Dimensioning against fatigue - stress-based approach or fracture mechanics?

2006 ◽  
Vol 220 (8) ◽  
pp. 1109-1120
Author(s):  
H-P Gänser ◽  
K Glinsner ◽  
W Eichlseder
Keyword(s):  
Crack Length ◽  
Crack Arrest ◽  
Accurate Estimate ◽  
Formal Structure ◽  
Structural Stress ◽  
Propagation Condition ◽  
Stress Concentrations ◽  
Notch Radius ◽  
Sharp Crack ◽  
Limiting Case

The non-propagation condition of a sharp crack introduced in the root of a generic notch is investigated. Thus, pre-existing flaws near structural stress concentrations are advantageously treated in a unified manner, giving in the limit of vanishing notch radius the case of a crack in an unnotched component, and in the limit of vanishing crack length the classical engineering case of a notched, crack-free component. Particular focus is put on the limiting case of vanishing crack length. The results are compared with the classical engineering strength estimate following the nominal stress concept. It is shown that both concepts may, with minor differences, be brought into the same formal structure. The transition between notch-like and crack-like behaviour of notches is investigated in some detail, giving an accurate estimate for the transition between both regimes as well as reliable lower bounds for the fatigue strength in both regimes with special consideration of crack arrest effects in flawed components.

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.

Shape optimization of the fillet under a bolt’s head

2019 ◽  
Vol 54 (4) ◽  
pp. 247-253
Author(s):  
Andrea Sorrentino ◽  
Davide Castagnetti ◽  
Andrea Spaggiari ◽  
Eugenio Dragoni
Keyword(s):  
Shape Optimization ◽  
Stress Reduction ◽  
Ad Hoc ◽  
Optimization Procedure ◽  
Element Model ◽  
Critical Issue ◽  
Stress Concentrations ◽  
Lower Stress ◽  
Notch Radius ◽  
Head Height

The stress concentration in the fillet under a bolt’s head is a critical issue for the fatigue life of this component. Remembering the variable notch radius solution observed in many biological structures to lower stress concentrations due to normal loads, this work proposes, examines and optimizes a double circular arc fillet, not re-entering the head. The work implements an ad hoc shape optimization procedure that combines a genetic algorithm (the particle swarm optimization) and a parametric, axisymmetric finite-element model of the bolt: by focusing on an M12 bolt, the analysis focuses on two issues: first, the optimization of the radius of each arc in the fillet; second, the optimization of the bolt’s head height and head’s diameter in combination with the fillet radiuses. By comparing the proposed solution with the fillet geometries for the bolt’s heads from the literature, it appears a noticeable stress reduction, about 14% lower than the standard circular fillet, combined with an easy manufacturing.

scholarly journals Approximating Isoneutral Ocean Transport via the Temporal Residual Mean

Fluids ◽  
2019 ◽  
Vol 4 (4) ◽  
pp. 179 ◽  
Author(s):  
Andrew L. Stewart
Keyword(s):  
Numerical Model ◽  
Potential Temperature ◽  
Accurate Estimate ◽  
Neutral Density ◽  
Advective Transport ◽  
External Reference ◽  
Temperature And Pressure ◽  
Limiting Case ◽  
Mean State

Ocean volume and tracer transports are commonly computed on density surfaces because doing so approximates the semi-Lagrangian mean advective transport. The resulting density-averaged transport can be related approximately to Eulerian-averaged quantities via the Temporal Residual Mean (TRM), valid in the limit of small isopycnal height fluctuations. This article builds on a formulation of the TRM for volume fluxes within Neutral Density surfaces, (the “NDTRM”), selected because Neutral Density surfaces are constructed to be as neutral as possible while still forming well-defined surfaces. This article derives a TRM, referred to as the “Neutral TRM” (NTRM), that approximates volume fluxes within surfaces whose vertical fluctuations are defined directly by the neutral relation. The purpose of the NTRM is to more closely approximate the semi-Lagrangian mean transport than the NDTRM, because the latter introduces errors associated with differences between the instantaneous state of the modeled/observed ocean and the reference climatology used to assign the Neutral Density variable. It is shown that the NDTRM collapses to the NTRM in the limiting case of a Neutral Density variable defined with reference to the Eulerian-mean salinity, potential temperature and pressure, rather than an external reference climatology, and therefore that the NTRM approximately advects this density variable. This prediction is verified directly using output from an idealized eddy-resolving numerical model. The NTRM therefore offers an efficient and accurate estimate of modeled semi-Lagrangian mean transports without reference to an external reference climatology, but requires that a Neutral Density variable be computed once from the model’s time-mean state in order to estimate isopycnal and diapycnal components of the transport.

A study of stress concentrations in solids with circular holes by three-dimensional special hybrid stress finite elements

1990 ◽  
Vol 25 (1) ◽  
pp. 29-35 ◽  
Author(s):  
Z-S Tian
Keyword(s):  
Finite Elements ◽  
Three Dimensional ◽  
Stress Concentrations ◽  
Equilibrium Equations ◽  
Circular Holes ◽  
Concentration Factors ◽  
Solid Finite Elements ◽  
Stress Concentration Factors ◽  
Limiting Case ◽  
Free Cylindrical Surface

Four kinds of 12-node solid finite elements with two parallel faces and one traction-free cylindrical surface have been developed based on the assumed stress hybrid method. Cylindrical coordinates are used so that the assumed stresses satisfy the equilibrium equations as well as the traction-free condition over the cylindrical boundary. In the limiting case of plane stress the assumed stresses also satisfy the compatibility conditions. Examples have indicated that these present special solid finite elements are far superior in predicting the distributions of the circumferential stresses and the stress concentration factors for solids with circular holes.

Assessment of Steels for Nuclear Reactor Pressure Vessels

1964 ◽  
Vol 86 (4) ◽  
pp. 393-401 ◽  
Author(s):  
A. Cowan ◽  
R. W. Nichols
Keyword(s):  
Large Body ◽  
Stress Rupture ◽  
Crack Arrest ◽  
Pressure Vessels ◽  
Small Scale ◽  
Specimen Thickness ◽  
Stress Concentrations ◽  
Flat Plates ◽  
Reactor Pressure ◽  
Arrest Temperature

Some of the materials problems associated with the use of mild steels in large gas-cooled reactor pressure vessels are discussed. Tests to failure of 5-ft-dia 0.36 percent carbon-steel vessels with through-thickness longitudinal slots, supported by tests on 7-ft-wide centrally slotted flat plates, have indicated that rapid failure at working-stress levels can only initiate from very long cracks, feet rather than inches in length. Of the mechanisms whereby realistic defects can grow to these sizes, brittle-crack propagation is considered the most important and this can be prevented by the maintenance of a minimum pressurization temperature, based on the crack-arrest temperature. The tests used to assess the crack arrest temperature of plates up to 4 in. thick are described; compared with tests on thinner specimens the thick plate gives arrest temperatures higher by approximately 10 deg C per in. of test-specimen thickness. A comparison is made of crack-arrest temperature and data given by small-scale tests, particularly the Charpy V-notch test. Mechanical limitations of creep deformation in some current designs have been more restrictive on design stress than the values allowed by the existing BS.1500. The test data quoted for stress-rupture and fatigue indicate that these modes of crack extension are not important in current designs. Possible magnitudes and effects of stress concentrations are quoted but, other than a large body of satisfactory service operation, there is little direct evidence of the effect of operating in the creep range on these stress concentrations. The importance of work of this type in justifying higher design stresses and more economic use of material is emphasized.

Strain concentration effects in large structures

1965 ◽  
Vol 285 (1400) ◽  
pp. 46-57 ◽  
Keyword(s):  
Steel Structures ◽  
Fracture Initiation ◽  
Simple Calculation ◽  
Crack Arrest ◽  
Stress Concentrations ◽  
Concentration Effects ◽  
Large Structures ◽  
Inclusion Distribution ◽  
Large Sections

When a large structure is subject to a temperature lower than the crack-arrest temperature of the particular steel from which it is built, the problem of fracture initiation at stress concentrations becomes of paramount importance. On the basis of the attainment of a critical local displacement as the criterion for fracture initiation, a theoretical analysis has been undertaken on some aspects of the fracture of large sections. In particular a simple calculation has been made for the fracture stress of a large plate containing a notch, and it has been shown that the results are identical for small stresses with those given by the fracture mechanics approach, which was developed from energetic considerations. If a steel is very brittle, then inclusions or small defects may act as the cracks from which final fracture is initiated; consideration has been given to the role of inclusion distribution on the behaviour of steel structures.

Comparison of Peak Stress-Based and Flaw Tolerance-Based Fatigue Analysis of a Cylinder With Variable Stress Concentrations

2014 ◽  
Author(s):  
Wolf Reinhardt ◽  
Gary L. Stevens
Keyword(s):  
Fatigue Life ◽  
Crack Growth ◽  
Growth Analysis ◽  
Notch Root ◽  
Stress Gradient ◽  
Fatigue Curve ◽  
Fatigue Analysis ◽  
Stress Concentrations ◽  
Notch Radius ◽  
Flaw Tolerance

To show the implications of using crack growth analysis to evaluate fatigue life, the case of a cylinder with an internal notch with a varying root radius is examined. The notch radius controls the stress gradient, where a steeper stress gradient is expected to result in slower crack growth and longer fatigue life. The notch root stress is made the same between specimens of different notch radius by scaling the applied load. As a result, the conventional fatigue analysis that calculates a fatigue usage factor from a fatigue curve based on stress at a point gives identical results for all specimens. A crack growth analysis, on the other hand, gives significantly different fatigue lives for the specimens because of the different stress gradients. On this basis of allowable fatigue life, the traditional fatigue curve-based approach is compared with the crack growth-based flaw tolerance approach. The relative conservatism of the two approaches as a function of various parameters, including stress gradient, is discussed.

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.

Coupling Analysis of Vibration and Fatigue Crack Propagation for a Breathing Cracked Beam

2012 ◽  
Vol 164 ◽  
pp. 338-343 ◽  
Author(s):  
Wei Cui ◽  
Jian Jun Wang
Keyword(s):  
Fatigue Crack ◽  
Fracture Mechanics ◽  
Crack Propagation ◽  
Crack Length ◽  
Fatigue Crack Propagation ◽  
Resonant State ◽  
Coupling Effect ◽  
Crack Arrest ◽  
Harmonic Excitation ◽  
Cracked Beam

The coupling effect of vibration and fatigue crack propagation for a cracked cantilever beam is studied in this paper. The dynamic characteristics and fracture mechanics parameters are calculated by using 2D 8-nodes elements in FEM code. The nonlinear dynamic behavior of breathing crack is described by a frictionless contact FEM model. Linear fracture mechanics theory is used to calculate the stress intensity factor. At resonant state, coupling effect is significant between vibration and crack propagation. The response of beam under harmonic excitation is extremely sensitive to the structure natural frequency decrease which is caused by crack length growth. An approach of sweeping crack length analysis is proposed in resonant response evaluation of cracked beam. Two numerical tests are calculated to investigate coupling effects at resonant state: crack arrest problem and crack unstable propagation problem.

Construction of Various Fatigue Design Master S-N Curves for Offshore/Marine Structures Using Battelle Structural Stress Method

2011 ◽  
Author(s):  
Jeong K. Hong
Keyword(s):  
Structural Integrity ◽  
Environmental Safety ◽  
Fatigue Tests ◽  
Structural Stress ◽  
Stress Concentrations ◽  
Marine Structures ◽  
Fatigue Design ◽  
Weld Toe ◽  
Weld Fatigue ◽  
Corrosive Environments

Reliable structural integrity evaluation is a critical part of the design process. Reliable structural integrity evaluation is especially important for large and complex structures such as buildings and offshore/marine installations that have significant implications for human and environmental safety. The design and careful evaluation of welded structures are essential in structural design since the stress concentrations at the welds have significant impact on the overall fatigue lives of the structures. A mesh-insensitive structural stress method has been developed and proven to be highly effective in correlating fatigue behaviors of welded joints by Battelle researchers. The Battelle structural stress method and related weld fatigue master S-N curve approach has been adopted by ASME and API in 2007 [1–2]. The design fatigue master S-N curve has been constructed by incorporating the results of more than 800 fatigue tests, which are clearly categorized as weld toe failure. In addition, a design master S-N curve for weld root failure has been constructed recently [3]. For offshore/marine structures, it is essential to consider weld fatigue damage in corrosive environments as well as that in air, and to understand the effects of techniques commonly applied to improve weld fatigue strength of the structures, e.g., hammer peening, toe grinding, and TIG dressing. In order to meet the industry’s increasing demand for reliably, fatigue resistant structures, design master S-N curves incorporating the effects of corrosive environments and weld improvement techniques have been constructed. These new curves are based on existing weld fatigue data from the literature and class bodies’ fatigue design documents.

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