ABSTRACT

When failure occurs to a component, the use of a systematic analytical approach is the first step in properly identifying the root cause of the failure. This approach should include acquisition of the pertinent background data and service history, followed by thorough visual examination, photographic documentation and laboratory evaluation as needed. It is imperative during the course of any such evaluation to take the known service history into account, particularly when the features do not clearly indicate the failure mode. It is equally important to conduct all of the necessary evaluations to properly identify the cause of the failure, and avoid taking shortcuts by omitting evaluations. When inaccurate background information is relied upon during the course of a failure investigation, or when critical evaluations are omitted, the test data and component features could be confusing or misleading. This article details the failure analysis process and illustrates potentially misleading details in failures involving stress corrosion cracking (SCC).

INTRODUCTION

Proper identification of the root cause of any failure is vitally important in order to prevent recurrence. The use of a logical and systematic failure analysis approach will minimize the likelihood that important features will be overlooked during the course of any such investigation. Even when a failure investigation is properly conducted, some of the obtained test data and failure features can be misleading or confusing when the background information, including the service history of the component, is not properly understood. There is a need for a systematic approach for identifying the root cause of any failure, and for recognizing details that could be potentially misleading in the absence of accurate background information.

THE FAILURE ANALYSIS PROCESS

Certain steps have been found effective in any material failure investigation. When properly followed, the failure analysis process will prove very effective in helping the analyst recognize the failure mode. With an understanding of the part design and service history, the root cause of the failure can be determined, and potential solutions can be recommended using solid engineering principles combined with common sense. Several different instruments and analytical techniques are used to characterize a failed component with regard to appearance, composition and properties, and a few conventional instruments are listed in Table 1 along with some of their applications.

Background Data Collection

The first step to any failure analysis is to gather as much background information as possible, as it relates to the failed component. Although sometimes limited, any information pertaining to the manufacturing and service history of the failed component should be obtained at the early stage of the investigation. Any unusual operating conditions may prove to be of particular importance in cases involving isolated failure incidents. Furthermore, details such as part information and identification, including pertinent drawings and specifications, should be collected at this stage. In cases involving machines and equipment, details about the maintenance schedule and repair history should be recorded, along with any anomalous service events. The frequency of the observed failure is also an important detail, as it can provide clues as to whether the failure is due to production lot or unusual service conditions.

Initial Inspection

The initial inspection involves thorough visual inspection and review of the failed component and adjacent components, including documentation of details such as wear patterns, unusual surface damage and corrosion features. I

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