Sudath C. Siriwardane

Structural Appraisal-based Different Approach to Estimate the Remaining Fatigue Life of Railway Bridges

The current fatigue life estimation methods of railway bridges are generally based on combinations of measured strain histories, Miner’s rule, and railway code provided fatigue curve. Even though the past measured strain histories are available for major bridges, most of the old bridges do not have past strain measurements. Furthermore, in case of existing railway bridges where the detailed loading history is known, Miner’s rule might provide incorrect results because of its omission of load sequence effect. These reasons hinder the usage of current methods to estimate the realistic remaining fatigue life of most of the existing railway bridges. Therefore, a new method is presented in this article to estimate remaining fatigue life of riveted railway bridges. The method mainly consists of predicted stress histories, recently developed sequential law and fully known Wöhler curve. Here, it is essential to use the fully known Wöhler curve as the related fatigue curve. Hence, the technique, which utilizes transfer of the partially known Wöhler curve to fully known curve, is also discussed in this article. Further, this method describes a reasonably accurate procedure to obtain the past stress histories from present day measured strains. Initially, the article describes the proposed method for remaining fatigue life estimation. Then the sequential law and associated fully known S-N curve determination procedure are verified by comparing theoretical fatigue life, damage behavior with real fatigue life and damage behavior of few materials. Finally, the remaining fatigue life of an existing railway bridge is estimated. Hence, validity and merits of the proposed method is confirmed by comparing the results with previous method-based fatigue lives.

New Combined High and Low-Cycle Fatigue Model to Estimate Life of Steel Bridges considering Interaction of High and Low Amplitudes Loadings

A new model is proposed in this paper for the estimation of the life of bridges subject to damage caused by high cycle fatigue combined with low cycle fatigue taking account of the interaction of high and low amplitude loadings. High cycle fatigue is caused by normal routine traffic (low amplitude) loading while low cycle fatigue is caused by extreme (high amplitude) loading situations produced by such as earthquakes. The model mainly consists of a new damage indicator and a new strain-life fatigue curve. Total strain is treated as the damage variable. The proposed model predictions were verified by comparing with fatigue test results for four materials reported in the literature. The proposed model was then applied to estimate the fatigue life of a bridge member subject to combined high and low-cycle fatigue damage caused by normal traffic and by earthquake loadings. The results of the case study, confirm the importance and applicability of the proposed model.

Structural Integrity Control of Ageing Offshore Structures: Repairing and Strengthening With Grouted Connections

Structural integrity control (SIC) is an increasingly important element of offshore structures. Not only is it used in newly built and existing offshore structures (e.g. oil and gas (O&G) production & process facilities (P&PFs), wind turbine installations, etc.), but SIC is also essential for ageing offshore platforms which are subjected to an extension of their design service life. In these cases, SIC programs must be performed to assess the platforms. If any significant changes in structural integrity (SI) are discovered, then it is essential to implement an appropriate strengthening, modification and/or repair (SMR) plan. Currently, welded and grouted repairs are mostly used for SMR. Although a welded repair may typically restore a structure to its initial condition, if the damage is due to fatigue loading and welded repairs have been carried out, then historical evidence reveals that there is a high potential for the damage to reappear over time. On the other hand, mechanical connections are significantly heavier than grouted connections. Consequently, grouted repairs are widely used to provide additional strength, for instance, to handle situations such as preventing propagation of a dent or buckle, sleeved repairs, leg strengthening, clamped repair for load transfer, leak sealing and plugging, etc. This manuscript examines current developments in grouted connections and their comparative pros and cons in relation to welded or mechanical connections. It also provides recommendations for future research requirements to further develop SMR with grouted connections.Copyright

An accurate fatigue damage model for welded joints subjected to variable amplitude loading

Researchers in the past have proposed several fatigue damage models to overcome the shortcomings of the commonly used Miners rule. However, requirements of material parameters or S-N curve modifications restricts their practical applications. Also, application of most of these models under variable amplitude loading conditions have not been found. To overcome these restrictions, a new fatigue damage model is proposed in this paper. The proposed model can be applied by practicing engineers using only the S-N curve given in the standard codes of practice. The model is verified with experimentally derived damage evolution curves for C 45 and 16 Mn and gives better agreement compared to previous models. The model predicted fatigue lives are also in better correlation with experimental results compared to previous models as shown in earlier published work by the authors. The proposed model is applied to welded joints subjected to variable amplitude loadings in this paper. The model given around 8% shorter fatigue lives compared to Eurocode given Miners rule. This shows the importance of applying accurate fatigue damage models for welded joints.

Hardness-Based Non-destructive Method for Developing Location Specific S-N Curves for Fatigue Life Evaluation

This paper first describes the importance of using location specific S-N curves for fatigue damage assessment of existing steel structures. It discusses the existing concepts and methods for developing S-N curves using empirical formulae and monotonic strength parameters, such as the ultimate tensile strength and hardness. It also discusses relationships among these monotonic parameters. Then it presents formulae for developing hardness-based full range S-N curves for medium strength steels. The formulae are verified using experimental data obtained from both monotonic and cyclic testing. Finally, it describes the advantages of these hardness-based formulae for developing location specific S-N curves as hardness testing is a non-destructive test which can be carried out on specific locations in structures.

VIBRATION MEASUREMENT-BASED SIMPLE TECHNIQUE FOR STRUCTURAL APPRAISAL OF STEEL BRIDGES

The structures experience increasing traffic volume and weight, deteriorating of components and large number of stress cycles. Therefore, assessment of the current condition (i.e. Structural Health Monitoring) of steel railway bridges becomes necessary. Most of the commonly available approaches for Structural Health Monitoring are based visual inspection and non-destructive testing methods. The visual inspection is unreliable as those depend on uncertainty behind inspectors and their experience. Also, the non-destructive testing methods are found to be expensive. Therefore, recent researches noticed that dynamic modal parameters or vibration measurements-based Structural Health Monitoring Methods are economical and may also provide more realistic predictions to damage state of civil infrastructure. Therefore this paper proposes a simple technique to locate the damage region of railway truss bridges based on measured modal parameters. The technique is discussed with a case study. Initially paper describes the details of considered railway bridge. Then methods and observations of visual inspection, material testing and in-situ load testing are discussed under separate sections. Then the development of validated FE model of the considered bridge is comprehensively discussed. Hence, variations of modal parameters versus position of the damage were plotted. These plots are considered as the main reference for locating the damage of the railway bridge in future periodical inspection by comparing the measured corresponding modal parameters. Finally the procedure of periodical vibration measurement and damage locating technique were clearly illustrated.

APPLICATION OF NEW DAMAGE INDICATOR-BASED SEQUENTIAL LAW TO ESTIMATE FATIGUE LIFE OF OFFSHORE JACKET STRUCTURES

The most of current fatigue life estimation methods of offshore jacket structures are based on Miner’s rule. Miner’s rule has always been acknowledged as a simplification that is easy to use in design where detailed loading history is unknown. However, under random amplitude loading, Miner ́s rule might provide incorrect results because of its omission of the loading sequence effect. Recently, a new damage indicatorbased sequential law has been proposed to capture the loading sequence effect more precisely than the Miner’s rule. As offshore steel structures are subjected to variable amplitude loading and the state of the art methodology available allows us to measure or predict the detailed loading history, it is advisable to utilize the new damage indicatorbased sequential law for offshore steel structures. Therefore, the objective of this study is to propose a sequential law employed new approach to estimate fatigue life of offshore steel jacket structures. The proposed approach consists mainly of the predicted stress histories, the recently developed sequential law and a technique for transferring the partially known S-N curve to a full range curve. Then the proposed approach is applied to predict fatigue life of an offshore jacket structure as a case study. The case study consists of 3D-modeling of a jacket platform, wave-structure interaction modeling, FEM-employed dynamic time history analysis, hot spot stress analysis, a technique for obtaining the full range S-N curve and fatigue life estimation based on the newly proposed sequential law. Finally, the proposed and the conventional approach ́s predicted fatigue lives are compared to confirm the significance of the new approach.

A Simple Criterion to Predict Fracture of Offshore Steel Structures in Extremely-Low Cycle Fatigue Region

This paper presents a simple criterion to predict the fracture of offshore steel structures due to the interaction effect of ductile fracture and fatigue, which is termed as extremely-low cycle fatigue (ELCF) fracture. The criterion has been obtained from further simplification of available cyclic void growth model (CVGM). The main advantage behind this simplified criterion (i.e. simple criterion) is that it can be easily utilized with commonly available elasto-plastic finite element (FE) packages than the previous CVGM criterion. Initially the simplified ELCF fracture criterion is clearly presented. And the associated ELCF fracture prediction methodology is also indicated. The simplified criterion is then employed to determine ELCF fracture of some structural models. Hence verification of the simplified criterion is confirmed by comparing the results with previous criterion-based estimations. Then the simplified criterion is applied to predict the ELCF fracture of a reduced beam section specimen. Finally, study tends to conclude that the simplified criterion provides reasonable accurate prediction to ELCF fracture of offshore steel structures where magnitude of triaxiality remains relatively constant.Copyright