Probabilistic Predictions for Piping Fatigue Including the Effects of Ultrasonic Examinations

2003 ◽  
Author(s):  
F. A. Simonen
Keyword(s):  
Crack Initiation ◽  
Fatigue Cracks ◽  
Wall Stress ◽  
Cyclic Stress ◽  
Ferritic Steels ◽  
Cyclic Stresses ◽  
Stress Cycles ◽  
Low Levels ◽  
Stress States ◽  
Failure Probabilities

This paper presents a methodology for estimating failure probabilities of piping welds that experience cyclic stresses and that are subject to ultrasonic examinations designed to detect growing fatigue cracks. Fatigue cracks can start as either preexisting fabrication flaws or as cracks initiated after an accumulation of stress cycles. Low levels of cyclic stresses and/or small numbers of cycles produce low failure probabilities, with the failures caused mainly by fabrication flaws. More severe cyclic stress conditions produce higher failure probabilities, with the failures caused mainly by fatigue cracks that initiate during the life of the component. Numerical results are presented to address both crack initiation and crack growth. The calculations cover both stainless and ferritic steels, inservice inspections with different inspection intervals, and stress states with and without high levels of through-wall stress gradients. It is shown that effective inspection programs can significantly reduce failure probabilities, and that such programs require suitable NDE sensitivities and adequate inspection frequencies.

scholarly journals Stiffness Reduction of Composite Laminates under Combined Cyclic Stresses

2001 ◽  
Vol 10 (3) ◽  
pp. 096369350101000 ◽  
Author(s):  
T. P. Philippidis ◽  
A. P. Vassilopoulos
Keyword(s):  
Normal Stress ◽  
Composite Laminates ◽  
Failure Modes ◽  
Low Cycle Fatigue ◽  
Cyclic Stress ◽  
Fibre Direction ◽  
Cyclic Stresses ◽  
Stiffness Reduction ◽  
Stress States ◽  
Modulus Reduction

Stiffness reduction due to fatigue of a [0/(±45)2/0]T Glass/Polyester (GRP) laminate under combined cyclic stress is investigated in this experimental study. Stress states combining all three components of in-plane stress tensor are induced by uniaxially testing specimens cut off-axis at various angles from the principal material coordinate system. Modulus reduction is related to the various failure modes exhibited under different states of combined stress. It is verified that shear and transverse normal stress induce more severe stiffness degradation compared to stress states where normal stress in the main fibre direction is dominant. For every loading condition and stress state, it is observed in general that stiffness decrease is more pronounced under lower stress levels than these inducing low cycle fatigue.

Low Cycle Fatigue Crack Initiation of Rotating Structures Under Biaxial Stress States

2009 ◽  
Author(s):  
Chao Zhang
Keyword(s):  
Stress State ◽  
Crack Initiation ◽  
Low Cycle Fatigue ◽  
Cyclic Stress ◽  
Biaxial Stress ◽  
Test Results ◽  
Primary Stress ◽  
Biaxial Stress State ◽  
Fatigue Characteristics ◽  
Stress States

Rotating structures can experience biaxial stress states with a wide range of biaxiality ratios on structure surfaces. Low cycle fatigue (LCF) crack initiation in such conditions demonstrates different fatigue characteristics in terms of crack orientation, fatigue life, etc. The biaxial stress states can be categorized into two types: in-phase and out-of-phase under which fatigue characteristics can be significantly different according to rig test results. This paper presents an investigation of LCF crack initiation under in-phase and out-of-phase biaxial stress states based on rig test results of a nickel alloy. The crack orientations are reviewed and discussed at different stress states. Relations of biaxial LCF life debit factor vs biaxiality ratio are derived (the debit factor is defined as a ratio of the LCF life at a biaxial stress state to the LCF life at corresponding uniaxial stress state which has same cyclic and mean stresses as the primary cyclic and mean stressees of the biaxial stress state). The rig test results showed that the crack orientation is usually normal to the primary stress vector under in-phase biaxial stress states but is inclined to the primary stress vector under out-of-phase stress states. As per the derived biaxial LCF life debit factors, the LCF life was found to be slightly reduced with increasing biaxiality ratios under in-phase biaxial stress states but significantly reduced under out-of-phase biaxial stress states compared with corresponding uniaxial primary stress states. The equivalent cyclic stress fatigue criterion is also employed to theoretically model the biaxial LCF life debit factor under in-phase biaxial stress states. The hydrostatic cyclic stress is included in the equivalent cyclic stress in order to take into account the hydrostatic cyclic pressure effects. The equivalent cyclic stress in the criterion can physically reflect the materials’ ductility reduction under in-phase multiaxial stress states.

Stress Ratio Effect on Fatigue Crack Initiation Mechanism of Magnesium Alloy AZ31

2021 ◽  
Vol 1016 ◽  
pp. 1003-1008
Author(s):  
Yoshikazu Nakai ◽  
Shoichi Kikuchi ◽  
Kaito Asayama ◽  
Hayata Yoshida
Keyword(s):  
Fatigue Crack ◽  
Magnesium Alloy ◽  
Crack Initiation ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Cyclic Stress ◽  
Initiation Mechanism ◽  
Alloy Az31 ◽  
Magnesium Alloy Az31 ◽  
Schmid Factor

Fatigue tests were conducted under several stress ratios, including negative maximum stress to elucidate the fatigue crack initiation mechanism of a magnesium alloy, AZ31. The specimen surface near the crack initiation site was analyzed by EBSD. On the basis of the results of EBSD analysis, it is concluded for an alternating cyclic stress condition (fully reversed cyclic stress) that fatigue cracks formed from grains where both the grain size and Schmid factor of the basal slip system are large, and that the crack initiation mechanism is based on irreversible slipping and unrelated to twinning. Under compression-compression fatigue test (R=10), cracks were formed along boundary of grains with large Schmid factor and misfit of both side grain are large. At the tip of the initiated crack, twin bands were observed.

Effect of high-frequency PCB laminate on thermal cycling behavior of electronic packaging structures

2022 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Lijuan Huang ◽  
Zhenghu Zhu ◽  
Hiarui Wu ◽  
Xu Long
Keyword(s):  
Fatigue Life ◽  
Thermal Cycling ◽  
Electronic Packaging ◽  
High Frequency ◽  
Solder Joints ◽  
Fatigue Cracks ◽  
Cyclic Stress ◽  
Content Type ◽  
Stress States ◽  
Packaging Structure

PurposeAs the solution to improve fatigue life and mechanical reliability of packaging structure, the material selection in PCB stack-up and partitioning design on PCB to eliminate the electromagnetic interference by keeping all circuit functions separate are suggested to be optimized from the mechanical stress point of view.Design/methodology/approachThe present paper investigated the effect of RO4350B and RT5880 printed circuit board (PCB) laminates on fatigue life of the QFN (quad flat no-lead) packaging structure for high-frequency applications. During accelerated thermal cycling between −50 °C and 100 °C, the mismatched coefficients of thermal expansion (CTE) between packaging and PCB materials, initial PCB warping deformation and locally concentrated stress states significantly affected the fatigue life of the packaging structure. The intermetallics layer and mechanical strength of solder joints were examined to ensure the satisfactorily soldering quality prior to the thermal cycling process. The failure mechanism was investigated by the metallographic observations using a scanning electron microscope.FindingsTypical fatigue behavior was revealed by grain coarsening due to cyclic stress, while at critical locations of packaging structures, the crack propagations were confirmed to be accompanied with coarsened grains by dye penetration tests. It is confirmed that the cyclic stress induced fatigue deformation is dominant in the deformation history of both PCB laminates. Due to the greater CTE differences in the RT5880 PCB laminate with those of the packaging materials, the thermally induced strains among different layered materials were more mismatched and led to the initiation and propagation of fatigue cracks in solder joints subjected to more severe stress states.Originality/valueIn addition to the electrical insulation and thermal dissipation, electronic packaging structures play a key role in mechanical connections between IC chips and PCB.

Aspects of Fatigue Crack Growth

1988 ◽  
Vol 202 (1) ◽  
pp. 19-29 ◽  
Author(s):  
M W Brown
Keyword(s):  
Fatigue Crack ◽  
Crack Growth ◽  
High Cycle Fatigue ◽  
Fatigue Cracks ◽  
Cyclic Stress ◽  
Cycle Fatigue ◽  
Linear Elastic ◽  
Propagation Law ◽  
Elastic Fracture ◽  
Low Levels

The propagation of fatigue cracks under low levels of cyclic stress can be modelled by linear elastic fracture mechanics. Elastic-plastic parameters must be used for the higher stress amplitudes of conventional S–N curves in both the low and the high cycle fatigue regimes. The occurrence of different modes of crack growth is discussed and the use of a propagation law to predict component lifetime is demonstrated.

Ultrahigh Plasticity Behavior of Metallic Materials in the Ultra-High-Cycle (or Gigacycle, Very-High-Cycle) Fatigue Regime

2015 ◽  
Vol 664 ◽  
pp. 231-245 ◽  
Author(s):  
Andrey Shanyavskiy
Keyword(s):  
Crack Initiation ◽  
Stress Level ◽  
Fatigue Cracks ◽  
Cyclic Stress ◽  
Test Material ◽  
Metallic Materials ◽  
Growth Duration ◽  
Transition Range ◽  
Very High ◽  
Temperature Surface

This research concerns the metals behavior as varying, with increasing cyclic stress level, in the transition range between the ultrahigh cycle (UHCF) and high cycle (HCF) fatigue regimes. Having analyzed the synergetics of surface crack initiation, the authors propose to identify the UHCF-to-HCF transition with a certain stress level corrected, with the respective dimensionless functions, for the effect of the environmental attacks, temperature, surface roughness of the test piece, etc. on the crack initiation behavior. Subsurface initiation of a fatigue crack occurs in the test-material susceptible of a deformation-induced transition to superplasticity state, which favors formation of the nanocrystalline zone bordered by a fine-granular area Another case of subsurface cracking is in that a local even area forms owing to the vortex-like deformation combined with the diffusion of the retained gases toward and into the discontinuity. A proposed equation is plainly descriptive of the subsurface growth of fatigue-cracks and allowing to acquire the crack growth duration from the fractography data.Nomenclature

Plastic Slip and Early Stage of Fatigue Damage in Polycrystals: Comparison between Experiments and Crystal Plasticity Finite Elements Simulations

2014 ◽  
Vol 891-892 ◽  
pp. 1711-1716 ◽  
Author(s):  
Loic Signor ◽  
Emmanuel Lacoste ◽  
Patrick Villechaise ◽  
Thomas Ghidossi ◽  
Stephan Courtin
Keyword(s):  
Fatigue Crack ◽  
Finite Elements ◽  
Crack Initiation ◽  
Fatigue Damage ◽  
Crystal Plasticity ◽  
Early Stage ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Plastic Slip

For conventional materials with solid solution, fatigue damage is often related to microplasticity and is largely sensitive to microstructure at different scales concerning dislocations, grains and textures. The present study focuses on slip bands activity and fatigue crack initiation with special attention on the influence of the size, the morphology and the crystal orientation of grains and their neighbours. The local configurations which favour - or prevent - crack initiation are not completely identified. In this work, the identification and the analysis of several crack initiation sites are performed using Scanning Electron Microscopy and Electron Back-Scattered Diffraction. Crystal plasticity finite elements simulation is employed to evaluate local microplasticity at the scale of the grains. One of the originality of this work is the creation of 3D meshes of polycrystalline aggregates corresponding to zones where fatigue cracks have been observed. 3D data obtained by serial-sectioning are used to reconstruct actual microstructure. The role of the plastic slip activity as a driving force for fatigue crack initiation is discussed according to the comparison between experimental observations and simulations. The approach is applied to 316L type austenitic stainless steels under low-cycle fatigue loading.

Influence of Interference and Clamping on Fretting Fatigue in Single Rivet-Row Lap Joints

2000 ◽  
Vol 123 (4) ◽  
pp. 686-698 ◽  
Author(s):  
K. Iyer ◽  
C. A. Rubin ◽  
G. T. Hahn
Keyword(s):  
Crack Initiation ◽  
Three Dimensional ◽  
Fretting Fatigue ◽  
Cyclic Stress ◽  
Lap Joints ◽  
Element Analysis ◽  
Out Of Plane ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element ◽  
Fretting Damage

Primary fretting fatigue variables such as contact pressure, slip amplitude and bulk cyclic stresses, at and near the contact interface between the rivet shank and panel hole in a single rivet-row, 7075-T6 aluminum alloy lap joint are presented. Three-dimensional finite element analysis is applied to evaluate these and the effects of interference and clamping stresses on the values of the primary variables and other overall measures of fretting damage. Two rivet geometries, non-countersunk and countersunk, are considered. Comparison with previous evaluations of the fretting conditions in similar but two-dimensional connections indicates that out-of-plane movements and attending effects can have a significant impact on the fatigue life of riveted connections. Variations of the cyclic stress range and other proponents of crack initiation are found to peak at distinct locations along the hole-shank interface, making it possible to predict crack initiation locations and design for extended life.

A Dislocation Model for Fatigue Crack Initiation

1981 ◽  
Vol 48 (1) ◽  
pp. 97-103 ◽  
Author(s):  
K. Tanaka ◽  
T. Mura
Keyword(s):  
Grain Size ◽  
Crack Initiation ◽  
Slip Band ◽  
Fatigue Crack Initiation ◽  
Strain Range ◽  
Material Surface ◽  
Dislocation Model ◽  
Plastic Strain Range ◽  
Stress Cycles ◽  
Distributed Dislocations

The slip band formed in a grain on the material surface is a preferential site for crack initiation during low strain fatigue of polycrystalline metals. The forward and reverse plastic flow within the slip band is modeled in the present study by dislocations with different signs moving on two closely located layers, and it is assumed that their movement is irreversible. Based on the model, the monotonic buildup of dislocation dipoles piled up at the grain boundary is systematically derived using the theory of continuously distributed dislocations. This buildup is associated with the progress of extrusion or intrusion. The number of stress cycles up to the initiation of a crack of the grain size order is defined as the cycle when the stored strain energy of accumulated dislocations reaches a critical value. The relation between the initiation life and the plastic strain range derived theoretically is in agreement with a Coffin-Manson type law, and that between the fatigue strength and the grain size is expressed in an equation of the Petch type.

On the Dynamics of Cracked Rotors: A Literature Survey

1990 ◽  
Vol 43 (1) ◽  
pp. 13-17 ◽  
Author(s):  
Jo¨rg Wauer
Keyword(s):  
Crack Initiation ◽  
Fatigue Cracks ◽  
Rotating Machinery ◽  
Literature Survey ◽  
Vibration Monitoring ◽  
Structural Elements ◽  
Cracked Rotor ◽  
Vibrational Behavior ◽  
Rotating Shafts ◽  
Fracture Damage

Propagating fatigue cracks can have detrimental effects on the reliability of rotating machinery. An early crack warning can considerably extend the durability of these very expensive machines, increasing their reliability at the same time. Vibration monitoring as a means of detecting crack initiation has been receiving much interest. A detailed study of the vibrational behavior of cracked rotating shafts, therefore, is an important problem for engineers working in the area of the dynamics of machines. This article presents a review of the field of the dynamics of cracked rotors, including the modeling of the cracked part of the structure and finding different detection procedures to diagnose fracture damage. The material should be helpful to scientists and researchers working in this area or planning to work in it in the future. Since the study of nonrotating, cracked structural elements obviously is relevant to the cracked rotor problem, the review can also be a basis for discussing the dynamics of cracked beams and columns.

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