Brittle Fracture

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed
in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating
crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them
indicates that the maximum crack velocity is 800 m s1, which corresponds to 0.32 times the shear wave speed. Fractography reveals
that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-pennyshaped
cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic
stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the
dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of 79 nm. Juxtaposition of the crack
velocity with this spacing suggests that the crack takes 1010 s for peak-to-peak propagation. This, and the estimated adiabatic temperature
rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation
formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack
tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic
oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed

The use of polymeric material such as polyethylene (PE) and polyamides (PA) made it possible to achieve significant profits in design construction times and installation costs. The objective of this study is to highlight the various mechanisms of rupture of polyethylene pipes in service and in laboratory conditions under fatigue and creep loadings. It is known is some cases that at least two mechanisms control PE pipe failures based on results cumulated in operating conditions. They are nominally ductile and brittle mechanisms respectively characterizing short and long-term failures. Several laboratory tests are used to extract design data for long-term failure-type prediction based on stress and time-to-failure relationship. It remains difficult to assess the relation between creep and fatigue loadings on one side. On the other side, the manufacturing process of the test specimens influences considerably the obtained performance for viscoelastic materials subjected to working conditions and environmental effects. Brittle-to-ductile transition is studied under fatigue crack propagation mode using an energy criterion. The brittle fracture damage zone is characterized by a single craze made up by locally drawn fibers and dispersed voids whereas ductile rupture is rather dominated by highly yielded material and significantly transformed matter as observed under polarized-light microscopy. The assessment of polyethylene pipe and polyamide parts failure mechanisms is to contribute to a better understanding of effects of other external chemical agents such as solvents in degrading the pipe overall resistance. Recent results from environmental stress cracking of PE pipe and exposed polyamide PA66 to detergent will be presented and correlated to mechanical properties degradation.