Material Failures – The Role Of Fractography In Determining The Cause
Just as cartography describes the study of maps, fractography describes the study of fracture surfaces. Many times, fractography is used to determine the cause of a failure. While it may seem a little like tea-leaf reading, fractography is a well-established science going back hundreds of years. Some of the first scientists to break materials in a laboratory reported the appearance of the fracture surfaces in addition to forces measured. It can be likened to the practice of medicine. While many patients may have the same disease, that disease may present itself differently in each patient. Likewise, while many materials may fail by fatigue, each material’s fracture surface may appear different, depending upon the circumstances that led to the failure.
Fractography can be summarized as a three-step process: First, the investigator tries to classify the failure as either “ductile” or “brittle”. (“Ductile fracture” is similar to the fracture of chewed gum as it is stretched to failure – there is a lot of deformation and the fracture surface isn’t flat. “Brittle fracture” is similar to the fracture of glass – there isn’t much deformation and the fracture surfaces are nearly flat). Second, the investigator tries to determine where the fracture originated. And third, once the origin is identified, the investigator looks closely for any sign of what started the failure. Typically, one looks for voids (bubbles) or inclusions (impurities). Also, the perpendicular surface to the origin is examined for any marks or scratches.
Some investigators may insist that fractography can only be performed using a scanning electron microscope (SEM). While this may be true during the third step of the typical fractographic study, the first and second steps can many times be accomplished with just the human eye. Many features of a failure mode can be seen without magnification, and fractography can be performed with a well-trained eye. For example, “beach markings”, a feature associated with fatigue, are almost always visible to the human eye.
In the example on the left, a pair of glasses worn to correct vision broke via fatigue. The beach marks in the fatigue zone can be seen without the use of an SEM. In other cases, a fracture surface may appear flat overall. This is an indicator of brittle failure that can be observed with the eye. Finally, in metals, a fracture surface may appear to sparkle in sunlight (or similar light), which often indicates microscopic areas of flat fracture consistent with brittle fracture.
In general, ductile failure is preferred over brittle failure. Ductile failure provides some warning before failing, as the material bends before breaking. In addition, and most importantly, ductile failure requires more energy to cause failure. If steel is heat-treated improperly so that it fails in brittle fashion rather than ductile fashion, the energy required to fail the steel is less. Ductile failure is good; brittle failure is bad. Therefore, if a fractography inspection determines that a normally ductile material failed in brittle fashion, we often conclude that the material was defective. If it failed in a ductile fashion, we often conclude that the material was not defective and that somehow the forces on it were higher than expected.
If the failure mode was fatigue failure, the focus is on the origin. Fatigue failure occurs when a material is subjected to repeated cyclic loading – for example, when a machine is run for several hours a day and turned off. Most of the time leading up to a fatigue failure is spent starting a crack that then grows rather quickly through the part, resulting in final failure. Consequently, the location where the crack started is most important, as it establishes the cause of the ultimate failure. Unfortunately, the features of interest at the origin are very small, and magnifications in excess of 1000 are needed to visualize them. This is why an SEM is needed to look at that area. In many instances, the part needs to be reduced in size in order to fit within the SEM, so the SEM investigation can be destructive.
In summary, fractography provides insight into the cause of a material fracture and can be used to determine whether a material defect contributed to the fracture. It can be performed by eye or with an optical microscope. However, an SEM can sometimes be required to examine the origin of fracture.
JAMES J. MASON, PhD, P.E. is a Senior Engineer at ARCCA specializing in the mechanical failure of consumer and industrial products/equipment and machinery design/failure, as well as metallurgical laboratory analyses. He is also experienced in failed consumer medical devices, such as orthopedic implants and plastics.
