Elements of Failure Analysis

Failure analysis is conducted to determine the root cause of failure. Sometimes these failures are catastrophic, e.g., Titanic. Other times the failures are a nuisance, e.g., failed o-ring in plastic faucet water valve. In both cases, the component failed unexpectedly, which can result in injury or death, not to mention financial loss due to unscheduled downtime. By using the information presented in the failed component a company could reduce, or eliminate, the possibility of re-occurrence of that type failure. This paper will discuss failure analysis in general terms and provide several case studies. The areas of failure analysis to be presented include typical tools, steps in conducting a failure analysis, theory of crack propagation, typical failure mechanisms, and case studies.

Introduction

It may sound like a bad joke, but what do manufacturers, insurance companies, and lawyers have in common? From an engineering viewpoint the common factor is providing engineering analysis to determine the root cause of why a component failed. Manufacturing companies want to save money, be more efficient, reduce down-time, and have proper preventive maintenance programs. Insurance companies do not want to pay a claim if abuse of the equipment was responsible for the failure and resulting claim. Lawyers need engineering data to assist in proving their case.

Failure analysis is a broad discipline that includes metallurgy and mechanical engineering. Some personal attributes of a good failure analyst include common sense, the willingness to expect the unexpected, and of course, a strong understanding of the engineering theory. Some of the typical tools include various forms of examination, e.g., visual and electronic. There are numerous steps in completing a failure analysis study and they should be performed in the proper sequence.

This paper introduces the above concepts and provides a few case studies showing how engineering knowledge and the ability to apply it work in these problem solving scenarios.

Typical Tools
Failure analysis provides insight into failure mechanisms if the analysis is thorough and accurate and all the necessary tests are performed. If the analysis is incomplete, then the wrong conclusions will be reached with possible serious future consequences. This paper only addresses a few of the tools, but they are all inter-related. There are several references the reader can obtain to become familiar with all the possible tools available.^{(1, 2, 3, 4)}

Visual exam
The overall condition of the component is quite important, beyond just looking at the fracture surface. It is important to determine the exposure of the entire component to the environment, which includes temperature, acid, tensile or compressive stresses, impact forces, corrosion, and wear. Just receiving a portion of the failed component, i.e., the fractured surfaces will not allow a fully justifiable conclusion to be determined. The author experienced this very concept a few years ago, which made the investigation quite challenging.⁽⁵⁾

Macroscopic exam
The initial view of the fractured surface provides many clues that will aid the failure analyst in determining the responsible failure mechanism. The presence of oxide on a portion of the fracture surface indicates a long exposure to the atmosphere, a smooth surface could indicate rubbing of the mating surfaces after fracture. Certain features will assist the failure analyst in where to concentrate the area of evaluation, e.g., ratchet marks and beach marks. There are several excellent references that can aid the reader. ^{(1, 6, 7)}