Three-point Flexural Loading Research Articles

Effect of impact and flexural loading on hybrid composite made of kevlar and natural fibers

In this work, four varieties of hybrid Fiber-Reinforced Polymer (FRP) panels made of kevlar-29 and natural fibers are studied. All panels have kevlar-29 face sheets and natural fiber core, such as jute, flax, sisal, and hemp. This research focuses on the behavior of these hybrid FRP panels under flexural and impact loading so that the panels can be explored for the structural/semi-structural members of army shelters, portable helipad, and roofing panels in high-altitude areas. Natural fibers are chemically treated with NaOH to improve hydrophobicity. The panels are vacuum bagged, the fiber volume fraction is 0.39, and the thickness is close to 4 mm. Three-point flexural loading using the universal testing machine and low-velocity impact loading up to 24 J under drop weight impact test setup is carried out to characterize the panels. Damage area, delamination, permanent deformation, indentation depth, energy absorbed, flexural strength, and modulus are measured. The hybrid flax/kevlar panel and hemp/kevlar panel, each resist impact with permanent deformation less than 0.5 mm up to 24 J. Without significant face sheet or core fiber breakage, the delamination is spread over a small radial distance of 18.5 and 24.5 mm, respectively. Interface matrix breakage causes delamination. The load vs deflection curve is almost linear under flexural loading, and specimens failed under compression at 240 MPa. The numerical simulation is also done using ANSYS and LS-DYNA for detailed study.

Study of the Determination of Three-Point Flexural Strength Loading in Resin and Reinforcing Ornamental Stones

Theoretical framework: Brazil is one of the world's largest producers of ornamental stones, and is therefore the richest country in granite diversity in the world. Improvements in processing and production techniques have been developed over the years to add value to the final product. The resistance of stones is a preponderant factor for their application in certain environments. Some stone industries try to improve the resistance of the material by resining or reinforcing (resin plus fiberglass mesh) on their material. Method/project/approach: the work studied the behavior of resined and reinforced stones (resin plus fiberglass mesh). three-point flexural strength tests were carried out on pegmatite stones and quartzites. They were divided into groups with just resin and others with resin plus fiberglass mesh. Results and conclusion: The results showed that in the group of Pegmatite stones it was possible to obtain a gain of up to 44% in material resistance when resined and reinforced. However, the Quartzites did not have a significant gain, in some cases they obtained a lower value when reinforced. Only one of the quartzites achieved a 14% gain in resistance. Therefore, it is necessary to carry out a preliminary study with laboratory tests before resining and reinforcing the ornamental stones for sold. Research implications: Compare the three-point flexural strength between samples of pegmatic and quartzite stones and evaluate the resistance of reinforced (resin plus fiberglass mesh) and resin (resin) stones. Originality/value: analyze the influence of resin and fiberglass mesh on the three-point flexural strength of ornamental stones.

An investigation on flexural properties of anti-tetrachiral auxetic structures fabricated by material extrusion

The present paper describes an investigation on flexural properties of anti-tetrachiral auxetic structures fabricated by material extrusion technique of additive manufacturing. Flexural performance of these structures is crucial in automotive crash box, robust shock absorbers, air seat cushions, sound absorbers, air filters, airfoil morphing, biomedical stent, non-pneumatic tyres, vibration attenuation, and impact detection sensors. Mechanical properties of auxetic structure depend on geometric parameters of its unit cell. Therefore, in the present study, influence of two geometric parameters namely cylinder radius and ligament thickness on modulus, strength and specific energy absorption (SEA) of in-plane and out-of-plane oriented structures under flexural loading has been investigated. Experimental plan is prepared using central composite design method of response surface methodology. Specimens of acrylonitrile butadiene styrene (ABS) material are fabricated using material extrusion technique. Each specimen is tested under three-point flexural loading as per ASTM D790. Experimental results are analysed using analysis of variance (ANOVA). It is found that both geometric parameters influence modulus, strength and SEA of in-plane and out-of-plane oriented structure significantly. For both orientations of structure, modulus and strength increase with decrease in cylinder radius and increase in ligament thickness, while SEA increases with increase in cylinder radius and decrease in ligament thickness. Further, optimization of geometric parameters is performed using grey relational analysis to maximize the responses. Regression models are also developed to predict the responses. The findings of present study are useful in maximizing flexural properties of anti-tetrachiral auxetic structures through selection of optimum geometric parameters of unit cell.

Degradation mechanism for mechanical property of carbon fiber reinforced epoxy composite under simulated seawater absorption coupled with flexural load

Property evolution of carbon fiber reinforced epoxy composite used in marine environment was investigated in this paper. The moisture absorption and flexural properties of carbon fiber composite laminate were studied under the combined pretreatment condition of simulated seawater and three-point flexural load. An in-situ observation system was used during mechanical test to analyze damage initialization and development on dry and wet composites. Theoretical analysis considering hydro-mechanical coupling was conducted, based on experimental data, moisture simulation and stress analysis. Stress distribution under flexural load was discussed using finite element method, and its effects on moisture diffusion and failure process were analyzed based on the theoretical and experimental results. It demonstrates that moisture absorption of composites is inhibited as flexural load is applied during moisture absorption. It is attributed to compressive stress concentration of composite on the loading region. The retention of flexural property shows slight increase under moisture coupled with flexural load, which is resulted from decreased moisture content. Failure process of composite during three-point flexural test after immersion is different from that of dry composite, especially for the position and number of crack formation and propagation. Weakened interlaminar interface and more large cracks are found after immersion coupled with flexural load, which are explained based on water degradation and initial damage at stress concentration region. It denotes that more attentions should be paid on failure process of wet composite structure in marine field.

Probabilistic flexural fatigue strength of ultra-lightweight cement concrete and high strength lightweight aggregate concrete

Recently, more attention has been paid to the development of lightweight aggregate concrete due to its many advantages, such as low density, good thermal insulation, and fire resistance. The fatigue performance of ultra-lightweight cement composite (ULCC) and lightweight aggregate concrete (LWAC) with comparable compressive and flexural strength was investigated and compared in this paper. The data of three-point flexural fatigue loading experiments were collected from the literature for 108 specimens sized as 100 mm × 76 mm × 406 mm. The least-squares regression (LR) and the orthogonal regression (OR) methods were adopted and compared to fit the experimental coefficients of the Wöhler equations. Two- and three-parameter Weibull distribution models were used and compared when evaluating the probabilistic fatigue life of ULCC and LWAC at given stress levels. Analysis results show that the coefficients of the Wohler equations fitted by LR and OR methods in this study are similar. For stress levels in the middle ranges, namely, approximate 0.65–0.8, the differences of the S-N curves by two and three-parameter models are slight. When stress levels are larger than 0.8 or smaller than 0.65, significant differences exist between curves fitted by the two models. The nonlinear curves obtained from the three-parameter model are deemed more consistent with the actual situation than the linear curves by the two-parameter model.

The behavior of steel-concrete-steel sandwich beams with different depths effect

This research studies the effect of beam depth on the behavior of steel-concrete-steel (SCS) sandwich beams. Four SCS sandwich beams measuring 100 mm wide and 1200 mm length were cast and tested up to failure under three-point flexural loading. The depths in the tested beams ranged from 150 to 300 mm in regular increments of 50 mm. All beams had top and bottom steel skin plates with thickness 4 mm. The shear connector configuration of the Truss using 10 mm diameter steel bars was used to directly connect the two external steel skin plates. The test results confirmed that increasing the depth of the SCS beams from 150 to 300 mm led to an increase in the first cracking load and the ultimate load by 33.5 to 66.5% and 59.2 to 64.2%, respectively. In addition to improving the mechanical properties, when the SCS beams depth was raised to 50%, the service stiffness and secant stiffness both improved by 226.9 and 190.4%, respectively. Moreover, the flexural toughness increased by 101.1% with the SCS sandwich beams depth rising by 25%.

Optimum Design of Flexural Strength and Stiffness for Reinforced Concrete Beams Using Machine Learning

In this paper, an optimization approach was presented for the flexural strength and stiffness design of reinforced concrete beams. Surrogate modeling based on machine learning was applied to predict the responses of the structural system in three-point flexure tests. Three design input variables, the area of steel bars in the compression zone, the area of steel bars in the tension zone, and the area of steel bars in the shear zone, were adopted for the dataset and arranged by the Box-Behnken design method. The dataset was composed of thirteen specimens of reinforced concrete beams. The specimens were tested under three-point flexure loading at the age of 28 days and both the failure load and the maximum deflection values were recorded. Compression and tension tests were conducted to obtain the concrete data for the analysis and numerical modeling. Afterward, finite element modeling was performed for all the specimens using the ATENA program to verify the experimental tests. Subsequently, the surrogate models for the flexural strength and the stiffness were constructed. Finally, optimization was conducted supporting on the factorial method for the predicted responses. The adopted approach proved to be an excellent tool to optimize the design of reinforced concrete beams for flexure and stiffness. In addition, experimental and numerical results were in very good agreement in terms of both the failure type and the cracking pattern.

High-speed synchrotron X-ray phase-contrast imaging for evaluating microscale damage mechanisms and tracking cracking behaviors inside cross-ply GFRCs

We integrated the high-speed synchrotron X-ray phase-contrast imaging (PCI) with a modified Kolsky compression bar loading platform to visualize the dynamic failure processes of cross-ply glass fiber reinforced composites (GFRCs). Four S-2 glass/SC-15 composite specimens were prepared, having similar thicknesses but different stacking sequences, namely as [012/9012], [9012/012], [08/908/08], and [908/08/908]. Three-dimensional synchrotron X-ray computed tomography and scanning electron microscopy (SEM) were employed to examine the microstructures and quantify the fiber volume fractions. Each specimen was notched and subjected to a dynamic three-point flexural loading. The onset of cracking close to the notch tip, crack propagation in 0° or 90° plies and their interface, crack opening, and ultimately failure of the specimen were captured by high-speed synchrotron X-ray PCI. Additional dynamic experiments were performed to determine the average time when the stress wave propagated through the specimen and correlate the X-ray images with the specimen's force-deflection response. Finally, the surface morphology of each specimen after the dynamic loading was imaged by SEM. Comparison between real-time X-ray images and post-fracture SEM images demonstrated the capability of the X-ray method to record damage evolution inside composites. Furthermore, the high spatial and temporal resolutions of the X-ray setup and edge enhancement by PCI enabled the identification of microscale damage features within 1 μs. Two damaging processes were identified, crack growth was quantified, and fracture toughness of the composites was evaluated. The method is deemed useful to reveal microscale damage mechanisms and track cracking behaviors inside cross-ply GFRCs under dynamic loading in real time.

Static and sustained loading behavior of a basalt FRP shell–concrete composite bridge deck: An experimental and numerical study

The fiber reinforced polymer (FRP)-concrete composite bridge deck is expected to be a competitive alternative to the reinforced concrete bridge deck owing to its great durability, low weight, and high loading efficacy. Based on previous studies, an optimized basalt FRP shell (BFRP)–concrete composite bridge deck was investigated experimentally and numerically in this study. Through the construction simulation test, the optimized BFRP shell was proved to be a qualified formwork with a 30.9% lower midspan deflection than the BFRP shell of previous studies and was lower than 1/800 of the net span. In the static test, the load capacities of deck specimens with depths of 200 mm and 180 mm reached 470 kN and 340 kN, respectively, with residual deflections lower than 7 mm. Meanwhile, the deck showed decent collaborative behavior, without visible interfacial debonding or slippage during the static test. Compared with the previously studied bridge deck, the bridge deck of the current study can achieve a higher short-term cost performance. After 175 days of three-point flexural sustained loading, the additional midspan deflections were 0.3 mm, 0.37 mm, and 0.46 mm, for load levels of 0.2, 0.25, and 0.3, respectively, which were less than 7% of their instantaneous elastic deflections. The finite element (FE) model was established using the Bailey-Norton model for the creep of the FRP and Bazant’s B-3 model for the creep of the concrete. The FE results were in good agreement with the experimental results, both in short- and long-term scenarios. Using numerical simulation with a load duration of 50 years, the allowable load level of this bridge deck reached 0.4, which meets the criteria for creep deflection in the ACI standard.

Structural Behavior of Fibrous-Ferrocement Panel Subjected to Flexural and Impact Loads.

Ferrocement panels, while offering various benefits, do not cover instances of low and moderated velocity impact. To address this problem and to enhance the impact strength against low-velocity impact, a fibrous ferrocement panel is proposed and investigated. This study aims to assess the flexural and low-velocity impact response of simply supported ferrocement panels reinforced with expanded wire mesh (EWM) and steel fibers. The experimental program covered 12 different ferrocement panel prototypes and was tested against a three-point flexural load and falling mass impact test. The ferrocement panel system comprises mortar reinforced with 1% and 2% dosage of steel fibers and an EWM arranged in 1, 2, and 3 layers. For mortar preparation, a water-cement (w/c) ratio of 0.4 was maintained and all panels were cured in water for 28 days. The primary endpoints of the investigation are first crack and ultimate load capacity, deflection corresponding to first crack and ultimate load, ductility index, flexural strength, crack width at ultimate load, a number of impacts needed to induce crack commencement and failure, ductility ratio, and failure mode. The finding revealed that the three-layers of EWM inclusion and steel fibers resulted in an additional impact resistance improvement at cracking and failure stages of ferrocement panels. With superior ultimate load capacity, flexural strength, crack resistance, impact resistance, and ductile response, as witnessed in the experiment program, ferrocement panel can be a positive choice for many construction applications subjected to repeated low-velocity impacts.

A Laboratory Study on the Flexural and Shear Behavior of RC Beams Retrofitted with Steel Fiber-Reinforced Self-compacting Concrete Jacket

In this research, the behavior of RC beams retrofitted with steel fiber-reinforced self-compacting concrete jacket (SFRSCC-J) is investigated. For this purpose, by constructing 25 different beams, flexural load-carrying capacity of beams retrofitted with different concrete jackets is examined. The variables studied are the type of concrete (conventional and self-compacting), the strength grade of concrete (C40, C50, and C60), and the presence or absence of reinforcement bar and steel fiber in the jacket. The beams were subjected to three-point flexural loading, and the response was evaluated in terms of load–displacement diagrams. The results show that by adding SFRSCC-J to the perimeter of the beams, their bearing capacity can be increased up to about 7.4 times to reach the yield load, and about 5.1 times to reach the ultimate load. Moreover, the combination of steel fibers (SFs) and steel rebar reduced the cracks more, and at the final moment, the cracks extended to the neutral axis of the cross-section. In the retrofitted beams with the combination of rebar and SFs, the bonding between the reinforcement and the concrete increased and the reinforcement bars slipped less into the specimen. Thus, with a better distribution of stress, besides the concentration of the cracks in the middle of the span, the diagonal cracks decreased, and as the load increased, crack lengths increased less.

Corrosion and Chloride Diffusivity of Reinforced Concrete Cracked under Sustained Flexure

This research discusses the chloride diffusivity of concrete as well as corrosion performance of rebars in cracked and uncracked states. Prismatic concrete specimens with two water-to-cement ratios, two concrete cover thicknesses with and without steel fibers were used. Three-point flexural loading was applied to form cracks and cracks were sustained by a bolt system. Half-cell potential and corrosion rate measurements were carried out following wetting - drying cycles in chloride environment which were continued for 80 weeks. The positive effects of lower water-to-cement ratio and greater cover depth were found to be surpassed by existence of cracks in concrete.

Linear energy storage and dissipation laws during rock fracture under three-point flexural loading

To investigate the energy storage and dissipation characteristics during rock flexure fracturing, a series of single cyclic loading–unloading flexural fracture tests was conducted on rectangular rock beams using the three-point loading technique. The results show that under different experimental unloading levels, the elastic and dissipation energies increase linearly as the input energy increases. On this basis, the linear energy storage and dissipation laws were obtained, which were immune to the experimental unloading level. The flexural energy storage coefficient and flexural energy dissipation coefficient were subsequently introduced to characterize the potentials of the rock for storing and dissipating energy, respectively. A positive or negative relationship between the two coefficients and the rock mechanics properties (the brittleness index and average flexural strength) was observed. The influence of pre-existing rock flaws on the linear energy storage and dissipation laws was discussed. In addition, highly quadratic increases in the elastic energy, dissipation energy, and input energy were observed with an increasing experimental unloading level. For the energy characteristics at rock failure (the failure point of the rock specimen), the peak input energy, peak elastic energy, and peak dissipation energy varied considerably under different experimental unloading levels, while the associated peak elastic dissipation index (the ratio of peak elastic energy to peak dissipation energy) remained constant. Similar to the flexural energy storage coefficient, the mean value of peak elastic dissipation index is also positively related to the rock mechanics properties. The linear evolution law of energy storage and dissipation not only permits a quantitative determination of the energy parameters at rock failure (the peak elastic energy and peak dissipation energy), but also relates the energy properties with the rock mechanics properties.

Analysis of behavior of composite material during loading tests

Composites are widely used as a material suitable for construction applications, but the operating conditions can lead to reduction of materials properties, damage initiation and collapse of the structure, so it is necessary to monitor condition of components and thus prevent its catastrophic failure. The unidirectional glass fiber reinforced polymer matrix composite (GFRP) was inspected with usage of non-destructive (NDT) acoustic emission during the static loading tests. The 0° test specimens were manufactured and tested under tensile and three-point flexural load for which the custom-made apparatus was used. Failure mechanisms were inspected by detailed analysis using scanning electron microscopy (SEM) to deduce the damage sequence. Tensile stress-initiated matrix cracking which induces formation of scarps, ribbons and riverlines followed by delamination and fiber failure with formation of radials. Flexural load showed cracks in matrix, delamination and typical compression regions in form of fiber microbuckling. Based on the speed of wave propagation and linear localization, damage processes were detected in form of lateral cracking, delamination and integrity loss during the tensile and flexural tests. The data obtained from mechanical testing were correlated with selected acoustic emission parameters (counts, events, amplitude and duration). Behavior of GFRP material during the tensile test can be described very well with usage of counts while the flexural test provides much less information. The evaluation of damage processes using event parameter based on the amplitude and duration proved to be problematic due to the noise interference. The Short-time Fourier transform (ST-FFT) and peak frequency was used for identification of failure modes. In case of tensile and flexural load the matrix cracking has lower frequencies while delamination and fiber failure have higher values.

Influence of salt freeze-thaw cycles on the damage and the following electrical and self-sensing performance of carbon nanofibers concrete

This paper aimed to study the damage of carbon nanofibers (CNFs) concrete during salt freeze-thaw cycles. The evolution of weigh, relative dynamic elasticity modulus and electrical resistance of the specimens were determined to evaluate the freeze-thaw damage. Moreover, the self-sensing performance of CNFs concrete under monotonic compressive loading, cyclic compressive loading and three-point flexural loading was studied. Water and 3.0% NaCl solution were selected as freeze-thaw mediums, while the freeze-thaw temperature cycles were conducted at −10 °C ∼ 10 °C. Results indicated that the mass, the relative dynamic elasticity modulus decreased and the resistance increased with the increasing freeze-thaw cycles. The dried specimens exhibited the best piezoresistive performance. Additionally, the sensitivity and linearity of piezoresistivity deteriorated after 300 freeze-thaw cycles (The deterioration of piezoresistivity after 300 salt freeze-thaw cycles was more obvious). The resistance of all specimens (except the specimens after 300 freeze-thaw cycles in NaCl solution) increased during the flexural loading. The water-saturated specimens showed the highest flexural self-sensing sensibility while the drying specimens presented the lowest flexural self-sensing sensibility. However, the resistance of NaCl freeze-thaw specimens decreased firstly before destruction during three-point flexural loading. When the concrete was destroyed, the resistance of all groups increased to a maximum value rapidly. As a result, CNFs concrete can be a kind of self-sensing material.

Stamping of high performance thermoplastic composite intrusion beams

In this study, an experimental investigation was performed to evaluate the potential of thermoplastic composites to be used for the fabrication of an automotive intrusion beam using the composite stamping process. To assess the potential of this technology, mechanical properties of several high performance thermoplastic composite materials were first evaluated on coupons. Due to their higher load and displacement at failure, the most promising materials for the fabrication of a composite intrusion beam were found to be glass fibre/polypropylene (GF/PP) and carbon fibre/polyamide 66 (CF/PA66) laminates. Then, stamped intrusion beams were manufactured from these composites using three different stacking sequences. The beams were later tested under three-point flexural loading. For all components, failure occurred at the attachment points. However, higher properties were obtained for ± 45 laminates for which shear-out failure could be avoided. From the load-displacement curves obtained, it can nevertheless be concluded that stamping of thermoplastic composites is promising for the fabrication of intrusion beams, but that further development will be required to avoid failure at the attachment points.

Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m3. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.

The fractography analysis of IN718 alloy after three-point flexure fatigue test

In this study, the high cycle fatigue (HCF) properties of IN718 superalloy with given chemical composition were investigated at three-point flexure fatigue test at room temperature. INCONEL alloy 718 is nickel-chromium-iron hardenable alloy and due to its unique combination of mechanical properties (high-strength; corrosion-resistant and so on) used for production of heat resistant parts of aero jet engine mostly. Mechanical properties of this alloy are strongly dependent on microstructure and on presence of structural features such are principal strengthening phase gamma double prime, gamma prime and due to its morphology less desired delta phases. The mentioned phases precipitate at various temperature ranges and Nb content as well. The three-point flexure fatigue test was performed on ZWICK/ROELL Amsler 150 HFP 5100 test equipment with approximate loading frequency f=150 Hz. The S – N (Stress – Number of cycles) curve was obtained after testing. With the help of scanning electron microscope (SEM), fractography analyses were performed to disclose the fracture features of specimens in different life ranges. The brief comparison of three-point flexure and push-pull fatigue loading modes and its influence on fatigue life is discussed as well.

The SEM Investigation of Inconel 718 Fatigue Process at Various Loading Conditions

Fracture surfaces of specimens broken by cyclic loading provide valuable information about individual stages of fatigue process. Changes in the loading conditions (character of fatigue loading, stress amplitude level, influence of cycle asymmetry ratio R, testing temperature, environment of test, etc.) and in the structure of tested material as well cause changes in the fatigue process, whose have effect on fracture surface resulting in change of fatigue properties. In this paper, authors describe the changes of fatigue process of nickel base superalloy Inconel 718 as a result of loading conditions change using SEM (Scanning Electron Microscopy) microfractography analysis of fractured surfaces. The various fatigue loading were, at the first, regular push-pull loading with asymmetry ratio R = -1 and frequency f = approx. 20 000 kHz (High Frequency and High Cycles fatigue Loading - HFL) and the second was three-point flexure loading with asymmetry ratio R = 0.116 / 0.507 and frequency f = approx. 150 Hz (Low Frequency and High Cycles fatigue Loading - LFL). All fatigue tests were done at room temperature.

Carbon nanotube/cement composites for crack monitoring of concrete structures

The feasibility of using a carbon nanotube/cement composite as a crack sensor for concrete structures is evaluated. First, the relationship between the crack width and electrical conductivity of the composite material was investigated. Then, this analysis was repeated for the composite embedded in reinforced mortar beams, using three-point flexural loading. A conductivity model for the cracked composite was introduced and compared with experimental results. In addition, to evaluate whether monitoring was feasible under the conditions of crack closure by external loading, the cracked mortar beam was compressed along the vertical direction with respect to the crack and the conductivity of the composites embedded in the beam was measured. The conductivity of the composite decreased with increasing crack width and its reliability could be improved by increasing its original conductivity.