Four-point Beam Bending Test Research Articles

Laboratory evaluation of loading frequency effects on HMA self-healing

Self-healing is an intrinsic property of bituminous materials with a significant effect on the performance and durability of asphalt mixtures. Also, research has shown that the self-healing phenomenon affects the fatigue endurance limit of asphalt mixtures. In order to determine the fatigue endurance limit in the laboratory, the four-point bending (4PB) beam test is conducted at low strain levels. The 4 PB beam fatigue test at low strain amplitudes is time-consuming because of the self-healing phenomenon. In this study, to determine self-healing effects on the laboratory fatigue life of HMA, the 4 PB fatigue test loading frequency was increased from 10 Hz to 25 Hz. Analysis of results showed that the effect of self-healing on fatigue life can be approximately specified. Based on the results, self-healing factors range from 2.51 to 9.45 and increase by decreasing the 4 PB fatigue test strain level.

Experimental evaluation of the basalt fibers and diatomite powder compound on enhanced fatigue life and tensile strength of hot mix asphalt at low temperatures

In the present research, a composite of basalt fibers and diatomite was used to enhance fatigue life and tensile strength of hot mix asphalt (HMA). Conducted experiments included a four-point bending beam tests at 20°C for determining the fatigue life of the mix, and indirect tensile strength test at −5°C for determining tensile strength and ultimate strain of the mixture. The results indicate that, in general, at higher levels of strain, fibers may not enhance fatigue life of asphalt mixes. Although a composite of basalt had not been effective in enhancing the fatigue life, the results indicate that the use of the fibers in reinforced mixes with diatomite (which were of higher stiffness) ended up with doubling the fatigue life. Investigating the results of indirect tensile tests at −5°C and looking into the range of strain at failure, it is found that the basalt fibers can significantly improve the properties of hot mix asphalt at low temperatures, and simultaneous use of diatomite and basalt fibers can well compensate for weaknesses of asphalt mixes at lower temperatures.

An investigation of material characterization of pultruded FRP H- and I-beams

ABSTRACTThis study presents the material characterization of pultruded fiber-reinforced polymer beams of different sizes using experimental and analytical methods. Various tests were performed to determine the material properties such as Young's and shear moduli using tensile and bending tests of coupons and beams. The stiffness measured from the four-point bending test of both beams is in close agreement with analytical methods. It is shown that the numerically obtained flexural response of the beam using ABAQUS software with material properties obtained from the four-point beam bending test is close to the experimental response.

Experimental study of recycled asphalt concrete modified by high-modulus agent

Property characterization of high modulus recycled asphalt concretes modified by high modulus agents based on experimental evaluation and comparison with normal recycled asphalt concretes was performed in this paper. Both normal recycled asphalt concretes and high modulus recycled asphalt concretes with percentages of reclaimed asphalt pavement (RAP) varying from 20% to 60% were prepared for property evaluation including dynamic modulus by simple performance tester (SPT), rutting stability by wheel loading test, moisture susceptibility by Marshall test and indirect tensile test, thermal cracking resistance by three-point beam bending test, and fatigue cracking resistance by four-point beam bending test. It is found that, both RAP and high modulus agents can improve the dynamic modulus and rutting stability of asphalt concrete but jeopardize the thermal cracking resistance and fatigue cracking resistance of recycled asphalt concrete. However, the high modulus agents can enhance the moisture susceptibility of recycled asphalt concrete while the RAP could harm the moisture susceptibility of recycled asphalt concrete. It is also noticed that the high modulus agents can reduce the harmful effects by RAP on the properties of recycled asphalt concrete. Meanwhile, increasing the asphalt content of high modulus recycled asphalt concrete can further improve its moisture susceptibility, thermal cracking resistance and fatigue cracking resistance while keep its dynamic modulus and rutting stability still much better than the normal recycled asphalt concrete. The evaluation results show that the high modulus asphalt concrete have more tolerance than the normal asphalt concrete for negative effects of RAP on the engineering properties. It is more promising to use large percentage of RAP in the high modulus recycled asphalt concrete than in the normal recycled asphalt concrete.

Influence of Aging, Temperature and Moisture Damage on the Stiffness of Asphalt Concrete through the Fatigue Process

The stiffness of asphalt concrete is considered as a key element in the fatigue process under load repetitions. In this study, an investigation was made in order to better understand the influence of long term aging, moisture damage, low testing temperature, asphalt content, and Microstrain level on the stiffness through the fatigue process of asphalt pavement. Repeated four-point flexure bending beam test in controlled strain mode has been implemented using Nottingham apparatus. Asphalt concrete mixtures were prepared using different percentages of asphalt cement, (optimum and 0.5% above and below the optimum). Asphalt concrete slab samples of (300x 400x 60) mm were prepared using roller compaction, beam specimens of (400x 50x 60) mm were cut from the slab samples. Beam specimens were divided into four groups, the first group was tested for fatigue life under the influence of three levels of micro strain (750, 400, and 250). The second group was tested at 30, 20, and 5 °C, while the third group was subjected to long-term aging, before testing for fatigue life. The fourth group was subjected to moisture damage impact, and then tested for fatigue life. The changes in stiffness per load cycles have been monitored. It was concluded that the stiffness is susceptible to moisture damage and aging, the increase in Microstrain level leads into a remarkable decrease in initial and failure stiffness's. The stiffness is susceptible to the testing temperature and asphalt content, lower testing temperature of 5o C exhibits higher stiffness value, while higher binder content has a negative impact on the stiffness.

Performance Characterization of Polymer Modified Asphalt Binders and Mixes

Fatigue sensitivity of four different asphalt binders and three different asphalt mixes was evaluated in the study. Binders were subjected to Linear Amplitude Sweep (LAS) test at three temperatures of 10, 20, and 30°C. Four-point beam bending test (4PBBT) was conducted on the asphalt mixes at a temperature of 20°C for strain amplitudes varying from 200 to 1000 microstrains. Tests like retained Marshall Stability and indirect tensile strength (ITS) were also carried out to judge the mix performance. Experimental studies demonstrated that elastomeric modified binder and mixes gave the best performance in fatigue. Plastomeric modification was found to be highly strain susceptible and resulted in poor fatigue performance. The fatigue life of stone mastic asphalt (SMA) was found to be almost five times higher than the dense graded mixes. For similar strain levels, the results of LAS test could be linearly correlated with the 4PBBT results.

Impact of Loading Frequency on the Fatigue behavior of SBS Modified Asphalt Mixtures

Fatigue cracking is considered as one of the most serious distresses associated with flexible pavements. Loading frequency has a great effect on fatigue behavior of asphalt mixes. Such that for standard mixes it is widely stated that the fatigue life decreases by increasing loading frequency when the test is conducted in deformation control mode. On the other side the effect of loading frequency on SBS modified asphalt mixes has not been investigated extensively. During fatigue tests on polymer modified mixes (PMA), temperature of the specimen increases. Higher temperature reduces the stiffness of asphalt concrete during the fatigue test and increases the fatigue life of the specimens and may affect the characterization of asphalt fatigue behavior. In this research an investigation was made to understand the fatigue behavior of SBS modified asphalt mixes and influence of loading frequency on PMA fatigue life in comparison with unmodified mixtures. The conventional bitumen with 60/80 penetration grade was used in this research and was modified with styrene-butadiene-styrene (SBS) at 5% by weight of the bitumen. Two different loading frequencies were applied and fatigue life for asphalt mixtures was determined using the four-point bending beam fatigue test based on phenomenological approach as well as dissipated energy concept. Results showed that the fatigue life of SBS-modified mixture is up to three times more than conventional mixtures. Temperature increase of PMA specimens during four-point bending beam test mainly affects the fatigue behavior in the test. Because of simultaneous effects of this temperature increase and healing phenomena, the difference between fatigue lives in two different loading frequencies is negligible, while this difference is significant in case of conventional mixtures.

Study on the impact of geotextiles on fatigue life of asphalt specimens

In this research, four types of geotextiles were selected and their impact on fatigue life and average flexural stiffness of hot mix asphalt (HMA) specimens was investigated. Fatigue tests were implemented by four-point bending beam test at strain control condition with haversine wave and 10 Hz frequency. The fatigue tests were carried out in four strain levels 250, 400, 650 and 1000 micro strain. Based on test results, the fatigue life of all specimens reinforced by four types of geotextiles was increased, but this increase in specimens of Geot Type 1 and Geot Type 2 was more than that of Iranian ones. Fatigue life was different at various strain levels for any type of geotextile materials and results showed that specimens made of reinforced geotextiles had higher performance at high strain levels. Results also expressed that geotextiles may reduce or increase the average flexural stiffness of specimens but they totally cause the improved quality of the asphalt pavement. Key words: Fatigue life, geotextiles, bending beam test, strain levels, flexural stiffness.

Toughening characteristics of plastic fiber-reinforced hot-mix asphalt mixtures

With the performance limitation of a conventional hot-mix asphalt mixture, the structural integrity of that is highly needed to be enhanced to increase the fatigue life of the mixture. Various geosynthetic alternatives have been widely utilized, such as geogrid, geotextile, or geomembrane layers at the bottom the mixture or on the top of a subgrade to improve the structural integrity of a hot-mix asphalt pavement system. Although reinforcing effects of such interlayers has been reported with some improvements in hot-mix asphalt pavements’ performances, such as mitigating rut or delaying reflective cracks; however, such alternatives do not enhance toughness, tensile strength, or shear strength of the hot-mix asphalt mixture itself due to that the interlayers are usually installed in between two layers in a pavement system and those interlayers do not mix with hot-mix asphalt mixtures. A new plastic fiber-reinforced hot-mix asphalt mixture was proposed in this study to provide more enhanced structural integrity of the mixture not only at the bottom of the mixture but also within the mixture. The developed mixture in this study led to significant enhancements in phenomenological toughness and fatigue life of that at least 1.5 times higher than those for conventional hot-mix asphalt mixtures as resulting from indirect cyclic fatigue tests in loading-control modes and four-point bending beam tests in displacement-control modes.

Analysis of strain error sources in micro-beam Laue diffraction

Micro-beam Laue diffraction is an experimental method that allows the measurement of local lattice orientation and elastic strain within individual grains of engineering alloys, ceramics, and other polycrystalline materials. Unlike other analytical techniques, e.g. based on electron microscopy, it is not limited to surface characterisation or thin sections, but rather allows non-destructive measurements in the material bulk. This is of particular importance for in situ loading experiments where the mechanical response of a material volume (rather than just surface) is studied and it is vital that no perturbation/disturbance is introduced by the measurement technique. Whilst the technique allows lattice orientation to be determined to a high level of precision, accurate measurement of elastic strains and estimating the errors involved is a significant challenge. We propose a simulation-based approach to assess the elastic strain errors that arise from geometrical perturbations of the experimental setup. Using an empirical combination rule, the contributions of different geometrical uncertainties to the overall experimental strain error are estimated. This approach was applied to the micro-beam Laue diffraction setup at beamline BM32 at the European Synchrotron Radiation Facility (ESRF). Using a highly perfect germanium single crystal, the mechanical stability of the instrument was determined and hence the expected strain errors predicted. Comparison with the actual strain errors found in a silicon four-point beam bending test showed good agreement. The simulation-based error analysis approach makes it possible to understand the origins of the experimental strain errors and thus allows a directed improvement of the experimental geometry to maximise the benefit in terms of strain accuracy.

Experimental Investigation of Grid-reinforced Asphalt Composites Using Four-point Bending Beam Tests

Four-point bending beam tests are applied to investigate the bending behavior of asphalt composites strengthened by grid reinforcements made of carbon, glass, and polyester fiber grids. Both experimental and analytical predictions are compared considering the deflection behavior of carbon grid-reinforced beams. This study especially shows significant structural improvement and better crack resistance of asphalt composite beams by carbon grids. Temperature and loading rate dependencies are also evaluated and compared for both reinforced and unreinforced asphalt composite beams. The maximum bending forces of carbon grid-reinforced beams significantly decrease with decreasing loading rate and increasing the temperature. The deflection behavior of asphalt composite beams reinforced by carbon grids is reasonably predicted using a bilinear damage-based model and a viscoelastic beam theory. The layer-parallel direct shear test is conducted for investigating the interlayer shear behavior of asphalt composite beams.

Prediction of In-Service Fatigue Life of Flexible Pavements Based on Accelerated Pavement Testing

Abstract Full-scale accelerated pavement testing (APT) is a supplement to typical laboratory procedures. As such, it leads to significant advances in practice and economic savings for the evaluation of new pavement configurations, stress level related factors, new materials, and design improvements. This type of testing closely simulates field conditions; however, it does not capture actual pavement performance because of the limited ability to address long-term phenomena. Therefore, to fully utilize the benefits of APT, there is a need for a methodology to predict the long-term performance of in-service pavement structures based on the generated test results in order to account for such differences. The bias correction factor (BCF) or bias correction functions should account for the quantifiable differences between the fatigue life of the pavement site under APT and the in-service conditions. The BCF represents an improvement of the currently used “shift factors” since they are more general and are based on laboratory testing or computer simulations. By applying the proposed methodology, APT performance results from a structure similar to an in-service structure can be used to perform four-point bending beam tests and structural analysis to obtain an accurate estimate of the necessary BCF to estimate in-service performance.

Wall friction in the compaction of pharmaceutical powders: measurement and effect on the density distribution

In this paper, the axial density profile of tablets of microcrystalline cellulose (MCC) powder compacted in nonlubricated die is investigated by finite element modelling (FEM). The Drucker–Prager/Cap model was adopted for the compaction behavior of powder. The material parameters of the model, including the die wall friction coefficient, were estimated from experimental data of die compaction where the initial density of powder is taken uniform. Changes of Young's modulus with density was measured with a four-point beam bending test. The results of the simulation of the compression and the decompression steps were used to calculate the axial density distribution. Comparison with the measured data presented in [A. Michrafy, M.S. Kadiri, J.A.D. Dodds, Wall friction and its effects on the density distri-bution in the compaction of pharmaceutical excipients, Chem. Eng. Research and Design, Vol. 81, Part A, September (2003)] is discussed.

Micromechanical modeling of softening behavior in steel fiber reinforced cementitious composites

The fracture of cementitious materials is governed by its softening behavior. When steel fibers are incorporated into concrete to improve its toughness, the softening behavior can be predicted in terms of the composite micro-parameters including the properties of fiber, matrix and interface as well as fiber size, length, distribution and volume fraction. In this paper, the bridging force provided by a steel fiber at any arbitrary angle to the crack is first modeled in terms of the micro-parameters. For a given fiber distribution, the averaged crack bridging force is then derived and employed to obtain the composite softening behavior. The model is verified at both the microscopic and macroscopic levels with experimental results from the fiber pull-out test and the four-point beam bending test. With the model, numerical simulations are carried out to study the effect of various parameters on the tensile softening behavior. These simulations can facilitate the choice of micro-parameters for the most cost-effective material design.

The Influence of Thermoplastic Film Interleaving on the Interlaminar Shear Strength and Mode I Fracture of Laminated Composites

Results are presented on the interlaminar shear strength (ILSS) and Mode I fracture toughness (GIC) of glass/epoxy laminates interleaved with thermoplastic polyester films using four-point bending and double cantilever beam tests, respectively. The ILSS equation from classical beam theory was modified to account for the increased film thickness. It was found that the ultimate failure load and ILSS could be doubled if a thermoplastic film of high ductility but low glass transition temperature is used. Good film/composite ply adhesion is necessary. Mode I fracture toughness is influenced by film thickness and interleaving with a 0.2 mm thick film increased the GIC by 40 percent. Interleaving with a thinner film (0.1 mm), however, resulted in adhesive failure and reduced fibre bridging.

Viscoelastic creep behavior of filament-wound case materials

The viscoelastic creep behavior of an S-901 glass/epoxy composite used in a solid rocket motor case is examined for a variety of loading conditions. Samples were removed from the forward dome region of a filament-wound motor case at fiber angles of ± 20 and ± 70 deg. They were tested at several stress levels in tension and four-point beam bending to determine the creep response through several creep-recovery cycles. Constant displacement rate tests also were conducted using the four-point beam bending test to evaluate hysteresis at relatively low stress levels. The behavior was found to be similar in many ways to that previously found for autoclaved, flat-plate samples tested at equivalent conditions. The creep behavior was found to follow a power law in time, D = D0 + /)/*, where D is the creep compliance (psi ~*), D0 is the initial elastic compliance, and D2 and n define the viscoelastic response characteristics. In the linear viscoelastic range, n = 0.19 and is in good agreement with data derived from the epoxy resin itself. At higher stress levels, the value of n increases due to microcracking within the composite. The largest difference in the creep-recovery behavior is exhibited between the first and second loading cycles, with n decreasing slightly with subsequent loading.