We need to invoke some general principles of fracture mechanics in order to explain why these failures occurred.  In particular, cast irons tend to be relatively low toughness materials, particularly when sharp or irregular graphite flakes are present.  There also exists a general inverse trend for fracture toughness to decrease as the tensile strength of alloys increases. This is illustrated in the tutorial example hyperlinked from this page. The metallographic information clearly indicates that the microstructure of the cast iron insulator caps changes from ferritic to pearlitic during the first arc strike, which also induces small cracks because of the rapid cooling and the ensuing brittle structure. This is analogous to autogenous welding (i.e. welding with no filler metal) and will give rise to the type of problem mentioned on the linked welding information page.  Compounding the decrease in fracture toughness arising from a change in microstructure and hardness, the presence of small defects will also be highly detrimental to fracture stress. Subsequent arc strikes can then readily lead to crack growth to critical sizes.

These effects could be relatively simply illustrated using the Griffiths energy approach to fracture, provided that we know the energy associated with continued growth of a sharp crack. We could then examine typical changes to fracture stress (compared with the tensile strength value of 278-355 MPa) arising from small defects and change in microstructure that influence the value of the materials resistance to crack growth (or fracture toughness expressed as J/m2) in the Griffiths formulation given below:

where R is fracture toughness in J/m2, E is the elastic modulus (= 172 GPa), v is Poisson's ratio (= 0.34) and a is induced defect length in mm.  The only problem with this approach lies in obtaining values for the resistance to continuing crack growth, expressed as J/m2. For instance, the dynamic tear energy for malleable cast iron, as a function of microstructure, is shown in Figure 1.
Toughness_vs_microstructure.JPG (100579 bytes)
Figure 1

The dynamic tear test is version of qualitative fracture toughness testing that provides better discrimination (through increased specimen size) between materials that would show low Charpy impact toughness values.  It also provides better notch toughness information for materials which show a rising R curve with increase in crack length (generally thinner, more ductile materials in plane stress).  However, values of both dynamic tear energy and Charpy impact energy are total energy values that include significant contributions from crack initiation processes, as well as crack growth, and hence cannot be used in the Griffiths equation. What Figure 1 does indicate, however, is a significant reduction in notch toughness of malleable iron with a change from an annealed ferrite structure to one that contains high as-cast pearlite percentages.

We need to rather use a stress intensity approach to calculating reduction in fracture strength as a function of defect size. Plane strain fracture toughness values applicable to this particular malleable iron are difficult to estimate, but a value of 20-30 MPam for an as-cast pearlitic structure is not unreasonable.  Stress intensity values can then be estimated for the case of edge cracks, using the formulation:

Use the applet below to calculate the fracture stress as a function of crack size and plane strain fracture toughness. At what crack length does the theoretical fracture stress start to decrease below the observed value of tensile strength?

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