Ouk Sub Lee

Reliability of Buried Pipeline Using a Theory of Probability of Failure

The reliability estimation of buried pipeline with corrosion defects is presented. The reliability of corroded pipeline has been estimated by using a theory of probability of failure. And the reliability has been analyzed in accordance with a target safety level. The probability of failure is calculated using the FORM (first order reliability method). The changes in probability of failure corresponding to three corrosion models and eight failure pressure models are systematically investigated in detail. It is highly suggested that the plant designer should select appropriate operating conditions and design parameters and analyze the reliability of buried pipeline with corrosion defects according to the probability of failure and a required target safety level. The normalized margin is defined and estimated accordingly. Furthermore, the normalized margin is used to predict the failure probability using the fitting lines between failure probability and normalized margin.

Dynamic Deformation Under a Modified Split Hopkinson Pressure Bar Experiment

This paper presents a modified split-Hopkinson pressure bar (SHPB) technique. The dynamic stress-strain behaviors were estimated at room temperature and subzero temperature to −75°C by using the conventional SHPB and compared with a modified SHPB technique. A computer simulation using a finite element algorithm is also performed to study the dynamic material responses. Furthermore, we attempt to find a proper material constitutive law by using the simulation process. It is suggested that the modified SHPB test used in this study can be successfully utilized to offer an experimental condition of a higher strain rate than that obtained from the conventional SHPB test.

The Reliability Estimation of Pipeline Using FORM, SORM and Monte Carlo Simulation with FAD

In this paper, the reliability estimation of pipelines is performed by employing the probabilistic method, which accounts for the uncertainties in the load and resistance parameters of the limit state function. The FORM (first order reliability method) and the SORM (second order reliability method) are carried out to estimate the failure probability of pipeline utilizing the FAD (failure assessment diagram). And the reliability of pipeline is assessed by using this failure probability and analyzed in accordance with a target safety level. Furthermore, the MCS (Monte Carlo Simulation) is used to verify the results of the FORM and the SORM. It is noted that the failure probability increases with the increase of dent depth, gouge depth, operating pressure, outside radius, and the decrease of wall thickness. It is found that the FORM utilizing the FAD is a useful and is an efficient method to estimate the failure probability in the reliability assessment of a pipeline. Furthermore, the pipeline safety assessment technique with the deterministic procedure utilizing the FAD only is turned out more conservative than those obtained by using the probability theory together with the FAD. The probabilistic method such as the FORM, the SORM and the MCS can be used by most plant designers regarding the operating condition and design parameters.

Determination of stress intensity factors and J-integrals using the method of caustics

Abstract Applications of the optical shadow method of reflective caustics to the measurement of the stress intensity factor and J -integral in various specimens are investigated. The necessary experimental requirements to help in determining an accurate stress intensity factor and J -integral are described. The ratios of r 0 (radius of initial curve)/ r p , ( plastic zone size ) and r 0 t (thickness of specimen) are found to be very important experimental parameters with which to obtain meaningful stress and/or strain intensities surrounding crack tips. The appropriate ranges to determine accurate values of stress intensity factor and J -integral for polycarbonate (compact tension) and aluminum (c-shaped tension) specimens are presented.

Micro-strain distribution around a crack tip by electron beam moiré methods

The microscopic deformation around a crack tip in a small tensile specimen of aluminum was measured by using electron beam moiré methods. The tensile test was carried out in a scanning electron microscope(SEM) by using specially designed small tensile testing apparatus for an SEM. A line grating with a pitch of 87 nm was written at the crack tip by electron beam lithography. SEM images of the grating contained(original) moiré fringes at certain magnifications without loadings on the specimen. The displacement and strain in the region 10 to 50 μ m from the crack tip were evaluated by analyzing the (mismatch) moiré fringes. Measured strains normal to loadings(longitudinal strains) near the crack tip were found to be comparable to those estimated using linear elastic fracture mechanics(LEFM), for the lowest load, and elastic-plastic fracture mechanics, at effective crack tip position located within the actual crack tip. The exponent of the strain singularity in the vicinity of the actual crack tip changed with applied stress, ranging between −0.64 and −1.0. The measured crack tip opening displacement(CTOD) values disagreed with CTODs based on the Hutchinson-Rice-Rosengren(HRR) field, the Dugdale model, and LEFM. The experimental plastic zones spread from the line of crack extension up to an angle of about 60°, as expected from LEFM theory.

Reliability estimation of buried gas pipelines in terms of various types of random variable distribution

This paper presents the effects of corrosion environments of failure pressure model for buried pipelines on failure prediction by using a failure probability. The FORM (first order reliability method) is used in order to estimate the failure probability in the buried pipelines with corrosion defects. The effects of varying distribution types of random variables such as normal, lognormal and Weibull distributions on the failure probability of buried pipelines are systematically investigated. It is found that the failure probability for the MB31G model is larger than that for the B31G model. And the failure probability is estimated as the largest for the Weibull distribution and the smallest for the normal distribution. The effect of data scattering in corrosion environments on failure probability is also investigated and it is recognized that the scattering of wall thickness and yield strength of pipeline affects the failure probability significantly. The normalized margin is defined and estimated. Furthermore, the normalized margin is used to predict the failure probability using the fitting lines between failure probability and normalized margin.

Dynamic Deformation Behavior of Rubber under High Strain Rate Compressive Loading

A specific experimental method, the split Hopkinson pressure bar (SHPB) technique is used to determine the dynamic material properties under the impact compressive loading condition with strain-rate of the order of 103/s~104/s. The dynamic deformation behavior of rubber materials widely used for the isolation of vibration from varying structures under dynamic loading is determined by using the Split Hopkinson Pressure Bar technique. The relationships between the stresses at transition points of rubber materials and the strain rate are found to be bilinear. However, an interesting relationship between the strains at transition points of rubber materials and the strain rate, which needs further investigation, is noted.

Failure Probability of Corrosion Pipeline with Varying Boundary Condition

This paper presents the effect of external corrosion, material properties, operation condition and design thickness in pipeline on failure prediction using a failure probability model. The predicted failure assessment for the simulated corrosion defects discovered in corroded pipeline is compared with that determined by ANSI/ASME B31G code and a modified B31G method. The effects of environmental, operational, and random design variables such as defect depth, pipe diameter, defect length, fluid pressure, corrosion rate, material yield stress and pipe thickness on the failure probability are systematically studied using a failure probability model for the corrosion pipeline.

Fatigue crack propagation between holes and particles

This study investigates the behaviour of a fatigue crack, propagating between holes or particles. These inhomogeneities are found to play a significant role on the fatigue propagation behaviour. The study is performed by simulations, both experimentally and numerically. The experiments show that holes or partly debonded particles on the average accelerate the crack growth, even though they may exert a decelerating effect, due to shielding, during some phase of a crack tip passage between the holes or particles. Besides debonding, a high material stiffness ratio between matrix and particles also tends to accelerate fatigue crack growth. The numerical simulations are able to explain the essential experimental results quite satisfactorily, even though a few seemingly anomalous experimental results are still not convincingly explained.

Residual Stress Interference by Two Micro-Vickers Indentations

An electron beam moiré method is employed to measure the residual strain produced by a Vickers indenter in a WC-4,7wt.% Co specimen. Line gratings 57μm wide by 45μm high, with a pitch of 87nm, are written by electron beam lithography. Two interior regions of the grating are loaded by the micro-Vickers indenter with 9.8N for 30s. The residual stress interference phenomena caused by two impressions of the Vickers indenter are estimated by using the displacement fringes recorded with the aid of an electron beam moiré (EBM) technique. The measured residual stresses are fitted to the theoretical values estimated by two available models such as Yoffe model and CME (Chiang-Marshall-Evans) model.