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  • 1.
    Atashipour, Seyed Rasoul
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Girhammar, Ulf Arne
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Challamel, Noël
    Université de Bretagne Sud, UBS – Institut Dupuy de Lôme, Centre de Recherche.
    Stability analysis of three-layer shear deformable partial composite columns2017In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 106-107, p. 213-228Article in journal (Refereed)
    Abstract [en]

    This paper is focused on the effect of imperfect bonding and partial composite interaction between the sub-elements of a box-type column on the critical buckling loads. The box column is modelled as a symmetric three-layer composite structure with interlayer slips at the interfaces, based on the Engesser-Timoshenko theory with uniform shear deformation assumptions. Linear shear springs or slip modulus is considered at the interfaces to model the partial interaction between the sub-elements of the structure. The minimum total potential energy principle is utilized to obtain governing equations and boundary conditions. A direct analytical solution of the original governing equations is presented for obtaining exact buckling characteristic equation of the three-layer partial composite column with different end conditions including clamped-pinned end conditions. Also, the coupled equations are recast into an efficient uncoupled form and shown that there is a strong similarity with those for the two layer element. It is shown that the obtained formulae are converted to the known Euler column formulae when the slip modulus approaches infinity (i.e. perfect bonding) and no shear deformations in the sub-elements are considered. A differential shear Engesser-Timoshenko partial composite model is also employed and critical buckling loads, obtained from an inverse solution method, are compared to examine the validity and accuracy level of the uniform shear model. Comprehensive dimensionless numerical results are presented and discussed

  • 2.
    Banco, N.
    et al.
    EPS-University of Girona.
    Gamstedt, E.K.
    Kungliga tekniska högskolan, KTH.
    Asp, Leif
    Costa, J.
    EPS-University of Girona.
    Mixed-mode delamination growth in carbon-fibre composite laminates under cyclic loading2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 15, p. 4219-4235Article in journal (Refereed)
    Abstract [en]

    Delamination growth under fatigue loads in real composite components generally develops in a non-constant propagation mode. The aim of the investigation described in this article was to develop a model capable of predicting the fatigue delamination growth in a general case, under varying mode mix conditions. The crack growth development in essentially unidirectional laminates of carbon-fibre reinforced epoxy was analysed in terms of the Paris law for different constant propagation modes: mode I (double-cantilever beam test), mode II (end-notched flexure test) and different mixed-modes I/II (mixed-mode bending test). The dependence of the Paris law parameters on mode mix is compared with the existing models in the literature. It is shown that these models do not reproduce the non-monotonic dependence on mode mix which has been observed in experimental data. Therefore, an improved phenomenological model is introduced and compared with the experimental data obtained by other researchers. To check the ability of the model to predict variable mixed-mode fatigue delamination, the mixed-mode end-loaded split test was employed and the experimental results were compared to the predictions of the model. The underlying mechanisms responsible for the dependency of the crack propagation rates on the degree of mode mix are also discussed on the basis of fractographic analysis.

  • 3. Eman, Jesper
    et al.
    Sundin, Karl-Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Spatially resolved observations of strain fields at necking and fracture of anisotropic hardened steel sheet material2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 13, p. 2750-2756Article in journal (Refereed)
    Abstract [en]

    In this work plastic strain localization, also referred to as necking, of press-hardened ultra-high strength steel is observed using digital speckle correlation. The region of the neck is studied during tensile tests of specimens specially designed to facilitate strain localization at an inner point of the material, thus avoiding edge effects on localization and fracture. By using measurements with a length scale small enough to properly resolve the neck, its growth and shape can be studied. Furthermore, the anisotropy of the material is investigated by examining specimens cut out at different angles to the rolling direction. It is seen that the local fracture strain of specimens cut out along the rolling direction is approximately twice as high as it is for specimens cut out perpendicular to the rolling direction.

  • 4.
    Girhammar, Ulf Arne
    et al.
    Royal Institute of Technology (KTH).
    Pan, Danhua
    Royal Institute of Technology (KTH).
    Dynamic analysis of composite members with interlayer slip1993In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 30, no 6, p. 797-823Article in journal (Refereed)
    Abstract [en]

    The exact and approximate analysis of composite members with partial interaction and subjected to general dynamic loading are presented. General closed-form solutions for the displacement functions and the various internal actions in the composite elements are presented for both the exact and approximate cases. The solutions reduce to the well-known values for the extreme cases of interlayer connection: non-composite and full composite action, respectively. An approximate solution for the eigenfrequencies in the exact analysis procedure is proposed and evaluated by comparison with the exact solution. These solutions show the effect of interlayer connection on the eigenfrequency. The exact and approximate analysis procedures are applied to simply-supported beams subjected to impulsive and step loadings to illustrate the difference in the solutions obtained by the two procedures. Very good agreement is obtained between the results of exact and approximate analysis of these particular cases

  • 5. Jonsén, Pär
    et al.
    Häggblad, Hans-åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Fracture energy based constitutive models for tensile fracture of metal powder compacts2007In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 20, p. 6398-6411Article in journal (Refereed)
    Abstract [en]

    Diametral compression test or the Brazilian disc test is commonly used to characterise the tensile strength of brittle materials. A general fracture model based on energy assumptions is proposed for simulation of the discrete and localised tensile fracturing process in metal powder. The characteristics of the tensile fracture development of the central crack in diametral tested specimen is numerically studied. The softening rate of the model is obtained from the corresponding rate of the dissipated energy. Finite element simulations of the diametral compression test are performed with the proposed tensile fracture model used in conjunction with a Cap model for the deformation of the powder material. The results agree reasonably with experiments.

  • 6. Kajberg, Jörgen
    et al.
    Lindkvist, Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Characterisation of materials subjected to large strains by inverse modelling based on in-plane displacement fields2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 13, p. 3439-3459Article in journal (Refereed)
    Abstract [en]

    A method for characterisation of materials subjected to large strains beyond the levels when plastic instability occurs in standard tension tests is presented. Thin sheets of two types of hot-rolled steel are subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera and by digital speckle photography (DSP) in-plane pointwise displacement fields are obtained. By numerical differentiation and assuming plastic incompressibility the equivalent plastic strain is determined. The characterisation performed in this paper consists of estimating material parameters in two constitutive models. These models are a piecewise linear plasticity model and a parabolic hardening model. By using inverse modelling including finite element analyses (FEA) of the tension tests the material parameters are adjusted to achieve a minimum in a so-called objective function. The objective function is basically a least-square functional based on the difference between the experimental and FE-calculated displacement and strain fields. Due to the large deformations an adaptive meshing technique is used in order to avoid highly distorted elements. The DSP- technique provided measurements, where the uncertainty of the equivalent plastic strain varied between 0.0015 and 0.0056. The maximum obtained strain was approximately 0.8. The true stress-strain curves based on the estimated parameters are validated in the low strain region by comparison with curves from standard tension tests

  • 7.
    Kajberg, Jörgen
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Wikman, Bengt
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Viscoplastic parameter estimation by high strain-rate experiments and inverse modelling: speckle measurements and high-speed photography2007In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 1, p. 145-164Article in journal (Refereed)
    Abstract [en]

    A methodology based on inverse modelling for estimating viscoplastic material parameters at high strain-rate conditions is presented. The methodology is demonstrated for a mild steel exposed for compression loading in a split Hopkinson pressure bar arrangement. By using dog-bone shaped specimens nonhomogeneous states of deformation are obtained throughout the entire deformation process. The resulting nonhomogeneous deformation of the specimens is evaluated using digital speckle photography (DSP) to give in-plane point-wise displacement and strain fields. The photographs are captured with a high-speed camera of image converter type, which acquire time resolved images during the impact loading. The experiments are simulated using finite element analysis (FEA), where the material model suggested by Johnson-Cook for high-strain rate conditions are utilised. Experimental and FE-calculated field information are compared in order to estimate the viscoplastic parameter in the Johnson-Cook material model. The estimation is performed by minimising least-square functions that contain the differences in displacements and strains, respectively. The quality of the estimated parameters is studied from statistical point of view.

  • 8.
    Katerelos, Dionysos T.G.
    et al.
    Luleå tekniska universitet, FORTH/ICE-HT, Patras, Institute of Chemical Engineering and High Temperature Processes, Ionian Islands Technological Educational Institute, Faculty of Music Technology, Department of Sound and Musical Instruments Technology.
    Krasnikovs, Andrejs
    Department of Strength of Materials, Riga Technical University, Riga Technical University, Latvia.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Variational models for shear modulus of symmetric and balanced laminates with cracks in 90-layer2015In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 71, p. 169-179Article in journal (Refereed)
    Abstract [en]

    Three analytical models with increasing complexity, all based on minimization of complementary energy, are compared in their ability to predict shear modulus reduction of laminates with intralaminar cracks in 90-layer. The very elegant and simple (and the less accurate) model by Hashin assumes linear out-of-plane shear stress distribution across the ply in all layers. The second model assumes exponential shape of these stresses in the constraint layer keeping linear assumption in the cracked layer. The model developed in the present paper accounts for nonlinear out-of-plane shear stress thickness distribution in all layers described by shape functions determined in the procedure of minimization. Increasing the complexity of the model the predicted shear modulus of the damaged laminate increases approaching to value obtained using finite elements (FE). Results show that for laminates with relatively thick cracked layers the stress state description in the cracked layer should be refined whereas for laminates with constraint layer thicker than the cracked layer more accurate stress description in the constraint layer is necessary. More accurate solutions could be derived using the described methodology, but the involved complexity and the numerical routines required for their application diminish their value comparing with direct FE solution

  • 9.
    Klisinski, Marek
    Luleå tekniska universitet.
    On constitutive equations for arbitrary stress-strain control in multi-surface plasticity1998In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 35, no 20, p. 2655-2678Article in journal (Refereed)
    Abstract [en]

    The structure of incremental constitutive equations in multi-surface plasticity is discussed with respect to different choices of state and control variables. The state and control variables can combine stresses and strains as long as they are decomposed into energy-conjugate parts. A general uniqueness condition is established for non-associated flow rules and any choice of control variables. Furthermore, proper tangent constitutive matrices are given within each loading/unloading region. The theory is demonstrated in a simple example involving Tresca's yield condition

  • 10.
    Kuriakose, Sunil
    et al.
    School of Aerospace Engineering, Georgia Institute of Technology, Atlanta.
    Talreja, Ramesh
    Variational solutions to stresses in cracked cross-ply laminates under bending2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 9-10, p. 2331-2347Article in journal (Refereed)
    Abstract [en]

    Approximate analytical solutions based on a variational approach are presented for stresses in two cross-ply laminates, [90m/0n]s and [0m/90n]s, with matrix cracks in the 90° layers, subjected to bending. The analysis assumes a plane stress state in longitudinal sections of the laminate and accounts for the lack of symmetry caused by partially closed crack planes or by cracks present on one side of the laminate mid-plane. Comparisons with finite element analysis for laminates of glass/epoxy and graphite/epoxy show that the stress components in the cracked layers in the regions of interest are in good agreement with analytical results. The model is therefore suitable for predicting matrix crack multiplication in addition to estimating the residual flexural stiffness. The results obtained show that the transverse normal stress at the 0/90 interface is compressive for [0/90]s laminate, while this stress is tensile in regions of the interface closer to the crack for the [90/0]s laminate. Thus the analysis suggests that delamination is possible under bending in the [90/0]s laminate.

  • 11.
    Li, Shujun
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Singh, Chandra Veer
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    A representative volume element based on translational symmetries for FE analysis of cracked laminates with two arrays of cracks2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 7-8, p. 1793-1804Article in journal (Refereed)
    Abstract [en]

    A methodology is proposed for the construction of a representative volume element (RVE) for analysis of laminated composites containing two arrays of ply cracks running in different directions. The only requirement is that the cracks in any ply are uniformly spaced, and if more than one ply of a given orientation is cracked, then the crack spacing of individual plies must only be in exact multiples of each other. The spacing of cracks in the two directions can be fully independent. The RVE is constructed through a systematic consideration of translational symmetries present in the cracked laminate. As a result, the boundary conditions on the RVE can be imposed without compromising accuracy. Examples of the application of the RVE methodology are given to illustrate its broad capability and a finite element (FE) stress analysis is performed for these cases to illustrate results such as the crack surface displacements, local stress fields and RVE-averaged elastic properties. For one case, the average properties are compared with experimental results, showing good agreement.

  • 12. Nilsson, K.-F.
    et al.
    Asp, Leif
    Alpman, J.E:
    Nystedt, L.
    Delamination buckling and growth for delaminations at different depths in a slender composite panel2001In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 38, no 17, p. 3039-3071Article in journal (Refereed)
    Abstract [en]

    A numerical and experimental investigation for delamination buckling and growth for slender composite panels loaded in compression is presented. The investigated panels consisted of 35 plies in a cross-ply layup with artificially embedded delaminations inserted after three, five or seven plies from the upper surface. The tests clearly and consistently showed that for all delamination depths, delaminated panels failed by delamination growth slightly below the global buckling load of the undamaged panel, whereas the undelaminated panels failed in compression at global buckling. The analysis was done with a finite element based computational model that accounts for contact between delaminated members and fracture mode separation and where crack propagation was simulated with a moving mesh scheme. For all delamination depths, the analysis showed a dramatic increase in the energy release rate when global buckling takes place. Features seen in the tests were captured in the computational analysis. Excellent agreement with tests was found for loads at which delaminated members buckle, the load for onset of delamination growth and the evolution of delamination, e.g., delamination shape and out-of-plane displacements.

  • 13.
    Nilsson, K.F.
    et al.
    Aeronautical Research Institute of Sweden.
    Asp, Leif
    Sjögren, A.
    Aeronautical Research Institute of Sweden.
    On transition of delamination growth behaviour for compression loaded composite panels2001In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 38, no 46-47, p. 8407-8440Article in journal (Refereed)
    Abstract [en]

    This paper presents a combined numerical and experimental study of slender composite panels loaded in compression with artificial delaminations at two different depths. The study was motivated by finite element analyses where this change in delamination depth induced a transition in the direction of delamination growth along with a change in the basic fracture modes and stability. Tests were then carried out to verify the transition in delamination growth. The predicted transitions were to a large extent also seen in the tests. The paper gives an outline of the computational model, which includes contact between delaminated members, calculation of energy release rate with fracture mode separation by an approximate as well as a reliable method for general layups, and moving mesh scheme to account for delamination growth. The experimental work includes monitoring of delamination growth by C-scan and acoustic emission along with a detailed fractographical study. The correlation between experimental observations and computed results are discussed in detail.

  • 14.
    Rodriguez, Juan Manuel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Carbonell, Josep Maria
    International Center for Numerical Methods in Engineering (CIMNE), Campus Nord UPC, Gran Capitán, s/n., 08034 Barcelona.
    Cante, J.C.
    Escola Tècnica Superior d’Enginyeries Industrial i Aeronàutica de Terrassa.
    Oliver, J.
    International Center for Numerical Methods in Engineering (CIMNE), Campus Nord UPC, Gran Capitán, s/n., 08034 Barcelona.
    Continuous chip formation in metal cutting processes using the Particle Finite Element Method (PFEM)2017In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 120, p. 81-102Article in journal (Refereed)
    Abstract [en]

    This paper presents a study on the metal cutting simulation with a particular numerical technique, the Particle Finite Element Method (PFEM) with a new modified time integration algorithm and incorporating a contact algorithm capability . The goal is to reproduce the formation of continuous chip in orthogonal machining. The paper tells how metal cutting processes can be modelled with the PFEM and which new tools have been developed to provide the proper capabilities for a successful modelling. The developed method allows for the treatment of large deformations and heat conduction, workpiece-tool contact including friction effects as well as the full thermo-mechanical coupling for contact. The difficulties associated with the distortion of the mesh in areas with high deformation are solved introducing new improvements in the continuous Delaunay triangulation of the particles. The employment of adaptative insertion and removal of particles at every new updated configuration improves the mesh quality allowing for resolution of finer-scale features of the solution. The performance of the method is studied with a set of different two-dimensional tests of orthogonal machining. The examples consider, from the most simple case to the most complex case, different assumptions for the cutting conditions and different material properties. The results have been compared with experimental tests showing a good competitiveness of the PFEM in comparison with other available simulation tools.

  • 15.
    Runesson, Kenneth
    et al.
    Chalmers University of Technology, Department of Structural Mechanics.
    Axelsson, Kennet
    Luleå tekniska universitet.
    Klisinski, M.
    Polish Academy of Sciences, Institute of Fundamental Technological Researeh.
    Characteristics of constitutive relations in soil plasticity for undrained behavior1992In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 29, no 3, p. 363-380Article in journal (Refereed)
    Abstract [en]

    Elastic-plastic relations which are subjected to the constraint of incompressibilily, that is pertinent to undruined behavior of soils, are assessed and compared with the relations for drained behavior. The tangent relations for mixed control variables (total stress and strain components) are established explicitly. The criterion signalling plastic loading, and elastic unloading, is considered in particular. A criterion for unique response, as expressed in terms of elastic and plastic loading, is formulated as a condition on the particular plastic modulus that corresponds to undrained behavior. Stability is discussed in terms of appropriate critical values of the plastic modulus. It appears that, for a frictional material, dilatancy has a stabilizing effect whereas contractancy may be destabilizing compared to drained behavior.

  • 16.
    Singh, Chandra Veer
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Analysis of multiple off-axis ply cracks in composite laminates2008In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 45, no 16, p. 4574-4589Article in journal (Refereed)
    Abstract [en]

    This paper presents a synergistic methodology to analyze damage behavior in composite laminates with transverse matrix cracks in plies of multiple orientations. The approach combines the strengths of micro-damage mechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to presence of transverse cracks. The micromechanics is performed on a representative unit cell using a three-dimensional finite element analysis to calculate the crack opening displacement (COD) accounting for the influence of the surrounding plies, the so-called constraint effect. This information is then incorporated in the CDM formulation dealing with laminates containing cracks in different ply orientations through a 'constraint parameter'. In CDM, a separate damage mode is defined for each type of crack and the expressions for engineering moduli of the damaged laminate are derived in terms of crack density and the constraint parameter. The COD and stiffness degradation predictions agree well with published experimental data for [0 / ± θ 4 / 0 1 / 2] s laminate configuration. To enable damage analysis of other configurations of [0 m / ± θ n / 0 m / 2] s laminate, a parametric study of the CODs is performed and using the computations a master equation is developed

  • 17.
    Singh, Chandra Veer
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Evolution of ply cracks in multidirectional composite laminates2010In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 47, no 10, p. 1338-1349Article in journal (Refereed)
    Abstract [en]

    This paper treats evolution of ply cracks in multidirectional composite laminates subjected to a quasi-static tensile load in the longitudinal direction. Starting with pre-existing ply cracks in off-axis plies, the formation of additional cracks is analyzed by an energy-based approach. A critical laminate energy parameter associated with formation of these cracks is defined and is evaluated using experimental data for a reference cross-ply laminate. The modeling approach requires crack surface displacements, which are calculated by a three-dimensional finite element (3-D FE) analysis performed on a suitable representative volume of the given cracked laminate. The model predictions agree well with experimental data for [0 / ± θ 4 / 0 1 / 2] s and [0 / 90 / ∓ 45] s laminates. A parametric study is conducted to evaluate effects of ply thickness and ply stacking sequence on damage evolution in [0 m / 90 n / ∓ θ p] s laminates.

  • 18.
    Singh, U.K.
    et al.
    Luleå tekniska universitet.
    Digby, Peter
    Luleå tekniska universitet.
    Application of a continuum damage model in the finite element simulation of the progressive failure and localization of deformation in brittle rock structures1989In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 25, no 9, p. 1023-1038Article in journal (Refereed)
    Abstract [en]

    A continuum damage model is generalized to simulate both the progressive failure and the formation of localization bands in brittle rock structures under both tensile and compressive loading. The model is implemented into a finite element code to simulate the constitutive behaviour of a number of brittle rock structures which might actually be found in the laboratory or in the field. The effect of closed cracks is included in the calculations. Since, in the present work, finitely extended bodies are considered, the damage in a given direction can no longer be supposed uniformly distributed throughout the body considered

  • 19.
    Singh, U.K.
    et al.
    Luleå tekniska universitet.
    Digby, Peter
    Luleå tekniska universitet.
    Continuum damage model for simulation of the progressive failure of Brittle Rocks1989In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 25, no 6, p. 647-663Article in journal (Refereed)
    Abstract [en]

    A constitutive model is developed in which a set of continuous field variables, called damage vectors are used to describe the anisotropic response of a brittle solid due to the growth of cracks under general applied loads. The model is tested numerically by studying the response of infinitely extended solids under a number of general plane strain loading conditions

  • 20.
    Tan, X.C.
    et al.
    Luleå tekniska universitet.
    Digby, P.J.
    Luleå tekniska universitet.
    Buckling of drill string under the action of gravity and axial thrust1993In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 30, no 19, p. 2675-2691Article in journal (Refereed)
    Abstract [en]

    A number of different equilibrium helical buckling configurations for a tubing or drill string confined within a cylindrical casing or hole of larger radius and buckled under static compressive forces are determined. Our work is more general and informative than earlier work since the solutions relating the buckling load and the postbuckling configuration are given explicitly for the string of weight and at any inclined positions. A consideration of the state of transient string buckling when the string undergoes sudden change in the helix radius and might lose contact with the hole wall is also proposed.

  • 21.
    Zhuang, Linqi
    et al.
    Department of Aerospace Engineering, Texas A&M University.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effects of Voids on Postbuckling Delamination Growth in Unidirectional Composites2014In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, no 5, p. 936-944Article in journal (Refereed)
    Abstract [en]

    This work examines the effects of manufacturing induced voids on the postbuckling behavior of delaminated unidirectional composites. In the finite element model developed, a through-width delamination is introduced close to one surface of a flat panel, and a void is placed in the delamination plane ahead of each delamination front. The panel is subjected to compression in the fiber direction. The postbuckling delamination growth is studied by calculating the strain energy release rate (SERR) using the virtual crack closure technique. Local stress analyses of the region near the delamination front are also performed to further investigate the void effects. It is found that although the presence of void does not significantly alter the postbuckling transverse displacement of the delaminated panel, the induced stress perturbation by the void affects the SERR. The Mode II SERR as well as the total SERR increase depending on the size of the void and its distance from the delamination front. Since the Mode I SERR shows non-monotonic behavior with the applied load, the effects of voids are studied on its maximum value.

  • 22.
    Östlund, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Berglund, Daniel
    Gestamp R&D.
    Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion2015In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 56, p. 1-10Article in journal (Refereed)
    Abstract [en]

    Strain localization and fracture initiation of elasto-plastic thin steel sheets is analyzed. A shell element enhanced by embedded discontinuities is developed to improve coarse-mesh accuracy in terms of fracture initiation prediction and to regularize the post-instability response. Discontinuities in the strain field are introduced when instability is signaled by a local bifurcation analysis. The enhancements are implemented for the Belytschko-Lin-Tsay shell element. This is combined with a stress based fracture criterion which relates the magnitude of the stress vector and the first invariant of the stress tensor. A robust experimental procedure based on full-field measurements enable direct calibration in stress space, and provides a flow curve up to large strains. Numerical examples involving tensile samples with different localization behavior are presented to demonstrate significant reduction of spurious mesh dependence. Moreover, the engineering feasibility of the direct use of a stress based fracture criterion in combination with the enhanced element is evaluated by comparison of measured and calculated fracture elongations.

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Output format
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  • asciidoc
  • rtf