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<p>The consultant reasoning about cracking knowledge (CRACK) system, which uses qualitative reasoning to analyze fatigue and fracture in steel bridges, is described. CRACK's architecture includes three reasoning levels-heuristic, qualitative, and quantitative-linked by a common representation of the bridge's physical structure. CRACK explains a failure by matching a crack progression sequence to the observed facts. The system establishes the type of problem using design critiques, predictive modeling, or failure analysis describes the problem by gathering information on the girder's geometry, service history, material properties, and observed symptoms. It then hypothesizes a cause, qualitatively simulates possible crack progression sequences to guide quantitative analysis, calculates the fracture mechanics to determine critical crack sizes and fatigue lives, and evaluates the hypothesis. Finally it states a conclusion. Several difficulties that arose in developing a simple, theoretically defensible, qualitative model which translates numerical relationships into a correct set of behaviors are discussed.</p>

W. K. Roddis and J. L. Martin, "CRACK: Qualitative Reasoning About Fatigue and Fracture in Steel Bridges," in IEEE Intelligent Systems, vol. 7, no. , pp. 41-48, 1992.
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