Elasto Plastic Fracture Mechanics

Once the crack is nucleated, crack growth can be modelled using the above-mentioned models, or from a more macroscopic point of view by using the techniques of elasto-plastic fracture mechanics (EPFM) (see, for example, Anderson, 1995, and Broberg, 1999). To the authors knowledge, few results are available of ductile fibre failure based on EPFM, most probably because the small size of fibre diameters invalidates some of the hypotheses on which this theory is based. 

Fracture of massive brittle and ductile pieces are rather well understood. By taking proper account of the microstructure as well as the micro- and macro-defects, most catastrophic and fatigue failures find a satisfactory explanation within the scope of the linear elastic fracture mechanics or the elasto-plastic fracture mechanics. Metallic filaments are particular and in many respects deserve a treatment of their own. Particular fabrication methods, such as drawing, melt spinning or crystallization from the vapor phase for whiskers are needed to obtain their small lateral dimensions. These processes may give rise to particular textures, intrinsic and extrinsic defects. Thermal treatments may modify or eliminate such defects but in many cases fracture is initiated by defects that stem from the fabrication process. Moreover, the small lateral dimensions, especially in micro-wires, make metallic filaments prone to external influences. Corrosive attacks may rapidly affect an important fraction of their cross-section. Hydrogen, for instance, which usually results in a severe embrittlement, may diffuse up to the core in a rather short time. 

It is very important, from one hand, to accept a hypothesis about the material fracture properties before physical model building because general view of TF is going to change depending on mechanical model (brittle, elasto-plastic, visco-elasto-plastic, ete.) of the material. From the other hand, it is necessary to keep in mind that the material response to loads or actions is different depending on the accepted mechanical model because rheological properties of the material determine type of response in time. The most remarkable difference can be observed between brittle materials and materials with explicit plastic properties. 

The mechanical behavior of Zr02-Ni system strongly depends on constitutional variation. The Ni-rich materials exhibit typical behavior of elasto-plastic deformation and ductile fracture similar to metallic material. The materials containing PSZ from 40-80 vol% mainly presents typically linear elastic behavior and macroscopic brittle fracture. However, the material with 60 vol% PSZ behaves as non-linear elastic behavior after the linear stage. 

Application of fracture mechanics to elasto-plastic materials... 

At present the shadow optical method represents a powerful tool in experimental fracture mechanics for the determination of stress intensity factors at the tips of quasistatically extending and fast running cracks, respectively. Further, the method of caustics can also be applied for mixed-mode crack problems as well as for the investigation of plastic zones around crack tips propagating in elasto-plastic materials. Moreover, due to the possibility of applying the method auf caustics in transmission as well as in reflection the treatment of cracked opaque model materials can also be performed. A comprehensive review about the shadow optical method of caustics has been given In reference [16]. 

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