Accelerated Test Procedures Research Articles

Analysis of accelerated failure times of rehabilitation liners subjected to a constant or variable pressure

Abstract Pipe rehabilitation liners are often installed in host pipes that lie below the water table. As such, they are subjected to external hydrostatic pressure. The external pressure leads to early deformation in the liners, which could ultimately lead to its failing or buckling before its expected service lifetime is achieved. Experiments involving long term buckling behavior of liners are typically accelerated lifetime testing procedures. In an accelerated testing procedure involving type I censoring a liner is often subjected to a constant external hydrostatic pressure and observed until it fails or for a certain time, t whichever occurs first. Liners that do not fail at time t are deemed censored observations. While a constant pressure is convenient to use in experimental situations, in reality pressure fluctuates under soil conditions over time depending on the water table. It is, therefore, desirable to study and compare accelerated life testing models under variable and constant pressure. Statistical analysis of data on accelerated time till buckling is based on the maximum likelihood procedure for censored or uncensored observations. It is known that maximum likelihood estimates of model parameters are asymptotically (for very large samples) unbiased and normally distributed. In practice, however, it is important to determine the applicability of these asymptotic assumptions for relatively small samples with censored observations under constant and variable pressure. In this study, the accelerated Weibull model for time till buckling under constant pressure is extended to variable pressure and the applicability of the model is investigated by simulation for different sample sizes and different levels of censoring. Data were generated through computer simulation and estimates of parameters were obtained using the maximum likelihood and Newton–Raphson methods. Results on the statistical properties (concerning distribution and bias) of model parameter estimates were used to make inference about the applicability of the accelerated life testing procedure for relatively small censored samples encountered in practice. Expressions from the model are given for computing the probability of survival until a given time or the survival time for a given probability of survival.

Thermal aging of painted galvanized steel after mechanical deformation

In the present work the effects of thermo-mechanical degradation on the protective properties of coil coated samples are studied. A cupping test was performed, with penetration depths of 2, 4 and 6 mm and a finite-element model was set to define the local strain for the different depths of penetration. Different thermal stresses were applied to accelerate the natural weathering: a thermal cycle with a maximum temperature of 45 °C was chosen to simulate the stress occurring in the outdoor exposure, a second thermal cycle (maximum temperature 95 °C) was proposed as 1-week accelerated testing procedure. The effects of the thermo-mechanical stress were studied by electrochemical impedance spectroscopy. The ageing procedure and the electrochemical measurements were useful to evaluate the barrier and adhesion properties for the various coating formulations and to define the influence of the different strain levels.

Accelerated Testing Procedures for Evaluating Separation Layer Performance in Open-Graded Base Courses

An accelerated testing procedure was developed to evaluate pumping and deformation performance of separation layers used between open-graded bases and lime-treated and untreated subgrade soils. A pneumatically driven, electronically controlled repeated loading laboratory apparatus was developed. A low plasticity clay and a silty clay till soil were tested. A nonwoven geotextile and a dense-graded base aggregate were evaluated as separation layers. Tests were conducted at various loading levels and durations to develop the testing procedure. Comparisons of specimen pumping and deflection performance for stabilized, separated specimens were made relative to nonseparated and nonstabilized control specimens. Ultimately, loading at levels upwards of five times that expected in the field, for short durations (20,000 cycles or less), was performed to evaluate separation layer performance. Separation layer performance under accelerated loading correlated well to long-term field loading level performance. The testing method was sensitive to the use of geotextiles for separation between soils and open-graded aggregates, though some pumping of fines still occurred. The dense-graded separation level showed a marked drop-off in performance as moisture contents increased above optimum. A distinct breakpoint in subgrade soil performance occurred at a cone index of 200 (California bearing ratio 4). Soils with strengths less than cone index of 200 performed poorly, whereas soils above that strength pumped and deformed minimally regardless of separation layer type. Based upon performance comparisons with nonseparated test cases, the use of a separation layer between lime-stabilized subgrades and open-graded aggregate bases is recommended.

A Method to Estimate the Rheological Properties of Aged Asphalt Binders without Actually aging them

ABSTRACT Asphalt binder stiffens due to aging during the mixing and lay down operations as well as during the service life of the pavement. The aging processes in the field have been simulated in the laboratory through the use of accelerated test procedures and tracked through rheological characterization of the aged asphalts. However, the laboratory aging procedures along with the rheological characterization of the aged samples are time-consuming processes. Some researchers have been seeking methods to shorten the time involved in generating this information, while others have been suggesting improvements in the test procedures to get reproducible and reliable data, and still others are attempting to find the procedures that would give information in the laboratory that is closer to the reality in the field. In all cases, it is the rheological properties of the aged binders that are used for characterizing the extent of aging. The present work is a step in the same direction but chooses a different approach. A method is proposed to shorten the time involved in generating the information on the aged binder rheology. The suggested procedure aids in estimating the rheological properties of aged asphalts without actually going through the aging process. In order to establish the procedure, rheological data on unaged asphalts are compared with rheological data on aged samples after the Rolling Thin Film Oven Test (RTFOT) and Pressure Aging Vessel (PAV)-aging. It is shown that estimates of the specification temperatures based on the high and intermediate temperature rheological properties of the aged asphalts can be obtained without actually aging the binders in the laboratory. By eliminating the laboratory aging procedures to obtain rheological properties of the aged asphalts, there is a great saving in time and the information is obtained rapidly.

Wearing-out of components in a variable environment

Abstract The probabilistic model for the wearing-out component’s performance is studied, based on the Markov-Additive Principle (MAP) of wearing-out. It is proved that under certain assumptions this principle leads to the accelerated life model (ALM), according to which the lifetime distribution functions (DFs) in some baseline (reference) and real environments are connected via a general time transformation function. The specific linear transformation function is widely used in the accelerated testing procedures. The change point in environment is considered and the remaining lifetime DF is derived in the formal way for various settings. The effect of shocks, increasing the components wear-out, is studied as well as the effect of an operation of the opposite nature that decreases wear-out. Different maintenance activities usually can be considered as wear-out decreasing. Several generalisations are presented.

Study of low-dose-rate radiation effects on commercial linear bipolar ICs

The results of a detailed study of the degradation of commercial linear bipolar ICs due to irradiation at four dose rates are presented. The time dependence of the degradation rate at the different dose rates is shown to be consistent with a model that describes a mechanism for defect generation in the devices used in this study. Based on this model, an accelerated test procedure for bipolar devices is proposed.

Insulation characterization in multistress conditions by accelerated life tests: An application to XLPE and EPR for high voltage cables

This paper focuses on accelerated testing procedures that can be applied to XLPE and EPR for high voltage cables and that are aimed to achieve the thermo-electrical endurance characterization of insulation. Several methods and techniques for single and combined thermo-electrical stress are illustrated; indices for the evaluation of electrical, thermal, and multi-stress endurance are reported and their derivation is shown. Indications on the feasibility of the design-stress inference from tests on laboratory specimens also are given. Experimental results mainly refer to cable models, but data relevant to full-sized cables, as well as to simple, flat specimens, are also considered.

Durability assessment of automotive coatings—Design and evaluation of accelerated tests

In this work the influence of different climatic parameters on the rate of gloss reduction was studied for a number of different automotive coatings. The purpose was to improve the reliability of accelerated test procedures by selecting test conditions that improve the simulation of the degradation processes that occur during outdoor exposure. Results from Florida exposure and accelerated exposure tests were evaluated using an acceleration factor based on the light dose needed to cause a certain reduction in gloss. The results indicate that agreement between accelerated tests and Florida exposure is improved when a test sequence simulating acid rain is included in the accelerated test cycle.

An evaluation of metallic coatings for erosive wear resistance in die casting applications

An accelerated testing procedure with a multi-pin die has been established for the study of the erosive wear of die materials and surface treatments in die casting applications, under production conditions. This test procedure utilizes the wear of core pins as a surrogate measure of die erosive wear, and is used to evaluate the efficacy of coatings and die surface treatments in reducing erosive wear in dies under actual production conditions. Metallic coatings of W, Mo, and Pt deposited on the test pins using the plasma source ion implantation technique (PSII) technology in the ion assisted deposition mode, were then evaluated for erosion resistance using this developed test procedure. The results of this evaluation on the effectiveness of these metallic coatings are presented here along with possible reasons for such coating behavior.

Characterization and modeling of nonlinear viscoelastic response of PEEK resin and PEEK composites

The viscoelastic properties of PEEK resin and unidirectional PEEK composite APC-2 were characterized by an accelerated testing procedure based on the time-temperature and time-stress superposition principles. The results obtained by creep and recovery tests are presented, and the constitutive modeling of the nonlinear viscoelastic response is discussed. The stress-dependent viscoelastic responses of the composite were modeled by a special case of Schapery's nonlinear viscoelastic equation with a kernel of the general power law. The relations for the temperature-time scale shift factors and stress-time scale factors are given. With the equations and parameters presented, the linear and nonlinear viscoelastic behaviors of APC-2 composite laminate from ambient temperature up to the glass transition temperature can be described analytically. Comparisons were made on the nonlinear properties of the 15° off-axis laminate, transverse laminate and pure resin. The results agree with the theory that the matrix octahedral shear stress is the main variable determining the nonlinearity of polymers and polymer-based composites.

Suitability of two rapid test methods for determining the alkali reactivity of sands

Five sands have been used in a study aimed at determining their reactivity potential using a number of test methods. The sands were used either in mortar bars or in concrete prisms containing non-reactive coarse aggregate, at various alkali levels, and subjected to accelerated test procedures. Expansion of mortar bars stored in 1 M NaOH solution at 80°C indicated the sands to be potentially reactive, whereas those made at 2·5 or 3·5% cement alkali levels and subjected to autoclaving for 4 or 5 h at 127°C did not produce deleterious expansions, except in the case of one sand. Microstructural studies have indicated that the mechanisms operating in the two methods could be different. Although some of the sands had petrographic features that could indicate potential reactivity, unfortunately, no controlled long-term experimental or field data are available related to the service performance of the sands in concrete structures. Further investigation is required to establish or verify the suitability of the rapid test methods for determining the reactivity of sands with more confidence.

Characterization of SiO/sub 2/ dielectric breakdown for reliability simulation

The authors analyze breakdown distributions of thin oxide MOS capacitors to reveal the limitations of accelerated procedures for reliability simulation at circuit level. The detailed analysis of the breakdown distributions corresponding to different stress voltages shows the presence of three modes of breakdown, associated with different kinds of defects. These breakdown nodes control the shape of the distributions at different oxide field ranges. The mean values of time-to-breakdown, charge-to-breakdown, and energy-to-breakdown and their dependence on oxide field are found to be directly related to the mode dominating the distribution. Since the shape of the breakdown distribution changes with stress voltage, an accelerated testing procedure must provide a way to extrapolate to operation conditions not only the mean value, but also the shape of the distribution. The results indicate that different physical mechanisms cause the breakdown at low and high fields.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stabilization of HgI2 X-ray detectors

The principal problem limiting the use of HgI2 X-ray detectors is instability. We developed an encapsulation procedure potentially capable of stabilizing devices for 20 years. We also developed an accelerated test procedure to accurately simulate many years of aging.

A viscoelastic fracture mechanics assessment of slow crack growth in polyethylene gas distribution pipe materials

A substantial amount of the plastic gas distribution pipe currently in service, as well as that anticipated for future use, is polyethylene (PE). While this material has an excellent record, due to a variety of abnormal loadings that can arise in long time service, some slow crack growth (SCG) related field failures have occurred. Accelerated test procedures and analysis methodologies to predict accurately the long term performance of PE gas pipes are therefore required. One such procedure is the three-point bend test conducted on a pre-cracked pipe segment that is known as the SCG test. The research reported here examines the validity of this test through the application of advanced fracture mechanics principles. Specifically, considering the viscoelastic behavior of PE materials and using the region of dominance concept, the delimitation of a linear elastic fracture mechanics (LEFM) interpretation of the SCG test was developed. Excluding the time spent in crack initiation, the long-term performance of the category of materials for which the assumptions of LEFM are valid can therefore be effectively predicted from SCG test results. Through illustrative examples, this paper demonstrates the veracity of the approach by comparing time to failure predictions for abnormal service loadings with actual gas distribution service experiences.

Laboratory simulation of space and salt water environmental effects on polymers

AbstractMost polymers are eroded rapidly (&gt;0.1 μm/h) in the low earth orbit (LEO) environment when facing the direction of flight. The rate of erosion varies little with the polymer type, except for fluoropolymers and silicones. Methods for simulating such an environment are reviewed. Secondly, the oxidation lifetime of stabilized and unstabilized polyolefins can be reduced 50 to 90 percent, with a, high yield of carboxylic acid (i.e. chain scission) oxidation products, when thermal oxidation at 70 to 90°C takes place in 0.01 to 0.1 molar sodium chloride solutions, as compared to thermal oxidation in air. Both sets of observations illustrate the need for detailed mechanistic investigations before accelerated test procedures can be developed to predict material lifetime.

A Method to Estimate the Lifetime of Solid Electrical Insulation

This paper describes a new statistical method of lifetime estimation of a solid electrical insulation using data from an accelerated aging test. The method simultaneously uses the maximum likelihood estimation and the Newton-Raphson methods. The novel aspect of this work is the introduction of a new threshold term in addition to well-known estimation techniques based on the inverse power model combined with the Weibull distribution. Thus, we can estimate the threshold value with a confidence interval. In order to reduce time and costs of voltage-lifetime test5, an accelerated testing procedure with type I censoring is considered.

The air mass zero behavior of inversion layer solar cells with a silicon nitride dielectric layer

The reliability of metal-insulator-semiconductor (MIS) inversion layer (IL) solar cells with a plasma-deposited silicon-nitride dielectric layer is investigated by irradiating these cells with a mercury super pressure lamp. In comparison to the air mass zero (AM0) sun spectrum this is a slightly accelerated testing procedure. The dependence of the dark and illuminated I-V characteristics of the cells on the irradiation time is monitored. As revealed by the experiments, the values of the short-circuit current, of the open-circuit voltage, and of the fill factor of these cells diminish by irradiation. As a consequence, a drastic decrease of the air mass one (AM1) total-area efficiency can be observed. The observed irradiation effects can be understood by introducing an inversion layer recombination current which is due to photon-generated interface states at the silicon nitride/thin silicon oxide/silicon interface.

The corrosion performance of electroless nickel: The role of accelerated test methods compared to actual process condition evaluations in oil and gas environments

SummaryThe corrosion resistance or protective properties afforded by coatings of nickel-phosphorus deposited from electroless nickel baths is an important reason these coatings are selected for many applications. Most industry and military specifications in effect today cite test procedures such as the 5% neutral salt spray, Acetic acid salt spray, Corrodkote or Kesternich as a measure of the corrosion performance of these coatings. These test methods, which have been used extensively in the automotive industries, are primarily designed to evaluate coating porosity and defects that allow contact of the corrosive environment with the base metal. They do not necessarily, however, quantify the corrosion performance of the coating. The acceptance of electroless nickel coatings into many industries, for applications where their long term corrosion performance in a specific process environment is a primary concern, is extremely important to their future growth. The risks associated with a premature failure of a plated part in these applications can be great for the user in terms of downtime and lost productivity. Ultimately such unforseen failures are reflected in a reluctance to consider electroless nickel coatings for new applications, impairing future market growth. This underscores the importance of adequate testing of these coatings before they are recommended for service. It also raises fundamental questions regarding the use of accelerated test procedures such as the ASTM B-117 (DIN 50021) salt spray test, compared to actual tests in process environments, as a primary basis for recommending electroless nickel coatings. This paper will explore these points in some detail.

Low frequency fatigue crack growth behavior of a470 class 8 rotor steel at 538°C (1000°F)

Fatigue crack growth rate data were developed at various frequencies and hold times at maximum load for A470 Class 8 steel at 538°C (1000°F) by using an accelerated test method which involves alternating test frequency and temperature. These data were consistent with fatigue crack growth rate data obtained from the same material and developed according to the ASTM specification E-647-T78. This result suggests that there is no transient effect associated with the alternating test frequency and temperature and that the accelerated testing procedure can be used to expedite the development of elevated temperature fatigue crack growth rate data at very low frequencies and long hold times. At 538°C (1000°F) fatigue crack growth properties with hold time developed from both 1T-CT and multiple-edge-craek tension specimens fall in the same scatter band on the da/ dN vs ΔK plot. This result indicates the applicability of ΔK to characterize the fatigue crack growth behavior with hold time at elevated temperature. Also, the model proposed by Saxena et al. was found to successfully predict the fatigue crack growth rate properties with 28 min hold time of the A470 Class 8 rotor steel at 538°C (1000°F).

Accelerated Tests for Evaluating Protein Stability in Frozen Skim Milk

ABSTRACTSkim milk was diafiltered (DF) to remove about 90% of the lactose, and major salts and vitamins were restored to normal levels. Protein stability at –9°C was monitored using two accelerated test procedures. One involved DF skim milk containing a normal level of carbohydrate and 3.9 times the normal level of calcium, and the other involved DF skim milk containing a normal level of calcium and one‐half the normal level of carbohydrate. Both approaches resulted in casein instability in about 1 wk at ‐9°C. These test procedures should be useful for rapidly assessing the effects of additives or processing on the stability of casein in frozen skim milk.