| 1. |
- Mousavi, Mahmoud, 1983-
(författare)
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Dislocation-based fracture mechanics within nonlocal and gradient elasticity of bi-Helmholtz type - Part II : Inplane analysis
- 2016
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 92-93, s. 105-120
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Tidskriftsartikel (refereegranskat)abstract
- Abstract This paper is the sequel of a companion Part I paper devoted to dislocation-based antiplane fracture mechanics within nonlocal and gradient elasticity of bi-Helmholtz type. In the present paper, the inplane analysis is carried out to study cracks of Modes I and II. Generalized continua including nonlocal elasticity of bi-Helmholtz type and gradient elasticity of bi-Helmholtz type (second strain gradient elasticity) offer nonsingular frameworks for the discrete dislocations. Consequently, the dislocation-based fracture mechanics within these frameworks is expected to result in a regularized fracture theory. By distributing the (climb and glide) edge dislocations, (Modes I and II) cracks are modeled. Distinctive features are captured for crack solutions within second-grade theories (nonlocal and gradient elasticity of bi-Helmholtz type) comparing with solutions within first-grade theories (nonlocal and gradient elasticity of Helmholtz type) as well as classical elasticity. Other than the total stress tensor, all of the field quantities are regularized within second-grade theories, while first-grade theories give singular double stress and dislocation density and classical elasticity leads to singularity in the stress field and dislocation density. Similar to gradient elasticity of Helmholtz type (first strain gradient elasticity), crack tip plasticity is captured in gradient elasticity of bi-Helmholtz type without any assumption of the cohesive zone. ", keywords = Crack; Inplane; Dislocation; Nonlocal elasticity of bi-Helmholtz type; Gradient elasticity of bi-Helmholtz type; Nonsingular, isbn = 0020-7683, doi=https://doi.org/10.1016/j.ijsolstr.2016.03.025
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| 2. |
- Mousavi, Mahmoud, 1983-
(författare)
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Dislocation-based fracture mechanics within nonlocal and gradient elasticity of bi-Helmholtz type Part I : Antiplane analysis
- 2016
-
Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 87, s. 222-235
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Tidskriftsartikel (refereegranskat)abstract
- Abstract In the present paper, the dislocation-based antiplane fracture mechanics is employed for the analysis of Mode III crack within nonlocal and (strain) gradient elasticity of bi-Helmholtz type. These frameworks are appropriate candidates of generalized continua for regularization of classical singularities of defects such as dislocations. Within nonlocal elasticity of bi-Helmholtz type, nonlocal stress is regularized, while the strain field remain singular. Interestingly, gradient elasticity of bi-Helmholtz type (second strain gradient elasticity) eliminates all physical singularities of discrete dislocation including stress and strain fields and dislocation density while the so-called total stress tensor still contains singularity at the dislocation core. Based on the distribution of dislocations, a fracture theory with nonsingular stress field is formulated in these nonlocal and gradient theories. Strain and displacement fields within nonlocal fracture theory are identical to the classical ones. In contrast, gradient elasticity of bi-Helmholtz type leads to a full nonsingular fracture theory in which stress, strain and dislocation density are regularized. However, the singular total stress of a discrete dislocation results in singular total stress of the plane weakened by a crack. Within classical fracture mechanics, Barenblatt’s cohesive fracture theory assumes that cohesive forces is distributed ahead of the crack tip to model crack tip plasticity and remove the stress singularity. Here, considering the dislocations as the carriers of plasticity, the crack tip plasticity is captured without any assumption. Once the crack is modeled by distributing the dislocations along its surface, due to the gradient theory, the distribution function gives rise to a non-zero plastic distortion ahead of the crack. Consequently, regularized solutions of crack are developed incorporating crack tip plasticity. ", keywords = Crack; Antiplane; Dislocation; Nonlocal elasticity of bi-Helmholtz type; Gradient elasticity of bi-Helmholtz type; Nonsingular, isbn = 0020-7683, doi=https://doi.org/10.1016/j.ijsolstr.2015.10.033
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| 3. |
- Ashwear, Nasseradeen, et al.
(författare)
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Optimization of modular tensegrity structures for high stiffness and frequency separation requirements
- 2016
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 80, s. 297-309
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Tidskriftsartikel (refereegranskat)abstract
- Tensegrities are cable-strut assemblies which find their stiffness and self-equilibrium states from the integrity between tension and compression. Low stiffness and coinciding natural frequencies are known issues. Their stiffness can be regulated and improved by changing the level of pre-stress. In vibration health monitoring, the first natural frequency is used as an indicator of better stiffness, but coinciding natural frequencies will be an obstacle in measuring and analysing the correct resonance. In this paper, the above two issues have been considered for modular tensegrity structures. The finite element model used considers not only the axial vibration of the components, but also the transversal vibration where non-linear Euler-Bernoulli beam elements are used for simulations. A genetic algorithm is used to solve the optimization problem, with a multi-objective criterion combination. The optimum self-stress of the tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others. Two approaches are used to find the optimal self-stress vector: scaling from a base module or considering all modules at once. Both approaches give the same optimum solutions.
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| 4. |
- Atashipour, Rasoul, 1983, et al.
(författare)
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A realistic model for transverse shear stiffness prediction of composite corrugated-core sandwich elements
- 2017
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 129, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- Several previous studies of the transverse shear stiffness of corrugated-core sandwich elements have demonstrated that the existing analytical formulations significantly overestimate this property, especially when these lightweight elements are made of composite sub-elements. Due to the recent widespread fabrication and use of composite sandwich elements in many fields of application, a more accurate and reliable formulation is needed. This paper deals with an accurate analytical model for predicting the transverse shear stiffness of composite sandwich elements with a structural corrugated core. The effect of the directional material properties of the sub-elements, including face sheets and core, as well as the effect of a low shear modulus in composite material constituents of the sub-elements, is taken into ac- count. It is shown that the transverse shear stiffness of corrugated web core elements is considerably lower than that predicted by the existing formulations. The present solution provides much more real- istic results, especially when the corrugated-core sandwich is made of composite constituents with low shear moduli. Different combinations of the main orthotropic material orientation of the faces and core are studied comparatively for different composite material choices corresponding to an innovative timber composite floor case. It is demonstrated that the planar direction of the main orthotropic material ori- entation of the core has a significant influence on transverse shear stiffness, whereas that of the faces is less important. Several finite element comparisons are made to ensure the reliability of the developed for- mulation. The importance of the disregarded shear deformation effects on the transverse shear stiffness component versus different geometrical parameters of the sandwich element is studied and discussed.
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| 5. |
- Atashipour, Rasoul, 1983, et al.
(författare)
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Stability analysis of three-layer shear deformable partial composite columns
- 2017
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 106-107, s. 213-228
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Bergström, Per, et al.
(författare)
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Scaling Behaviour of Strength of 3D-, Semi-flexible-, Cross-linked Fibre Network
- 2019
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 166:July 2019, s. 68-74
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropic, semi-flexible, cross-linked, random fibre networks are ubiquitous both in nature and in a wide variety of industrial materials. Modelling mechanical properties of such networks have been done extensively in terms of criticality, mechanical stability, and scaling of network stiffnesses with structural parameters, such as density. However, strength of the network has received much less attention. In this work we have constructed 3D-planar fibre networks where fibres are, more or less, oriented in the in-plane direction, and we have investigated the scaling of network strength with density. Instead of modelling fibres as 1D element (e.g., a beam element with stretching, bending and/or shear stiffnesses), we have treated fibres as a 3D-entity by considering the features like twisting stiffness, transverse stiffness, and finite cross-link (or bond) strength in different deformation modes. We have reconfirmed the previous results of elastic modulus in the literature that, with increasing density, the network modulus indeed undergoes a transition from bending-dominated deformation to stretching-dominated with continuously varying scaling exponent. Network strength, on the other hand, scales with density with a constant exponent, i.e., showing no obvious transition phenomena. Using material parameters for wood fibres, we have found that the predicted results for stiffness and strength agree very well with experimental data of fibre networks of varying densities reported in the literature.
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| 7. |
- Biel, Anders, 1977-, et al.
(författare)
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Strength and toughness in shear of constrained layers
- 2018
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 138, s. 50-63
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Tidskriftsartikel (refereegranskat)abstract
- Confined layers may fracture in shear. This occurs, for example in adhesive joints and composite materials. A common mechanism for shear fracture is the formation of shear hackles associated with an expansion of the layer. This makes shear toughness and strength depend on the constraint of the expansion. By constraining the expansion using external loading in experiments, the expansion is reduced but not totally inhibited. The experiments are evaluated using the path independent properties of the J-integral. It is shown that the shear toughness increases for the more constrained case. Thus, from a strength analysis perspective, ignoring the expansion leads to a conservative estimate of the fracture properties. Extrapolation of the evaluated properties to totally inhibited expansions gives the traction separation relation and the fracture toughness for a layer in simple shear.
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| 8. |
- Borodulina, Svetlana, et al.
(författare)
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Effect of fiber and bond strength variations on the tensile stiffness and strength of fiber networks
- 2018
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 154, s. 19-32
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Tidskriftsartikel (refereegranskat)abstract
- As fiber and bond characterization tools become more sophisticated, the information from the fiber scale becomes richer. This information is used for benchmarking of different types of fibers by the paper and packaging industries. In this work, we have addressed a question about the effect of variability in the fiber and fiber bond properties on the average stiffness and strength of fiber networks. We used a fiber scale numerical model and reconstruction algorithm to address this question. The approach was verified using the experimental sheets having fiber data acquired by a fiber morphology analyzer and corrected by microtomographic analysis of fibers in these sheets. We concluded, among other things, that it is sufficient to account for the average bond strength value with an acceptable number of samples to describe dry network strength, as long as the bond strength distribution remains symmetric. We also found that using the length-weighted average for fiber shape factor and fiber length data neglects the important contribution from the distribution in these properties on the mechanical properties of the sheets.
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| 9. |
- Bortot, Eliana, et al.
(författare)
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Analysis of viscoelastic soft dielectric elastomer generators operating in an electrical circuit
- 2016
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683. ; 78-79:1, s. 205-215
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Tidskriftsartikel (refereegranskat)abstract
- A predicting model for soft dielectric elastomer generators (DEGs) must consider a realistic model of the electromechanical behaviour of the elastomer filling, the variable capacitor and of the electrical circuit connecting all elements of the device. In this paper such an objective is achieved by proposing a framework for reliable simulations of soft energy harvesters. In particular, a simple electrical circuit is realised by connecting the capacitor, stretched periodically by a source of mechanical work, in parallel with a battery through a diode and with an electrical load consuming the energy produced. The electrical model comprises resistances simulating the effect of the electrodes and of the conductivity current invariably present through the dielectric film. As these devices undergo a high number of electro-mechanical loading cycles at large deformation, the time-dependent response of the material must be taken into account as it strongly affects the generator outcome. To this end, the viscoelastic behaviour of the polymer and the possible change of permittivity with strains are analysed carefully by means of a proposed coupled electro-viscoelastic constitutive model, calibrated on experimental data available in the literature for an incompressible polyacrylate elastomer (3M VHB4910). Numerical results showing the importance of time-dependent behaviour on the evaluation of performance of DEGs for different loading conditions, namely equi-biaxial and uniaxial, are reported in the final section.
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| 10. |
- Bremberg, Daniel, et al.
(författare)
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A numerical procedure for interaction integrals developed for curved cracks of general shape in 3-D
- 2015
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 62, s. 144-157
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a numerical procedure for the evaluation of interaction energy integrals used to extract mixed-mode stress intensity factors. The interaction energy integral is expressed as a domain integral, and the proposed numerical procedure delivers accurate results for three-dimensional cracks with curved crack fronts and curved crack surfaces for a rather general set of integration domains. It is clearly shown that, when the curvature of the crack surface becomes sufficiently large, special care must be taken in the evaluation of both the volume and the area integrals involved. To improve the accuracy in the evaluation of the former, a composite rule for the Gaussian quadrature scheme is employed. Four benchmark geometries with available analytical solutions are considered. Firstly, mesh design parameters for planar cracks with straight and curved crack fronts are established. Secondly, non-planar cracks with straight and curved crack fronts are employed to examine the accuracy of the numerical procedure.
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