Crack Initiation and Propagation in Static Loaded Fracture Mechanics Tests in Steels Containing Atomic Hydrogen

2020 ◽  
Author(s):  
Philippa L. Moore ◽  
Menno Hoekstra ◽  
Alex Pargeter
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Plastic Zone ◽  
Crack Growth ◽  
Crack Extension ◽  
Low Alloy Steels ◽  
Ductile Tearing ◽  
Initiation Fracture Toughness ◽  
Stable Tearing

Abstract Hydrogen is well known to have a detrimental influence on the ductility of low alloy steels, reducing the fracture toughness. Standard test methods to characterize fracture toughness of steels in terms of ductile tearing resistance curves have not been developed to account for any hydrogen-driven contribution to the crack extension, Δa. Simply plotting J or CTOD against Δa is not necessarily appropriate for defining the initiation fracture toughness for tests performed in a hydrogen-charging environment. This paper explores a method to further analyse experimental data collected during fracture toughness tests, which allows the contribution of plasticity (i.e. when blunting precedes ductile tearing) to be considered separately from the initiation of crack extension (which could be by stable tearing and/or by hydrogen-driven crack extension). The principle is based on the assumption that a crack growing by a hydrogen-driven mechanism in a quasi-static fracture mechanics test performed in environment may not be associated with significant ductility in the plastic zone (which would accompany crack growth by stable tearing). The analytical method presented in this paper compares the different points of deviation from linear behavior of the components of J, to isolate the effects of ductility within the plastic zone from pure crack extension. In this way, the point of crack initiation can be defined in order to determine the relevant initiation fracture toughness; whether by blunting and stable tearing, or by hydrogen-driven crack growth. This approach offers a screening method which is illustrated using examples of fracture mechanics specimens tested in environments of varying severity (air, seawater with cathodic protection, and sour service). This method can be used to identify the relevant definition of initiation fracture toughness while allowing for a combination of ductile tearing, hydrogen-driven crack extension, or both, to be present during the test.

Determination of Crack-Initiation in Fracture Toughness Testing Using an Experimental Key-Curve Methodology

2021 ◽  
Author(s):  
S. Pothana ◽  
G. Wilkowski ◽  
S. Kalyanam ◽  
J. K. Hong ◽  
C. J. Sallaberry
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Power Law ◽  
Direct Current ◽  
Electric Potential ◽  
Crack Mouth Opening Displacement ◽  
Ductile Tearing ◽  
Curve Method ◽  
Unloading Compliance

Abstract A new approach was implemented to confirm the start of ductile tearing relative to assessments by other methods such as direct-current Electric Potential (d-c EP) method in coupon specimens. This approach was developed on the Key-Curve methodology by Ernst/Joyce and is similar to the ASTM E-1820 Load Normalization procedure used to determine J-R curves directly from load versus Load-Line Displacement (LLD) record of the test specimen. It is consistent with Deformation Plasticity relationships for fully plastic behavior. Using this Experimental Key-Curve method, crack initiation can be determined directly from load versus LLD data or load versus Crack-Mouth Opening Displacement (CMOD) obtained from a fracture test without the need for additional instrumentation required for crack initiation detection. It is based on the fact that plastic deformation of homogeneous metals at the crack tip follows a power-law function until the crack tearing initiates. Crack tearing initiation is determined at the point where the power-law fit to the load versus plastic part of CMOD or LLD curve deviates from the total experimental load versus plastic-CMOD or LLD curve. The procedure for fitting of the data requires some care to be exercised such that the fitted data is beyond the elastic region and early small-scale plastic region of the Load-CMOD or Load-LLD curve but include data before crack initiation. An iterative regression analysis was done to achieve this, which is shown in this paper. The iterative fitting in this region typically results with a coefficient of determination (R2) values that are greater than 0.990. This method can be either used in conjunction with other methods such as direct-current Electric Potential (d-c EP) or unloading-compliance methods as a secondary (or primary) confirmation of crack tearing initiation (and even for crack growth); or can be used alone when other methods cannot be used. Furthermore, when using instrumentation methods for determining crack-initiation such as d-c EP method in a fracture toughness test, it is good to have a secondary confirmation of the initiation point in case of instrumentation malfunction or high signal to noise ratio in the measured d-c EP signals. In addition, the Experimental Key-Curve procedure provides relatively smooth data for the fitting procedure, while unloading-compliance data when used to get small crack growth values frequently has significant variability, which is part of the reason that JIC by ASTM E1820 is determined using an offset with some growth past the very start of ductile tearing. In this work, the Experimental Key-Curve method had been successfully used to determine crack tearing initiation and demonstrate the applicability for different fracture toughness specimen geometries such as SEN(T), and C(T) specimens. In all the cases analyzed, the Experimental Key-Curve method gave consistent results that were in good agreement with other crack tearing initiation measuring method such as d-c EP but seemed to result in less scatter.

Evolution of Plasticity in Relation to Ductile Tearing in 304(L) Stainless Steel

2010 ◽  
Author(s):  
Andrew P. Wasylyk ◽  
Andrew H. Sherry
Keyword(s):  
Fracture Toughness ◽  
Plastic Zone ◽  
Crack Growth ◽  
Structural Integrity ◽  
Plastic Zone Size ◽  
Compact Tension ◽  
Image Correlation ◽  
Ductile Tearing ◽  
Integrity Assessment ◽  
Unloading Compliance

In the structural integrity assessment of structures containing defects, ductile tearing and plastic collapse are treated as competing failure mechanisms. The validity of fracture toughness measurements in test specimens is limited by the development of plasticity ahead of the crack tip. Compact Tension (CT) specimens are commonly used to characterise the ductile fracture toughness. Two sizes of CT specimens (thickness 25 and 15mm) were tested using the unloading compliance technique and the J-Resistance curve characterised. Concurrently, the development of the plastic zone was monitored on the surface of specimens using digital image correlation. This enabled the plastic zone size to be correlated with the evolution of crack growth. It was found that in both specimens no crack growth had occurred prior to plastic yielding of the un-cracked ligament on the specimen surface.

Fracture and Fatigue Testing of Micro-Sized Materials for MEMS Applications

2005 ◽  
Author(s):  
Kazuki Takashima ◽  
Timothy P. Halford ◽  
Yakichi Higo
Keyword(s):  
Fracture Toughness ◽  
Fatigue Crack ◽  
Fatigue Life ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Amorphous Alloy ◽  
Crack Initiation ◽  
Crack Growth ◽  
Testing Machine ◽  
Fatigue And Fracture

We have developed a new type of mechanical testing machine for micro-sized specimens, which can apply a small static or cyclic load, and have investigated fracture and fatigue crack growth behavior of micro-sized specimens. Cantilever beam type specimens (10 μm × 10 μm × 50 μm), with notches were prepared from thin films of a Ni-P amorphous alloy by focused ion beam machining. Fatigue and fracture toughness tests were carried out in air at room temperature using the mechanical testing machine. Fatigue and fracture testing was completed successfully for micro-sized cantilever specimens. Once fatigue crack growth occurs, rapid sample failure was observed in these micro-sized specimens. This indicates that the fatigue life of micro-sized specimens is mainly dominated by crack initiation. This also suggests that even a micro-sized surface flaw can be a fatigue crack initiation site which will shorten the fatigue life of micro-sized specimens. As a result of fracture toughness tests, plane strain criteria for small scale yielding were not achieved for this amorphous alloy. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This suggests that fracture mechanics is still valid for such micro-sized specimens.

Assessment of Power Plant Components With Flaws and Defects Operating in the Long-Term Creep Range

2019 ◽  
Author(s):  
Magdalena Speicher ◽  
Thorben Bender ◽  
Andreas Klenk ◽  
Falk Mueller ◽  
Christian Kontermann ◽  
...  
Keyword(s):  
High Temperature ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Material Behavior ◽  
Critical Crack Length ◽  
Creep Crack ◽  
Combining Data ◽  
Crack Problems

Abstract Originating from defects and flaws in high temperature components crack initiation and crack propagation under service conditions can occur. Fracture mechanics data and procedures are needed to study crack problems and to support an advanced remnant life evaluation. During subsequent research in the past 35 years, data were determined for different high temperature materials. Methodologies and concepts taking into account the specific material behavior were developed in order to be able to describe crack initiation and crack growth and have appropriate assessment methods available. For creep crack initiation two criteria principles were used and for creep crack growth assessment based on the integral C* parameter were applied. Furthermore, a method for determination of critical crack length was developed allowing decisions whether modified stress analysis methods are sufficient or more complicated fracture mechanics methods are needed. To provide data and methodologies in a user-friendly way, a program system combining data and methods was implemented. The paper describes developed features and shows comparisons to other methods. The methods can be applied for design purposes as well as remnant life assessments.

Estimation of Through-Wall Cracking Frequency of RPV Under PTS Events Using PFM Analysis Method for Identifying Conservatism Included in Current Japanese Code

2014 ◽  
Author(s):  
Kazuya Osakabe ◽  
Koichi Masaki ◽  
Jinya Katsuyama ◽  
Genshichiro Katsumata ◽  
Kunio Onizawa
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Structural Integrity ◽  
Probabilistic Approach ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Analysis Method ◽  
Integrity Assessment ◽  
The U.S

To assess the structural integrity of reactor pressure vessels (RPVs) during pressurized thermal shock (PTS) events, the deterministic fracture mechanics approach prescribed in Japanese code JEAC 4206-2007 [1] has been used in Japan. The structural integrity is judged to be maintained if the stress intensity factor (SIF) at the crack tip during PTS events is smaller than fracture toughness KIc. On the other hand, the application of a probabilistic fracture mechanics (PFM) analysis method for the structural reliability assessment of pressure components has become attractive recently because uncertainties related to influence parameters can be incorporated rationally. A probabilistic approach has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. According to the PFM analysis method in the U.S., through-wall cracking frequencies (TWCFs) are estimated taking frequencies of event occurrence and crack arrest after crack initiation into consideration. In this study, in order to identify the conservatism in the current RPV integrity assessment procedure in the code, probabilistic analyses on TWCF have been performed for certain model of RPVs. The result shows that the current assumption in JEAC 4206-2007, that a semi-elliptic axial crack is postulated on the inside surface of RPV wall, is conservative as compared with realistic conditions. Effects of variation of PTS transients on crack initiation frequency and TWCF have been also discussed.

Simulation of Ductile Tearing in a Dissimilar Material Weld up to Pipe Wall Break-Through

2010 ◽  
Author(s):  
Philippe Gilles ◽  
Alexandre Brosse ◽  
Moi¨se Pignol
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Wall Thickness ◽  
Surface Crack ◽  
Pipe Wall ◽  
Ductile Crack ◽  
Pipe Wall Thickness ◽  
Ductile Tearing ◽  
Ductile Crack Growth ◽  
Computational Analyses

This paper presents ductile initiation calculations and growth simulations of a surface crack up to pipe wall breakthrough. For validation purpose, one of the two BIMET configurations is selected. The EC program BIMET has been carried out to analyze the ductile tearing behavior of DMWs through experiments and computational analyses. In the mock-up, the initial defect is an external circumferential defect located close to the weld-ferritic interface, with a depth of one third of the wall thickness. During the test, the crack extended up to two third of the pipe wall thickness. The aim of the study is to simulate the crack initiation and growth, to compare the results with the experimental records and to continue the ductile crack growth up to pipe wall break-through.

Crack Tip Shielding from a Plastic ‘Inclusion’

2011 ◽  
Vol 465 ◽  
pp. 1-8
Author(s):  
M. Neil James ◽  
C.J. Christopher ◽  
Yan Wei Lu ◽  
K.F. Tee ◽  
Eann A Patterson
Keyword(s):  
Plastic Deformation ◽  
Fracture Mechanics ◽  
Plastic Zone ◽  
Crack Growth ◽  
Displacement Field ◽  
Driving Force ◽  
Applied Load ◽  
Elastic Stress ◽  
Ductile Material ◽  
Crack Tip Shielding

This paper presents a very brief overview of the philosophy underlying a plastic inclusion approach to defining the boundary stresses imposed on the applied elastic stress or displacement field by the plastic deformation attendant on crack growth in a ductile material. It leads to two new fracture mechanics parameters, KR and KS. KR defines a retardation component arising from wake contact and the Poisson’s contraction associated with the plastic zone, whilst KS describes a compatibility-induced component arising from shear at the elastic-plastic interface. These additional components imply that KF is not directly comparable with KI, as it describes the net driving force on the crack from the applied load.

Fatigue Analysis of Clad and Unclad Thick-Wall Structures

1982 ◽  
Vol 22 (01) ◽  
pp. 151-156
Author(s):  
Theodore Gottlieb ◽  
Tarlochan Mann
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
High Stress ◽  
Cyclic Strain ◽  
Inconel 625 ◽  
Thick Wall ◽  
Low Alloy Steels ◽  
Aisi 316L ◽  
Clad Steel

Abstract It is common practice to clad steel components with a relatively thin layer of a stainless material to prevent corrosion economically. Little, however, has been published regarding the effect of such cladding on fatigue published regarding the effect of such cladding on fatigue life in areas of localized high stress. Large valves that are pressure-cycled often and offshore equipment, such as pressure-cycled often and offshore equipment, such as risers, tensioners, and wellhead flanges that are loaded cyclically by ocean currents and waves, must be analyzed for fatigue life during design. Unlike storage vessels, drilling and completion hardware generally has areas of relatively high stress concentrations because of abrupt section size changes, threads, grooves for seals, bolt holes, and other stress-concentrating geometries. While yielding or rupturing is a function of bulk stresses, fatigue life is a function of peak stresses, which typically are highest on the surface of an area of stress concentration. It has been determined that both the metallurgical characteristics of the cladding and the pressure/load history can be varied to enhance or diminish significantly the fatigue life of a clad steel component. The results and conclusions of this study are based on laboratory studies. Axial fatigue tests (R=0.05) were performed using a side-notched fatigue specimen that produces combined axial and bending stresses in the notched area. Specimens of AISI 4130 (dt HRc 20) were tested unclad and with the notched area clad with Inconel 625 or AISI 316L. Each set of specimens was tested both unpreloaded and preloaded to produce localized yielding at the notched surface only.The findings of this study are applicable to components subject to failure by fatigue and corrosion fatigue and sour service steel components that become locally work-hardened either in service or during overload proof testing as required by most API specifications. Introduction Fatigue failure of a homogeneous, unflawed metal occurs in two stages:nucleation of a stable crack andcrack growth until failure occurs. The nucleation portion is the result of alternating strain of a magnitude portion is the result of alternating strain of a magnitude sufficient to cause the formation and the coalescence of dislocations to form a crack. Crack growth can be predicted by fracture mechanics techniques. predicted by fracture mechanics techniques. Although fatigue curves often are plotted with alternating stress on the abscissa and cycles to failure on the ordinate, it is actually the cyclic strain that determines fatigue life. Fatigue prediction methods therefore must relate calculated stresses to cyclic strain. Stress vs. strain relationships are complex and include at least the following variables: part geometry, grain size, microconstituents, cold working coefficient, direction of forces, magnitude of forces, and strength and modulus of the material. It is seen that fatigue is associated strongly with the metallurgy of the materials being tested. Purpose of Study Purpose of Study The purpose of this study was to develop data and evaluate an analytical technique to predict fatigue life of thick-wall, clad and unclad, pressure vessels in the long- and short-cycle fatigue mode. More specifically, data were generated to simulate high-pressure wellhead equipment fabricated from quenched and tempered low alloy steels. Claddings studied were the austenitic-nickel-base Inconel 625 and iron-base AISI 316L. Both these cladding materials have substantially different metallurgical properties from those of low alloy steel. Since fatigue failures generally result from peak surface stresses, nucleation of fatigue cracks will occur in the cladding. The cladding therefore controls the fatigue life of the vessel since crack nucleation comprises the majority of the total cycles compared to crack growth. SPEJ P. 151

Embedded Crack Treatments and Fracture Toughness Evaluation Methods in Probabilistic Fracture Mechanics Analysis Code for the PTS Analysis of RPV

2004 ◽  
Author(s):  
Kunio Onizawa ◽  
Katsuyuki Shibata ◽  
Masahide Suzuki ◽  
Daisuke Kato ◽  
Yinsheng Li
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Treatment Method ◽  
Evaluation Methods ◽  
Probabilistic Fracture Mechanics ◽  
Mechanics Analysis ◽  
Toughness Evaluation ◽  
Embedded Crack ◽  
Fracture Toughness Evaluation

Using the probabilistic fracture mechanics analysis code PASCAL, we studied the treatment method of an embedded crack and the fracture toughness evaluation methods on the probability of crack initiation and fracture of a reactor pressure vessel (RPV). For calculating the stress intensity factor (SIF) of an embedded crack, the ASME and CRIEPI procedures were introduced into the PASCAL code. The CRIEPI method enables us to calculate the SIF values at three points on the crack tip. Under a severe pressurized thermal shock (PTS) condition, the crack growth analysis methods with different SIF calculation points and crack growth directions are compared. To evaluate precisely the fracture toughness after neutron irradiation, the new fracture toughness curves based on the Weibull distribution were incorporated into the PASCAL code. The calculated results with these new curves showed little difference in the conditional probabilities of RPV fracture as compared to the curve currently used in the U.S.

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