What is required to understand how to tell the difference between a macro-scale ductile and a macro-scale brittle crack is a basic understanding of whether shear (sliding) or normal (crack opening) stresses are more likely to create a crack in a given component. This does actually require an understanding of how the component is loaded in service. Service loads are usually the primary factor in determining the stress state of the component. Residual stresses sometimes also become an important factor in determining the stress state and thus how the crack may have happened.
Again, we see that there is room for specialists who know how to sort out the complicated cases. However, a strong basis for being able to distinguish a macro-scale brittle from a macro-scale ductile crack may be obtained by looking at six simple loading geometries in a few basic shape configurations.
These include axial loading of a cylindrical rod or bar (Figure 1), compression loading of a cylindrical bar, bending of a rectangular beam (Figure 2), torsion of a cylindrical shaft (Figure 3), contact stresses due to the intense pressure created in a bearing ball and its associated races (Figure 4), and direct shear stresses such as experienced by rivets holding two plates together. The next columns in this series will address these basic loading geometries in components of simple shapes.
The only other detail remaining in gaining a basic understanding of how to tell ductile from brittle fracture is learning to interpret micro-scale features. A complete fracture analysis, often required to determine areas of responsibility in field failures, needs to determine both macro- and micro-level crack features.
The macro-level features generally shed the most light on how the part was loaded while the micro-level features shed the most light on whether the material was processed properly.
After reviewing the basic loading geometries, this column will move on to cover aspects of microfractography. A scanning electron microscope is generally (although not always) needed to perform microfractography. In order to interpret microfractographic data, an understanding of physical metallurgy and an ability to interpret microstructure data is required.
The two basic types of micro-scale brittle fractures (Figures 5 and 6) and the one basic type of micro-scale ductile cracking are shown here (Figure 7).
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