Brittle failure embrittlement, fracture mechanics

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. 

The temperature corresponding to CTy = CTbf is referred to as the transition temperature. Ioffe s explanation of cold embrittlement can be characterised in a number of ways. It is well known that absolute brittle failure of metals is not encountered in practice. As a rule, traces of slip deformation are seen on the surface of a material that has undergone brittle failure. However, signs of plastic deformation on the fracture surfaces of macroscopically brittle specimens do not by themselves constitute incontestable proof that deformation precedes fracturing, as opposed to both mechanisms occurring simultaneously. 

This type of attack does not require any specific environment to take place, since it can take place simply in neutral or acidic wet environments. Failure due to hydrogen is named hydrogen embrittlement since it leads to a brittle-Uke fracture surface. Indeed, the ductility of the bulk metal does not change, but the propagation of the crack is due to the mechanical stresses induced in the lattice by hydrogen accumulated near the crack tip. If hydrogen is present in the metal lattice before the application of loads a delayed fracture may occur, i. e. the steel does not fail when the load is applied, but after a certain time. 

The use of a light or optical microscope to examine fracture surfaces is virtually indispensable as a first step in understanding failure mechanisms of plastic parts. Visual/microscopic inspection of a failed component may assist in narrowing down the cause of failure. A specimen is commonly checked for surface imperfections, embrittlement, extent and location of cracking, nature of cracking (ductile or brittle), chalking, crazing, discoloration, contamination, etc. 

---