Microscopically brittle fracture

In some environments and under certain conditions, a microscopically brittle fracture of materials can occur at levels of mechanical stress that may be far below those required for general yielding or those that could lead to significant damage in the absence of an environment. This susceptibility also depends on the chemical composition and... 

In some environments and under certain conditions, a microscopically brittle fracture of materials can occur at levels of mechanical stress that may be far below the level required for general yielding or those that cause significant damage in the absence of an environment. The susceptibility also depends on the chemical composition and microstructure of the alloy. This form of corrosion requires an interaction between the electrochemical dissolution of the metal, hydrogen absorption, and the mechanical loading conditions (stress, strain, and stain rate) (73). The nature of these fracture modes varies from one class of material to another. However, all fracture modes are largely similar to one another. 

A cleavage fracture (microscopically brittle fracture) occurs (almost) without microscopic deformation perpendicular to the largest tensile stress. Bonds between the atoms break. In face-centred cubic metals, the ductility is so large that cleavage fracture can occur in extreme cases only. In body-centred cubic metals, cleavage fracture can occur at low temperature or high strain rates in ceramics, cleavage fracture is the standard case. 

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

Paraffin is an odorless and tasteless, translucent, colorless, or white solid. It feels slightly greasy to the touch and may show a brittle fracture. Microscopically, it is a mixture of bundles of microcrystals. Paraffin burns with a luminous, sooty flame. When melted, paraffin is essentially without fluorescence in daylight a slight odor may be apparent. 

Pressure vessels are also subject to cyclic stress. Cyclic stress arises from pressure and/or temperature cycles on the metal. Cyclic stress can lead to fatigue failure. Fatigue failure, discussed in more detail in Module 5, can be initiated by microscopic cracks and notches and even by grinding and machining marks on the surface. The same (or similar) defects also favor brittle fracture. 

The microscopic aspect of the surface where the crack propagates due to stress corrosion and that where the final brittle fracture occurs are rather different. 

Mechanical testing on the foam is performed to measure the compressive stress and the resiliency. The results from diese tests are shown in table 2, in which the properties of extruded polystyrene foam (XPS) and commercial starch-based loose-frll foams (Eco-foam and Mater-Bi) together with EPS loose-fill foam (Pelaspan Pac) are added for conq>arison. The values of XPS are obtained from tests on typical XPS retail packaging trays. The table shows that the compressive stress reached with potato starch foam is comparable with that of XPS. Through the cell structure of the potato starch foam (high cell density, very small cells) a good resiliency can be obtained, although pure starch plastics exhibit brittle fracture behavior. This brittle fracture still is present on the microscopic scale of the individual cells but due to the cell density, the foam exhibits resiliency on macroscopic scale. 

Shear fracture (microscopically ductile fracture) occurs by plastic deformation with slip in the direction of planes of maximum shear stress (see sections 3.3.2 and 6.2.5). Therefore, it occurs only in ductile materials. In most cases, shear fracture is associated with large macroscopic deformations, as, for example, in a tensile test. However, if this is prevented by the component geometry, the component may fail macroscopically brittle, but still with a shear fracture. This may happen if there are notches or cracks in the material (see chapters 4 and 5). 

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