• Tribology and Lubricants
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• v.30 no.5
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• pp.271-277
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• 2014

Hydraulic reciprocating seals have been widely used to prevent fluid leakage and to provide lubricant film on counter surface in various hydraulic system. The degradation of the seal may cause the catastrophic failure of the hydraulic system. To assess the durability of the seals and the compatibility with counter surface, accelerated life testing (ALT) has been typically employed from industry. However, ALT often takes up to a few months to cause a failure of the seals, and therefore, there is a need to develop more efficient ALT methods. In this work, the degradation characteristics of polyurethane (PU) seals from field test are investigated and they are compared to those from the component and bench tests, with an aim to contribute to the development of ALT method. From the comparison of the cross-sectional profiles of the sealing surface of the PU specimens before and after the tests, both wear and compression set are found to be responsible for degradation of the PU seals. It is also shown that the major wear mechanisms of the PU seals from the field is abrasive wear and formation of pits. The component and bench tests performed in this work are shown to reproduce such wear mechanisms, and therefore, those test methods can be used as an ALT method for PU seals. In particular, the bench test proposed in this work may be effectively utilized to assess the durability and the compatibility of the seals with the counter surface. The results of this work are expected to aid in the design of ALT for PU seal.

• Journal of IKEEE
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• v.21 no.3
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• pp.252-255
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• 2017

$Mo_xW_{1-x}Si_2$ heaters were fabricated by self-propagating high-temperature synthesis (SHS) process and post sintering process. To validate the reliability of the $Mo_xW_{1-x}Si_2$ heaters, the accelerated life test (ALT) was conducted, and then lifetime to $Mo_xW_{1-x}Si_2$ heaters was estimated by using Minitab programs. Also, the failure analysis of $Mo_xW_{1-x}Si_2$ heaters after ALT was performed through electrical and structural properties. As the results, it was confirmed that the dominant failure mode of $Mo_xW_{1-x}Si_2$ heaters is the crack formation in heaters and the delamination of protective $SiO_2$ layers.

• Journal of Korean Society for Quality Management
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• v.24 no.4
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• pp.29-43
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• 1996

This paper identifies these data by the data diagnosis in lognormal distribution and presents the method to obtain exact parameter estimates and confidence intervals of regression line. The life-stress relationship uses Arrhenius model and life data generate Class-H insulation complete data by simulation. Also, the method to estimate parameters uses least squares estimation and externally Studentized residuals can be used as test statistics for identifing outliers. And influential cases are identified by Cook's distance. This research is intended to obtain the useful information for the life of products and test method, to save time and costs, and to help optimum accelerated life test plans.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.335-342
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• 2004

The failures such as flaking, wear, crack and seizing caused by the high contact pressure accompanied with sliding motion between inner or outer ring and ball are potential failures of ball bearing. In this research, we have qualitatively selected the efficient test items through the analysis of the life and potential failures of ball bearing. The bearing's failures are related closely to the whole system in using the bearing. So, the bearing itself requires an estimation of life in order to opreate the system safely. We have tested ten ball bearings. Our research has applied both radial and axial direction force of maximum torque conditions simultaneously for the accelerated life testing. The result is established by employing the weibull plot and compared the predicted life of ball bearing to the experimental result.

• The Korean Journal of Applied Statistics
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• v.9 no.2
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• pp.119-134
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• 1996

Accelerated life testing (ALT) of a system is commonly used to reduce time and cost. ALT is achieved by subjecting the test systems to more severe conditions than the normal ones to obtain estimates of life distribution under normal condition. The major interest of this research is to use a model of incorporating the common environmental effect on the components serially linked into a system-so called frailty model for the system life time distribution under each stress and to discuss the related data analysis and comparison of the model with the generally used one. The profile likelihood is used to get an initial values required to compute maximum likelihood estimates and simulation is carried for comparison.

• Transactions of The Korea Fluid Power Systems Society
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• v.1 no.1
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• pp.10-16
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• 2004

The failures such as leakage and burst stemmed from the repetitive motions of bending and stretching of the assembly of hydraulic hose in construction machines, agriculture machines, vehicles, and industrial heavy machines can induce big troubles. Therefore, the hydraulic hose itself eventually requires an estimation of life to operate the hydraulic system safely. In this research, we have qualitatively selected the efficient test items by the analysis of the life and potential failures of hydraulic hose. We have used more than seven of hydraulic hoses simultaneously for the research. We have applied impulse pressure and half omega flexing motions to the accelerated life testing Test results have been expressed by employing weibull plot.

• Tribology and Lubricants
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• v.31 no.2
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• pp.35-41
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• 2015

A water-pump located in the cooling area of a car circulates cooling water. A particular bearing element, known as a water-pump bearing, installed in the rotating part carries the entire load. The failure of this water-pump bearing has a direct impact on the failure of the automobile engine, and so securing its reliability is crucial. Several researchers have examined the design principles of the water-pump bearing, but there are no reports on the life characteristic of the bearing yet. Herein, we report the construction of test equipment to reproduce the spalling of the roller contact, which is the main failure mode of the chosen water-pump bearing. We chose the radial load as an accelerated stress factor and validated the failure mode by monitoring the surface defects. We conducted the accelerated life test after determining the accelerated stress level through a combination of finite element analysis and a preliminary test. In the life tests, we used an accelerometer to perform failure diagnosis. In the last stage of this study, we present a statistical reliability analysis. Thus, we fully estimated the shape parameter of the water-pump bearing, accelerating level on the load , and the lifetime (MTTF and B₁₀ life) under real use conditions, and finally proposed an interval estimation value considering the uncertainty of the estimated value.

• International Journal of Reliability and Applications
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• v.15 no.2
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• pp.125-150
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• 2014

Accelerated life tests (ALTs) are extensively used to determine the reliability of a product in a short period of time. Test units are subject to elevated stresses which yield quick failures. ALT can be carried out using constant-stress, step-stress, progressive-stress, cyclic-stress or random-stress loading and their various combinations. An ALT with linearly increasing stress is ramp-stress test. Much of the previous work on planning ALTs has focused on constant-stress, step-stress, ramp-stress schemes and their various combinations where the stress is generally increased. This paper presents an optimal design of ramp soak-stress ALT model which is based on the principle of Thermal cycling. Thermal cycling involves applying high and low temperatures repeatedly over time. The optimal plan consists in finding out relevant experimental variables, namely, stress rates and stress rate change points, by minimizing variance of reliability function with pre-specified mission time under normal operating conditions. The Burr type XII life distribution and time-censored data have been used for the purpose. Burr type XII life distribution has been found appropriate for accelerated life testing experiments. The method developed has been explained using a numerical example and sensitivity analysis carried out.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1387-1390
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• 2008

Pneumatic cylinder is widely used in the various industrial fields. Reliability Study of this field is very important part to the related companies. In this study, we want to predict the life of pneumatic cylinder using Cox (or proportional hazards) model. Used in biomedical applications, the Cox model can be used as an accelerated life testing model. We considered working pressure and temperature as stress factors. The statistical software is used to analyze and forecast the life data.

• Korean Journal of Metals and Materials
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• v.47 no.12
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• pp.819-827
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• 2009

This work is concerned with a statistical analysis of factors affecting the irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels for core internals of pressurized water reactors (PWR). The microstructural and environmental factors were reviewed and critically evaluated by the statistical analysis. The Cr depletion at grain boundary was determined to have no significant correlation with the IASCC susceptibility. The threshold irradiation fluence of IASCC in a PWR was statistically calculated to decrease from 5.799 to 1.914 DPA with increase of temperature from 320 to $340^{\circ}C$. From the analysis of the relationship between applied stress and time-to-failure of stainless steel components based on an accelerated life testing model, it was found that B2 life of a baffle former bolt exposed to neutron fluence of 20 and 75 DPA was at least 2.5 and 0.4 year, respectively, within 95% confidence interval.