In earlier columns, we looked at some photos of broken parts to gain an appreciation for the fact that ductile and brittle fractures may be more complicated than the commonly known method of looking for obvious changes in component shape might indicate. We discovered that a more powerful method of classifying ductile and brittle component fractures is by determination of what types of stresses allow the cracks to happen. Specifically, if shear stresses cause the crack, then deformation precedes and accompanies the crack event and a ductile crack occurs. Thus, we saw that transverse cracks on cylindrical shafts caused by torsion are indeed ductile cracks, even though it is sometimes difficult to see any obvious deformation even with close and careful inspection.
If “normal” or “crack opening” stresses allow the crack to happen, then the deformation that does occur is usually more limited and may be confined over much of the crack surface to a very small portion of the overall volume of the component. Normal stresses promote brittle cracks. Note again that any deformation that happens after the crack event due to the totally new stress state that results from the discontinuity in the part shape (previously described chain link), does not change the fact that the crack event of interest was in fact brittle on the macro, or visible, scale.
This again allows us to see the importance of understanding the difference between brittle and ductile material and a brittle and ductile crack. Ductile materials often undergo brittle cracking, especially when the determination of ductile or brittle is made on the macro scale alone. Obviously, ductile materials often do actually crack in a ductile manner. Inherently brittle materials, on the other hand, rarely crack in a ductile manner.
Why should you as a heat treater care about ductile and brittle cracks? Well, basically, money. If someone wants to hold you responsible for incorrect heat treating that they claim was the “cause” of a brittle failure, it would be nice if you or one of your staff knew how to tell the difference between a truly brittle fracture and one that only appears to the untrained person to be brittle. Why take the heat for something you did not do? Some basic home study can provide the foundations to the average person to be able to understand how to distinguish a macro-scale brittle crack from a macro-scale ductile crack. And unless the part is so tiny that a microscope is required to see it, no expensive tools are required to make this determination. A reasonably good pair of eyeballs (or poorer eyeballs enhanced with a magnifying glass) and basically trained brain are adequate.
This does not mean that there are no “head scratchers.” Sometimes damage to the component does indeed make it difficult to figure out what is going on. There is still room in the complex world of technology for us specialists to actually earn our keep! Don’t agree to take the blame for a brittle crack unless someone has demonstrated that the crack was really brittle by both macro (what we have been talking about) and micro (to be covered later) scale criteria!
The next installment will discuss how to tell the difference between a macro-scale ductile and a macro-scale brittle crack.