Three-point Bending Configuration Research Articles

Experimental and theoretical studies on the shear resistance of reinforced engineered cementitious composite beams with GFRP rebars and natural fibers

In this work, an attempt is made to produce a ductile construction system using natural fiber-based engineered cementitious composite (ECC) beams where the conventional longitudinal steel reinforcements are replaced using glass fiber-reinforced polymer (GFRP) bars. In total, sixteen medium-scale reinforced ECC beams were tested under an un-symmetric three-point bending configuration to generate shear failure. The parameters considered as a part of the experimental program include (a) type of longitudinal reinforcement (steel or GFRP) and (b) type of natural fiber (flax, hemp, kenaf and pineapple). In addition, a detailed analytical study was performed using the Architectural Institute of Japan (AIJ) method, and a modified equation was proposed for predicting the shear behavior of FRP reinforcement using ECC beams. Test results reveal that the use of GFRP reinforcements resulted in the reduction of peak load from 33.6% to 65.4% when compared to similar steel-reinforced R-ECC beams. However, the failure mode was highly ductile due to the excellent fiber bridging mechanism, multi-crack formation and effective stress redistribution at the crack tip. The predictions obtained from the modified AIJ method improved the accuracy of shear capacity FRP-reinforced ECC sections when compared to the experiments.

Tensile constitutive relationship of UHPFRC from crack hinge based inverse analysis of notched beams

The structural applications of ultra-high performance fiber reinforced concrete (UHPFRC) in the construction industry can be promoted if proper constitutive models can be established for design. The various tension test methods for UHPFRC are being adopted to develop a complete stress-crack width or strain response for design. This study aims to understand the fracture behavior of UHPFRC by testing notched beams under a three-point bending configuration. This testing method helps in quality control checks and obtaining complete tension stress-crack opening behavior that can be used for design. The tensile stress-crack width response of UHPFRC is obtained using the load-CMOD response. The variables considered in this study are the volume fraction of fibers (1 % and 2 %), types of steel fibers (straight, combination of straight and hooked ended), and different cross-section dimensions (100×100×500 mm and 75×75×500 mm). A crack hinge-based inverse analysis method is used for UHPFRC to obtain the complete tensile stress-crack opening response. Stress-strain relations from inverse analysis and direct tension stress-crack opening results are compared and found to be similar. A data-driven tension stress-crack opening response UHPFRC is verified by simulating the girder's response using nonlinear finite element analysis (NFEA). Thus, the proposed inverse analysis can be used to obtain the multi-linear tensile stress-crack opening response which is adequate for capturing the structural response.

Influence of fatigue damage on the impact performance of toughened-interleave carbon fibre epoxy composite laminates

The impact performance of mechanically fatigued carbon fibre-reinforced polymer (CFRP) composite laminates with and without thermoplastic toughening particles has been investigated through a joint experimental and numerical campaign. Prismatic carbon fibre/epoxy specimens with added toughening particles at the ply interfaces were mechanically fatigued with different load amplitudes leading to a loss in apparent stiffness of 10% to 15%. Following the fatigue loading, high resolution computed tomography (CT) has been performed, revealing varying degrees of delamination starting from the free edges along the length of the specimens. In addition, transverse matrix cracks are observed in all fatigued specimens regardless of the amount of delamination. The residual ballistic impact performance of the damaged specimens has been investigated in a dynamic three-point bending configuration at three different velocities, showing a significant detrimental effect of even barely visible initial delamination. In addition, post-mortem CT scans and detailed finite element simulations are used to gain further insight into the mechanisms by which the two types of initial damage affected the impact resistance of the laminates. Finally, repeating the same experiments on a similar CFRP system without discrete toughening particles showed that their inclusion significantly increased the impact resistance both with and without prior fatigue damage.

Numerical Simulation of Transverse Crack on Composite Structure Using Cohesive Element

Due to their anisotropic behavior, composite structures are weak in transverse direction loading. produces transverse cracks, which for a laminated composite, may lead to delamination and total failure. The transition from transverse crack to delamination failure is important and the subject of recent studies. In this paper, a simulation of transverse crack and its transition to delamination on cross-ply laminate was studied extensively using a cohesive element Finite Element Method (FEM). A pre-cracked [0/90] composite laminate made of bamboo was modeled using ABAQUS/CAE. The specimen was in a three-point bending configuration. Cohesive elements were inserted in the middle of the 90° layer and in the interface between the 0° and 90° layer to simulate transverse crack propagation and its transition to delamination. A load–displacement graph was extracted from the simulation and analyzed. As the loading was given to the specimen, stress occurred in the laminates, concentrating near the pre-cracked region. When the stress reached the tensile transverse strength of the bamboo, transverse crack propagation initiated, indicated by the failure of transverse cohesive elements. The crack then propagated towards the interface of the [0/90] laminates. Soon after the crack reached the interface, delamination propagated along the interface, represented by the failure of the longitudinal cohesive elements. The result of the numerical study in the form of load–displacement graph shows a consistent pattern compared with the data found in the literature. The graph showed a linear path as the load increased and the crack propagated until a point where there was a load-drop in the graph, which showed that the crack was unstable and propagated quickly before it turned into delamination between the 0o and 90° plies.

Shear behaviour of reinforced concrete beams strengthened with ultra-high performance fiber reinforced concrete (UHPFRC)

Ultra-high performance fiber reinforced concrete (UHPFRC) is an advanced cementitious composite with high compressive strength and low permeability. Due to its excellent mechanical properties and superior durability, UHPFRC is considered promising for strengthening of the existing concrete bridges. In order to examine its strengthening efficiency for shear capacity, an experimental study is carried out on shear-deficient beams without stirrups. Strengthening method comprising precast UHPFRC laminates being glued with epoxy resin on two lateral sides of the reinforced concrete beams, is examined. To investigate the robustness of the system under severe exposure conditions, some beams are subjected to freeze-thaw (FT) cycles. Beams are tested to failure under three-point bending configuration. Test results show that for epoxy resin bonding, UHPFRC shear strengthening is a promising method to increase the load and deformational capacity, and to limit the crack openings. The load capacity is doubled, and the deformational capacity is increased by around 60%. After exposure to 30 FT cycles, the strengthening efficiency and fracture behaviour of UHPFRC composite beams seem not to be affected. It seems that the interfacial bond strength is sufficient to prevent premature debonding between UHPFRC and NC, which under combined action of environmental exposure (e.g. FT) and mechanical loading might become a challenge. Finally, a finite element model is developed to predict and understand the shear behaviour of the reference and strengthened beams. In general numerical results show good agreement with the experimental results in terms of failure pattern and peak load prediction once the perfect bond model is used for the interface between UHPFRC and NC. In order to better understand the role of governing parameters on the shear capacity of the composite member, parametric studies are conducted focusing on the role of varying UHPFRC softening behaviour and UHPFRC-concrete interface properties.

Measurements and estimation of frequency-dependent multi-scale viscoelastic damping properties of materials

The state-of-art viscoelastic property measurement and estimation methods at different frequencies/length scales inherently couple structural effects or sometimes lose accuracy for relevant parameters at smaller length scales in the material. We report a unified approach to measuring and estimating the frequency-dependent viscoelastic damping properties across different frequency scales independently of the specimen size effect and boundary conditions. The Dynamic Mechanical Analysis (DMA) method with three-point bending configuration is extended to higher-fidelity modeling in terms of time–frequency scales and material micro-mechanical scales together. The methodology demonstrates ways to identify the possible sources of undesired dynamics in the measurement process and eliminate those effects. It includes exciter tip contact dynamics in three-point bending configuration, anharmonic energy transfer to different frequencies (fractal bifurcation), and unmodelled artifacts or noise. Critically reviewing the standard DMA procedure with normal mode synthesis, this paper proposes a time–frequency Fourier spectral method that resolves a broader frequency range that extends to even acoustic/ultrasonic wave regimes of viscoelastic damping. An algorithm proposed in this paper with the advanced viscoelastic model with multi-parameters (standard linear solid model) estimates the fundamental material parameters at small length scales, which is more insightful to obtain a frequency-dependent correlation of material response to general dynamic loading. Stiffness and damping properties are estimated from measured data and validated for self-consistencies via different controls regarding static preload, specimen resonance, filtered noise, and model fidelity across the frequency range, and certain consistency and convergence properties of a series solution. The unified approach with a single test will be helpful in the design and evaluation of a wide variety of materials and their applications in vibration and acoustic problems. The proposed technique further demonstrates a novel way to extend low-frequency measurement to high-frequency response modeling.

Effect of cyclic loading on the microstructure of thin zirconia tapes

The effect of cyclic loading on the microstructure and the phase composition of thin zirconia tapes was studied. Zirconium oxide stabilized with 3 mol% of yttrium was used for preparing the tapes by the gel-tape casting method. The static and cyclic strengths of the samples with a thickness of approximately 0.2 mm were tested. The average static strength of 1233 MPa was determined in a three-point bend configuration. The maximal flexural stress during cyclic loading had to be kept below 900 MPa to ensure the failure-free performance of the tapes - (107 cycles). No evidence of an excessive transformation of the tetragonal microstructure induced by cyclic loading was found. The transformation toughening mechanism of the tetragonal microstructure was not significantly affected by cyclic loading. Based on the results, thin flexible 3Y-TZP parts possess a low risk of mechanically induced excessive transformational microstructural changes during the service life of the part.

Quantifying combined effects of in-plane and out-of-plane constraint in an aluminium alloy

Fracture load of a component can depend on the geometry as well as the material properties due to the effect of constraint on the plastic region which forms around the crack tip prior to fracture. The constraint has two components: in-plane constraint, due to the size and geometry of the component, and out-of-plane constraint, mainly due to its thickness but also affected by other geometric parameters. There are currently methods to quantify both types of constraint separately as well as suggested parameters that quantify both simultaneously. However, experimental programmes that objectively compare the applicability of each are few and far between, especially on the out-of-plane constraint. In this paper we report the results of a substantial experimental programme making use of single edge notched bend (SENB) aluminium alloy specimens to study the combined effects of in-plane and out-of-plane constraint. These experiments involved testing the specimens in a three-point bending configuration with nine combinations of thickness and crack length. This allowed fracture loads to be measured then converted to stress intensity factor K by the use of three-dimensional finite element simulations. Common constraint parameters such as the Q parameter and plastic zone volume were extracted from these simulations. The results showed that K varied between 31 and 55 MPa√m for various levels of in and out-of-plane constraint. Both the magnitude of Q and the plastic zone volume increase with reducing plastic constraint. We showed that both Q and plastic zone volume could be used to quantify the combined effects of in and out-of-plane constraint, however the uncertainty associated with the plastic zone volume is less than that of Q.

RETRACTED: The influence of fiber RC beams under flexure on the chloride-induced corrosion

Corrosion of rebars is the main cause of the deterioration of reinforced concrete structures. In an aggressive marine environment, rebar corrosion occurs due to chloride ion penetration of the reinforced concrete (RC). This paper presents the findings of an experimental study conducted to determine the influence of fiber reinforcement on the chloride-induced corrosion and flexural behaviour of reinforced concrete beams. Uncracked and cracked reinforced concrete beams were exposed to a 16.5% chloride solution in a cyclic immersion for three years, causing them to corrode naturally. The half-cell potential test was carried out to monitor the corrosion potentials, and flexural tests were carried out in a three-point bending configuration to assess the structural performance. Gravimetric tests revealed that bars in conventional concrete had higher degrees of corrosion than those in fiber-reinforced concrete with a variation of 11 %. With increasing crack widths (max 0.2–0.3 mm), there was a tendency toward early corrosion initiation, in contrast, fiber addition delayed the process of corrosion initiation by 6 %. Regarding flexural performance, fiber-reinforced beams are significantly better than conventional concrete beams, especially steel fiber incorporated concrete beam's residual flexural strength 16 % more than the conventional concrete beams.

Flexural characterization and ductility assessment of small-scale mortar beams reinforced with 3D-printed polymers

Additive Manufacturing (AM) is recently getting tremendous attention in many scientific fields. In this research, a civil engineering application of AM is investigated, where thirty small-scale cement mortar prisms reinforced with 3D-printed thermoplastics were tested in flexure. Rebar-like reinforcement were printed using a fused deposition modeling (FDM) printer. Five different thermoplastics were used with three different surface morphologies, obtained by introducing lugs on the rebar surface. The lugs were intended to enhance the reinforcement-matrix bond. Beams were tested in a three-point bending configuration to assess their load–deflection response and flexural ductility. In general, the introduction of lugs increased the load-carrying capacity of the specimen, between 40 and 120%. The ductility of beams was calculated based on displacement and energy methods. Due to the lack of a definite yield point in such composites, modifications to well-established ductility calculation methods are proposed here. The results indicate the capability of thermoplastics to reinforce mortar beams in flexure. Surface morphology has shown to be significantly influential for most of the thermoplastics considered in this study in terms of strength gain and ductility.

Quantitative analysis of the structural health of railway turnouts using the acoustic emission technique

Rail defects such as fatigue cracks have been one of the leading root causes of a number of derailments in the past. Cracks that initiate and propagate below the surface are difficult to detect using traditional non-destructive testing (NDT) methods. Acoustic emission (AE) is a more effective method for detecting and monitoring crack growth in rails online. This study investigates the applicability of AE for quantifying damage propagation in austenitic cast manganese steel used in manufacturing railway turnouts. The relationship between AE and crack growth rate in austenitic cast manganese steel samples that were fatigue tested in a three-point bending configuration was investigated by evaluating the AE activity with respect to direct current potential drop (DCPD) measurements and scanning electron microscopy (SEM) fractographic analysis of the tested samples. From the results obtained, it was not possible to observe a clear relationship between AE activity and the actual crack growth rate. Based on the SEM fractographic analysis, this is likely due to the plasticity occurring at the tip of the fatigue crack in the tested samples. This is plausible since the cast manganese steel samples had been cut off from a plate that had not been previously work hardened. The effect of carbides present in the microstructure is an additional contributing factor. Further tests should be carried out on cast manganese steel samples that have been work hardened prior to fatigue testing.

Influence of Type of Filler and Bitumen on the Mechanical Performance of Asphalt Mortars.

This article presents a new methodology of analysis based on a fast-running experimental procedure to characterise the mechanical response of asphalt mortars in terms of stiffness, ductility, and fatigue resistance. This was achieved using the DMA (Dynamic Mechanical Analyser) three-point bending configuration. The study was carried out by considering the employment of different types of fillers such cement and CaCO3 and different types of binders such as conventional asphalt binder (B35/50) or modifided polymer-modified bitumen (PMB 25/55–65). From the results of this study, the filler was found to have a greater influence on the stiffness and ductility of the asphalt material, while bitumen had a higher effect on the fatigue life of the asphalt mortar. Fatigue life was observed to increase with the use of a polymer-modified binder, while a lower degree of permanent deformation and higher bearing capacity achieved by the use of cement instead of calcium carbonate as active fillers.

Effect of petrographic composition and chemistry of aggregate on the local and general fracture response of cementitious composites

This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions.

Flatwise to Upright Build Orientations under Three-Point Bending Test of Nylon 12 (PA12) Additively Manufactured by SLS.

This paper describes in detail a series of static tests conducted in a three-point bend configuration on three build orientations (i.e., 0°, 45°, and 90°) of additively manufactured Nylon 12 (PA12) specimens produced with a powder refresh ratio of 50%, using a benchtop industrial SLS platform, Formlabs Fuse 1. The bending strength and flexural elasticity moduli are determined following ISO 173 specifications and by employing a more complex equation that considers the influence of large deflections as per ISO 14125 indications. Statistical variability of experimental data is considered and compared to the results from the literature. Through a fractographic SEM study, the damage morphologies of tested specimens are analyzed and associated with the recorded load-deflection curves for an accurate perception of build orientation-dependent anisotropy in bending properties of AM PA12 SLS specimens. A surprising result of this investigation is that the specimens built with 45° orientation showed superior modulus elasticity in flexure but a low bending strength compared to flatwise oriented specimens. In addition, a Weibull reliability quantification of bending strength is adapted to pinpoint the effects of internal 3D printing flaws (contained within a characteristic highly-stressed volume of material) over the failure probability of the three build orientations in question.

Mechanical fracture parameters of concrete drill-core specimens supported by a slenderness ratio study

The detailed analytical and experimental investigation of the fracture behaviour of quasi-brittle materials is an endeavour which has been ongoing worldwide for many years. Such materials are usually characterized in terms of their mechanical fracture parameters, which are determined based on the evaluation of quasi-static fracture experiments. One of the most commonly used building materials with a quasi-brittle response is concrete, which is most often based on a cement matrix. It is sometimes also necessary to characterize concrete included in existing structures. In this case, test specimens are obtained by core drilling, and the investigation is conducted with the requirement to maximize the number of parameters obtained while minimizing the number of test specimens drilled from the structure. This paper focuses on the mechanical fracture parameters of core-drilled specimens taken from a selected concrete structure. Tests were performed on cylindrical specimens with a chevron-notched stress concentrator in the three-point bending configuration in order to determine modulus of elasticity, fracture toughness and fracture energy. Subsequently, theoretical compressive strength was estimated and tests for the determination of compressive strength values were performed focusing on dependence on the slenderness ratio, i.e. the relationship between the compressive strength and the length to diameter ratio of the cylindrical specimens. In relation to the obtained mechanical fracture parameters, selected specimens were analysed and three-dimensionally characterized via high-resolution X-ray computed tomography.

Mixed mode fracture behavior of short-particle engineered wood

The present article discusses the fracture behavior of medium-density fiberboard (MDF) and particleboard (PB), under mixed mode loading conditions. With wide applications in construction, marine and furniture industry, MDF and PB are two well-known, wood-based composites that are made of wood particles and fibers, adhered together using a standard polymer glue. Our examination of the elastic properties of these wooden samples in transverse and longitudinal directions via the digital image correlation (DIC) technique sheds light on the elastic isotropy of the employed MDF and PB sheets. Moreover, with the introduction of an improved three-point bend configuration for in-plane wood fracture tests, the brittle behavior of engineered wood is evidenced, which validates the utilization of the linear elastic fracture mechanic (LEFM) concepts herein. Therefore, within the framework of LEFM, we apply the maximum tangential strain (MTSN) criterion to predict the fracture initiation angles and fracture toughness for wooden specimens. Finally, in a comprehensive Weibull statistical analysis over the MDF and PB fracture test data, it is shown that, the mode II test results can be well-estimated based on the value of KIf, using the MTSN criterion. This is especially beneficial when the probability of mode II fracture is sought after, given the mode I statistical parameters.

Characterization of material mechanical properties using strain correlation method combined with virtual fields method

The isotropic and anisotropic behaviors are considered as the important formats of the constitutive behaviors, and can also be called the global properties. To improve the identification ability of virtual fields method (VFM) when the global properties are unknown, this paper proposes the strain correlation method (SCM) to determine the global properties before the parameter identification using the VFM. Firstly, the basic principle of SCM is described in detail. Then, the feasibility and accuracy of SCM are verified through the numerical experiments based on the three-point bending configuration and the real experiment of polymethyl methacrylate (PMMA). The influence of the additive Gaussian white noise, local errors in the strain fields, and missing data at the specimen edges on the characterization results are evaluated. The results show that the SCM has good noise immunity and lower accuracy requirements for the strain fields. As an application, the mechanical properties of Ti-6Al-4V alloys fabricated by selective laser melting (SLM) are characterized by the SCM. The results show that the alloys are isotropic, and the isotropic VFM is utilized to determine the mechanical parameters. By using the SCM, the accuracy of identification results can be improved for the isotropic or bidirectional reinforced orthotropic materials when using VFM.

Influence of rock inclusion composition on the fracture response of cement-based composite specimens

This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions.

Effects of strain rate and bond preparation for dissimilar materials in energy dispersive applications

Composite materials, typically consisting of two or more dissimilar materials adhered together in layers, are frequently used for energy absorption applications. The interface and material characteristics strongly influence the global energy absorptive capability of the composite. This research focuses on ceramic-polymer interfaces and, in particular, links between the properties of the composite, the interface and the separate materials. After characterisation of the materials, the effects of impact speed and bond condition were considered for a polymer-ceramic bond in a threepoint bend configuration. Specimens were loaded in a screw-driven machine at 0.05 mm s-1 and through projectile impact at speeds of approximately 50 m s-1. Screw-driven experiments were performed at ambient and sub ambient conditions, with the temperature chosen to simulate the expected polymer performance in the gas gun experiment, making use of the equivalence of rate and temperature for polymers.

Lime-Based Mortar Reinforced by Randomly Oriented Short Fibers for the Retrofitting of the Historical Masonry Structure.

Recent seismic events prompted research to develop innovative materials for strengthening and repair of both modern and historic masonry constructions (buildings, bridges, towers) and structural components (walls, arches and vaults, pillars, and columns). Strengthening solutions based on composite materials, such as the Fiber Reinforced Polymers (FRP) or the Fiber Reinforced Cementitious Matrix (FRCM), have been increasingly considered in the last two decades. Despite reinforcement made of short-fibers being a topic that has been studied for several years from different researchers, it is not yet fully considered for the restoration of the masonry construction. This work aims to experimentally investigate the enhancement of the mechanical properties of lime-based mortar reinforced by introducing short glass fibers in the mortar matrix with several contents and aspect ratios. Beams with dimensions of 160 mm × 40 mm × 40 mm with a central notch were tested in three-point bending configuration aiming to evaluate both the flexural strength and energy fracture of the composite material. Then, the end pieces of the broken beams were tested in Brazilian and compressive tests. All the tests were performed by a hydraulic displacement-controlled testing machine. Results highlight that the new composite material ensures excellent ductility capacity and it can be considered a promising alternative to the classic fiber-reinforcing systems.