Page 200 - Fiber Fracture

STRENGTH AND FRACTURE OF METALLIC FILAMENTS 185

INTRODUCTION

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.
Metallic filaments, however, are not always full of such defects. Whiskers with
diameters of a few micrometers even approach the picture we have from an ideal
crystal. The absence of even intrinsic crystalline defects gives rise to the well-known
size effect in the rupture stress. Interestingly, also in the case of a completely disordered
crystal structure such as amorphous metals, which apart from a few alloys of complex
composition can only be produced in filamentary form, an extremely high resistance to
fracture and fatigue is observed.
Similarly, in polycrystalline wires extremely high rupture stresses which by far
exceed the values observed in the bulk metals may be obtained through the strain hard-
ening resulting from the drawing process. Recrystallization treatments of polycrystalline
wires reduce the intrinsic defect concentrations and increase the grain size, give rise as
in bulky samples to soft structures. Interesting size effects in the yield stress and the
crack initiation in fatigue become, however, apparent in micro-wires when the number
of grains on a cross-section becomes small (oligocrystalline microstructure).

FAILURE DUE TO FABRICATION AND EXTERNALLY INTRODUCED
DEFECTS

Drawing Defects, Nonhomogeneous Microstructure and Texture

The majority of wires and metallic filaments are produced by drawing. The extreme
cold work to which the metal is subjected modify its microstructure and in most cases
introduces a strong fiber texture. Both the microstructure and the texture are known
to have a strong influence on the mechanical behavior of the wires and are in many
cases exploited to achieve the desired properties. Moreover, the deformation and the
material flow are very nonhomogeneous and depend on the form of the die, the friction
between wire and die, and the strain hardening capacity of the metal. This gives rise to
heterogeneous microstructures and may cause macroscopic structural defects.