Mater Struct Research Articles

Retraction: Fracture investigation of U‐notch made of tungsten–copper functionally graded materials by means of strain energy density, H. Mohammadi, H. Salavati, Y. Alizadeh, A. Abdullah, F. Berto, Fatigue and Fracture of Engineering Materials and Structures, Volume 40, issue 12, pp. 1984–1993

The above article, published online on 12 April 2017 in Wiley Online Library (wileyonlinelibrary.com) has been retracted by agreement between the authors, the journal and John Wiley and Sons, Ltd. The retraction has been agreed due to considerable overlap with a previously published article (1).(1) Mohammadi H, Salavati H, Alizadeh Y, Berto F. (2017) Prevalent mode II fracture assessment of inclined U‐notched specimens made of tungsten‐copper functionally graded material. Theoretical and Applied Fracture Mechanics, 89, 90–99.REFERENCEMohammadi H, Salavati H, Alizadeh Y, Abdullah A, Berto F. Fracture investigation of U‐notch made of tungsten–copper functionally graded materials by means of strain energy density. Fatigue Fract Engng Mater Struct. 2017;40(12):1984‐1993. doi:10. 1111/ffe.12616

Exact Solutions and Symmetry Analysis of a Boussinesq Type Equation for Longitudinal Waves Through a Magneto-Electro-Elastic Circular Rod

In this work the longitudinal wave equation through magneto-electro-elastic circular rod is studied analytically. Painlevé test is performed to check integrability of the equation. Exact solitary wave solutions are found by homogeneous balance method and Tanh method and are plotted using suitable values of physical parameters. A solution similar to an earlier related work (Xue et al. in Smart Mater Struct 20:105010, 2011; Ma et al. in Int J Comput Sci Issues 10 (1): 62–68, 2013) is obtained. However a new solution is obtained as well. Lie symmetry analysis has been done and similarity reduction of the equation is presented. Dependence of the solution on the longitudinal wave velocity and the dispersion parameter related to the transverse Poisson effect are obtained numerically. Numerical results related to phase velocity etc for different materials have been derived and compared.

Multiple cracking model in a 3D GraFEA framework

In this work, a thermodynamically consistent three-dimensional (3D) small strain-based theory to describe the deformation and fracture in quasi-brittle and brittle elastic solids is presented. The description of fracture at a material point resembles the microplane fracture approach developed by Bažant et al. (J Eng Mech 126(9):944–953, 2000, J Eng Mech 122(3): 245–254, 1996), but the present theory has the following novel features: (a) a probabilistic description of fracture propagation is used, developing evolution equations for the probability of a microcrack occurring at a given location and (b) a kinematical approach to modeling crack opening and closing. The new 3-D constitutive theory, in which elements were recently proposed by Srinivasa et al. (Mech Adv Mater Struct 80(27–30):2099–2108, 2020), has been computationally implemented within a Graph-based Finite-Element Analysis (GraFEA) framework developed by Reddy and colleagues (Khodabakhshi et al. in Meccanica 51:3129–3147, 2016, Acta Mech 230:3593–3612, 2019), and it has also been implemented into the dynamics-based Abaqus/Explicit (Reference manuals. Simulia-Dassault Systemes, 2020) finite element program through a vectorized user–material subroutine interface. Our computational approach for fracture modeling is intra-element-based, which is central to the GraFEA approach rather than inter-element fracture, as is done in cohesive zone-based numerical methods, together with selective non-locality where the non-locality is only for probability evolution motivated by population dynamic models that allows us to perform efficient implementation of the code without special elements or other numerical artifacts. Several homogeneous deformation cases for fracture in cementitious and brittle elastic materials were modeled, and the response obtained from the constitutive theory and its finite element implementation are qualitatively similar to that obtained in the literature. In particular, we show that our computational procedure is able to model crack closure in solids in a robust, relatively simple and elegant manner instead of relying on a previously developed method of decomposing the stored energy into “positive” and “negative” portions (Amor et al. in J Mech Phys Solids 57(8):1209–1229, 2009, Miehe et al. in Int J Numer Meth Eng 83:1273–1311, 2010).

Load–Displacement Behavior of Helical Shape Memory Alloy Spring Actuators with Small Spring Diameter to Wire Diameter Ratios

A design methodology for fabricating and predicting the load–displacement behavior of shape memory alloy (SMA) spring actuators has been previously created and validated (Nicholson et al., Smart Mater Struct 23(12):13–23, 2014). However, it was found in this work that for springs with a spring index (ratio of spring diameter to wire diameter) below five, the load–displacement response of shape memory spring actuators no longer fits the previously established model. This paper explores the use of a correction factor in the governing equations to account for the plastic deformation that occurs when fabricating springs with very small spring indices. The plastic deformation induces an effective reduction in the volume of the transforming material, thus reducing the load-bearing and actuation capabilities of the spring. A semi-analytical solution to this problem is found which can be used to predict the load–displacement behavior. This is accomplished through a reduction in the effective wire diameter based on the approximate shear strain during loading. This approach is consistent with observations during thermomechanical cycling in SMAs, where the phase transformation remained mostly unhindered despite large residual inelastic and/or plastic strains, and has direct application in the design of actuators with small spring indices by quantifying the drop in actuation force and stroke.

Nonlinear dynamic analysis of temperature-dependent functionally graded magnetostrictive sandwich nanobeams using different beam theories

In the present study, nonlinear dynamic analysis of an embedded functionally graded sandwich nanobeam (FGSNB) integrated with magnetostrictive layers is investigated. The core layer of FGSNB, which is subjected to a time-dependent transverse load, is made of a two-constituent functionally graded material that the material properties of the functionally graded nanobeam are temperature dependent and assumed to vary in the thickness direction. The modified couple stress theory is taken into account so as to consider the small-scale effects. The surrounding elastic medium is simulated as visco-Pasternak foundation to study the effects of damping, shear and elastic effects of surrounded medium. Using energy method and Hamilton’s principle, the governing motion equations and related boundary conditions are obtained for different beam theories. Finally, the differential quadrature as well as Newmark-β methods are employed to obtain the nonlinear dynamic response of the functionally graded magnetostrictive sandwich nanobeam (FGMSNB), and therefore deflection-response curves are plotted to study the effects of small-scale parameter, surrounding elastic medium, magnetostrictive layers, geometrical parameters, material compositions of core layer, environment temperature and boundary conditions and nonlinear terms graphically. The results indicate that the magnetostrictive layers play a key role on the dynamic behavior of the FGMSNB. Moreover, comparing results with those obtained in Ghorbanpour Arani and Abdollahian (Mech Adv Mater Struct, 2017. https://doi.org/10.1080/15376494.2017.1387326 ) shows the importance of the nonlinear terms. The obtained results in this paper can be used as sensor and actuator in the sensitive applications.

Design and Analysis of Artificial Muscle Robotic Elbow Joint Using Shape Memory Alloy Actuator

Artificial muscle is one of the more prominent topics in modern robotics as it can be applied to robotic arms, electric vehicles and wearable robots (Shahinpoor et al. in Smart Mater Struct 7:15–30, 1998; Jani et al. in Mater Des 56:1078–1113, 2014). The advantages of Shape Memory Alloy (SMA) artificial muscle are lightness and high energy density. The high energy density allows the actuator to make powerful motions. Meanwhile, SMA wire contracts 6% of its length, which means that the required displacement cannot be achieved by a simple connection. To resolve these disadvantages, the SMA wires are coiled in a diamond-shaped structure. If the electric current is given by contracting wires in the longitudinal direction, the actuator can exert force and displacement in the diagonal direction. As the crossed tendon finds its minimal length when actuated, the rotation angle converges to 90°. Parameters related with the rotating motion were selected, such as SMA wires’ diameter and length, distance between the crossed part and elbow part, size of the diamond-shaped structure, friction, etc. To determine the maximum force of the actuator, a graphical method was used, which is similar to the yield strength determination (0.2% offset). Because the robotic elbow joint is connected by the tendon, the connections between links are flexible, and without motor it does not generate any sound or noise during operation. The robotic elbow joint using the SMA actuator is designed and analyzed, which can rotate 86.7° and generates maximum 56.3 N force.

Modeling of Hydrogen Effects on the Thermomechanical Behavior of NiTi-Based Shape Memory Alloys

NiTi Shape Memory Alloys (SMAs) orthodontic wires are subject to complex chemical loading in oral cavities. In the worst case, fractures are observed. Hydrogen effects are suspected, by analogy with the hydrogen embrittlement in Ti alloy systems. Several mechanisms have been observed for the case of steels, including hydride formations, hydrogen-enhanced-strain-induced vacancies (HESIV), hydrogen-enhanced-decohesion (HEDE), and hydrogen-enhanced-localized-plasticity (HELP). A degradation of the mechanical properties of NiTi arches appears due to the presence of hydrogen. Hydrogen atoms, which come from the chemical environment of buccal cavity, are supposed to diffuse in the interstitial sites naturally present in the crystallographic structure of NiTi SMAs as the same way than in steels. For instance, the maximum strain decreases, the area of the hysteresis becomes smaller with the increasing of hydrogen concentration in SMA archwires and fatigue life becomes shorter. Accounting for effects of hydrogen diffusion on the NiTi behavior, a coupled chemo-thermomechanical constitutive model needs to be formulated. The work of Lachiguer et al. (in: Smart Mater Struct 25(11):1–11, 2016) proposes material parameter dependencies to the normalized concentration of hydrogen in a NiTi constitutive law. The main limitation of this model is that the hydrogen concentration can only be considered as homogeneous. Nano-indentation tests carried out on NiTi wires charged with hydrogen highlighted an heterogeneous distribution of hardness (which is related to the hydrogen concentration). It becomes then necessary to take into account the gradient of hydrogen distribution. To this end, the weak form of equilibrium equations for each field (thermal, mechanical and chemical) through 2D domain is discretized and numerically solved by finite element method. A special finite element with coupled degrees of freedom (displacements, temperature, and hydrogen concentration) is developed and implemented in the Abaqus finite element software through the UEL subroutine. Numerical tests with complex loadings are carried out. Obtained results are discussed showing the relevance of the adopted approach.

Experimentations on the retrofitting of damaged Single Step Joints with Self-Tapping Screws

When assessing the roof of existing buildings, engineers may be confronted with structural joints badly preserved, for instance the damaged Single Step Joint (SSJ) located at the foot of timber trusses. Since the early appearance of failure modes in this traditional carpentry connection may lead to the collapse of the whole timber truss, the retrofitting of damaged SSJ is then required as an economically-viable intervention to stabilize the roof structure. In consequence, the retrofitting of damaged SSJ with Self-Tapping Screws (STS) has been conducted through the Experimental Campaign in order to explore further different possibilities offered by this recent intervention technique (Sobra et al. in Historical earthquake-resistant timber framing in the mediterranean area, pp 359–369, 2016). To this end, two strategies, noted R1 and R2, have been proposed to retrofit the SSJ specimens with STS, which had been previously damaged due to both failure modes, namely the crushing at the front-notch surface and the shear crack in the tie beam (Verbist et al. in Mater Struct, 2017. https://doi.org/10.1617/s11527-017-1028-4 ). Afterwards, the SSJ specimens retrofitted with STS have been tested under monotonic compression in the rafter in order to pull out their mechanical behaviour encompassing the failure modes, the force–displacement response, the stiffness of the connection, and the ultimate normal force in the rafter. By comparing the mechanical behaviour of retrofitted SSJ specimens with the initial ones from Verbist et al. (2017), the performances of both retrofitting strategies with STS have been discussed. Furthermore, the impact of the shear row splitting on the mechanical behaviour of retrofitted SSJ has been evaluated, by providing some warnings to engineers when intervening in existing timber trusses featuring natural damage such as the shrinkage splitting.

Mechanical behaviors of hierarchical cellular structures with negative Poisson’s ratio

An investigation of the mechanical behaviors on hierarchical re-entrant honeycomb structures was undertaken using finite element method. The hierarchical structure with a hierarchy order n (n ≥ 1) was constructed by replacing each vertex of a re-entrant hexagonal structure of hierarchy order n − 1 with a smaller re-entrant hexagon with identical geometry aspect ratio. The Poisson’s ratio and energy absorption capacity of re-entrant structures of different hierarchy orders were studied under various compression velocities. The minimum Poisson’s ratios of the first-order (n = 1) and second-order (n = 2) hierarchical re-entrant structures were − 1.581 and − 1.823, respectively; they were 32.9 and 53.2% lower than that of a zeroth-order hierarchical structure (i.e., conventional re-entrant hexagon). The second-order hierarchical structure exhibited the highest rate of increase in energy absorption capacity with an increasing compression velocity. The plateau stresses of the first- and second-order hierarchical structures were lower than that of the zeroth-order hierarchical structure; however, the second-order hierarchical structure exhibited the highest energy absorption capacity at high compression velocity rate (v > 40 m/s). The energy absorption capacities of the first- and second-order hierarchical structures proposed in the present study are 1.5 and 1.8 times, respectively, higher than those of the hierarchical re-entrant structures proposed in our previous work (Li et al. in Smart Mater Struct 26:025014, 2017).

Corrigendum

Fatigue & Fracture of Engineering Materials & StructuresVolume 40, Issue 12 p. 2204-2204 CORRIGENDUMFree Access Corrigendum This article corrects the following: Short-crack modelling of the effect of corrosion pits on the fatigue limit of 12% Cr steel G. Härkegård, Volume 38Issue 9Fatigue & Fracture of Engineering Materials & Structures pages: 1009-1016 First Published online: April 14, 2015 First published: 04 April 2017 https://doi.org/10.1111/ffe.12593AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Short-crack modelling of the effect of corrosion pits on the fatigue limit of 12% Cr steel. Vol. 38, Issue 9, 1009–1016, Article first published online: 14 April 2015. Errors in two equations in Härkegård (2015)1 are corrected here. Equation (12), on p. 1011 erroneously reads it should read Equation (17), on p. 1012 erroneously reads it should read References 1Härkegård, G. (2015) Short-crack modelling of the effect of corrosion pits on the fatigue limit of 12% Cr steel. Fatigue Fract Engng Mater Struct, 38, 1009– 1016. doi: 10.1111/ffe.12305 Volume40, Issue12December 2017Pages 2204-2204 ReferencesRelatedInformation

Non destructive assessment of in situ concrete strength: comparison of approaches through an international benchmark

Strength assessment of concrete in existing structures is a key issue. Many non destructive techniques (NDT) are available which can provide information about the material condition and several approaches can be used to derive strength estimates from NDT test results. However, common practice suffers from many drawbacks: (a) it is difficult to ascertain the level of accuracy/confidence of concrete strength estimates, (b) one lacks established guidelines for estimating the concrete strength variability, (c) the best way to combine NDT methods and coring remains an open issue. The RILEM TC 249 “Non destructive in situ strength assessment of concrete” is addressing these problems. A benchmark was carried out in order to compare (a) how experts define and can carry out an NDT investigation program and (b) how experts derive strength values from the NDT measurements. The benchmark was based on synthetic simulations which reproduced a synthetic data set corresponding to a grid of twenty 3 m-high columns defining a single-storey building structure. The comparison of the various strategies and the analysis of results enabled identification of the most influential parameters that define an investigation approach and influence its efficiency and accuracy. A special emphasis was placed on the magnitude of the measurement error. A model of the investigation strategy is proposed, that will be detailed in future guidelines. The reader is invited to refer to a companion paper (Alwash et al. in Mater Struct, 1. doi: 10.1617/s11527-016-0962-x ), based on more extensive Monte-Carlo simulations of the same case-study, which analyzes and quantifies the efficiency of the investigation strategies with respect to both average strength and strength variability assessment, in order to draw conclusions based on rational analyses rather than on the element of chance.

Shape factors for concrete shrinkage and drying creep in model B4 refined by nonlinear diffusion analysis

The source of shrinkage and drying creep is the drying process. From the diffusion analysis of drying one can estimate the shrinkage strain. Same as drying, the shrinkage times scale as the square of the effective specimen thickness (or size), D, commonly characterized by the volume-surface ratio. But there is also an additional effect of cross section shape. In the creep and shrinkage prediction model B4 (a new Draft RILEM Recommendation, Mater Struct 48:753–750, 2015), the shape effect is taken into account by shape factor $$k_{\rm{s}}$$ multiplying D. However, because of the strong nonlinearity of the diffusion equation for drying, the optimal $$k_{\rm{s}}$$ values depend also on the environmental humidity. In model B4, as well as its predecessors since 1975, the $$k_{\rm{s}}$$ values have been specified for typical shapes, i.e. the slab, cylinder, prism, sphere and cube, with values calculated approximately for only one relative humidity—65 %. Here the $$k_{\rm{s}}$$ values for the same typical shapes are calculated with greater accuracy and for different environmental humidities—30, 40, 50, 60, 70 and 80 %, which allows interpolation in between. The $$k_{\rm{s}}$$ values for the typical shapes range from 1.00 to 1.41.

Influence of the cross section shape on the behaviour of SRG-confined prismatic concrete specimens

The effectiveness of the steel-reinforced grout (SRG) jacketing technique in increasing both strength and deformation capacity has been substantiated by experimental evidence (Thermou and Pantazopoulou in Fib Struct Concr J 8: 35–46, 2007; Thermou et al. in ECCOMAS Thematic Conference, 2013; Thermou et al. in Mater Struct J, 2013). What it has not been addressed yet is the influence of the cross section shape on the behaviour of SRG-confined prismatic unreinforced concrete specimens. An experimental study was carried out where 18 concrete small scale specimens were tested to failure under concentric uniaxial compression load. A single layer of the steel-reinforced fabric was applied to circular, square and square specimens with rounded edges. Test results demonstrated that both strength and deformation capacity increased as the shape of the cross section changed from square to circular. In case of the square specimens, the gain in compressive strength was satisfying, whereas the ductility increase was more significant compared to that of the square specimens with rounded edges. An analytical expression for the lateral confining pressure exerted by the composite system was derived based on the assumption that the steel cords were treated as well-anchored steel stirrups placed at a distance equal to the spacing between the steel cords. The ability of the various steel and fiber-reinforced polymer confinement models from literature to predict the normalized compressive strength of non-circular specimens confined with SRG jackets was also explored.

Assessment of fibre content and 3D profile in cylindrical SFRC specimens

The inductive method from Torrents et al. (Mater Struct 45(10):1577–1592, 2012, 1) is used to assess the fibre content and orientation in steel fibre reinforced concrete. Despite several advantages, the method presents limitations. On one hand, it was conceived for cubic specimens, which complicates its application in existing structures given the difficulty to obtain cubic cores. On the other hand, it only shows the fibre orientation in the three axes perpendicular to the faces of the specimen, being impossible to derive the orientation in other directions with these results. Moreover, it only gives average values without providing any information on the scatter or probabilistic distribution of the fibre orientation—a parameter that may be essential to for design and to explain differences in the behaviour of concretes apparently with the same average fibre distribution. The objective of this paper is to propose an assessment of the fibre content and orientation profile using the inductive method and cylindrical specimens. First, a modification of the method is proposed. Then, new equations are deducted to generalize the test to samples with different shapes and to assess the anisotropy level as well as the directions with the maximum and the minimum fibre contribution. An extensive experimental program and FEM simulations are performed to validate and determine the accuracy of the formulation developed. The results show that the execution of only one additional measurement per specimen is enough to determine the fibre probabilistic profile in all in-plane directions with a high accuracy.

An Experimental Study on Fracture Energy of Plain Concrete

In this study, the concrete fracture energy was obtained using the three point notched beam test method developed by Hillerborg et al. (Cem Concr Res 6(6):773–782, 1976). A total of 12 notched concrete beams were tested under two different loading conditions: constant stroke control and constant crack mouth opening displacement (CMOD) control. Despite individual fracture energies obtained from the two different loading conditions showing some variation, the average fracture energy from both loading conditions was very similar. Furthermore, the results obtained support the idea that a far tail constant “A” could change the true fracture energy by up to 11 %, if it is calculated using CMOD instead of LVDT. The far tail constant “A” is determined using a least squares fit onto a straight line according to Elices et al. (Mater Struct 25(148):212–218, 1992) and RILEM report (2007). It was also observed that the selection of the end point can produce variations of the true fracture energy. The end point indicates the point in the experiment at which to stop. An end point of 2 mm has been recommended, however, in this study other end points were also considered. The final form of the bilinear softening curve was determined based on Elices and Guinea’s methods (1992, 1994) and RILEM report (2007). This paper proposes a bilinear stress–crack opening displacement curve according to test results as well as the CEB-FIP model code.

Peeling tests for assessing the cohesion and consolidation characteristics of mortar and render surfaces

Standard principles for applying the so-called Scotch tape test for assessing the surface cohesion characteristics of materials were explained in an earlier paper on testing stone consolidation effects (Drdacký et al., Mater Struct 45(4):505–520, 2012). The method has also been used for evaluating the strengthening of surface mortar layers, though it is not supported by any standard or reliably verified recommendations for its application. As in the case of stone testing, its applicability can be overestimated, and arbitrary use of the test without adequate knowledge and sufficient understanding can lead to results and assessments that are not intercomparable or reproducible, and which are also in many cases incorrect and severely biased. This paper reports on the results of a recent study aimed at establishing limits for the application, reliable procedures, and a “standard” protocol for testing the cohesion characteristics of brittle and quasi-brittle materials, mainly mortars. This paper discusses the effects of various surface qualities of mortar or rendering, and suggests recommendations for performing peeling tests on mortar surfaces. It presents a mathematical procedure suitable for evaluating the results. Again the recommendation is to perform repeated peeling on an identical place on a surface in order to eliminate the effect of the natural decrease in the material detached from the subsurface layers, which might be incorrectly interpreted as a consolidation effect. The examples and the validation measurements presented here include laboratory measurements.

Isogeometric analysis of laminated composite and sandwich plates using a layerwise deformation theory

We present an isogeometric finite element formulation for static, free vibration and buckling analysis of laminated composite and sandwich plates. The idea behind this work is to associate an isogeometric analysis (IGA) with a layerwise theory [A.J.M. Ferreira. Analyis of composite plates using a layerwise deformation theory and multiquadrics discretization. Mech Adv Mater Struct 2005;12(2):99–112]. Isogeometric analysis based on non-uniform rational B-spline (NURBS) basic functions were recently proposed to preserve exact geometries and to enhance very significantly the accuracy of the traditional finite elements. B-splines basic function (or NURBS) is used to represent for both geometric and field variable approximations, which provide a flexible way to make refinement and degree elevation. They enable us to achieve easily the smoothness with arbitrary continuity order compared with the traditional FEM. The layerwise theory assumes a first-order shear deformation theory in each layer and the imposition of displacement continuity at the layers interfaces. This permits to remove shear correction factors and improves the accuracy of transverse shear stresses. Intensive numerical studies have been conducted to show the highly efficient performance of the proposed formulation.

Rheology of concrete: a study case based upon the use of the concrete equivalent mortar

A deeper and wider knowledge of the rheology of concrete could be obtained if a variety of absolute rheological parameters were measured. Although a valuable rheological classification can be induced from concrete rheometers, they do not give the same absolute values of the rheological parameters. Moreover, rheological tests with concrete require a large volume of material. The aim of this work is to use the concrete equivalent mortar (CEM) method (Schwartzentruber and Catherine, Mater Struct 33:475–482, 2000) to get concrete rheological information with an absolute rheometer. Additionally some graph tools that resume the results from several rheological tests are suggested for the design of concretes. CEM have been formulated to test concrete formulations with an absolute rheometer. Steady flow measurements of a CEM corresponding to a self-compacting concrete (SCC) clearly reveal characteristic non-linear viscoplastic behavior which it is not shown by conventional tests used to characterize concretes. The thixotropic behavior of a concrete is well-established using three different rheological tests that can be made with an absolute rheometer testing its corresponding CEM. These tests reveal aspects of the strength and kinetics of the micro-structure that are not observable when thixotropy of concrete is semiquantified with conventional methods. Only with a CEM is possible to make oscillatory shear tests. In this way the viscoelastic behavior of the concrete can be characterized. Results of practical interest are so obtained. For example, the necessity of vibration at rest application to the fresh concrete can be established from frequency sweep tests of CEM in the linear viscoelastic region. The correlation between rheological behaviors of a concrete and its corresponding CEM has been supported. Steady flow and thixotropy rheographs of CEM are suggested as tools for an easy and fast determination of adequate formulation for specific applications.

Locating the acoustic source in an anisotropic plate

By analyzing the arrival times of guided waves, the acoustic source in a plate is predicted. Solving this problem is important for continuous health monitoring of structures. Several techniques based on the triangulation principle have been proposed for this purpose but they do not work for anisotropic plates. The popular triangulation technique assumes that the wave velocity is the same in all directions, which is not true for anisotropic plates. An alternative method based on the optimization scheme was proposed by Kundu et al. (Kundu T, Das S and Jata KV. Point of impact prediction in isotropic and anisotropic plates from the acoustic emission data. J Acoust Soc Am 2007; 122(4): 2057–2066; Kundu T, Das S, Martin SA and Jata KV. Locating point of impact in anisotropic fiber reinforced composite plates. Ultrasonics 2008; 48(3): 193–201; Kundu T, Das S and Jata KV. Health monitoring of a thermal protection system using lamb waves. Struct Health Monit: Int J 2009; 8(1): 29–45; Kundu T, Das S and Jata KV. Detection of point of impact on a stiffened plate by the acoustic emission technique. Smart Mater Struct 2009; 18(3): 1–9) to locate the point of impact in anisotropic plates by analyzing the times of arrival of the ultrasonic signals received by the passive sensors attached to the plate. Recently, Hajzargarbashi et al. (Hajzargarbashi T, Kundu T and Bland S. An improved algorithm for detecting point of impact in anisotropic inhomogeneous plates. Ultrasonics 2010; 51(3): 317–324.) improved that technique. Following their modification, in this article, the acoustic source point in an anisotropic plate is predicted from the acoustic emission data with some additional modifications. Experiments are carried out with a carbon–epoxy plate. A parallel pre-stressed actuator is used as the acoustic source and the acoustic signals at different locations are received by adhesively bonded acoustic sensors. The source point is then predicted and compared with its actual location. Related theory and computed results of wave propagation in anisotropic plate are also presented and the theoretical predictions experimentally verified at frequencies higher than what is typically generated by the simple impact phenomenon.

The Effect of Specimen Size on Strength and Other Properties in Laboratory Testing of Rock and Rock-Like Cementitious Brittle Materials

The effect of specimen size on the measured unconfined compressive strength and other mechanical properties has been studied by numerous researchers in the past, although much of this work has been based on specimens of non-standard dimensions and shapes, and over a limited size range. A review of the published literature was completed concentrating on the presentation of research pertaining to right cylindrical specimens with height:diameter ratios of 2:1. Additionally, new data has been presented considering high strength (70 MPa) cement mortar specimens of various diameters ranging from 63 to 300 mm which were tested to failure. Currently, several models exist in the published literature that seek to predict the strength–size relationship in rock or cementitious materials. Modelling the reviewed datasets, statistical analysis was used to help establish which of these models best represents the empirical evidence. The findings presented here suggest that over the range of specimen sizes explored, the MFSL (Carpinteri et al. in Mater Struct 28:311–317, 1995) model most closely predicts the strength–size relationship in rock and cementitious materials, and that a majority of the empirical evidence supports an asymptotic value in strength at large specimen diameters. Furthermore, the MFSL relationship is not only able to model monotonically decreasing strength–size relationships but is also equally applicable to monotonically increasing relationships, which although shown to be rare do for example exist in rocks with fractal distributions of hard particles.