Influence Of Load Level Research Articles

Effects of load and high temperature on uniaxial compressive properties of desert sand concrete

In order to explore the influence of load and high temperature on the uniaxial compressive properties of desert sand concrete (DSC), the axial compressive strength test of desert sand concrete after different loads and temperatures was carried out, and the stress-strain curve was measured. The influence of load level, temperature grade and cooling method on the mass loss rate, ultrasonic velocity and uniaxial compressive performance of desert sand concrete after high temperature was analyzed. The results show that as the temperature increases, the mass loss rate of desert sand concrete gradually increases, the axial compressive strength gradually decreases, the peak strain increases significantly, the stress-strain curve tends to be flat, and the elastic modulus decreases continuously. The "pseudoplastic platform" near the axial compressive strength of the stress-strain curve of desert sand concrete after high temperature is more obvious. Based on the two-stage constitutive model of concrete, the constitutive model of desert sand concrete considering the influence of temperature and load level is established. The research results can provide technical support for the performance evaluation of desert sand concrete after high temperature.

A statistical comparison between calculated and experimentally evaluated crack spacing measures given in practise codes for the design of reinforced concrete elements made with self-healing concrete

Available design codes provide formulations to evaluate the maximum spacing between the cracks, which then is used to calculate the crack width in concrete structures. This paper discusses the parameters controlling the crack spacing and develops an experimental test program on a set of industrial-scale reinforced concrete elements cast with self-healing concretes to be tested under flexural actions. This study provides a wide picture of the limits of maximum, minimum, and average spacing occurring in the beams. A comparison is highlighted between the calculated and experimentally evaluated measures. It was observed that the provisions given in Eurocode 2 and Model code 2010 present a good approach for the calculation, always with a small degree of overestimation for concrete without fibres. On the other hand, the values calculated using recommendations from NFP 18-710, the overestimation is higher. The influence of loading levels seems to not affect the number of cracks with an increase in concrete cover. The experimentally evaluated ranges to relate maximum and minimum spacing with the average value in a loaded region are given. No influence of using self-healing agents was detected.

Processing and rheological properties of polyol/cellulose nanofibre dispersions for polyurethanes

Polyols are widely used as precursors, compatibilisers, and viscosity modifiers in the polymer, food, and paint industries, and their rheological properties are critical for engineering the optimal performance of final products and enabling new and diverse applications. This study investigates the mechanical and viscoelastic properties of a polyether polyol filled with long thin cellulose nanofibres (CNF). It provides insights into the influence of loading levels on mechanisms of dispersion, interaction, and gelation behaviour. The steady-shear rheological measurements revealed four different regimes of flow behaviour associated with increasing viscosity and yield stress with increasing loading of CNF in the polyol. The oscillatory-shear rheology enabled a better understanding of interactions of CNF dispersed in the polyol and the determination of the threshold concentration of CNF for percolation network formation. The correlation of mechanical properties of the polyol nanocomposites with Shih et al. suggests the network formation of intraflocs of fractal dimension two at the higher loadings of CNF.

Numerical Investigation of the Influence of Fatigue Testing Frequency on the Fracture and Crack Propagation Rate of Additive-Manufactured AlSi10Mg and Ti-6Al-4V Alloys

Advances in machine systems and scanning technologies have increased the use of selective laser melted materials in industrial applications, resulting in almost full-density products. Inconsistent mechanical behavior of components under cyclic stress is caused by microstructure and porosity created during powder melting. The extended finite element method, XFEM, was used to imitate crack propagation utilizing an arbitrary fracture route to study fatigue crack growth in additively produced fatigue specimens. The influence of loading level and testing frequency on fatigue life was studied using fracture energy rate curves. Micro-computed tomography (µ-CT) scans offer 2D images in angular increments. There are several ways to reduce the number of faces and vertices. Opensource software was used to isolate the cylindrical shell from interior pores and create finite element models from µ-CT projections. All simulations were on supposedly cylindrical fatigue specimens made by selective laser melting (SLM) based on previous experimental results of the authors. Crack propagation rate curves were utilized to evaluate the effects of loading level and testing frequency. At larger loads, the fracture area increases abruptly at 3E3 cycles, then stabilizes at 4E4 cycles in Al alloys in comparison to Ti-6Al-4V alloys. Crack propagation rate curves may be used to determine Paris constants based on the applied stresses.

Experimental Analysis of Matrix Cracking in Glass Fiber Reinforced Composite Off-Axis Plies under Static and Fatigue Loading.

The inter-fiber failure of glass fiber-reinforced epoxy specimens with four different fiber angles was analyzed. Flat specimens were subjected to static and fatigue loading considering different load levels and load ratios. Damage investigation in terms of crack density measurement was performed by transmitted white light imaging using a digital camera and LED illumination from the back of the specimen on a servo-hydraulic testing machine. Static and fatigue results were examined with respect to crack initiation and crack growth, considering the special case of bonding yarns parallel to the fiber directions. The bonding yarns act as stress concentrations, influencing the early cracking behavior, and complicate the detectability of cracks growing underneath or next to the bonding yarns. In cyclic loading, the influence of load level, load ratio, mean stress, fiber orientation, and ply thickness was the focus of the experimental campaign. Cyclic cracking behavior in terms of initiation and growth was analyzed based on the applied loading conditions and laminate configurations. It was found that halving the ply thickness nearly doubled the amount of microcracks in case of high loads. For low loads, no such effect was observed up to loading cycles. Experimental findings on individual crack growth confirmed that crack interaction started for crack spacings less than four times the ply thickness and that subsequent crack growth shifted into regions of larger local crack spacing.

Effects of applied load and its direction on the permeability behavior of concrete: A case study

The applied load has significantly impacted on the permeability behavior of concrete. Although many investigations have reported the impact of loading level on the permeability, the influence of loading direction on the permeability received little consideration in previous publications. Therefore, this work gives a comprehensive study on the influence of loading level and its direction on water and chloride permeability. Applied loading experiment, water absorption experiment and chloride diffusion test were conducted in this work. The findings showed that the applied load increased the specific crack area and pore size distribution, and that the permeability increased as the applied loading level and duration increased. The water and chloride penetration whose direction is vertical to the applied loading direction is greater than that whose direction parallels the loading direction; in addition, the applied load direction shows larger effects on water ingress than on chloride ingress. Capillary absorption coefficient and chloride diffusion coefficient grow linearly as total porosity increases, and the chloride diffusion coefficient is in direct proportion to the capillary absorption coefficient. When the loading direction is vertical to water penetration, the capillary absorption coefficient of concrete after 75 % fc and 85 %fc increases by 25.76 and 28.27 (g/m2h1/2), compared with the case that the loading direction is parallel with water penetration. This paper further gives a relationship between concrete permeability and the induced loading damage.

Research on Variation Law of Back-Calculation Modulus of Asphalt Pavement Layer

Based on in-situ FWD testing and laboratory dynamic modulus test, the influence of loading level, position and pavement temperature on FWD back calculated modulus and the relation between bacalculated modulus and dynamic modulus was analysed. The structure combination of asphalt layer was determined based on pavement temperature profile and the composite modulus was calculated based on laboratory modulus test and in-situ measured pavement temperature. The results show that loading level had little influence on bacalculated modulus and testing position had much more influence on bacalculated modulus than loading level. The backalculated modulus and composite modulus decreased with temperature increase, when temperature was low backalculated modulus was greater than composite modulus, when temperature was in room temperature backalculated modulus was equal to composite modulus and when temperature was high backalculated modulus was less than composite modulus. For relation between bacalculation modulus and temperature, temperature correction coefficient for bacalculation modulus was developed and for relation between backcalculation modulus and composite dynamic modulus, correction coefficient model between them was developed.

Freeze⁻Thaw Durability of Strain-Hardening Cement-Based Composites under Combined Flexural Load and Chloride Environment.

Cement-based materials are usually not exposed to an independent deterioration process but are exposed to a combination of mechanical load and environmental effects. This paper reports the frost resistance durability of strain-hardening cement-based composites (SHCC) under combined flexural loading at different levels and under chloride attack. The loss of mass, dynamic elastic modulus, and microstructure characteristics of SHCC specimens were determined, and the influence of loading level on frost resistance was analyzed. In addition, the effect of freeze–thaw action on the flexural performance and diffusion properties of chloride in SHCC under the combined loads was investigated. The results show that the process of degradation was accelerated due to the simultaneous action of flexural loading and freeze–thaw cycles in the chloride environment, and SHCC suffered more serious damage at a higher loading level. However, flexural strength decreased by only 13.87% after 300 freeze–thaw cycles at load level S = 0.36. The diffusion properties of chloride in SHCC under constant flexural loading were affected by the freezing and thawing cycle. The free chloride concentration Cf increased with the development of freezing and thawing at the same diffusion depth, and a bilinear relationship was found between the chloride diffusion coefficient Dc and the number of freeze–thaw cycles.

Influence of loading level on the bearing capacity of RC columns strengthened by jacketing

Compressed reinforced concrete elements are quite common in construction. An example of such elements is the columns of industrial buildings, which are loaded with eccentric force, for example, from a bridge crane, columns of multistory buildings, structures subjected to compression and bending. When the reinforced concrete structure becomes unfit to future operation, it is often more economical and easy to retrofit it than to change it for a new one. Most of these structures are influenced by certain loads and unloading the structures before strengthening is not always possible. Therefore, in this work reinforced concrete structures strengthened by jacketing are investigated. To simulate the real life conditions, the loading level of 65-70% of the not strengthened column bearing capacity was maintained during jacketing. This load level simulates the actual service load on the structure. The bearing capacity load and serviceability limit state of not strengthened and jacketed reinforced concrete columns were investigated and the experimental results are presented in this paper.

Quantification of self-heating and its effects under cyclic tests on a bituminous binder

This paper studies the reduction in complex modulus when a bituminous binder is subjected to the fatigue test, along with the ability of this material to recover such a loss in modulus. It has been demonstrated in the literature that during a fatigue test, a rapid and reversible decrease of complex modulus can be observed at the beginning of the test. This reversible decrease is not correlated with fatigue and has been explained by the existence of reversible phenomena, such as thixotropy, self-heating, nonlinearity, etc. The aim of this work is to quantify the self-heating and determine its share in the reversible reduction of complex modulus under cyclic loading. A strain sweep test has been developed with different loading steps, followed by rest periods after each loading phase. The loading level increases with each step, and the loading phases allow observing the modulus decrease, while the rest periods are introduced to observe modulus recovery after each loading level. A complete recovery of modulus means that the modulus diminution is due to a reversible phenomenon however a partial recovery of modulus notifies the presence of nonreversible phenomenon (such as damage). In using this approach, the influence of loading level on both complex modulus decrease and recovery can be examined. In order to quantify self-heating during testing, the sample temperature is measured by embedding a thermocouple probe inside the sample during its manufacturing. To obtain the relationship between complex modulus and temperature, the complex modulus test results have been interpreted herein. A thermoviscoelastic numerical simulation was conducted to determine the temperature field in the material and quantify the variation in complex modulus with respect to self-heating. Results show a temperature variation of 11°C inside the sample. A comparison between experimental and numerical results indicates that the numerical model perfectly predicts the variations in both temperature and complex modulus until a certain strain level, where the damage, thixotropy or other factors have not appeared yet. It is also observed that the reversible complex modulus decrease can be explained by self-heating.

The influence of loading levels on strength of RC columns strengthened by RC jacketing

The influence of loading levels on strength of RC columns strengthened by RC jacketing

Simplified Analytical Approach to Evaluate the Nonlinear Dynamics of Elastic Cylindrical Shells Under Lateral Blast Loads

This paper presents an analytical solution to predict the nonlinear forced vibrations of elastic thin-walled cylindrical shells under suddenly applied loads. Interest in this problem is motivated by effects due to explosions on fluid-storage metal tanks. The model is based on the energy criterion due to Lagrange, in which the kinematic nonlinear relations are assumed using Donnell's simplified shell theory. Solution is achieved as a series summation in terms of trigonometric functions in the axial and circumferential directions, whereas the degrees of freedom depend on time. A blast load is assumed to represent effects due to explosions on the shell as time-dependent pressures with a given circumferential distribution (a cosine square distribution in terms of the central angle). The procedure is validated by comparison with a nonlinear finite element model under the same load conditions. The influence of load level and shell geometry on the transient response is investigated by mean of parametric studies. Good accuracy is found in the results for the range of shells which are representative of horizontal, fuel storage tanks in the oil industry.

Railway track performance study using a new testing facility

This paper presents results from full-scale laboratory research on railway tracks using the geopavement and railway accelerated fatigue testing (Graft) facility at Heriot-Watt University. A typical ballast depth of 300 mm has been considered, overlying subgrade soil (a typical situation on site). Sub-ballast has not been used, to ensure that only ballast and subgrade behaviour are investigated. The influence of load level and subgrade stiffness for a typical UK track is presented, and from the results a new railway track settlement equation is proposed that fits the measured data in Graft well. The proposed equation can predict track settlement in Graft after a number of cycles for any applied load, and for the clay and subgrade Young's modulus considered in this paper. The threshold stress concept used in fine-grained subgrade materials, granular materials and within railway track design processes has been considered, and a new track parameter (t) within the track settlement equation has been proposed to account for this. The equation has been compared with other previously published equations with different parameters that are based on both laboratory and field measurements, and similarities have been found. This paper demonstrates how the Graft facility can accurately simulate the railway track response.

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.

Influence of Load Level on Performance of Standalone Vertical Axis Wind Turbine System

The influence of load level on the performance of a stand-alone, straight-wing, vertical axis wind turbine system in terms of its power generation capability and output fluctuation during operation is numerically investigated using a dynamic simulation model. The system is mainly operated at a constant tip-speed ratio, i.e., at a maximum power coefficient point, by load control in proportion to the square of rotational speed. Thus, the steady-state performance and dynamic behavior of the system under four load levels are analyzed. Then, the dynamic system performance under different load levels is discussed. This work shows that, under high mean wind speed conditions, a low load level is undesirable because it would cause an increase in the output fluctuation. However, under low mean wind speed conditions, a low load level could be effective in improving the power generation capability, with only a slight increase in the output fluctuation.

Experimental behaviour of high-strength concrete columns in fire

Twelve high-strength concrete columns were tested in fire under two heating rates at different load ratio and restraining levels. Imposing axial restraint against column thermal expansion during fire causes additional axial forces to be generated and develops excessive buckling deformations. Therefore, an early failure can take place which may lead to partial failure of the building during fire. This paper reports an integrated parametric experimental study, in which the influence of load level and degree of restraint are investigated together with their interactions. The paper contains main results and conclusions including measured concrete and reinforcement temperatures, axial deformation, temperature failure and additional restraining forces. It is found that, the maximum value of axial deformation is independent of the rate of heating and it is dependent on the load that has been applied during the test. Furthermore, the failure time of columns tested under low heating rate is almost twice the failure time of columns tested under high heating.

Experimental behaviour of high-strength concrete columns in fire

Twelve high-strength concrete columns were tested in fire under two heating rates at different load ratio and restraining levels. Imposing axial restraint against column thermal expansion during fire causes additional axial forces to be generated and develops excessive buckling deformations. Therefore, an early failure can take place which may lead to partial failure of the building during fire. This paper reports an integrated parametric experimental study, in which the influence of load level and degree of restraint are investigated together with their interactions. The paper contains main results and conclusions including measured concrete and reinforcement temperatures, axial deformation, temperature failure and additional restraining forces. It is found that, the maximum value of axial deformation is independent of the rate of heating and it is dependent on the load that has been applied during the test. Furthermore, the failure time of columns tested under low heating rate is almost twice the failure time of columns tested under high heating.

Cyclic Response of Geotextile-Reinforced Soil

ABSTRACT Reinforced soils exhibit an improvement in strength and deformation characteristics under monotonic loading conditions, due to the additional “pseudo” confinement caused by the lateral restraint and shear stress mobilization along the soil-inclusion interface. In this paper, the influence of the lateral restraint on the cyclic behavior of the soil is investigated. Experimental results obtained from cyclic triaxial tests on a partially saturated reinforced silt are presented. The reinforcement consists of horizontal discs of woven and non-woven geotextile equally spaced along the specimen. The influence of loading level, geotextile type, reinforcement spacing, and specimen diameter are examined. An analytical procedure to interpret the results, based on the confinement enhancement model, is proposed. In the procedure, the effect of the additional equivalent confinement on both the strength and stiffness of the soil is considered.

The influence of loading levels, temperature and environment on fibre failure during the fatigue of an SiC fibre reinforced Ti-MMC

Abstract The fatigue crack propagation characteristics of Ti-6Al-4V reinforced with SM 1240 SiC fibres have been investigated under a number of different loading conditions and at elevated temperatures of 300 and 500 °C, in air and in vacuum. Both the initial applied stress intensity factor range (ΔK) and the initial applied maximum stress intensity factor (Kmax) have been found to be important factors, which determine whether the initial loading conditions cause catastrophic failure of the specimen, or whether the bridging fibres in the wake of the crack reduce the effective value of ΔK at the crack tip by an amount sufficient to cause the fatigue crack to arrest. At the elevated temperatures of 300 and 500 °C in air, crack growth resistance decreases progressively with temperature increase. In vacuum, however, a decrease in fatigue crack growth resistance is seen only at a test temperature of 500 °C.

Shear flow rheological properties, fiber damage, and mastication characteristics of aramid‐, glass‐, and cellulose‐fiber‐reinforced polystyrene melts

AbstractAn experimental study of the viscosity and principal normal stress difference of a polystyrene melt filled with aramid (Kevlar), glass, and cellulose fibers is reported. The influence of loading level and mastication on the rheological properties is discussed. The effects of mixing and mastication on fiber damage are considered. Glass fibers break down rapidly to very small aspect ratios, while aramid shows a “kinked” structure, with kinks occurring every 100 μm. A mechanism is proposed for fiber breakage based on buckling during rotation in shear flow. It is found that addition of fibers increases the viscosity in the same manner as a reduction in temperature, and data may be superposed by reduced plotting. This indicates that the viscosity increase is due solely to enhanced viscous dissipation in the matrix and not to interparticle forces as is the case with smaller particles. The principal normal stress difference increases at fixed shear stress with fiber loading. The extent of increase depends upon fiber loading, aspect ratio, and modulus.