This work presents an investigation of fracture and fatigue in thin ribbons of a nickel base metallic glass:​ Ni₇₈Si₁₀B₁₂. The fracture and fatigue crack propagation behaviour of this high tensile strength (2100 MPa) and high toughness (67 MPa√m) amorphous alloy is of interest for two reasons:​ firstly, the alloy has no normal metallurgical microstructure, and secondly, the alloy shows an unusual form of plastic deformation which proceeds by nucleation and propagation of localized shear bands. On uniaxial tensile loading, failure of uniform ribbons occurs instantaneously at the yield stress by shear rupture through an intense shear band inclined at 45 degrees to the loading axis.

The development of a local plastic zone at the crack tip in single edge-notched specimens under monotonic tensile loading has been investigated by a replication technique. Under plane stress conditions, these plastic zones are dominated by elongated shear bands. Dugdale's 'strip yield' model offers a reasonable description of the zone size and displacement at the crack tip.

The propagation of shear cracks has also been studied by carrying out 'trouser leg' tear tests. The tear energy is found to depend strongly on specimen thickness and testing temperature. Three types of shear band are observed near the tip of the tear crack;​ one type is associated with the bending of the testpiece while the others are evidence of types II and III shear deformation. A model based on the geometry of these bands has been developed and used to obtain an estimate of the plastic work involved in tearing. Good agreement is found between the calculated and measured tear energies.

The relationship between fatigue crack growth per cycle, da/dN, and the alternating stress intensity factor, AK, has been determined as a function of R-ratio. For growth rates in the range 10⁻⁶-5 × 10⁻⁴ mm/cycle, the Paris Law (with an exponent m≈2) is obeyed independent of the R-ratio. The mechanisms of fatigue crack extension are shown to depend on the deformation microstructure of the alloy. At intermediate ΔK's, decohesion of 'parabolic' shear bands produces faceted fracture morphologies. At near threshold values of ΔK, growth rates deviate from the Paris Law, producing an extremely low ΔK_TH. This is accompanied by an unusual dependence on the R-ratio. As the R-ratio is raised, ΔK_TH increases and growth rates decrease. It is proposed that the applied mechanical factors modify the local crack driving force. This is confirmed by establishing a correlation between ΔK_eff and the crack tip opening displacements.