed with the experimental ones.

• Progress in Superconductivity and Cryogenics
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• v.6 no.1
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• pp.6-11
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• 2004

The influences of mixed strain mode of bending-tension on the critical current. Ic in externally reinforced Bi-2223 tapes and their interaction were investigated in this study. A test fixture which providing a mixed deformation mode of bending-tension to HTS tapes has been newly devised. When the total strain induced in the tape in the mixed strain mode was expressed by the superposition of the bending and tensile strains the irreversible strain for the critical current degradation of Bi-2223 tapes increased, as compared with the simple bending mode case. The $I_c$ degradation at the region exceeding the irreversible strain showed a medium between the simple bending case and the simple tension case. As the initial bending strain imparted increased , namely as the diameter of mandrel adopted decreased. the apparent irreversible strain in Bi-2223 tapes increased . but the increment became smaller As a result. it can be found that the tension to be applied to bent Bi-2223 tapes during cabling should be smaller. as the mandrel diameter becomes smaller.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.1
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• pp.9-14
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• 2012

This paper investigates the prediction of the dynamic strain response using acceleration response only. Two methods are proposed for the strain prediction; one is based on beam theory and the other is calculated by the frequency response function between acceleration and strain. First, it is estimated the dynamics of the simple notched beam, including the non-linearity, through the uni-axial vibration testing. Then, the dynamic strain response is predicted under two different methods using acceleration response. The validation of proposed methods is conducted by the comparison between measured strain and predicted values. The comparison reveals that the proposed method based on the FRF between acceleration and strain is more reliable one than that stemmed from beam theory and the maximum relative error is less than 8 %.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1442-1445
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• 2005

Several test methods, including micro strain/deformation measurement techniques, have been studied to more reliably measure the micro properties in micro/nano materials. Therefore, in this study, the continuous measurement of in-plane tensile strain in micro-sized specimens of thin film materials was introduced using the micro-ESPI technique. TiN and Au thin films 1 and $0.47\;\mu{m}$ thick, respectively, were deposited on the silicon wafer and fabricated into the micro-sized tensile specimens using the electromachining process. The micro-tensile loading system and micro-ESPI system were developed to measure the tensile strain during micro-tensile test. The micro-tensile stress-strain for these materials was determined using the algorithm for continuous strain measurement. Furthermore, algorithm for enhancing the sensitivity to measurement of in-plane tensile strain was suggested. According to the algorithm for enhancement of sensitivity, micro-tensile strain data between interfringe were calculated. It is shown that the algorithm for enhancement of the sensitivity suggested in this study makes the sensitivity to the in-plane tensile strain increase.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.1
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• pp.167-174
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• 2003

Fatigue damage of 2.2Ni-1Cr-0.5Mo steel used fir high strength pressure tubes and vessels was evaluated using uniaxial specimens subjected to strain-controlled fatigue loading. Based on the fatigue test results from different strain ratios of -2. -i 0, 0.5, 0.75, the fatigue damage of the steel was represented by using a cyclic strain energy density. Mean stress relaxation depended on the magnitude of the applied strain amplitude. The high pressure vessel steel exhibited the cyclic softening behavior. Total strain energy density consisting of the plastic strain energy density and the elastic tensile strain energy density described fairly well the fatigue life of the steel, taking the mean stress effects into account. Compared to other fatigue damage parameters, fatigue life prediction by the cyclic strain energy density showed a good correlation with the experimental fatigue lift within a factor of 3.

• Computers and Concrete
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• v.3 no.6
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• pp.375-388
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• 2006

Poor strength test results are sometimes not an indication of low concrete quality, but rather inferior testing quality. In a compression test, the strain distribution over the ends of the specimen is a critical factor for the test results. Non-uniform straining of a concrete specimen leads to locally different compressive stresses on the cross-section, and eventual premature breaking of the specimen. Its effect on a specimen can be quantified by comparing the compressive strength results of two specimens, one subjected to uniform strain and another to a specified strain gradient. This can be done with the help of a function that relates two parameters, the strain ratio and the test efficiency. Such a function depends on the concrete strength and cross-sectional shape of the specimen. In this study, theoretical relationships between the strain ratio and test efficiency are developed using a concrete stress-strain model. The results show that for the same strain ratio, the test efficiency is larger for normal strength concrete than for high strength concrete. Further, the effect of the strain gradient on the test result depends on the cross-sectional shape of the specimen. Implementation of the results is demonstrated with the aid of two examples.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.33-36
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• 2000

The dynamic recrystallization (DRX) of medium carbon steels (SCM 440 and POSMA45) was studied with torsion test in the temperature range of $900-1100^{\circ}C$ and the strain rate range of $5.0x10^{-2}\;-\;5.0x10^0/sec$. To establish the quantitative equations for DRX, the evolution of flow stress curve with strain was analyzed. The critical strain (${\varepsilon}_c$) and strain for maximum softening rate ( ${\varepsilon}^{*}$) could be confirmed by the analysis of work hardening rate ($d{\sigma}/d{\varepsilon}\;=\; \theta$). The volume fraction of dynamic recrystallization ($X_{DRX}$) as a function of processing variables, such as strain rate ( $\dot{\varepsilon}$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\varepsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction, the ${\varepsilon}_c$, ${\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A respectively. The transformation-effective strain-temperature curve for DRX could be composed. It was found that the calculated results were agreed with the experimental data for the steels at any deformation conditions.

• International Journal of Concrete Structures and Materials
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• v.10 no.3
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• pp.307-323
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• 2016

The nonlinear behaviors of recycled aggregate concrete (RAC) frame structure are investigated by numerical simulation method with 3-D finite fiber elements. The dynamic characteristics and the seismic performance of the RAC frame structure are analyzed and validated with the shaking table test results. Specifically, the natural frequency and the typical responses (e.g., storey deformation, capacity curve, etc.) from Model 1 (exclusion of strain rate effect) and Model 2 (inclusion of strain rate effect) are analyzed and compared. It is revealed that Model 2 is more likely to provide a better match between the numerical simulation and the shaking table test as key attributes of seismic behaviors of the frame structure are captured by this model. For the purpose to examine how seismic behaviors of the RAC frame structure vary under different strain rates in a real seismic situation, a numerical simulation is performed by varying the strain rate. The storey displacement response and the base shear for the RAC frame structure under different strain rates are investigated and analyzed. It is implied that the structural behavior of the RAC frame structure is significantly influenced by the strain rate effect. On one hand, the storey displacements vary slightly in the trend of decreasing with the increasing strain rate. On the other hand, the base shear of the RAC frame structure under dynamic loading conditions increases with gradually increasing amplitude of the strain rate.

• Structural Engineering and Mechanics
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• v.54 no.2
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• pp.199-219
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• 2015

This paper presents the non-destructive evaluation of a high-speed railway bridge using train-induced strain responses. Based on the train-track-bridge interaction analysis, the strain responses of a high-speed railway bridge under moving trains with different operation status could be calculated. The train induced strain responses could be divided into two parts: the force vibration stage and the free vibration stage. The strain-displacement relationship is analysed and used for deriving critical displacements from theoretical stain measurements at a forced vibration stage. The derived displacements would be suitable for the condition assessment of the bridge through design specifications defined indexes and would show certain limits to the practical application. Thus, the damage identification of high-speed railways, such as the stiffness degradation location, needs to be done by comparing the measured strain response under moving trains in different states because the vehicle types of high-speed railway are relatively clear and definite. The monitored strain responses at the free vibration stage, after trains pass through the bridge, would be used for identifying the strain modes. The relationship between and the degradation degree and the strain mode shapes shows certain rules for the widely used simply supported beam bridges. The numerical simulation proves simple and effective for the proposed method to locate and quantify the stiffness degradation.

• Geomechanics and Engineering
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• v.16 no.2
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• pp.205-215
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• 2018

Strain-based criteria are known as a direct method in determining the stability of the geomechanical structures. In spite of the widely use of Sakurai critical strain criterion, it is so conservative to make use of them in rocks with initial plastic deformation on account of the considerable difference between the failure and critical strains. In this study, a new criterion has been developed on the basis of the failure strain to attain more reasonable results in determining the stability status of the tunnels excavated in the rocks mostly characterized by plastic-elastic/plastic behavior. Firstly, the stress-strain curve was obtained having conducted uniaxial compression strength tests on 91 samples of eight rock types. Then, the initial plastic deformation was omitted making use of axis translation technique and the criterion was presented allowing for the modified secant modulus and by use of the failure strain. The results depicted that the use of failure strain criterion in such rocks not only decreases the conservativeness of the critical strain criterion up to 42%, but also it determines the stability status of the tunnel more accurately.