Direct Tensile Loading Research Articles

Using artificial neural network to predict the fracture properties of the interfacial transition zone of concrete at the meso-scale

Concrete is a multi-phase heterogeneous material in which the interfacial transition zone (ITZ) between aggregates and mortar significantly affects the cracking behaviour of concrete, especially under tensile load. In this paper, the artificial neural network (ANN) method is applied in predicting the fracture properties of ITZ in concrete. To form the data pool for the training of the ANN, a large number of two-dimensional (2D) meso-scale fracture simulation of concrete under direct tensile load is conducted. Cohesive crack elements are used in simulating arbitrary cracking in the ITZ and mortar. After the verification of the trained ANN model, the tensile strength and fracture energy of the concrete ITZ are predicted by using the RILEM direct tensile test results, i.e., stress–displacement curve, as the input for the ANN. It has been found that, the trained ANN performs well in predicting the ITZ properties and the computed stress–displacement curve together with the optimized ITZ fracture parameters has a good agreement with that from the RILEM test results. The randomness of aggregates has little effect on the predicted ITZ tensile strength while it becomes slightly bigger on the predicted ITZ fracture energy. The tensile strength ratio of ITZ to mortar are calculated 0.38–0.47 and the fracture energy ratio of ITZ to mortar are 0.18–0.58. These inversely predicted fracture properties of concrete ITZ can be useful complementation to the existing dataset and 2D fracture simulation of concrete structures.

Cyclic tensile behavior of high strain hardening UHPC analyzed by acoustic emission techniques

Understanding the cyclic tensile behavior of high strain hardening ultra high performance concrete (UHPC) is important in structural design and for predicting its damage state. In this study, acoustic emission (AE) analysis was applied to monitor the damage evolution process of high strain hardening UHPC under direct tensile cyclic loading at different objective tensile strain levels. The test results show that when the objective strain is in the elastic state, the tensile cyclic stress–strain curves show marginal change, whereas when it is in the strain hardening state, the residual strain and the stiffness of UHPC are reduced by increasing the number of cycles and finally tends toward stability, the maximum stress remains stable. The AE analysis results can help to explain the evolution of the mechanical properties of high strain hardening UHPC under cyclic tensile loading and proves the existence of the Kaiser effect, which can be used to quantify the deformation and the degree of damage of UHPC. The stiffness of UHPC under cyclic tension is controlled by the nominal debonding length of the steel fibers. Finally, the relationship between the total tensile strain and the loading stiffness degradation ratio under cyclic tension was proposed and could be employed in evaluate of stiffness of the high strain hardening UHPC under cyclic tension in some extent.

A new development cracked chevron notched direct tension method for determining the mode I fracture toughness of rocks

A new development has been proposed to measure the tensile fracture toughness (KIc) of rocks. An innovative configuration of chevron notch was applied acting as a cracked chevron notched direct tension (CCNDT) method to determine the mode I fracture toughness, which also could be suggested as a direct method to specify the tensile fracture toughness. In this method, the initial crack is generally created in the form of a chevron notch by a simple rotary saw, and afterward, the crack is subjected to the direct tensile load. Using the chevron notch in the desired specimen is mainly due to the advantages of the sub-critical crack growth in brittle materials. In the present study, a new expression is proposed to calculate the fracture toughness of CCNDT specimens. Various finite element analyses are performed using different chevron crack geometries to obtain the minimum dimensionless stress intensity factor of the CCNDT specimen. The analytical approach based on Bluhm's slice synthesis method is also implemented for evaluating the minimum dimensionless stress intensity factor of the CCNDT specimens. Furthermore, several experiments are carried out on Hornfels using CCNDT and ASTM standard fracture toughness; single-edge notched bend (SENB) tests to verify the proposed method. The geometrical parameters of the CCNDT specimens are also provided to meet the plane strain fracturing condition.

An Experimental Investigation of the Behavior of Strain-Hardening Cement-Based Composites (SHCC) under Impact Compression and Shear Loading

The ductile behavior of strain-hardening cement-based composites (SHCC) under direct tensile load makes them promising solutions in applications where high energy dissipation is needed, such as in earthquakes, impacts by projectiles, or blasts. However, the superior tensile ductility of SHCC due to multiple cracking does not necessarily point to compressive and shear ductility. As an effort to characterize the behavior of SHCC under impact compressive and shear loading relevant to the aforementioned high-speed loading scenarios, the paper at hand studies the performance of a particular SHCC and its constituent, cement-based matrices using the split-Hopkinson bar method. For compression experiments, cylindrical specimens with a length-to-diameter ratio (l/d) of 1.6 were used. The selected length of the sample led to similar failure modes under quasi-static and impact loading conditions, necessary to a reliable comparison of the observed compressive strengths. The impact experiments were performed in a split-Hopkinson pressure bar (SHPB) at a strain rate that reached 110 s−1 at the moment of failure. For shear experiments, a special adapter was developed for a split-Hopkinson tension bar (SHTB). The adapter enabled impact shear experiments to be performed on planar specimens using the tensile wave generated in the SHTB. Results showed dynamic increase factors (DIF) of 2.3 and 2.0 for compressive and shear strength of SHCC, respectively. As compared to the non-reinforced constituent matrix, the absolute value of the compressive strength was lower for the SHCC. Contrarily, under shear loading, the SHCC demonstrated higher shear strength than the non-reinforced matrix.

Damage mechanism of ultra-high performance fibre reinforced concrete at different stages of direct tensile test based on acoustic emission analysis

The two strain-hardening stages for ultra-high performance fibre reinforced concrete (UHPFRC) under direct tensile loading are the swift strain-hardening (stage II) and widely held strain-hardening (stage III) stages. A direct tensile test with acoustic emission (AE) registration and air permeability measurement was conducted on three UHPFRC types. StageII displays more superior crack-width control, higher tensile stiffness, and better impermeability compared to stage III. Based on the analysis of three AE parameters, the damage mechanism of UHPFRC at different tensile stages was identified. The AE source location provides strong evidence of multiple randomly distributed micro-defects (<0.01 mm) at stage II. The AE average frequency and energy can distinguish three UHPFRC damage patterns (matrix cracking, fibre–matrix debonding, and fibre pullout) and evaluate their contributions at different tensile stages. Matrix cracking is dominant at stage II. Matrix cracking and fibre–matrix debonding is roughly balanced at stage III. The large-scale fibre–matrix debonding is generated accompanied by marginal fibre pullout at the strain-softening stage. The stress state of steel fibres at UHPFRC crack sections at stagesII and III was also analysed. The configuration of the strain-hardening stage is determined by the relationship between the fibre–matrix bond strength and the tensile stress of the steel fibres at the crack sections.

Modelling micro-cracking behaviour of granite during direct tensile test using cohesive GBM approach

We employed a cohesive grain-based model (GBM) to investigate the mechanical behaviour and micro-cracking response of Neuhauser granite under direct tensile loading. The microstructural properties of Neuhauser granite were incorporated into the numerical model, and hourglass-shaped specimens were generated based on the geometrical configuration of specimens. We proposed an alternative calibration procedure, and the numerical observations revealed that the cohesive GBM could accurately reproduce the failure behaviour of physical specimen. At the same time, the conventional GBM failed to return a reliable output. The calibration procedure was augmented by examining the impacts of the random distribution of minerals and tensile loading rate on the fracture behaviour of GBM specimens. Besides, pre-cracked granite was generated to investigate the influence of pre-existing cracks on mechanical and fracture behaviour. By increasing the flaw inclination angle, the peak tensile strength of both single and double flaw specimens was increased. The micro-cracks initiated from flaw tips and extended horizontally towards the side of specimens. At low flaw inclination angles, the macroscopic tensile fracture was almost identical to that of intact rock while the fracture pattern was different at high flaw angles.

Experimental Study on Damage Evolution Behavior of Self-Compacting Rubberized Concrete under Direct Tensile Fatigue Loading

Fatigue properties of self-compacting rubberized concrete (SCRC) under axial tension are rare. To explore the fatigue damage evolution of SCRC, the direct tensile fatigue test and residual mechanical test were conducted. According to the experimental results, the linear relationship between the fatigue life and secondary strain rate also exists in tensile fatigue test of SCRC, and the slope value is similar to the compressive and direct tensile fatigue test of ordinary concrete. Further, the downtrend of the residual secant modulus is more obvious than the residual strength. In addition, the direct tensile fatigue damage evolution of SCRC under different stress levels can be predicted by the Isojeh’s model.

On the use of natural curauá reinforced cement based composites for structural applications

The construction industry demand for environmentally friendly and energy-efficient solutions has driven researchers to matching non-traditional materials, as natural resources, with modern building technologies. The present work investigated the mechanical behavior and cracking mechanisms of a cementitious composite reinforced with natural curauá unidirectional fabric towards its use as reinforcement for a structural concrete element. The experimental program included direct tension, bending and cyclic tests in cement-based composites reinforced with curauá natural fabrics. Different specimen scales were tested under direct tensile loading in order to study the cracking mechanisms and strain sensitivity to size effect. A structural beam externally reinforced with this newly developed composite was tested under bending and an analytical model was proposed to corroborate the experimental tests. The proposed model gave very accurate prediction of the moment-curvature. The composite used as a structural reinforcement behaved efficiently, resulting in a rebar yielding delay, providing to the beam a higher ductility and a flexural strength gain of about 16%.

Tensile characterization of graphene nanoplatelets (GNP) mortar using acoustic emissions

This study characterizes tensile behavior of graphene nanoplatelets (GNPs) reinforced cementitious composites using acoustic emissions (AE). Two acoustic sensors were attached to dog-bone specimens that were cast using GNP nano-reinforced mortar composites at concentration levels of 0% to 0.5 % by weight of cement. The specimens were tested under direct tensile loading and the average tensile strength was calculated. AE parameters such as average frequency (AF) and rise time over amplitude (RA) were used to analyze the cracking mode. Average RA was studied to investigate the effect of GNPs reinforcement. A correlation between average acoustic energy and the tensile strength was established. In addition, the acoustic emissions data was analyzed for the localization of cracks along the length of the specimens. Results indicated that the addition of GNPs increased the tensile strength of the cementitious composites by 8%–48%. In addition, the AE analysis revealed a delay in crack initiation shear stress transfer on the matrix -GNPs interface for mortar composites with GNPs and was able to localize cracks as they are initiated.

On Applicability of Some Indirect Tests for Estimation of Tensile Strength of Anisotropic Rocks

The tensile strength of rocks plays a noteworthy role in their failure mechanism, and its determination can be beneficial in optimizing the design of the rock structures. Schistose rocks due to their inherent anisotropy in different foliation directions show a diverse strength at each direction. The purpose of this work was to compare and assess the tensile strength of phyllite, which was obtained in direct and indirect tensile tests in different foliation directions. To this end, several phyllite specimens with different foliation angles (0o, 30o, 45o, 60o, and 90o) related to the loading axis (β) were prepared. Finally, the direct tensile test, diametrical and axial point load tests, Brazilian test, and Schmidt hammer test were conducted on 188 samples. The results of the experimental tests revealed that the maximum and minimum tensile strengths in direct tensile testing tension were directly related to the angles of 0o and 90o. Also it was observed that the Brazilian tensile strength overestimated the tensile strength. Furthermore, an exponential correlation was introduced between the direct tensile strength and the Brazilian tensile strength.

Experimental analysis of the tensile behaviour of textile reinforced cementitious matrix composites using distributed fibre optic sensing (DFOS) technology

This work presents an experimental study of the mechanical behaviour of textile reinforced cementitious matrix composites (TRCMC) subjected to direct tensile loading. Distributed optical fibres embedded in the core of the composites were used as strain sensors. The measurements were based on the Rayleigh backscattering principle. The sensors were able to measure the strain in the specimen during the test at different locations because of their spatial resolution and reduced dimensions and thus make it possible to analyse and identify the behaviour of the matrix, the textile, and the textile/matrix interface. Six configurations were tested and analysed in this study: two types of matrices with three reinforcement ratios.The local and global behaviours were analysed, and the test results during the three typical stages of the mechanical behaviour of TRCMC composites are explained. The parameter controlling the behaviour of the crack propagation phase is identified, and an explanation of its influence on crack distribution is highlighted. The validity of the law of mixtures and the hypotheses of the textile/matrix interface were examined and compared to experimental results. The influence of the reinforcement ratio was also identified. The obtained results allow a better understanding of the local and global mechanical behaviours of TRCMC composites and the assessment of present theoretical hypotheses in literature related to these behaviours.

Restrained Shrinkage Mechanism of Ultra High Performance Concrete

The understanding and controlling of the restrained shrinkage is critical for the application of ultra high performance concrete (UHPC). This study made an analysis of restrained shrinkage mechanism of four kinds of UHPCs based on the direct tensile test accompanied with acoustic emission (AE) source location, free shrinkage test and restrained shrinkage test. The effects of UHPC tensile properties (strain softening or strain hardening) and high performance calcium sulphoaluminate (HCSA, a new kind of expansion agent) dosages (0%, 3% and 6% by mass of total binder) were investigated. The restrained shrinkage cracking mechanism of strain hardening UHPC and strain softening UHPC was analyzed based on AE analysis method under direct tensile loading. The results indicates that strain hardening UHPC shows multiple micro-defects to relax the restrained tensile stress marginally while strain softening UHPC shows several hairline cracks to relax the restrained tensile stress evidently. In ASTM C1581-04 ring test, the restrained shrinkage of strain hardening UHPC with HCSA dosage of 0%, 3% and 6% at 80d is 141 με, 96 με and 16 με, respectively. The HCSA expansion agent can effectively reduce the restrained shrinkage of UHPC and the influence on the structure.

Investigation on fractal characteristics of salt rock under different tensile conditions

In order to study the characteristics of fractal dimension of salt rock and the change of its physical properties under different tensile conditions, the experiment of indoor indirect tensile, direct stretching, three-point bending and tensile pressure alternately was carried out on the pure salt rock, and the whole process was monitored by acoustic emission test system and the fracture scanning was performed after the test. The results showed that: (1) the fractal characteristics of the rock fracture roughness under different tensile conditions are related to the mechanical behavior. The higher the tensile strength, the smaller the fractal dimension of the cross section, and the smaller the roughness of the section. (2) under the four tensile conditions, the fractal dimension of the acoustic emission of salt rock shows a dimension reduction with the increase of stress and the release of energy, and the dimension reduction process is related to the mechanical characteristics of salt rock. Under the alternating stress path, the fractal feature of the acoustic emission location point of the salt rock appears more late, and the fractal dimension of the peak time is smaller than that under the direct tensile loading and unloading stress path, indicating that under the alternating stress path of tension and pressure, the internal damage of the salt rock is more aggregated; (3) the fractal dimension of the roughness and the fractal dimension of the roughness under the same tension condition. The results of acoustic emission fractal dimension calculation are highly similar.

A new approach of rock tension−compression cyclic measurement

On the basis of the MTS815 rock mechanics test system, a new measurement set-up was developed to study the tension−compression cyclic behaviour of rocks. The new measurement set-up implements a completely reversed loading condition that is direct tensile stress loading and compressive stress loading. On the basis of the experimental observation, the developed measurement set-up can be easily applied for investigating the tension−compression cyclic behaviour of rock material. It is capable of monitoring the microcracking process by combining the developed measurement set-up and acoustic emission testing system. The axial misalignment of experimental set-up and the cylinder rigidity are the main factors affecting the availability of the new measurement method. When using this approach, the axial lines of the testing machine, the developed measurement set-up and the rock sample need to be aligned. The cylinder rigidity should be far greater than the stress level applied in the test.

Experimental Investigation of Mechanical Behaviorsof Self-Compacting Concrete under Cyclic DirectTension.

Self-compacting concrete (SCC) is increasingly applied in the construction industries due to its advantages of outstanding workability and eco-friendliness. However, few experimental studies on the mechanical behaviors of SCC under cyclic direct tension are available in the literature. In this work, experimental investigations of mechanical behaviors of SCC under cyclic direct tension were conducted. Especially, direct tensile load equipment was developed. It is found that the envelope stress-strain curve under cyclic direct tension is close to that under monotonic direct tension; however, it is different from that in a compressive case. It is also revealed that the ratio of unloading strain to irreversible strain is approximately linearly dependent on unloading strain. The evolutions of energy dissipation for SCC under both cyclic direct tension and compression are similar to those of normally vibrated concrete. In addition, Poisson’s ratio is observed to be nearly 0.21 for SCC. Furthermore, a damage model enabling characterization of the stress-strain curve under both monotonic and cyclic loading was proposed and verified against experimental results. Therefore, the results in this work provide original research material for studying and modelling the mechanical behaviors of SCC under uniaxial cyclic direct tension.

Delamination mechanism of GFRP sheet bonded on the reinforced concrete beams

Fiber reinforced polymer (FRP) has been developed to be applied for a strengthening of the deteriorated structures. In the form of a sheet, the FRP may be applied for the strengthening of the structures by bonding it to the concrete surface. Glass composed FRP (GFRP) sheet is most commonly used due to its relatively lower cost compared to the other FRP materials. GFRP sheet is applied externally by bonding it on the concrete surface. The strengthened structures should be monitored periodically to ensure the health of the strengthened structures. Regarding the development of monitoring system of the strengthened structure, it is important to study the delamination phenomenon of the bonded GFRP. Therefore the delamination mechanism is important to be clarified. Many studies have been done to investigate the bonding characteristics of GFRP sheet under direct tensile loading. However, the studies on the bonding characteristics of GFRP sheet on the strengthened beams due to flexural loadings are still limited. A series of concrete beams strengthened with GFRP sheet on extreme tension surface were prepared. The beam specimens ware loaded under four-point bending test gradually up to the ultimate capacity. Results indicated that prior to final delamination, a local delamination occurred which was indicated by the suddenly decreasing of an applied load. The delamination of the GFRP sheet may be initiated by the flexural cracks occurred on the beams.

STRENGTH OF COMPOSITE YARN UNDER BIAXIAL LOADING

The present paper is concerned with two types of periodic unit cell of composite yarn with different geometry but the same material properties. Their macroscopic response under tensile and compressive loading in the transverse direction and their combination are plotted in the graphs. Based on stress-strain curves the failure envelopes are constructed. A simple maximum stress criterion and linear softening law is used in the adopted progressive damage analysis to outline the softening part of stress-strain diagrams. Finally the impact of selected representative volume element is observed through the comparison of results gained for both designed periodic unit cells in the microlevel, meaning the level of yarn.

Exploratory investigation of grouted bar-in-duct connections under direct tensile load

Grouted dowel connections are widely used to resist tension induced by in-plane straining actions and to provide ductility to precast concrete load bearing wall structures. Yet, such connections have not yet been duly investigated. Current design code and standard provisions treat it similar to a simple bar-in-concrete detail. The present study reports the findings of 22 full-scale tests undertaken to examine the bond behaviour of grouted connections under monotonic tensile loading. The examined test parameters included the bar embedment length, bar material and corrugated duct. Results indicate that grouted connections behave quite differently from the bar-in-concrete model assumed in design codes, primarily due to the confinement mechanism of the duct, which provides restraint to lateral expansion of the grout. Varying the embedment length seemed to affect both the bond stress and slip, but did not influence the failure mechanism of the connections. The experimental results were used to calibrate two well-known bond-slip analytical treatments, predictions of which appear to be in good agreement with experimental results. The experimental and analytical findings provide an enhanced understanding on the behaviour of such connections, highlighting the need for revisions in future relevant design code provisions.

On the direct experimental measurement of mortar fracture toughness by a compression-to-tensile load transformer (CTLT)

The tensile or Mode I fracture toughness is one of the most important strength parameters of mortar which is usually measured experimentally in the laboratory. Several widely accepted indirect methods have been used for toughness measurement but they estimate it and do not provide the real and precise fracture toughness of mortar. In this paper, the direct tensile fracture toughness of mortar is determined through a compression to tensile load transferring device (called Compression-to-Tensile Load Transformer (CTLT)) in the laboratory. This sophisticated device is developed to use a specially prepared Bond Shaped Sample (BSS) containing edge cracks. These samples are carefully prepared and subjected to the direct tensile loading by the special loading device. The direct fracture toughness results are verified by performing the Notched Brazilian Disc (NBD) tests (the standard indirect tests). The effect of crack length and crack overlapping on the stress concentration at the crack tip is analyzed numerically using finite element method. However, comparison of these results can provide a good basis for the validity of the direct toughness result. The experimentally observed fracture propagation pattern is also in good accordance with those obtained by the standard finite element method.

A Guide for Using Mechanical Stimulation to Enhance Tissue-Engineered Articular Cartilage Properties.

The use of tissue-engineered articular cartilage (TEAC) constructs has the potential to become a powerful treatment option for cartilage lesions resulting from trauma or early stages of pathology. Although fundamental tissue-engineering strategies based on the use of scaffolds, cells, and signals have been developed, techniques that lead to biomimetic AC constructs that can be translated to in vivo use are yet to be fully confirmed. Mechanical stimulation during tissue culture can be an effective strategy to enhance the mechanical, structural, and cellular properties of tissue-engineered constructs toward mimicking those of native AC. This review focuses on the use of mechanical stimulation to attain and enhance the properties of AC constructs needed to translate these implants to the clinic. In vivo, mechanical loading at maximal and supramaximal physiological levels has been shown to be detrimental to AC through the development of degenerative changes. In contrast, multiple studies have revealed that during culture, mechanical stimulation within narrow ranges of magnitude and duration can produce anisotropic, mechanically robust AC constructs with high cellular viability. Significant progress has been made in evaluating a variety of mechanical stimulation techniques on TEAC, either alone or in combination with other stimuli. These advancements include determining and optimizing efficacious loading parameters (e.g., duration and frequency) to yield improvements in construct design criteria, such as collagen II content, compressive stiffness, cell viability, and fiber organization. With the advancement of mechanical stimulation as a potent strategy in AC tissue engineering, a compendium detailing the results achievable by various stimulus regimens would be of great use for researchers in academia and industry. The objective is to list the qualitative and quantitative effects that can be attained when direct compression, hydrostatic pressure, shear, and tensile loading are used to tissue-engineer AC. Our goal is to provide a practical guide to their use and optimization of loading parameters. For each loading condition, we will also present and discuss benefits and limitations of bioreactor configurations that have been used. The intent is for this review to serve as a reference for including mechanical stimulation strategies as part of AC construct culture regimens.