The nitrogen gas tension test. Part 2: failure mechanism

In this study, the mechanism of concrete failure in the nitrogen gas tension test was investigated through a series of experiments. First, the nitrogen gas tension test was carried out two types of specimens: solid cylinders and hollow cylinders. The test results clearly showed that there was no significant difference in the gas pressure at failure between the solid specimen and the hollow specimen. Since a tension crack occurring on the surface of the concrete specimen at a gas pressure almost equal to the tensile strength of the concrete might play a key role in understanding the failure mechanism, a failure criterion based on linear elastic fracture mechanics (LEFM) was consequently developed. The nitrogen gas tension test was newly carried out on cylindrical specimens with circumferential notches of various depths. Though LEFM was found to be useful in developing an understanding of the mechanism of concrete failure, the experimental results indicated that it was not really valid for specimens with notch depths deeper than some critical size (critical notch depth). However, based on the experimental observation that the concrete specimen failed at its tensile strength at notch depths smaller than the critical notch depth, a modified LEFM based failure mechanism was proposed taking into account the notch sensitivity of the concrete.     

Size effect and boundary conditions in the Brazilian test: Experimental verification

The effect of both size specimen and boundary conditions on the splitting tensile strength, determined from the Brazilian test, were studied experimentally. A total of 110 splitting tests of granite and mortar specimens were performed, using cylindrical and prismatic specimens of sizes between 17 mm and 300 mm. To analyze the effect of the boundary conditions, the specimens were tested with different widths of load bearing strip in the range of size recommended by the standards. The influence of the rupture mode (stable or unstable crack propagation) on the splitting tensile strength was also explored. The results of the tests were compared with the theoretical predictions obtained from a closed form analytical expression based on the cohesive crack model. The validity of the classical limit strength theory for larger size specimens was also analyzed. The results indicate that the splitting tensile strength depends strongly on specimen size and on the boundary conditions of the test. As the size of the specimen increases and the relative width of the bearing strip decreases, the splitting strength tends asymptotically to the minimum value coincident with the tensile strength. The dependence of the Brazilian test on the specimen size and boundary conditions closely follows the predictions of the cohesive crack model.

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Résumé L'influence de la taille et des conditions de bords sur la résistance mécanique à la traction indirecte (essai brésilien) ont été déterminés à partir des essais de compression diamétrale. On a essayé un total de 110 éprouvettes, en granite et en mortier. Les échantillons employés étaient prismatiques et cylindriques, de dimensions comprises entre 17 mm et 300 mm. Afin d'analyser l'effet des conditions d'essai, les échantillons ont été testés avec différentes largeurs d'appuis, dans l'intervalle conseillé par la norme. L'influence du mode de rupture (propagation stable ou instable) sur la résistance mécanique à la traction a été aussi testée. On a comparé les résultats des essais avec les prévisions théoriques obtenues moyennant une expression analytique fermée qui s'appuie sur le modèle de la fissure cohésive. La validité de l'approche de la contrainte limite a été analysée pour des éprouvettes de plus grande taille. Les résultats montrent que la résistance à la traction des éprouvettes est étrotement liée à la taille et aux conditions de l'essai. Quand la taille de l'éprouvette augmente et la largeur relative de l'appui diminue, la résistance atteint une limite qui correspond à la résistance à la traction. L'essai de compression indirecte (essai brésilien) dépend de la taille de l'éprouvette et des conditions de l'essai, en accord étroit avec les prévisions du modèle de la fissure cohésive.

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Editorial Note Prof. M. Elices and Prof. J. Planas are RILEM Senior Members. Both participate in the work of RILEM Technical Committee QFS (Size effect and scaling of quasibrittle fracture) Prof. Elices also participates in RILEM TC 147-FMB (Fracture mechanics applications to anchorage and bond) and 148-SSC (Test methods for the strain softening response of concrete). Prof. G. Guinea is the 1994 Robert l'Hermite medallist.     

Size effect and boundary conditions in the brazilian test: theoretical analysis

Splitting strength determined in the Brazilian test is assumed to be a property independent of size and uniquely related to the intrinsic material strength. However, as was experimentally demonstrated by various authors, the splitting strength depends on the specimen size. In this paper, the size effect in the Brazilian test is analyzed theoretically using a nonlinear fracture model based on cohesive crack concepts and the results obtained are compared with the classical strength limit approach. Two important variables are studied: the load-bearing strip and the geometry of the specimen. From the numerical results a closed form expression is proposed, dependent on the width of the bearing strip and on geometry. The results confirm that splitting strength decreases with the specimen size, tending towards an asymptotic solution for large size specimens. Within the size range analyzed (0.1 m to 2.5 m diameter for typical concrete) the splitting strength can vary by up to 25% in cylindrical specimens and by up to 35% in prismatic square section specimens, although this size effect is strongly dependent on the load-bearing strip. For widths of bearing strip smaller than 4% of the specimen diameter, the effect of the specimen size is negligible and the splitting strength approaches the tensile strength for any practical specimen size.     

Effect of strain gradients on the size effect of concrete in uniaxial tension

A series of uniaxial tension experiments has been conducted to investigate the size effect on strength and fracture energy of quasi brittle materials like concrete and sandstone. This paper focuses on the results of the concrete tests, and specifically deals with the variation of the nominal strength for specimens of six different sizes in a scale range of 1:32. It was found that under given experimental conditions, the nominal strength strongly depended on the specimen size. More important however, is the fact that most of this size effect could be attributed to strain gradients which were present in the cross section of the specimens. These strain gradients were caused by the specimen shape, load eccentricity and material inhomogeneity. Through a combination of experimental data and a simple linear elastic analysis, the importance of strain gradients with respect to the ultimate load level could be visualized. This leads to the conclusion that studying a material size effect is not possible without taking into account structural stress/strain gradients. This revised version was published online in July 2006 with corrections to the Cover Date.     

混凝土梁式构件弹性模量的应变率效应研究

首先给出了梁式构件三点弯曲振动时材料弹性模量和跨中截面应变率的表达式,通过三点弯曲梁测试装置研究了素/钢筋混凝土梁弯曲振动时跨中最大应变率与动态弹性模量的关系,并进一步通过试验结果拟合出了考虑应变率效应的素/钢筋混凝土梁弹性模量提高系数的表达式.试验结果表明:在梁式构件的三点受弯振动中,随着跨中最大应变率由10-5/s增长到10-3/s,素混凝土梁的弹性模量增长了大约7.8%,钢筋混凝土梁的弹性模量增长了约10%.另外,由得到的拟合曲线和其他模型对比可知,应变梯度的存在一定程度上影响了混凝土材料弹性模量的应变率效应.     

Plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression

This paper presents the investigation and modeling of plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression. Also the effect of the cross-section shape of specimens on these parameters is investigated. For this purpose, the experimental results of cyclic stress–strain behavior of circular, square and rectangular concrete prisms confined by FRP-composites are presented and examined. Besides, the effect of repeated complete cycles on the plastic strain and stress deterioration is studied. The results show that the relation between plastic strain and first unloading strain is linear and depends only on the two parameters of concrete strength and the shape of cross-section, but the stress deterioration depends only on the first unloading strain. The predictive formulations for plastic strain and stress deterioration are presented and compared with the experimental results.     

Tensile cracking in concrete and sandstone: Part 2—Effect of boundary rotations

This paper contains the second part of our work on tensile cracking that has been conducted over the past years. The behavioural of concrete and sandstone subjected to uniaxial tension is decribed. In part 1 of this paper [1], a numerical lattice model is presented, which is used for explaing fracture mechanisms in heterogenous materials like concrete and sandstone. In the lattice model, the material is schemitzed as a lattice of brittle, breaking beam elements. Cracking is simulated by removing, in each step, the element with the highest stress—to—strength ratio. It was shown that the amount of detail included in the numerical model has a significant effect on the ductility in the load-displacement diagram. In this paper, special attention is given to the effect of varying the rotational stiffiness of the loading platens in uniaxial tensile tests. Experiments are presented of cylindrical specimens of various sizes loaded in tension between non-rotating (fixed) and freely-rotating boundaries. The lattice model is used for a qualitative analysis of the subject. The results suggest that the uniaxial tensile test between freely rotating boudaries yields a (safe?) lower bound for the fracture energy of concrete.     

The response of a confined gas to a thermal disturbance: rapid boundary heating

Summary An inert compressible gas confined between infinite parallel planar walls is subjected to significant heat addition at the boundaries. The wall temperature is increased during an interval which is scaled by the acoustic time of the container, defined as the passage time of an acoustic wave across the slab. On this time scale heat transfer to the gas occurs in thin conductive boundary layers adjacent to the walls. Temperature increases in these layers cause the gas to expand such that a finite velocity exists at the boundary-layer edge. This mechanical effect, which is like a time-varying piston motion, induces a planar linear acoustic field in the basically adiabatic core of the slab. A spatially homogeneous pressure rise and a bulk velocity field evolve in the core as the result of repeated passage of weak compression waves through the gas. Eventually the thickness of the conduction boundary layers is a significant fraction of the slab width. This occurs on the condition time scale of the slab which is typically a factor of 10⁶ larger than the acoustic time. The further evolution of the thermomechanical response of the gas is dominated by a conductive-convective balance throughout the slab. The evolving spatially-dependent temperature distribution is affected by the homogeneous pressure rise (compressive heating) and by the deformation process occurring in the confined gas. Superimposed on this relatively slowly-varying conduction-dominated field is an acoustic field which is the descendent of that generated on the shorter time scale. The short-time-scale acoustic waves are distorted as they propagate through a slowly-varying inhomogeneous gas in a finite space. Solutions are developed in terms of asymptotic expansions valid when the ratio of the acoustic to conduction time scales is small. The results provide an explicit expression for the piston analogy of boundary heat addition.     

Sensitivity of plastic strain localization zones to boundary and initial conditions

 The plastic strain localization phenomena are studied with the use of the theory of viscoplasticity and finite element method. The aim of the study is to check if the strain localization is sensitive to variation of initial and boundary conditions. Total and local plastic deformation energy as candidate response functionals for sensitivity analysis are discussed by selected numerical examples of wave propagation problems. The paper considers sensitivity of the form and intensity of plastic strain localization to the variation of the above parameters. Different response functionals for structures demonstrating plastic strain localization were proposed and studied. Received: 4 February 2002 / Accepted: 10 October 2002     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.     

An effect of sudden circumferential strain on the laminar boundary layer on a rotating cylinder in an axial flow

Summary The laminar boundary layer which develops on a rotating thin cylinder fitted with an aft-section rotating with an angular velocity different from that of a fore-section is examined numerically. This problem concerns a relaxation process of the boundary layer subjected to a sudden circumferential rate of strain. Two methods are adopted: one is to regard the flow on the aft-cylinder as perturbations of that on the fore-section, and the other is to approximate the discontinuous change in the angular velocity by the cumulative normal distribution function. It is shown that the flow fields are largely influenced by the degree of favorable pressure gradient produced in the boundary layer. Especially, in case of flow passing onto the aft-section with a smaller angular velocity, adverse pressure gradient is induced immediately after the junction between two cylinders, and when the degree of the discontinuity is increased, flow separation can be provoken.With 11 Figures     

Thermo-hygro-mechanical behavior of early-age concrete deck in composite bridge under environmental loadings. Part 2: strain and stress

Restraint of volume change in concrete accompanies the development of stress in concrete structures. When concrete is placed in the field, the stress development becomes very complicated at early ages due to the hydration and environmental interaction. This study investigates quantitatively the effects of various factors on the stress variation of early-age concrete decks in composite bridges under environmental loading. The test members were made to exhibit the early-age behavior of a composite bridge. The effects of parameters related to thermal and drying shrinkage stress were analyzed through a numerical model and compared with measured data. The risk of transverse cracking in early-age concrete decks was evaluated in the numerical parametric study. The present study provides better understanding of the behavior of early-age composite bridge under environmental loading, which can be efficiently utilized to reduce the risk of cracking at early ages.     

Fracture energy and strain softening of concrete as determined by means of compact tension specimens

Fracture mechanics parameters of concrete are determined by means of the compact tension (CT) test. The effects of ligament length, rate of loading and concrete composition on the specific fracture energy G_F and the strain-softening diagram are investigated. As a first approximation of the real softening behaviour of concrete, a bilinear strain softening diagram is used in a finite-element analysis. A parameter study shows that several bilinear diagrams can represent the real behaviour equally well. With the bilinear softening diagram, a good agreement between both calculated and measured load-displacement curves and G_F-values is obtained. The determined strain-softening diagrams are transformed into a normalized presentation. For each investigated testing condition, characteristics shapes of this normalized strain-softening diagram are obtained.     

The effect of defects in tubes: Part 1. Mode I delamination resistance

This paper investigates the use of coupon specimens to study the severity of defects in filament would tubes. Composites based on two types of resin are studied, an epoxy and a vinylester. First, the properties of the unreinforced resins are measured. Next the properties of flat filament wound unidirectional glass fibre reinforced composite plates using these resins are determined, and in particular their delamination resistance under mode I loading. These results are then compared with those determined on specimens taken from filament wound tubes with different winding angles (±30°, ±45° and ±60°) and diameters.     

The effect of single-step low strain and annealing of nickel on grain boundary character

Combinations of a low tensile strain and annealing temperature have been successfully processed to double the fraction of special boundaries, Σ_sp, in commercially pure (C.P.) nickel. Processing sequence of one step of 6% strain followed by a 900 °C anneal for 10 min, has resulted a special boundary fraction (3 ≤ Σ ≤ 29) of 74.7%, compared to the as-received material of 36.5%. A large proportion of this increased in special boundaries were found to be made up of annealing twins Σ3 and its twin variants Σ3ⁿ. Deformation applied to the sample alone without heat treatment, caused the fraction of special boundaries, to decrease, the Σ_sp value falling, for example to 25.6% at 12% strain. Energy minimization processes such as grain boundary migration and grain growth were suggested as means of enhancing the formation of annealing twins.