Some Results on the Estimation of Fatigue Resistance of Welded Joints (original) (raw)
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Determination of Some Parameters for Fatigue Life in Welded Joints Using Fracture Mechanics Method
Journal of Materials Engineering and Performance, 2010
In this work, the parameters stress intensity factor (SIF), initial and final crack lengths (a i and a f), crack growth parameters (C and m), and fatigue strength (FAT) are investigated. The determination of initial crack length seems to be the most serious factor in fatigue life and strength calculations for welded joints. A fracture mechanics approach was used in these calculations based on SIF which was calculated with the finite element method (FEM). The weld toe crack was determined to be equal to 0.1 mm, whereas the weld root crack’s length was varied depending on the degree of the weld penetration. These initial crack length values are applicable for all types of joints which have the same crack phenomenon. As based on the above calculated parameters, the new limits of FAT for new geometries which are not listed yet in recommendations can be calculated according to the current approach.
Fracture mechanics-based estimation of fatigue lives of welded joints
2013
The effects of the lack of penetration flaw and misalignment on fatigue life of cruciform welded joints made of low-alloy steel were studied experimentally and theoretically. It was found that two locations of fatigue fracture were possible under cyclic tension loading, depending on the relative magnitude of the misalignment. In the absence of misalignment, all fatigue failures occurred as a result of fatigue growth of cracks emanating from the weld root. In the presence of misalignment, fatigue life depended on the fatigue growth of cracks growing from the weld toe. It has been shown that the entire fatigue life can be modeled as a fatigue growth of cracks starting either from the weld toe or the weld root. The initial crack size was selected as a small crack characteristic for a given material, i.e., being dependent only on the material. The weight function method was used to calculate the required stress intensity factors.
A simple expression to estimate the fatigue endurance of welded joints
MATEC Web of Conferences
A simple expression is proposed to estimate the fatigue endurance of welded joints that can be used to understand and analyze in a simple way the influence of the main geometrical, mechanical and material effects (weld geometry, local geometry, material properties, residual stresses and size of defects). The proposed expression was derived from the results of the analysis of the fatigue strength of welds studied by means of a fracture mechanics approach that takes into account the fatigue behavior of short cracks by using the resistance curve method. For that purpose numerical simulation of transversely stressed butt, T and cruciform joints were performed.
Fatigue & Fracture of Engineering Materials & Structures, 2007
Residual stresses and weld defects play a major role in the fatigue behaviour of welded structures, so these effects need to be accounted for in a theoretical analysis. A simplified engineering procedure based on linear-elastic fracture mechanics is applied to estimate the fatigue behaviour, particularly the limit of endurance. Local geometrical irregularities and pre-existing flaws, which are typical for this kind of weld, are covered by an overall notch intensity factor instead of a specific stress intensity factor, so the initial flaw size is not needed explicitly in the analysis. The effect of residual stresses can be easily included. The cut-compliance method was applied to measure the residual stress distribution on the cross-section of the weld. A welded T-joint was used as a benchmark. Unexpectedly, compressive residual stresses were found to prevail in the root region. According to the analysis, they contribute to the endurance limit of the considered joint by about 50%. This result was confirmed by fatigue tests where a significant decrease in the fatigue strength after a post-weld stress relieving heat treatment was observed.
Fatigue Crack Propagation Life Calculation in Welded Joints
2009
The determination of fatigue strength of welded joint across the board hasbig draw to evaluate fatigue life of welded joints. In spite of considerable fatigue designdata which exist for welded joints in the recommendations, the studies for the effect ofcrack growth parameters C, m and initial crack length determinations of weldedstructures are still not clear and have not been discussed enough. Therefore, this paperaims to present procedures to find the FAT for welded geometries and determine initialcrack depth. The new recommended limits of FAT for new geometries not listed yet inrecommendations can be calculated according to backward calculations. Initial crackand crack growth parameter are determined
Fatigue strength modelling of high-performing welded joints
International Journal of Fatigue
This paper investigates the fatigue strength modelling of high-performing welded steel joints. The work considers the strain-based, notch stress, averaged strain energy density approaches, and linear elastic fracture mechanics. A comparison of the methods with experimental data shows that the predictions vary significantly for different modelling assumptions. Only the microstructure-sensitive strain-based approach can predict the fatigue life of various weld geometries and plate thicknesses. The explicit modelling of the localised plasticity using the microstructure-dependent representative volume element is required for accurate prediction of the short crack initiation and growth periods, which dominate the fatigue life modelling of high-performing welds.
Fatigue behavior of welded joints Part 2: Physical modeling of the fatigue process
Welding Journal, 2006
The fatigue process in fillet welded joints is discussed and modeled. As a first approximation, a pure fracture mechanics model was employed to describe the entire fatigue process. The model is calibrated to fit the crack growth measurements obtained from extensive testing on fillet weld joints where cracks emanate from the weld toes. Emphasis is laid on the choice of growth parameters in conjunction with a fictitious initial crack size distribution in order to obtain both reliable crack growth histories and predictions of the entire fatigue life. The model has its shortcomings in describing the damage evolution at low stress ranges due to the presence of a significant crack initiation period in this stress regime. As an alternative to the fracture mechanics model, a two-phase model (TPM) for the fatigue process was developed and calibrated. The number of cycles to crack initiation was modeled by a local strain approach using the Coffin-Manson equation, whereas the propagation phase...
Fatigue Life Prediction of Welded Joints?A Re-Assessment
Fatigue & Fracture of Engineering Materials and Structures, 1987
Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM AK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified.
This work is part of a global study performed, among a research partnership between three industries and a research laboratory, on the development of a method which allows to better estimate the fatigue life of welded structures (armoured vehicles, ships, Floating Production Storage Off-loading units, wrecking cranes, cars …) submitted to variable loading conditions. The complexity of these structures has lead to adopt a multiscale approach, based on the use of finite element codes associated to various levels of modelling, going from the global cartography of damaged zones to the local calculation with cracks inserted in the models. The aim of this project is to develop an industrial process, avoiding successive re-meshing, being an efficient and easy tool to apply. It is also open enough to provide tools allowing the engineer to assess crack initiation, propagation until failure. The crack initiation is calculated by the use of a multi-axial fatigue damage criterion based on the local approach. Coupled with an extension of the Line Spring Method, multi-initiation of fatigue cracks in welds and through crack growth are then considered, in order to calculate the stress intensity factors for various loads and geometries. Furthermore, sets of tools were developed to predict crack bifurcation and take into account the influence of the loading history on fatigue crack growth, such as crack growth retardation effect, as a result of overloads. This approach is then applied to an overall aluminium welded structure experiment, which was designed to allow several cracks to initiate and propagate. Local micro-geometries and residual stresses were measured at weld toe, as needed for local stress calculations. Furthermore, a complete instrumentation and test of this welded structure allowed to determine precisely crack initiations and to follow crack propagation. The results are in good agreement with calculations and point out the industrial necessity to measure the local characteristics of welds and to control the quality for fatigue design.