Obvious & Not So Obvious Ductile & Brittle Fractures

by Debbie Aliya

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?

Figure 1

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.

Figure 2

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.

Figure 4 shows another “tricky” part. We are looking at the fracture surface of a shaft loaded in torsion. We see the smear marks that we might think happened after the crack had completely fragmented the shaft, perhaps due to rubbing of the mating fracture face. There is no necking. The diameter is unchanged at the crack area. So, this is a brittle crack, right? Think again! This shaft was loaded in torsion, so the crack opening stresses, the normal stresses, act on helical planes that are at 45 degrees to the axis of the shaft. Brittle cracks are helical when they are created in torsion. Prove this to yourself by twisting a carrot or a piece of chalk. These are inherently brittle materials, and they will not usually break in any way but in a helical shape.

Figure 3

The planes where shear stresses can act in torsional loading are the disk-shaped transverse planes and the rectangular longitudinal (radial) planes. This steel was quite hard (HRC42), but it was highly stressed and twisted off in a ductile manner. In fact, if you look at the cylindrical surfaces, which may have longitudinal grinding marks for example, you can often see evidence of twisting, even though there is no obvious change in shape to the casual observer.

We conclude that when we see smear marks like this, (unless it was a fatigue crack caused by rotating bending, which has a totally different stress state and, in general, does not create such a flat fracture surface) they actually were created during the actual separation process, and this is a macro scale ductile part!

Understanding how a component fails is an important step in understanding why a component fails. In order to understand how a component cracks, it is important to understand what loading geometry or geometries could have been responsible for the fracture. It is equally important to understand how high the load was or how fast the component was loaded, and the basic loading geometries, including tension, compression, bending, torsion, contact stresses and direct shear. The failure analyst must strive to learn to “read” the fragment shapes to determine what loading geometry was actually present. This is a key to being able to properly determine whether the component was installed and used per the design intent.

Figure 4

We looked at the previous 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.

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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.

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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!

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.

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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).

Debbie Aliya

Debbie Aliya Debbie Aliya is the owner and president of Aliya Analytical, Inc. in Grand Rapids, Mich., and specializes in failure analysis and prevention. She has a BS in Metallurgy and Materials Science from Carnegie Mellon University and an MS in Materials Science and Engineering from Northwestern University. She is also an IMT associate.