Fatigue Life Estimation of Pitted Specimens by Means of an Integrated Fracture Mechanics Approach

2016 ◽  
Author(s):  
Nicolas O. Larrosa ◽  
Mirco D. Chapetti ◽  
Robert A. Ainsworth
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Crack Length ◽  
Driving Force ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Stress Concentrations ◽  
Engineering Structures ◽  
Premature Failure ◽  
Critical Crack Length

The synergistic nature of corrosion and fatigue is one of the main reasons for the premature failure of engineering structures and components. The decrease in fatigue life of specimens subjected to aggressive environments is likely to be attributed to local, pit-induced, stress concentrations that cause premature initiation of fatigue cracks. In this work, we have developed a predictive approach to assess the life of specimens containing pits assuming the pit both as a crack and as a smooth notch. The proposed approach assumes that even though the critical place for crack initiation seems to be the pit mouth, once the crack initiates, during propagation, the location of the hot spot shifts according to the location of the crack tip and due to the redistribution of stresses and strains. An integrated fracture mechanics approach that compares the driving force of the crack emanating from the pit and the evolution of the material threshold to crack propagation with crack length is proposed. The material threshold is estimated from the plain fatigue endurance limit, the position d of the strongest microstructural barrier and the SIF threshold for long cracks. The effective driving force is assessed by means of parametric FEA. This approach considers the influence of the pit geometry on the stress field surrounding the crack providing a more realistic estimate of the applied driving force. The maximum applied stress range as a function of number of cycles (S-N curves) have been estimated for different configurations (stress level, initial crack length, location at the crack front) assuming that failure of the component will be given when the critical crack length is reached. The procedure has been first developed and used to assess deep pits, as these are the most detrimental and common configuration encountered in real Oil and Gas applications.

STRUCTURAL STRESS METHOD FOR FATIGUE DESIGN OF CONCRETE FILLED STEEL TUBULAR T-JOINTS UNDER AXIAL LOADING

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Idris A. Musa
Keyword(s):  
Fatigue Life ◽  
Three Dimensional ◽  
Axial Loading ◽  
Fe Model ◽  
Structural Stress ◽  
Stress Concentrations ◽  
Engineering Structures ◽  
T Joints ◽  
Tubular Joints ◽  
Concrete Filling

Steel tubular structural members are being widely used in various engineering structures. The steel tubular joints will have fatigue problem when subjected to repetitive loading. Fatigue strength is one of the key factors that control the design of steel tubular joints in structures subjected to frequent loading. Research has shown that concrete filling of the steel tubes can effectively reduce stress concentrations at the joint. In this study, the structural stress method which involves the through-thickness stress distribution, has been employed to estimate the fatigue life of concrete filled steel tubular (CFST) T-joints under axial loading in the brace. A Finite Element (FE) model has been developed using ABAQUS. The three-dimensional 8-node hexahedral element has been employed in the FE model. The structural stresses have been extracted and the fatigue life of the joint has been estimated. The results have been verified using experimental results reported in the literature. The current study showed that the structural stress method can effectively predict reliable fatigue life in concrete filled steel tubular (CFST) T-joints.

scholarly journals The Influence of Longitudinal Stiffeners on Weld Toe Stress Concentrations in Multi Planar Tubular KK Joints

2007 ◽  
Vol 348-349 ◽  
pp. 409-412
Author(s):  
Charles O. Woghiren ◽  
F.P. Brennan
Keyword(s):  
Stress Concentration ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Stress Concentrations ◽  
Out Of Plane ◽  
Invaluable Tool ◽  
Weld Toe ◽  
Dimensional Parameters ◽  
Stress Concentration Factors ◽  
Jack Up

This paper reports a parametric stress analysis of various configurations of rack plate stiffened multi-planar welded KK joints using the finite element method. The KK joint finds application in the leg structure of offshore Oil & Gas jack-up platforms. The rack plate is a dual purpose element of the joint because it firstly functions as a stiffener which reduces the stress concentration at the brace/chord intersection. This could be an immense contribution to the increase in fatigue life of the joint but other hot spot sites are introduced to the joint. The rack is also used for raising and lowering of the jack-up hull which gives the jack-up platform its jacking capability. Over 120 models using a combination of shell and solid elements were built and analysed within ABAQUS. Non-dimensional joint geometric parameters; β, γ and . were employed in the study with . being defined as the ratio of rack thickness to chord diameter. Stress Concentration Factors (SCFs) were calculated under applied axial and OPB (out-of-plane-bending) loading. Three critical SCF locations were identified for each load case, with each location becoming the most critical based on the combination of the non-dimensional parameters selected for the joint. This is important as careful design can shift the critical SCF from an area inaccessible to NDT to one that can be easily inspected. The SCF values extracted from the models were used to derive six parametric equations through multiple regression analysis performed using MINITAB. The equations describe the SCF at the different locations as a function of the non-dimensional ratios. The equations not only allow the rapid optimisation of multi-planar joints but also can be used to quickly identify the location of maximum stress concentration and hence the likely position of fatigue cracks. This in itself is an invaluable tool for planning NDT procedures and schedules.

Fatigue Life Predictions of Smooth and Notched Specimens Based on Fracture Mechanics

1981 ◽  
Vol 103 (2) ◽  
pp. 91-96 ◽  
Author(s):  
M. H. El Haddad ◽  
T. H. Topper ◽  
T. N. Topper
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Intensity Factor ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Propagation Model ◽  
Actual Length ◽  
Notched Specimens ◽  
Material Condition ◽  
Length Constant

An elastic plastic fracture mechanics solution for short fatigue cracks in smooth and notched specimens is presented which admits plasticity by replacing the conventional stress term with a strain term and accounts for the propagation of very short cracks by the introduction of an effective crack length which is equal to the actual length increased by length l0, the length constant l0 is characteristic of the material and material condition and is calculated from the smooth specimen endurance limit and the long crack threshold stress intensity. Crack growth results for cracks in both elastic and plastic strain fields of notched specimens when interpreted in terms of this strain based intensity factor showed excellent agreement with elastic long crack data. This intensity factor when combined with a propagation model that includes all stages of crack growth also successfully predicted the total fatigue life of the smooth and notched specimens studied here. The predicted propagation life of elliptical and circular notched specimens is in all cases within 50 percent of the actual fatigue lives.

scholarly journals Comparison Between Different Approaches for the Evaluation of the Hot Spot Structural Stress in Welded Pressure Vessel Components

2009 ◽  
Author(s):  
Martin Muscat ◽  
Kevin Degiorgio ◽  
James Wood
Keyword(s):  
Fatigue Life ◽  
Pressure Vessel ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Design Stage ◽  
Structural Stress ◽  
Notch Stress ◽  
Weld Toe ◽  
Section Viii ◽  
Division 2

Fatigue cracks in welds often occur at the toe of a weld where stresses are difficult to calculate at the design stage. To circumvent this problem the ASME Boiler and PV code Section VIII Division 2 Part 5 [1] uses the structural stress normal to the expected crack to predict fatigue life using elastic analysis and as welded fatigue curves. The European Unfired Pressure Vessel Code [2] uses a similar approach. The structural stress excludes the notch stress at the weld toe itself. The predicted fatigue life has a strong dependency on the calculated value of structural stress. This emphasizes the importance of having a unique and robust way of extracting the structural stress from elastic finite element results. Different methods are available for the computation of the structural hotspot stress at welded joints. These are based on the extrapolation of surface stresses close to the weld toe, on the linearisation of stresses in the through-thickness direction or on the equilibrium of nodal forces. This paper takes a critical view on the various methods and investigates the effects of the mesh quality on the value of the structural stress. T-shaped welded plates under bending are considered as a means for illustration.

Fatigue Modeling of Friction-Stir-Welded (FSW) Butt-Joints for Aerospace Applications

2019 ◽  
Author(s):  
Muhammad A. Wahab ◽  
Vinay Raghuram
Keyword(s):  
Fatigue Life ◽  
Aluminum Alloys ◽  
Fracture Mechanics ◽  
Crack Propagation ◽  
Fatigue Cracks ◽  
Strength Criterion ◽  
Fatigue Properties ◽  
Interface Element ◽  
Friction Stir ◽  
Stress Ratios

Abstract Among the recent research Friction-Stir-Welding (FSW) has been adopted worldwide as one of the dominant processes for welding lightweight aerospace Aluminum alloys. Al-2195 which is one of the new generation Aluminum alloys has been used in the external tank of the space shuttles. Aerospace fabricators are continuously pursuing FSW-technologies in its efforts to advance fabrication of the external tanks of the space shuttles. The future launch vehicles with reusable mandates require the structures to have excellent fatigue properties and improved fatigue lives. The butt-welded specimens of Al-2195 and Al-2219 are fatigue tested according to ASTM-E647. The effects of stress ratios, use of corrosion preventive compound (CPC), and the applications of periodic overloading on fatigue lives are investigated in this study. Scanning-electron-microscopy (SEM) is used to examine the criticality of the failure surfaces and the different modes of crack propagation that could have been initiated into the materials. It is found that fatigue life increases with the increase in stress ratio, and results show an increase in fatigue life ranging over 30% with the use of CPC, and the fatigue life increases even further with periodic overloading; whereas crack-closure phenomenon predominates the fatigue fracture. Fracture mechanics analysis and crack similitude was modified for fatigue cracks by Paris. Numerical studies using FEA has produced a model for fatigue life prediction scheme for these structures, where a novel strategy of the interface element technique with critical bonding strength criterion for formation of new fracture surfaces has been used to model fatigue crack propagation lives. The linear elastic fracture mechanics stress intensity factor is calculated using FEA and the fatigue life predictions made using this method are within 10–20% of the experimental fatigue life data obtained. This method overcomes the limitation of the traditional node-release scheme and closely matches the physics of the crack propagation.

Fracture Resistance of Ship Longitudinal Members Including Fatigue Crack

2013 ◽  
Author(s):  
Yann Quéméner ◽  
Chien-Hua Huang ◽  
Chi-Fang Lee
Keyword(s):  
Fatigue Crack ◽  
Fatigue Life ◽  
Fatigue Crack Growth ◽  
Crack Length ◽  
Crack Growth ◽  
Fracture Resistance ◽  
Crack Tip ◽  
Critical Crack ◽  
Critical Crack Length ◽  
Extreme Loads

This study investigates the fracture failure of longitudinal members including cracks. Specifically, this study employs the failure assessment diagram methodology to assess the conditions of failure at the crack tip. Based on various crack configurations, this study establishes the analytical formulations of the crack-tip condition that are validated using finite element analyses. In addition, the material toughness is expressed in terms of crack-tip opening displacement. This study evaluates the failure stress of representative cracked members as a function of the crack length. This enables determining critical crack lengths corresponding to the maximum stresses derived from extreme loads. Finally, this study uses simplified fatigue crack growth analyses to characterize the critical crack length in terms of fatigue life. For members located in the deck and bottom regions, the critical crack lengths correspond to the end of the assessed fatigue life. Therefore, the fracture resistance of the longitudinal members is satisfactory as it will not cause the premature loss of the component. This study also provides analytical formulations for crack-tip conditions that could be employed in a reliability study linking fatigue crack growth and fracture under extreme loads.

Prediction of the Fatigue Life of the Vibrational Sieve Supporting Beam

2017 ◽  
Vol 755 ◽  
pp. 274-278
Author(s):  
Jelena M. Djoković ◽  
Ružica R. Nikolić ◽  
Jan Bujnak
Keyword(s):  
Fatigue Life ◽  
Crack Length ◽  
Crack Propagation Rate ◽  
Working Life ◽  
Propagation Rate ◽  
Initial Crack ◽  
Initial Crack Length ◽  
Critical Crack ◽  
Critical Crack Length ◽  
Remaining Fatigue Life

Vibrational sieves are exposed to exceptionally high cyclic loading during their working life and that is the reason why the special attention should be paid to estimates of the fatigue life of their structural elements, as well as to design with respect to fatigue fracture. Any change in the design appearance of the structural component must be carefully analyzed, since even the rearrangement of the components' elements layout, with keeping all the sizes constant, can cause serious consequences to the particular component's fatigue life. The research subject considered in this paper is the remaining fatigue life of the carrying beam, as well as the suggestions for improving the working life of the vibrational sieve. The Paris' law was used for estimates of the average crack propagation rate. From the presented diagram of the initial crack length versus the remaining working life one could see that the fatigue life decreases with the crack length increase. When the initial crack length reaches a value a little less than a half of the critical crack length, the remaining fatigue life starts to drop abruptly.

Research on Fatigue Life of Autofrettaged Thick-Walled Cylinder in Probabilistic Fracture Mechanics

2012 ◽  
Vol 249-250 ◽  
pp. 36-39 ◽  
Author(s):  
Ai Jun Chen ◽  
Zi Chu Cha ◽  
Zhi Qun Wang
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Bauschinger Effect ◽  
Fatigue Cracks ◽  
Analysis Method ◽  
Intensity Factors ◽  
Probabilistic Fracture Mechanics ◽  
Best Fit ◽  
Elliptical Cracks ◽  
Walled Cylinder

Based on the theory of probabilistic fracture mechanics and Monte Carlo simulation, reliability analysis method for fatigue life of autofrettaged thick-walled cylinder was given. The forms of fatigue cracks in bore of autofrettaged thick-walled cylinder were considered as semi-elliptical cracks. The autofrettage residual stress solution was suitable for the thick-walled cylinder made of steel with strain hardening and Bauschinger effect. The stress intensity factors of thick-walled cylinder were calculated according to weight function method. The analysis of the examples showed that lognormal distribution is the best fit for fatigue life. Finally, the fatigue life of autofrettaged thick-walled on the condition of different reliabilities and confidences were presented.

scholarly journals FATIGUE LIFE ANALYSIS OF RAIL-WELDS USING LINEAR ELASTIC FRACTURE MECHANICS

2019 ◽  
Vol 6 (1) ◽  
Author(s):  
Mequanent M. Alamnie ◽  
Yalelet Endalemaw
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Fatigue Cracks ◽  
High Stress ◽  
Fatigue Cracking ◽  
Linear Elastic Fracture Mechanics ◽  
Frequency Interval ◽  
Element Analysis ◽  
Linear Elastic ◽  
Elastic Fracture

The initiation and growth of fatigue cracking is mainly due to high stress concentration, heterogeneity and poor quality of weld. The detection and rectification of such weld defects are major concerns of rail network managers to reduce potential risk of rail breaks and derailments. To estimate the fatigue life of welded joints and to analyze the progress of fatigue cracks, a fracture mechanics-based analysis and fatigue models were developed using Finite Element Analysis. The initial flaw is obtained from a sample weld using ultrasonic flaw detecting machine test. Linear Elastic Fracture Mechanics (LEFM) approach based on the Paris law was applied to determine critical crack size and the number of cycles to failure using FRANC3D software. The inspection interval of rail welds before fracture (failure) was suggested based on reliability and life cycle analysis that correspond with minimum overall cost and frequency interval. It is recommended that fracture-based models in combination with reliability analyses can be a sustainable infrastructure decision-making algorithm.

Sign in / Sign up

Export Citation Format

Share Document