Four-point Bending Tests Research Articles

Shear behavior of FRP-UHPC composite beams enhanced by FRP shear key: Experimental study and theoretical analysis

To investigate the shear behavior of FRP (fiber reinforced polymers)-UHPC (ultra-high performance concrete) composite beams, four-point bending tests were conducted on seven FRP-UHPC specimens and two FRP-NSC (normal strength concrete) specimens, having different width and depth of concrete flange as well as FRP shear key (FSK) spacing. The slip between FRP profiles and concrete flange was controlled by employing FSK and epoxy resin bonded hybrid connection. The failure pattern, load-deflection/strain curves, and sliding response of composite beams were analyzed to study the influence of concrete type, FSK spacing, width and thickness of concrete slab. The results indicate that FRP-UHPC composite beams exhibited shear failure, while FRP-NSC composite beams experienced bending-shear failure. The composite beams demonstrated shear-lag effect, which became more pronounced with the increasing of the concrete slab width. The use of UHPC, reducing FSK spacing, and increasing the size of cross-section of concrete flange can effectively enhance the shear performance and reduce interface sliding. Formulae were developed to predict the shear capacity and deflection, considering shear deformation. The results predicted by the formulae developed match well with the experimental results.

Research on the relationship between comprehensive characteristics of cracks and stiffness of RC beams

The relationship between crack comprehensive characteristics and the stiffness of RC beams is investigated through experimental tests and numerical simulations. A synthetic parameter, the surface damage ratio (SDR), is proposed to represent the comprehensive characteristics of cracks, including crack width, length and location. Crack propagation in RC beams is simulated using a dual-damage parameter plastic damage model. A method for calculating the width and length of cracks is developed based on integral point strains and element equivalent length. The accuracy of this method is verified by comparing it with the results of four-point bending tests on RC T-beams and rectangular beams. Correlation analysis indicates that the cracks in different zones have varying impacts on beam stiffness, with crack length in the bending area and crack width in the shear area having a greater impact than others. To consider the comprehensive characteristics of cracks and their impact on beam stiffness, the SDR, the ratio between the entire crack area and the intact area of the beam surface, is proposed. The proposed parameter is shown to increase linearly with load, resulting in beam stiffness degradation following a power law. The accuracy and applicability of the proposed SDR and its correlation with load and stiffness are verified through load tests on rectangular beams with different reinforcement ratios using Cervenka’s test. Parameter analysis has shown that the section modulus, concrete strength, reinforcement arrangement, and ratios significantly influence how beam stiffness varies with SDR. The presented simulation model for concrete crack evolution, combined with the synthetic parameter of crack characteristics, and the correlation analysis, provide a valuable approach for the safety evaluation of concrete beams during their service life.

Ex Vivo Biomechanical Comparison of Four Techniques to Tibiotarsus Osteosynthesis in Adult Laying Hens (Gallus gallus domesticus).

To assess the biomechanical parameters of intact tibiotarsi (INT) and tibiotarsi with a 5-mm segmental diaphyseal defect repaired using four osteosynthesis techniques: a locking plate (LP), a plate-rod combination, an external skeletal fixator (one end-threaded positive-profile pin per fragment) with an intramedullary pin tie-in (TIF 1), and an external skeletal fixator (two end-threaded positive-profile pins per fragment) with an intramedullary pin tie-in (TIF 2). Sixty tibiotarsi from 30 adult laying hens were allocated into five groups for nondestructive dynamic torsion and four-point bending tests, followed by failure tests. Nondestructive dynamic tests evaluated stiffness over time in torsion and bending. Torsion destructive tests provided maximum torque and rotation values, whereas the four-point bending tests provided the yield load, maximum bending load, and maximum displacement. The INT group showed higher torsional stiffness and maximum torque but similar bending stiffness, torsional strength, and bending strength in one or more groups. LP and TIF 2 exhibited the highest similarity frequencies among the treatment groups, whereas the TIF 1 group displayed lower stiffness and strength for most of the evaluated parameters. Similar results for LP and TIF-2 groups suggest the biomechanical equivalence of these methods for tibiotarsal osteosynthesis in adult hens.

Fracture mode classification of UHPC based on deep learning and wavelet time–frequency spectrum derived from acoustic emission waveform

Fracture mode categorization is essential in identifying the failure stage, evaluating the damage characteristics, and providing references for structural maintenance. Acoustic emission (AE), as a passive nondestructive technique, presents great potential in damage characterization of cementitious materials and structures. In this study, a convolutional neural network (CNN) and gated recurrent unit (GRU) integrated model for tensile, shear, and mixed cracking mode classification of ultra-high-performance concrete (UHPC) was proposed and verified based on AE waveform features. First, four-point bending and direct shear tests for UHPC specimens were carried out, respectively, to generate AE waveform signals corresponding to different cracking modes (tensile, shear, and mixed ones). Subsequently, the collected one-dimensional AE signals were converted into two-dimensional time–frequency spectrum by continuous wavelet transform. And an AE dataset was constructed after manual labeling based on the corresponding relationships between cracking modes and wavelet time–frequency spectra. Then, an automatic CNN-GRU automatic classification model was established using wavelet time–frequency spectrum images as input variables, where the CNN extracts spatial image features, and the GRU screens time dependencies and realizes final classification. The proposed CNN-GRU model achieves an accuracy of 96.17% for cracking mode classification of UHPC, which is superior both in computational accuracy and efficiency to lightweight CNN and AlexNet. Classification outcomes for UHPC specimen under four-point bending and direct shear tests revealed the proportion of tensile cracks progressively decreased with the increase of damage, while shear cracks exhibited the opposite trend. The percentage of mixed mode cracks maintained relatively stable throughout the failure stages. The model proposed in this study presents favorable performance to reveal the underlying damage evolution mechanism during UHPC fracture, which can also provide references for the cracking mode classification of other cementitious materials and structures.

Short cross-laminated timber plates subjected to out-of-plane loads

The use of cross-laminated timber (CLT) panels has increased significantly, driven by the demand for more sustainable structural solutions. When used in short spans, the load capacity of CLT panels can be significantly enhanced; however, their behaviour and failure mechanisms differ from those of large spans. This study evaluates the mechanical behaviour of short-span CLT panels made from Pinus taeda L. under out-of-plane loads, through experimental analyses on full-scale panels. Fourteen industrially manufactured panels were tested in four-point bending tests, using adhesive weights of 250 and 220 g/m². The values of maximum bending moment, shear force, and their respective resistances were analysed based on the observed failure modes. The transverse modulus of elasticity was determined using strain gauges. It was found that short panels, with a span-to-depth ratio of 9 ≤ l/h ≤ 12, exhibited bending moment and shear force failures, with no significant differences between adhesive weights. Discrepancies in shear resistance and stiffness values compared to existing standards suggest the need for reduced-specimen testing for greater accuracy.

Experimental and analytical assessment of the bending behaviour of floating inflated membrane beams

The study aims to study the bending and failure behaviour of the inflated membrane beams under floating conditions to support their application in floating platforms. Bending tests are conducted to assess the structural stiffness and ultimate bearing capacity of the beam with various diameters and inflated pressures. With the elastic modulus determined through four-point bending tests, a new analytical formula based on Winkler elastic foundation theory is then developed to predict the deflection of the beams. The results revealed that the beams initially undergo total submersion, followed by end turn-up and anti-arching deformation in the loaded section. The bearing capacity of the beams increases with the increase in internal pressure and diameter. Localised wrinkling is a typical failure mode at low internal pressures, while at high pressures, the failure mode shifts to boundary failure, characterised by submerging in water. The experimental and analytical results show quite good agreement, confirming the accuracy of the current analysis. Overall, this paper contributes to understanding the bending and failure behaviour of inflated membrane beams under floating conditions and supports the offshore structure safety design.

Mechanical properties of beech wood treated with malic acid-based polyester

Abstract While chemical modification enhances wood’s resistance to deterioration and dimensional stability, it often results in alterations to the mechanical properties, limiting its engineering applications. This study focuses on the in situ esterification of beech wood using malic acid/polyol mixtures and evaluates its impact on mechanical properties. The results of the compression tests yielded limited information, characterized by a notable degree of variability as indicated by the high standard deviation. The four-point bending tests conducted here revealed an increase in the modulus of elasticity (MOE). However, this improvement in MOE was accompanied by a decrease in the modulus of rupture (MOR), indicating a trade-off between stiffness and strength. To better understand the mechanisms affecting the treated wood’s mechanical properties, we compared the experimental and theoretical glass transition (Tg) of the polymers with material stiffness. X-ray computed tomography revealed that treatment increases specimen density and creates a gradient, with higher density near the surface, potentially contributing to increased stiffness. These findings suggest a nuanced impact of the in situ esterification process using malic acid/polyol mixtures on the mechanical properties of beech wood.

Flexural behaviour of stone slabs with prefabricated prestressed CFRP-reinforced stone strips

A strengthening technique incorporating prefabricated, prestressed, carbon-fibre-reinforced polymer (CFRP)-reinforced stone strips for stone slabs was proposed. To investigate the flexural behaviour of strengthened stone slabs, an experimental campaign consisting of 11 stone-slab specimens was conducted by considering the reinforcement ratio, the prestress level, the length of stone strip and the strengthening method as the main parameters. By installing strain gauges on the CFRP bars and stone slabs, the stress-transfer efficiency between different components of the strengthened stone slab during prestressing and strengthening processes were monitored. The results showed that the temporary device could effectively maintain the prestress imposed in the CFRP bars and transfer it to the stone slabs to be strengthened. Further four-point bending tests indicated that the prefabricated stone strips shifted the failure mode of bare stone slabs from brittle fracture to ductile, with obvious deflection and multiple flexural cracks. The load plotted against deflection responses of the strengthened slabs showed three stages, and a significant increase in the load-carrying and deformation capacities was observed when compared to those of the bare stone slabs. Finally, simplified theoretical models for predicting the flexural strength and initial stiffness of the strengthened stone slabs were proposed and assessed.

Life Cycle Assessment and Experimental Mechanical Investigation of Test Samples for High-Performance Racing Boats

This paper investigates the environmental impact and mechanical performance of two composite sandwich structures, named Series 1 and Series 2, used in high-performance racing boats. Mechanical tests, including four-point bending and drop impact tests, were performed. It was found on a general basis that Series 2 has higher load-bearing capacity and limited deflection. Series 1, which has a higher density, was able to absorb more impact energy but was more susceptible to damage. A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impact associated with the materials, considering also the testing phase, which plays an important role in the life cycle of materials and structures for advanced marine applications. In addition, two performance indexes were introduced to correlate the mechanical and environmental properties of the analyzed materials. This study emphasizes the importance of considering the testing phase in LCA, as the energy-intensive nature of mechanical testing contributes significantly to the overall environmental impact. The introduced indexes allow for a more comprehensive understanding of the balance between mechanical performance and environmental sustainability. The findings suggest a trade-off between mechanical performance and sustainability, calling for further research into recyclable composites and greener manufacturing processes to balance these competing priorities.

Biaxial bending strength of thin silicon dies in the ring-on-ring test by considering geometric nonlinearity and material anisotropy

The ring-on-ring test (RoR) is a standard biaxial bending test specified in ASTM C1499-19 and ISO 17167. This test has been utilized for characterizing the biaxial bending strength of silicon dies or wafers to eliminate the die edge chipping effect in the four-point bending test. However, judging from the literature, when testing thin silicon dies, the test is subject to geometric nonlinear effects. This study aims to investigate this nonlinear mechanics in the RoR test using experimental, theoretical, and numerical methods while considering silicon material anisotropy. A 2D-isotropy model of a nonlinear finite element method (NFEM) simulation with specimen elastic modulus of 130 GPa is utilized and verified by experiments and a 3D-anisotropy model in terms of deformation (or displacement) and stresses. Based on the 2D-isotropy NFEM solutions, the fitting equations of correction factors to the theoretical solution are proposed and implemented on determining the biaxial bending strength of 10 mm × 10 mm silicon dies ranging from 57 μm to 297 μm in thickness. It is found that those proposed fitting equations are independent on the test specimen thickness, radius, and materials but not on the radii of the loading and supporting rings. It has also been successfully demonstrated that the RoR test using the theory associated with the correction factor equations can be easy to use to determine the biaxial bending strength of the thin silicon dies that frequently failed in the nonlinear range.

Enhancement of Mechanical Properties of Geopolymer Ferrocement through Incorporation of a Novel Binder and through Optimization of Alkaline Activator Ratios

Geopolymer ferrocement is geopolymer mortar, reinforced with wire mesh. The ratios of binder composition and alkaline activator, with which the geopolymer has been prepared, have profound influence on the mechanical performance of geopolymer ferrocement. In this study the influence of a binder material Alccofine and variation in ratios of alkaline activators have been studied for their effect on mechanical properties of geopolymer ferrocement. Alccofine was replaced with varying proportions of Ground Granulated Blast-Furnace Slag (GGBS). The molarities of NaOH and NaOH/Na₂SiO₃ ratios were also variated to find their influence on strength and crack control of ferrocement, when combined with single, double, and triple-layers of ferro mesh configurations. Tests such as direct shear test, three-point bending, and four-point flexural tests were conducted to to measure shear strength, flexural strength and to estimate region of pure bending. It was observed that the mix with 20% Alccofine and 80% GGBS, an A/B ratio of 0.5, and a NaOH/Na₂SiO₃ ratio of 1.5 yielded a compressive strength of 50 MPa at 90 days of geopolymer ferrocement. Also the triple-layer mesh configuration exhibited the best results, achieving 10 N/mm² in shear strength and 12 N/mm² in flexural strength, compared with other configurations. These findings indicate that by incorporation of Alccofine, optimizing binder composition, activator ratios and reinforcement, the strength parameters of geopolymer ferrocement can be significantly improved.

Numerical Investigation of Strain Variations along Steel Pipelines in Four-Point Bending Tests

Numerical Investigation of Strain Variations along Steel Pipelines in Four-Point Bending Tests

Experimental research on the flexural properties and pore structure characteristics of engineered geopolymer composites prepared by calcined natural clay

The synthesis of high ductility geopolymer composites using excavated natural clay is an economical and environmentally friendly method to realize the utilization of excavated natural clay. This paper presents an experimental research on the flexural properties and pore structure characteristics of engineered geopolymer composites (EGC) prepared by calcined natural clay. The EGC with different slag and fiber contents were prepared and the four-point bending tests were conducted on the EGC. The effect of slag and fiber content on the flexural strength, flexural deflection and flexural toughness were discussed. The microscopic pore structure characteristics of EGC were further analyzed. The optimal mixture ratio of calcined clay-based EGC with the highest flexural toughness was proposed. The results show that fiber can significantly enhance the flexural properties and cracking control ability of EGC. The ultimate flexural strength, flexural deflection, flexural strengthening index and flexural toughness of EGC all increase with increasing fiber content. The results also show the addition of 10.0% slag can increase the flexural strength and flexural toughness, while excessive slag content reduces the flexural properties. The EGC with 10.0% slag content and 2.5% fiber content exhibits excellent flexural properties and cracking control ability. The ultimate flexural strength and flexural toughness index reach 9.75 MPa and 486.11, showing the highest flexural toughness.

Crack growth stabilization in the eccentric three-point end-notched flexure test of solid wood using side-grooved samples

A three-point eccentric end-notched flexure (3EENF) test was conducted on Sitka spruce to characterize the Mode II resistance curve (R-curve). To extend the range for stabilized crack growth, grooves were cut along both side surfaces of the sample to reduce the region around the neutral axis. The R-curve was determined by varying the initial crack lengths, and fracture toughness values were measured at both the onset and during growth. Additionally, the effect of side grooves on stabilizing crack growth in the 3EENF test was investigated by comparing numerical simulation results with those from four-point end-notched flexure (4ENF) tests. The grooves effectively stabilized crack growth, and the stabilization range was equivalent to that of the 4ENF test when the initial crack length was appropriately determined. The grooved sample exhibited behavior indicative of a longer crack than the actual length.

Full-field analysis of semi-transparent cenosphere-filled composites using backlight illumination

This study introduces a novel approach for measuring deformations in semi-transparent composites using digital image correlation (DIC). Unlike traditional approaches relying on artificial speckle patterns, this method uses backlighted cenosphere particles dispersed in a semi-transparent matrix as intrinsic speckle patterns, eliminating the need for surface painting. The effectiveness of the intrinsic pattern is evaluated by comparing global parameters, such as speckle size, distribution, coverage, Shannon entropy and Mean Intensity Gradient, with those of a conventional pattern. Results indicate a relationship between speckle characteristics and measurement accuracy. A factorial design is employed to optimize DIC analysis parameters, including facet size, grid spacing, and filtering, resulting in a significant reduction in noise. Rigid displacement and four-point bending tests confirm the pattern’s traceability, with results compared against a numerical model. This approach lays the groundwork for future investigations of transparent and semi-transparent materials using DIC, allowing comprehensive mechanical characterization without altering material transparency.

Explanation of edge defect influence on sapphire bending strength scatter using the coupled criterion

Sapphire fracture is studied by means of four-point bending tests on mirror-polished millimetric specimens having their cristallographic a→-axis oriented along the specimen length. The scattering of bending strengths between 500MPa and 750MPa is mainly due to edge defects of some tens of microns resulting from the specimen cutting process. Crack initiation occurs from an edge defect along a a-plane, perpendicular to the direction of maximum tensile stress, and further deviates along weaker m-planes. Numerical simulations of edge defect-induced crack initiation based on the coupled criterion reveal that the material sensitivity to edge defect-induced crack initiation mainly depends on Irwin’s length. For Irwin’s lengths larger than twice the defect depth, the bending strength is the same as the one obtained without defect. By retrieving the bending strength variation as a function of the defect depth measured experimentally, the proposed approach enables the identification of sapphire a-plane 850±90MPa tensile strength for a Gc=15Jm−2 critical energy release rate.

Mechanical behaviour of rigid-flexible combined structures: Aluminium-inflated membrane beams for application in floating photovoltaic platform

This study aims to investigate the bending and failure behaviour of aluminium-inflated membrane beams for their applications in floating photovoltaic platforms. Four-point bending tests are conducted for a range of inflated pressures and two different deck heights to assess their effect on structural stiffness and ultimate bearing capacity. Meanwhile, the surface-based fluid cavity method is employed to develop the finite element model with the material properties determined by independent coupon-level tests. The bearing capacity of the aluminium-inflated membrane beam is positively correlated with the internal pressure and deck height. The midspan strain distribution is similar to those of the traditional four-point bending beam with the upper part undergoing compression and the lower part experiencing tension, however, the structural behaviour at the failure stage is different. Failure typically occurs due to localised depressions at the loading points on the aluminium deck, ultimately leading to structural failure. The numerical model closely matches the experimental data for the initial inflated and bending configurations, exhibiting a deviation of only 0.10 % to 0.46 % in diameter and 0.32 % to 5.57 % in equivalent bending stiffness. A parametric study shows that the loading properties of the beam are more sensitive to the internal pressure than the deck height and thickness.

Ductility and cracking behavior of UHPC-RC composite beam under bending test based on acoustic emission parameters

Ultrahigh-performance concrete (UHPC) possesses improved mechanical properties due to its high strength, durability, and toughness. Currently, there is a single method for detecting the cracking behavior and evaluating the ductility of the UHPC-reinforced concrete (RC) composite beam. As an important part of structural non-destructive monitoring techniques, the acoustic emission (AE) technique can be used to analyze the damage pattern of the structure by the variation characteristics of its individual parameters. On this basis, four-point bending tests were carried out on the RC beam and UHPC-RC composite beam, and the AE technique was used for ductility analysis and cracking behavior monitoring. The results showed that the UHPC material enhanced the flexural capacity of the UHPC-RC composite beam. The AE ring counts and energy parameters could be used to analyze the cracking process of the UHPC-RC composite beam during flexural damage. The proposed new ductility indices, AE ring counts ductility index (ARD) and AE energy ductility index (AED), differed from the displacement ductility indices within the range of 2 %, providing a new method for calculating ductility indices of concrete structures. During the failure stage, the UHPC-RC composite beam produced more tensile cracks at the early stage of cracking and the stage of damage compared to the RC beam. In addition, the b value of the RC beam had a large range of fluctuations during the failure stage, indicating that the RC beam produced more micro-cracks and macro-cracks and more damage compared to the UHPC-RC composite beam.

Flexural behavior of circular concrete-filled steel tube members reinforced with annular stiffener

This paper presents a study on the flexural performance of a novel concrete-filled steel tube (CFST) member reinforced with an internal annular stiffener. The annular stiffener is composed of curved steel plates, and the circumferential plate serves as the interface connector. Four-point bending tests were conducted on the four new composite beams and two traditional CFST counterparts to investigate the flexural performance of the composite members. The failure modes, deflection distribution, flexural strength, strain distribution, and moment-curvature relationship were analyzed. The test results showed that the new composite members exhibited higher bending capacity and stiffness than their CFST counterparts. Then, a finite element (FE) model was established and validated using the test results to numerically investigate flexural behavior. The available design models for predicting the flexural capacities of the composite beams was compared. Design formulae considering the stiffener contributions were deduced to calculate the flexural strength of the composite beams based on the plastic-stress distribution method. It was demonstrated that the developed FE modeling appropriately simulated the structural behavior of the composite members. Parametric studies indicated that the use of high-strength concrete was inefficient. In addition, the comparison results indicated that the proposed design formulae were feasible for predicting the bending capacity of composite members.

Assessing pavement characteristics: An in-depth evaluation of waste engine oil and Buton rock asphalt composite modified asphalt and its mixture

To investigate the cooperative influence of waste engine oil (WEO) and Buton rock asphalt (BRA) on asphalt characteristics, the WEO/BRA composite modified asphalt and its mixture with varied dosages were prepared. The high-, moderate-, and low-temperature assessments of WEO/BRA composite modified asphalt were conducted through temperature sweep tests, multiple stress creep and recovery (MSCR), and bending beam rheometer (BBR) tests. The softening point difference was used to analyze the storage stability of composite modified asphalt. Based on the Fourier transform infrared spectroscopy (FTIR) and fluorescence microscope (FM) test, the microscopic feature and chemical constitution of WEO/BRA composite modified asphalt were analyzed. Rutting test, low-temperature bending test, immersed Marshall test, freeze-thaw splitting test, and four-point fatigue bending test were carried out to verify the pavement properties of the WEO/BRA composite modified asphalt mixture. The results indicated that WEO improved the capability in low-temperature conditions and fatigue properties of asphalt modified with BRA. BRA enhanced the high-temperature property of asphalt modified with WEO. The synergistic effect of WEO, BRA, and neat asphalt was a physical reaction, and these modifiers were uniformly distributed in the asphalt. Meanwhile, WEO/BRA composite modified asphalt had excellent storage stability for the single modified asphalt. The test results of the asphalt mixture verified the results, indicating that WEO/BRA composite modified asphalt can be considered a new green and durable asphalt pavement material.