Pekka Heino, Tommi Peussa, and Kimmo Kaski
In this subproject we have studied dynamic fracture of disordered solids. The research is based on numerical modeling employing either mesoscopic models [7,10,8] or molecular dynamics simulation technique .
The microstructure of amorphous materials such as PMMA, plexiglass, that has been used in recent experiments is disordered. We applied mesoscopic models with topological disorder to study the role of disorder in dynamic fracture. The models consist of either 2 dimensional triangular elements or interacting mass sites forming a 2 dimensional network. Disorder was generated by choosing the initial locations for nodes or mass sites at random and by solving the problem of nearest neighbors with Voronoi polygons. In the simulations the bending of the daughter cracks was quantitatively in agreement with experimental results. In addition, the simulated branching angle for the longest daughter cracks was in agreement with the experiments. In the future dynamic fracture in topologically disordered systems will be studied in three dimensions, too.
In addition, we have studied the role of correlated density disorder being present in many fibrous materials, e.g. paper. Two dimensional correlation was generated by adding elliptic ``flocs'' to the plane describing the strength of interaction between neighboring mass sites. We found the floc orientation to affect the crack propagation in a dynamic system. When the floc orientation was parallel to straining, flocs could arrest a propagating crack and its branches. In the opposite case, floc orientation being perpendicular to straining, flocs could arrest only the main crack and the branches were able to continue to propagate resulting crack curving. In addition, we studied the scaling of the maximum strength of the system with its size and found similar behavior with other studies.
The role of viscoelasticity was studied by introducing a viscoelastic term into the mesoscopic models. Viscoelasticity was seen to be an effective mechanism for crack arrest. In addition inclusion of viscoelasticity reduced the number of daughter cracks and decreased their length.
As an example of microscopically described viscoelastic material we have studied mechanical properties of copper using the effective medium theory and the molecular dynamics method. So far we have concentrated on ordered system  but in the future the role of disorder will be studied.