Fatigue Life Of Steel Structures Research Articles

Estimation of stress concentration factor at stop-hole repair strengthened by bonded patches

Fatigue cracks detected in existing steel structures during periodic inspections are often tentatively repaired by drilling holes, called stop-holes, at crack tips. However, in some cases, fatigue cracks may reoccur at the edge of stop-holes, depending on the crack length and applied stress range. Although numerous studies have demonstrated the efficacy of patch plates in strengthening cracked steel plates and extending fatigue life of steel structures, there is a limited literature on the effectiveness of stop-hole repair strengthened by bonded patches. Particularly, the impact of the bonded patch configuration and position on the fatigue strength improvement remains unexplored. This study investigated the influence of the bonded patch configuration and position on the stress concentration factor of a stop-hole by finite element analysis. In this study, the material of bonded patch plates is assumed to be steel. Furthermore, this study presents a fatigue design equation for stop-hole repair strengthened by bonded patches, considering bonded patch configuration and position based on the fracture mechanics. The proposed design equation enables the design of patch configuration and the evaluation of remaining fatigue life to repair fatigue cracks in existing structures.

An improved crack growth model of corrosion fatigue for steel in artificial seawater

Corrosion will greatly reduce the fatigue life of steel structures. Based on the Paris equation and continuum damage mechanics, an improved crack growth model is obtained by introducing corrosion damage into the Paris equation. The derivation and calculation procedure of the improved Paris equation are discussed in detail. The proposed model was used to calculate the corrosion fatigue crack growth length of S690 in 3.5 wt% NaCl solution. The model demonstrates good accuracy and predicts the corrosion fatigue life of T-welded joint specimens in artificial seawater.

Debonding development in cracked steel plates strengthened by CFRP laminates under fatigue loading: Experimental and boundary element method analysis

Carbon fiber reinforced polymer (CFRP) laminates can effectively enhance the fatigue life of steel structures. However, few studies have investigated the influence of crack-induced debonding in the CFRP-steel interface on the CFRP strengthening efficiency and the relationship of crack propagation and debonding development. This study experimentally and numerically – with the boundary element method (BEM) – investigated crack propagation and debonding development in CFRP-strengthened cracked steel (Q345, Q460, and Q690) plates. The fatigue test specimens were subjected to a maximum stress of 50% steel yield stress and a stress ratio of 0.1, during which real-time changes in CFRP strain distribution were recorded by digital image correlation (DIC). The CFRP strain gradient calculations showed that crack-induced debonding was crack length-dependent. The numerical fatigue life results were in good agreement with the experimental data when considering debonding and overestimated the experimental data without considering debonding, demonstrating the necessity of considering crack-induced debonding in calculation and design. Furthermore, a relationship was obtained between crack propagation and debonding development.

Computation of stress concentration factor of tubular joints for the fatigue analysis of steel structures

The offshore structures are welded tubular structures which are continuously subjected to cyclic environmental loads like waves, winds, currents, earthquakes etc. These cyclic environmental loads (predominantly wave loads) will induce time varying cyclic stresses on the offshore structures causing fatigue damages even at very low nominal stress levels. Fatigue failure is defined as the tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of intensity considerably below the normal strength. Fatigue is very critical at the welded joints, because the stresses at the welded regions are several times higher than the nominal stress. Stress Concentration Factors (SCFs) converts the nominal stress into higher stresses known as hot spot stresses. These SCFs are very sensitive in the computation of fatigue life of steel structures. A small difference in SCFs can cause a huge difference in fatigue life of steel structures. Different empirical methods give different values of SCFs for the same element, hence different fatigue life. This study aims to investigate the stress concentration factor of different configurations of tubular joints numerically using Finite Element Analysis (ANSYS) under certain boundary conditions and compare the result with the SCF obtained from empirical method (using parametric equations).

A Damage Criterion to Predict the Fatigue Life of Steel Pipelines Based on Indentation Measurements

Abstract A study was conducted to investigate the effects of surface microhardness on different phases of fatigue damage. This helps to estimate the evolution of the material resistance from microplastic distortions and gives pertinent data about cumulated fatigue damage. The objective of this work is to propose a damage criterion, associated with microstructural changes, to predict the fatigue life of steel structures submitted to cyclic loads before macroscopic cracking. Instrumented indentation tests (IIT) were conducted on test samples submitted to high cycle fatigue (HCF) loads. To evaluate the role of the microstructure initial state, the material was considered in two different conditions: as-received and annealed. It was observed that significant changes in the microhardness values happened at the surface and subsurface of the material, up to 2 µm of indentation depth, and around 21% and 7% of the fatigue life for as-received and annealed conditions, respectively. These percentages were identified as a critical period for microstructural changes, which was taken as a reference in a damage criterion to predict the number of cycles to fatigue failure (Nf) of a steel structure.

Towards a prognosis of fatigue life using a Two‐Stage‐Model

AbstractNominated for the Bernt Johansson Outstanding Paper Awards at Nordic Steel 2019This paper presents a Two‐Stage‐Model for determining the fatigue life of steel structures. The model follows the distinction between crack initiation and crack propagation according to the phenomenological background. The first stage uses the local strain‐life approach to predict the number of cycles for crack initiation Ni considering the cyclic material behaviour as well as the load‐time history. The second stage calculates the crack propagation based on a linear‐elastic fracture mechanics approach. Using X‐FEM in combination with cyclic loading, an arbitrary, solution‐dependent crack path is achieved to determine the number of cycles for propagation Np. The Two‐Stage‐Model was applied to butt welds to determine their total fatigue life Nf in terms of a proof‐of‐concept study. The study investigated the ability of the model to consider the influence of the weld geometry, i.e. plate thickness and weld imperfections (excess and vertical offset). The results are presented by means of S‐N curves for initiation life, propagation life and total fatigue life. A comparison with experimental results provided in the literature shows that the Two‐Stage‐Model can reflect the weld quality of butt welds in terms of fatigue life.

A statistical model for lifespan prediction of large steel structures

The fatigue life of steel structures under operating conditions inevitably depends on various random factors. Among the most influential factors are the characteristics of load cycles, such as stress means and amplitudes. A knowledge of their probability distribution is thus crucial for fatigue life analysis and prediction. Finite probabilistic mixture models have previously been used for this purpose. This paper presents a study of the possible benefits of mixture models with log-normal components, using a large experimental data set from the slew bearing substructure of a stacker. The study shows that for this particular situation, the log-normal mixture model performs significantly better than Gaussian mixtures, and thus can be used as a suitable model in similar areas of application.

Fatigue behaviour of CFRP strengthened steel plates with different degrees of damage

An experimental and analytical study was conducted to further investigate the effectiveness of the carbon fibre reinforced polymer (CFRP) plates in extending fatigue life of steel structures. Different lengths of artificial cracks were introduced to represent different degrees of fatigue damage. The experimental results demonstrated that the CFRP patches could effectively slow down the crack growth and prolong the fatigue life. A theoretical model was developed to predict the fatigue life of tested specimens. Thereafter, a parametric study was carried out to investigate the fatigue behaviour of steel plates with a wider range of damage degrees. This study extends the understanding of CFRP repair at different stages of crack propagation and provides some useful suggestions for the strengthening method.

Assessment of fatigue and selection of steel on constructions of steel bridges welded according to Eurocode 3

In this study, results of X-ray radiographic examinations in situ of butt welds in 155 steel railway bridges in Poland are presented. In 34 bridges, cracks in 437 joints were found. Distribution and size of stresses in a section of weld weakened with those weld unconformities were determined as well as brittle fractures and fatigue cracks were discussed. Fatigue classes of elements without welds and butt welds, according to which concentration of stresses at selection of steel for brittle cracking and analysing of structures exposed to fatigue according to PN-EN 1993-1-10 and PN-EN 1993-1-9 should be made, were presented. In order to estimate the fatigue life of steel structures with imperfections, the author described his own fatigue investigation of steel of diversified ageing and four groups of butt welds. Fatigue strength curves obtained from particular tests were compared with standard curves.

Experimental study on repair of fatigue cracks at welded web gusset joint using CFRP strips

This paper presents repair methods of fatigue cracks using CFRP strips. In particular, the subject of repair is fatigue cracks initiated at welded web gusset joints, which are the typical details in steel bridges. Several repair methods were investigated experimentally focusing on weld details. In addition, more effective repair methods were also investigated using combination of CFRP strips and drill-holes. As a result, it was found that fatigue life after repair was significantly improved. Therefore, the authors confirmed the feasibility of the proposed technique as a useful repair method to improve fatigue life of steel structures.