Many engineers think it is pretty easy to distinguish a ductile from a brittle crack. However, many people have incomplete understandings of these concepts.
Look at the following two pictures of a broken steel tensile test coupon and a broken bolt.
Figure 1 shows a necked down area right around the crack. This is a sure sign of a ductile fracture. Figure 2 shows a crack which followed the thread root, but did not have any readily observable change in shape. Hence, it is an obvious brittle crack.
Many people think that necking or visible deformation of the part means it is a ductile crack, and lack of such readily visible deformation means it is a brittle crack. But this is not necessarily the case. Let’s look at some trickier parts.
The chain link in Figure 3 used to have a symmetrical shape and be a continuous flattened loop. But now there is a crack, and the shape has obviously changed from the as-manufactured symmetry. So this is a ductile crack, right?
The question here related to the crack process itself, and not what happens after the crack has completely separated the previously connected material. A chain link is loaded in tension, so there would have been tensile stresses acting to pull the material apart along its length. In this case, the majority of the crack surface is more or less perpendicular to the length of the material on either side. The deformation is not associated with the crack process; rather the deformation happened after the crack let go, which now allowed bending stresses to be created in what is shown as the lower portion of the link. The portions of the link near the crack are actually pretty straight, as they were at the time the link was made. It is not possible to see in this single view, but the material adjacent to the crack is not necked down at all. This was a brittle crack, in a ductile material. The fact that the material was ductile is shown by the deformation elsewhere in the link.
Of course, it would require a complete analysis to determine if there was some local embrittlement at the crack area. But my guess is that there was not. The crack was found through additional examination to have beach marks typical of a fatigue crack. A fatigue crack is by definition one that propagates below the nominal yield strength, even though locally, the stress must be above the tensile strength, or no crack could grow. In any case, it is not possible to have macro scale (visible) deformation when the high stress is so localized. A fatigue crack has only very limited (“microscale”) ductility in the layers immediately adjacent to the crack.
A good definition of a macroscale brittle crack is one that grows due to “normal stresses,” which are not the opposite of “abnormal stresses,” but are the stresses that cause crack opening. The “opposite” or alternative to normal stresses is shear stresses. Shear stresses are sliding stresses. Shear stresses are what allow deformation to happen.
The chain link had shear stresses in it, but they were probably only about half of the level of the normal stresses in this type of loading. The shear stresses in tensile loading are at 45 degrees to the normal stresses. In fact, we can see a tiny “shear lip” at 45 degrees to the main part length. This final portion to separate along the upper edge of the fracture surface was macroscale ductile. Some people might call this a “slant fracture.” Until there was very little connecting ligament left, the shear stresses were not able to cause any large scale deformation. The level of the local normal stress was obviously high enough to initiate a crack even when most of the section was intact. Note that this link was used in a meat processing plant, and harsh chemicals are used to clean the equipment. It is likely that corrosion allowed the crack to initiate in the first place.
Check back next week (7-14) for Part 2.