Direct Tension Research Articles

Deep learning-based classification of damage-induced acoustic emission signals in UHPC

Acoustic emission (AE) is advantageous in studying and monitoring the damage state of UHPC, and its performance can be further improved if various damage-induced AE signals are classified accurately. The deep learning method is promising to enhance the classification accuracy, but it is limited by the absence of the corresponding AE dataset. In this study, fiber pullout tests and direct tension tests (DTTs) were performed to explore the time–frequency features of various damage-induced AE signals in UHPC. The AE sources can be classified into five categories: matrix cracking, fiber debonding, fiber sliding, fiber scraping, and matrix spalling. A corresponding AE dataset was established after manual pre-labeling, pseudo-labeling with transfer learning, and mix-up data augmenting. Several lightweight convolutional neural networks (CNNs) were trained from scratch based on the dataset, with modified time–frequency spectrograms as data input. The performances of CNN-based classification models are superior to traditional shallow-learning methods. The popular lightweight CNN structure of ResNet18 achieves the highest overall accuracy of 93.94% among adopted CNN structures such as GoogleNet, ResNet18, EfficientNet-b0, and MobileNetV2. The AE signal classification results of DTTs show that fiber-pullout damage and matrix cracking occur intensively at the strain-hardening phase and increase with fiber content. That suggests the increasing fiber bridging and multi-crack propagation in the uniaxial tension of UHPC with higher fiber content.

An experimental investigation on fatigue characteristics of granite under repeated dynamic tensions

Rock engineering is prone to fatigue failure under cyclic disturbances caused by repeated dynamic loadings such as earthquakes, blasting and impacts. These cyclic dynamic loadings could be tensile under repeated implosions , excavation induced unloading and reflection of compressive waves at free surfaces or interfaces. However, compared with cyclic dynamic compression, the rock fatigue characteristics under cyclic dynamic tension remain poorly understood. To investigate the dynamic tensile fatigue characteristics of rock, cyclic dynamic Brazilian tests and direct tension tests were conducted on granite using the split Hopkinson pressure bar and the split Hopkinson tension bar, respectively. The testing results showed that the peak stress of granite decreases, while the peak strain increases with increasing impact number under a given impact stress. The fatigue thresholds of granite in the cyclic dynamic Brazilian and direct tension tests are 0.71 and 0.49 times the critical impact stress, respectively, indicating that the rock is more prone to fatigue failure under dynamic direct tension. The total dissipated energy for fatigue failure of rock increases linearly with fatigue life. The damage accumulation features defined by energy dissipation and strain are distinct, and the damage calculated by strain can display nonlinear characteristics of damage evolution. In addition, with increasing impact stress, the failure surface roughness of the specimens in cyclic dynamic Brazilian tests increases, while that in direct tension tests decreases. The findings in this study could facilitate the understanding of rock fatigue characteristics and provide a basis for stability evaluation and disaster prevention in rock engineering.

Notes on further refining de Sitter swampland conjecture with inflationary models

In this paper, we challenge some inflationary models with the recent new conjecture of the swampland program, namely, further refining the de Sitter swampland conjecture. So far, different inflation models have been studied with refined dS swampland conjecture in various conditions. Some of these models were in line with this conjecture, and some were in direct tension with it. Therefore, a modified model that combines the first and second conditions of refined dS conjecture was proposed. It has a limitation on its constant parameters such as a , b = 1 − a and q . We challenge several inflation models with the new conjecture, which contradicts the refined dS conjecture. We discuss the constraints and adjustments of each of a , b = 1 − a , and q and the compatibility or incompatibility of the inflation models with further refining the de Sitter swampland conjecture. We will compare the results with the models studied by refined dS conjecture. We conclude that many inflation models are fully compatible with the mentioned conjecture. If the new conjecture is accurate, these inflationary models live in the landscape instead of the swampland. • Further refining de sitter swampland conjecture. • Minimal gauge inflation model. • Minimal plateau inflationary model. • Gauged Nambu–Jona–Lasinio inflation. • limitations applied to its free constant parameters.

Numerical Investigation of Brittleness Effect on Strength and Microcracking Behavior of Crystalline Rock

Brittleness has a significant influence on rock failure under compression; however, the mechanism is rarely comprehensively discussed. This study numerically investigates the brittleness effect on microcracking behavior of crystalline rock using a grain-based model implemented into a two-dimensional particle flow code, with a focus on the discussion of how rock brittleness affects the failure mechanism. The simulated failure mode changes from tension to shear with decreasing rock brittleness, which is consistent with previous laboratory test results. As the brittleness gradually decreases in the model, the grain boundary (GB) tensile crack to shear crack ratio increases, and the corresponding fractures change from vertical or subvertical to an angle about 45° along the vertical direction. The propagation and coalescence of generated microcracks result in a transition of failure pattern from splitting to shear under uniaxial compression with a decreasing brittleness level in the rock. A transition from GB tensile crack to shear crack is also observed under direct tension when the brittleness index gradually decreases. The tension to shear transition mechanism is closely related to the relative strength of the mineral grain and mineral bonding. The relative strength of mineral and mineral bonding could be used as a parameter to characterize rock brittleness from a microscale viewpoint.

Evaluation of the significance of different mix design variables on asphalt mixtures’ cracking performance measured by laboratory performance tests

Cracking is one of the primary distresses for asphalt pavements. This study is motivated to investigate the significance of the various mix design variables in determining the cracking performance of asphalt mixtures including both intermediate and thermal cracking performance, contributing to a better understanding of the mix design to improve the mixture cracking performance and promoting the development and implementation of the performance related specification and performance based specification. Total 20 mixtures were tested and evaluated in this study. The most current state of the practice mixture performance tests, including the Complex Modulus (E*) test, Semi Circular Bending (SCB) test, Disk Shaped Compact Tension (DCT) test and Direct Tension Cyclic Fatigue (DTCF) test were conducted on these mixtures for characterization of their cracking performance. The advanced statistical analysis methods were employed on the generated data base to investigate the significance of the mix design variables on determining the cracking performance parameters measured from the laboratory tests. The correlations between the various cracking performance parameters measured from the tests were also evaluated in this study. The results show that the binder related mix variables play the significant role for determining both the low temperature and durability cracking performance of asphalt mixture. For evaluation of the mixture fatigue cracking performance, several aggregate related variables in addition to binder variables are identified as the significant factors. Correlation analysis shows that mixture rheological based cracking parameter G-Rm does not show the strong correlations with neither the facture indices measured from the DCT and SCB tests nor the fatigue parameters measured from the DTCF test. This study also develops a simplified linear regression model as a predesign tool for evaluation and prediction of mixture cracking performance in general.

Experimental Investigation into the Capacity of Concentrically Loaded Steel Connections with Pretensioned High-Strength Bolts and Longitudinal Fillet Welds in Combination

This paper presents the results of an experimental study aiming to investigate the behavior of steel connections that combine pretensioned high-strength bolts and longitudinal fillet welds on a common faying surface. A total of 75 double-shear tension splices were tested under direct tension loading to quantify the effect of various connection variables on the load-deformation behavior of the connection. These variables include the (1) bolt pattern (2×2 and 2×3), (2) bolt size (3/4 in. and 1 in.), (3) bolt grade (ASTM F3125 Grade A325, A490, and F1852), (4) bolt pretensioning method (turn-of-nut and tension control bolts), (5) faying surface class (Class A and B), and (6) weld/bolt strength ratio. The variation in the connection characteristics covered a wide range of weld/bolt strength ratios from 0.50 to 2.00. The bolts were installed in oversized holes, and the specimens were assembled in a negative bearing condition to allow for a maximum slip distance. The load-deformation behavior of the combination connections was recorded and compared to that of the bolted- and welded-only control specimens. In all tests, the addition of welds increased the capacity of the connection. The investigation shows that the capacity of the combination connection with pretensioned high-strength bolts and longitudinal fillet welds can be computed by adding the capacities of the individual connecting elements while considering the strain compatibility.

Method of formulating 3D-printable strain-hardening alkali-activated composites for additive construction

One of the major limitations of the current 3D-concrete-printing technology is the incorporation of reinforcement. Furthermore, there is a need to decrease the ecological footprint of printable concrete. As a possible solution for these challenges, this paper presents a 3D-printable strain-hardening alkali-activated composite (3DP-SHAAC) that shows pseudo-ductile behaviour under direct tension. The developed 3DP-SHAAC is composed of a one-part (just-add-water) alkali-activated binder made of slag (GGBFS), fly ash (FA) and solid activators. The one-part alkali-activated binder eliminates the need for elevated temperature curing and handling of corrosive alkaline liquids. At first, an optimum matrix was identified by studying the effects of FA to GGBFS ratio on the rheological properties and compressive strength. Subsequently, the optimum matrix was reinforced by PVA fibres to make the 3DP-SHAAC, and printing performance and rheological properties were evaluated. In addition, the influences of curing temperature on the compressive, flexural and tensile performances of the printed specimens were also investigated. The results were compared with those obtained for the mould-cast specimens. The 3DP-SHAAC exhibited superior flexural performance, higher tensile strength, and comparable tensile strain capacity to the mould-cast counterpart. Further, the curing temperature had influence on the mechanical properties of both 3D-printed and mould-cast SHAACs. The underlying reasons for the differences are discussed.

Desiccation characteristics and direct tension attributes of thin clayey soil containing discrete natural fibers

The use of thin clayey soil as a liner plays an important role in many geotechnical and geo-environmental engineering applications, such as open channel and reservoir sealant, contaminant barrier etc. Their functional performance and sustainability depend primarily on the desiccation characteristics of these liners and barriers. A number of studies have been undertaken to quantify the degree of improvement achieved by using natural and synthetic fiber reinforcement. However, there is a lack of studies to understand the desiccation behavior of reinforced clay. This study aimed to explore the desiccation and cracking behavior of clayey soil reinforced with two natural fibers (coir and jute fiber) in addition to the degree of improvement in tensile strength. A series of direct tension and desiccation cracking tests have been conducted in the laboratory on clay-coir and clay-jute fiber mixes. The results demonstrate that when coir and jute fibers are used, the tensile strength of fiber-reinforced soil rises by up to 475 percent and 215 percent, respectively, when compared with the tensile strength of unreinforced soil at the same moisture content. Desiccation test results also show that blending of fibers reduces the breadth and depth of cracks significantly. The characteristics of unreinforced and fiber-reinforced clayey soil under desiccation and direct tension are briefly discussed in this paper. Findings of the present study will be important for professionals dealing with clay liners and trying to reduce cracking problems associated with drying soil.

Performance Evaluation of Asphalt Mixtures with 100% EAF and BOF Steel Slag Aggregates Using Laboratory Tests and Mechanistic Analyses

The purpose of this paper is to evaluate fatigue cracking and rutting performance of two asphalt mixtures with 1) Electric Arc Furnace (EAF) and 2) Basic Oxygen Furnace (BOF) steel slag as 100% recycled aggregate in comparison to hot-mix asphalt concrete mixture (HMA). Fatigue cracking performance was assessed through 3D FEM structural analysis methods of Layered Viscoelastic Continuum Damage (LVECD) based on Viscoelastic Continuum Damage (VECD) material modeling derived through laboratory dynamic modulus and direct tension cyclic fatigue tests. Rutting resistance was evaluated by using laboratory Incremental Repeated Load Performance Deformation (IRLPD) and full-scale Accelerated Pavement Tests (APT). The results of laboratory testing and mechanistic analysis showed better rutting resistance of asphalt pavement with EAF aggregates in comparison to HMA pavement while pavement with BOF aggregates exhibited about the same performance. Additionally, asphalt pavement with BOF aggregates, HMA, and pavement with EAF aggregates manifested fatigue cracking performance in descending order. The findings of this study can be used as a guide for the development of recycled asphalt mixtures containing slag aggregates acquired from steel production.

Modified softened membrane model for ultra-high-performance fiber-reinforced concrete solid and hollow beams under pure torsion

Limited research has focused on the development of rational theoretical models for evaluating the pure-torsional behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams. To fill this research gap, this study aimed to develop a modified softened membrane model for torsion (SMMT). Based on the direct tension force transfer model, a UHPFRC tensile constitutive model was derived by considering the fiber pull-out behavior. The proposed constitutive model was implemented in the SMMT, and the modified SMMT was found to be useful in predicting the pure-torsional behavior of UHPFRC beams with solid and hollow cross-sections. Moreover, the theoretical torsional responses were compared with the experimental results, and the experimental data reported in previous studies were used to verify the applicability of the proposed model. The results indicate that the estimated analytical responses matched well with the experimental outcomes, and the modified SMMT considered the influences of both the wall thickness and excellent UHPFRC material properties. The proposed model is expected to be effective for further investigation of the torsional mechanism and design methodology of UHPFRC members under pure torsion.

An Experimental Investigation and Computer Modeling of Direct Tension Pullout Test of Reinforced Concrete Cylinder

Executing the obligation of strengthened concrete is essential in investigating load exchanges from concrete to the inner reinforcing bar. The bond–displacement conduct and extreme pullout quality for pullout samples are essential information related to the durability of RC structures. The slip in the interface is basically due to a contrast in stresses between concrete and reinforcement. This distinction brings about the start of the split in encompassing concrete. This study examined the simple pullout solid 3D cylinder model strengthened by a reinforced steel bar, considered a line element for bond–slip conduct. The non-linear finite element model utilizing ANSYS software was established to concentrate on the concrete and steel reinforcement bond. Material nonlinearity because of cracking, crushing of concrete, and the steel reinforcing bar’s yielding were investigated. Test results showed that: a prediction model for early-age bond stress–slip relationship between steel bars and concrete was proposed based on modeling, which showed good agreement with test results. The precision of this model is explored by contrasting the finite element numerical analysis and that anticipated from test consequences of pullout examples. Immense homogeneity between the model and test results was found. This study could provide more accurate bond properties for structural analysis and design.

Steady-state deformation of asphalt concrete

• An expression has been obtained for the steady-state deformation rate of the asphalt concrete. • The steady-state deformation rate depends on the viscosity, a stress, a temperature and an activation energy. • The graphs for “the steady-state deformation rate – a temperature” are similar. • “The stress substitution principle” has been proposed. • The test for creep is an alternative method for determination of the materials viscosity. The steady-state deformation rate of an asphalt concrete is investigated in details within a wide range of a stress variation (from 0.0081 MPa to 3.0 MPa) and a temperature variation (from + 60 °C to −24 °C). A hot dense fine-grained asphalt concrete of type B accepted for the investigation has been prepared with the use of a road viscous bitumen (4.8 % by weight of a dry mineral material) of the grade 100–130 under penetration. The bitumen was produced by the Pavlodar petrochemical plant (Kazakhstan) by the direct oxidation method from an oil of the Western Siberia (Russian Federation). The investigation of the steady-state deformation of the asphalt concrete has been performed by testing of samples (50x50x150 mm) for creep at direct tension. A load and a temperature have been kept as the constant ones from the beginning to the end of testing of all the samples (241 units). The tests of the asphalt concrete samples for creep have been performed in a device, which has been specially invented and assembled. It has been found out that the tested asphalt concrete is deformed under the impact of a constant stress and at a constant temperature during some period in a steady-state regime, i.e. it is deformed with a constant rate; the dependence of a steady-state deformation rate on a stress is described by power function with a high accuracy at all the considered temperatures; a fundamental expression has been obtained which connects the steady-state deformation rate with its viscosity, stress, temperature and activation energy of the viscous deformation; a simplified expression has been obtained for the steady-state deformation rate of the asphalt concrete for the practical use; a similarity has been shown for the dependence of the steady-state deformation rate on a temperature at various stresses in semi-logarithmic coordinates; “the stress substitution principle” has been formulated. The results obtained in this work have shown that the viscous flow and the steady state deformation (steady-state creep) of the elastoviscoplastic materials as the physical phenomena can be explained on the basis of the kinetic theory of liquids (Ya.I. Frenkel) and the theory of rate processes (H. Eyring). The test for the creep can be considered as an alternative method of the experimental determination for the viscosity of the materials.

Comparison between Direct Tension, Four-Point Flexure, and Simplified Double-Punch Tests for UHPC Tensile Behavior

Comparison between Direct Tension, Four-Point Flexure, and Simplified Double-Punch Tests for UHPC Tensile Behavior

Experimental investigation on the performance of engineered spiral fiber: Fiber pull-out and direct tension tests

Recent researches indicate promising characteristics of Engineered Spirally Deformed steel Fiber (SDF) for reinforcing cementitious matrixes. However, the available research includes only limited SDF pull-out tests (mostly vertically aligned fibers) and flexural beam tests on Spirally Deformed Fiber Reinforced Concrete (SDFRC). This paper performed a complete single SDF pull-out and several direct tension tests on dog-bone SDFRC specimens with a proposed fully hinged apparatus to fill the research gaps. Various test parameters were considered in the SDF pull-out study, including matrix strength (20 MPa and 30 MPa) as well as fiber embedment length (10 mm, 15 mm, and 20 mm) and inclination angle (0˚, 15˚, 30˚, 45˚, and 60˚) of the fiber. SDFRC strength (20 MPa and 30 MPa) and fiber volume fraction (0.15 % and 0.25 %) were considered in the direct tension tests of SDFRC. Spiral fibers presented promising energy absorption capacity. Consistent with the outcomes of other fiber pull-out studies, the current study found that the matrix strength positively correlates with peak and post-peak pull-out load, while the fiber embedded length and inclination angle correlated inversely with loads. However, unlike other fully deformed fibers, the inverse correlation between the fiber embedded length and loads is considerably less pronounced. The direct tension tests revealed that such a fiber sustains load bearing even in significant cracks (widths ≥ 15 mm).

Island mirages

We point out a loophole in the proof that the Island Finder conditions [1] are sufficient for the existence of islands. We explore examples which satisfy the conditions, but have no islands, which we call island mirages. We also describe and give resolutions to two new puzzles associated with islands, including a direct tension with the quantum Bousso bound.

Experimental methods for the analysis of the tensile behavior of concrete joints

AbstractAdhesion plays an important role in the evaluation of hydraulic structures with unreinforced concrete joints. The experimental determination of joint tensile strength as a quantifiable parameter is not standardized, resulting in a variety of test setups found in literature. The present paper highlights advantages and disadvantages of three of the most common tension tests for plain concrete and concrete joints through both theoretical and laboratory experimental analysis on specimens with artificial joints. Splitting tension tests were found to be inexpensive but tend to overestimate the adhesive strength of weak joints. Direct tension tests require an elaborate test setup but may yield information on the tension softening behavior. Pull‐off tests stand out for their ability to yield in‐situ results but deliver inconclusive results.

Experimental evaluation and component model for single anchored blind-bolted concrete filled tube connections under direct tension

This paper presents an experimental programme of single anchored blind-bolted concrete filled square steel tube connections under tension to understand the joint behaviour, with emphasis on combined failure mode. A total of eight full-scale specimens were fabricated and tested. Parameters including infill concrete, length and diameter of anchored blind-bolt, tube wall thickness and concrete strength were considered. The experimental programme is accompanied by bolt preload tests and a range of material tests for all the elements in the connection. Primarily, three failure modes of the connection were identified, viz., tube wall deformation, bolt fracture and combined failure. It was observed that, apart from the beneficial effect of bolt anchorage length in enhancing the connection performance, the combined failure mode may be preferred over other failure modes for higher ductility and collapse prevention. Concrete strength is identified as the primary influential factor determining the failure modes. A component model based on spring theory is developed for prediction of global force-deformation behaviour of the bolted connection. Based on this analytical model, the strength and stiffness of such a complex connection can be predicted with good accuracy.

Optimal design of a low-cost high-performance hybrid fiber engineered cementitious composites

Engineered cementitious composites (ECC) is a fiber reinforced high-performance cementitious material with an ultra-high tensile strain capacity (above 3%) and excellent durability. However, such excellent strain capacity far exceeds some engineering requirements and is usually not fully utilized. Meanwhile, its high cost limits it application only in local engineering. In this study, a low-cost high-performance hybrid fiber ECC (HF-ECC) with both polypropylene (PP) fiber and polyethylene (PE) fiber was manufactured. Both macroscopic and microscopic mechanism analyses were conducted to investigate the effect of fiber characteristics and proportions on the performance of HF-ECC. The results show that the ultimate tensile strain of HF-ECC can reach 5% even the volume fraction of PP and PE fibers account for 1%, respectively. Its fiber cost is 43.8% lower than that of pure PE-ECC, which reduces the corresponding total cost by 27.5%. This paper presents an in-depth investigation of the characteristics of HF-ECC, direct tension, compression, fiber-bridging, pore structure, fiber distribution, and fiber dispersion. Finally, the optimal design of the HF-ECC is conducted through pseudo-strain-hardening indices and multi-criteria evaluation.

Bond Behavior of Steel Cords Embedded in Inorganic Mortars.

This paper investigates the bond behavior of steel cords embedded in inorganic matrices. A series of pull-out tests were carried out on individual galvanized steel cords embedded in either a cementitious or lime-based mortar matrix and the corresponding bond-slip relationships were derived. The quality of bond between cord and mortar was found to be critically affected by the workability of the mortar and its ability to create adequate composite action along the entire embedment length of the cord. The more workable lime-based mortar was found to guarantee a better interaction with the steel cord, in terms of initial bond stiffness, maximum bond strength, and post-peak behavior. The experimentally derived bond–slip relationships were subsequently integrated in a 3D non-linear finite element framework and used to determine the constitutive relationship of a surface-based cohesive contact between cord and mortar. The cohesive bond behavior was used to conduct a series of parametric studies on cords embedded in a lime-based mortar and examine the stress development within specimens with cords of different embedment lengths and subjected to different loading conditions (i.e., pull-out and direct tension). The active ‘Stress Transfer Zone’ was found to be about 125 mm, while an ‘Effective Transfer Radius’ of approximately 3.5–4 mm was identified. The numerical investigation implemented in this paper enabled one to study key interaction properties of steel reinforced grouts and can assist the design of more effective strengthening solutions.

Evaluation of Mode-I Fracture Toughness of Asphalt Mixtures with Symmetric Geometry Specimen at Intermediate Temperature.

Mode I fracture (tensile type) is the common cracking mode of asphalt pavements, which is caused by thermal cyclic loading or traffic. Some studies allow the analysis of the fracture modes by means of standardized tests, some of which are limited, difficult, with little repeatability or do not generate an adequate tension state. In this paper, mode I fracture toughness of asphalt mixtures with symmetric geometry specimens at intermediate temperature is evaluated. Experimental results from direct tension test and simulations on asphalt mix specimens subjected to intermediate temperatures of 10, 20 and 30 °C, mode I load rates (0.5, 1 and 2 mm/min) and notches (2 and 3 cm) were compared to find the variables that reflect the operating conditions of the asphalt mix. Results showed that shear stresses are 8.12% lower in the simulations with respect to the tests, while the load-deformation curves show 30% and 35% variation, where the temperature of 20 °C, the notch of 2 cm and the loading speed of 1 mm/min are the conditions that best represent the stress state of the test; moreover, it manages to consider the elastic and viscous components of the material.