Fractography Using the SEM
When metallurgical engineers discuss the history of failure analysis and the analysis of the surface of a failed sample (known as fractography), the discussion is usually divided into two time periods: before the development of the scanning electron microscope (SEM) and after. This sums up the importance of the SEM in determining the cause of failures through the examination of the microscopic features present on the fracture surfaces which were created while the material was failing.
The SEM has several important characteristics that make it the optimum instrument for fractography. First, the SEM is capable of very high magnifications, able to magnify features from 10 to 100,000 times. This is critical because many of the microscopic features cannot be resolved at magnifications much below several thousand times. For comparison, most optical microscopes are limited to a maximum magnification of 1000 times. Secondly, the SEM has tremendous depth of field. The field of focus on a SEM is several hundred times greater than that of an optical microscope, and approaches that of the human eye. Anyone who has used an optical microscope in a science classroom can remember how difficult they are to focus. One can either focus on the high spots or the low spots, but never on both. The SEM can keep the high and low spots in focus, which is critical because even the smoothest fracture truly is a mountain range of high peaks and low valleys when viewed at high magnifications.
Once viewed on the SEM, each fracture reveals its own characteristic pattern of microscopic features that are easily photographically documented. But what do all these features represent? What do they tell you about why the component failed?
Each failure mode or type of failure has a characteristic set of fractographic features. While some of the details of each may differ depending on the exact material that failed, or the exact conditions of the failure, the features are very similar in most circumstances. This allows the experienced metallurgical engineer to determine the fracture mode with SEM examination. Since some fractures may show different features in different areas, the experience of the engineer is critical in determining the direction and sequence of these features to understand the cause of failure.
Ductile Dimples: uniform from tension (left), elongated from shear (right).

1000X1000X
DIMPLES are indicative of a ductile overload failure. They look like craters on the moon, as seen in the photograph. Dimples indicate the loads applied to cause the failure of the component are in excess of the tensile strength of the material, and that the material has ductility, or the ability to deform. When dimple features are present and ductile overload has occurred, the component typically shows macroscopic signs of deformation, such as bending, necking down, or stretching. Visually, the surface is generally not flat, and may have several layers or deep valleys on the fracture surface. The metal microscopically pulls and stretches, creating microscopic voids. The voids then further stretch until they break in half. Each dimple is half of a microscopic void. Uniform equiaxed voids indicate the loading is in tension, while elongated or stretched voids indicate shear or bending load. This failure mode is often referred to as microvoid coalescence.
INTERGRANULAR features indicate the fracture has progressed through the grain boundaries in the material, and not through the grains. They give a very distinctive "rock candy" appearance as seen in the photograph. Optically, the fracture may have a sparkly appearance, as each of the smooth grains easily reflects light. Macroscopically, the components generally will not show deformation or stretching, since intergranular fractures typically are a brittle failure mode. While some materials will fail intergranularly as a matter of course, these are generally pure metals. Most materials fail intergranularly because a weak phase is present along the grain boundaries, making the grain boundaries weaker than the grains and thus the easiest crack path. The hardened case in a carburized steel generally has fine carbides along the grain boundaries, creating an intergranular failure in the case, with a ductile dimpled fracture in the core. Likewise, some tool steels or exotic materials with grain boundary phases will fail intergranularly. Stress corrosion cracking, a time dependent failure mode which requires the presence of a corrosive environment and an applied or residual stress, can run intergranularly in some materials and some environments. But perhaps the most common and most worrisome intergranular failure mode is hydrogen embrittlement. Hydrogen typically from a plating or cleaning operation is absorbed into the metal and congregates along the grain boundaries, creating an intergranular failure mode in a material which would be expected to fail in a ductile manner at much higher stress levels.
Intergranular (left): with features similar to rock candy. Cleavage (right): with fan-like features.

300X 1200X
CLEAVAGE fracture is a transgranular failure mode where neither dimples nor grain boundaries are evident. Cleavage features are flat and fanlike, as seen in the photograph. Similar to intergranular fracture, it is a brittle failure mode, and thus no macroscopic signs of deformation or stretching are typically present on the component. Like ductile overload, it is an overload failure mode indicating the stresses applied to the component exceed the tensile strength of the material. Materials with low fracture toughness exhibit brittle cleavage fracture. These include steels that fail at temperatures below the ductile to brittle transition temperature, and those experiencing temper embrittlement. Cleavage cracking progresses through the material very rapidly, often creating a rapid, unforgiving and often dangerous failure.
Fatigue: with fine parallel lines called striations.

2000X
FATIGUE fracture is caused when stresses lower than the tensile strength of the material are applied cyclically. Cyclic loading can be vibrational, rotational, bending or similar, and can occur over 100 cycles, or 10,000,000 cycles. The key element is cyclic loading. After an incubation time for the microscopic strain to build up locally in the component, the crack initiates, and propagates by tearing a very small distance during each cycle of loading. Each tear is called a striation, and the cracking creates a pattern of fine lines across the fracture surface as seen in the photograph. This is a very unique pattern. The striations are generally more easily distinguished on ductile materials, such as stainless steel or aluminum, and less readily noticeable on high strength or low toughness materials. Unfortunately, with each cycle of loading, the fractures often rub together and damage the striations replacing them with wear and smearing. Macroscopically, fatigue failures tend to be very planar, with no deformation present, and typically start at a notch, corner or defect. After each loading cycle, the fatigue crack grows and reduces the usable cross section of the material. After a time, the remaining cross section of material can no longer support the applied loads and the fracture completes itself in a single overload event. That is why ductile overload dimples are often also found at some locations on a fracture surface of a component failing by fatigue.
SEM Fractography is another tool that we at MEI utilize to determine the cause of fracture and thus gain an understanding of the failure scenario and all the related causative factors. Only when all these have been identified can the proper corrective actions be implemented, so that future failures can be prevented.


A2LA Accreditation: What It Means for You
Materials Engineering recently achieved accreditation by the American Association for Laboratory Accreditation (A2LA). This organization is a nonprofit organization that is the recognized leader for accreditation of testing laboratories. A2LA has audited our quality system to ISO/IEC Guide 25 "General Requirements for Accreditation of Testing Laboratories". This specification is the generally recognized standard for testing laboratories, and includes the principles of ISO 9000 but also contains tougher requirements for qualification and competence of testing personnel and procedures, insuring that a laboratory has the technical abilities to conduct testing, and not just having the proper paperwork in order.
Accreditation can be thought of as formal independent recognition that a laboratory is competent to carry out specific tests using recognized test procedures, trained personnel and calibrated equipment.
As part of the accreditation process, we successfully completed an audit by A2LA's team of independent auditors, and will continue to be audited biannually. The scope of the audit includes a complete review of the quality system, test procedures, policies, training, equipment calibration and manuals. The auditor also watches over the shoulders of our staff as we conduct testing to insure our competence to perform testing. These audits provide opportunities for continual assessment of ourselves, allowing us to maintain the high quality of our laboratory services.
A2LA accreditation to ISO Guide 25 insures:
• Testing is conducted in compliance to nationally recognized test procedures, including ASTM and SAE test specifications.
• Internal procedures are in place to insure continued proper calibration of test equipment using NIST traceable standards.
• All testing is performed by trained, knowledgeable skilled professionals, who follow the clearly defined test procedures.
• Internal audits are conducted to insure procedures and policies are being followed.
• Any testing subcontracted by MEi will be conducted only by companies who have a similar commitment to quality.
• Reports and technical records will be maintained in such a manner to allow verification of test results and traceability well into the future.

In addition to the accreditation, Materials Engineering, Inc., participates in collaborative testing programs. In these programs, identical samples are tested by laboratories across the country, with the results statistically analyzed. This insures our test methods yield results similar to other testing laboratories.
Our desire is that through our accreditation and participation in collaborative testing program, you and your customers to have full confidence in the testing and analysis you entrust us to conduct.
We will be happy to provide a copy of our scope of accreditation to customers whom require such documentation as part of their quality system requirements. The scope of accreditation includes failure analysis, SEM/EDS, hardness and microhardness, metallography and microstructural evaluation.
For more information on ISO Guide 25, contact A2LA at 301-670-1377.


MEi Presentation to Local ASM Chapter
MEi President and Principal Engineer, Bill Durako, addressed the Rockford Chapter of ASM International during their January Dinner Meeting. ASM International is the largest technical society dedicated to Materials and Metallurgical Engineering. Bill's presentation was titled "Case Studies in Failure Analysis" and presented some of the more unusual and interesting projects he has worked on through the years. The case studies came from legal, insurance and industrial failures, and covered items including golf clubs, bicycles, arson investigation, gas line explosions, automotive dashboards, construction equipment, washing machine hoses and food packaging products.
Perhaps the most interesting involves a design related case where the inadvertent changes in geometry and material thickness on a pull tab food can caused the can to "explode" when opened. The hands on demonstration startled the audience when they heard the loud noise produced when that can was improperly opened.
The presentation was meant to be as entertaining as educational. If you would like one of our engineers to speak at your company or technical society, please give us a call. If you are interested in ASM international, their national headquarters and membership information are 1-800-336-5152 or www.asm-intl.org.


The "What Is It?" Contest
The scanning electron microscope (SEM) is a powerful tool, capable of magnifications up to 180,000 times. It allows us to reveal information which is critical to metallurgical investigations, such as fracture modes and surface characteristics.
The SEM can also be fun to play with, because it allows one to view the surface of anything at high magnification with great depth of field. All of us have been amazed by the pictures of various insect parts, especially the eye of a fly.
In our contest, we take a look at an object on the SEM that should be familiar to all of you. In this issue's contest, we have three different fabrics which we ask you to identify. Please note the differences in magnification on the photographs, as one is much smaller than the other two. As there are three photographs, we thought it only fair to give you some help. Not necessarily in order, the three fabrics are A) famous for sweaters B) from the orient C) the "fabric of our lives" Can you identify all three?


600X 600X 1200X
Please fax, mail or e-mail us (don't call) with your answer. We will draw a winner from all correct entries received by June 6. The correct answer and the winner will be published in the next issue Of Materials Interest.
The prize is a $50 restaurant gift certificate, so put on your thinking caps.
Results:
Last issue, we showed photographs of a natural substance from our 50th state. Many of you correctly identified the substance as lava, or lava rock. We were fascinated by the needle-like structure formed upon solidification. Our winner, drawn at random from the correct entries, was Mark Hlinak, Analytic Chemist from CR Industries in Elgin, Illinois. Mark received a gift certificate for dinner at Bob Chinn's Crabhouse in Wheeling. CR Industries, once known as Chicago Rawhide, is the world's leading supplier of fluid sealing devices for the truck, automotive, farm equipment, aircraft, heavy machinery and machine tool industries. CR also supplies seals for aerospace missiles, earth moving equipment, appliances and a wide variety of pumps, hydraulic systems, motors and subassemblies. Congratulations, Mark!