Craig J. Schroeder
When a medical device fails, it can adversely affect the patient’s safety, resulting in pain, suffering, or delayed treatment. Such failures can not only harm the reputation of the device manufacturer but also lead to litigation, recalls, or delivery delays stemming from quality concerns. Understanding why a part failed is critical to preventing similar failures from reoccurring.
When studying a failed part, the analyst must consider a broad range of possible failure modes. Although some failures can be attributed to a single primary cause, multiple secondary factors are also common contributors. The failure analyst must evaluate all of the available evidence to prepare a hypothesis about the causes of the failure.
The most common type of failure is a component fracture. Fractures often have the most serious consequences, especially when load-bearing parts lose their ability to carry their intended load. When a medical device fractures, debris can enter into the patient’s body if an open wound is present, resulting in injury. Other types of failures may be related to distortion, wear, or corrosion. An artificial joint, for example, can corrode and exhibit wear inside the patient’s body.
A well-equipped materials laboratory has most of the tools necessary for effectively analyzing a component that has experienced these types of failures. Such tools include low-power stereomicroscopes, metallographic equipment, hardness testers, spectrometers, and scanning electron microscopes with energy dispersive x-ray spectroscopy (SEM-EDS), to name a few.
|Figure 1: In this photo of the fractured torque body, the black arrows indicate the locations where the part fractured, while the red arrow indicates a saw cut surface.|
The process of analyzing a failed part starts with the collection of background information. It is important to know what the specified requirements for the part and the material are. That information is often available in the form of a part drawing and referenced material specifications. It is also important to understand the expected performance of the part and how the performance of the failed part compares with its expected performance. Any changes made to the manufacturing process should be reported to the failure analyst. Examples include vendor, design, material, and thermal processing changes.
The next step in the failure analysis process is the visual examination of the part. Features to be noted, recorded, and photographed in the visual examination include, but are not limited to, fractures, fracture origin regions, damage to the part, the presence of residues, corrosion products, and corrosion pits. In some cases, nondestructive testing may be warranted if cracks are not readily visible or if they may be present below the surface. Chemical and hardness testing is performed in most cases to verify whether the part met the specified requirements. If enough material is available, tensile and impact testing is desirable to help understand the metal's inherent mechanical properties.
|Figure 2: A low-power magnification of the fractures.|
SEM analysis is a critical step in the metallurgical analysis of failed parts. The electron microscope is the best means to verify the fracture mode. Visual and low-power light microscopy is often not adequate for revealing the true morphology of a fracture.
For example, the fracture surface of a torque body used in a medical device application is presented in Figure 1. The black arrows in the photograph indicate the locations where the part fractured. The red arrow indicates the presence of a saw-cut surface. The sections of the fractured part were arbitrarily identified as A and B for the purposes of the investigation. Usually, the analyst provides a scale, a ruler in this case, to help the reader understand the approximate size of the part. Without such a scale, it can be hard to determine just how big the part is.
It’s critical for the analyst to carefully document, record, and identify the pieces of the failed part and the damage present in its as-received condition. Because the part may be sectioned and damaged during the investigation, it can be important to know its as-received condition when it is handled, excised, and examined in the laboratory.
|Figure 3: A SEM image of the fracture surface.
As shown in the low-power stereomicroscopy image presented in Figure 2, closer inspection of the fractures revealed that the part exhibited a relatively rough texture. It is often at this stage in the investigation that features of interest, such as potential fracture origins, are identified and selected for closer inspection, as indicated by the arrows in this view.
The fracture morphology and other features studied using SEM analysis can help the analyst to determine whether the failure occurred as a result of thousands or millions of cyclic stresses, whether corrosion or foreign contaminants played a role in the failure, or whether other types of embrittling phenomena may have played a role. The SEM image shown in Figure 3 revealed that the failed part in this study exhibited a fracture morphology consisting of what is known as a quasicleavage, or brittle transgranular cracking. This damage verified that the fracture occurred as a single overload event.
Metallography is another important step in the metallurgical failure analysis process. Examination of a part’s microstructure can help verify whether the part was subjected to proper thermal processes. Metallography can also identify whether anomalies were present in the material that could have had a deleterious effect on the part’s performance. In this case, the microstructure was judged to be normal for the given manufacturing material, as highlighted in Figure 4. Typically, the analyst will also identify the etchant that was used to reveal the microstructure and the magnification at which the image was taken.
Weighing the Evidence
|Figure 4: The metal's microstructure, revealed using an etchant known as Vilella’s reagent, consisted of martensite.|
After all the information has been collected and the analysis is complete, a final conclusion can be made based on the evidence. The investigation can be thought of as a forensic puzzle. The more pieces of the puzzle that are in place, the more conclusive the investigation will be. It is often tempting to ask the metallurgist to minimize the amount of testing to save time and money. It may also be tempting to withhold background information about the part for fear of biasing the metallurgist’s final conclusion.
However, it should be understood that restricting the amount of testing and withholding important information effectively removes pieces of the puzzle that can prove critical to achieving the correct final conclusion. If the reasons for failure are not properly understood, corrective actions to prevent future failures may be ineffective. When the investigation is complete, it is often up to the parties involved to collectively determine the best course of corrective action to prevent similar events from occurring again.
Craig J. Schroeder, is a senior engineer, metallurgy at New Berlin, WI–based Element Materials Technology. Specializing in failure analysis project management and training, he has experience with a range of materials, including carbon and low-alloy steels, stainless steels, aluminum alloys, cast iron, titanium, and nickel. Previously, he was a senior metallurgy engineer at Element Materials Technology and a materials engineer at Rexnord Technical Services. Schroeder received BS and MS degrees in materials engineering from the University of Wisconsin in Milwaukee. Reach him at [email protected].