Key Engineering Materials Vols. 488-489

Abstract: A general purpose direct BEM code has been developed for three-dimensional crack problems in piezoelectric structures. Special 3D non-continuous crack tip elements and several techniques for determining crack tip parameters were implemented. To calculate the electromechanical energy release rate for a virtual crack extension, the θ-method is employed, which was originally suggested by BONNET for linear elastic materials. The paper presents the generalization and numerical realization of the θ-method to 3D piezoelectric cracks. The great advantage of the θ-method is the direct computation of energy release rate, whereas the way via K-factors and the IRWIN matrix is more complicated. The efficiency and accuracy of the technique are shown for various example problems by comparing with analytical solutions.

Abstract: Since welded constructions are widely used in engineering, a certain flaws in welded joints may occur either in process of welding or in exploitation period. Easiest way to prolong working life of such welded construction is to repair welded joint to eliminate possibility of construction failure. Process of repair welding usually gives heterogeneous welded joints because during process of repair additional material is introduced into welded joint, resulting in heterogeneity from the presence of materials in welded joint point of view. Such difference in materials usually results in yield strength difference between materials, represented with mismatch ratio, and it is commonly present in welds where high strength low-alloyed (HSLA) steels were welded. Since I butt welded joints are very common in welding, a systematic investigation of such welds is performed and presented in this paper. Therefore in this investigation the influence of present material in heterogeneous weld and geometry of weld is investigated in context of fracture resistance of welded joint represented as yield load solutions in the first place. A flaw in form of crack was implemented in such heterogeneous weld and using finite element method yield load solutions for different combinations of weld geometry and material strength are obtained and presented in form of diagrams.

Abstract: The aim of this work is to propose a non-invasive technique aimed at assessing the evolution of damage in elongated bones that could be caused, for instance, by metabolic diseases. The technique exploits signals related to mechanical guided waves traveling along the bone to characterize its mechanical properties in a total non-destructive manner. Such properties can be used as an indicator of the damage level present in the bone. The preliminary results motivate future studies

Abstract: Stress guided waves in the sonic and ultrasonic regime are acknowledged as a powerful too lto inspect pipes in a non-invasive manner.A key point of the inspection procedure is related to the dispersive behavior of guided waves, that for agiven pipe is defined by the so-called dispersion curves. Such behavior, is generally predicted bymeans ofanalytical formulations. However, when the geometry of the pipe cross-section is not axially symmetric,such as in the presence of an open internal crack running along the pipe length, analytical formulations fail. Here, the computation of the guided waves properties for such a scenario is addressed via a SemiAnalytical Finite Element (SAFE) formulation in which the open crack is modeled at the mesh level.Different crack depths are considered and their effect on the waves dispersion curves are highlighted.The results could be of interest in pipeline inspection and monitoring.

Abstract: In this paper simple engineering methods for a fast and close approximation of stress intensity factors of cracked beams and bars, subjected to bending moment, normal and shear forces, as well as torque, are examined. As far as the circular cross section is concerned, comparisons are made on the base of numerical calculations. The agreement between the present results and those previously published is discussed. New formulae for calculating the stress intensity factors are proposed.

Abstract: This study is an extension of the paper by E. Viola and A. Marzani [1] where the eigenfrequencies and critical loads of a single cracked beam subjected to conservative and nonconservative forceshave been investigated. Here the aim is to analyze the dynamic stability of T cross section beams withmultiple cracks. A doubly cracked Euler-Bernoulli beam subjected to triangularly distributed subtangential forces, which are the combination of axial and tangential forces, is considered. The governingequation of the system is derived via the extended Hamilton’s principle in which the kinetic energy, theelastic potential energy, the conservative work and the nonconservative work are taken into account. Thelocal flexibility matrix for a beam with T cross-section is used to model the cracked section. The resultsshow that for given boundary conditions cracked beams become unstable in the form of either flutter ordivergence depending on the crack parameters, the nonconservativeness of the applied load as well as theinteraction of the two cracks.

Abstract: Tensile failure of metals often occurs through void nucleation, growth and coalescence. This work is concerned with the study of plastic nanovoid cavitation in face-centeredcubic (FCC) crystals at finite temperature. In particular, the Quasicontinuum (QC) method,suitably extended to finite temperatures, is taken as the basis for the analysis. We specificallyfocus on nanovoids in copper single crystals deforming in uniaxial and triaxial tension. Thecomplex structure of dislocations around the nanovoid and the evolution of stress, deformationand temperature of the sample is described in the present work.

Abstract: Total hip replacement is one of the most common techniques in orthopaedic surgery, and one of the most important surgical advances of the last XX century. Normally, implant is fixed to bone by means of a polymer material known as bone cement, building an interface between implant and bone regions. Microscopically, two interfaces can be distinguished, namely, bone-cement and implant-cement interfaces. One of the main causes of failure is implant loosening due to fatigue of one of the two microscopic interfaces. In this work, a micromechanical analysis of bone-cement interface under cyclic forces is introduced. Both bone and cement are considered using different models based on fatigue damage over a statistically representative volume element (RVE) of the microstructure. This technique allows to homogenize mechanical stresses of the RVE yielding the effective macroscopic behavior of the bone-cement interface, avoiding experimental fitting case to case, once the interface geometry and mechanical characterization of the involved phases are known.

Abstract: Nanoindentation is considered to be a very promising experimental approach to measuring the ideal shear strength since the stressed volume beneath the sharp indenter may be defect-free. The local shear component of the stress reaches its maximum value at some close distance from the indenter in the bulk. The value of the stress can reach the ideal shear strength and, consequently become high enough to nucleate dislocations. This process might be detected as a pop-in on the nanoindentation load-displacement curve. To model the nanoindentation test for that purpose, three different approaches have been used in this works. The first approach is based on the analytical Hertzian solution of the stress field beneath the nanoindenter where only a continuum mechanics is taken into account. The second concept is based on the numerical solution without crystallographic considerations and the third one respects the fact that the dislocation generation in the substrate is subjected to crystallographic rules. The aim of this article is to compare all these concepts by their application to the nanoindentation process performed on selected bcc and fcc metallic substrates.

Abstract: A precise description of the stress and deformation fields in a cracked body is provided using multi-parameter fracture mechanics based on the approximation of the fields by means of the Williams’ power series. This paper presents a detailed analysis of the stress field in a wedge-splitting test geometry specimen aimed at the calculation of coefficients of the higher order terms (up to 14) of the Williams’ expansion. The numerical study is conducted with the use of a conventional finite element package; however, for processing of the results an over-deterministic method is employed. Special attention is paid to the influence of boundary conditions of the test geometry on the values of the coefficients of the higher order terms of the Williams’ series. The results are compared to data from the literature; a strong effect of the boundary conditions is observed.