Failure Mechanism Of Specimens Research Articles

An innovative strategy for improving ductility of seawater sea-sand engineered cementitious composites beam reinforced with GFRP bar

Seawater sea-sand engineered cementitious composites (SS-ECC) members with glass fiber reinforced polymer (GFRP) bars have inferior ductility due to the brittle nature of GFRP bars. In the current research, an enlarged head anchorage system (EHAS) was recommended to improve the ductility of GFRP bars reinforced SS-ECC beams. To explore the bond characteristics between SS-ECC and GFRP bars with EHAS, a total of 39 specimens were tested by pull-out loading. The influences of enlarged head shape, stirrup spacing and SS-ECC’s strength grade on the bond characteristics were experimentally investigated. GFRP longitudinal bars in specimens with EHAS presented rupture failure rather than anchorage failure. EHAS could effectively realize the slip hardening of GFRP bars in low strength SS-ECC, and ensure the reliable bond of GFRP bars in high strength SS-ECC. Additionally, the failure mechanism of specimens under pull-out loading was explored by finite element analysis (FEA). The pull-out resistance was mainly provided by EHAS in force transition stage and slip hardening stage. The influences of fiber reinforced polymer (FRP) bar type on the bond performance were investigated by FEA. EHAS could realize the slip-hardening behaviors of aramid, basalt and carbon FRP bars in normal strength SS-ECC. More importantly, to verify the effectiveness of the proposed ductility enhancement strategy, SS-ECC beams reinforced with GFRP bars were tested by four-point bending. These beams with different enlarged head shapes possessed excellent ductility and strong ultimate deformation capacity. It was recommended that low strength SS-ECC and GFRP bars with EHAS were jointly utilized to enhance the ductile performance of the beams.

Experimental and theoretical studies on 3D printed short and continuous carbon fiber hybrid reinforced composites

3D-printed carbon fiber composites hold significant potential in aerospace applications because of their lightweight, high strength and complex structure fabrication capabilities. However, additive manufacturing characteristics such as high porosity, anisotropy and continuous fiber content significantly affect the mechanical properties of printed parts. The aim of this study is to investigate the influence of hybrid printing of short and continuous carbon fibers on the mechanical properties and failure mechanisms of composites and to develop a model to predict elastic properties considering porosity. First, mechanical testing and characterization of short and continuous fiber reinforced polyamide (nylon) print filaments are performed. The results indicate that the elastic modulus and ultimate strength of continuous carbon fiber filaments reach 60 GPa and 1500 MPa, respectively, while the strength and elastic modulus of short carbon fiber filaments reach 60 MPa and 1 GPa, respectively. Then tensile specimens with different continuous fiber orientations and different continuous fiber placement sequences are printed and tested using a multi-material 3D printing technique. The specimens are characterized before and after testing using scanning electron microscopy and microscopy to assess the porosity and failure mechanisms of specimens with different configurations. The results show that the mechanical properties of the printed parts are much lower than those of the print filaments, which proves the serious negative impact of the material extrusion process on the mechanical properties of the printed structural parts. Finally, an analytical method for predicting the elastic behavior of printed composites is developed by introducing the porosity factor in the volume-averaged stiffness model. For continuous and short fiber filled composites with different fiber contents and printing orientations, the predicted results are in agreement with the experiments, and the prediction error is greatly reduced from 30 % to <5 %.

Bond Properties of CFRP Externally Bonded Reinforcement on Groove in Concrete

Externally bonded reinforcement on groove (EBROG) is a significant reinforcement technology proposed by researchers to enhance the bond properties of reinforced concrete structural members. To understand the influence of groove size on concrete specimens of different strength, a total of 60 concrete specimens with 4 different strengths were cast with the single shear test in this paper, among which 48 EBROG specimens and 12 specimens with externally bonded reinforcement method (EBR) were used as the control group. The failure modes and failure mechanisms of specimens with various sizes and reinforcement methods were analyzed. Additionally, the test results of ultimate load, load–displacement curves, and bond-slip curves for specimens with different groove sizes were compared. The effectiveness of EBROG method in enhancing the ultimate load capacity at the bond interface of the specimens is proved. Furthermore, in situations where the volume of the groove was kept constant, the specimens with lower concrete strength and deeper groove exhibited superior bond properties. Also, the influence of groove width on bond properties was better than that of groove depth. Finally, the test results in this paper were compared with the prediction of the existing EBR and EBROG models to evaluate the performance of different models, and based on the original model, a prediction model for EBROG method in the groove region with higher accuracy was proposed.

Investigating the failure mechanism of X-shaped non-persistent joints under uniaxial loading: Experimental and numerical analysis

In this paper, rock samples containing non-persistent cracks of ‘‘X’’- shape were prepared and experimentally studied under uniaxial compression. Then, the numerical simulations of these tests were established using dynamic finite element software (LS-DYNA). Various joint configurations are assumed and their lengths are considered to change from the center of “X” in different rock specimens. The cracks propagation patterns, the failure processes, and energy absorptions during the tests are studied. The gypsum samples’ dimensions were 10 × 10 × 5 cm and one “X” shaped non-persistent crack was provided inside each specimen. The tensile strength of gypsum samples was 1 MPa. The notch's lengths are similar in some three cases while the length of one wing of the crossest joint varied in two other cases. In two residual cases, the length of the two wings of the crossest joint was varied. The opening of the crack was 1 mm. Angel between the notch trajectories was 90°. The model was subjected to an applied axial load rate of 0.05 mm/min. Results show that the failure mechanism of specimens was affected by notch length. The larger notch has a dominant effect on the breakage mode and failure strength. Newborn tensile cracks originated at the notch tip and propagated parallel to the loading axis to merge with the model boundary. Both the crack initiation stress and breakage stress were decreased by increasing the notch length. The absorbed energy has a minimum value when two large notches with a length of 6 cm exist in the model while it has a maximum value when notch lengths are equal to 2 cm. The increase in notch length resulted in a decrease in the difference between strains associated with crack initiation stress and final stress, indicating progressive breakage. Whereas the difference between strains associated with crack initiation stress and final stress was increased by decreasing the notch length, therefore delay failure occurred after crack initiation in the model. The results obtained from both laboratory experiments and numerical simulations show good agreement in the failure progression.

Effect of build interruption during laser powder bed fusion process on structural integrity of Ti-6Al-4V

The growing concern for global sustainability highlights the need to address waste generation in metal additive manufacturing (AM) processes. Even with proper planning, unexpected incidents such as software glitches and power outages may interrupt AM processes by pausing the build sequence, leading to a waste of resources (i.e. time, money, and materials). The layer-wise nature of AM processes allows for interrupted builds to be resumed from the stopping point. Nevertheless, interrupting the build process (i.e. pausing and resuming) can compromise the structural integrity of the fabricated parts, potentially leading to localized failure near the interruption location. The objective of this study is to investigate the effect of build interruption during the laser beam powder bed fusion process on the structural integrity of Ti-6Al-4V specimens. Tensile and fatigue tests were performed to assess any possible contribution of build interruption to the specimen failures under monotonic and cyclic loading, and the results were compared with their control counterparts, fabricated without interruption. Tensile and fatigue failures as well as the propagation of a secondary crack were observed at the interruption location. Our experimental observations indicate that when large process-induced defects are present, build interruption plays a peripheral role in the fatigue life and failure mechanism of specimens, with defects being the overriding factor. None of the heat-treated specimens exhibited any tensile or fatigue failures at the interruption location, suggesting that an appropriate heat treatment can be effective in reducing the risk of localized failure near the interrupted location.

Study on mechanical behavior and failure mechanism of specimens with single joint subjected to dynamic cyclical loading

AbstractJointed rock masses are a common phenomenon in rock engineering and are vulnerable to cyclic loads caused by earthquakes, blasts, and traffic loads. Therefore, it is crucial to comprehend the fatigue characteristics of jointed rock masses under dynamic cyclic disturbance for long‐term stability assessments of rock engineering structures. This paper aims to investigate the effects of distinct joint inclinations under pre‐cyclic loading on the strength and failure characteristics of rock‐like specimens using experiments and numerical simulations. The results indicate that cyclic loading decreases the strength of the specimen, and the strength characteristics of the specimen are more apparent as the joint inclination angle becomes smaller. Furthermore, insights into different upper limit stress fatigue behaviors were gained by analyzing strength characteristics, crack propagation characteristics, and failure modes. Numerical results show that the damage degree of cyclic loading at high‐stress levels is linked to the joint inclination angle and carrying capacity of specimens, and high cycle stress levels have an impact on the failure mode of the specimen.

Mechanical properties and constitutive model of steel fiber-reinforced rubberized concrete

This study aims to assess the mechanical behavior of steel fiber-reinforced rubberized concrete (SFR-RuC) with and without mix optimization through experimental investigations, and eighteen concrete mixtures were manufactured for test specimens. Through mix optimization, SFR-RuC with a steel fiber volume fraction no more than 1.5% and a rubber particle volume substitution as high as 20% could successfully achieve a compressive strength of 49 MPa-56 MPa. Cubic and uniaxial compressive tests, direct shear tests and four-point flexure tests were conducted on all specimens. The results showed that optimized SFR-RuC had overall improved mechanical properties and more ductile failure than nature concrete, and the elastic modulus and compressive, flexural and shear strength were enhanced by 13%, 16%, 25% and 50% respectively, when 10% rubber particles and 1.5% steel fibers were added. The peak strain, toughness and post-peak deformation of optimized SFR-RuC under compression increased with the steel fiber volume fraction as well as the rubber particle volume substitution, while the peak deflection, toughness and ductility under flexure and shear increased mainly with the steel fiber volume fraction. An obvious synergistic action was shown between the rubber particles and the steel fibers, through Scanning Electron Microscope (SEM) analyses on the microstructure, as well as test observations on the failure mechanism of specimens. Based on uniaxial compressive test results, formulas were proposed for the compressive strength, elastic modulus and peak strain of optimized SFR-RuC, and a modified stress–strain model was developed based on the classical concrete stress–strain model in the Chinese code (GB50010-2010), for a detailed assessment of the complete stress–strain response.

Numerical analysis of square concrete-filled double skin steel tubular columns with rubberized concrete

Structural engineers have assessed the possibility of replacing part of natural aggregates with rubber particles in concrete in recent years to obtain sustainable structural members. It has been confirmed that the use of rubber particles can enhance ductility and energy absorption capacity of concrete. By contrast, the compressive capacity of rubberized concrete (RuC) is usually lower than the normal concrete (NC). Confining RuC with steel tubes can be considered as an appropriate solution for reaching to a ductile member with adequate strength. This paper, therefore, investigates the behavior of rubberized concrete-filled double skin steel tubular (RuCFDST) short columns by performing a series of nonlinear finite element (FE) analyses. The developed FE models were verified against the experimental test results by comparing the axial load–displacement curves, ultimate axial strength, and failure mechanism of specimens. The validated FE model was used to investigate the effects of geometric and material properties on the behavior of RuCFDST columns with 5%, 15%, and 30% rubber contents, and to compare the performance of RuCFDST and concrete-filled double skin steel tubular (CFDST) columns. The results showed that using RuC for filling square CFDST columns could significantly enhance the ductility. By contrast, CFDSTs presented greater axial strengths than RuCFDSTs. Reduced confining stresses between the outer tube and the concrete core were displayed when RuC was applied compared to CFDST.

Experimental study on seismic performance of precast hollow shear walls

SummaryThis paper puts forward a new type of the precast hollow shear wall. Focusing on seismic performance, the low‐frequency cyclic loading tests of five precast specimens and two cast‐in‐situ specimens in full size are conducted for comparison. The yielding and failure mechanism of specimens are analyzed and compared. The reliability of the vertical connection of precast hollow shear wall structure under the seismic effect is studied. The effects of axial compression ratio, single reinforcement connection in the central part, and confined stirrup for lapped reinforcements in boundary elements are also tested through experiments. The result shows that the failure mechanism of the precast specimen is basically same as the cast‐in‐situ specimen. The hysteretic curve of the precast specimen is plumper. And the energy dissipation capacity of precast specimen is superior to that of cast‐in‐situ specimen. In addition, the experimental data show that the buckled stirrup encryption for lapped steel in boundary elements will not increase the seismic performance of the proposed precast specimen. On the whole, the seismic performance of the proposed precast hollow shear wall can meet the safety requirements. Hence, it can be used in actual engineering project replacing cast‐in‐situ shear wall.

A numerical study of acoustic emission characteristics of sandstone specimen containing a hole-like flaw under uniaxial compression

For a better understanding of acoustic emission (AE) characteristics and failure mechanism of specimens containing hole-like flaw, a series of uniaxial compression tests are carried out by using the bonded-particle model (BPM). In the BPM, the AE simulation combined with moment tensor inversion is adopted. The results indicate that the stress drop in the stress–strain curve corresponds to a sudden increase of AE counts, which results from the propagation and coalescence of micro-cracks. The shape of hole-like flaw does not have a significant influence on the magnitude distribution of AE event. The b-value of three models is similar, i.e. 1.68, 1.83, and 1.75 corresponding to circular, square, and horseshoe flaw respectively. The forces concentration at upper and bottom of the hole-like flaw is dominated by tensile, and the fracture path only contains micro-tensile cracks. While the forces acting on lateral sides of the hole-like flaw are dominated by compressive. The fracture path contains both micro-tensile and micro-shear cracks and the failure nature is shear and compaction, although the number of micro-tensile crack is predominant.

Experimental investigation of stress transfer and failure mechanism between existing concrete and CFRP grid-sprayed PCM

Abstract Carbon fiber reinforced polymer (CFRP) grid and sprayed polymer cement mortar (PCM) has been widely applied to retrofit and strengthen concrete structures. This paper focused on the stress transfer and failure mechanism between the concrete and CFRP grid with sprayed PCM. Pull-out tests of the single grid point in CFRP grid and plate-type specimens consisting of concrete, PCM and CFRP grid were carried out. Types of CFRP grids, grid interval and whether or not to use primer were selected as analysis parameters. The maximum tensile loads, failure modes and strains in the CFRP grid were obtained. The stress transfer process was investigated through analysis of the changing process for the resistant action of horizontal grid and the bond action of vertical grid. The stress transfer mode and its influence parameters were investigated based on the strain share ratios of grid points. In addition, the failure mechanisms of specimens were discussed through mechanical analysis. The research results indicated that the tensile strength of grid point was no more than 50% of the tensile strength of CFRP grid. The stress transfer mode was not constant, and it would gradually change with the increase of tensile load. The failure mechanism of the specimen could be evaluated based on the difference among the ultimate tensile action of CFRP grid, bond capacity between concrete and PCM with CFRP grid and the anchorage capacity of CFRP grid in the PCM. In order to make full use of the tensile strength of CFRP grid, the bond capacity between concrete and PCM with CFRP grid and the anchorage capacity of CFRP grid in the PCM should be larger than the ultimate tensile action of CFRP grid.

Parametric analysis of composite sinusoidal specimens under quasi-static crushing

ABSTRACTThis paper aims to build the finite element model of the composite sinusoidal specimens and to carry out the parametric analysis. In this paper, the damage behaviour and the energy-absorbing results of composite sinusoidal specimens have been studied by quasi-static crushing experiments. The failure mechanisms of specimens under quasi-static crushing is further analysed. A numerical simulation has been performed by using the finite element model code LS-DYNA. The numerical results, in terms of load -displacement data, have been compared against experimental data, and good agreement has been found. Moreover, a sensitivity study has been carried out by varying material properties in order to assess their influence on the numerical results, and the material parameter selection scheme is optimised based on the constructed corresponding response surfaces. The results show that the response surface model has passed the test of goodness of fit, and the optimisation method can effectively assist the finite element modelling, and greatly decrease the numbers of trial and error.

Multi-scale computational analysis of unidirectional carbon fiber reinforced polymer composites under various loading conditions

A multi-scale computational analysis based on representative volume element (RVE) modeling and molecular dynamics (MD) simulations is developed to investigate the microscopic failure mechanisms of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites. The average properties of the 200-nm thickness interphase region between fiber and matrix are characterized through MD simulations and an analytical gradient model. The results demonstrate that the interphase region has higher Young’s modulus and strength, compared to the bulk matrix. This stiffened interphase region influences the composite response significantly. Specifically, the traditional two-phase model with zero-thickness interface fails to capture the stress-strain behavior compared to the experimental data. However, by adding the interphase region to a modified RVE model, the accuracy of simulation results will be improved significantly. Furthermore, a coupled experimental-computational micromechanics approach is adopted to calibrate and validate the cohesive parameters of the interface. By including the cohesive interface, our modified RVE model accurately captures the failure strength of the composites. Finally, different failure mechanisms for specimens are investigated using our multi-scale computational framework. The results show that the failure modes of UD CFRP composites are very complex and multiple failure mechanisms co-exist depending on the loading conditions, agreeing well with our experimental analyses.

Static and dynamic responses of a novel Al nanocomposite foam/sandwich structure under bending, impact and quasi-static compression tests

The aim of this study is to analyze mechanical properties of a new Al sandwich structure with a foam core reinforced by 0.75 wt% silicon carbide nanoparticles. The reinforced core as the main component of the sandwich structure is examined by nanoindentation, quasi-static compression, impact and three-point bending tests. The behavior of the nanocomposite foam core sandwiched with AA3103 facing sheets is also analyzed under three-point bending test. The results showed that the silicon carbide nanoparticles play an important role in enhancing the Young’s modulus and hardness of the metallic matrix, static compressive strength, energy absorption of the foam core as well as load carrying capacity and maximum deflection of the sandwich structure. However, they have no significant influence on the morphological features, impact and bending properties of the foam core. The effectiveness of the silicon carbide nanoparticles was dependent on the dominant deformation mode and failure mechanism of specimens under the applied loadings.

Buckling length analysis for compression chord in cold-formed steel cantilever truss

The lateral buckling of a cold-formed steel truss structure was investigated. Both an experimental test and finite element analysis presented a good correlation in the failure mechanism of specimens. The critical buckling load of the compression truss chord was analyzed using a Southwell plot and the analytical method. The transfer matrix method and the governing equation of an Euler column were used to analyze the critical buckling load of a multi-step, uniform chord memberwith lateral spring supports. Expressions for the buckling length factor presented according to AISI (2012) and the EN 1993-1-3 (2006) specification were compared by experimental testing and the analytical method. The buckling length factorof the experimental test and the analytical model confirmed that the buckling length of a compressed truss chord was less than the distance between the lateral bracing.

Influence of heat treatment on the wear life of hydraulic fracturing tools

Wear phenomenon has caused severe damage or failure of fracturing tools in oil and gas industry. In this paper, influence of heat treatment on the mechanical properties and wear resistance of fracturing tool made of lamellar graphite grey cast iron were investigated. The surface composition and microstructure were characterized by X-ray diffraction (XRD) and metallographic microscope. Sliding wear tests were performed to study the tribological behavior. Tests results showed that wear rates of treated specimens decreased by 33 %. Besides, worn morphology and wear debris were analyzed using Scanning electron microscope (SEM) and Energy dispersive X-ray spectra (EDS). Wear failure mechanisms of specimens were identified. Furthermore, on-site experiment results indicated that wear loss of treated samples decreased by 37.5 %. The wear life of hydraulic fracturing tools can be improved obviously by the heat treatment.

Tensile Properties and Failure Mechanism of a New 3D Nonorthogonal Woven Composite Material

Tensile properties and failure mechanism of a newly developed three-dimensional (3D) woven composite material named 3D nonorthogonal woven composite are investigated in this paper. The microstructure of the composite is studied and the tensile properties are obtained by quasi-static tensile tests. The failure mechanism of specimen is discussed based on observation of the fracture surfaces via electron microscope. It is found that the specimens always split along the oblique yarns and produce typical v-shaped fracture surfaces. The representative volume cell (RVC) is established based on the microstructure. A finite element analysis is conducted with periodical boundary conditions. The finite element simulation results agree well with the experimental data. By analyzing deformation and stress distribution under different loading conditions, it is demonstrated that finite element model based on RVC is valid in predicting tensile properties of 3D nonorthogonal woven composites. Stress distribution shows that the oblique yarns and warp yarns oriented along the x direction carry primary load under x tension and that warp yarns bear primary load under y tension.

The bearing behavior and failure characteristic of CFRP laminate with cutout under shearing load: Part II. Numerical simulations

In part I of this work, the mechanical performance of carbon fiber reinforced polymer (CFRP) laminate with cutout of three different shapes (circular, curved-diamond, rounded-rectangular) subjected to shear load was investigated experimentally. This part describes the finite element (FE) simulations of the problem investigated. An efficient laminate-leveled computational model is employed to provide an insight on the failure mechanisms of specimens. The load bearing procedures of specimens, both at pre-buckling and post-buckling stage, are simulated. The characteristics of post-buckling progressive damage undergoing within specimens obtained from modeling are compared with the experimental observations in detail. It is found that the damage model is capable of reproducing the post-buckling progressive damage with complicated coupling effects within thin-walled composite structure costing low computational resources, and good agreement is in general obtained between the numerical simulations and experimental results. Through this comparative study, the progressive damage events within the structure with different shaped cutout help reader in better understanding the bearing performance and underlying failure mechanisms of CFRP laminate with cutout.

Effect of Specimen Thickness on Fatigue Limit in Carbon Composites

The fracture behavior and fatigue limit in notched specimens of C/C composites were investigated. Also, the effect of specimen thickness on fatigue limit was discussed. Two plates of different thicknesses of plates of C/C composites using fine-woven carbon fiber laminates with α=0°/90° direction were used for testing. α is the angle between the carbon fiber direction and specimen axis. The crack growth behavior and failure mechanism of specimens are derived from the shear damage in the fiber bundle and matrix. Slits of several sizes were cut on both sides of a test section and different geometries of the specimens were prepared. Specimens with slits and blunt-notches were used to compare the fatigue strength. The fatigue limit is related to the method of making the plate of carbon composites. Large sizes of voids are observed in the case of specimens of thinner thickness. The fatigue limit was related to the void fraction, and thinner specimens showed a lower fatigue limit.

Seismic Performance of T-Shaped CFT Column to Steel Frame Beam Connection – Experimental Study

The focus of this research program is T-shaped CFT central column to steel frame beam connection. 3 joints with strong columns-weak beams and 1 joint with strong beams-weak columns 1:2 scale specimens were tested under constant axial loads and cyclic horizontal loads. Overall impact of axial force ratio and beam to column linear stiffness ratio on joint failure mechanism, hysteretic behavior, deformation ductility, and energy dissipation capability was investigated. Results showed that the failure mechanism for specimens with strong columns-weak beams was local buckling of the steel beam flanges and formation of the plastic hinges. There was minimum damage on the concrete column and joint panel zone. For a specimen with strong beams-weak columns, there was local buckling fracture on steel tube above and below the joint panel zone. Crushing of the core concrete was also observed with formation of the column hinges. It was found that both axial forces and beam to column linear stiffness ratio had impacts on joint capacity and ductility behavior of the specimens. Experiment results showed that the joint models had deformation ductility factor between 3.39 and 3.91 and viscous damping ratio between 0.46 and 0.51.