• Elastomers and Composites
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• v.36 no.1
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• pp.30-36
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• 2001

The objective of this study was to investigate factor that influences the development of anisotropy in carbon black filled natural rubber vulcanizates. Chain orientation affects tensile strength, stiffness. Parallel sample shows low stress at low deformation, but have high stiffness at high deformation compared to isotropic or perpendicular samples. This study shows that natural rubber(NR) exhibits much larger tensile anisotropy at high strains than SBR. It seems that the parallel sample of NR is dominated by orientation effect at high strains. This oriented chain is expected to act as nuclei for following crystallization during second stretching and facilitates the strain-induced crystallization.

• Advances in aircraft and spacecraft science
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• v.3 no.1
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• pp.29-43
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• 2016

Composite materials are rapidly gaining popularity as an alternative to metals for structural and load bearing applications in the aerospace, automotive, alternate energy and consumer industries. With the advent of thermoplastic composites and advances in recycling technologies, fully recyclable composites are gaining ground over traditional thermoset composites. Stamp forming as an alternative processing technique for sheet products has proven to be effective in allowing the fast manufacturing rates required for mass production of components. This study investigates the feasibility of using the stamp forming technique for the processing of thermoplastic, recyclable composite materials. The material system used in this study is a self-reinforced polypropylene composite material (Curv$^{(R)}$). The investigation includes a detailed experimental study based on strain measurements using a non-contact optical measurement system in conjunction with stamping equipment to record and measure the formability of the thermoplastic composites in real time. A Design of Experiments (DOE) methodology was adopted to elucidate the effect of process parameters that included blank holder force, pre heat temperature and feed rate on stamp forming. DOE analyses indicate that feed rate had negligible influence on the strain evolution during stamp forming and blank holder force and preheat temperature had significant effect on strain evolution during forming.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.326-329
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• 2005

Since the required strength of forged part is achieved by work hardening with the accumulation of plastic strain during the cold working, severe load can be exerted on die system. So, dies are liable to the early fracture for the non-heat treated steel forging in comparison with the conventional mild steels. Therefore, it is necessary to lower the flow stress of steels as much as possible during forging steps. Bauschinger effect can be utilized to lower flow stress during forging steps by giving the tensile prestrain on the forging billet during wire drawing step. In the present study, the prestrain effect on Bauschinger phenomenon is studied to avoid difficulties with application of non-heat treated cold forging steels in practice.

• Transactions of the Korean Society of Automotive Engineers
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• v.18 no.1
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• pp.58-65
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• 2010

For the improvement of fuel consumption, the study on the use of lightweight material or thinner sheet have been carried out in automotive industry. With the need for the use of thinner sheet, the dent resistance became one of the major concern in th design of exterior panels in automotive industry. Many studies have been carried out for the dent resistance by experiment or quasi-static numerical simulation. In this study, the dent formation behavior is investigated by dynamic finite element analysis using ABAQUS. Dent formation may be affected by many factors such as sheet thickness, material properties, pre-strain, and sheet curvature. The effect of these factors on dent resistance is investigated. From the analysis following three conclusions are derived. First, dent resistance become hard as the sheet curvature radius increases. Second, dynamic dent resistance is mainly affected by bending stress rather than tensile stress. Third, the pre-strain itself do not give any guidance for dynamic dent resistance and dynamic dent resistance have to be decided considering the strain hardening and thickness reduction together. The results are considered to be reliable and useful to improve the dent damage of automotive panels.

• Advances in nano research
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• v.5 no.4
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• pp.337-357
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• 2017

Knowledge of thin films mechanical properties is strongly associated to the reliability and the performances of Nano Electro Mechanical Systems (NEMS). In the literature, there are several methods for micro materials characterization. Bulge test is an established nondestructive technique for studying the mechanical properties of thin films. This study improve the performances of NEMS by investigating the mechanical behavior of Nano rectangular thin film (NRTF) made of new material embedded in Nano Electro Mechanical Systems (NEMS) by developing the bulge test technique. The NRTF built from adhesively-bonded layers of basalt fiber reinforced polymer (BFRP) laminate composite materials in Nano size at room temperature and were used for plane-strain bulging. The NRTF is first pre-stressed to ensure that is no initial deflection before applied the loads on NRTF and then clamped between two plates. A differential pressure is applying to a deformation of the laminated composite NRTF. This makes the plane-strain bulge test idea for studying the mechanical behavior of laminated composite NRTF in both the elastic and plastic regimes. An exact solution of governing equations for symmetric cross-ply BFRP laminated composite NRTF was established with taking in-to account the effect of the residual strength from pre-stressed loading. The stress-strain relationship of the BFRP laminated composite NRTF was determined by hydraulic bulging test. The NRTF thickness gradation in different points of hemisphere formed in bulge test was analysed.

• Korean Journal of Materials Research
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• v.13 no.1
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• pp.64-68
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• 2003

The 1.6 vol% and 2.5 vol% TiNi/6061Al composites were fabricated by permanent mold casting for investigating the effect of heat treatment on tensile strength for composites. The tensile strength without T6 treatment at 293 K was increased with increasing the volume fraction of TiNi fiber and at 363 K the higher the pre-strain, the higher the tensile strength. The tensile strength of the composite with $T_{6}$ treatment at 293 K was found to increase with increasing both the amount of pre-strain and the volume fraction of TiNi fiber and was higher than that without $T_{6}$ treatment. It should be noted that the tensile strength 2.5vol%TiNi/6061Al composites rolled at a 38% reduction ratio was the maximum value of 298 MPa. The tensile strength of composites decreased with increasing the reduction ratio over 38% because of the rupture of TiNi fiber.

• Transactions of Materials Processing
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• v.15 no.2 s.83
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• pp.118-125
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• 2006

API steel is used for line-pipe to transport the oil and natural gas. As the recent trends in the development of API steel are towards the use of larger diameter and thicker plate, many researches have been studied to achieve higher strength, higher toughness and lower yield ratio in API steel. However, the strength of API steel after pipe forming is changed depending on the competition of the Bauschinger effect and work hardening which are affected by the strain history during pipe forming process. So, the purpose of this study is to investigate the influence of microstructure on the Bauschinger effect for API steel. To change the microstructure of API steel we have changed the hot rolling condition and the amounts of V and Cu addition. The compression-tensile test and the microstructure observation by OM and TEM were conducted to investigate the yield strength drop and the correlation between the Bauschinger effect and microstructure of API steel. The experimental results show that the increase of polygonal ferrites volume fraction increases the Baushcinger effect due to the back stress which comes from the increase of mobile dislocation density at polygonal ferrite interior during the compressive deformation. The hot rolling condition was more effective on the Bauschinger effect in API steel than the small amount of V and Cu addition.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2006.04a
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• pp.914-921
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• 2006

To describe the effect of the numerous and various oriented microcracks on the compressive and tensile concrete behaviors, the directional nonlocality is defined. The plasticity model using multiple failure criteria is developed for RC planar members in tension-compression. The crack damages are defined in the pre-determined reference orientations, and then the total crack damage is calculated by integrating multi-oriented crack damages. To describe the effect of directional nonlocality on the anisotropic tensile damage, based on the existing test results, the nonlocal damage factor is defined in each reference orientation. The reduced compressive strength in the cracked concrete is defined by the multi-oriented crack damages defined as excluding the tensile normal plastic strain from the compressive equivalent plastic strain. The proposed model is implemented to finite element analysis, and it is verified by comparisons with various existing panel test results.

• Journal of Welding and Joining
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• v.23 no.3
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• pp.34-39
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• 2005

It is required for the steel materials used in the sour environment to have sufficient resistance to hydrogen induced cracking(HIC). For line pipe steels, HIC resistance could be varied during pipe making process due to the large plastic deformation applied in the thick-wall pipe. In order to figure out such effect, HIC tests were performed not only in the plate condition but in the pipe condition and their results were compared in terms of cracking ratio. Test results demonstrated a detrimental effect of plastic deformation to HIC resulting in a substantial increase in the cracking ratio after pipe forming process. All of the cracks found in the pipe material were located in the outer layer of pipe where the tensile strain was resulted during pipe forming stage. In order to understand the HIC resistance of the pipe but in the plate condition, it was suggested to pre-strain the plate to some extent before the HIC test.

• Journal of the Korean Geosynthetics Society
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• v.2 no.1
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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.