Accelerated Test Procedures Research Articles

Method for accelerated testing of wind turbine gearbox bearings based on frictional energy in the rolling contact

This paper investigates a method for a novel accelerated test procedure for rolling bearings in wind turbine gearboxes. Established test procedures e. g. highly accelerated lifetime tests (HALT) mainly reflect fatigue damage and not slip-induced damage patterns (e.g. smearing). A commonly used criterion to rate the severity of an operating point regarding slip-induced damage is the frictional energy. Frictional energy is introduced into the rolling contact as soon as there is a significant combination of simultaneously occurring slip and pressure. Up to now only critical thresholds for this criterion that must not be exceeded have been identified on small-scale component test rigs. However, the permissible amount and duration of overshoots of the critical threshold that lead to damage in the actual application are not understood yet. Therefore, the aim is to conduct tests on full-size test rigs in which frictional energy is applied until slip-induced damage occurs. In order to perform these tests in a reasonable time, it is essential to accelerate the test procedures. Thus, this paper introduces a method for accelerated test procedures based on frictional energy in the rolling contact. The requirement is that the same cumulated frictional energy as in field operation is applied in a shorter time on the test rig. A further requirement is that the frictional power in the accelerated test procedure never exceeds the maximum frictional power occurring in the field. This paper shows the theoretical background regarding frictional energy and the transfer to the test procedure.

Carbonation behaviour of concretes with alternative binders at different CO2 concentrations

AbstractIn order to reduce CO2 emissions in the construction sector, alternative binders are attracting more and more attention. Whether these alternative binders contain a lower clinker‐cement ratio or no Portland cement clinker at all, to be truly sustainable they must not compromise durability and must therefore be durable.The progress of carbonation and the associated reduction of the pH value have a major influence on the corrosion protection of steel reinforcement, especially as such “low carbon” concretes are typically less resistant against CO2 exposure. Previous research has shown that accelerated tests at elevated CO2 concentrations can lead to altered carbonation behavior, especially in systems low in Ca(OH)2.In this study, hydration products and carbonation resistance of various alternative cements and binders, such as composite cements with calcined clays or modified steel slag, alkali‐activated materials, C$SMA cement and a C‐S‐H binder were investigated. The carbonation was studied on mortar samples stored in three different CO2 concentrations ‐ atmospheric storage (∼0.04 vol.%), 0.3 vol.% and 1.0 vol.% CO2. It was found that the binders can be divided into three clusters on the basis of their hydration products and w/CaO or w/(CaO + MgOeq + Na2Oeq + K2Oeq) ratios, which in turn provide information on the carbonation behavior. For each cluster a different CO2 concentration can be recommended to achieve a realistic carbonation behavior in accelerated test procedures.

Recommendation of RILEM TC 271-ASC: New accelerated test procedure for the assessment of resistance of natural stone and fired-clay brick units against salt crystallization

This recommendation is devoted to testing the resistance of natural stone and fired-clay brick units against salt crystallization. The procedure was developed by the RILEM TC 271-ASC to evaluate the durability of porous building materials against salt crystallization through a laboratory method that allows for accelerated testing without compromising the reliability of the results. The new procedure is designed to replicate salt damage caused by crystallization near the surface of materials as a result of capillary transport and evaporation. A new approach is proposed that considers the presence of two stages in the salt crystallization test. In the first, the accumulation stage, salts gradually accumulate on or near the surface of the material due to evaporation. In the second, the propagation stage, damage initiates and develops due to changes in moisture content and relative humidity that trigger salt dissolution and crystallization cycles. To achieve this, two types of salt were tested, namely sodium chloride and sodium sulphate, with each salt tested separately. A methodology for assessing the salt-induced damage is proposed, which includes visual and photographical observations and measurement of material loss. The procedure has been preliminarily validated in round robin tests.

Solar Photovoltaic Modules’ Performance Reliability and Degradation Analysis—A Review

The current geometric increase in the global deployment of solar photovoltaic (PV) modules, both at utility-scale and residential roof-top systems, is majorly attributed to its affordability, scalability, long-term warranty and, most importantly, the continuous reduction in the levelized cost of electricity (LCOE) of solar PV in numerous countries. In addition, PV deployment is expected to continue this growth trend as energy portfolio globally shifts towards cleaner energy technologies. However, irrespective of the PV module type/material and component technology, the modules are exposed to a wide range of environmental conditions during outdoor deployment. Oftentimes, these environmental conditions are extreme for the modules and subject them to harsh chemical, photo-chemical and thermo-mechanical stress. Asides from manufacturing defects, these conditions contribute immensely to PV module’s aging rate, defects and degradation. Therefore, in recent times, there has been various investigations into PV reliability and degradation mechanisms. These studies do not only provide insight on how PV module’s performance degrades over time, but more importantly, they serve as meaningful input information for future developments in PV technologies, as well as performance prediction for better financial modelling. In view of this, prompt and efficient detection and classification of degradation modes and mechanisms due to manufacturing imperfections and field conditions are of great importance towards minimizing potential failure and associated risks. In the literature, several methods, ranging from visual inspection, electrical parameter measurements (EPM), imaging methods, and most recently data-driven techniques have been proposed and utilized to measure or characterize PV module degradation signatures and mechanisms/pathways. In this paper, we present a critical review of recent studies whereby solar PV systems performance reliability and degradation were analyzed. The aim is to make cogent contributions to the state-of-the-art, identify various critical issues and propose thoughtful ideas for future studies particularly in the area of data-driven analytics. In contrast with statistical and visual inspection approaches that tend to be time consuming and require huge human expertise, data-driven analytic methods including machine learning (ML) and deep learning (DL) models have impressive computational capacities to process voluminous data, with vast features, with reduced computation time. Thus, they can be deployed for assessing module performance in laboratories, manufacturing, and field deployments. With the huge size of PV modules’ installations especially in utility scale systems, coupled with the voluminous datasets generated in terms of EPM and imaging data features, ML and DL can learn irregular patterns and make conclusions in the prediction, diagnosis and classification of PV degradation signatures, with reduced computation time. Analysis and comparison of different models proposed for solar PV degradation are critically reviewed, in terms of the methodologies, characterization techniques, datasets, feature extraction mechanisms, accelerated testing procedures and classification procedures. Finally, we briefly highlight research gaps and summarize some recommendations for the future studies.

A simple suggested approach to reduce the testing time of concrete surface abrasion using ASTM C1138

The ASTM C1138 standard underwater abrasion test can be considered as the best available experimental tool to qualitatively evaluate the abrasion resistance of concrete surfaces in hydraulic structures. It introduces an accelerated test procedure to simulate the high speed motion of water and waterborne materials on concrete surfaces. However, the test period is relatively long, which is composed of 6 time steps of 12 h each, totaling a complete 72 testing hours for each sample. Thus, although of the simple test procedure and good potential to simulate real abrasion in hydraulic structures, it is difficult to perform and requires a long time if many concrete samples are needed to be compared. At the aim of investigating the possibility of reducing the total testing time of ASTM C 1138, an experimental work was conducted in this research. Three concretes with low strength (LSC), normal strength (NSC) and high strength (HSC) were tested at different rotational speeds of 800, 1200 and 1600 rpm. The test results showed that increasing the test speed from the standard 1200 rpm–1600 rpm resulted in an average increase in an abrasion weight loss of 42.6–47.0%, which encourages the possibility of reducing the total testing time by 40 % if the test is performed at a speed of 1600 rpm. The test results of this study and others from the literature showed that the relationship between the abrasion weight loss and testing time (up to 72 h) can adequately be represented using a linear correlation. The R2 values of the linear correlations of 16 different concrete mixtures ranged from 0.936 to 0.999. Based on the obtained results, a simplified modified test procedure was suggested to reduce the total testing time from 72 h to 15 h.

Influence of initial material properties on the degradation of mortars with low expansion cements subjected to external sulfate attack

External sulphate attack consists in reactions between sulphate ions and hydration products causing crystallisation pressures then macroscopic expansion and cracking. The penetration of sulphates within the material is the rate-limiting step. Thus, laboratory tests can last a few years before giving usable results. Moreover some cement-based materials show relatively long latency period and low expansion, hence the need for accelerated testing procedure and/or alternative indicator to shorten the response time. It is possible to shorten the response time by stimulating the penetration of sulphates. This can be achieved by drying the material at 60 °C then to saturate it with a sulphate solution. The impact of this thermal pre-conditioning on the sulphate attack phenomenology still remains unclear. In this study, two sulphate-resisting cements, one Portland cement with low-C3A content (SR3) and a CEM III/B cement, and two blended slag-fly ash cements CEM V were subjected to external sulphate attack. The behaviour of mortars with and without thermal pre-conditioning was analysed. The microstructure of tested specimens was also studied using TGA, Hg-porosimetry, X-ray tomography, and XRD. Pre-conditioning did not influence the chemical properties of formed sulphate-rich phases but modified the microstructure of mortars with different trends depending on cement type. CEM V mortars with pre-conditioning showed finer porosity and lower volumes of precipitation. These modifications of mortar microstructure were consistent with the macroscopic behaviour of mortars with and without thermal pre-conditioning.

Comprehensive semi-empirical approach to describe alkali aggregate reaction (AAR) induced expansion in the laboratory

Describing alkali-aggregate reaction (AAR) induced expansion is a very complicated task, yet very useful to forecast and understand the potential of further deterioration of affected concrete. Back in the 90s, Larive developed an empirical model to describe AAR-kinetics in the laboratory. Although relatively well accepted by AAR community, Larive's equation is based on empirical mathematical parameters that do not have a clear physicochemical meaning and thus cannot directly describe AAR-induced development for a variety of materials and exposure conditions. This work proposes a novel semi-empirical model based on Larive's works and using five measurable parameters that directly affect AAR-induced development such as aggregate's type and nature/reactivity, alkali content of the concrete, temperature and external relative humidity. A comprehensive laboratory test campaign along with literature data are used to calibrate the proposed model. Coefficients from each of the above parameters are then established according to their influence on the chemical reaction. Results show that the proposed model may accurately describe AAR-induced expansion in the laboratory for a wide range of materials and exposure conditions without further calibration. Moreover, it can help identifying time periods and associate expansion levels for distinct accelerated test procedures.

Microstructure and Degradation of Mortar Containing Waste Glass Aggregate as Evaluated by Various Microscopic Techniques.

The primary aim of this article is to focus on the alkali-silica reaction (ASR) in mortar specimens containing coloured waste glass used as an aggregate. Mortar expansion was measured using the ASTM C 1260 accelerated test procedure until the specimens disintegrated. Special attention was paid to the microscopic examination of the damaged mortar. Various methods were used for this purpose, including optical microscopy in reflected and transmitted light with one and two crossed polarizers. The specimens were also subjected to the scanning electron microscopy observations with energy dispersive spectroscopy (SEM-EDS). The data obtained from these techniques provided information on the mechanism of glass-containing mortar degradation due to ASR and also allowed the comparison of different microscopic techniques in terms of the information they can provide on ASR occurrence.

Sulfate resistance of portland-limestone cement concrete systems: Linking laboratory and field performances

The work presented in this paper comprises an extensive testing program on the resistance of portland-limestone cement (PLC) concrete systems to external sulfate attack. Several mixture parameters, namely interground limestone content, C3A content, and SCM type and replacement level, were evaluated in a wide range of experimental conditions designed to link, replicate, and predict laboratory and field performances. The presented results have highlighted the role that the experimental parameters play in the degradation process, which were shown to be particularly pronounced in systems of low sulfate resistance with high limestone contents, owing to their high propensity for thaumasite formation. The findings presented will be crucial to evaluate/develop an accelerated testing procedure that provides a better correlation with actual field performance of concrete structures.

Effects on Reinforced Concrete Slabs by Environmental Loads

Throughout the development of accelerated testing procedures, the behavior of pure concrete specimens was evaluated for a comparison in the different effects that are generated between concrete specimens with resistance to the compression of 20 MPa (200 kg/cm2) regarding water/cement (w/c) 0.62, being this compressive the employee minimum value according to Mexican regulations and which is currently used in the construction of structural elements of the housing and specimens with compressive strength of 25 MPa (250 kg/cm2) with ratio water/cement (w/ c) of 0.55, considering that this resistance corresponds to the recommended minimum value according to criteria of durability in coastal areas to the same regulations. The different concrete structures early deterioration has been the cause of constant study in recent years, finding that their durability differs as specified in the Mexican regulations. Damages that occur in elements directly exposed to environmental influences, as in the case of roof slabs, are attributed, mainly, to errors and defects in the design stages or execution in the construction process and not to the effect of these charges. Visualizing the effect of conditions of moisture, temperature, and solar radiation on this type of structure requires years of direct observation. The mass loss and degradation of the exposed surface, as well as the penetration of the carbonation front, were established as measurement parameters. The significant mass loss registred was 0.374% in the specimens with an f’c of 20 MPa and a minor value of 0.046% in the specimens with an f’c of 25 MPa. Regarding the carbonatation front, a value of 10.7 mm was registered in the short edges of the trial specimens with an f´c of 20 MPa and 7.3 mm for the specimens with an f´c of 25 MPa. Even though the mass loss percentages were less than 1%, and considering that carbonatation triggers corrosion processes that lead to the early deterioration of reinforced concrete structures, the results obtained indicated that the increase to the compressive strength by decreasing the w/c relation, besides favoring the concrete mechanical properties, it can also increase its durability; demonstrating the necessity of designing concrete mixtures that attend the local weather conditions, and even more in the significant exposed elements such as roof slabs.

Specifying resistance against carbonation-induced corrosion by performance

The determination of performance levels for carbonation resistance from modelling or from benchmarking current national provisions does not give consistent results. Current modelling tends to inaccurately determine values for concretes containing cements with less than 55% Portland cement clinker. This is probably due to the input from standard carbonation tests, where the relative humidity of 57% to 65% significantly reduces further hydration. Data from sheltered natural exposure sites show that concrete made from slowly reacting cements perform much better than predicted by modelling. Testing the carbonation resistance of concretes conforming to the recommendations in BS 8500-1 is not normally necessary, but in the author's opinion there are a few situations, identified in the paper, where carbonation testing is advisable. As it is almost essential that performance results are obtained in a reasonable timescale, carbonation-resistance requirements should be specified in terms of accelerated test criteria. Specifying the performance requirements given in this paper for the accelerated test procedure (prEN 12390-12) will be safe but very conservative for slowly reacting cements. Where carbonation resistance is specified by performance, it is prudent to specify, in addition to the performance requirements, limiting values, until sufficient experience is obtained from specifying durability by performance.

Comparison of the mechanical deterioration behavior of C/BMI composite under hygro-thermal or vacuum-thermal cycling

The effects of vacuum-thermal cycling and hygro-thermal cycling on the fiber/matrix interface characteristics of a carbon fiber reinforced bismaleimide (C/BMI) composite were investigated to assess the degradation behavior of the mechanical property of C/BMI composite in low earth orbit (LEO) and atmosphere environment. The feasibility of using one comprehensive accelerated test procedure capable of representing the LEO and atmosphere environmental effects simultaneously for the aerospace plane was explored for the first time. A simplified mathematical approach and the corresponding model was supplied firstly to prove the reliability and effectiveness of the widely-used SEM observation method in the assessment of fiber matrix interface properties. The dominating degradation mechanism of the C/BMI composites caused by vacuum-thermal cycling in LEO environment is the interfacial sliding induced by thermal stress, whereas the thermal oxidation and decomposition of the matrix is the main degradation mechanism after hygro-thermal cycling in atmosphere environment. Candidate composite materials for aerospace plane must be tested in atmosphere and LEO environments separately in the environmental simulation experiments before their application.

Selected methods of accelerated durability tests of systems and subsystems of high mobility vehicles

The analysis of selected methods useful in accelerated testing of systems and subsystems of high mobility vehicles has been presented. The paper contains information about a few methods applied in calculating the load signal in time and frequency domians: Constant Amplitude Loading – CAL, Peak Valley Extraction – PVE, Block Load Sequence – BLS, Statistical Exceedance Method – SEM, Increasing Load Frequency – ILF, Multiaxial Peak Valley Extraction – MPVE, Time Correlated Fatigue Analysis – TCFA, Simple PSD Scaling, Shock Response Spectrum – SRS, Extreme Response Spectrum – ERS, Fatigue Damage Spectrum – FDS. Parameters such as pseudo – damage, equivalent loads and equivalent mileage have been presented as well. Those parameters are useful when the influence of road conditions on durability of the vehicle is compared. An initial assessment has been presented on the example of a tested vehicle of high mobility. The high mobility wheeled vehicle was tested in various on- and off-road conditions at the proving ground test center. After tests the signal of load acting on a chosen element of the vehicle frame was inspected, corrected and analyzed. The goal of testing was to find the method to calculate the remaining useful life of the main part of the vehicle. The results obtained have proved the validity of the adopted accelerated testing procedure for this type of vehicles.

Accelerated lifetime testing of reinforced polymer gears

The main advantages of polymer gears in comparison to metal gears are low manufacturing costs for mass production, vibration damping and no need for lubricant. In the literature and guidelines, the allowable gear endurance limits for bending and contact stresses are mainly given for polyamides (PA) and polyacetals (POM). The large number of suitable polymer gear materials is available, but the standards offer little support for the lifetime calculations of polymer gears from other materials. Therefore testing of gear geometry and materials combinations cannot be avoided in the design of an optimal gear drive. However, gear testing is very time consuming and expensive, especially when testing several different material combinations at different testing conditions. By applying the accelerated testing procedure gear test time and costs can decrease significantly. Determination of the gear temperature during meshing is needed for the precise calculation of plastic gears. The presented temperature calculation model is using the Hachmann–Strickle temperature model, however with corrected and improved input parameters, which were determined from the test results. Accelerated tests were conducted on different combinations of reinforced and unreinforced commercially available materials: PA6, PA66, POM and PPS. Glass and carbon fiber were used for reinforcement. The research goal was characterization of different material pairs with friction coefficient, time strength, wear and failure mechanism in the relation to speed of rotation and load level. (Less)

What could and should be done differently: failure-oriented-accelerated-testing (FOAT) and its role in making an aerospace electronics device into a product

High operational reliability of an electronic material or a device intended for aerospace applications is critical, and, in the author’s opinion, cannot be assured, if the underlying physics of failure is not well understood and the never-zero probability of failure is not predicted and made adequate for the particular material, device and application. The situation is the same in some other areas of electronics materials engineering, such as military, medical, or long-haul communications, where high level of reliability is required. The situation is different in today’s commercial electronics, where cost and time-to-market are typically more important than high reliability. Failure-oriented-accelerated-testing (FOAT) of aerospace electronics materials and products and its role in making a viable device into a reliable product is addressed and discussed vs. very popular today highly-accelerated-life-testing (HALT). The differences of the two accelerated test procedures and objectives is briefly discussed. FOAT is an essential part of the recently suggested probabilistic design for reliability (PDfR) approach in electronics engineering. It is argued that high (adequate) reliability level of aerospace electronics materials and devices cannot be achieved and assured, if their never-zero probability-of-failure is not quantified for the given (anticipated) combination of the loading conditions (stresses, stimuli) and time in operation. It is the application of the FOAT, the heart of the highly effective and highly flexible PDfR concept, that should be employed and mastered, when high reliability of a material or a device is imperative. The general concepts are illustrated by numerical examples. They are based on an analytical modeling approach, as the FOAT models are.

Accelerated Fretting Wear Tests for Contacts Exposed to Atmosphere

Engineering components can be subjected to normal and/or rotational fretting wear with contacts that are intermittently exposed to the atmosphere. Exposure to the environment may lead to the alteration at the contact due to the changing role of third body particles such as hard oxides which can result in abrasion. The abrasion due to hard oxide particles differs for the closed contact and intermittently opened contact. In the former scenario, the oxides are compacted into tribo-film, while in the latter case they remain loose, leading to bigger role of abrasion. Standard fretting test setup employed to estimate the fretting wear characteristic operates with a constant load such that the contact remains closed between the counter surfaces and does not simulate the opening and closing of the contacts as observed in certain applications. The forceful interruptions to the experiments to simulate open and close condition of the contact require considerable amount of time and effort. In this paper, an accelerated test procedure is proposed and investigated to capture the effect of intermittent opening of the contact without stopping the experiments. A test rig is designed to simulate the opening and closing conditions, and tests were performed with abrasive diamond-like particles. Friction and wear results are compared with those of intermittently contact opening conditions along with operating wear mechanisms. Scanning electron microscope analysis showed that the wear mechanism observed in the case of fretting with intermittent opening of contact is similar to that of fretting with diamond-like abrasives at the contact.

Cable Test and Breakdown Voltage Determination of Joysense Cable Insulation

Cross-linked Polyethylene (XLPE) has been used as the insulation for polymeric power cables for its superior advantages. This type of cable insulation are famously known and used for their good dielectric properties, mechanical properties, thermal properties, and probability to be utilized at high temperature. This study is of four (4) parts; designing suitable method for cable test, accelerated testing procedures applied to XLPE insulation for high voltage cables, online partial discharge determination, and aging test. To study the insulation durability to AC high voltage operation, the breakdown strength and aging were investigated under different setting of temperature. The breakdown voltages of XLPE were measured at different temperatures of 30<sup>0</sup>C, 50<sup>0</sup>C and finally at 70<sup>0</sup>C. Lastly, the aging effect of cable insulation was observed by conducting the AC breakdown voltage test after the aging process. Results showed that the breakdown voltage and aging of XLPE cables will decrease with increase of temperature setting.<em> </em>

Characterizing Fatigue Failure Behavior of Modified Asphalt Binders from Linear Amplitude Sweep Test

The fatigue characteristics of asphalt binder contributes to the fatigue resistance of asphalt concrete and pavements. The linear amplitude sweep (LAS) test (AASHTO TP 101) has been developed as the accelerated testing procedure for fatigue specification of asphalt binders in the United States. Recently a newly energy-based fatigue failure definition and unified failure criterion have been proposed for improving the LAS procedure and data interpreting. However, these new improvements need more verification and calibration on more materials with different modifiers for distinguishing the fatigue performance. In this paper, the improved LAS testing procedure was applied on totally two neat asphalt binders and six modified asphalt binders, which covered the high modulus asphalt binders (HMABs), SBS modified binder, high viscosity (HV) binder and crumb rubber (CR) modified binder. Based on the simplified-viscoelastic continuum damage (S-VECD) modeling approach, three material properties of linear viscoelastic property, damage characteristic curve and failure criterion were characterized for fatigue life prediction of all tested binders. Test results indicates that the CR binder demonstrated the highest failure strain followed by the SBS and compound SBS+HV binders, HMABs and neat binders. Further results of predictive fatigue life also verified the best fatigue life of CR binder followed by SBS+HV binders, SBS binder and HMABs. The HV additives to SBS binder was found to linearly increase the binder fatigue lives and the HMAB-RA showed better fatigue performance than HMAB-PR. Generally, the fatigue life ranking determined from the multiple-loading rates of LAS tests is consistent to the failure strain results and thus, the failure strain identified from the standard LAS-5 test could be used as a fatigue performance parameter.

Giant Radiolytic Dissolution Rates of Aqueous Ceria Observed in Situ by Liquid-Cell TEM.

The dynamics of cerium oxide nanoparticle aqueous corrosion are revealed in situ. We use innovative liquid-cell transmission electron microscopy (TEM) combined with deliberate high-intensity electron-beam irradiation of nanoparticle suspensions. This enables life video-recording of materials reactions in liquid, with nm resolution. We introduce image quantification to measure detailed rates of dissolution as a function of time and particle size to be compared with literature data. Giant dissolution rates, exceeding any previous reports for chemical dissolution rates at room temperature by many orders of magnitude, are discovered. The reasons for this accelerated dissolution are outlined, including the importance of the radiolysis of water preceding the ceria attack. Electron-water interaction generates radicals, ions, and hydrated electrons, which assist in hydration and reductive dissolution of oxide minerals. The presented methodology has the potential to become a novel accelerated testing procedure to compare multiple nanoscale materials for relative aqueous durability. The ceria-water system is of crucial importance for the fields of catalysis, abrasive polishing, environmental remediation, and as simulant for actinide oxide behaviour in contact with liquid for nuclear engineering.

Accelerated testing of cementitious materials for resistance to physical sulfate attack

Sulfate attack can cause serious damage to concrete structures. Susceptibility of binders to chemical sulfate attack is often tested, however, no standard method exists for evaluation of concrete’s resistance to physical sulfate attack. Traditional field testing of concrete’s resistance to sulfate attack may take several years. Thus development of an accelerated testing procedure is required. The aim of this research is to evaluate suitable exposure conditions and deterioration evaluation methods suitable for accelerated testing. A number of methods for assessment of the level of deterioration were assessed. It was found that 100 thermal cycles between 5 and 30°C while immersed in 30wt% sodium sulfate solution were sufficient to assess the resistance of a wide range of mortar mixes. The mass loss proved as the most effective measure of the deterioration level.