Accelerated pavement testing and modeling of reflective cracking in pavements

In the field of reflective cracking initiation and propagation in pavement structures, an Accelerated Pavement Testing (APT) has been developed. The numerical method is also supported. Numerical results obtained by calculations made with the finite element CESAR-LCPC software are then compared with the experimental data. The reflective cracking APT experience is the first step in the development of a new laboratory equipment to evaluate the design of bituminous layers to repair pavements. The experiment was performed on a 30 m track where eight discontinuities were established by cutting-off the sub-bases in order to ensure a crack development in the bituminous layer. The conditions that promote crack propagation were reproduced by the application of heavy periodic loads using the Fabac ALT-APT test rig. The width of the bituminous layer was decreased to facilitate the observation of the crack propagation, while avoiding creep of the material. The structure has a large number of sensors that measure longitudinal and transverse deformations at the bottom and at the surface of the bituminous layer. The deflections and the displacements are measured at several levels of the structure, as well as the temperature and the lateral edge cracking which is monitored by cracking sensors on both sides of the track. Preliminary results on two joints were used to determine the mechanisms of rupture of the structure. Four more tests with improved sensors have completed the information. Experimental and numerical results are compared in order to validate the analysis of the fatigue process (debonding, damage and cracking) in full scale pavement.     

Effect of full-size and down-scaled accelerated traffic loading on pavement behavior

Accelerated pavement testing (APT) is an effective testing procedure to evaluate asphalt pavements. With APT it is possible to determine and measure the structural response and pavement performance under a controlled, accelerated damage accumulation in a compressed period of time. However, different types of APT technologies can lead to different results. Full-size loading devices simulate road traffic accurately, but are expensive, while down-scaled size simulators are cost effective, nevertheless further away from reality. In this work, two types of APT mobile load simulators with different loading characteristics are compared with respect to pavement response in the field and in the laboratory. The MLS10 is a full-size simulator, whereas the MMLS3 is a one-third scale device. The relationship between the devices was studied in terms of the measured strains induced by both machines in the same pavement. Therefore, a testing field was instrumented with strain gauges and first trafficked with MLS10. Later, a slab of the instrumented pavement was cut off the road and tested in the laboratory with the smaller MMLS3. Furthermore, the structure of the pavement was modelled with a viscoelastic finite element method model and the moving loads of both machines were simulated considering size, speed and approximate footprints of their tires. As for the pavement materials, the properties of the different asphalt layers were determined in the laboratory. Experimentally acquired strain data were used to validate the models. Stress fields under different loading and environmental conditions were analysed and compared. The evaluation shows that the models can predict the pavement response under different loading conditions. However, they still need to be improved to increase the accuracy under different conditions. Further, the analysis of the strains show that both load simulators induce a different stress–strain situation and scaling of the pavement should be considered.     

Laboratory scale erosion testing of a wear resistant glass–ceramic

Abstract

The testing of erosion resistant materials is usually performed on laboratory scale test rigs under accelerated wear conditions. The validity of this is questionable because of the dependence of wear mechanisms on a variety of impact parameters. Data are presented that have been obtained from such tests on a glass–ceramic (Silceram) which has been developed as a low-cost erosion resistant lining material. Various impact conditions have been investigated, including impact angle, particle velocity, and impact frequency. The data concerning the effects of particle velocity show very good agreement with one particular erosion model, although there is an apparent dependence on other test variables.

MST/611     

Fatigue crack growth testing at negative stress ratios: discussion on the comparability of testing results

It is an accepted fact in fatigue community that compressive loads contribute to fatigue crack growth. Evidences range from fatigue crack growth under fully compressive loads to effects of compressive underloads to negative stress ratio loading. Because the crack closes under compression and the crack flanks transmit compressive stresses, the loading situation is completely different to those of tensile loading. The present paper addresses the comparability of crack growth testing procedures at negative stress ratios. It reveals that compressive loading at the crack tip differs in different specimens for an equal maximum stress intensity factor K_max and negative stress ratio R. Furthermore, the crack length can significantly influence the loading conditions at the crack tip for tension–compression loading. Depending on the specimen type and crack length, a negative force ratio may lead to a change of algebraic sign of the stresses at the crack tip or not. As a consequence, the comparability of available literature results for R ≤ 0 tests is not ensured. Proposals to improve the comparability of tension–compression crack growth testing will be given.     

Thermo-mechanical fatigue testing of a rhenium-free single-crystalline super alloy

Abstract

Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys.

In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.     

Quasi-static fracture toughness testing of a single CT specimen at two test conditions

Abstract

It is proposed that a single CT specimen can be used for determining J ₀.₂ at two testing conditions, provided it can be ensured that the crack tip plastic zones for the two tests do not interfere. This is achieved by extending the crack at the end of the first fracture test by fatigue cycling at ambient temperature to obtain the starting crack for the second test. This method has been validated by testing thermally aged CT specimens of modified 9Cr - 1Mo steel at 653 K and 803 K. The J-Δa values obtained by a multispecimen method at a specific temperature were on a single curve irrespective of whether the data were generated from the first test or second test on that sample. Also, the J-Δa curves obtained using a single specimen normalisation method from data on first and second tests were within the expected specimen to specimen variation.     

Structural response of grid-reinforced bituminous pavements

Grid reinforcement is becoming a standard construction and rehabilitation technique to improve the performance of bituminous pavements. Currently, selection of the appropriate grid type and position is based on empirical criteria or derived from the results of laboratory tests which consider a single aspect of the mechanical behavior of the grid-reinforced systems. An improvement in the existing design and testing approaches could be obtained considering the actual response of grid-reinforced systems under vehicular loads. An instrumented pavement section was constructed to achieve this objective by installing a glass fiber polymer grid (FP) and a carbon fiber/glass fiber grid (CF) inside a double-layered asphalt surfacing along an in-service road. This pavement is part of a wider project which also involves a RILEM inter-laboratory test on the same reinforced systems. The pavement response to falling weight deflectometer (FWD) and real-scale truck loads was measured using pressure cells and asphalt strain gauges installed inside the pavement. A layered elastic theory (LET) model was adopted to perform both back-calculation of layer moduli and forward-calculation (simulation) of pavement stress and strain. The FWD and the real-scale tests yielded congruent results highlighting that the strain field inside the double-layered surfacing was considerably reduced by the installation of the CF/glass fiber grid whereas the glass FP grid was probably too stiff, potentially leading to interface debonding. The LET model proved to be a simple and effective tool for a first-approach analysis of the reinforcement pavement response.     

Diametral compression testing of metal matrix composites

Abstract

For the design and optimisation of advanced materials, the possibility of reliably testing very small amounts of material or laboratory scale simple shape specimens is particularly interesting. To satisfy such requirements, this work presents a new procedure for mechanical testing of metal matrix composites. The method is essentially a diametral compression test of cylindrical specimens and represents an improvement of the ‘Brazilian’ test. The mechanisms leading to fracture of composites have been investigated via the mechanical behaviour of different metal matrix composites (A354–SiC_p A354–A1₂O₃ short fibres) and consideration given to the role of the major parameters involved (both test and material related). The results of the diametral compression test, in terms of rupture strength have been compared with those of conventional tensile tests and characterisation of the microstructure via light and electron microscopy has been carried out. Evaluation of the results led to the conclusion that the failure of the materials is due to a tensile mechanism, according to elasticity theory, and that under identical testing conditions, for all the composites examined, the ratio of ultimate tensile strength/rupture strength is constant. The method developed constitutes a useful and rapid tool for both the selection of raw materials and the optimisation of metal matrix composite manufacturing processes.

MST/2060     

Schadensakkumulationshypothesen zur Lebensdauervorhersage bei schwingender Beanspruchung. Teil 1. – Ein kritischer Überblick—

Cumulative Damage Theories for the Prediction of Fatigue Life . Most fatigue data are determined in constant stress amplitude tests. Therefore they are not applicable directly for the prediction of fatigue life under service loads: A ?cumulative damage theory”? is necessary. For about 350 program test series (blocked 8 stress level tests) the cumulative damage sum Σ n_i/N_i at failure is calculated. The mean value of this ratio is near 1,0 and thus agrees with Miner's rule; however the scatter is extremely high. Tests in bending give significantly lower damage sums than tests under axial loads. Furthermore about 130 random and flight by flight tests are analysed. Next, modified linear damage theories are discussed and it shows that only theories which take residual stresses into account will improve the accuracy. A relative fatigue life estimation is proposed, where one test under service conditions is the basis and Miner's rule is used as a transfer function.     

Schadensakkumulationshypothesen zur Lebensdauervorhersage bei schwingender Beanspruchung. Teil 2. – Ein kritischer Überblick—

Cumulative Damage Theories for the Prediction of Fatigue Life . Most fatigue data are determined in constant stress amplitude tests. Therefore they are not applicable directly for the prediction of fatigue life under service loads: A “cumulative damage theory” is necessary. For about 350 program test series (blocked 8 stress level tests) the cumulative damage sum Σ n_i/N_i at failure is calculated. The mean value of this ratio is near 1,0 and thus agrees with Miner's rule; however the scatter is extremely high. Tests in bending give significantly lower damage sums than tests under axial loads. Furthermore about 130 random and flight by flight tests are analysed. Next, modified linear damage theories are discussed and it shows that only theories which take residual stresses into account will improve the accuracy. A relative fatigue life estimation is proposed, where one test under service conditions is the basis and Miner's rule is used as a transfer function.     

Round robin on creep fatigue crack growth testing for verification of ASTM standard 2760-10

Abstract

This paper presents early results from an ongoing round robin study to verify the newly developed ASTM standard E-2760-10 on creep fatigue crack growth testing. This round robin is an international effort with 18 participants from 11 countries from Asia, North America and Europe. All participants are testing compact type, C(T) specimens according to the procedure described in the standard. The test material is P91 taken from a section of retired steam pipe that was heat treated to rejuvenate the microstructure. All background materials data such as tensile, creep deformation and creep fatigue crack formation data are already available on the test material. Creep fatigue crack growth testing was conducted as part of a pilot program at 625°C using C(T) specimens under constant load amplitude conditions at two load levels. The tests are conducted at hold times of 0, 60 and 600 s. Creep fatigue crack growth rates are analysed using the two methods included as part of the test standard. The data from the various participants will be assessed to determine the expected levels of precision and bias in creep fatigue crack growth data obtained using the ASTM standard. Modifications to the current version of the standard will be proposed and balloted as needed from the round robin results.     

Optical fibre-based sensors for distributed strain monitoring of asphalt pavements

Strain distribution of asphalt pavement varies in transverse and longitudinal directions, and distresses, such as cracks, ruts and settlements, often occur randomly, which can be efficiently measured by distributed optical fibre sensing technology. As bare optical fibre is weak to resist shear and torsion forces during pavement construction, the protective technique is required. Therefore, a flexible asphalt-mastic packaged optical fibre sensor was developed in this research for distributed strain monitoring of asphalt pavement. Theoretical analysis on strain transfer of the optical fibre-based sensors embedded in asphalt pavement was conducted to improve the design of the protective layer and remove the strain transfer error. Afterwards, laboratory tests on the asphalt concrete beam were carried out to validate the performance of the sensor. Finally, the proposed sensors were applied to detect the in situ performance of urban asphalt pavement under temperature and traffic loads. The results indicate that the proposed optical fibre sensor detects the distributed strain of asphalt pavement effectively, and the in situ data show significant effects of temperature and traffic loads on asphalt concrete course. This research contributes to the full-scale monitoring and health assessment of large-span pavement.     

A BIST ram architecture with parallel testing in a microprogram rom

Abstract

Symmetrically simple test patterns for parallel testing of row/column pattern‐sensitive faults in RAMs are proposed in this paper. Based on the concept of the maximum leakage current among the RAM cells, the worst case testing becomes the critically efficient method for RAM testing. Due to the simplicity of generating test patterns, we can implement a BIST RAM with test procedures stored in a microprogram ROM to reduce the cost and time of testing. The test complexity of row/column pattern‐sensitive faults is reduced to O(N) in an N bits RAM as compared with O(N^{3/ 2}) of [1].     

Modelling of creep effect on moisture warping and stress developments in concrete pavement slabs

Abstract

One of the major reasons for the fatigue failure in concrete pavements attributes to the curling and warping deformations and the traffic loads, while creep effect can reduce such deformation and consequently the stress generated in slabs. The literature on the influence of creep effect on slab warping and stress generation is found rare. In this study, a test set-up was designed to measure the flexural creep of concrete beams exposed to sealed and drying conditions. The measured creep properties were then incorporated in finite element analysis to model the creep effect on warping deformation and stress generated in slabs under the conditions of bounded and unbounded with base. It is found that creep effect is significant in slab bonded with base, it reduces warping deformation and stress by 36 and 45%, respectively. The total stress is reduced by 34%. Therefore, it is of importance to take into account creep effect when conducting deformation and stress analysis in concrete pavement slabs.     

A novel high-throughput fatigue testing method for metallic thin films

Abstract

Thin films are used in a wide variety of computing and communication applications although their fatigue behavior and its dependence on alloying elements are not very well known. In this paper, we present an experimental implementation of a novel high-throughput fatigue testing method for metallic thin films. The methodology uses the fact that the surface strain amplitude of a vibrating cantilever decreases linearly from the fixed end to the free end. Therefore, a thin film attached to a vibrating cantilever will experience a gradient of strain and corresponding stress amplitudes along the cantilever. Each cantilever can be used to extract a lifetime diagram by measuring the fatigue-induced damage front that progresses along the cantilever during up to 10⁸ load cycles.     

The use of quasi-static testing to obtain the low-velocity impact damage resistance of marine GRP laminates

The use of more economical quasi-static testing to predict the dynamic impact behaviour of marine GRP composites has been evaluated by comparing equivalent impact and quasi-static test results. Static tests predicted well the initial impact behaviour and onset of delamination damage, which is likely to be the key impact resistance design variable. However, only very conservative estimates of the final fibre failure and total energy absorption capacity were achieved, except for the thickest specimens where fibre damage occurred at similar loads for both static and impact tests. Quasi-static testing avoided the problems associated with oscillations in the force signal, and delamination force showed a very strong linear relationship with laminate thickness^3/2 as predicted by two simplified models. It was inferred that the undamaged and delaminated responses are not strain rate dependant, but that the fibre failure mechanisms are.     

The importance of the compression test procedure for plastic drums for dangerous goods

Packagings for the transport of dangerous goods are tested by four tests:- free fall, leakproofness, hydraulic pressure and stacking. However, the test procedures can be interpreted in different ways, particularly the stacking test. Some test authorities apply a guided load to the top of the package and others, including the LNE in France, use non-guided loads. The objective of recent research carried out at LNE was to evaluate the risk (in terms of stack stability for dangerous goods packagings) if the stacking tests are performed with guided loads rather than non-guided loads. The stacking tests were carried out on a limited number of plastic drums (200 of four types) submitted for qualification testing using the above four tests, with either guided or non-guided loads. From the results we conclude that:

• i for the transport of dangerous goods of Danger Group I, the field of use is not altered when tested by either procedure.
• ii with goods of Danger Groups II and III, the use of either procedure does not alter the position as far as transport is concerned. However, there is a risk in warehousing: the non-guided procedure being preferred.

     

An innovative testing technique for assessing the VHCF response of adhesively bonded joints

In the last few years, the use of adhesive joints for structural applications has rapidly increased and adhesives are more often subject to fatigue loads during their in‐service life. In presence of a rapidly varying load, such as a high‐frequency vibration, adhesively bonded joints may undergo fatigue lives in the Very High Cycle Fatigue (VHCF) region that are significantly larger than those investigated in usual high‐cycle fatigue tests. The present paper proposes an innovative testing technique for performing accelerated fully reversed tension‐compression VHCF tests on adhesive butt‐joints. The procedure for the design of the adherends is described and then experimentally validated. Ultrasonic VHCF tests are finally carried out on a cyanoacrylate butt‐joint up to 10⁹ cycles: experimental results show that the proposed testing equipment permits an effective assessment of the VHCF response of the adhesive in a limited testing time.     

Strategies for Formulation and Evaluation of Emulsions and Suspensions: Some thermodynamic Considerations

Abstract

The difficulties in the development of reasonably stable emulsions and suspensions and the problems associated with the evaluation of such products are, for the most part, related to their non-equilibrium state. A conceptual understanding of the thermodynamics of these dispersions, particularly their surface and/or interfacial free energy, will facilitate successful formulation of such disperse systems. A strategy that fails to utilize surface behavior to stabilize thermodynamically unstable formulations is unlikely to be productive whereas a strategy based on an appreciation of the surface properties of the system is much more likely to succeed. Stability testing of liquid disperse systems is one of the most difficult problems faced by pharmaceutical and cosmetic chemists. The scientist is often asked to predict the shelf-life of a product or choose between experimental formulations based on estimates of how well they will hold up with time. While there are no standardized tests available to determine stability, a conceptual understanding of the thermodynamics of these systems, particularly their interfacial properties, can provide some insight into the expected types of instability, and equally important into the most sensible tests for predicting shelf-life.