Browsing by Author "Brödling, Nils Christian"

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Open Access
Multiscale modeling of fracture and deformation in interface controlled materials
(2007) Brödling, Nils Christian; Arzt, Eduard (Prof. Dr. phil.)
Many nanostructured metals are characterized by scale dependent mechanical properties and by size effects due to geometrical confinement. Dislocation activities, interface mediated plasticity, and macroscopic yielding are quite different from those in unconstrained metals. The role of interfaces for the material properties and for the governing deformation mechanisms remains unclear despite the large efforts made in experimental and theoretical investigations. Here we approach the effect of geometrical confinement on the atomic and on the mesoscopic scale. We elucidate size effects on failure mechanisms and on scale dependent plasticity of nanostructured dual phase composite materials with the aid of computer simulations. Cleavage failure of dual phase layered materials is simulated with a mesoscopic model to clarify the scaling behavior of the materials fracture toughness. The model accounts for the confinement effect that a layer geometry imposes on the collective dislocation behaviour near a moving crack tip. The critical layer thickness at which the bulk fracture toughness of the elastic-plastic material is reached as well as the bulk fracture toughness itself increase with the cohesive strength of the interface, but become smaller for higher yield strengths. The main conclusion drawn in this work is that fracture toughness as a function of layer thickness saturates gradually if dislocation activity is dispersed, dilute and not compact around the crack tip. It increases abruptly with the thickness when dislocation activity right at the crack tip is possible and a compact, shielding dislocation array forms near the crack tip. Furthermore this work provides preliminary understanding of the governing mechanisms that control the limiting length scale for the strengthening of bioinspired metallic nanocomposits. Large-scale molecular dynamics simulations are performed to investigate the plastic deformation behavior of a bioinspired metallic nanocomposite which consists of hard nanosized Ni platelets embedded in a soft Al matrix. The simulation results are analyzed with respect to the prevailing deformation mechanisms quantifying the contribution of dislocation-based plasticity and interface-mediated interfacial slip as a function of the nanostructural scaling. The results of the simulations show that interfacial sliding contributes significantly to the plastic deformation despite a strong bonding across the interface. Critical for the strength of the nanocomposite is the geometric confinement of dislocation processes in the plastic phase. The confinement effect strongly depends on the length scale and the morphology of the metallic nanostructure. The main conclusion drawn for this material is that below a critical length scale, the softening caused by interfacial sliding prevails, giving rise to a maximum strength at the optimum size.