Hsin Yang Chung

Reliability-based optimal inspection for fracture-critical steel bridge members

To prevent fatigue failure of members in steel bridges, one usually needs to perform frequent periodic bridge inspections and employ detailed inspection methods. This is especially true for fracture-critical members or details. Carrying out these inspections puts a large burden on a transportation agency’s bridge maintenance budget. A systematic reliabilitybased method for inspection scheduling is proposed to yield the most economical inspection strategy for steel bridges that, at the same time, guarantees an acceptable safety level through the planned service life. A methodology is presented for evaluating the fatigue reliability of a specified detail classified according to AASHTO fatigue categories. A Miner’s rule approach is used to evaluate the fatigue reliability. The inspection scheduling problem is modeled as an optimization problem with a welldefined objective function that includes the total expected cost of inspection, repair, and failure formulated on the basis of an event tree framework and appropriate constraints in inspection intervals and minimum (target) structural reliability. An optimal inspection-scheduling plan can thus be obtained for any specified fatigue details (fracture-critical details) in steel bridges. Examples presented demonstrate the advantage of the reliabilitybased optimal inspection scheduling in cost saving and structural reliability control over alternative periodic inspection plans. Two numerical examples for a steel bridge in Texas are presented to demonstrate the proposed reliability-based optimal inspection scheduling.

Determining 2D notch SIFs by the image-correlation method

This study evaluates the two-dimensional (2D) stress intensity factors (SIFs) of a sharp V-notch using the image-correlation experiment and least-square method for isotropic materials. First, the Williams eigenfunction and complex displacement function approach are deduced into a least-square form, and then displacement fields from the image-correlation experiment are substituted into the least-square equation to evaluate the 2D SIFs. Compared with the SIFs from finite element and body force methods, the least-squares method can be used to calculate SIFs more accurately, if more than two displacement terms are included. The SIFs calculated from this least-squares method are not sensitive to the maximum or minimum radius of the area from which data is included. The major advantage of the proposed method is that the procedure is simple and systematic, so it can be applied to any finite element or experimental methods that obtain displacement fields.

Evaluations of mixed-mode stress intensity factors for load-carrying fillet welded cruciform joints using the least-squares method

This study presents the numerical and experimental applications of using the least-squares method (LSM) to evaluate the mixed-mode stress intensity factors (SIFs) of load-carrying fillet welded cruciform joints. In the numerical application, fillet welded cruciform joints of different combinations of geometric parameters (including weld leg size, initial lack of penetration size, and plate thickness) were systematically studied using the finite element method and the LSM. The derived SIF results were used to develop the general formulae of the opening mode (mode-I) and shearing mode (mode-II) SIFs, K I and K II, respectively. To validate the numerical results, five fillet welded cruciform joint specimens were fabricated and tested by a non-contact optical experiment using a commercial digital camera and the digital image correlation technique for image processing. The numerical and experimental mode-I SIF results were in good agreement with the results from the existing BS 7910 formula for fillet welded cruciform joints. In addition, the suggested non-contact optical experiment is simple and portable, making it well-suited for measuring mixed-mode SIFs directly from the welded details on site. This is very useful for fracture and fatigue evaluation and analysis of weld in existing or newly designed structures, such as steel bridges and offshore structures.

Considerations of NDT quality in fracture-critical inspections for steel bridges

Publisher Summary The chapter discusses the considerations of non-destructive testing (NDT) quality in fracture-critical inspections for steel bridges. NDT techniques are routinely used in bridge inspection. However, detecting accuracy, accessibility, cost, and consequences of false indication are all factors to consider when selecting the NDT techniques to be used. A probabilistic method is proposed to help select the most suitable NDT technique and an associated optimal schedule for fracture-critical member/detail inspections on a specified steel bridge. The probability of detection (POD) curve for each NDT technique that denotes its detection accuracy is employed in the chapter. By combining the probability calculations based on the use of the POD curve together with numerical Monte Carlo simulations for crack propagation in the fracture-critical detail, a total cost function is formulated that includes the expected cost of inspections, repair and failure resulting from the chosen NDT method, and alternative inspection schedules.