• Journal of Welding and Joining
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• 제8권4호
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• pp.69-75
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• 1990

In this paper, ultimate compressive strength of TMCP 50HT steel plates (yield stress .sigma.$_{o}$=36kg/mm$^{2}$) with HAZ softening is studied. Finite element method formulated by the author is applied to analyze the elasto-plastic large deflection behaviour of the plates. The influence of HAZ softening breadth, welding direction and slenderness ratio on the ultimate compressive strength is investigated. The results obtained are summarized as 1) With the increasing of the HAZ softening breadth, early plasticity on the plates is formed and then the ultimate compressive strength is decreased, in which about 8% of the ultimate strength for the plate with h/t=4(h: HAZ softening breadth, t: plate thickness) was reduced comparing with no HAZ softening. 2) The large decrease of the ultimate strength for the case that the welding direction is normal to the loading direction is occurred than the case that the welding direction is parallel to the loading direction. 3) The influence of HAZ softening on the ultimate compressive strength is serious for thick plates, while it may be negligible for thin plates.s.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.778-779
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• 2006

Manufacturing technologies of micro parts were studied in nano grained Al-1.5mass%Mg alloy. During compressive test at $300^{\circ}C$, the Al alloy showed stain softening phenomenon by grain boundary sliding regardless of strain rate. Micro spur gear with ten teeth (height of $200{\mu}m$ and pitch of $250{\mu}m$) was fabricated with sound shape by micro forging. During micro forging, increase of applied stress induced by friction between material and die surface was effectively compensated by decrease of stress by strain softening behavior and as a result, flow stress increased only about 50 MPa more than that in compressive test

• International Journal of Concrete Structures and Materials
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• 제18권3E호
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• pp.191-198
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• 2006

The objective of this study is to examine fracture characteristics of concrete at early ages, i.g. critical stress intensity factor, critical crack-tip opening displacement, fracture energy, and bilinear softening curve based on the concepts of effective-elastic crack model and cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By experimenting with various strengths and ages, load-crack mouth opening curves were obtained, and the results were analyzed by linear elastic fracture mechanics and FEM(finite element method). The results from the test and analysis showed that critical stress intensity factor and facture energy increased while critical crack-tip opening displacement decreased with concrete aging from 1 day to 28 days. Four parameters of bilinear softening curve from 1 day to 28 days were obtained from a numerical analysis. The obtained fracture parameters and bilinear softening curves at early ages from this study are to be used as a fracture criterion and an input data for the finite element analysis of concrete at early ages.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1995년도 가을 학술발표회 논문집
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• pp.185-193
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• 1995

The softening behaviors of concrete have been the object of numerous experimental and numerical studies, because the load carrying capacity of cracked concrete structure is not zero. Numerical studies are devoted to the investigation of three-dimensional softening behaviors of concrete on the basis of a viscoplastic theory, which may be able to represent the effects of plasticity and also of rheology. In order to properly describe material behaviors corresponding to different stress levels, two surfaces in stress space are adopted; one is a yield surface, and the other is a failure or bounding surface. When a stress path reaches the failure surface, it is considered that the softening behaviors are initiated as micro-cracks coalesce and are simulated by assuming that the actual strain increments in the post-peak region are less than the equivalent viscoplastic strain increment. The experimental studies and the finite element analyses have been carried out under the displacement control. Numerically simulated results indicate that the model is able to predict the essential characteristics of concrete behaviors such as the non-linearity, stiffness degradation, different behaviors in tension and compression, and specially dilatation under uniaxial compression.

• Elastomers and Composites
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• 제42권3호
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• pp.168-176
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• 2007

카본블랙이나 실리카 등으로 보강된 고무 가황체는 순수한 초기상태에서 하중(부하)를 가하고 제거하는 반복과정에서 응력은 점점 연화되어 초기상태에서 얻어진 응력보다 작게 나타난다. 이러한 응력 연화 현상을 Mullins 효과라고 부른다. 이러한 응력 연화 거동을 이론적으로 표현하기 위하여 Ogden-Roxburgh 등이 손상 파라미터를 이용하여 제안한 pseudo-elastic 개념을 적용하여 보강제가 함유된 고무 가황체의 변형률 에너지 함수를 구하였다. 카본블랙으로 보강된 NR 가황체를 이용하여 준정적 반복 부하 시험을 실시하였으며, pseudo-elastic 모델에서의 손상 파라미터가 제하 및 재 부하 시 응력-변형률 곡선에 어떠한 영향을 주는가와 더불어 손상 파라미터의 두 가지 변수인 r과 m의 물리적 의미를 파악하였다. 또한 보강제 함량을 달리하여 제작한 고무 가황체의 응력연화 변형률 에너지 함수를 결정하고 비교하였다.

• 한국지반신소재학회논문집
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• 제2권1호
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• pp.57-68
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• 2003

Strain-softening model is developed to characterize the interface behavior of geomembrane with geotextile and geosynthetic clay liner(GCL). The model proposed in this research is calibrated by using data from direct shear tests conducted on smooth and textured geomembrane. The research is divided into two regions, pre-peak and post-peak, to take into account of strain-softening effect. Although slight difference between measured and back calculated data is observed under high normal stress, good agreements, in general, are found from back calculations. Especially, good consistency is observed in the case of low normal stress. Based on the results, it can be concluded that the proposed model can be a reasonable constitutive law to figure out the behavior of strain-softening between interfaces of geomembrane. In addition, DSC(Disturbed State Concept) model is also presented for further application in geosynthetic interfaces.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.427-733
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• 2003

Adiabatic shear bands have been observed in the serrated chip during high strain rate metal cutting process of medium carbon steel and titanium alloy. The recent microscopic observations have shown that dynamic recrystallization occurs in the narrow adiabatic shear bands. However the conventional flow stress models such as the Zerilli-Armstrong model and the Johnson-Cook model, in general, do not predict the occurrence of dynamic recrystallization (DRX) in the shear bands and the thermal softening effects accompanied by DRX. In the present study, a strain hardening and thermal softening model is proposed to predict the adiabatic shear localized chip formation. The finite element analysis (FEA) with this proposed flow stress model shows that the temperature of the shear band during cutting process rises above 0.5T$\sub$m/. The simulation shows that temperature rises to initiate dynamic recrystallization, dynamic recrystallization lowers the flow stress, and that adiabatic shear localized band and the serrated chip are formed. FEA is also used to predict and compare chip formations of two flow stress models in orthogonal metal cutting with AISI 1045. The predictions of the FEA agreed well with the experimental measurements.

• 소성∙가공
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• 제9권1호
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• pp.72-79
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• 2000

Dynamic recrystallization (DRX), which may occur during hot deformation, is important for the microsturctural evolution of 304 stainless steel. Especially, the current interest in modelling hot rolling demands quantitative relationships among the thermomechanical process variables, such as strain, temperature, strain rate, and etc. Thus, this paper individually presents the relationships for flow stress and volume fraction of DRX as a function of processing variables using torsion tests. The hot torsion tests of 304 stainless steel were performed at the temperature range of 900~110$0^{\circ}C$ and the strain rate range of 5x10-2~5s-1 to study the high temperature softening behavior. For the exact prediction of flow stress, the equation was divided into two regions, the work hardening (WH) and dynamic recovery (DRV) region and the DRX region. Especially, The flow stress of DRX region could be expressed by using the volume fraction of DRX (XDRX). Since XDRX was consisted of the critical strain($\varepsilon$c) for initiation of dynamic recrystallization (DRX) and the strain for maximum softening rate ($\varepsilon$*), that were related with the evolution of microstructure. The calculated results predicted the flow stress and the microstructure of the alloy at any deformation conditions well.

• Coupled systems mechanics
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• 제4권2호
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• pp.115-136
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• 2015

We present for one-dimensional model for elastoplastic bar with combined hardening in FPZ - fracture process zone and softening with embedded strong discontinuities. The simplified version of the model without FPZ is directly compared and validated against analytical solution of Bazant and Belytschko (1985). It is shown that deformation localizes in an area which is governed by the chosen element size and therefore causes mesh sensitivity and that the length of the strain-softening region tends to localize into a point, which also agrees with results obtained by stability analysis for static case. Strain increases in the softening domain with a simultaneous decrease of stress. The problem unloads elastically outside the strain-softening region. The more general case with FPZ leads to more interesting results that also account for induced strain heterogeneities.

• 한국생산제조학회지
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• 제26권2호
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• pp.193-198
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• 2017

Rubber materials are widely used for anti-vibration in various industries such as railways, automobile, and aviation. However, various factors hinder the accurate prediction of mechanical properties and lifetime of these materials. Particularly, a stress softening phenomenon Mullins effect greatly affects the accuracy of test results by reducing the initial peak stress. Although the Mullins effect has been studied previously, research on its temperature dependence is lacking. In this study, we performed experiments to estimate the temperature dependence of the Mullins effect. Dumbbell specimens made of natural rubber (NR65) was mounted on a stress softening tester and placed in a heat chamber, where they were tested at temperature of 25, 50, and $80^{\circ}C$. Further, five test sets, each consisting of 10 loading/unloading cycles were sequentially performed at predetermined time intervals. Based on the test results, we assessed the effect of temperature and time interval on stress softening and recovery.