| 1. |
- Cedergren, Joakim, et al.
(författare)
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Three-dimensional analysis of compaction of metal powder
- 2002
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Ingår i: Mechanics of Materials. - 0167-6636. ; 34:1, s. 43-59
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional compaction simulations of geometries with circular and quadratic cross-sections are reported using dilatant viscoplastic material models representing a ductile steel powder. The simulations are compared with experiments using in situ measurements of the wall-tractions within the die. Two different porous viscoplastic material models have been used: the combined Fleck-Kuhn-MeMeeking (FKM) and Gurson model and the Shima-Oyane model. The contact between the die and the powder was modelled with Coulomb-type friction. Effects such as the applied traction-density response and average relations for the upper, lower and the normal tractions against the die wall are compared. Comparisons between the numerical results and experimental results show that both material models can well be used to capture the overall density distributions for the geometries studied and that many of the coarse three-dimensional features of the powder-pressing process are well described by the simulations. (C) 2002 Elsevier Science Ltd. All rights reserved.
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| 2. |
- Ottosen, Niels Saabye, et al.
(författare)
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Discontinuous bifurcations in a nonassociated Mohr material
- 1991
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Ingår i: Mechanics of Materials. - 0167-6636. ; 12:3-4, s. 255-265
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Tidskriftsartikel (refereegranskat)abstract
- Discontinuous bifurcations of an elastic-plastic material obeying Mohr's yield criterion and a nonassociated flow rule and subjected to general three-dimensional loading are investigated. The first possibility for bifurcation is identified and expressed in terms of the critical hardening modulus and the corresponding orientation of the slip planes. In the case of perfect plasticity the Mohr and Roscoe solutions for the orientation of the slip planes are derived. The corresponding strain rate fields are determined and it is shown that the Roscoe strain rate field differs significantly from the other fields. Moreover, the stress rate field is continuous across the singular surface for the Roscoe solution.
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| 3. |
- Ristinmaa, Matti
(författare)
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Void growth in cyclic loaded porous plastic solid
- 1997
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Ingår i: Mechanics of Materials. - 0167-6636. ; 26:4, s. 227-245
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Tidskriftsartikel (refereegranskat)abstract
- In low-cycle fatigue, where plastic strains are of great importance, final ductile fracture depends upon the mechanisms of void growth and coalescence of voids. A cell model is used to simulate a periodic array of initial spherical voids and this model is subjected to different loads that include cyclic loading. Three different types of matrix material are simulated: elastic-perfectly plastic, isotropic hardening and kinematic hardening. The cell model results are compared with the approximate constitutive equations for a voided material suggested by Gurson. The simulations show that the unspecified parameters introduced by Tvergaard in the Gurson yield function depend on the hardening behavior of the matrix material. For a perfectly plastic matrix material, the parameters q1 = 1.5 and q2 = 1.02 provide very close predictions for a variety of loadings. However, for isotropic or kinematic hardening matrix materials these parameters result in an inferior agreement and a much closer accuracy is obtained by adopting q1 = 1.5 and q2 = 0.82. This suggests that the parameter q2 depends on the hardening behavior of the matrix material. For kinematic hardening of the Gurson model, it is shown that Ziegler's hardening rule is superior to Prager's hardening rule. Finally, the void shape change due to loading is studied and it is found that this change has an insignificant effect on the response.
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| 4. |
- Aidanpää, Jan-Olov, et al.
(författare)
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One-dimensional model for the transition from periodic to chaotic motions in granular shear flows
- 1993
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 16:1-2, s. 153-161
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Tidskriftsartikel (refereegranskat)abstract
- A simple one-dimensional mechanical model to simulate some aspects of the dynamics of granular flow is suggested. The model consists of a visco-elastic packet bouncing between two oscillating walls. The motion of the walls is prescribed to be harmonic. The amplitude and frequency of the wall motion are related to the concentration and shear rate, respectively. The dynamics of the system is studied for various parameter combinations and it is shown that periodic as well as chaotic motions are possible for different parameters and initial conditions. Results are presented in the form of time histories, power spectral densities, phase diagrams, Poincare plots and bifurcation diagrams. This simple one-dimensional model presents many features that are analogous to those observed in the two-dimensional simple shear flow of disks.
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| 5. |
- Akshantala, Nagendra V., et al.
(författare)
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A mechanistic model for fatigue damage evolution in composite laminates
- 1998
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 29:2, s. 123-140
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Tidskriftsartikel (refereegranskat)abstract
- We propose a mechanistic model which is capable of describing the evolution of transverse cracking in cross ply laminates subjected to cyclic tension in the longitudinal direction. The key feature of the model is that it incorporates delamination associated with transverse cracks in a manner that induces further formation of transverse cracks as delamination grows in fatigue. A variational approach is taken to estimate the stresses in the region between transverse cracks, and these are found to be accurate away from the crack planes when comparison is made with finite element computations. The evolution of transverse crack density and the associated overall elastic moduli changes predicted by the model are in agreement with experimental results
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| 6. |
- Ganghoffer, J.F., et al.
(författare)
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A micromechanical model of the martensitic transformation
- 1998
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Ingår i: Mechanics of Materials. - 0167-6636. ; 27:3, s. 125-144
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Tidskriftsartikel (refereegranskat)abstract
- A micromechanical model of the martensitic transformation at the grain scale has been established, considering the more specific case of ferrous alloys. The transformation proceeds through the formation of successive variants of the product phase within a unit cell representative of a grain; interactions between neighbouring grains are simulated by the choice of periodic boundary conditions. From a thermodynamical analysis, a selection rule for the order and orientation of the forming martensitic variants has been established, based on internal stresses anisotropy. These concepts have been implemented into a two-dimensional finite element simulation of the transformation, considering an elastoplastic behaviour of both parent and product phases. Morphological and crystallographical features of the transformation are considered: one variant consists of a thin layer of elements within the mesh that can form with four possible discrete orientations. Simulation results show the development of the plate pattern as a combination of the influence of both external load and internal stresses built during the progress of the transformation. These are related to global evolutions of transformation plasticity vs. transformation progress. Comparison with experiments show a similar form of the evolutions of the total strain; however, the model overestimates the strain levels. The possible reasons for this discrepancy are discussed.
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| 7. |
- Kumar, Rajesh Sai Santosh, et al.
(författare)
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A continuum damage model for linear viscoelastic composite materials
- 2003
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 35:3-6, s. 463-480
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a constitutive model for linear viscoelastic orthotropic solids containing a fixed level of distributed cracks. The model is formulated in a continuum damage mechanics framework using internal variables taken as second rank tensors. Use is made of the correspondence principle for linear viscoelastic solids to define a pseudo strain energy function in the Laplace domain. This function is then expressed as a polynomial in transformed strain and tensorial damage variables using the integrity bases restricted by the initial orthotropic symmetry of the material. The constitutive relationships derived in the Laplace domain are then converted to the time domain by using the inverse Laplace transform. The model is applied to the specific case of cross-ply laminates with transverse matrix cracks. The material coefficient functions appearing in the model are determined by a numerical (finite element) method for one cross-ply laminate configuration at one damage level. Predictions of the viscoelastic response are then made for the same laminate at other damage levels and for other cross-ply laminate configurations at different damage levels. These predictions agree well with independently determined time variations of properties by an analytic method (Kumar and Talreja, 2001, Linear viscoelastic behavior of matrix cracked cross-ply laminates. Mechanics of Materials 33 (3), 139-154) as well as with the numerically calculated values. Extension of the model to incorporate effects of transient temperature, physical aging and moisture is outlined
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| 8. |
- Kumar, Rajesh Sai Santosh, et al.
(författare)
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Linear viscoelastic behavior of matrix cracked cross-ply laminates
- 2001
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 33:3, s. 139-154
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Tidskriftsartikel (refereegranskat)abstract
- The linear viscoelastic behavior of matrix cracked symmetric cross-ply laminates is studied. A lower bound solution and an approximate 3D solution to the properties are obtained by using the elastic-viscoelastic correspondence principle. The accuracy of Schapery's approximate Laplace inversion technique in the solution procedure is discussed
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| 9. |
- Lacy, Thomas E., et al.
(författare)
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Gradient concepts for evolution of damage
- 1999
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 31:12, s. 831-860
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Tidskriftsartikel (refereegranskat)abstract
- While low-order measures of damage have sufficed to describe the stiffness of bodies with distributed voids or cracks, such as the void volume fraction or the crack density tensor of Vakulenko, A.A., Kachanov, M., 1971., addressing the growth of distributed defects demands a more comprehensive description of the details of defect configuration and size distribution. Moreover, interaction of defects over multiple length scales necessitates a methodology to sort out the change of internal structure associated with these scales. To extend the internal state variable approach to evolution, we introduce the notion of multiple scales at which first and second nearest-neighbor effects of nonlocal character are significant, similar to homogenization theory. Further, we introduce the concept of a cutoff radius for nonlocal action associated with a representative volume element (RVE), which exhibits statistical homogeneity of the evolution, and flux of damage gradients averaged over multiple subvolumes. In this way, we enable a local description at length scales below the RVE. The mean mesoscale gradient is introduced to reflect systematic differences in size distribution and position of damage entities in the evolution process. When such a RVE cannot be defined, the evolution is inherently statistically inhomogeneous at all scales of reasonable dimension, and the concept of macroscale gradients of internal variables is the only recourse besides micromechanics. Based on a series of finite element calculations involving evolution of 2D cracks in brittle elastica arranged in random periodic arrays, we examine the evolution of the mean mesoscale gradients and note some preliminary implications for the utility of such an approach.
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| 10. |
- Li, Chunlin, et al.
(författare)
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Deformation of brittle rocks under compression - with particular reference to microcracks
- 1993
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 15:3, s. 223-239
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Tidskriftsartikel (refereegranskat)abstract
- A constitutive model for brittle rocks has been developed based on the analysis of microcracks. The macroscopic deformation of a rock is decomposed into three components: response of the rock matrix, closure of open cracks and deformation due to the fracture of microcracks. The main contribution of this approach is the prediction of macroscopic deformation induced by microcracking. The basic model units are sliding cracks. Wing cracks are initiated at tips of sliding cracks and propagated in the major loading direction. The relationships between the compressive stress, the length of the growing crack and deformation due to the fracture of cracks are established. A deformation solution to an elliptic crack is employed to describe the closure effect of an open crack under compression. The total deformation caused by the closure and fracture of cracks is obtained by the sum of the components of individual cracks. In this model, the nonlinear behavior of deformation results either from the closure of open cracks at low stress levels, or from the fracture of microcracks at high stress levels. Hysteresis is captured under cyclic loading. Comparisons of the model simulations with experimental data are presented and show good agreement in the prefailure stage.
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