Numerical investigation of concrete subjected to high rates of uniaxial tensile loading

The present article is concerned with the response of structural concrete prisms to high rates of uniaxial tensile loading. The numerical investigation carried out is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses. This program is known to yield realistic predictions to the response of a wide range of plain- and reinforced-concrete structural forms subjected to arbitrary static and earthquake actions. Furthermore, its application has recently been successfully extended in predicting the response of plain-concrete prism elements under high rates of uniaxial compressive loading. The main feature of the FE program is that it incorporates a 3D material model which is characterized by both its simplicity and its attention to the actual physical behaviour of concrete in a structure. Its analytical formulation is based on the assumption that the material properties of concrete are independent of the applied loading rate (strain rate) thus attributing the effect of the applied loading rate on the prism's response to inertia. The validation of this assumption is based on a comparative study between numerical and experimental data which reveals good agreement. This constitutes a major departure from current thinking as regards material modelling of concrete under high-rate loading. In addition, the available data (numerical and experimental) show that the response of the concrete prism elements depends on a number of parameters linked to geometry and material properties of the structural forms under investigation as well as the testing method adopted. This dependence explains, to a significant extent, the scatter that characterizes the available experimental data, and it also suggests that both experimental and numerical results describe structural rather than material behaviour thus raising questions regarding the validity of the use of such data in the constitutive modelling of concrete-material behaviour under high-rate loading conditions.     

Structural optimization based on preconditioned conjugate gradient analysis methods

An efficient method for structural optimization is presented. Instead of classical direct decomposition methods, Preconditioned Conjugate Gradient (PCG) methods, in conjunction with two proposed starting-vector generation schemes, are used to solve the systems of linear equations associated with the finite element analysis and behaviour sensitivity analysis problems. These inherently iterative analysis procedures are then used to carry out the analyses needed at the beginning of each stage in an approximation concepts approach to structural optimization. This technique has been implemented in a research program and used to solve a collection of minimum weight truss sizing design problems subject to static deflection and stress constraints. The effectiveness of the PCG methods of analysis in structural optimization is demonstrated. Comparison among different preconditioners is made. The effect of the proposed starting-vector generation schemes is shown. The comparative merits of analytical sensitivity analysis and finite difference sensitivity analysis, when using PCG methods of analysis, are assessed. The parallel computation potential of PCG methods is discussed. Because of the iterative nature of PCG analysis methods and the prospects they offer for parallel computation, it is found that PCG analysis methods show promise in the context of structural optimization.     

Dynamic Properties of Cortical Bone Tissue: Izod Tests and Numerical Study

Bone is the principal structural component of a skeleton: it assists the load-bearing framework of a living body. Structural integrity of this component is important; understanding of its mechanical behaviour up to failure is necessary for prevention and diagnostic of trauma. In dynamic events such as traumatic falls, involvement in car crash and sports injuries, bone can be exposed to loads exceeding its structural strength and/or fracture toughness. By developing adequate numerical models to predict and describe its deformation and fracture behaviour up to fracture, a detailed study of reasons for, and ways to prevent or treatment methods of, bone fracture could be implemented. This study deals with both experimental analysis and numerical simulations of a cortical bone tissue and its response to dynamic loading. Two areas are covered: impact Izod tests for quantifying a bone's behaviour under impact loading, and a 2-D finite-element model simulating these tests. In the first part the effect of three different parameters - a cortex position, a notch depth and an energy level - on the bones tissue's response to dynamic loading was investigated. Specimens cut from anterior, posterior, medial and lateral cortex position were tested at two different levels of energy for two notch depths. In the second part, a 2D numerical model for the impact Izod test was developed using the Abaqus/Explicit finite-element software. A fully transient formulation employs an initial angular velocity of the hammer together with the real dimensions and material properties of the specimen and the impacting hammer. Three different constitutive material models - linear-elastic, elastic-plastic and viscoelastic - were implemented to compare respective results for impact parameters and fracture force. The obtained experimental results emphasize that bovine femur cortical bone has a nearly uniform fracture energy character with regard to cortex position. The simulation results showed a good agreement of the viscoelastic model with the experimental data.     

Obtaining evidence of structural safety using a fibre optical monitoring system on the example of the “Wave of Bern”

In 2004, the roof structure of the main train terminal in Bern was extended to cope with increasing passenger volume. For this reason, an innovative architectural structure was erected across the passenger platforms. Although the wave-shaped roof with its choice of light materials was designed with diligence and care, there were some remaining uncertainties to the full extent of the roof behaviour under the given wind exposure, which could not be resolved using the conventional theoretical design approach alone. To clear these minor uncertainties, the structural monitoring department of Basler & Hofmann was approached to install a fibre optical structural monitoring system on the most exposed steel/glass roof structure (GD 5) of the train terminal, so that the influence of high wind loads on its behaviour could be monitored. The ridge purlin of the main roof is subject to vertical and horizontal loads from various loads, which sometimes act on the structure with eccentricity. Therefore, the purlin is subject not only to bending but also to torsion stresses. Therefore, it was important to carry out multidirectional vibration monitoring of this ridge purlin. A variety of different sensors was used to register the static structural and dynamic behaviour. The vibrations of the purlin induced by vertical loading were measured using optical strain sensors (System OSMOS); the torsion behaviour was monitored using inclinometers and accelerometers. The latter sensors were also used to register any wind induced wobbling of the outer edges of the steel/glass roof. The correlation between wind speed and vibration behaviour was accomplished by using a wind anemometer which was installed on the roof structure. The aim of the monitoring system was to obtain a clear understanding of the behaviour of the roof, especially when it was subject to strong wind loads. The results were needed to make reliable predictions of the long term aerodynamic stability of the roof. In consequence, structural monitoring can be used on innovative structures to verify the theoretically expected behaviour and gain confidence in the stability of the structure. In the long term, monitoring can be used as effective asset management tool to optimise spending on maintenance and repair. In case of unforeseen events such as very high snow loads, monitoring could act either as alarm system or allow for the structure to being operated safely.     

Dependability modelling and evaluation by using stochastic Petri nets: application to two test cases

The systems modelling and the assessment of their performances are the two key phases of any reliability or risk analysis study. However, it is well known and admitted that most of the methods devoted to this goal are unsuitable if the physical behaviour of the systems cannot be made independent from the probabilistic behaviour, as is the case for dynamic process systems. To overcome this inefficiency, some alternative methods have recently appeared. But they are not very approachable and could be, for this reason, ignored by most of the practitioners. Among these methods, however, simulation methods should be confirmed. From this point of view, the authors propose a straightforward approach to this problem by using stochastic Petri nets on a simple and well-known dynamic system from the literature. In addition, for testing this approach, the authors have carried it out on a system with periodically tested components. The results obtained are compared to those already published, the limits of the method are underlined and its efficiency in solving this kind of problem is being examined.     

Size-scale and slenderness influence on the compressive strain-softening behaviour of concrete

In this article the compressive mechanical behaviour of quasi-brittle materials is analysed by means of experimental tests and by using an ad hoc algorithm for numerical simulations based on the Pseudo-traction and the Boundary-element methods. The experimental analysis is carried out on specimens with three different size-scales, three different values of slenderness and two boundary conditions. The numerical analysis was carried out by taking into account the initial random crack distribution, considering the mutual crack interaction, the crack–boundary interaction and the internal friction between the faces of the cracks. The numerical results, in good agreement with the experimental data, highlight the characteristic strain-softening behaviour of quasi-brittle materials, and the influence of size-scale and slenderness on the structural response. By observing the evolution of the crack patterns, it is possible to emphasize, both experimentally and numerically, the transition from crushing to splitting collapse by increasing the specimen slenderness, as well as the transition from ductile to brittle behaviour by increasing the specimen size-scale.     

Mechanical and structural studies of low density polyethylene

This paper is one of a series concerned with the complete characterisation of the creep behaviour of oriented polymers, the correlation of creep behaviour with other mechanical properties and the interpretation of such data in the light of present structural knowledge. Sheets of oriented low-density polyethylene were prepared from initially isotropic sheets by cold-drawing, cold-drawing followed by heat-treatment at 55° C, drawing at a temperature of 55° C and hot-drawing at temperatures in the range 90 to 100° C. At each draw ratio, specimens were cut at angles of 0°, 45° and 90° to the draw direction. For each specimen, the variation of longitudinal and lateral strain with time, during uniaxial tensile creep at 20° C, was measured simultaneously by direct extensometer methods, for a wide range of applied stresses. All the materials exhibited complex anisotropic non-linear viscoelastic behaviour. The methods of presenting such data are discussed and the results are presented in some detail. Many similarities in the creep behaviour of the cold- and hot-drawn materials are noted. However, marked differences are apparent in the non-linearity and creep rate of the 45° specimens from these two materials at high draw ratio. These, and other effects found at high draw ratio, are discussed with reference to the structural studies reported in part 1. At low draw ratio, it is shown that the anomalous behaviour of the modulus in the draw direction, reported previously for cold-drawn material, may also be found in the hot-drawn material, although at a different creep time. On the basis of obvious differences in wide-angle X-ray patterns other workers had previously predicted that the anomalous mechanical behaviour of cold-drawn LDPE was probably unique. The anomalous behaviour of the hot-drawn material is also explained in terms of the structures discussed in part 1.     

The continuous crack flexibility model for crack identification

The presence of a crack in a structural member introduces a local flexibility that affects its dynamic response. Moreover, the crack will open and close in time depending on the loading conditions and vibration amplitude. The changes in dynamic characteristics can be measured and lead to an identification of the structural changes which eventually might lead to the detection of a structural flaw. The results of various independent evaluations of changes in the natural frequency of vibrations of cracked structural elements are reported. A crack model of a continuous flexibility, found with fracture mechanics methods using the displacement field in the vicinity of the crack developed recently is used here. The analytical results for the cracked elements behaviour based on the continuous crack flexibility vibration theory were correlated with numerical solutions, the lumped-crack beam vibration analysis and experimental results obtained on aluminium and steel beams with open cracks.     

Prediction of steel fibre reinforced concrete under flexure from an inferred fibre pull-out response

Steel fibre-reinforced concrete (SFRC) is being used in a variety of structural applications, yet there is still considerable debate how to express and evaluate flexural toughness for design purposes. This is holding back the material's development as a permanent structural material. Existing beam and slab test methods have problems with variability or their application in structural design. Furthermore, existing models of SFRC flexural behaviour do not fully capture what happens at the cracked section in terms of the fibre-matrix interactions. Typical of these approaches is the modelling of the tension zone from single fibre pull-out tests, which is problematic in measurement of the load-displacement relationship, the interaction of groups of fibres and the extensive testing required to cover all permutations of fibre geometry. An alternative approach is proposed where the average pull-out response of the fibres bridging the cracked zone is inferred from flexural beam tests. The characteristic load versus crack-mouth opening displacement behaviour for a particular fibre concrete then forms part of the stress and strain/displacement profile in a flexural analysis to predict moment capacity in a design calculation. The model is explained together with its validation by comparing the predicted load-displacement response for a range of fibre volumes in sprayed and cast SFRC. It is concluded that the analysis of beam load/deflection curves to infer the fibre pull-out response is a viable approach. It offers a promising solution to the need for a flexural design model combined with a practical method of characterizing the tensile contribution of steel fibres.     

RBS/channelling study of ion-implanted and proton-exchanged LiNbO3:Nd:MgO planar waveguides

We present a structural study of two Nd³⁺/MgO codoped LiNbO₃ waveguides fabricated by two different methods: ion-implantation and proton-exchange. Rutherford backscattering spectrometry in combination with channelling was used to study the influence of the waveguide fabrication method on the possible modification of the LiNbO₃ crystal structure within the waveguiding layer. The results show that the ion-implanted waveguide mainly maintains the same properties as the virgin crystal, apart from a deterioration of a thin surface layer, whilst the proton-exchanged waveguide exhibits a quite different behaviour, although its surface is less damaged.     

Main problems encountered in the study of cryogenic generators

There is general agreement worldwide on the basic structure of cryogenic generators, with a hot stator and a rotor with superconducting field winding at very low temperature (4 to 5 K). The main problems arising from cryogenic generator development especially concern the rotor: the problem of the electromagnetic and mechanical behaviour of the shield required for protecting the superconductor against the induction changes associated with any possible disturbances; the problem of the superconductor and helium cooling system behaviour associated with the particular operating conditions of a cryogenic generator; the problem of structural materials; technological problems arising from helium admission in the rotor. At the present stage of studies, one can positively state that the cryogenic generator is feasible, but its competitiveness with respect to conventional generators has not been established. It is obvious that its chances of development could be greatly increased if a certain number of parallel research efforts succeeded in the following fields: development of superconductors with a high critical temperature; development of structural materials, both metallic and non-metallic; reduction of refrigerator costs; and increased reliability.     

Der Zugscherversuch Für Metallklebstoffe. Neue Untersuchungsmethoden und Ergebnisse

The Single Lap Shear Test for Structural Adhesives – New Experimental Methods and Results The single lap shear test with the thick adherend specimen, a method for measuring the shear stress – shear strain behaviour for structural adhesives, is evaluated in a combined experimental and analytical study. New methods are described for obtaining mechanical characteristics needed by engineers for the design and analysis of adhesivebonded structural joints. Results from the use of this methods are shown to give consistent shear modulus and strength data as a function of adhesive bond thickness.     

Design oriented analysis of imperfect truss structures—part II—approximate analysis

An approximate analysis method for calculating local and system geometric imperfection influences on the displacements, stresses and buckling load of imperfect truss structures, is developed. The structural response, as a function of the design variables, is expressed in this approximate analysis which is much faster and cheaper than the ‘accurate’ analysis. The accurate analysis for a particular design is used as a base for each approximate analysis which then gives the response information needed for other designs in the neighbourhood about the base point design. Comparison of the approximate behaviour predictions with accurate results based on the more refined analysis methods shows that the quality of the proposed approximate analysis procedure is adequate for guiding a structural design procedure. It is evident that the approximate analysis is capable of describing the nonlinear behaviour of the structure, but when nonlinearity increases, the quality of the approximate analysis tends to deteriorate.     

Time gap effect on bond strength of 3D-printed concrete

An advancing technology that combines the concrete extrusion with a motion control to create structures with complex geometrical shapes without the need for formwork is known as 3D concrete printing. Since this technique prints layer by layer, the time taken to reach the same position in the subsequent layer is important as it will create an anisotropic property that has a weaker tensile strength at the bond interface of the two printed filaments. Through rheological measurement, which reveals the material deformation and flow behaviour, it is possible to examine the material structural build-up due to time-gap effect by measuring at different time delay. This paper focuses on investigating the time-gap effect on the printed filament with rheological and observation at macroscopic-scale to understand the material behaviour of the initial and subsequent printed layer during its fresh phase. Rheological experiment findings reveal that the tensile strength of the printed specimen is correlated to the material modulus at the initial layer.     

STATISTICAL EVALUATION OF FRACTURE TOUGHNESS TEST DATA

Abstract— Application of fracture mechanics for verification of the fracture resistance of structural components and pipelines is well established in the offshore industry, and development of reliability analysis methods and calibration of assessment procedures is now in progress. One important parameter in fracture mechanics evaluations is the fracture toughness—a parameter characterized by large scatter, sensitivity to fabrication conditions and large costs related to testing. This has raised the need for efficient methods for characterization of the fracture toughness test data in terms of characteristic values and parametric distribution functions.
Two relevant data sets are investigated with respect to their statistical properties and the ability of different distribution functions to fit the data.
Three methods for estimation of characteristic values for fracture toughness are described. Their relevance and capabilities are investigated by numerical simulations with sample data sets drawn randomly from a larger population.
It is concluded that the distribution functions based on a physical model of the fracture have disadvantages such as unstable behaviour for small data sets and lack of capability in providing certain estimations. The log-normal distribution showed a stable and predictable behaviour for sample sizes down to 3, the methods for calculation of distribution parameters and characteristic values with specified confidence levels are demonstrated to be satisfactory.     

Multi-scale detection of failure in planar masonry thin shells using computational homogenisation

This paper presents a computational homogenisation-based technique for localisation detection in planar masonry shells. A computational homogenisation procedure is used for the in-plane and the out-of-plane behaviour of masonry walls taking the periodicity of the material into account. The quasi-brittle nature of the masonry constituents results in initial and damage-induced (evolving) anisotropy properties with localisation of damage at both the structural and fine scales. Using a closed-form damage model at the mesoscopic scale, it is shown that a structural scale localisation criterion based on the acoustic tensor adapted to shell kinematics allows to detect the structural scale localisation. This detection identifies average preferential cracking orientations consistent with the stacking mode of masonry for both in-plane and out-of-plane failure. This approach is illustrated by examples of bed joint and stair-case failure, and its subsequent integration in multi-scale nested computational schemes is discussed.     

Non-linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension

The recent development of inorganic based composites as low-cost materials in reinforced concrete structural strengthening and precast thin-walled components, requires the creation of models that predict the mechanical behaviour of these materials.Textile Reinforced Mortar (TRM) shows complex stress–strain behaviour in tension derived from the heterogeneity of its constituent materials. This complexity is mainly caused by the formation of several cracks in the inorganic matrix. The multiple cracking leads to a decrease in structural stiffness. Due to the severe conditions of the serviceability limit state in structural elements, the prediction of the stress–strain curve is essential for design and calculation purposes. After checking other models, an empirical nonlinear approach, which is based on the crack control expression included in the Eurocode 2, is proposed in this paper.Following this scope, this paper presents an experimental campaign focused on 31 TRM specimens reinforced with four different reinforcing ratios. The results are analysed and satisfactorily contrasted with the presented non-linear approach.     

建筑结构用再生混凝土水平受力构件研究进展

建筑结构水平构件混凝土用量较大,且对混凝土力学性能要求相对不高,更适合再生混凝土的结构化应用。本文总结分析了国内外学者及笔者所在课题组近年来在再生混凝土材料性能、钢-再生混凝土黏结性能以及钢筋再生混凝土板、梁和钢-再生混凝土组合板、组合梁等水平受力构件力学性能方面的研究成果。结果表明:研究学者针对再生混凝土的材料性能进行了三十余年的试验研究与机理分析,积累了充足的试验数据,并提出了相对可靠的预测模型,为其在建筑结构中的应用奠定了基础;通过对钢筋再生混凝土板与梁的受弯、受剪以及长期性能的试验研究与理论分析,提出了成套的钢筋再生混凝土水平受力构件设计方法,现已纳入了再生混凝土结构技术规程;近十年来,研究学者对钢-再生混凝土组合板与组合梁的受弯、受剪以及长期性能进行了试验研究与有限元分析,发现再生混凝土在组合结构中的应用是可行的,但相关研究尚待深入。未来仍需对再生混凝土水平受力构件设计方法的可靠度进行系统研究,并拓展再生混凝土水平受力构件的疲劳性能、耐久性能及抗火性能等研究。     

Implicit modelling and mesh refinement for complex structures

When analysing complex structures it is rarely feasible to represent explicitly all the structural features which may contribute significantly to local or global structural performance. This paper presents a variety of methods for representing features which are below the economic analysis scale. Two approaches are described—referred to as implicit stiffness calculation and implicit mesh refinement. The former is essentially approximate and uses relatively simple formulae and computations. The latter involves quite extensive subsidiary calculation, but can lead to numerically exact simulation of fine mesh behaviour in so far as it affects neighbouring structure. Stiffness equivalence principles are stated and a number of computation schemes are presented and illustrated with simple two-dimensional examples.