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What types of FAD analyses are covered by Signal FFS?
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Known flaw assessment, limiting flaw assessment, limiting load assessment and fatigue crack growth assessments.

April 2014

The Failure Assessment Diagram Explained

The Failure Assessment Diagram (FAD) is the most commonly used method for determining the acceptability of crack-like flaws in structures. The FAD extends the crack stability assessment to structures experiencing both brittle and ductile fracture. It is a plot with a limiting curve and points representing the structure of interest.

The figure below shows a sample FAD. The x-axis of the plot is the load ratio (Lr), which is the ratio between the reference stress and the material yield strength. The reference stress is proportional to the far-field stress and is computed based on the loading condition, the component geometry and the crack configuration. The y-axis of the plot is the toughness ratio (Kr) which is the ratio of the stress intensity factor (KI) computed for the primary and secondary loads (residual stresses) and the fracture toughness of the material (KIC). The through-thickness stress profile is incorporated in the computation of Lr - Kr.  The computed Lr-Kr point illustrates the stability of the examined flaw. Signal FFS automates analyses using the FAD with a few simple steps.


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