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logo SMC logo SMC  NLIN logo SMC  NLIN  PROJ4 Small scale plasticity in metallic materials
   
Contact Thierry J. Massart
   
Keywords small scale plasticity, gradient plasticity, phase transformations, grain refinement, variable confinement, thin films
   
Collaborations
  • Prof. T. Pardoen, Université Catholique de Louvain, Belgium
  • Prof. S. Godet, Chemicals and Materials Department, ULB, Belgium
   
Scientific motivation Size effects play an important role in the behaviour of freestanding thin metallic films. This research focuses on the simulation of the yield increase and of the decrease in ductility observed at such small scales.

Classical plasticity theories do not account for the increase of hardening associated to a size reduction of microstructural features typically in the micrometer range. Phenomenological formulations have been developed to incorporate such effects by introducing a contribution of the strain gradients in the constitutive setting, among which the higher order extension of the J2 theory of plasticity. A generalized effective plastic strain rate is therefore defined, incorporating the plastic strain and its gradients as well as intrinsic length parameters setting the scales at which plastic gradients play a role. The implementation of such descriptions allows assessing the effect of specific boundary conditions on the plastic flow at the boundary of a region in which plastic flow occurs. Such conditions have to be motivated from the physical understanding of the dislocation mechanics. An interface between different and strongly bonded phases is usually modelled as impenetrable to dislocations by preventing any plastic deformation along the interface, while natural boundary conditions are applied at unconfined interfaces.

   
Transformation induced plasticity Size effects in phase transformation induced plasticity

The objective of this research project consists in analysing the potential size effects responsible for hardening enhancements by means of transformation induced plasticity in multi-phase steels. A simplified microstructural description is used with a single autenitic inclusion inside a ferritic matrix, and which partially transforms into martensite. Evolving plastic confinement conditions can be used at the forming martensitic boundary to simulate transformation, together with an eigenstrain.

NL 4 Fig1

Based on a isotropic strain gradient plasticity formulation (Fleck-Hutchinson formulation), the various contribution to hardening are analysed, leading to the following conclusions

  • the use of a strain gradient plasticity theory leads to much larger strengthening related to the TRIP effect than when using a classical plasticity theory,
  • the boundary conditions specified on the plastic flow play a major role regarding the impact of the transformation strain,
  • when the plastic flow is unconstrained, the transformation strain has a significant overall effect on the strengthening only when using the full Fleck-Hutchinson theory. In other cases, most of the strengthening comes from the evolving higher order composite effect associated to the presence of a new hard inclusion. 
     
Interfacial effects Interfacial effects in small scale plasticity

The impenetrability of interfaces present in materials may in some circumstances evolve along the loading conditions (evolving grain boundaries, appearing twin boundaries, phase transformations). Grain boundaries in polycrystals evolve from fully impenetrable to dislocations at low dislocation density towards partially transparent when a steady state recovery/dislocation multiplication process develops, as illustrated in the Figure below. Using evolving confinement at pre-defined interfaces in microstructures allows simultaneously capturing the increasing yield strength and loss of strain hardening capacity associated to a decreasing grain size.

NL 4 Fig2

   
Thin films plasticity Size effects in thin films plasticity

Size effects play an important role in the behaviour of freestanding thin metallic films. This research focuses on the simulation of the yield increase and of the decrease in ductility observed at such small scales.

     
Support
  • Walloon Region
  • F.R.S.-FNRS
     
Selected publications
  • [1] L. Mazzoni-Leduc, T. Pardoen, T.J. Massart, Strain gradient plasticity analysis of transformation induced plasticity in multi-phase steels, International Journal of Solids and Structures, 45, 5397-5418, 2009.
  • [2] L. Mazzoni-Leduc, T. Pardoen, T.J. Massart, Analysis of size effects associated to the transformation strain in TRIP steels with strain gradient plasticity, European Journal of Mechanics A/Solids, Available on-line, 2010.
  • [3] T.J. Massart, T. Pardoen, Strain gradient plasticity analysis of the grain size dependent strength and ductility of polycrystals with evolving grain boundary confinement, In Preparation, 2010.
  • [4] T.J. Massart, T. Pardoen, Grain boundary confinement in multi-phase polycrystals and its application to dual phase steels, In Preparation, 2010.