• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.868-880
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• 2020

One of the main concerns in the structural integrity of offshore pipelines is mechanical damage from external loads. Pipelines are exposed to fatigue failure in welded joints due to geometric discontinuity. In addition, fatigue loads such as currents, waves, and platform motions may cause significant plastic deformation and fracture or leakage within a relatively low-cycle regime. The 2007 ASME Div. 2 Code adopts the master S―N curve for the fatigue evaluation of welded joints based on the mesh-insensitive structural stress. An extension to the master S―N curve was introduced to evaluate the low-cycle fatigue strength. This structural strain method uses the tensile properties of the material. However, the monotonic tensile properties have limitations in describing the material behavior above the elastic range because most engineering materials exhibit hardening or softening behavior under cyclic loads. The goal of this study is to extend the cyclic stress-strain behavior to the structural strain method. To this end, structural strain-based procedure was established while considering the cyclic stress-strain behavior and compared to the structural strain method with monotonic tensile properties. Finally, the improved prediction method was validated using fatigue test data from full-scale girth-welded pipes.

• 대한기계학회논문집A
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• 제24권12호
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• pp.3033-3039
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• 2000

Mechanical properties of the materials used for transportations and industrial machinery under high stain rate loading conditions have been required to provide appropriate safety assessment to these mechanical structures. The Split Hopkinson Pressure Bar(SHPB) technique with a special experimental apparatus can be used to obtain the material properties under high strain rate loading condition. There have been many studies on the material behavior under high strain rate compressive loading compared to those under tensile loading. In this paper, mechanical properties of the aluminum alloy, Al6061-T6, under high strain rate tensile loading were determined using SHPB technique.

• 한국농공학회논문집
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• 제46권1호
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• pp.41-51
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• 2004

FRP re-bar in concrete structures could be used as a substitute of steel re-bars for that cases in which aggressive environment produce high steel corrosion, or lightweight is an important design factor, or transportation cost increase significantly with the weight of the materials. But FRP fibers have only linearly elastic stress-strain behavior; whereas, steel re-bar has linear elastic behavior up to the yield point followed by large plastic deformation and strain hardening. Thus, the current FRP re-bars are not suitable concrete reinforcement where a large amount of plastic deformation prior to collapse is required. The main objectives of this study in to evaluate the tensile behavior and the fracture mode of hybrid FRP re-bar. Fracture mode of hybrid FRP re-bar is unique. The only feature common to the failure of the hybrid FRP re-bars and the composite is the random fiber fracture and multilevel fracture of sleeve fibers, and the resin laceration behavior in both the sleeve and the core areas. Also, the result of the tensile and interlaminar shear stress test results of hybrid FRP re-bar can provide its excellent tensile strength-strain and interlaminar stress-strain behavior.

• Structural Engineering and Mechanics
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• 제38권3호
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• pp.333-347
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• 2011

This paper shows the solution for an orthotropic disk under the plane strain condition obtained with complex stress functions. These stress functions were induced by Lekhnitskii and expanded by one of the authors. Regarding diametrical compression test, the finite element method poses difficulties in representing the concentrated force because the specimens must be divided into finite elements during calculation. On the other hand, the method shown in this study can exactly represent this force. Some numerical results are shown and compared with those obtained under the plane stress condition for both stress and displacement. This comparison shows that the differences between the tensile stresses occurred under the plane strain condition and also that the differences under a plane stress condition increase as the orthotropy ratio increases for some cases.

• 대한기계학회논문집
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• 제17권7호
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• pp.1772-1782
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• 1993

The characteristics of metal matrix composite under dynamic tension at high strain rates up to the order of $10^3/sec$ is studied by using newly developed apparatus. The composite material processed in this research is aluminum-alumina metal matrix composites, arid fabricated by compocasting with the fiber volume fraction from 5 to 20%. The whisker and matrix material used in this paper were ${\delta}-Al_2O_3$ and Al-6061, respectively. The mechanical tests performed in this research are low and high strain rate tensile test. At low strain-rate tensile test, the modulus of elasticity and the ultimate tensile strength of the composites were improved about 77 pct. and 55 pct., respectively comparing with the unreinforced materials. At strain-rate from $10^{-3}\;to\;10^3/s$, the effect of strain-rate on the modulus, ultimate strength, flow stress is determined. Also the effect of strain rate on the modulus, ultimate tensile strength, flow stress and elongation to failures were investigated.

• 한국자동차공학회논문집
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• 제13권2호
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• pp.127-134
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• 2005

This paper introduces a newly developed high speed material testing apparatus for tensile tests at the strain rate up to 500/sec. The tensile properties of sheet metals are indispensable for the accurate crashworthiness analysis of auto-bodies since the local strain rate reaches to 500/sec in the car crash. An appropriate experimental method has to be developed to acquire the tensile properties at the intermediate strain rate ranged from 0.003/sec to 200/sec. Tensile tests of various different steel sheets for an auto-body were perform ed to obtain the dynamic properties with respect to the strain rate. The dimensions of specimens that can provide the reasonable results were determined by the finite element analysis. A special jig fixture of a load cell is designed to reduce the load ringing phenomenon induced by unstable stress propagation at the high strain rate. Stress-strain curves were acquired for each steel sheet from the dynamic tensile test and utilized to obtain the relationship of the stress to the strain rate.

• 소성∙가공
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• 제20권4호
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• pp.309-315
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• 2011

The behavior of metallic materials at high strain rates shows different characteristics from those in quasi-static deformation. Therefore, the strain rate should be considered when simulating crash events. The objective of this paper is to evaluate the dynamic tensile characteristics of SPRC440 as a function of the volume fraction of phases. As-received SPRC440 is composed of ferrite and pearlite phases. However, ferrite and martensite phases were observed after heat treatment at $730^{\circ}C$ and $780^{\circ}C$ for 5 minutes, as expected by calculations based on the curves from dilatometry tests. High cross-head speed tensile tests were performed to acquire strain-stress curves at various strain rates ranging from 0.001 to $300\;s^{-1}$, which are typical in real vehicle crashes. It was observed that the flow stress increases with the strain rate and this trend was more pronounced in the as-received specimens consisting of ferrite and pearlite phases. It is speculated that the dislocation density in each phase has an influence on the strain rate sensitivity.

• Geomechanics and Engineering
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• 제16권1호
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• pp.35-47
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• 2018

This paper investigates the mechanisms of tunnel spalling and massive tunnel failures using fracture mechanics principles. The study starts with examining the fracture propagation due to tensile and shear failure mechanisms. It was found that, fundamentally, in rock masses with high compressive stresses, tensile fracture propagation is often a stable process which leads to a gradual failure. Shear fracture propagation tends to be an unstable process. Several real case observations of spalling failures and massive shear failures in boreholes, tunnels and underground roadways are shown in the paper. A number of numerical models were used to investigate the fracture mechanisms and extents in the roof/wall of a deep tunnel and in an underground coal mine roadway. The modelling was done using a unique fracture mechanics code FRACOD which simulates explicitly the fracture initiation and propagation process. The study has demonstrated that both tensile and shear fracturing may occur in the vicinity of an underground opening. Shallow spalling in the tunnel wall is believed to be caused by tensile fracturing from extensional strain although no tensile stress exists there. Massive large scale failure however is most likely to be caused by shear fracturing under high compressive stresses. The observation that tunnel spalling often starts when the hoop stress reaches $0.4^*UCS$ has been explained in this paper by using the extension strain criterion. At this uniaxial compressive stress level, the lateral extensional strain is equivalent to the critical strain under uniaxial tension. Scale effect on UCS commonly believed by many is unlikely the dominant factor in this phenomenon.

• Journal of Welding and Joining
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• 제22권4호
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• pp.50-58
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• 2004

In this study, high temperature tensile properties of high strength steels(POSTEN60, POSTEN80) were investigated. The three-dimensional thermal elastic-plastic analyses were conducted to investigate the characteristics of welding residual stresses in welds of high strength steels on the basis of thermal and mechanical properites at high temperature obtained from the experiment. According to the results, high temperature tensile strength of POSTEN60 steel deteriorated slowly to 10$0^{\circ}C$. As the temperature went up, the tensile strength became better because of blue shortness, and it deteriorated radically after reaching to the maximum value around 30$0^{\circ}C$. For the POSTEN80 steel, high temperature tensile strength deteriorated slowly to 20$0^{\circ}C$. As the temperature went up the tensile strength became better and it deteriorated slowly to $600^{\circ}C$ after reached to the maximum value around 30$0^{\circ}C$. Strain of high strength steels at the elevated temperature increased radically after the mercury rose to $600^{\circ}C$. The strain hardening ratio of POSTEN60 steel was larger then that of POSTEN80 steel at the elevated temperature as in the case at the room temperature and it became smaller radically after the mercury rose to 40$0^{\circ}C$. And, in the welding of high strength steels, increasing tensile strength of the steel (POSTEN60

• Advances in Computational Design
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• 제7권4호
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• pp.281-295
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• 2022

Tensile property is one of the excellent properties of ultra-high performance concrete (UHPC), and uniaxial tensile test is an important and challenging mechanical performance test of UHPC. Traditional uniaxial tensile tests of concrete materials have inherent defects such as initial eccentricity, which often lead to cracks and failure in non-test zone, and affect the testing accuracy of tensile properties of materials. In this paper, an original integrated design scheme of mould and end fixture is proposed, which achieves seamless matching between the tension end of specimen and the test fixture, and minimizes the cumulative eccentricity caused by the difference in the matching between the tension end of specimen and the local stress concentration at the end. The stress analysis and optimization design are carried out by finite element method. The curve transition in the end of specimen is preferred compared to straight line transition. The rationality of the new integrated design is verified by uniaxial tensile test of strain hardening UHPC, in which the whole stress-strain curve was measured, including the elastic behavior before cracking,strain hardening behavior after cracking and strain softening behavior.