683 research outputs found
Finite strain logarithmic hyperelasto-plasticity with softening: a strongly non-local implicit gradient framework
No full textThis paper addresses the extension of a Eulerian logarithmic finite strain hyperelasto-plasticity model in order to incorporate an isotropic plastic damage variable that leads to softening and failure of the plastic material. It is shown that a logarithmic elasto-plastic model with a strongly non-local degrading yield stress exactly preserves the structure of its infinitesimal counterpart. The strongly non-local nature of the model makes it an attractive framework for the numerical solution of softening plasticity problems. Consistent constitutive tangent operators are derived for the particular case of hyperelasto-J2-plasticity, which are exactly equal to the corresponding infinitesimal tangent operators. The finite element implementation, along with the geometrically nonlinear contributions and the incremental solution strategy, is outlined. A benchmark example is solved, illustrating the main differences between the purely elasto-plastic case and the case with plastic damage. Finally, the main model characteristics and its practical use are emphasized
Modelling the Rate- and Temperature-Dependent Micro-Mechanical Behaviour of Carbon Fiber Reinforced PVDF
No full textThe conditions to which fiber reinforced plastics (FRPs) are exposed in state of the art applications are becoming more extreme, for example in the offshore oil and gas industry. Therefore,
the ability to predict the long-term behaviour, and thereby identifying the failure mechanisms,
of fiber reinforced plastics is of great importance. Especially under these extreme conditions,
the contribution of the matrix plays an important role and a detailed description of its behaviour
is required. In oil and gas applications, polyvinylidene fluoride (PVDF) is used because of
its excellent gas barrier properties. In this work the rate- and temperature-dependent micromechanical behaviour of carbon fiber reinforced polyvinylidene fluoride is studied.
The behaviour of the composite is studied by using a micro-mechanical model that is composed
of individually modelled carbon fibers embedded in a PVDF matrix. The time- and temperature dependent behaviour of PVDF is captured by the Eindhoven Glassy Polymer (EGP) constitutive
model [1]. This model enables the description of the intrinsic behaviour of the semi-crystalline
matrix over a range of applied strain rates and temperatures using a single set of material parameters. The characterization of these material parameters, requires a set of experimental data
obtained from uniaxial compression and tensile tests performed at different temperatures and
applied strain rates. To describe the material behaviour of the individually modelled carbon
fibers, an elastic orthotropic material model is employed. Off-axis tensile tests of the composite
led to the observation that the interface behaviour between matrix and fiber must be incorporated in the micro-mechanical model as well. Subsequently, an interface between the matrix
and fiber is added to the model by using cohesive zone interface elements. The behaviour of
these interface elements is described by an appropriate constitutive relation.
It is demonstrated what influence the rate- and temperature dependence of PVDF, described
by the EGP-model, and the behaviour of the interface elements have on the micro-mechanical
response of the composite
Machten van tien : mechanische sterkte relateren en relativeren
No full textHet begrip mechanische sterkte is een zeer oud wetenschappelijk begrip dat indirect overal zijn sporen nalaat in onze geschiedenis en onze hedendaagse cultuur. De mens heeft altijd getracht om het begrip te doorgronden en er optimaal gebruik van te maken voorde meest uiteenlopende doeleinden. Deze enorme voorkennis diepgaand exploiteren met moderne wetenschap, technologie en simulatietechnieken zal het begrip in deze eenentwintigste eeuw wellicht letterlijk en figuurlijk levendiger maken dan ooit. Daar waar vroeger 'mechanische sterkte' erkend werd als een inherente eigenschap van materialen en structuren, proberen we vandaag de fundamenten van de complexe onderliggende realiteit bloot te leggen. Het succes van deze evolutie wordt gedragen door de ladder van de machten van tien, waarbij de verbanden tussen het nano-, micro-, meso- en macroniveau een essentieel uitgangspunt vormen. 'Multi-scale mechanics' (figuur 1) is een begrip aan het worden dat de rode draad vormt bij tal van wetenschappelijke ontwikkelingen in dit vakgebied. Het hoe en het waarom van deze ontwikkeling, het belang voor de industriele technologie en voor de wetenschap in het algemeen, vormen de kern van dit artikel
Preface to special issue on Multiscale computational homogenization : from microstructure to properties
No full text
A composite dislocation cell model to describe strain path change effects in BCC metals
No full textSheet metal forming processes are within the core of many modern manufacturing technologies, as applied in, e.g., automotive and packaging industries. Initially flat sheet material is forced to transform plastically into a three-dimensional shape through complex loading modes. Deviation from a proportional strain path is associated with hardening or softening of the material due to the induced plastic anisotropy resulting from the prior deformation. The main cause of these transient anisotropic effects at moderate strains is attributed to the evolving underlying dislocation microstructures. In this paper, a composite dislocation cell model, which explicitly describes the dislocation structure evolution, is combined with a BCC crystal plasticity framework to bridge the microstructure evolution and its macroscopic anisotropic effects. Monotonic and multi-stage loading simulations are conducted for a single crystal and polycrystal BCC metal, and the obtained macroscopic results and dislocation substructure evolution are compared qualitatively with the published experimental observations.Mechanical, Maritime and Materials Engineerin
Mechanics of dislocation pile-ups : a unification of scaling regimes
No full textThis paper unravels the problem of an idealised pile-up of n infinite, equi-spaced walls of edge dislocations at equilibrium. We define a dimensionless parameter that depends on the geometric, constitutive and loading parameters of the problem, and we identify five different scaling regimes corresponding to different values of that parameter for large n. For each of the cases we perform a micro-to-meso-upscaling, and we obtain five expressions for the mesoscopic (continuum) internal stress. The upscaling method we illustrate here can be made mathematically rigorous, as we show in the companion paper (Geers et al., 2013. Asymptotic behaviour of a pile-up of infinite walls of edge dislocations. Arch. Ration. Mech. Anal. 209, 495–539). The focus of the present paper is on the mechanical interpretation of the resulting internal stresses. In the continuum limit we recover some expressions for the internal stress that are already in use in the mechanical community, as well as some new models. The results in this paper offer a unifying approach to such models, since they can be viewed as the outcome of the same discrete dislocation setup, for different values of the dimensionless parameter (i.e., for different local dislocations arrangements). In addition, the rigorous nature of the upscaling removes the need for ad hoc assumptions.
Keywords: Dislocations; Pile-up; Internal stress; Plasticity; Upscalin
Deformation and failure kinetics of polyvinylidene fluoride: Influence of crystallinity
No full textThe present study investigates the effect of processing conditions on the yield kinetics, such as rate dependence of the yield stress and creep rupture, of polyvinilidene fluoride. Samples were compression molded with cooling rates varying from 100°C/s to 0.5°C/min, or isothermally crystallized at temperatures varying from 20 to 120°C. Deformation kinetics were studied over a wide range of strain rates and temperatures. It is shown that for all conditions the yield response is well represented by the Ree–Eyring model. Moreover, the activation volumes and activation energies are independent from the processing conditions. The effect of processing is fully covered by a simple relationship between the rate factors and the degree of crystallinity. Subsequently, the versatility of this relationship is demonstrated by experimental validation
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