| 1. |
- Alfredsson, Bo, et al.
(författare)
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Low temperature creep in a high strength roller bearing steel
- 2016
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 100, s. 109-125
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Tidskriftsartikel (refereegranskat)abstract
- Noticeable low temperature creep was established for a bainitic and a martensitic microstructure of the 100CrMnMo8 high strength roller bearing steel. The response revealed primary creep that differed between the microstructures, following a power law for martensite and the logarithmic description for bainite. The detected creep was pressure sensitive, higher in tension than in compression for the same stress level, following the strength differential effect (SDE) at material yielding. Two models were proposed where the stress variable for the pressure effect was based on the Drucker-Prager yield function and deviatoric creep strains were derived from a non-associated von Mises potential. Model parameters were determined from experimental series on the respective microstructure. When the models were evaluated against the experiments the accuracy was of the same order as the effects of different heat treatment batches and different load application rates. The importance of different material parameters in the descriptions was discussed.
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| 2. |
- Dusthakar, D. K., et al.
(författare)
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Laminate-based modelling of single and polycrystalline ferroelectric materials – application to tetragonal barium titanate
- 2018
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Ingår i: Mechanics of Materials. - : Elsevier BV. - 0167-6636. ; 117, s. 235-254
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Tidskriftsartikel (refereegranskat)abstract
- The present contribution deals with the development of a laminate-based model designed to study the single and polycrystalline tetragonal ferroelectric material behaviour. Laminate-based models are micromechanically motivated and consider the volume fraction of the distinct ferroelectric variants directly in their formulation. At first, a single crystal laminate-based model is established by considering the average strain and polarisation compatibility conditions. A suitable thermodynamic electric Gibbs energy and a rate-dependent dissipation equation are postulated to capture the dissipative hysteretic material response. The update of the inequality constrained volume fractions is solved by adopting a Fischer–Burmeister-type algorithm in combination with a Newton–Raphson scheme. Following the single crystal formulation, a homogenisation procedure based on random orientation of the individual grains in a polycrystalline aggregate is considered. The material properties and the polarisation switching response of the randomly oriented individual grains are averaged using a finite element framework in order to study the macroscopic polycrystalline behaviour. A parameter fitting procedure based on experimental data for single crystalline response, taken from the literature, is detailed and the material model as well as the algorithmic scheme are verified by solving representative boundary value problems. Moreover, the finite element based simulation results are compared with newly generated experimental hysteresis data for a barium titanate piezoceramic.
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| 3. |
- Ehrlin, Niklas, et al.
(författare)
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Flow stress model for hydrogen degraded Inconel 718
- 2018
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 119, s. 56-64
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Tidskriftsartikel (refereegranskat)abstract
- For life time estimation, it is desirable to capture the lowering of yield strength and premature failure that some alloys exhibits when subjected to hydrogen. For this, a mechanism based material model has been developed to simulate the hydrogen enhanced localized plasticity (HELP) for the superalloy IN718. The model accounts for the increase in mobility of moving dislocations during plastic deformation, whenever hydrogen is present in the material. Tensile tests performed at four different strain rates: 5 x 10(-5), 5 x 10(-4), 5 x 10(-3) and 5 x 10(-2) s(-1) show a difference in yield behaviour between hydrogen pre-charged and uncharged samples. No strain rate dependency of the hydrogen effect could be concluded. Two different hydrogen charging methods have been used: cathodic charging with molten salt as electrolyte, and high temperature gas charging. No differences in the tensile response could be seen between the two different charging methods. The proposed model was fitted against the experimental curves using a minimizing method and model parameters were obtained. Comprising iteratively updated parameters, the model is suited for implementation in finite element software.
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| 4. |
- Ghassemali, Ehsan, 1983-, et al.
(författare)
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Effect of cold-work on the Hall–Petch breakdown in copper based micro-components
- 2015
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 80, part A, s. 124-135
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Tidskriftsartikel (refereegranskat)abstract
- Effects of substructural dimensions on the mechanical properties of micro-pins produced by an open-die micro-extrusion/forging process were studied. Micro-pins of diameter 0.3 mm were manufactured from copper strips, having different initial grain sizes. Micro-compression tests on the micro-pins revealed no significant size effect, even if the number of grains over the diameter of the micro-pins falls below its critical value. However, relaxation of the as-formed substructure using recovery annealing led to a surprising drop in the flow stress of the micro-pins. This was explained and attributed to the number of subgrains over the diameter of the micro-pins, showing the important role of subgrains rather than grains in determining the mechanical properties.
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| 5. |
- Hazar, Selcuk, et al.
(författare)
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Mechanical modeling of coupled plasticity and phase transformation effects in a martensitic high strength bearing steel
- 2018
-
Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 117, s. 41-57
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Tidskriftsartikel (refereegranskat)abstract
- The stress and strain induced solid to solid phase transformation of retained austenite in a martensitic high strength bearing steel has been studied. Monotonic tension experiments that were carried out at different temperatures using this high strength steel showed that not only the strain induced but also the stress induced phase change plays a crucial role in the phase transformation of retained austenite to martensite. In the material model, plastic deformation was defined using the Drucker Prager yield surface through a nonassociated flow rule accompanied by nonlinear kinematic and isotropic hardening. The hardening was coupled with stress and strain induced phase transformations. A nonlinear elastic effect based on elastic dilation was included in the constitutive model by extending the bulk modulus with a second order term. For the finite element analysis, the material model was written as a user defined material subroutine (UMAT). The numerical simulations were done using ABAQUS and compared to monotonic tension, compression and cyclic experiments. The results showed that the strength differential effect and the volumetric change under loading are closely related to the transformation of retained austenite to martensite. At low temperatures the effect of stress induced phase transformation on yield strength was noticeable. It was concluded that at certain temperatures both strain and stress induced phase transformations significantly affect mechanical behavior of the high strength steel.
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| 6. |
- Hämäläinen, Pyry, et al.
(författare)
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On the determination of transverse shear properties of paper using the short span compression test
- 2017
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 107, s. 22-30
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Tidskriftsartikel (refereegranskat)abstract
- The present paper explores the short span compression tester (SCT) as a means to experimentally determine the transverse shear moduli of paper. These moduli, which are known to be difficult to determine by any other means, are of importance for the behavior of paper during tissue manufacturing and in the converting and embossing of paperboard. Testing was conducted on paper of two different grammages both in MD and in CD. By applying the Timoshenko-Engesser theory for buckling of shear compliant materials, estimates of the transverse shear moduli were obtained through the measured SCT values and standard measurements of the Young's modulus and the thickness. These estimates were evaluated by detailed FE-analyses of the SCT setup incorporating initial geometrical imperfections representative for real test conditions. It was found that the Timoshenko-Engesser theory gives estimates of the transverse shear moduli that are within an accuracy well applicable for most engineering purposes. The results suggest that the method is at least as accurate as any other, more involved, method that could be used for the purpose.
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| 7. |
- Johansson, Joakim, 1983, et al.
(författare)
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Effect of microstructure on dynamic shear localisation in Alloy 718
- 2017
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Ingår i: Mechanics of Materials. - : Elsevier BV. - 0167-6636. ; 109, s. 88-100
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Tidskriftsartikel (refereegranskat)abstract
- The effect of microstructure on the strain localisation behaviour during dynamic shear deformation of Ni-Fe-base superalloy Alloy 718 was investigated by a combination of experiments and numerical simulations of Split-Hopkinson tests of specimens with top-hat geometry. The results show that strain localisation is promoted by small grain size and precipitation hardening, whereas large grains and solution annealing largely supressed the formation of shear bands. In addition, a smaller fillet radius facilitated the initiation of shear bands. Finite element simulations were able to reproduce the most important features of both global load–displacement histories, and presence of shear localisation. In particular, detailed simulations of the local deformation during localisation showed that temperatures exceeding 750?°C (locally above 1000?°C) and strain rates in the order of 2?105 s?1 were reached in the band.
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| 8. |
- Larsson, Ragnar, 1960, et al.
(författare)
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Damage growth and strain localization in compressive loaded fiber reinforced composites
- 2018
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Ingår i: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 127, s. 77-90
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Tidskriftsartikel (refereegranskat)abstract
- To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking which is typically induced in compression. We propose a new set of continuum damage models that are able to predict fiber kinking response under compression. A structure tensor based formulation is established at the unidirectional ply level, where the elastic material response is governed by transverse isotropy. To consider geometrical effects in conjunction with fiber kinking instability, a continuum damage formulation at finite strain is developed. The fracture area progression includes a convective and a local damage production involving a finite progression speed. In this framework, two damage evolution models are considered; one non–local model including the gradient damage effect and a local one, without the gradient enhancement. The models are implemented in a FE–code and validated for a compression loaded specimen. The models are computationally robust and can predict the localized nature of fiber kinking. A thorough sensitivity study is presented to show how the different formulations influence the predicted responses.
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| 9. |
- Lindgren, Lars-Erik, et al.
(författare)
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Improved and simplified dislocation density based plasticity model for AISI 316 L
- 2017
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 108, s. 68-76
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Tidskriftsartikel (refereegranskat)abstract
- A previously published dislocation density based flow stress model has been refined and made more consistent with underlying physical assumptions. The previous model included many temperature dependent parameters that are taken as constant in the current work. The model has also been simplified with respect to dynamic strain aging. Additional contributions to flow stress from the Hall-Petch effect and solute hardening have now been explicitly included in the model. Furthermore, the dynamic recovery part of the model has been improved.
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| 10. |
- Maniprakash, S., et al.
(författare)
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Experimental investigation, modelling and simulation of rate-dependent response of 1-3 ferroelectric composites
- 2016
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Ingår i: Mechanics of Materials. - : Elsevier BV. - 0167-6636. ; 94, s. 91-105
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Tidskriftsartikel (refereegranskat)abstract
- Further development and design of piezoelectric composites enhances the improved use of piezoelectric materials and devices by overcoming their brittleness. In order to engineer this class of materials and to predictably simulate its behaviour, a computationally efficient constitutive model is established in this paper. This contribution deals with the development of a model for piezoelectric composites to capture their effective behaviour. We first discuss a three-dimensional fully coupled electromechanical rate-dependent model for the response of ferroelectric ceramics. Secondly, a simple homogenisation approach is applied to capture the behaviour of composites for various volume fractions of PZT fibres under different loading frequencies. Following this, a finite element formulation is applied in order to study the behaviour of composites. Finally, these two approaches are compared with experimental results.
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