• Proceedings of the Korean Reliability Society Conference
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• 2011.06a
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• pp.125-130
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• 2011

발전 설비는 기대 수명동안의 안정성을 확보하기 위하여 해당 규격에 부합하도록 설계하여 건설된다. 하지만 가동 중 다양한 복합 환경에 노출됨에 따라 구조물을 이루고 있는 재료의 열화 현상이 가속화되어 예기치 못한 파손이 발생할 수 있다. 기계적 물성은 재료의 기계적 거동을 나타내는 주요 척도가 되며 이는 신뢰성 및 안전과 직결된다. 하지만 기존의 역학물성을 측정하는 대부분의 시험법들은 특정 크기의 시편을 요구하고 파괴적인 시험법이기 때문에 가동 중 시설물에 적용하기가 불가능하였다. 이러한 한계점을 극복하고자 비파괴적이고 정량적인 시험이 가능한 연속압입시험법이 최근 각광받는 시험법으로서 많은 연구자들에 의해 연구되고 있다. 이 시험법은 시험 대상물의 형상에 제약을 받지 않으며 시험 절차가 매우 간단하다는 장점을 가진다. 또한 대상의 국소 부위에 시험할 수 있어 취약 부위 판별이 가능하다. 본 연구에서는 대표응력-대표변형률 기법을 통하여 인장물성을 평가하고, 압입 하중 차이를 이용하여 소재에 존재하는 잔류응력을 평가하는 기법을 소개한다. 또한, 연속압입시험을 이용하여 실제 발전소 파이프의 취약부위로 알려진 용접부에 대하여 인장물성 및 잔류응력을 측정함으로써 실제 산업체의 신뢰성 평가가 적용할 수 있음을 확인하였다.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.2
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• pp.158-162
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• 2004

Reliability of welded structures in power plant facilities is very important, and their reliability evaluation requires exact materials properties. But, the conventional PQR (Procedure Qualification Record) can hardly reflect the real material properties in the field because the test is only done on specimens with simulated welding. Therefore, a continuous indentation technique is proposed in this study for simple and non-destructive testing of in-field structures. This test measures the indentation load-depth curve during indentation and analyzes the mechanical properties such as the yield strength, tensile strength and work hardening index. This technique has been applied to evaluate the tensile properties of the weldment in the main steam pipe and hot reheater pipe in power plants under construction and in operation.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.165-172
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• 2013

Power generating unit structures are designed and built to meet standard to secure its safety for expected life time. As the structures have been exposed to combined environment, degradation of structure material is accelerated and it can cause unexpected damage; evaluating precise mechanical properties of weak site like welded area is an essential research area as it is directly connected to safety issues. Existing measuring technique like tensile test requires specific size in testing specimen yet it is destructive method which is hard to apply on running structures. To overcome above mentioned limitation, IIT is getting limelight as it is non-destructive and simple method. In this study, latest technique is introduced to evaluate tensile property and residual stress by analyzing stress field occurs under the indenter while IIT is performed. Test on welded area, the weak site of nuclear structures have been practiced and confirmed that IIT can be usefully applied to evaluate integrity in industry.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.95-100
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• 2019

Nanoindentation has been widely used for evaluating mechanical properties of nano-devices, from MEMS to packaging modules. Residual stress is also estimated from indentation tests, especially the Knoop indenter which is used for the determination of residual stress directionality. According to previous researches, the ratio of the two stress conversion factors of Knoop indentation is a constant at approximately 0.34. However, the ratio is supported by insufficient quantitative analyses, and only a few experimental results with indentation depth variation. Hence, a barrier for in-field application exists. In this research, the ratio of two conversion factors with variation in indentation depth using finite elements method has been attempted at. The magnitudes of each conversion factors were computed at uniaxial stress state from the modelled theoretical Knoop indenter and specimen. A model to estimate two stress conversion factor of the long and short axis of Knoop indenter at various indentation depths is proposed and analyzed.

• Journal of Power System Engineering
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• v.8 no.2
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• pp.12-17
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• 2004

In this study, to verify the reliability of the indentation for measuring welding residual stresses to get in method we compared the results of the method with those measured by the saw cutting method. Also for the same purpose we used a 4-point bending test and confirmed the reliability of indentation method. The specimens used in this experiment are made with the same conditions for hot reheater pipes and cold reheater pipes used in the electric power plant. Therefore we could know that the results of the methods showed the reliability of the method to obtain welding residual stresses.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.7 no.1
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• pp.41-47
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• 2011

Residual stress is the key parameter for reliability and lifetime assessment because it can reduce the fatigue strength and fracture properties of industrial structures. Recently, instrumented indentation testing (IIT) has been widely used for evaluating it, since it does not need specific specimen and time-consuming procedure. However, conventional Oliver-Pharr method, which is used for calibrating contact depth to analyze indentation load-depth curve, cannot estimate plastic pile-up between indenter and surface of specimen. Here, we introduce f parameter which is the ratio of contact depth and maximum depth, to consider pile-up height. And, its application for evaluating residual stress of weldment is introduced.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.4
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• pp.388-393
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• 2013

• The Monthly Technology and Standards
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• s.1
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• pp.54-58
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• 2002

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.4
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• pp.328-333
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• 2003

It has been widely recognized that the residual stress of the weldment affect the integrity of steel structures and cause an initiation of crack in the welded regions. Since the power plants adopt a variety of welding processes, it is necessary to know the distribution and magnitude of residual stresses. This paper has attempted to investigate the validity of the saw cutting method and the indentation method to measure the residual stresses in the steel plates. The residual stresses in the specimens of hot reheater pipes, cold reheater pipes and feed water lines in power plants were determined by the saw cutting method and the indentation method. The data were compared and reviewed for the validity of the methods.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.5
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• pp.306-314
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• 2006

Instrumented indentation technique is a new way to evaluate nondestructive such mechanical properties as flow properties, residual stress and fracture toughness by analyzing indentation load-depth curves. This study evaluated quantitatively the flow properties of steels and residual stress of weldments. First, flow properties can be evaluated by defining a representative stress and strain from analysis of deformation behavior beneath the rigid spherical indenter and the parameters obtained from instrumented indentation tests. For estimating residual stress, the deviatoric-stress part of the residual stress affects the indentation load-depth curve, so that by analyzing the difference between the residual-stress-induced indentation curve and residual-stress-free curve, the quantitative residual stress of the target region can be evaluated. The algorithm for flow property evaluation was verified by comparison with uniaxial tensile test and the residual stress evaluation model was compared to mechanical cutting and ED-XRD results.