Repeated Load Research Articles

Solution validation technique for optimal shakedown design problems

Modern computer technology allows conjoining shakedown theory, optimization and ever stricter standardized design requirements in a single mathematical problem formulation. However it raises a question of reliability: easily achieved solution should not be taken for granted but should be adequately assessed. This paper focuses on the physical validation technique for optimal shakedown design problem solution in the aspect of Melan theorem (statics) and residual deformation compatibility (kinematics). For that purpose Rosen gradient projection method is used. Optimization problem of bending circular, symmetric plate at shakedown, which is subjected by a variable repeated load, is considered for illustration of the validation technique.

Stiffness and deformation properties of spent coffee grounds based geopolymers

Spent Coffee Grounds (CG) are the granular organic waste material from the food and beverage industry resulting from the brewing of coffee, and are often disposed to landfills. Fly Ash (FA) is the primary by-product of coal-combustion power-plants used for energy generation. Slag (S), on the other hand, is an industrial waste generated from iron-ore refineries. In this research, geopolymers comprised of various mix designs of CG, FA, and S were developed via alkaline activation to create a green construction material out of these traditional waste materials. The intended use for this recycled construction material was as a subgrade fill, which if successfully implemented would considerably reduce the need of disposing such waste materials into landfills. In this article, key design parameters including Unconfined Compressive Strength (UCS), resilient moduli (MR) and permanent deformation characteristics of CG-geopolymer remoulded cylindrical specimens are evaluated using simulated long-term Repeated Load Triaxial (RLT) tests. Regression parameters used in two MR prediction models, namely the Bulk Stress Model and 2-Parameter Model, were determined for the CG geopolymers. The developed 3-Parameter Model was found to be valid for predicting the MR of CG geopolymers. When cured at 21°C, the MR for CG+FA geopolymers is generally lower than that of CG+S geopolymer specimens. Curing at 50°C reduces this difference of MR values between FA and S geopolymers. At 50°C curing, the MR of some mixes, notably 70CG:30FA+90Na2SiO3:10NaOH, 70CG:30S+90Na2SiO3:10NaOH, and 70CG:30S+50Na2SiO3:50NaOH decreases after 28days of curing while gaining increased UCS. This indicates that these geopolymers can gain improved ductility while developing high strengths under optimum curing conditions. This article evaluates key design parameters including the MR for CG geopolymer specimens, which enables coffee grounds to be used as green recycled construction materials in roads.

Elevated temperature, micro-compression transient plasticity tests on nanocrystalline Palladium-Gold: Probing activation parameters at the lower limit of crystallinity

In this study, we probe the activation parameters and time-dependent plastic behavior of a nanocrystalline Palladium-Gold alloy by means of elevated temperature micropillar compression experiments. All tests were conducted on a stable, displacement-controlled in situ SEM indenter. A series of strain rate jump tests, where the strain rates were varied from 10−4 s−1 up to 10−2 s−1, allowed us to extract strain rate sensitivity and average activation volumes in the order of 8 b3. Repeated load relaxation tests yielded lower activation volumes in the range of 4 b3. From the variation of test temperatures up to 125 °C, we extracted an apparent activation energy of ∼0.77 eV. The possible deformation mechanisms at such extremely small crystallite sizes are discussed within the framework of pertinent literature.

Calcium and bone metabolism across women's life stages. Stress fracture in female athletes.

Stress fractures are common overuse injuries in athletes, especially in long-distance runner, jumping sports and gymnastics in female athletes. The pathology of stress fracture mainly includes repeated biomechanical loading, which leads to bone micro-damage and failure of bone structure. Female athlete triad(FAT), which includes low energy availability with or without an eating disorder, functional hypothalamic amenorrhea, and osteoporosis, is a serious healthcare concern in female athletes. For prevention and treatment of stress fracture, management of biomechanical factors modifying the load applied to a bone is important, and especially in female athletes, management for FAT is one of the important factors. This report outlines the current knowledge on the epidemiology, diagnosis, treatment and prevention of stress fracture, relating to FAT in female athletes.

Efficient Identification of Naval High-Speed Craft Shock Mitigation Seat Modal Parameters From Drop-Test Data

Naval high-speed craft (HSC) operating in moderate to high seas experiences high-g and repeated shock loading at the seat–deck interface. These conditions are known to pose a serious potential for injury to the occupants. While various shock-mitigating seats are commercially available; their designs are in many cases quite different, and quantifying their shock attenuation characteristics can be challenging. The need for a standard test platform and experimental analysis methodology to investigate HSC seat effectiveness is a major objective of research being conducted by Carleton University's Applied Dynamics Laboratory (ADL) in partnership with Defence Research and Development Canada-Atlantic (DRDC Atlantic). A drop tower was designed and manufactured for testing HSC seats in order to characterize their shock-mitigating effectiveness by simulating the severe conditions of a slam impact at sea. Further, in order to identify seat dynamic parameters from drop-test data, the eigensystem realization algorithm (ERA), a modal-analysis-based system identification method, was applied to efficiently extract the modal parameters. The technique was shown to successfully extract the damping ratio as well as the damped and undamped natural frequencies of the seats from impact test data. The evaluated dynamic properties of the seats can subsequently inform decisions related to the design and/or procurement of commercially available seats.

Investigating the creep response of asphalt mixtures under waveform loading

The objective of this study was to examine the effect of waveform loading, truck speed levels and varying load pulse duration on the permanent deformation of asphalt mixture. The effect of vehicle speed and axle configuration was converted to pulse period using the length of total space influence domain divided by speed of the vehicle. Same effect was reproduced in uniaxial repeated load permanent deformation test by computing the pulse width and pulse period to evaluate the response of asphalt mixtures. Test was conducted at a single stress level and two temperature levels. Open-graded and dense-graded asphalt mixtures were prepared using two crushing mechanisms (single-stage and two-stage crushed aggregates). The study revealed that pulse duration and axle type related to any waveform have a significant influence on the permanent deformation of the asphalt mixtures as pulse duration depends on the speed of a vehicle and its axle configuration. The two-stage crushed material performed better in the laboratory as compared to the single-stage crushed material, which is in line with the previous study. Statistical analysis revealed that artificial neural network (ANN) can better predict the permanent deformation in asphalt mixtures than other techniques. However, as the data become more complex, ANN’s ability to predict with more accuracy reduces.

Using Genetic Programming to Predict Plastic Strain in Subgrade Soils Under Repeated Loading

Abstract Accumulated plastic strain in subgrade soils under repeated loading is an important factor for the design and analysis of flexible pavement. Accurate prediction of the plastic strain is dependent on the constitution of a predictive model. This study discusses the plastic strain properties in subgrade soil under repeated loading and develops a predictive model using a genetic programming (GP) method. Repeated load triaxial tests were conducted under various confining pressures, stress levels, and moisture contents. To develop the predictive model, a total of 475 records were randomly divided into three datasets (159, 158, and 158 records) for training, validation, and application purposes. The results showed that the GP method was applied successfully to develop a predictive model that uses stress level, load repetition, dry unit weight, liquid limit, plastic index, and clay content as inputs and has plastic strain as the output. The R2 values for the training, validation, and applied datasets were 0.9551, 0.9494, and 0.9489, respectively. This study demonstrates and confirms the superior performance of GP over traditional prediction methods when applied to predict plastic strain in subgrade soils. The model and algorithms proposed in this study provide a good foundation for further development when more robust datasets of other types of soils are available.

Permanent Deformation Characteristics of Coarse Grained Subgrade Soils under Train-Induced Repeated Load

This paper presents the results of a laboratory experiment that aimed to characterize the permanent deformation behavior of coarse grained soils. To evaluate the effects of the cyclic stress amplitude, initial mean stress, and initial stress ratio on the permanent axial deformation, six series of repeated load triaxial tests were performed. The results indicate that permanent deformation of coarse grained soils increased with increasing cyclic stress amplitude. In particular, for relative low cyclic stress levels, accumulation rate of permanent deformation decreased progressively with number of cycles and eventually reached an equilibrium state. The initial stress ratio was also found to obviously facilitate the buildup of axial deformation since it means higher deviatoric stress as the mean pressure kept constant. As the initial stress ratio was less than the slope of static failure line, the experimental results indicated that the increase of initial mean stress enhanced the capability of resisting deformation. A simplified mechanistic empirical prediction model was proposed, which predicted the permanent deformation as product of four independent functions about cyclic stress amplitude, initial mean stress, initial stress ratio, and number of load cycles. Satisfactory predictions of the permanent deformation behavior of coarse grained soils were obtained with the proposed model.

Enhancement in the Load-Carrying Capacity of Reinforced Concrete Corbels Strengthened with CFRP Strips under Monotonic or Repeated Loads

The present research investigates the effectiveness of using unidirectional carbon fiber reinforced polymer as an external strengthening technique to increase the load carrying capacity of existing reinforced concrete corbels subjected to monotonic or non-reversed repeated loads. 20 normal weight reinforced concrete double-sided corbels were cast for this purpose, 15 of them were strengthened with carbon fiber strips. The variables studied were: the width and orientation of carbon strips and the load history schemes used to apply the non-reversed repeated loading. It was found that the external strengthening with carbon strips improved the capacity of corbels. The enhancement in the load carrying capacity for the corbels strengthened with 50 mm strips and tested under monotonically loads were 11%, 15% and 27% for the horizontal, inclined and mixed orientations respectively. While for the non-reversed repeated loaded corbels, the enhancement in the load carrying capacity was about 11%, 18% and 21% for the horizontal, inclined and mixed orientations respectively.

STRUCTURAL SHAKEDOWN: A NEW METHODOLOGY FOR ESTIMATING THE RESIDUAL DISPLACEMENTS

A vector of residual forces of the ideally elastic-plastic structure at shakedown is obtained by solving the static analysis problem. A unique distribution of the residual forces is determined if the analysis is based on the minimum complementary deformation energy principle. However, the residual displacements developing in the shakedown process of ideally elastic-plastic structures under variable repeated loads can vary non-monotonically. Nevertheless mathematical models for the optimization problems of steel structures at shakedown must include the conditions for strength (safety) and stiffness (serviceability). Residual displacements determined by the plastic deformations are included in the stiffness conditions; therefore to improve the optimal solution it is necessary to determine upper and lower bounds of the residual displacement variations. This paper describes an improved methodology for estimating the variation bounds of the residual displacements at shakedown.

IV fosphenytoin in obese patients: Dosing strategies, safety, and efficacy.

Previous studies evaluated the disposition of IV phenytoin loading doses and found that obese patients had increased drug distribution into excess body weight, larger volumes of distribution, and longer half-lives when compared to their nonobese counterparts. We assess the safety and efficacy of fosphenytoin loading doses in patients with different body mass indices (BMIs). A retrospective chart review was conducted in 410 patients who received fosphenytoin. Patients were divided into 2 groups: BMI <30 (nonobese) and BMI ≥30 (obese). Patient demographics, fosphenytoin dose administered in mg/kg body weight, renal and liver function tests, fosphenytoin drug levels, and pre- and post-fosphenytoin administration vital signs were collected to assess for adverse events. Necessity of additional antiepileptic loading doses was used as a surrogate for clinical efficacy. The median dose of fosphenytoin administered was 19 mg/kg (interquartile range 15-20). The most frequently encountered adverse event was hypotension, which occurred in 39% of the cohort. Using a Bonferroni adjustment for multiple comparisons, there were no differences in adverse events between the 2 groups. The need for additional antiepileptic loading doses was not different between the 2 groups (p = 0.07). The incidence of adverse events and the need for repeat loading antiepileptic medications was similar between the 2 groups. From our findings, the patients in our study did not receive empiric loading dose adjustments and the current method of loading fosphenytoin achieves similar outcomes, regardless of the patient's BMI.

Case studies on optimal location and sizing of renewable energy generators in distribution system

Optimal location and sizing of different types of renewable energy generation system at the distribution networks for the purpose of active power loss minimization is revealing much attention of electric power utilities in the recent days. In the present work, optimal location and sizing of different types of renewable energy generators (REGs) have been proposed for IEEE 33-bus test system. The vastness of the REGs is determined by minimizing the power loss in the distribution system. Power distribution losses have been minimized with the determination of optimal location as well as optimum size of distributed generator. In this work, various case studies such as active power loss, reactive power loss, combined real and reactive power, optimum power factor, and multi REGs placements have been considered for optimal location of REGs. A simple but efficient Repeated Load Flow (RLF) technique has been used to ascertain optimal location of REGs. The simulation results obtained from this RLF technique have also been compared with an outcome of other approach results. The proposed approach has been validated on IEEE 33-bus and 69 radial distribution test systems.

Features of chronic hepatitis C course in pregnant women

Aim. To study features of the course of chronic hepatitis C virus infection in pregnant women and its effect on pregnancy.Methods. The study included 111 pregnant women: 67 with chronic hepatitis C and 44 healthy women. The mean age was 28 years. The number of pregnancies among examined women ranged from 1 to 8. All pregnant women had no concomitant therapeutic pathology and various addictions (alcohol, nicotine, drugs). The viral load in pregnant women with chronic hepatitis C ranged from 3.18×102 to 2.4×107 IU/mL.Results. Alanine aminotransferase and bilirubin levels in the group of chronic hepatitis C and in healthy pregnant women did not exceed the normal range and were not statistically different from each other. In repeated pregnancies viral load of hepatitis C virus was lower, compared with the first pregnancy: median [25%; 75%] = 5.202 [4.079; 6.364] and 6.658 [5.708; 7.380], respectively (p106 IU/mL). At the same time threatened miscarriage, intrauterine hypoxia and preeclampsia were registered more often.Conclusion. In the first pregnancy the viral load is higher than in repeated pregnancies; pregnancy in women with chronic hepatitis C without concomitant diseases does not cause activation of the inflammatory process in the liver; in pregnant women with chronic hepatitis C, especially at higher viral loads, medical history remarkable for obstetric diseases are more likely detected.

Compressive fatigue damage and failure mechanism of fiber reinforced cementitious material with high ductility

The fiber reinforced cementitious material with high ductility has potential use in particular environments and structures that undergo repeated or fatigue loads. In this study, a series of monotonic and fatigue tests were performed to investigate the compressive fatigue behavior of this material. It is found that the fatigue life of this material is higher than that of plain concrete and steel fiber reinforced concrete under the same stress level. In addition, the failure deformation of fiber reinforced cementitious material with high ductility under fatigue load was larger than the monotonic envelope, while the envelope coincides with the monotonic loading curve for concrete or fiber reinforced concrete. The failure surface and damage process were investigated and a new failure mode of polyvinyl alcohol fiber with crushed end was discovered. The fatigue failure surface could be divided into three regions, including fatigue source region, transition region and crack extension region.

Evaluation of microstructure and damage evolution for asphalt pavements in an advanced repeated load permanent deformation test using X-ray computed tomography

It is significant to capture the microstructure characteristics and evolution of asphalt mixtures since it is well related to the macroscopic pavement performance. The X-ray Computed Tomography (CT) technique is a suitable candidate to gather and analyse the image information of the internal structure for materials. An advanced repeated load permanent deformation (ARLPD) test that could largely simulate the confining pressure and temperature gradient of actual pavements was conducted on several typical newly constructed and overlay pavement structures. CT and digital image processing techniques were used to characterise the evolution behaviour of microstructure and damage in asphalt layers before and after testing at high temperatures. Combining the macroscopic pavement performance with microstructure evolution, the failure mechanism for different pavements and layers was finally discussed. It is found from the ARLPD test that newly constructed hot mix asphalt pavements show much better rutting resistance than the overlay pavements. The middle asphalt layer and the overlay layer, respectively, contribute approximate half of the total rut depth for newly constructed and overlay pavements. After testing, the change of air void content varies in different layers and pavements, indicating different rutting mechanisms (densification or shear flow). In the middle asphalt layer of overlay pavements, some cracks are formed with a great increase in air void aspect ratio and a significant decrease in air void number. Using the crack density as an indicator, the correlation of permanent deformation and cracks at high temperatures is preliminarily found.

Developing an optional multiple repeated load test to evaluate permanent deformation of asphalt mixtures based on axle load spectrum

An optional multiple repeated load (OMRL) test, capable of simulating the axial load spectrum in real pavement, was developed to evaluate the rutting susceptibility of asphalt mixture. This new test was conducted at the temperature of 60°C under multiple stresses determined by a simplified axle load spectrum considering the pavement management system (PMS). A pre-stage dynamic creep test was employed to determine the loading cycles of the first increment. Two kinds of asphalt mixtures were compared in the OMRL test with varying orders of multiple stresses and they were also tested in dynamic creep test under five different loading levels. As the test results show, loading sequences have a significant impact on the high-temperature performance of asphalt mixtures. The curve of permanent strain rate versus stress level could fit well with a power-law model in the dynamic creep test, and a similar result can be obtained by the OMRL test. However, the OMRL test shows higher efficiency than the traditional dynamic creep test. Besides, the secondary stage of the OMRL test fits a broken-line model since various stress levels are conducted on the specimen. The newly proposed lowest average strain rate (LASR) and multiple flow number (MFN) could evaluate the rutting resistance of asphalt mixtures under a more realistic loading condition. Overall, this OMRL test provides a better understanding of the rutting behavior of asphalt mixtures to improve the mix design and field performance.

Estimation of resilient modulus of unbound granular materials using Clegg impact value and field stress levels

Resilient modulus (Mr) is an important property that controls the performance of pavement materials under dynamic wheel loads. Mr can be determined in the laboratory from repeated load triaxial tests and is defined as the ratio of the deviator stress to the recoverable strain after a number of load applications. Inherently, it is a challenge to perform repeated load triaxial tests as routine tests due to their rather complicated, time-consuming and expensive procedure. Hence, researchers have attempted to develop empirical estimation models based on the mechanical properties of pavement materials, such as the California bearing ratio and the unconfined compressive strength or physical properties of materials. This study examines the correlation between the Clegg impact value (CIV) and the Mr of unbound granular materials (UGM) of pavements. The Clegg impact hammer test provides a parameter based on the response of the material to dynamic, rather than static or gradual, loading. Fourteen different unbound granular materials with particle sizes ranging from 7 mm to 19 mm used in the study. The results indicate that there exist a good to strong correlation between the resilient modulus Mr and Clegg impact hammer value, CIV, with the highest R2 value of 0.82 and the average R2 value of 0.76, from 48 stress levels used in Mr tests.

Evaluating the rutting resistance of asphalt mixtures using a simplified triaxial repeated load test

A Simplified Triaxial Repeated Load Test (STRT) is reported to evaluate the rutting resistance of asphalt mixtures under field conditions. The laboratory tests and field measurements are conducted to verify the feasibility of the newly developed test method to characterize the permanent deformation of asphalt mixtures. The sensitivity of D1, λ (regression parameters in the Stephen Price model) and flow number FN measured by conventional Triaxial Repeated Load Test (TRT) and STRT is compared. Then the correlations between D1, FN and the rutting depth of a selected asphalt pavement are analyzed. The results indicate that the varying confinement in the STRT has significant effects on the permanent deformation of asphalt mixture. Repetitive tests prove that parameters measured by STRT share the similar stability with TRT. D1 can be used as an alternative indicator to assess the rutting resistance of mixtures, particularly for the polymer modified asphalt mixtures. D1 and FN measured by both STRT and TRT have good relevance with the rutting depth of the asphalt pavement, while the STRT makes a better distinction of permanent deformation between field cores with different rutting depths. As a result, the newly developed STRT turns out to be feasible and can better understand the dynamic creep properties of asphalt mixtures.

Innovative performance-based design of slope stabilizing piles for a railway embankment

The paper presents a simplified displacement-based approach for the design of a slope stabilising system. On the top of the slope, a railway ballasted track induces repeated dynamic loads on the subgrade soil, that, in conjunction with other cyclic environmental loads (seasonal temperature and soil moisture variation) induce a progressive accumulation of soil displacements, despite the safety factors for the slope, if computed by means of traditional limit equilibrium methods, are all larger than one. The design procedure started then from the analysis of preliminary (pre-operam) monitoring data, in order to define the shape and amplitude of the soil displacement field, by interpreting them within a viscoplastic constitutive framework. This allowed to conveniently optimise the slope stabilising system (consisting in FRC piles connected to each other by means of a precast FRC beam and reinforced by pre-stressed ground anchors) with the aim of reducing the soil displacement rates below a prescribed threshold. The focus of the work, rather than presenting advanced constitutive or numerical simulations, is on the design approach, which allows the designer to get a reliable and safe design solution, even in lack of rich and sophisticated site characterisation, as it is often the case in real applications.

Development of a New Mechanistic Empirical Rutting Model for Unbound Granular Material

AbstractThis paper proposes a new mechanistic-empirical rutting (MER) model to evaluate the permanent deformation (PD) behavior of unbound granular material (UGM). To characterize the stress dependence of rutting behavior in UGM, the MER model incorporated a softening stress term and a hardening stress term into the Tseng-Lytton model, which is based on the Drucker-Prager plastic yield criterion. Repeated load triaxial tests were performed on two types of UGMs in this study, employing seven stress states to calibrate the model coefficients, and two stress states to validate the accuracy of the model predictions. The correlations of the two incorporated stress terms with the accumulated permanent strains were established based on the triaxial test results. It was found that the correlations are fitted by power functions with 0.97–0.99 R2 values. The proposed MER model was compared with the existing UGM rutting models, including the MEPDG model, Korkiala-Tanttu model, and UIUC model in terms of differences ...