Series Of Three-point Bending Tests Research Articles

Improving the mechanical performance of foam material based on bioinspiration

Inspired from foam-like biological construction, artificial foam, which is typically an ultralight and multifunctional material, has been developed for engineering applications. However, the presence of constructed porous materials leads to an extremely weakened load-bearing capacity. Therefore, we propose encasing the outside of bare foam with high-strength fabric to enhance the foam’s mechanical properties. A series of three-point bending tests are conducted to examine the enhancement in bending resistance generated by a high-strength composite fabric. A comparison between bare and encased foams shows that encasing bare foam with high-strength fabric significantly increases the foam’s load-bearing capacity, especially in terms of its bending resistance while retaining its intrinsic lightweight advantages.

Effect of Cyclic Loading on Mode I Fracture Toughness of Granite under Real-Time High-Temperature Conditions

Under hot dry rock development, rock formations undergo the combined challenges of cyclic loading and high temperatures, stemming from various sources such as cyclic hydraulic fracturing and mechanical excavation. Therefore, a fundamental understanding of how rocks fracture under these demanding conditions is fundamental for cyclic hydraulic fracturing technology. To this end, a series of three-point bending tests were conducted on granite samples. These tests entailed exposing the samples to cyclic loading under varying real-time high-temperature environments, ranging from 25 °C to 400 °C. Furthermore, different upper load limits (75%, 80%, 85%, and 90% of the peak load) obtained in monotonic three-point bending tests were used to explore the behavior of granite under these conditions. The analysis encompassed the study of load–displacement curves, elastic stiffness, and mode I fracture toughness under cyclic loading conditions. In addition, the microscopic features of the fracture surface were examined using a scanning electron microscope (SEM). The findings revealed notable patterns in the behavior of granite. Cumulative vertical displacement in granite increased with the growing number of cycles, especially at 25 °C, 200 °C, and 300 °C. This displacement exhibited a unique trend, initially decreasing before subsequently rising as the cycle count increased. Additionally, the critical damage threshold of granite exhibited a gradual decline as the temperature rose. As the temperature ascended from 25 °C to 200 °C, the damage threshold typically ranged between 80% and 85% of the peak load. At 300 °C, this threshold declined to approximately 75–80% of the peak load, and at 400 °C, it fell below 75% of the peak load. Within the temperature ranging from 25 °C to 300 °C, we noted a significant increase in the incidence of cracks, crystal microfracture zones, and the dislodging of mineral particles within the granite as the number of cycles increased.

Flexural behavior of fiber reinforced composites sandwich panels with a reused wooden core

The increasing use of wood products has inevitably resulted in a fast accumulation of waste wood, which is commonly landfilled or recycled for energy or products with low added value. In the present study, a concept has been adopted to reuse the waste wood formwork as the core of FRP sandwich panels. The waste wood formwork has been firstly sliced into wood strips of a specific width, which are then flipped by 90o and bonded together to form a plate as the sandwich core. Such a recycling routine appears as a promising method due to (1) minimum pre-treatment for processing the waste wood; and (2) less likelihood of the hazardous chemicals leakage because of the covered GFRP skins. The flexural behavior of such sandwich panels has been evaluated through a series of three-point bending tests with the thicknesses of GFRP layer and the core plate as the main parameters. The test results indicate that the GFRP-waste wood sandwich panels failed by either rupture of the tensile GFRP sheet or crushing of the compression side. The bending stiffness and flexural strength improves with the increase in the thicknesses of the core plate and GFRP layer, and the panels with three-layer GFRP sheets have the bending stiffness and flexural strength up to 11.0 and 4.4 times higher than that of the core plate. The analytical and numerical predictions on the flexural properties of the sandwich plate show in good agreement with that from the tests, validating the adopted the models.

Influence of 3D Microstructure Pattern and Infill Density on the Mechanical and Thermal Properties of PET-G Filaments.

This study aims to evaluate the thermal and mechanical performances of PET-G thermoplastics with different 3D microstructure patterns and infill densities. The production costs were also estimated to identify the most cost-effective solution. A total of 12 infill patterns were analysed, including Gyroid, Grid, Hilbert curve, Line, Rectilinear, Stars, Triangles, 3D Honeycomb, Honeycomb, Concentric, Cubic, and Octagram spiral with a fixed infill density of 25%. Different infill densities ranging from 5% to 20% were also tested to determine the best geometries. Thermal tests were conducted in a hotbox test chamber and mechanical properties were evaluated using a series of three-point bending tests. The study used printing parameters to meet the construction sector's specific needs, including a larger nozzle diameter and printing speed. The internal microstructures led to variations of up to 70% in thermal performance and up to 300% in mechanical performance. For each geometry, the mechanical and thermal performance was highly correlated with the infill pattern, where higher infill improved thermal and mechanical performances. The economic performance showed that, in most cases, except for the Honeycomb and 3D Honeycomb, there were no significant cost differences between infill geometries. These findings can provide valuable insights for selecting the optimal 3D printing parameters in the construction industry.

Experimental and Hybrid FEM/Peridynamic Study on the Fracture of Ultra-High-Performance Concretes Reinforced by Different Volume Fractions of Polyvinyl Alcohol Fibers.

In this study, a series of three-point bending tests were carried out with notched beam structures made of polyvinyl alcohol (PVA) fiber-reinforced ultra-high-performance concrete (UHPC) to study the effect of volume fractions of PVA fibers on the fracture characteristics of the UHPC-PVAs. Furthermore, in order to meet the increasing demand for time- and cost-saving design methods related to research and design experimentation for the UHPC structures, a relevant hybrid finite element and extended bond-based peridynamic numerical modeling approach is proposed to numerically analyze the fracture behaviors of the UHPC-PVA structures in 3D. In the proposed method, the random distribution of the fibers is considered according to their corresponding volume fractions. The predicted peak values of the applied force agree well with the experimental results, which validates the effectiveness and accuracy of the present method. Both the experimental and numerical results indicate that, increasing the PVA fiber volume fraction, the strength of the produced UHPC-PVAs will increase approximately linearly.

Analysis of tensile strength and fracture toughness of ZrB2–SiC ceramic from three-point bending samples with edge cracks

ZrB2–SiC ceramic is one of most promising high temperature resistant materials in the field of thermal protection, and its strength and toughness are two key mechanical indicators which determine its application prospects. How to conveniently and accurately obtain its strength and toughness is of great value for engineering applications. In this paper, the tensile strength and fracture toughness of the ZrB2–SiC ceramic were obtained according to the boundary effect model (BEM) from a series of three-point bending tests of ZrB2–SiC ceramic samples. Then, the BEM method and normal distribution methodology were employed for analysis of strength and toughness of ZrB2–SiC ceramic. Results show that there exists a linear relationship between maximum loads Pmax and effective area Ae, and the slope of the straight line passing through the coordinate origin is the tensile strength ft. The tensile strength ft and fracture toughness KIC of ZrB2–SiC ceramic can be predicted simply from a set of three-point bending tests according to the BEM method. Similarly, the maximum loads Pmax for three-point bending samples can also be predicted and determined when the ft, KIC and average grain size are available. Results indicate that the grain sizes, ft and KIC of the ZrB2–SiC ceramic follow the law of normal distribution. Satisfactory results of the predicted ft and KIC between the upper and lower bounds with 95% reliability can be achieved based on BEM method.

Experimental and numerical analysis of a novel curved sandwich panel with pultruded GFRP strip core

Curved sandwich structure made of glass fiber-reinforced polymer (GFRP) composite is emerging in the construction of non-linear architectures and wind turbine blades. In this work, a novel curved sandwich panel with pultruded GFRP strip core was proposed and studied. Compared with the common core materials such as foam and wood, pultruded GFRP strips can provide a higher stiffness and strength, particularly when used in large-scale sandwich structures. The proposed sandwich panel was manufactured through vacuum-assisted resin transfer molding process. A series of three-point bending tests was conducted to study the flexural behavior. The effect of surface thickness on the load-carrying capacity and failure mode was addressed. Then, a finite element analysis was conducted via ABAQUS and validated by the experimental results. The parametric study was performed to address the key design parameters. Both experimental and numerical results indicated that with the increased surface thickness, the failure mode could transition from the damage of glue seam to the transverse cracking inside the pultruded GFRP strip. Moreover, the load-carrying capacity can be improved with the thickened face sheet. In the end, this work successfully demonstrated the use of multilayer shell elements in the finite element modelling of the cracking behavior of glue seams.

Experimental and numerical characterization of flexural properties and failure behavior of CFRP/Al laminates

Fiber metal laminates (FMLs) are the assembly of composite materials and metallic sheets in an alternating arrangement. In this paper, mechanical performance and failure mechanisms of Carbon fiber-reinforced epoxy/Aluminum Laminates (CARALL) under three-point bending were investigated via experimental and numerical methods. A series of three-point bending tests are conducted to study the bending behavior of different CARALLs under fiber reinforced plastic composite/metal alternative stacking sequence. Results indicates that a comparatively high stiffness (109.70 GPa) and strength (1673 MPa) of 3/2 (3 aluminum sheets and 2 carbon fiber/epoxy laminates) FMLs are achieved in comparison to other stacking sequence of FMLs. With the increase of metal volume fraction, the bending properties are weakened, suggesting the carbon fiber composite layers play important roles in load bearing. Furthermore, bending properties and damage behavior of two fiber orientations (unidirectional and cross-ply) in 3/2-FMLs are investigated by Linde failure criterion, which is in agreement with the experimental load–deflection curves and microstructural analysis.

Mechanical properties of laterally loaded threaded rods embedded in softwood

At present, the mechanical properties of laterally loaded threaded fasteners with large diameters embedded in timber elements remain unknown. An experimental study of laterally loaded threaded rods with wood screw threads embedded perpendicular to grain in softwood elements (spruce and pine glulam and spruce LVL) is presented in this paper. Embedment tests with the load acting parallel and perpendicular to grain were carried out and the embedment strength and stiffness were quantified. For some test series, the experimental embedment strengths were lower compared to the predictions according to Eurocode 5 in terms of both mean and characteristic values. This finding indicates that the predictions by Eurocode 5 are not always conservative. To investigate the effect of the thread, additional series of embedment tests were carried out with smooth dowels featuring a diameter approximately equal to the core diameter of the threaded rods. Finally, the yielding moment of threaded rods was quantified based on a series of three-point bending tests of threaded rods. The experimentally determined yielding moment was significantly higher than the prediction of Eurocode 5.

Prediction of crack paths in ferroelectrics with anisotropic fracture toughness

Crack paths in ferroelectrics under combined electromechanical loading have been investigated both experimentally and numerically in the past. The influence of electric fields on the deflection behavior being of particular interest, specimens exhibiting a pronounced curvature of crack growth have been predominantly considered. In this regard, a series of three-point bending tests with eccentric notch has recently provided valuable experimental results under various electric loading conditions below the coercivity. Numerical simulations, however, nowadays do not match these crack paths satisfactorily. One important aspect is the distinctive and well-known anisotropy of fracture toughness in ferroelectrics. While isotropic simulations in anisotropic structural materials are demonstrably not suitable for predicting crack paths, different modeling approaches accounting for anisotropic crack resistance associated with global and local directions of polarization are investigated in this paper. Numerical simulations of crack paths are based on a new J-integral related criterion, incorporating the anisotropic elastic, piezoelectric and dielectric properties just as the fracture toughness and are compared to recent experimental findings.

Flexural size effect of ultra-high ductile concrete under different damage and ductility levels

Ultra-high ductile concrete (UHDC) is known for its excellent tensile strain capacity. However, investigations on the size effect of UHDC still remain scarce. This study aims to investigate the size effect on the flexural behavior of UHDC with various levels of specimen size, initial notch and ductility. A series of three-point bending tests were conducted on normal concrete beams and UHDC beams with different fiber volume fractions (1% and 2%) and various sizes of artificial notch (notch/beam depth ratio, 0.10 and 0.23). The results indicate that, with the help of strong fiber bridging, the fracture toughness and processing zone diameter of materials are enhanced by orders of magnitude, subsequently size effect becomes less significant and disappeared when materials change from brittle to ductile. Finally, the variation observed in the fracture processing zone (FPZ) of materials was used to explain the negligible size effect of UHDC.

Developing an innovative curved-pultruded large-scale GFRP arch beam

An arch glass fiber reinforced polymer (GFRP) I-beam with a 600 mm height was developed based on the latest curved-pultrusion technique to overcome the most critical design issues of large-scale GFRP beams, including excessive deflections and premature buckling failures. To achieve this goal, a review was first conducted regarding the complex buckling behaviors of pultruded GFRP beams, building a theoretical basis for the development of the curved beam. Then, a series of three-point bending tests for beams were conducted at full scale, in which the typical failure modes, load-carrying capacities and deflection and strain data were obtained. Compression flange delamination was found to be the dominant failure mode. The load–strain curves for flange and web plates demonstrated that the proposed beams were exempt from local buckling issues. Additionally, an analytical study and finite element modeling were carried out. Excellent agreement between experimental, analytical and numerical studies was observed. The design approach for conventional straight profiles is readily applicable to curved beams as the difference is limited. In the end, a 20-m-long full-scale GFRP pedestrian bridge was designed, constructed and tested. The great potential of the proposed curved-pultruded GFRP arch beam was successfully demonstrated.

Structural performance of 3D-printed composites under various loads and environmental conditions

Sandwich-structured composites are in high demand in various industries, and additive manufacturing has proven its ability to meet this demand. As a result of the advances in three-dimensional (3D) printing techniques, 3D-printed polymers have received considerable attention in fabrication of sandwich structures with complex geometries. This paper is concerned with design, manufacturing, and analysis of the 3D-printed sandwich-structured components which experienced various loadings and environmental conditions. The core structure plays a major role in the in-plane behavior of lattice composites, therefore in this study, sandwich specimens with two types of core topologies made of two common and similar 3D printing filaments, acrylonitrile butadiene styrene (ABS) and acrylonitrile styrene acrylate (ASA), were manufactured. Based on the applications of sandwich-structured parts, they might experience different temperatures in their service life. In order to determine effects of thermal environment, we conducted accelerated thermal aging within temperatures of 22-60 °C, which is below glass temperature of the examined materials. Based on a series of three-point bending tests, the failure behavior of the original and aged components are determined, and the effects of temperature change on the bending behavior of 3D-printed sandwich parts are discussed. The experimental practice revealed that ASA with honeycomb core specimens indicated highest stability under bending load after thermal aging. The current study sheds lights on durability of 3D-printed sandwich structural elements, and the obtained results demonstrate feasibility of 3D printing technology in fabrication of thermal-stable sandwich structures.

Experimental and numerical fracture analysis of the plain and polyvinyl alcohol fiber-reinforced ultra-high-performance concrete structures

In this paper, a new procedure for manufacturing the plain and polyvinyl alcohol (PVA) fiber-reinforced ultra-high-performance concrete (UHPC) is introduced to improve its workability and to reduce its shrinkage behavior. To determine its fracture characteristics, a series of three-point bending tests are performed with the notched beam structures made of the produced UHPCs. In addition, with the rising demand for time and cost saving design methodologies associated with R&D experimentation for many industrial materials, such as UHPC structures, numerical modelling and simulation have become an essential tool. Therefore, a relevant discrete-level numerical modelling approach based on bond-based peridynamics is proposed to predict the fracture behaviors of the UHPC and UHPC-PVA structures. In the proposed method, the structures are discretized by uniform meshless nodes linked by standard peridynamic bonds, and then the bonds are randomly associated with mechanical and fracture parameters of the UHPC and PVA materials according to their corresponding volume fractions. With this approach, the reinforcement of the PVA fibers on the UHPC materials is expressed by the bonds with the parameters of PVA materials, which greatly simplifies the modelling process of randomly distributed PVA fibers reinforced UHPC structures. The comparison between numerical and experimental results validates the effectiveness and accuracy of the present method.

Study on fracture mechanics of granite based on digital speckle correlation method

Crack propagation length (CPL) and stress intensity factor (SIF) are key parameters to solve rock failure issue when using fracture mechanics. In order to study the evolution law of the two factors during the mode Ⅰ fracture evolution, a series of three-point bending tests of granite specimens with central cracks were carried out. During the loading process, digital speckle correlation method (DSCM) was used to monitor the full field deformation. The crack opening displacement is first determined through the full field displacement data. Then, this value is compared with those calculated by ASTM and Tada equations. Additionally, a virtual extensometer is arranged at the crack tip. The crack tip position and CPL are determined by the measurement hybrid method. Results show that the crack length evolution can be divided into three stages: crack initiation, crack tip formation and crack propagation. At the 90% peak load, the crack propagation rate increases rapidly until the rock failure. During this period, the crack growth consists of 87% of the total length. While the increasing rate of SIF presents quick-slow-quick evolution law, and SIF reaches the maximum at the peak load. When the crack forms, CPL and SIF all experience rapid increase at the deformation point and reach the maximum during the instable failure of rock.

Properties of self-compacting concrete containing olive waste ash

Reasonable and standardized use of local and waste resources is of great importance for the economic development in the world. The utilization of waste materials can result in enhanced properties and eco-friendly concrete, at the same time. In this study, it is aimed to evaluate the effect of olive waste ash on the mechanical and fracture properties of self-compacting concrete. For this purpose, self-compacting concrete mixes were prepared by the utilization of olive waste ash, in three different proportions and with two different dosages of superplasticizer. Alongside compressive and tensile strengths analysis, a series of three-point bending tests on notched beams were conducted for all mixes. Moreover, initial defects of all self-compacting mixes were evaluated by using the X-ray computed tomography method. Findings showed that using olive waste ash in self-compacting concrete mainly affects the tensile strength and fracture energy for the two different contents of superplasticizer.

Electric and acoustic activity in notched fiber-reinforced concrete beams under three-point bending

An experimental study is presented, aiming to comparatively consider the electric and acoustic activity in fiber-reinforced concrete beams subjected to three-point bending. The study is part of an ongoing project attempting to detect phenomena considered as warning signals of upcoming catastrophic fracture, in case brittle materials are loaded mechanically at levels approaching the critical ones. A mature, well established monitoring tool (Acoustic Emissions) is used, in conjunction with a relatively new sensing technique, which is based on the detection of very weak electric currents (known as Pressure Stimulated Currents) produced during the various damage processes which precede (and lead to) the macroscopic fracture of brittle materials. In this direction, series of three-point bending tests with notched beam-shaped specimens made of concrete (either reinforced with metallic or plastic fibers or without any reinforcing agents) were implemented. It is concluded that, independently of the nature of the reinforcing fibers, both monitoring techniques employed in the experimental protocols (namely, the Acoustic Emissions and the Pressure Stimulated Currents) follow pretty faithfully the respective time evolution of the mechanical quantities and, what is more important, they provide well distinguishable features that can be considered as pre-failure indicators.

A biomechanical study of the role of sitagliptin on the bone characteristics of diabetic rats

An experimental protocol is described aiming to explore the in­fluence of Type 2 Diabetes Mellitus on the biomechanical response of the bone tissue and, also, to quantify the potential beneficial role of a pharma­ceutical treatment, based on sitagliptin, a diabetes drug that increases the levels of natural substances called incretins. Twenty eight male, 10-week old Wistar rats were used, divided into three groups, i.e., the control one, the group including the diabetic rats and, finally, the group including the diabetic rats which were treated using sitagliptin. The biomechanical study was based on a series of three-point bending tests of the femora of the sacrificed rats and the analysis of the experimental data was implemented in terms of the actual geometry of the fractured cross-section. It was concluded that diabetic bones undertake larger forces despite the fact that the “diameter” of their cross-section was some­­how smaller. On the contrary, the slope of the load-deflection curve (cor­responding to a measure of the stiffness) of diabetic bones is slightly lower compared to the control bones. Finally, it seems that treating diabetic animals with sita­gliptin only partly reverses the effect of Type 2 Diabetes Mellitus on their bone tissue, at least concerning its strength and stiffness.

Study of the interface behaviour between fictile tubules bricks and mortar: numerical and experimental analysis

This paper presents an investigation on the interaction between hollow clay bricks (caroselli) and mortar, aiming at obtaining frictional properties needed for a full description of the composite masonry material derived from these two constituents. A series of three-point bending tests is carried out on prism-shaped specimens representing typical arrangements of caroselli within mortar. Since no building code or guideline exists regulating this kind of tests, the specimens are built respecting the 1÷4 ratio between the transversal dimension and the length of the prism requested for mortar specimens subjected to three-point bending tests. The frictional properties of the interaction between the two constitutive materials are then derived through a series of back analyses concerning four different hypotheses on the actual degree of transmission of tangential stresses between caroselli and mortar. The numerical analyses are performed with the commercial finite element software ABAQUS. The obtained results sustain the experimental findings.

Evaluation of carbon fiber reinforced cementitious matrix as a recyclable strengthening material

The use of fiber-reinforced polymer (FRP)-epoxy resin is a common retrofitting technique usually employed by engineers. In the research presented in this paper, a new cement-based material was developed introducing chopped carbon fiber. The carbon-fiber-reinforced cementitious matrix (C-FRCM) containing a cement-based matrix and embedded carbon-fiber mesh is an alternative solution to FRP-epoxy resin, addressing cost, durability, and reversibility issues. The flexural performance of carbon-fiber-reinforced cementitious matrix plates with chopped carbon fibers was explored in this study. A series of three-point bending tests were conducted to investigate the effects of chopped carbon fiber content. The measured mechanical properties of the proposed C-FRCM mix were compared with those made of commercial formulations. The flexural strengths and failure modes of the carbon-fiber-reinforced cementitious matrix plates were obtained and recorded during tests, and then the interlaminar shear strengths were calculated. Upon the completion of three-point bending tests, scanning electron microscopy was used to observe the microscopic appearance of tested specimens. The optimum content of chopped carbon fibers in the cement-based materials was determined based on the structural behavior and microstructural analysis. The results also show that the carbon fiber mesh is effective in strengthening the carbon-fiber-reinforced cementitious matrix.