Multiaxial fatigue life assessment of a vertical-lift bridge connection using strain rosette data

Fatigue-induced damage is one of the most common types of damage experienced by civil engineering structures subjected to cyclic loading such as bridges. The applicability of multiaxial nonproportional fatigue life assessment is shown using strain measurements collected from a welded gussetless truss connection of a vertical-lift bridge. Methods for uniaxial loading and multiaxial nonproportional loading are compared. It is shown that non-proportional loading can cause a significant decrease in the estimates of remaining fatigue life. Civil structures are subject to different types of cyclic loading such as vehicular, mechanical, and wind loads. Such dynamic effects can cause cyclic loading and unloading with response stresses above a certain endurance limit. Continuous application of this types of stresses may induce microcracking that can eventually propagate and produce failure of the member or the structure. This type of internal damage is known as fatigue and has been found to be cumulative and irreversible. Small stress amplitude cycles that result in elastic deformations lead to longer fatigue life estimates. This type of fatigue is also known as high cycle fatigue (HCF). On the other hand, repeated plastic deformations in each stress cycle are characteristic of low cycle fatigue (LCF), such as deformations can occur in extreme seismic events. Fatigue can result in substantial financial losses and structural failures compromising the safety of users. Fatigue research initiated in the early 1800’s when Jean-Victor Poncelet studied this phenomenon as a failure mechanism establishing the term fatigue in 1829. Then, W.J.M. Rankine investigated the failure of railway axels in 1843 (Samuel and Weird, 1999). In 1858, quantification of fatigue lifetimes was introduced by August Wöhler (Wohler 1858). His work resulted in the now widely used S-N curves where S is the uniaxial stress level that a material can withstand, and N is the number of cycles. These S-N curves are developed under cyclic axial loading tests. Therefore, directly applicable when a structural component subjected to uniaxial stresses is assessed for remaining fatigue life. Welded connections are generally more susceptible to fatigue cracking compared to bolted connections. Factors such as weld defects, residual stresses, discontinuities, and lack of fusion can reduce the fatigue life of a connection (Haghani, Al-Emrani, and Heshmati, 2012). Guidelines and provisions for fatigue evaluation of welded structures are available in the American Association of State Highway and Transportation Officials (AASHTO 2018b) specifications for highway bridges, the federal highway administration (FHWA 2015), the American Railway Engineering and Maintenance-of-way Association specifications for rail bridges (AREMA 2016), the American Society for Testing and Materials design and evaluation standards(ASTM 2000), and the German Institute for Standardizations (DIN in German) DIN4112. In general, these specifications suggest the estimation of stress ranges using a combination of loads and distribution factors. The resultant stress range is then used in combination with S-N curves to estimate fatigue strength. However, these uniaxial fatigue evaluation methods are in many cases insufficient for large in service structures with complex geometry and connections subjected to multiaxial, non-proportional loadings. Multiaxial loading is common in structures with complex geometries and independently varying loadings such as bridges, machines, aircrafts, and rollercoasters. Such structures can have multidirectional transfer of forces subjecting components to multiaxial stresses. Multiaxial loading can be proportional (or in phase) or non-proportional (out-ofphase). The direction of principal stresses or strains remains constant with respect to the direction of cyclic loading in proportional multiaxial loading while the principal axes directions change over time for nonproportional loading. A method for fatigue life prognosis and fatigue life prediction for complex structures such as lift bridges is presented. The method proposed was initially evaluated in a rollercoaster connection (Puerto-Tchemodanova et al., 2019). The critical plane method is used for the estimation of remaining fatigue life using strain rosette data. Strains collected from strain rosettes in a gussetless truss bridge connection are used to determine the critical plane orientation using Findley’s criteria (Findley 1958). The critical plane is defined as the plane that represent the most damaging fatigue orientation leading to the least fatigue life (Bannantine and Socie, 1992). This approach consists of examining the detailed stress and strain states on all potential critical planes of a component based on a previously determined fatigue criterion. Stresses at the critical plane location are used for estimation of the number of stress reversals induced by live loads and the number of associated cycles using the rain-flow method (Socie 1993). Uniaxial and multiaxial fatigue analysis methods proposed for non-proportional loading are compared. The critical plane method is used for the estimation of multiaxial fatigue life and compared to procedures used in the current AASHTO’s manual for bridge evaluation. It is shown that non-proportional loading and the location of the critical plane can result in a significant decrease in the estimates of remaining high cycle fatigue life. Therefore, current methodologies used in complex geometries and based on uniaxial stresses for the estimation of fatigue life can overestimate the fatigue strength or life of a member. The methodology proposed in this research is anticipated to be used for real-time fatigue prognosis and evaluation tools for bridge