Four-point Bending Method Research Articles

Mechanical properties analysis of the naval ship similar model with an integrated sandwich composite superstructure

The research object of the article is a similar model of a naval ship with an integrated sandwich composite superstructure. The similarity model of this naval ship is based on the similarity theory, which is obtained by scaling down the actual naval ship. Based on the theoretical guidance of the double-beam method, by using the static loading test of the four-point bending method, the article analyzes the magnitude and trend of the strain, stress, and deformation of the measuring points on the naval ship model under different load cases. The main measuring points of the similar naval ship model are located in the typical cross-sections, each deck, and the interface between the steel main hull and the integrated sandwich composite superstructure. The article mainly explores the changes of longitudinal strain and stress of the measuring points with height on three typical cross-sections, and the changes of longitudinal strain and stress with the ship's length on the interface, and the deformation of the top deck of the superstructure. Combining with the results of finite element numerical simulation, the complicated mechanical properties of the naval ship model with an integrated sandwich composite superstructure are explored by mutual verification with the experiments. Some valuable conclusions are obtained for the construction and design of the naval ship with an integrated sandwich composite superstructure.

The Impact of Water and Road Salt with Anti-Caking Agent on the Stiffness of Select Mixes Used for the Road Surface.

An original experimental method was used to investigate the influence of water and road salt with anti-caking agent on the material used in pavement construction layers. This method allowed for monitoring material changes resulting from the influence of water and road salt with anti-caking agent over time. The experiment used five different mineral road mixes, which were soaked separately in water and brine for two time intervals (2 days and 21 days). Then, each sample of the mix was subjected to tests of the complex module using the four-point bending (4PB-PR) method. The increase in mass of the soaked samples and the change in value of the stiffness modulus were analyzed. Exemplary tomographic (X-ray) imaging was performed to confirm the reaction of the road salt and anti-caking agent (lead agent) with the material. Based on measurements of the stiffness modulus and absorption, the correlations of the mass change and the value of the stiffness modulus were determined, which may be useful in estimating the sensitivity of mixes to the use of winter maintenance agents—e.g., road salt with anti-caking agent (sodium chloride). It was found that the greatest changes occur for mixes intended for base course layers (mineral cement mix with foamed asphalt (MCAS) and mineral-cement-emulsion mixes (MCE)) and that the smallest changes occur for mixes containing highly modified asphalt (HIMA).

Experimental Study on Flexural Capacity of Corroded RC Slabs Reinforced with Basalt Fiber Textile

This experimental study investigated the flexural performance of corroded reinforced concrete (RC) slabs strengthened with basalt textile-reinforced mortar (BTRM) and basalt fiber-reinforced polymers (BFRP). Ten RC slabs were designed to achieve the expected corrosion levels (8% mass loss for moderate corrosion and 16% mass loss for severe corrosion) by accelerated corrosion methods. Two slabs served as reference specimens, and eight slabs were strengthened with BFRP or BTRM. The specimens were loaded to failure by the four-point bending method. The corrosion ratio, strengthening materials and the number of layers were tested for comparison. The failure modes, flexural capacities, load–deflection curves and deformation performances of the slabs were obtained from experiments. It was found that the use of BTRM layers was more effective in improving the flexural response than the use of the same amount of BFRP layers externally bonded with the corroded RC slabs under a state of serviceability. The results also showed that the strengthening effects of BFRP and BTRM were affected by the initial corrosion ratio and the number of textile layers. In a moderate state of corrosion, the flexural capacities and deflection capacities of RC slabs strengthened by BFRP and BTRM were increased substantially; the flexural capacities were increased by 27.81%~61.85%. In a severe corrosion state, the increase in flexural capacity of strengthened slabs is marginal but the increase in ductility indexes was 18% to 35% compared with the corresponding control slabs. By increasing the number of textile layers from three to five, the increments of the flexural capacity of strengthened slabs are almost doubled. Finally, the calculated results of the flexural capacity of the corroded RC slabs strengthened with BFRP and BTRM were found to be in good agreement with the experimental results.

Elevated thermal conditioning effect on flexural strength of GFRP laminates: An experimental and statistical approach

This study aims to investigate the flexural strength and damage formation in glass fibre reinforced polymer (GFRP) laminates for different thicknesses (2, 3 and 4 mm), small span length variations (50.8 and 56 mm), crosshead speeds (1 and 10 mm/min), thermal conditioning duration (3 and 6 h) and thermal conditioning temperatures (27 °C (room temperature), 50 °C, 100 °C and 150 °C). A closed muffle furnace used for thermal conditioning of GFRP specimens. A quasi-static transverse load applied using four-point bend method. GFRP plates are translucent; hence high-intensity background light is used to observe the formed damage and its extent after four-point bend test. The most effectively influencing parameters among GFRP laminate thickness, span length, thermal conditioning temperature, thermal conditioning time and crosshead speeds on the flexure strength of GFRP plates analysed by adopting the two-parameter Weibull distribution statistical method. Flexural strength of the GFRP laminates severely degraded when the exposed elevated temperature approached the glass transition temperature (Tg) 100 °C. Exposure duration affected the flexural strength. GFRP composites exposed for 6 h showed comparatively better flexural strength regain than those GFRP specimens exposed for 3 h. GFRP laminate thickness showed a more significant influence on the flexural strength compared to small span length variation. GFRP laminate with 2 mm thickness showed highest flexural strength compared to 3 mm and 4 mm thick GFRP specimens while the span length 50.8 mm showed higher flexural strength compared to GFRP laminates with 56 mm span length. Crosshead speed also affected the flexural strength and GFRP specimens tested at 10 mm/min showed higher flexural strength compared to specimens tested at 1 mm/min. For specimens tested at room temperature the cumulative failure probability curves agglomerated at a single region in the graph. However, as the temperature and laminate thickness increased these curves dispersed over the entire graph. Experimental flexural strength showed good agreement with the theoretical flexural strength calculated from gamma function.

WITHDRAWN:

Abstract Macro polypropylene fiber (MPPF) with aspect ratios of 30, 38, and 43 is used to prepare concrete with volume fractions of 0.5% and 0.8%, respectively. The workability and mechanical properties of fiber reinforced concrete are studied systematically. In order to obtain the ductility of concrete, three types of methods are selected, that is four-point bending method (trabecular beam bending), three-point bending method (notched beam bending), and square panel method. The results indicate that the density and slump of concrete can be reduced significantly by adding MPPF, and high fiber content increases the reduction. The 28 d-compressive strength of MPPF concrete is 17% lower than that of SF (steel fiber) concrete at the same fiber volume content. Moreover, the flexural tests reveal that MPPF reduces the initial crack strength of concrete, but the toughening effect of MPPF exceeds that of SF. The observation and analysis of SEM suggest that the phenomenon is induced by the dispersion, fluffiness, and friction of polypropylene fiber in concrete. Finally, the load-deflection curves of MPPF concrete have a longer platform section than SF concrete. The energy absorption of the concrete can be increased by 17.41%∼57.43%, which shows outstanding toughening effect.

Research on water transport behaviors and hydration characteristics of internal curing pavement concrete

This paper presents the influence of internal curing agent (super absorbent polymer, SAP) on the water transport behaviors and hydration characteristics of pavement concrete. The 1H nuclear magnetic resonance (NMR) test was performed to quantitatively analyze the transformation rule of different forms of water, and the variation rule of relative humidity in different vertical depths of concrete slab was also studied. Moreover, the isothermal calorimetry and thermogravimetry method were adopted to study the hydration process of internal curing pavement concrete. The flexural strength of specimens with SAPs were evaluated by the four-point bending method. Finally, the enhancement mechanism of SAP on the properties of pavement concrete was revealed through analyzing the water transport behaviors, hydration characteristics and microstructure characteristics. The results show that specimens with SAPs of smaller particle size could keep the humidity compensating and hydration enhancing process for a long time. The humidity gradient in the slab could be significantly reduced that might mitigate the humidity warping stress distributed in the slab. Although SAP delayed the hydration process of binding materials in the early age, it could greatly improve the hydration degree in the long term, which helps improving the crack resistance and the compactness of pavement concrete. The adhesion of aggregate-cement matrix interfacial transition zone (ITZ) and the compactness of microstructure was improved during the internal curing process.

Mechanics-based acceleration for estimating thermal fatigue life of electronic packaging structure

A simple mechanics-based approach is extremely anticipated to efficiently perform the equivalent estimations for thermal fatigue life of electronic packaging structures. In this paper, a cyclic loading in the form of four-point bending is applied and the mechanical fatigue life are measured for five groups of wafer level chip scale packaging (WLCSP) structures with different displacement amplitudes at a constant temperature of 125 °C. By conducting the conventional thermal cycling experiments, the thermal fatigue lives of the packaging structures are obtained for four stress levels due to different constraint schemes. With consideration of temperature effect within the temperature range of thermal cycling, the constitutive parameters of the lead-free Sn-3.0Ag-0.5Cu (SAC305) solder material are reasonably assessed by the framework of the unified creep-plasticity model to reflect the plastic and creep deformation at different temperatures, which is the essential perquisite of finite element simulations for fatigue life analysis. The increment of equivalent accumulative plastic strain is numerically calculated for each loading cycle for both four-point bending and thermal cycling by finite element simulations. By following the same slope in the double logarithmic coordinates, both the mechanical and thermal fatigue life are found to linearly decrease with regard to the increment of equivalent accumulative plastic strain, despite of certain discrepancy in intercept. This make possible the equivalence of accelerations between mechanics-based four-point bending and thermal cycling method.

Temperature-dependent plasticity and fracture mechanism under shear loading in metallic glass

The fracture surface morphology under shear loading is important for understanding the plasticity and the deformation mechanism of metallic glasses (MGs). However, it is difficult to carry out the shear fracture test for most MGs by conventional test methods due to their limited glass forming ability. In this work, through a unique anti-symmetrical four-point bend method, the shear fracture test is carried out under a quasi-static loading at a wide temperature range from 220 K (0.31Tg, Tg: glass transition temperature) to 620 K (0.88Tg) for a Zr-based MG. Basing on the examination on the fracture surface morphologies, we found three different fracture mechanisms with temperature: at lower temperatures (220 ~ 260 K, 0.31 ~ 0.37 Tg), both flower-like and shear-driven vein patterns appear on the fracture surface, which displays a mix of shear- and dilatation- dominated fracture; at intermediate temperatures (300 ~ 570 K, 0.42 ~ 0.80 Tg), the shear-driven vein patterns dominate on the fracture surface, indicating the shear-dominated fracture; at higher temperatures (600 ~ 620 K, >0.80 Tg), a mass of melt droplets indicate the transition from inhomogeneous to homogeneous deformation. From the Weibull statistical analysis on the main fracture features at intermediate temperatures, we found that the size of shear-driven vein patterns increases and the data distribution is sparser with the temperature increasing. The temperature dependence of fracture surface morphology as well as the relationship between the fracture surface feature and the microstructural evolution in MG were also interpreted from the shear transformation zone theory.

The Failure Mechanism of 3D C/C Composite Under Compression-Shear Coupled Loads: An Experimental Study

Material strength under complex stress states is vital for structure design. This paper studied the strength and failure behaviour of 3D C/C composites under compression-shear coupled loads. Experiments were conducted using a modified anti-symmetric four-point bending (MAFPB) method and off-axis compression method. Two dominant failure modes were observed; 1) shear and 2) compression. Mode transitions under relatively low shear/compressive rations were also observed. Results show that the material exhibits significantly lower failure stress under shear-compressive load compared to compressive or shear strengths alone. Further analysis revealed that meso-scale geometry characteristic including tow crimps and interfacial cracks are the main inducements: (1) tow crimps lead to local bending moment and increase local shear stress, (2) matrix-tow cracks formed under shear load degrade the lateral support of axial tows, (3) matrix fibre splitting and local buckling within tows induced by compressive load further reduce the shear strength of axial tows. Failure stresses in off-axis tests are lower compared to those in MAFPB tests due to the existence of bi-axial compression. These findings show that shear failure is a weakness for the 3D C/C composite and can provide insights for further material design and structural strength analysis.

Flexural behaviour of the functionally graded concrete beams using two-layers and three-layers configuration

Studies related to the bending behaviour of functionally graded concrete (FGC) beams are still investigated to explore the possibility of the more cost-efficient casting techniques and to produce high-performance FGC beam elements simultaneously. Previous researches show that the manufacture of FGC beams by combining two concrete mixes in two layers can increase the stiffness of the elements but it reduces the beam ductility. Research concentrating on the FGC beams with additional numbers of concrete layers was analysed in this study. Four pieces of FGC beams measuring 120 x 240 x 2200 mm were prepared with concrete strength of 25 MPa (NA); 30 MPa(NB); 30-20 MPa (GBA); and 30-20-30 MPa (GBAB). The specimens were tested in the laboratory using a four-point bending method. The experimental data consisting of the load increments and the mid-span deflections are further analysed to compare the resulting (1) maximum load-carrying capacity; (2) maximum deflection; (3) beam stiffness; (4) moment-curvature; and (5) ductility. The result shows that (1) the GBAB exhibited 0.83% greater maximum loads than the GBA; (2) the GBAB deflected 29.52% lesser than the GBA; (3) the GBAB posses stiffness 12.03% greater than the GBA; (4) the GBAB exhibited higher resulting yield points and ultimate state than the GBA at the moment-curvature relationship; and (5) the GBAB was 20.06% less ductile than the GBA.

Numerical and experimental investigation on the shear resistance of functionally graded concrete (FGC) beams

Functionally graded concrete (FGC) is one of the most innovative materials combining two or more different concrete strength in building elements to optimise the construction cost. Studies focussing on the behaviour of graded concrete in flexural strength have been conducted by several researchers, whilst the research on the shear behaviour of FGC beam has not been initiated yet. Investigation on the shear behaviour of RC graded concrete beam is necessary due to the fabrication process could create a cold-joint that lower the load-carrying capacity of the element. Delamination possibly presents on the casted concrete layer during the loading and turns the beam behaviour more brittle in action. To further investigate these hypotheses, a laboratory testing and a numerical analysis are conducted in this research. Three pieces of RC beams with an identical dimension of 120 x 240 x 2200 mm are prepared with the concrete strength of 20 MPa, 30 MPa, and 20-30 MPa. The specimens are then tested using a four-point bending method in the age of 28 days. A set of strain gauge is mounted in stirrups surface to record the strain response on each incremental load. To validate the result, the specimens are also modelled in three-dimension using Abaqus. The results show that there is a significant difference in the stress-strain curve resulted from the experimental works and the model simulation. It is due to the limitation of programme to model the real interaction between the rebars and the concrete. In reality, the presence of aggregate interlocking, concrete contribution in withstanding compression stress, dowel effect improves the shear resistance of beams. Additionally, the interfacial failure did not present during the testing owing to the support of stirrups that confine the transition zone of concrete layers.

Evaluation of Effect of Mechanical Stress on Stacking Fault Expansion in 4H-SiC P-i-N Diode

Single Shockley stacking faults (SSFs) expand from basal plane dislocations (BPDs) under forward current operation of 4H-SiC bipolar devices, giving rise to a reliability deterioration mode called “bipolar degradation”. Several groups have proposed models for the expansion of SSFs, in which the SSFs expand when electron-hole pair recombination takes place at BPDs. Maeda proposed a formulation of SSF expansion that includes stacking fault energy. However, the mechanisms by which mechanical stress affects the expansion of SSFs are unclear. In this paper, we evaluated the “expansion threshold current” of bar-shaped SSFs in a mechanical stress field using a p-i-n diode fabricated on 4H-SiC. To confirm the effect of mechanical stress on the threshold current for bar-shaped SSF expansion, a SiC-p-i-n diode was evaluated by the four-point bending method. Experimental results show that the threshold current of SSFs decreases or increases by more than 100 A/cm2 depending on the direction of the applied stress of SSFs. This result indicates that mechanical stress is an important factor for SiC bipolar device design.

Proposal of a Modified Four-Point Bending Method for Determining Interfacial Fracture Energy at Electrode/Electrolyte Interfaces in SOFCs

This study proposes the use of all-ceramic modified four-point bending specimens consisting of a thin porous electrode layer sandwiched between two dense electrolyte beams to determine the fracture energy of the electrolyte/electrode interface. The sandwich arrangement was effective to prevent the premature vertical cracking of the electrode and to eliminate the use of polymeric adhesives for metallic stiffeners which restricted the testing temperature range. The experiments conducted at room temperature using the all-ceramic layered specimens demonstrated that the proposed four-point bending method successfully induced delamination between the electrode and electrolyte producing the characteristic plateau behavior in the load-displacement records and enabled the interfacial fracture energy to be evaluated based on the load level at the plateau. The interfacial fracture energies were determined to be 5.6 J/m2 for the interface of NiO-8YSZ/3YSZ and 0.8 J/m2 for LSCF/8YSZ using the identical testing method and specimen configuration.

Piezoresistive strain sensor based on monolayer molybdenum disulfide continuous film deposited by chemical vapor deposition

In this paper, a centimeter-scale monolayer molybdenum disulfide (MoS2) film deposition method has been developed through a simple low-pressure chemical vapor deposition (LPCVD) growth system. The growth pressure dependence on film quality is investigated in this LPCVD system. The layer nature, electrical characteristic of the as-grown MoS2 films indicate that high quality films have been achieved. In addition, a hydrofluoric acid treated SiO2/Si substrate is used to improve the quality of the MoS2 films. Piezoresistive strain sensor based on the monolayer MoS2 film elements is fabricated by directly patterning metal contact pads on MoS2 films through a silicon stencil mask. A gauge factor of 104 ± 26 under compressive strain is obtained by using a four-point bending method, which may inspire new possibilities for two-dimensional (2D) material-based microsystems and electronics.

Adhesion and Material Properties Between Polyimide and Passivation Layers for Polymer/Metal Hybrid Bonding in 3-D Integration

This paper investigates polymer/metal hybrid bonding structure using conventional polyimide and modified polyimide-like material. Adhesion properties between bonding material and passivation layer were evaluated by four-point bending method, atomic force microscopy, water contact angle measurement, and X-ray photoelectron spectroscopy. Various annealing temperatures were applied to verify the reliability under thermal stress. The results show that the modified polyimide-like material with the curing temperature below 250 °C has better adhesion than the conventional polyimide. This modified polyimide material can be used in low-temperature hybrid bonding technology for advanced integration.

Evaluation of the Out-of-Plane Shear Properties of Cross-Laminated Timber

The out-of-plane shear properties of cross-laminated timber (CLT) substantially influence the overall mechanical properties of CLT. Various testing methods and theories related to these properties have recently been developed. The effects of the number of layers (three and five layers) and testing method (short-span three- and four-point bending tests) on the out-of-plane shear properties of CLT were evaluated. The out-of-plane shear strength values were calculated based on different theories for comparison. The failure mode in the short-span four-point bending (FPB) method was mainly the rolling shear (RS) failure in the cross layers, indicating that the FPB method was appropriate to evaluate the RS strength of CLT. The out-of-plane shear capacity obtained using the three-point bending (TPB) method was higher than that tested by the FPB method. The testing methods significantly influenced the out-of-plane shear capacity of the three-layer specimens but not that of the five-layer specimens. With an increase in the number of layers, the out-of-plane shear strength of the specimens decreased by 24%. A linear correlation was found among the shear strength values obtained from different theories.

Bending performance of TRC-strengthened RC beams with secondary load under chloride erosion

Textile reinforced concrete (TRC) has good bearing capacity, crack resistance and corrosion resistance and it is suitable for repairing and reinforcing concrete structures in harsh marine environments. The four-point bending method was used to analyze the influence of the salt concentration, the damage degree and the coupled effect of the environment and load on the bending performance of TRC-strengthened beams with a secondary load. The results showed that as the salt concentration increased, the crack width and mid-span deflection of the beam quickly increased, and its bearing capacity decreased. As the damage degree increased, the early-stage crack development and mid-span deflection of the beam were less affected and the ultimate bearing capacity significantly decreased. In addition, the coupled effect of the environment and load on the beams with a secondary load was significant. As the sustained load increased, the ultimate bearing capacity of the strengthened beam decreased, and cracks developed faster in the later stage. In addition, the mid-span deflection of the beam decreased at the same load level because of the influence of the initial deflection due to the sustained load corrosion.

Vanadium-doped molybdenum disulfide film-based strain sensors with high gauge factor

This paper reports the piezoresistive performance of the two-dimensional (2D) material of vanadium (V)-doped molybdenum disulfide (MoS2) films based on sulfurization of sputtered Mo thin films. I–V characteristics indicate that V atom doping indeed decreases the resistivity of MoS2. Strain sensors based on V-doped MoS2 resistive elements were fabricated. By using a four-point bending method, a gauge factor (GF) of 140 under compressive and tensile strain conditions was obtained. The piezoresistive effect of V-doped MoS2 with different V sputtering conditions was also investigated. The doping method introducing V atoms as dopants is found to play an important role in enhancing piezoresistive performance.

Fatigue tests, self-healing of polymer-matrix composites in unmanned aerial vehicles

Abstract This publication presented the main issues related to fatigue of polymer composite materials. It was featured a fatigue stress test based on composite sample, made of carbon fiber-reinforced polymer, using the four-point bending method. The test was carried out with the initial load and using positive load cycles. The perspectives of diagnostics and self-healing of composite materials, including intelligent materials, were also presented.

Evaluation of the bending properties of modified fast-growing poplar glulam based on composite mechanics

Currently, the bending properties of glulam made from fast-growing poplar barely meet the requirements for application. In this study, the bending properties of glulam made from preservative alkaline copper quaternary (ACQ)-treated and phenol-formaldehyde resin reinforced poplar in different laminate configurations were full-scale tested via a four-point bending method. Theoretical models including stiffness model and rigidity model under different loading modes were founded based on the mechanical analysis of composite materials to predict the Young’s modulus of bending and bending strength. The Young’s modulus and bending strength of modified fast-growing poplar glulam was greatly enhanced compared with untreated ones, which can meet the standard requirements for symmetrical mixed-grade composition glulam grade E85-F255. The Young’s modulus was predicted with the rigidity model with high accuracy. The relative error was below 12%. The modified stiffness model with correction factors for normal stress and interlayer tension shearing stress can also accurately predict the failure mode and bending strength.