scholarly journals Fatigue Performance of Welded Joints Under Variable Amplitude Loading Spectra

2019 ◽  
Author(s):  
Xu Liu ◽  
Yan-Hui Zhang ◽  
Bin Wang
Keyword(s):  
Welded Joints ◽  
Plate Thickness ◽  
Fatigue Cracking ◽  
Damage Parameter ◽  
Fatigue Performance ◽  
Mean Stress ◽  
Variable Amplitude ◽  
The Mean ◽  
The Impact ◽  
Loading Spectrum

Abstract Offshore pipelines are generally subjected to variable amplitude (VA) loading in service due to waves or ocean currents. Welded joints often represent the most critical locations for fatigue cracking. Use of the current fatigue design guidance, for example, BS 7608, to assess fatigue performance of the welded joints in such structure may lead to inaccurate estimates depending on the nature of the VA loading spectrum. Further studies on the effect of VA loading spectra on fatigue performance of welded joints are needed. In this research, both uniaxial and 3-point bending fatigue tests were performed on non-load carrying fillet welded plates under VA loading spectra to investigate the effects of mean stress and the type of VA loading spectra. The influence of plate thickness was also investigated. Test results suggest that the spectrum with a high constant maximum tensile stress (cycling-down) could significantly degrade fatigue performance of welded joints, with the damage parameter D only at around 0.5. The severity of this type of loading spectrum depends on the mean stress level and the plate thickness. An analytical model has been developed to predict fatigue crack propagation (FCP) by considering the interaction of stresses in the loading spectrum. The model considers the impact of the mean stress generated by the preceding load on FCP in the subsequent cycles. FCP predicted by the model shows a good agreement with the experimental data.

Time-Variant Reliability Assessment of FPSO Hull Girder With Long Cracks

2006 ◽  
Vol 129 (2) ◽  
pp. 81-89 ◽  
Author(s):  
Efrén Ayala-Uraga ◽  
Torgeir Moan
Keyword(s):  
Brittle Fracture ◽  
Fatigue Cracking ◽  
Stiffened Panel ◽  
Stress Effect ◽  
Mean Stress ◽  
Stiffened Panels ◽  
Hull Girder ◽  
Mean Stress Effect ◽  
Hull Structure ◽  
The Mean

An efficient time-variant reliability formulation for the safety assessment of an aging floating production storage and offloading (FPSO) vessels with the presence of through-thickness cracks (i.e., long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. Although it has been found that stiffened panels are tolerant to fatigue cracking, the safety of such structural components with the presence of long cracks may be threatened when exposed to overload extreme conditions, i.e., brittle or ductile fracture may occur. The probability of brittle fracture of an aging hull structure, i.e., a stiffened panel at the bottom plating with the presence of long cracks is studied in this paper. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation procedure presented herein. An analytical model is established for determining the equivalent long-term stress range including the mean stress effect. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. In the present case study it is found that the time-invariant approach is a good approximation when dealing with the time-variant reliability problem. One of the main conclusions drawn from this study is that the still-water mean stress has a significant effect on the failure probabilities of stiffened panels with long cracks under brittle fracture mode.

scholarly journals Determination of the Fatigue Service Life of Welded Joints Using the Spectral Method Defined in the Frequency Domain

2020 ◽  
pp. 53-58
Author(s):  
Michał Böhm
Keyword(s):  
Fatigue Life ◽  
Service Life ◽  
Frequency Domain ◽  
Spectral Method ◽  
Yield Point ◽  
Welded Joints ◽  
Mean Stress ◽  
The Mean

The paper discusses presently experienced problems concerned with forecasting the fatigue life of welded joints in terms of the spectral method defined in the frequency domain. In addition, the article presents the primary assumptions of the spectral method and describes the issue related to the recognition of the mean stress value and loads above the material yield point.

scholarly journals The Effect of Mean Load for S355J0 Steel with Increased Strength

Metals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 209 ◽  
Author(s):  
Roland Pawliczek ◽  
Dariusz Rozumek
Keyword(s):  
Fatigue Life ◽  
Mean Value ◽  
Mean Stress ◽  
Good Compliance ◽  
New Model ◽  
Proposed Model ◽  
Calculation Results ◽  
The Mean ◽  
Increased Strength ◽  
The Impact

The paper presents an algorithm for calculating the fatigue life of S355J0 steel specimens subjected to cyclic bending, cyclic torsion, and a combination of bending and torsion using mean stress values. The method of transforming cycle amplitudes with a non-zero mean value into fatigue equivalent cycles with increased amplitude and zero mean value was used. Commonly known and used transformation dependencies were used and a new model of the impact of the mean stress value on the fatigue life of the material was proposed. The life calculated based on the proposed algorithm was compared with the experimental life. It has been shown that the proposed model finds the widest application in the load cases studied, giving good compliance of the calculation results with the experimental results.

In-Service Fatigue Performance of SCR Girth Welds Installed by Reeling

2012 ◽  
Author(s):  
Philippe P. Darcis ◽  
Eduardo Aguilar ◽  
Emma Erezuma ◽  
Israel Marines-Garcia ◽  
Eduardo A. Ruiz ◽  
...  
Keyword(s):  
Time History ◽  
Cumulative Damage ◽  
Fatigue Performance ◽  
Variable Amplitude Loading ◽  
Variable Amplitude ◽  
Failure Initiation ◽  
Service Conditions ◽  
Girth Welds ◽  
Damage Rule ◽  
Loading Spectrum

Interest arises on verifying the SCR girth welds fatigue response to a more representative loading spectrum of the actual in-service conditions and after reel-lay deformation. It is important to determine if the actual riser component’s qualification, without pre-straining and under constant amplitude loading, evidences discrepancies with in-service conditions, in terms of fatigue strength. This situation has motivated the full scale S-N fatigue performance evaluation of SCR girth welds under constant and variable amplitude loading, and after reel-lay simulations. A CMn steel X65 pipe 10.75” outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure developed for girth welds manufacturing involved the use of the Lincoln STT® process for the root pass and the GMAW process for the fill and cap passes. Reeling trials were performed at Stress Engineering Services, Houston, U.S.A.. A dedicated commercial software was used to simulate the variable amplitude loading spectrum, which is representative of a SCR Touch Down Point (TDP) in West of Africa at a water depth of 1,200 m (3937 ft) and a FPSO as production platform. The experimental approach was focused on estimating the damage introduced by reeling and by loading cycles of various magnitudes in the riser service time history. Results of strained and unstrained specimens, tested at constant and variable amplitude, have been compared, and the cumulative damage rule typically used by Riser fatigue designers has been evaluated (i.e. Miner’s linear cumulative damage rule). Systematic fractographic investigations were performed on all the samples after testing to identify their fatigue failure initiation causes.

Fatigue Analysis of Preloaded Threaded Fasteners

2011 ◽  
Author(s):  
Sayed A. Nassar ◽  
Rania A. Ali
Keyword(s):  
Fatigue Strength ◽  
Fatigue Test ◽  
Tensile Load ◽  
Fatigue Performance ◽  
Mean Stress ◽  
Damage Analysis ◽  
Data Set ◽  
Threaded Fasteners ◽  
The Mean ◽  
Set Up

This study investigates the fatigue performance of a preloaded threaded fastener under two scenarios of axial cyclic in-service loading. Since the fastener mean stress level would naturally affect the fatigue performance, various methods are investigated for their suitability for use to generate mean-stress adjusted S´-N´ fatigue curves. The maximum in-service cyclic tensile load that may externally be applied to the joint for a desired fatigue performance is also investigated. Limited fatigue test data set-up is generated for comparison with various methodologies for generating the mean stress-adjusted fatigue strength curves of A286 fasteners. Cumulative damage analysis is also performed in accordance with commonly used damage theories.

Hybrid Riser Resistance to In-Service Fatigue Loading

2014 ◽  
Author(s):  
Francesco Iob ◽  
Elisabetta Mecozzi ◽  
Luigi Di Vito ◽  
Philippe Darcis ◽  
Israel Marines-Garcia ◽  
...  
Keyword(s):  
Cross Flow ◽  
Complex Loading ◽  
Fatigue Loading ◽  
Fatigue Performance ◽  
Variable Amplitude Loading ◽  
Variable Amplitude ◽  
Riser Pipe ◽  
Wide Range ◽  
Girth Welds ◽  
Loading Spectrum

Riser systems are subjected to complex loading spectra. A wide range of amplitude loading are induced by naturally complex sea states. The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (riser pipe and its girth welds or threaded connections). The present paper reports numerical calculations and a comparison of the actual fatigue loading experienced by different riser systems, Steel Catenary Risers (SCR), Lazy Wave Riser (LWR), Hybrid Risers (HR). For the Hybrid riser system, a good fatigue resistance, due to the vessel and wave first motion decoupling, is obtained. On the other hand, the phenomenon of cross flow vibration induced by Vortex Induced Vibrations (VIV), could significantly affect the hybrid riser fatigue performance. In this case, a sensitivity analysis has been performed to evaluate the influence of different parameters, on the applied fatigue loading, like: riser tension, hydro diameter (i.e. external pipe diameter including coating), riser wall thickness, current velocity. The HR variable amplitude loading spectrum, derived from in-service conditions, referred to Gulf of Mexico scenario, has been calculated and applied in laboratory tests on girth welds of X65, 10.75″ OD, 25.4 mm WT riser. Furthermore, the fatigue performance of these tests has been compared to analogous tests on samples subjected to constant and variable amplitude loading available from previous work on SCR system[1].

Asteroid and Comet Impact Speeds upon Mars: Significance for Panspermia and the Supply of Organics

1997 ◽  
Vol 161 ◽  
pp. 197-201 ◽  
Author(s):  
Duncan Steel
Keyword(s):  
Probability Distributions ◽  
Regions Of Interest ◽  
Significant Fraction ◽  
Organic Chemicals ◽  
Impact Speed ◽  
The Mean ◽  
The Impact

AbstractWhilst lithopanspermia depends upon massive impacts occurring at a speed above some limit, the intact delivery of organic chemicals or other volatiles to a planet requires the impact speed to be below some other limit such that a significant fraction of that material escapes destruction. Thus the two opposite ends of the impact speed distributions are the regions of interest in the bioastronomical context, whereas much modelling work on impacts delivers, or makes use of, only the mean speed. Here the probability distributions of impact speeds upon Mars are calculated for (i) the orbital distribution of known asteroids; and (ii) the expected distribution of near-parabolic cometary orbits. It is found that cometary impacts are far more likely to eject rocks from Mars (over 99 percent of the cometary impacts are at speeds above 20 km/sec, but at most 5 percent of the asteroidal impacts); paradoxically, the objects impacting at speeds low enough to make organic/volatile survival possible (the asteroids) are those which are depleted in such species.

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