Assessment of Constraint Effects on the Local Behaviour of a Propagating Crack Under Cyclic Loading

2012 ◽  
Author(s):  
M. R. Goldthorpe ◽  
A. H. Sherry
Keyword(s):  
Cyclic Loading ◽  
Best Practice ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Crack Tip ◽  
Element Model ◽  
Compact Tension ◽  
Stress Constraint ◽  
T Stress ◽  
Constraint Effects

During operation, reactor components experience a range of static and cyclic loading that have the potential to result in environmental-fatigue crack initiation and growth. Recent experimental work has indicated that the ASME XI fatigue ‘in air’ design curves are non-conservative for fatigue cracks propagating in primary water environments at fixed temperatures of relevance to the plant. The approach adopted to assess these tests has, to date, followed current best practice: in which global Linear Elastic Fracture Mechanics (LEFM) loading parameters are used to quantify crack growth rates. To help establish an improved understanding of these data, and to assist in their application to assess plant components, a local crack-tip finite element model has been developed. The model incorporates material constitutive behavior that simulates cyclic deformation of austenitic steel, can take account of plasticity-induced crack closure and can take into consideration cracks in structurally-representative geometries via the T-stress constraint parameter. The results of studies using the model suggest that highly compressive values of the T-stress constraint parameter tend to promote less severe reverse loading of the crack tip compared with high constraint geometries such as pre-cracked compact tension and bend test specimens. These findings indicate that rates of corrosion-fatigue in actual structural geometries might be different from those observed in pre-cracked test specimens.

Investigation of Constraint Effects on Fracture Toughness for CC(T) Specimens

2004 ◽  
Author(s):  
Dieter Siegele ◽  
Igor Varfolomeyev ◽  
Kim Wallin ◽  
Gerhard Nagel
Keyword(s):  
Fracture Toughness ◽  
Crack Tip ◽  
Temperature Shift ◽  
Local Stress ◽  
Significant Loss ◽  
Reference Temperature ◽  
T Stress ◽  
Local Stress State ◽  
Constraint Effects ◽  
Crack Tip Constraint

Within the framework of the European research project VOCALIST, centre cracked tension, CC(T), specimens made of an RPV steel were tested and analysed to quantify the influence of local stress state on fracture toughness. The CC(T) specimens demonstrate a significant loss of crack tip constraint resulting in a considerable increase in fracture toughness as compared to standard fracture mechanics specimens. So, the master curve reference temperature, To, determined on the basis of CC(T) tests performed in this study is about 43°C lower than To obtained on standard C(T) specimens. Finite element analyses of the tests revealed that the above experimental finding is in a good agreement with the empirical correlations between the reference temperature shift and the crack tip constraint as characterised by the T-stress or Q parameter (Wallin, 2001; Wallin, 2004). The results of this work are consistent with a number of other tests performed within the VOCALIST project and contribute to the validation of engineering methods for the crack assessment in components taking account of constraint.

Fatigue Crack Initiation and Growth in Stainless Steel Pipe Welds

2009 ◽  
Author(s):  
D. Green ◽  
R. D. Smith ◽  
J. P. Taggart ◽  
D. Beardsmore ◽  
S. Robinson
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Thermal Loading ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Cyclic Hardening ◽  
Element Model ◽  
Fatigue Tests ◽  
Welded Pipe ◽  
Pipe Work

Thermal fatigue cracks have been found in austenitic pipe work in many pressurised water reactors, caused by thermal cycling due to the passage of water at different temperatures along the pipe inner surface. The rates of crack initiation and growth for this situation are not well understood because of the stochastic nature of the temperature fluctuations. Therefore, large allowances must be made when assessing the integrity of this pipe work to this failure mechanism. Improved assessment of crack initiation and growth could enable increased plant availability, and better safety cases. A programme of work has been completed consisting of fatigue tests on thick 304L butt-welded pipe specimens, and accompanying predictions of crack initiation and growth. In each test, uniform thermal cycles were generated using a water jet on a small area of the pipe. The magnitude of the cycles differed between the tests. Crack initiation and growth were monitored using a dye penetrant technique, applied to the pipe inner and outer surfaces, together with destructive examination. Crack initiation predictions were made using fatigue data derived from mechanical fatigue tests on the same material as in the pipe specimens. Good predictions were made using a strain-life endurance curve at a temperature corresponding to the average temperature of the metal surface during the thermal cycle. Crack growth predictions were based on an inelastic finite-element model accounting for cyclic hardening, and an enhanced R5 procedure (1) with crack closure taken into account. A linear elastic fracture mechanics definition of a Paris law for crack growth was used, and plastic redistribution effects were included. Predictions were good for all of the experimental scenarios carried out. A further experimental and analytical programme is in hand using the same experimental arrangements, concerning variable amplitude thermal loading.

Critical Strain Energy Density along the Curved Front of the Growing Fatigue Crack

2005 ◽  
Vol 482 ◽  
pp. 307-310
Author(s):  
Tomáš Denk ◽  
Vladislav Oliva ◽  
Aleš Materna
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Cracks ◽  
Anomalous Behavior ◽  
Fatigue Threshold ◽  
Stress Constraint ◽  
Short Cracks ◽  
T Stress ◽  
Residual Fatigue ◽  
Two Parameter ◽  
Different Levels

A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Cohesive Release Loads in a General Finite Element Model of a Propagating Crack

2009 ◽  
Vol 417-418 ◽  
pp. 517-520 ◽  
Author(s):  
A. Fontana ◽  
M. Minotti ◽  
Pietro Salvini
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Crack Tip ◽  
Reference Value ◽  
Element Model ◽  
Cohesive Model ◽  
Plastic Materials ◽  
T Stress ◽  
Speed Mode ◽  
Elastic Plastic Materials

High speed MODE I crack growth in elastic-plastic materials, involving large scale plasticity and dynamic effects connected to rapid propagation, is faced through a cohesive model to tune force nodal release. The stress resisting to the opening of the edges in the cohesive zone should account of effective stress field ahead crack tip. In this paper a reference value is accounted: it represents the maximum closing stress measured at the crack tip, where the cohesive effects begin. A bi-parametric analytical formulation of stress distribution ahead the crack tip is suggested. The bi-parametric formulation is able to extrapolate the stress at the tip whatever is the T-stress (i.e. the stress acting in the direction of fracture propagation), thus completely defining the cohesive loads.

Research on the Mechanism of Fatigue Crack Initiation of X60 Pipeline Steel after Per-Tension Deformation

2012 ◽  
Vol 197 ◽  
pp. 798-801
Author(s):  
Yu Rong Jiang ◽  
Mei Bao Chen
Keyword(s):  
Fatigue Crack ◽  
Cyclic Loading ◽  
Brittle Fracture ◽  
Crack Initiation ◽  
Pipeline Steel ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Fracture Characteristics ◽  
Metallic Inclusions ◽  
The Matrix

It is impossible to keep pipelines free from defects in the manufacturing, installation and servicing processes. In this paper, pre-tension deformation of X60 pipeline steel was employed to experimentally simulate the influence of dents and the mechanism of fatigue crack initiation of X60 pipeline steel after per-tension deformation under cyclic loading were investigated. The results indicate that the mechanism of fatigue crack initiation is the typical cleavage fracture characteristics and the cracks mainly initiates from the non-metallic inclusions which was the local brittle fracture materials such as MnS inclusion. With the pre-tension deformation increase, the yield strength of the matrix was increased and the toughness decreased due to the work-hardening effect. With the effects of the non-metallic inclusions larger, the fatigue cracks initiated from the non-metallic inclusions easier.

The Role of the Constraint in the Case of Short Cracks

2005 ◽  
Vol 482 ◽  
pp. 303-306
Author(s):  
Pavel Hutař ◽  
Stanislav Seitl ◽  
Zdeněk Knésl
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Cracks ◽  
Anomalous Behavior ◽  
Fatigue Threshold ◽  
Stress Constraint ◽  
Short Cracks ◽  
T Stress ◽  
Residual Fatigue ◽  
Two Parameter

A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

scholarly journals Development and validation of a high constraint modified boundary layer finite element model

2011 ◽  
Vol 2 (2) ◽  
pp. 228-236
Author(s):  
Matthias Verstraete ◽  
W. De Waele ◽  
Stijn Hertelé
Keyword(s):  
Boundary Layer ◽  
Finite Element ◽  
Crack Tip ◽  
Element Model ◽  
Theoretical Background ◽  
Small Scale ◽  
T Stress ◽  
Development And Validation ◽  
Weak Influence ◽  
Crack Tip Stress

When a notched structure is loaded, its behaviour is not only affected by the material propertiesbut also by the geometry (of both the structure and the defect) and loading condition, alternatively termedas constraint condition. Therefore, the relation between the failure behaviour of a small scale fracturemechanics test and a full scale structure needs to be elucidated.In an attempt to understand and describe such relationships, the crack tip stress fields are analysed bymeans of finite element simulations and compared for several test specimen geometries. A reference forcomparison is the crack tip stress field obtained from a high constraint reference geometry, further called amodified boundary layer model.First, this article provides some theoretical background on the modified boundary layer model. Second, thedevelopment of a 2D model is outlined in detail, focussing on the mesh design in the vicinity of the crack tipand the applied boundary conditions. Afterwards, an analytical and numerical validation is provided, basedon the level of the applied load and, on the other hand, on the magnitude of the crack tip stress fields.Finally, this validated model is used for the comparison of several constraint parameters. This comparisonindicates a weak influence of the T-stress on the Q-parameter for positive T-stresses. In contrast, negativeT-stresses result in more pronounced negative Q-values.

The Effect of T-Stress on Crack-Tip Plastic Zones Under Cyclic Mixed-Mode Loading Conditions

2013 ◽  
Author(s):  
Qays Nazarali ◽  
Xin Wang
Keyword(s):  
Stress Intensity ◽  
Cyclic Loading ◽  
Stress Field ◽  
Plastic Zone ◽  
Mixed Mode ◽  
Crack Tip ◽  
Mode I ◽  
Loading Conditions ◽  
T Stress ◽  
Plastic Zones

In this paper, the influence of T-stress on crack-tip plastic zones under mixed-mode I and II loading conditions under cyclic loading is examined. The crack-tip stress field is defined in terms of the ranges of mixed-mode stress intensity factors and the T-stress using William’s series expansion. The crack-tip stress field is incorporated into the Von Mises yield criteria to develop an expression that determines the cyclic crack-tip plastic zone. Using the resultant expression, the cyclic plastic zone is obtained for various combinations of mode II to mode I stress intensity factor ratios and levels of T-stress. For the purpose of demonstrating the significance of the T-stress, this paper further analyses the plastic zone size for center cracked plate (CCP) specimen subjected to bi-axial mixed-mode cyclic loading.

Degradation of YSZ/EUCOR TBC Coating System during High Temperature Low Cycle Fatigue Tests

2016 ◽  
Vol 258 ◽  
pp. 420-423 ◽  
Author(s):  
Ivo Šulák ◽  
Karel Obrtlík ◽  
Ladislav Čelko ◽  
Pavel Gejdoš
Keyword(s):  
High Temperature ◽  
Cyclic Loading ◽  
Bond Coat ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Fatigue Crack Initiation ◽  
Thermal Barrier ◽  
Coating System ◽  
Air Plasma ◽  
Polycrystalline Nickel

Thermal barrier coatings are widely used to protect the substrate from high temperature and extremely aggressive environments in gas engines. In the present article, authors have been studied degradation of complex thermal barrier coating system deposited on polycrystalline nickel superalloy IN 713LC. The substrate material was grit blasted with alumina (Al2O3) particles prior to air plasma deposition of CoNiCrAlY bond coat. Top coat consists of conventional zirconia (ZrO2) stabilized by yttria (Y2O3) -YSZ ceramic in combination with a eutectic nanocrystalline ceramic Eucor made of zirconia (ZrO2), alumina (Al2O3) and silicia (SiO2) –in the ratio of 50/50 in wt. %. The top coat was deposited using water stabilized plasma. Test specimens with the TBC coating system were fatigued under strain control condition in fully reversed symmetrical push-pull cycles at 900°C in air. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating hardness and thickness were measured. Fracture surface and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. TBC delamination was observed at the top coat/bond coat interface after cyclic loading at high temperature. Fatigue crack initiation sites are documented. Majority of fatigue cracks start from the surface and top coat/bond coat interface.

Damage behavior of Cu–Ta bilayered films under cyclic loading

2007 ◽  
Vol 22 (9) ◽  
pp. 2478-2482 ◽  
Author(s):  
X.F. Zhu ◽  
G.P. Zhang ◽  
J. Tan ◽  
Y. Liu ◽  
S.J. Zhu
Keyword(s):  
Cyclic Loading ◽  
Fatigue Cracks ◽  
Damage Zone ◽  
Crack Tip ◽  
Damage Mechanism ◽  
Grain Rotation ◽  
Damage Behavior ◽  
Loading Tests ◽  
Cyclic Plastic Strain ◽  
Cyclic Loading Tests

Damage behavior of Cu–Ta bilayered films bonded to polyimide (PI) substrates has been investigated by cyclic loading tests. Experimental results show that fatigue cracks preferentially initiated in the Ta layer close to the Ta–PI interface and propagated into the Cu layer perpendicular to the interface. The alignment of nanometer-sized Cu grains resulted from the potential GB sliding combined with a small amount of grain rotation was found in the damage zone ahead of the crack tip, and that is suggested to be a likely damage mechanism to accommodate cyclic plastic strain ahead of the fatigue crack tip of the submicrometer-thick Cu layer.

Sign in / Sign up

Export Citation Format

Share Document