A Review of Accelerated Test Models

Engineers in the manufacturing industries have used accelerated test (AT) experiments for many decades. The purpose of AT experiments is to acquire reliability information quickly. Test units of a material, component, subsystem or entire systems are subjected to higher-than-usual levels of one or more accelerating variables such as temperature or stress. Then the AT results are used to predict life of the units at use conditions. The extrapolation is typically justified (correctly or incorrectly) on the basis of physically motivated models or a combination of empirical model fitting with a sufficient amount of previous experience in testing similar units. The need to extrapolate in both time and the accelerating variables generally necessitates the use of fully parametric models. Statisticians have made important contributions in the development of appropriate stochastic models for AT data [typically a distribution for the response and regression relationships between the parameters of this distribution and the accelerating variable(s)], statistical methods for AT planning (choice of accelerating variable levels and allocation of available test units to those levels) and methods of estimation of suitable reliability metrics. This paper provides a review of many of the AT models that have been used successfully in this area.Comment: Published at http://dx.doi.org/10.1214/088342306000000321 in the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Methodology for designing accelerated aging tests for predicting life of photovoltaic arrays

A methodology for designing aging tests in which life prediction was paramount was developed. The methodology builds upon experience with regard to aging behavior in those material classes which are expected to be utilized as encapsulant elements, viz., glasses and polymers, and upon experience with the design of aging tests. The experiences were reviewed, and results are discussed in detail

Coupled thermo-mechanical damage modelling for structural steel in fire conditions

This paper aims at developing a coupled thermo-mechanical damage model for structural 6 steel at elevated temperatures. The need for adequate modelling of steel deterioration behaviour 7 remains a challenging task in structural fire engineering because of the complexity inherent in 8 the damage states of steel under combined actions of mechanical and fire loading. A fully three9 dimensional damage-coupled constitutive model is developed in this work based on the hypothesis 10 of effective stress space and isotropic damage theory. The new coupling model, adapted from 11 an enhanced Lemaitre’s ductile damage equation and taking into account temperature-dependent 12 thermal degradation, is a phenomenological approach where the underlying mechanisms that govern 13 the damage processes have been retained. The proposed damage model comprises a limited number 14 of parameters that could be identified using unloading slopes of stress-strain relationships through 15 tensile coupon tests. The proposed damage model is successfully implemented in the finite element 16 software ABAQUS and validated against a comprehensive range of experimental results. The 17 damage-affected structural response is accurately reproduced under various loading conditions and 18 a wide temperature range, demonstrating that the proposed damage model is a useful tool in giving a 19 realistic representation of steel deterioration behaviour for structural fire engineering applications

Time-Dependent Reliability Framework for Durability Design of FRP Composites

The life-cycle performance, durability, and aging characteristics of Fiber Reinforced Polymer (FRP or Structural Composites) have been of keen interest to the engineers engaged in the FRP design, construction, and manufacturing. Unlike conventional construction materials such as steel and concrete, the design guidelines to account for the aging of FRP are somewhat scattered or not available in an approved or consistent format. Loss of strength over time or aging of any structural material should be of concern to engineers as the in-service lifespan of many engineering structures is expected to exceed 100 years. Use of durability strength-reduction factors or factors of safety (aka knock-down factors) is a common way to account for the anticipated in-situ site conditions during the FRP design phase; however, the considerations for FRP service life is often ignored or smeared into knock-down or safety factors. The individual or combined effect of these factors can be arbitrary and can lead to the system’s premature failure (or overdesigns), rendering FRP commercial application unreliable (or cost-prohibitive). Reliability or risk-based approach to the development of strength reduction factors has been successfully applied in modern Load, and Resistance Factor Design codes (e.g., highway bridge design specifications), and an original design framework (i.e., a set of ideas, tools or techniques that forms the basis for filling in the final details) incorporating the time-dependent behavior of FRP composites (e.g., decrease of mechanical strengths with an increase of variability with aging) is proposed. The research presented herein utilizes available natural and accelerated aged test databases to develop a relationship between the probability of failures (using reliability index and confidence intervals to measure reliability) and the desired service life of FRP members. The proposed framework illustrates how to use time-dependent reliability techniques to account for environmental and physical effects. For environmental effects, developing a direct relationship of reliability index with time-dependent durability works better, and for physical effects, indirect inclusion of probability in projecting the time (or cycles) to failure is more effective. The techniques presented in this research, along with three real-life design examples and a case study (i.e., the basis of design), can be readily used by design professionals to ascertain an adequate life cycle performance of FRP while maintaining a consistent component or system-level reliability. The intent is to allow others to refine this knowledge bank and to further the professional FRP design practice in a consistent, rational manner leading to the adaptation of formal codes and specifications. Although the presented data and associated findings primarily refer to pultruded glass fiber reinforced polymers (GFRP) in Vinylester resin, the presented framework can be easily extended to other structural composites. The report entails thorough documentation of published analytical and experimental formulations for various modes of FRP failures due to the typical aging process (e.g., moisture, temperature, alkalinity, and sustained loading, and a combination thereof) along with an associated sampling of durability strength reduction factors. Critical reviews of deterministic and stochastic methods are conducted, and gaps in the current approach to determining durability factors for FRP systems have been identified. A Basis of Design (BOD) for vinylester/polyester-based GFRP in a submerged marine condition using an accelerated test database with illustrative design examples has also been included for a better understanding of the proposed time-dependent reliability-durability concept. Understanding how an FRP system’s reliability changes over its life-span, designers will be able to confidentially choose the most suitable durability strength reduction factors, or factors of safety, that will meet their design’s target service life-span without exceeding strength or service limit states. Since absolute safety is not possible, all FRP members must be designed for a specific acceptable risk of failure. The research illustrates a unique set of techniques for determining FRP composites\u27 durability strength reduction factors, or threshold design values, by integrating durability characteristics developed in the laboratory tests with desired service lives and commonly acceptable risks of failure. Due to the limited availability of complete durability datasets, vast applications, varieties of FRP composites, and the enormity of calibration efforts required, this research proposes additional work to determine the final durability recommendations for the general use of FRP composites. However, this unique research forms a rational tool for designing specific FRP composites that are consistent with other modern design codes, takes into account their target service lives (e.g., 10, 50, 100 years), and bridges the gap between traditional deterministic FRP design methods and state of the art risked-based design philosophies

Lifetime Test for Optical Transmitters in the ATLAS Liquid Argon Calorimeter Readout System

Accelerated lifetime test has been carried out for 147 days on the custom-made optical transmitters used in the ATLAS Liquid Argon Calorimeter front-end electronics readout system. The lifetime of these optical transmitters is estimated to be greater than 200 years and exceeds the design goal for the LHC. The random failure rate has been estimated at 9.6´10-7 per hour at 90% confidence level

Accelerated Life Testing Of Subsea Equipment Under Hydrostatic Pressure

Accelerated Life Testing (ALT) is an effective method of demonstrating and improving product reliability in applications where the products are expected to perform for a long period of time. ALT accelerates a given failure mode by testing at amplified stress level(s) in excess of operational limits. Statistical analysis (parameter estimation) is then performed on the data, based on an acceleration model to make life predictions at use level. The acceleration model thus forms the basis of accelerated life testing methodology. Well established accelerated models such as the Arrhenius model and the Inverse Power Law (IPL) model exist for key stresses such as temperature and voltage. But there are other stresses like subsea pressure, where there is no clear model of choice. This research proposes a pressure-life (acceleration) model for the first time for life prediction under subsea pressure for key mechanical/physical failure mechanisms. Three independent accelerated tests were conducted and their results analyzed to identify the best model for the pressure-life relationship. The testing included material tests in standard coupons to investigate the effect of subsea pressure on key physical, mechanical, and electrical properties. Tests were also conducted at the component level on critical components that function as a pressure barrier. By comparing the likelihood values of multiple reasonable candidate models for the individual tests, the exponential model was identified as a good model for the pressure-life relationship. In addition to consistently providing good fit among the three tests, the exponential model was also consistent with field data (validation with over 10 years of field data) and demonstrated several characteristics that enable robust life predictions in a variety iv of scenarios. In addition the research also used the process of Bayesian analysis to incorporate prior information from field and test data to bolster the results and increase the confidence in the predictions from the proposed model

Short Beam Shear Strength Evaluations of GFRP Composites: Correlations Through Accelerated and Natural Aging

Fiber Reinforced Polymers (FRP) composites have been materials of interest in replacing or reinforcing steel, wood, and concrete, but lack of understanding of degradation under physical and chemical aging is a main concern. Through many years of research, the understanding of aging or durability of GFRPs has improved. To be able to evaluate aging related degradation rates, an accelerated aging methodology under varying environments is introduced. Accelerated aging is a concept used to age composites in a lab controlled environment under varying pH conditions (2 to 13) and temperatures (~ -20° to +160°F). Once acceleratedly aged testing is completed, Arrhenius relationships and Time-Temperature Superposition principles can be used to correlate the accelerated data with the naturally aged data to create strength reduction (knock-down) factors for 100-year service life. In this work, accelerated and natural aged data for glass fiber reinforced vinyl-ester composites was collected through in-lab testing and literature data. Knowing that interlaminar shear strength (ILSS) is the most detrimental mechanical property, this work was solely focused on the degradation of ILSS of glass fiber reinforced vinyl-ester under varying pH environments and temperatures. The degradation of ILSS in composites has been found to follow two aging trends. Most of the ILSS degradation occurs within the first 3-10 years of service followed by a more gradual trend. The focus of this report is to understand the reason behind a large amount of strength loss in the initial service life. Accelerated testing was also performed on vinyl ester composites with different thicknesses, as well as pure vinyl ester samples. Examining how degradation occurs with varying thicknesses and the resin system apart from the composite is very crucial in understanding the reasons behind aging. To achieve 100-year service knock-down factors, a correlation between acceleratedly aged and naturally aged data was formulated. In this study, the correlation was possible in a neutral pH environment due to the lack of natural aged data in alkaline and acidic environments. Therefore, alkaline and acidic environment knock-down factors are based solely on the plots shifted with acceleratedly aged data. Through hundreds of samples tested, alkaline environment is found to be the most detrimental compared to other chemical aging conditions, especially under elevated temperatures (~150°F). Under high alkaline (pH ~ 13) conditions, significant strength loss of up to 70% within the first thirty days of aging was observed, especially under high temperature conditions. 100-year service knock-down factors were arrived at using the Arrhenius relationship. This relationship is formed through reaction rates based solely on temperature dependency

Accelerated life test of high luminosity AlGaInP LEDs

Specific tests to assess reliability of high luminosity AlInGaP LED for outdoor applications are needed. In this paper tests to propose a model involving three parameters: temperature, humidity and current have been carried out. Temperature, humidity and current accelerated model has been proposed to evaluate the reliability of this type of LED. Degradation and catastrophic failure mechanisms have been analyzed. Finally we analyze the effect of serial resistance in power luminosity degradation