by E. Philip Dahlberg About the Author E. Philip Dahlberg is Senior Metallurgical Consultant in the Houston office of Failure Analysis Associates, 3637 West Alabama, Suite 145, Houston, Texas 77027. He has spent 20 years in the field of mechanical metallurgy, specializing in applications of electron microscopy to failure analysis. He received his Ph.D. from the University of Florida and undergraduate degrees from Northwestern University and Shimer College in Illinois. He is licensed as a Professional Engineer in Texas, Washington, and Illinois. Introduction No self-respecting criminologist will leave the scene of a crime without a careful inspection for fingerprints. Their discovery and subsequent identification will often lead the investigator to the culprit and to a solution of the crime. Similarly, the detailed markings on a fracture surface, the fractureprint, so to speak, will often provide the forensic metallurgist or failure analyst with important clues as to why a structure failed. Engineering alloys used for structural applications are in general complex, multi-component, multi-phase aggregates of metal grains, non-metallic inclusions, impurities and voids. The alloys have a fabrication or cast texture and microscopic deformation structure. They contain local variations in chemistry, electronic potential, and residual stress. When a metal fractures, a crack traverses this complex structure separating it into two or more parts. The crack interacts with the microstructural elements in specific ways which depend on the stress, strain, strain rate, temperature, and environmental conditions which existed at the time of failure, and therefore, it adopts a unique surface topography. The process of examination and analysis of the fracture surface topography is called fractography. (1) Fractography can be carried out at several levels; visually with the unaided eye, at low magnification using an optical microscope, or at high magnification with a scanning or transmission electron microscope (SEM or TEM). By carefully comparing the macroscopic and microscopic details of the fracture surface with those of well documented "pedigreed" fractures generated under known laboratory conditions, the reasons for service failures can often be identified and proper corrective measures instituted to insure that replacement of the failed parts will not result in additional problems or catastrophe. The purpose of this paper is to describe fractography techniques as used in routine failure analysis and present selected examples of particular fracture prints as observed on service failures using the scanning electron microscope. Failure Analysis Techniques The failure of an engineering structure is always a surprise. Structures are designed and built to stay together, and the aim of failure analysis is to eliminate the unexpected by making known those factors which combined to cause the fracture. This requires the use of a wide variety of tools and techniques.(2,3,4) While it is often of critical importance, the analysis of surface topography using fractographic techniques is one part of failure analysis and its use must be integrated with the other tools and procedures available to the failure analyst.(5,6) Proper failure analysis combines the results of fractography with a thorough understanding of the part's design, fabrication, service history, chemical composition and microstructure, and mechanical properties under the specific stress and environmental conditions existing at the time of failure. Often the analytical work must include complex exemplar tests to verify proposed failure mechanisms. Fracture of an engineering structure occurs when the sum of the stresses exceed design limits, fracture usually occurs after general yielding and plastic instability. More typically, however, brittle catastrophic failures occur at nominal stresses below design limits and are associated with the presence of an initiating defect, flaw, or crack. When the local stresses at the edge of such a defect exceed the fracture toughness of the material, unstable fracture occurs. The initiating defect can be a material flaw such as a large non-metallic inclu sion, a fabrication flaw such as a heat treat crack, a casting defect, weld porosity, or a machining mark. Similarly, it can be a crack which has initiated and grown to critical size by mechanical mechanisms such as fatigue, or by chemical means like corrosion. In the typical component failure there are two distinct regions on the fracture surface. One is the initiating defect or stable crack and the other the unstable fast fracture. It is imperative, therefore, that a careful macro-examination of the entire available fracture surface be carried out in order to locate these two areas. The initiating defect, the fast fracture, and the transition between the two all can contain clues or fractureprints which will aid in understanding the failure. High magnification photographs of these areas are important to accurate failure analysis. © 2007 Stork Metallurgical Consultants, Inc.
