Probabilistic Approach for Fatigue Life Assessment Based on S-N Curve

2010 ◽  
Author(s):  
Zhigang Wei ◽  
Robert E. Kurth ◽  
Thomas P. Forte
Keyword(s):  
General Trend ◽  
Standard Procedure ◽  
Probabilistic Approach ◽  
Pressure Vessels ◽  
Life Assessment ◽  
Engineering Structures ◽  
Fatigue Life Assessment ◽  
Fatigue Design ◽  
Fatigue Data ◽  
Cycles To Failure

Components in pressure vessels and pipes are usually subjected to mechanical and thermal cyclic loadings, which cause fatigue failure. The statistic and probabilistic assessment of these components based on S – N or ε–N curves is of great importance for fatigue design. Recently, the standard practice, as adopted by ASTM, BS, DNV and many other standards, for statistical analysis of linear or linearized stresslife (S – N) and strain-life (ε–N) fatigue data has been critically reviewed. The shortcomings of the standard procedure based only on the variation of cycles have been clearly demonstrated by examining the general trend of a large amount of S-N data. A new deterministic statistical method based on the equivalency between the changes of stress range and cycles to failure has been subsequently proposed and validated. In this paper a probabilistic approach based on the equivalency method is developed to quantify the uncertainty of engineering structures subjected to inherent randomness in material properties, and its effectiveness is also demonstrated.

scholarly journals Fatigue life assessment regarding different influences on the HCF/VHCF behavior of a martensitic steel

2018 ◽  
Vol 165 ◽  
pp. 20004
Author(s):  
Igor Milošević ◽  
Benjamin Seisenbacher ◽  
Gerhard Winter ◽  
Florian Grün ◽  
Martin Kober
Keyword(s):  
Crack Initiation ◽  
Life Assessment ◽  
Special Treatment ◽  
Initiation Mechanism ◽  
Component Size ◽  
Fatigue Life Assessment ◽  
Fatigue Data ◽  
Gradient Based ◽  
Actual Material ◽  
Cycles To Failure

Modern applications require a special treatment when the conventional specimen size is much larger than the component size. Additional to that, high sophisticated materials are used for highly loaded components. Often the conventional fatigue limit is exceeded and loads are applied in the VHCF regime. Focus was put on the lifetime calculation and the implementation of investigated fatigue data of a X5CrNiCuNb-16-4 type steel. Two specimen geometries with diameters D7.5=7.5 mm and D2.5=2.5 mm were tested at R=-1, at room temperature and up to 109 cycles to failure. The application of different software tools (FEMFAT, fe-safe) showed several issues based on the current results. Results showed a change of crack initiation mechanism to subsurface crack initiation at approx. 2x106 cycles to failure. The gradient based correction of the reference fatigue data showed a good applicability up to 2x106 cylces. The application of fe-safe allows the flexible modification of S/N parameters over the whole cycle range. The usage of the actual material configuration introduced several important questions regarding the fatigue data and the implementation into lifetime calculation tools.

scholarly journals Fatigue life assessment regarding different influences on the HCF/VHCF behavior of a martensitic steel

2018 ◽  
Vol 165 ◽  
pp. 20004 ◽  
Author(s):  
Igor Milošević ◽  
Benjamin Seisenbacher ◽  
Gerhard Winter ◽  
Florian Grün ◽  
Martin Kober
Keyword(s):  
Crack Initiation ◽  
Life Assessment ◽  
Special Treatment ◽  
Initiation Mechanism ◽  
Component Size ◽  
Fatigue Life Assessment ◽  
Fatigue Data ◽  
Gradient Based ◽  
Actual Material ◽  
Cycles To Failure

Modern applications require a special treatment when the conventional specimen size is much larger than the component size. Additional to that, high sophisticated materials are used for highly loaded components. Often the conventional fatigue limit is exceeded and loads are applied in the VHCF regime. Focus was put on the lifetime calculation and the implementation of investigated fatigue data of a X5CrNiCuNb-16-4 type steel. Two specimen geometries with diameters D7.5=7.5 mm and D2.5=2.5 mm were tested at R=-1, at room temperature and up to 109 cycles to failure. The application of different software tools (FEMFAT, fe-safe) showed several issues based on the current results. Results showed a change of crack initiation mechanism to subsurface crack initiation at approx. 2x106 cycles to failure. The gradient based correction of the reference fatigue data showed a good applicability up to 2x106 cylces. The application of fe-safe allows the flexible modification of S/N parameters over the whole cycle range. The usage of the actual material configuration introduced several important questions regarding the fatigue data and the implementation into lifetime calculation tools.

Fatigue of Tubular Joints: Hot Spot Stress Method Revisited

2012 ◽  
Vol 134 (3) ◽  
Author(s):  
Pingsha Dong ◽  
Jeong K. Hong
Keyword(s):  
Hot Spot ◽  
Life Assessment ◽  
Structural Stress ◽  
T Joints ◽  
Hot Spot Stress ◽  
Tubular Joints ◽  
Fatigue Design ◽  
Fatigue Data ◽  
Section Viii ◽  
Remaining Life Assessment

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap, and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.

Fatigue Life Analysis of Polyurethane Bending Stiffeners

2005 ◽  
Author(s):  
Fre´de´ric Demanze ◽  
Didier Hanonge ◽  
Alain Chalumeau ◽  
Olivier Leclerc
Keyword(s):  
Fatigue Life ◽  
Effective Strain ◽  
Fatigue Behaviour ◽  
Life Assessment ◽  
Test Results ◽  
Critical Areas ◽  
Fatigue Life Assessment ◽  
Fatigue Data ◽  
Life Analysis ◽  
Fatigue Life Analysis

Following some experiences of bending stiffeners fatigue failures during full scale tests performed at Flexi France on flexible pipe and stiffener assemblies, Technip decided to launch in 1999 a major research program on fatigue life analysis of bending stiffeners made of Polyurethane material. This fatigue life assessment is now systematically performed by Technip for all new design of flexible riser bending stiffeners. This totally innovative method comprises a number of features as follows: Firstly fatigue behaviour of polyurethane material is described. The theoretical background, based on effective strain intensity factor, is detailed, together with experimental results on laboratory notched samples, solicited under strain control for various strain ratios, to obtain fatigue data. These fatigue data are well fitted by a power law defining the total number of cycles at break as a function of the effective strain intensity factor. The notion of fatigue threshold, below which no propagation is observed, is also demonstrated. Secondly the design used by Technip for its bending stiffeners, and most of all the critical areas regarding fatigue for these massive polyurethane structures are presented. Thirdly the methodology for fatigue life assessment of bending stiffeners in the critical areas defined above is discussed. Calibration of the strain calculation principle is presented versus finite element analysis. Based on all fatigue test results, the size of the equivalent notch to be considered at design stage, in the same critical areas, is discussed. Finally, a comprehensive calibration of the methodology according to full and middle scale test results is presented. The present paper is therefore a step forward in the knowledge of fatigue behaviour of massive polyurethane bending stiffener structures, which are critical items for flexible risers integrity, and widely used in the offshore industry. The confidence in bending stiffeners reliability is greatly enhanced by the introduction of this innovative methodology developed by Technip.

Study of Fatigue Properties of Materials through Cyclic Automated Ball Indentation and Cyclic Small Punch Test Methods

2017 ◽  
Vol 734 ◽  
pp. 273-284 ◽  
Author(s):  
Raghu V. Prakash
Keyword(s):  
Fatigue Property ◽  
Test Method ◽  
Life Assessment ◽  
Remaining Life ◽  
Small Punch Test ◽  
Small Punch ◽  
Punch Test ◽  
Ball Indentation ◽  
Fatigue Data ◽  
Cycles To Failure

One of the important inputs while estimating the remaining life of critical components is the fatigue property of materials. Fatigue data, in the form of stress vs. cycles to failure (or) strain vs. cycles to failure (or) fatigue crack growth rate data is used to predict the residual life. Material’s fatigue property degrades with time and usage; hence, it is appropriate to use the current properties for remaining life assessment. Often the quantity of material available for generating fatigue data is limited, especially, if the material is scooped out of existing component of a power plant. Further, fatigue response being probabilistic in nature, requires multiple specimens to be tested at any given stress/strain levels. This has prompted us to develop test procedures to determine the fatigue data of materials from a limited volume of material. This paper presents the results of cyclic ball indentation test method as well as cyclic small punch test method that is used to generate the fatigue data at different stress levels. There are several fine details relating to these test techniques – viz., establishing a equivalent damage criteria for failure life with standard LCF/HCF test specimens. The influence of one of the variables, viz., friction at the specimen-tool interface of a small punch test is investigated through numerical simulation and the results are presented here.

scholarly journals A Probabilistic Method for the Fatigue Life Assessment of Powder Metallurgy Parts of Aircraft Engines

1995 ◽  
Author(s):  
R. Krafft ◽  
S. Mosset
Keyword(s):  
Fatigue Life ◽  
Powder Metallurgy ◽  
Scale Effect ◽  
Probabilistic Method ◽  
Probabilistic Approach ◽  
Aircraft Engine ◽  
Experimental Results ◽  
Life Assessment ◽  
Fatigue Life Assessment ◽  
Stress Dependent

This paper will present a probabilistic approach developed in order to assess the fatigue life of aircraft engine parts (turbine disks) obtained by powder metallurgy technique. First of all, the main issues will be pointed out and the theoretical principles of the method will be described. Then the design implications and the experimental correlation will be emphasised. The scale effect is a major concern for the fatigue life assessment of a powder metallurgy part. It no longer allows the designer to directly evaluate the life of a massive part from experimental results based on small specimen fatigue tests as it is done in the classical methodology. In order to correctly describe this scale effect, incubation sites (inhomogeneities like ceramic inclusions) must be characterised. The size of these inhomogeneities and their positions in the part appeared to be the most relevant parameters. Hence the methodology developed at Snecma integrates the scale effect scatter through a binomial probability distribution as well as a temperature and stress dependent life evaluation for each inhomogeneity size and position. The life calculation of a part implies an analysis of its whole volume and surface. An iterative process determines the number of cycles corresponding to a global reliability level requirement for the part. The complete methodology is then validated by comparing the calculated initiation distribution with experimental results on small specimens and test disks.

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