For safe operation of high-consequence structures such as airplanes, ships, trains, chemical plants, electricity-generating units, nuclear reactors, oil and gas pipelines, and pressure vessels, periodic inspection using nondestructive evaluation (NDE) technology and a deterministic approach to modeling fatigue crack growth has been mandated by government in the energy and transportation sectors of the nation’s economy since the 1970s. Recent advances in web-based computing, direct measurement-based NDE, and a stochastic approach to remaining fatigue life cycle prediction model have made it possible to not only enhance the credibility of fatigue life prediction but also shorten the turn-around time between field-based NDE and office-based modeling, analysis, verification and integrity assessment back to the field for decision making. To illustrate this new concept in preventing structural failure and extending useful life of high-consequence systems, we first recount a lesson learned in the history of the deployment of the U.S. nuclear submarine fleet, where the emphasis was on the continuous monitoring of 100% of pipe and vessel welds from their initial placement to the discovery of tangible signs of fatigue damage way before the onset of service disruption. Using two crack length vs. fatigue life cycle plots, one being based on the deterministic and the other a stochastic model, we summarize the contrast between the two models in their ability to deliver a credible prediction of the remaining fatigue life cycle based on a periodic or continuous inspection mode. In conjunction with that summary, we answer an important question in designing an NDE-based inspection strategy, namely, whether the inspection should be periodic or continuous. We show in this paper that the key to the success of a continuous monitoring system for aging structure is an NDE capability in measuring not only the initial crack length and the initial crack growth rate, but also their standard deviations. We conclude with a remark that a continuous direct-measurement-based NDE inspection system, when coupled with a finite element modeling and analysis capability, is capable of monitoring not only surface but also subsurface cracks.

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