Development of Functional Relationships for Fatigue Crack Growth Reconstruction Using Fracture Surface Striations, 18-R9587

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Principal Investigators
Daniel B. Garcia
Richard A. Page
Harold G. Saldana

Inclusive Dates:  10/01/05 – 10/01/06

Background - Quantitative fractography is an effective tool used in failure analysis for determining a component failure mode; however, its true value becomes evident in providing a sense of the in-service load history. Inherent to aircraft component failures and readily observed in failure analyses are localized microscopic incremental crack advance features known as fatigue striations. Striation observations and measurements are often incorrectly interpreted due to cycle accumulation per incremental crack advance, intermittent cycles, variations in local microscopic growth rates, and poor Scanning Electron Microscope (SEM) quality.

Approach - The objectives of this research consisted of developing practical tools and techniques that implement the relationship that exists between microscopic fatigue striations and macroscopic fatigue crack growth rates. This also includes providing documentation of the repeatable procedures necessary to generate striation measurements of high quality and objectivity. The existing SwRI methods for crack growth reconstruction were updated with modern, practical, and repeatable analysis techniques. The approach was divided into three primary areas consisting of analyzing fracture surfaces from existing fatigue crack growth and spectrum crack growth experiments, developing the analysis tools, and applying the acquired knowledge to previously researched fatigue critical locations. Key variables that were studied include: loading (constant versus variable amplitude or spectrum), material – (Al 7075-T7351), and geometry (corner crack, surface cracks, thru-cracks, and stress concentrators).

To accomplish the objectives, a team consisting of a principal investigator, SEM operator, and Institute scientist was assembled. A total of 13 different specimens were observed for striations, and the data were then used to develop and validate a model. After developing confidence in the model, it was then applied to an existing failure analysis.

Accomplishments - The results show that component fatigue crack growth data can be edited or filtered to remove cycles that will not generate a striation, and these results compare favorably with the striation-based crack growth reconstruction. The filter implements an empirical relationship determined from the striation spacing of fatigue crack growth rate data and applies it to a spectrum crack growth analysis to predict the striation-based crack growth curve. The filter is intuitive, easily adaptable, and works efficiently and accurately. These results provide techniques and tools that can be adapted to future failure analysis and damage tolerance analysis comparisons.

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