Static Flexural Strength Research Articles

Flexural fatigue analysis of self-compacting concrete beams

Results of an investigation of the probability distributions and flexural fatigue performance of self-compacting concrete beams are presented. The fatigue tests were carried out with a 100 kN capacity closed-loop servo-controlled actuator on 100 × 100 × 500 mm size beam specimens under four-point flexural loading. The static flexural strength of the self-compacting concrete was determined before the fatigue tests. The test data have been used to examine the distribution of fatigue life of self-compacting concrete at different stress levels. It has been shown that the statistical distribution of fatigue life of self-compacting concrete at a given stress level can be modelled by the two-parameter Weibull distribution with its distribution parameters estimated by different methods of analysis. The results indicate that there was lower variability in the distribution of fatigue life of self-compacting concrete compared to that of normally vibrated concrete. The fatigue life data have also been presented in the form of an S–N diagram and the two-million-cycles fatigue strength of self-compacting concrete has been found to be marginally higher than that of normally vibrated concrete. The fatigue life data of normally vibrated concrete used for comparison were obtained from the literature.

Flexural fatigue analysis of hybrid fibre-reinforced concrete

This paper presents the results of an investigation conducted to examine the probability distributions and flexural fatigue performance of hybrid fibre-reinforced concrete (HyFRC) containing different proportions of steel and polypropylene fibre. An experimental programme was planned to obtain the fatigue lives of HyFRC, in which approximately 435 beam specimens of size 100 × 100 × 500 mm were tested under four-point flexural loading. Approximately 225 static flexural strength tests were also conducted to facilitate fatigue testing. The specimens incorporated different proportions of steel and polypropylene fibres in the ratio of 25:75%, 50:50% and 75:25% by volume at each fibre volume fraction of 0·5%, 1·0% and 1·5%. The fatigue test data were used to establish probability distributions of HyFRC at different stress levels. The gradual replacement of steel fibres by polypropylene fibres in the HyFRC mix was found to significantly reduce the variability in the distribution of fatigue life of HyFRC compared with steel fibre-reinforced concrete containing comparable mono steel fibres in terms of size and shape. To examine the flexural fatigue performance of HyFRC, the test results are presented in the form of S–N diagrams; two million cycles of fatigue strengths/endurance limits were determined for different fibre mix combinations corresponding to each fibre volume fraction. The test results indicate that a combination of 50% steel fibres and 50% polypropylene fibres is the most appropriate for both reduction in variability and flexural fatigue performance in terms of the two-million cycle fatigue strength/endurance limit of HyFRC.

Effect of Thermal-Cold Cycling on Properties of Glass Fiber/ Epoxy Resin Composite for Wind Turbin Blade

The weight loss rate, FTIR characteristic, static mechanical properties and dynamic mechanical properties of neat epoxy resin and glass fiber/ epoxy resin laminates (GF/EP) after thermal-cold cycling were studied in this paper. The rule of static tensile and flexural strength of neat epoxy resin and GF/EP after ageing was investigated through the static mechanical properties analysis. The ageing mechanism of GF/EP was studied by analyzing the storage modulus (E′), loss modulus (E"), loss factor of GF/EP and FITR of epoxy resin. The results show that the weight loss rate increases with the ageing time，which is up to 9% after 400 times ageing cycle. The strength and flexural strength of GF/EP decreased by 6% and 21% after ageing, while the tensile strength of neat epoxy resin increases by 8% and its flexural strength increases slightly after ageing. The mechanism analysis reveals that internal defects of GF/EP would be induced by thermal-cold cycling stress, which results in the interface bonding strength dropping and the performance reducing.

Prediction of Mean and Design Fatigue Lives of Self Compacting Concrete Beams in Flexure

In this paper, result of an investigation conducted to study the flexural fatigue characteristics of self compacting concrete (SCC) beams in flexure are presented. An experimental programme was planned in which approximately 60 SCC beam specimens of size 100 × 100 × 500 mm were tested under flexural fatigue loading. Approximately 45 static flexural tests were also conducted to facilitate fatigue testing. The flexural fatigue and static flexural strength tests were conducted on a 100 kN servo-controlled actuator. The fatigue life data thus obtained have been used to establish the probability distributions of fatigue life of SCC using two-parameter Weibull distribution. The parameters of the Weibull distribution have been obtained by different methods of analysis. Using the distribution parameters, the mean and design fatigue lives of SCC have been estimated and compared with Normally vibrated concrete (NVC), the data for which have been taken from literature. It has been observed that SCC exhibits higher mean and design fatigue lives compared to NVC.

Synthesis and study of physical properties of dental light-cured nanocomposites using different amounts of a urethane dimethacrylate trialkoxysilane coupling agent

Objective The purpose of this work was the study of the effect of the amount of a urethane dimethacrylate silane (UDMS) coupling agent on physical properties of dental light-cured resin nanocomposites based on Bis-GMA/TEGDMA (50/50 wt/wt) matrix and Aerosil OX50 as filler. Methods Silica nanoparticles (Aerosil OX 50) used as filler were silanized with 5 different amounts of UDMS 1.0, 2.5, 5.0, 7.5 and 10 wt% relative to silica. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (60 wt%) were mixed with a Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of light cured composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by DMA analyzer. Measurements were taken in samples stored, immediately after curing, in water at 37 °C for 24 h. Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water of 75 vol% for 30 days. Thermogravimetric analysis of composites was performed in nitrogen atmosphere from 50 to 800 °C. Results Almost all of used amount of silane remained chemically bounded on the surface of silica particles, forming a layer around them, which have dense accumulation of methacrylate groups. No significant statistic difference was found to exist between the degree of conversion values of composites with different silane contents. The composite with the lowest amount of UDMS (1.0 wt%) showed the lower flexural strength value, the higher static and dynamic elastic modulus values and the higher sorbed liquid value and solubility. Significance The optimum concentration of UDMS seems to be that of 2.5 wt%. Higher concentrations of UDMS did not improve the properties of composites.

Synthesis and study of properties of dental resin composites with different nanosilica particles size

Objectives The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical–mechanical properties of the composites. Methods Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C. Results As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm 2 is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed. Significance The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.

Physical Properties of Waste Lignin/HDPE Composites

The effective utilization of lignin waste has been a main concern in the bio-ethanol industry which uses straw and stalks as feedstock. In this study the lignin waste was incorporated into high density polyethylene (HDPE) to make composites by extruded pellet fabrication and thermoform process. With the increasing of the lignin content from 55% to 75%, the static flexural strength of the resulting blends was hardly influenced, but the tensile strength decreased 42%. The mechanical and dimensional stability tests both showed that the addition of 4% MAPE promoted the interfacial bonding between lignin and HDPE. The DSC analysis showed the presence of lignin scarcely influences the thermal behavior of pure HDPE and the composites.

Bio-Ethanol Lignin Waste/HDPE Composites: Preparation and Mechanical Properties

The effective utilization of lignin waste has been a main concern in the bio-ethanol industry which uses straw and stalks as feedstock. In this study the lignin waste was incorporated into high density polyethylene (HDPE) to make composites by extruded pellet fabrication and thermoform process. With the increasing of the lignin content from 55% to 75%, the static flexural strength of the resulting composites was hardly influenced, but the tensile strength decreased 42%. Even though it was, the mechanical properties of lignin/HDPE composites were superior to those of traditional medium density fiberboard (MDF). Scanning electron microscopy showed that addition of 4% MAPE promoted the interfacial bonding between lignin and HDPE. Therefore, the macro performance also shows an improvement to mechanical properties.

Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites

Objectives The purpose of this study was to evaluate the effect of the amount of 3-methacryloxypropyl-trimethoxysilane (γ-MPS) coupling agent on some physical–mechanical properties of an experimental resin composite for understanding the optimum amount of silanization. Methods Silica nanoparticles (Aerosil OX 50) used as filler were silanized with 5 different amounts of γ-MPS 1.0, 2.5, 5.0, 7.5 and 10 wt% relative to silica. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (60 wt%) were mixed with a Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of light cured composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by DMA analyzer. Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution. Thermogravimetric analysis was performed in air and in nitrogen atmosphere from 50 to 800 °C. Results At lower silane amounts used (1.0, 2.5 wt%) the silane molecules must have a parallel orientation relative to the silica surface. At higher silane amounts (>2.5 wt%) silane molecules form a layer around the filler particles which now have to occupy a random, parallel and perpendicularly orientation relative to the silica surface. No significant statistic difference was found to exist between the flexural strength and flexural modulus values of composites with different silane contents. Dynamic elastic modulus E′ showed a maximum value for the composite contained 5 wt% silane. The composites with the higher amounts of silane showed the lower values for the tan δ at the T g revealing that these composites have better interfacial adhesion between filler and matrix. Significance The amount of silane used for the silanization of silica particles affect the orientation of the silane molecules relative to the silica surface. This seems to affect the dynamic mechanical properties of composites.

Flexural creep behavior of discontinuous thermoplastic composites: Non-linear viscoelastic modeling and time–temperature–stress superposition

Flexural creep behavior of nylon 6/6, polypropylene and high-density polyethylene long fiber thermoplastic (LFT) composites was studied according to ASTM D-2990. Neat polymers were tested for baseline data and compared with the 40 wt.% E-glass reinforced LFTs, all processed by compression molding. All materials exhibited non-linear viscoelasticity and showed a succession in creep resistance consistent with static flexural yield strength. A four parameter empirical model used for short fiber thermoplastics (SFT), proposed by Hadid et al., was found to provide an excellent fit to the experimental data. Time-compliance data from flexural creep and dynamic mechanical analysis (DMA) were combined to utilize short-term flexural creep tests to predict lifetime of the composites. A time–temperature–stress superposition (TTSSP) procedure was used, where stress-based vertical shifts were applied in addition to horizontal shifts used in a traditional time–temperature superposition (TTSP). Master curves obtained by this method projected the long-term creep properties, the order of creep resistance being consistent with the flexural creep data.

Real-time Detection and Analysis of Damage in High-performance Concrete under Cyclic Bending

In order to quantitatively evaluate the damage level in high-performance concrete (HPC) with pozzolanic minerals under constant amplitude cyclic loads, three methods for real-time damage detection are employed in the present work, i.e., dynamic modulus instrument, real-time strain collector, and digital speckle correlative method (DSCM). Six mechanical parameters at different numbers of loading cycles are real-time captured by these three methods. For a maximum applied fatigue stress equal to 70% of the static flexural strength, a cohesive crack is detected on the specimen surface by the DSCM system from 10% of concrete fatigue life. The nucleation and propagation of the cohesive crack is reflected by the change of the strain concentration zone in 2-dimensional strain fields. The experimental results show that the admixtures of Class F Fly Ash (FA) and S95 Ground Granulated Blast-furnace Slag (GGBS) in high proportions increase the strain and cohesive-crack opening displacement as well as remarkably improve the fatigue performance of HPC.

Dimensionsstabilität und Kriechverhalten von wärmebehandelten mitteldichten Faserplatten für den Außenbereich

A series of commercial phenol-formaldehyde bonded MDF panels were exposed to a post-manufacture heat-treatment at various temperatures and durations using a hot press and just enough pressure to ensure firm contact between the panel and the press platens. Physical properties and static flexural strengths of the post heat-treated MDF panels were evaluated according to ASTM D 1037 (2002). The results indicated that the post-manufacture heat-treatment of the exterior MDF panels resulted in improvement of thickness swelling. Water absorption and linear expansion properties were adversely affected by the heat-treatment. Modulus of rupture and modulus of elasticity values of the heat-treated panels decreased with increasing treatment temperature. A series of three 12-week creep tests were performed in climatic chambers conditioned at 65% RH, 90% RH, and cyclic 65–90% RH, all at a steady temperature of 20 °C. The creep tests generally followed procedures as specified in ASTM D 6815-02a (2002). Creep deflections of the panels increased with increasing temperature of the post heat-treatment.

Flexural fatigue strength prediction of steel fibre reinforced concrete beams

Results of an investigation conducted to study the fatigue strength of steel fibre reinforced concrete (SFRC) containing fibres of mixed aspect ratio are presented. Approximately eighty one beam specimens of size 500 mm x 100 mm x 100 mm were tested under four-point flexural fatigue loading in order to obtain the fatigue lives of SFRC at different stress levels. About thirty six static flexural tests were also carried out to determine the static flexural strength of SFRC prior to fatigue testing. The specimens incorporated 1.0, 1.5 and 2.0% volume fraction of corrugated steel fibres. Each volume fraction incorporated fibres of two different sizes i.e. 2.0 x 0.6 x 25 mm and 2.0 x 0.6 x 50 mm by weight of the longer and shorter fibres in the ratio of 50% - 50%. Fatigue life data obtained has been analysed in an attempt to determine the relationship among stress level, number of cycles to failure and probability of failure for SFRC. It was found that this relationship can be represented reasonably well graphically by a family of curves. The experimental coefficients of the fatigue equation have been obtained from the fatigue test data to represent the curves analytically

Glass FRP-Bonded RC Beams under Cyclic Loading

Ten beams bonded with glass fiber reinforced polymer (GFRP) laminates were tested under cyclic loading with the load range and the FRP reinforcement ratio as test parameters. The maximum load level during cyclic loading was 55%, 65% and 75% of the static flexural strength while the minimum load level was kept constant at 35%. Deflections of the beams at the end of 525000 cycles were found to increase by 16% and 44% when the maximum load level was increased from 55% to 65% and 75% of the static flexural strength, respectively. Beams with FRP reinforcement ratios of 0.64% and 1.28% were found to exhibit lesser deflections of about 15% and 20%, respectively, compared to a similar beam without FRP reinforcement. An analytical approach based on cycle-dependent effective moduli of elasticity of concrete and FRP reinforcement is presented and found to predict the deflections of the test beams well.

Flexural Fatigue Strength of Steel Fibrous Concrete Beams

The paper presents results of an investigation conducted to study the fatigue strength of steel fibre reinforced concrete (SFRC) beams in flexure. An experimental programme was planned in which about two hundred and thirty three flexural fatigue tests were conducted at different stress levels and stress ratios to obtain the fatigue lives of SFRC containing different volume fractions of steel fibres. One hundred and sixty six complimentary static flexural tests were also conducted to obtain the static flexural strength of SFRC beams prior to fatigue testing. The specimens of size 100 × 100 × 500 mm were used for static flexural as well as flexural fatigue tests and incorporated 0.5, 1.0 and 1.5% volume fraction of corrugated steel fibres of size 0.6 × 2.0 × 30 mm. The concept of equivalent fatigue life was then applied to the fatigue life data to incorporate the effects of stress level, stress ratio and survival probability into the fatigue equation. The results indicate that the distribution of equivalent fatigue life of SFRC can approximately be described by the two-parameter Weibull distribution. The material coefficients of the fatigue equation have been determined corresponding to different survival probabilities so that the fatigue equation can be used to predict the flexural fatigue strength of SFRC.

The effect of applied stress on damage mode of 3D C/C composites under bend-bend fatigue loading

The bend-bend fatigue behavior of 3D integral braided carbon/carbon composites (3D C/C) was examined. Fatigue test was conducted under load control at a sinu-soidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 203 MPa (92% of the static flexural strength), the lag loops of fatigue load-displacement were transformed from elasticity to anelasticity and the flexibility of specimens were enhanced with increase in applied stress. It is revealed that the interfacial sliding abrasion played an important role in the fatigue failure process, and the extent and speed of sliding abrasion were con-trolled by the level of applied stress.

Properties of steel fibrous concrete containing mixed fibres in fresh and hardened state

The paper presents results of an experimental investigation carried out to study the properties of plain concrete and steel fibre reinforced concrete (SFRC) containing fibres of mixed aspect ratio. An experimental programme was planned in which various tests such as inverted cone time, Vebe time and compaction factor were conducted to investigate the properties of plain concrete and fibre reinforced concrete in the fresh state. Compressive strength, split tensile and static flexural strength tests were conducted to investigate the properties of concrete in the hardened state. The specimen incorporated three different volume fractions, i.e., 1.0%, 1.5% and 2.0% of corrugated steel fibres and each volume fraction incorporated mixed steel fibres of size 0.6 × 2.0 × 25 mm and 0.6 × 2.0 × 50 mm in different proportions by weight. Complete load deflection curves under static flexural loads were obtained and the flexural toughness indices were obtained by ASTM C-1018 as well as JCI method. A fibre combination of 65% 50 mm + 35% 25 mm long fibres can be adjudged as the most appropriate combination to be employed in SFRC for compressive strength, split tensile strength and flexural strength. However, better workability was obtained as the percentage of shorter fibres increased in the concrete mix.

Flexural fatigue strength of steel fibrous concrete containing mixed steel fibres

This paper reports investigation conducted to study the fatigue performance of steel fibre reinforced concrete (SFRC) containing fibres of mixed aspect ratio. An extensive experimental program was conducted in which 90 flexural fatigue tests were carried out at different stress levels on size 500 mm × 100 mm × 100 mm SFRC specimens respectively containing 1.0%, 1.5% and 2.0% volume fraction of fibres. About 36 static flexural tests were also conducted to determine the static flexural strength prior to fatigue testing. Each volume fraction of fibres incorporated corrugated mixed steel fibres of size 0.6 mm × 2.0 mm × 25 mm and 0.6 mm × 2.0 mm × 50 mm in ratio 50:50 by weight. The results are presented both as S-N relationships, with the maximum fatigue stress expressed as a percentage of the strength under static loading, and as relationships between actually applied fatigue stress and number of loading cycles to failure. Two-million-cycle fatigue strengths of SFRC containing different volume fractions of mixed fibres were obtained and compared with plain concrete.

The effect of voids on the flexural fatigue performance of unidirectional carbon fibre composites developed for wind turbine applications

The influence of voids in the initiation and propagation of static flexural and flexural fatigue failures of unidirectional carbon fibre composite was investigated. Vacuum pressure in the vacuum bag oven cure process was varied to produce void levels in the range 0.5–6%. The voids were characterised using image analysis and used to explain the static flexural strengths and flexural fatigue results. It was observed that mean void content and aspect ratios were less effective in explaining the mechanical property results than void size distributions. A strong correlation between large voids (area > 0.03 mm 2) and a detrimental effect on the mechanical properties was found. This was attributed to the effect of these voids on the crack propagation in the resin rich inter-ply regions.

Probability of fatigue failure of steel fibrous concrete

The paper presents a study on the fatigue strength of steel fibre reinforced concrete (SFRC) at various stress levels. To obtain the fatigue lives of SFRC at various stress levels beam specimens of size 500 mm × 100 mm × 100 mm were tested under four-point flexural fatigue loading. Static flexural tests were also carried out to determine the static flexural strength of SFRC prior to fatigue testing. The specimen incorporated three volume fractions, namely 0·5, 1·0 and 1·5% of corrugated steel fibres of size 0·6 mm × 2·0 mm × 30 mm. All the fatigue tests were conducted on a 100 kN closed-loop universal testing machine. Fatigue life data thus obtained were analysed in an attempt to determine the relationship for SFRC between stress level S, number of cycles to failure N and probability of failure Pf for SFRC. It was found that this relationship can be represented reasonably well graphically by a family of S–N–Pf curves for SFRC containing different volume fractions of fibres. The experimental coefficients of the equation were determined from the fatigue test data to represent the S–N–Pf relationships analytically.