Role of Constraint in Specimen Geometries When Evaluating Fracture Toughness/Material Fracture Resistance for a Surface-Flawed Elbow

2019 ◽  
Author(s):  
S. Kalyanam ◽  
G. Wilkowski ◽  
F. W. Brust ◽  
Y. Hioe ◽  
E. Punch
Keyword(s):  
Surface Crack ◽  
State Of The Art ◽  
Crack Depth ◽  
Bending Moment ◽  
Compact Tension ◽  
Fe Analysis ◽  
Significant Finding ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Variable Surface

Abstract The fracture behavior of a circumferential surface crack in an elbow was evaluated using past data from the International Piping Integrity Research Group (IPIRG-2) Experiment 2-4. The elbow tested was nominal 16-inch diameter Schedule 100 TP304 material, which was solution-annealed after final fabrication. The elbow was loaded with an in-plane-closing bending moment and internal pressure of 15.51 MPa (2,250 psig) at 288 C (550 F). The surface crack was 180-degrees on the ID surface and centered on the extrados, but after fatigue precracking the depth was variable and the greatest was at about 45-degrees from the extrados. FE analysis of the IPIRG-2 elbow test was conducted with a state-of-the-art and precise 3D FE mesh (including variable surface crack depth, variable thickness, and initial elbow ovalization). The flaw depth for the single-edge notch tension (SENT) tests was selected to be equivalent to the deepest point in the elbow specimen crack front that provided the largest J-value in the elbow experiment, i.e., ao/W = 0.68. Comparison of the J-value for initiation (Ji) and crack-tip-opening displacement (CTODi) at crack initiation suggested that there was a slight difference in constraint between an identical depth SENT specimen (a/W = 0.68 with the same L-R orientation as the surface crack in the pipe) and an elbow with a circumferential surface crack (a/t = 0.68) [Ji was 0.368 MN/m, (2.1 ksi-inch) in the SENT tests, while it was 0.490 MN-m (2.8 ksi-inch) in the elbow test]. The more significant finding in this work was that the compact tension (C(T)) test Ji-value was much higher at 1.086 MN/m (6.2 ksi-inch) or ∼3 times higher. The elbow to SENT to C(T) specimen comparison illustrates very large differences in constraint between these geometries. From past work by several researchers it was determined that the constraint in C(T) specimens gives Ji-values that agree well with a circumferential through-wall crack in a straight pipe, but this difference with surface-cracked elbow or pipe is envisaged to be new information to the international research community. Additionally, from state-of-the-art FE analysis of the 180-degree surface-cracked elbow test it was found that the maximum J-value occurs at a position that was about 45-degree away from the extrados location. This trend showed that caution should be exercised when selecting the crack locations for elbow integrity evaluation, since for shorter flaw lengths it may be more critical to consider a crack that is closer to the 45-degrees from the extrados, which could be true for fracture as well as stress corrosion cracking (SCC) elbow evaluations.

Study of Constraint Issues in Elasto-Plastic Fracture Analysis Using Experimental and Finite Element Simulation

2017 ◽  
Author(s):  
Md Ibrahim Kittur ◽  
Krishnaraja G. Kodancha ◽  
C. R. Rajashekar
Keyword(s):  
Finite Element ◽  
Mild Steel ◽  
Compact Tension ◽  
Specimen Thickness ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
3D Fea ◽  
The Comparative Study ◽  
Crack Tip Opening

In this investigation, the variation of J-integral considering Compact Tension (CT) specimen geometry varying a/W and σ using 2D and 3D elasto-plastic Finite Element (FE) analysis have been studied. Further, the investigation has been done to examine the relationship between the J and δ for varied a/W and σ. The plane stress and plane strain elasto-plastic FE analyses have been conducted on the CT specimen with a/W = 0.45–0.65 to extract the J and Crack-tip Opening Displacement (CTOD) values for mild steel. The comparative study of the variation of dn with a/W of mild steel with earlier results of IF steel is carried out. The study clearly infers the effect of yield stress on the variation of the magnitude of dn with reference to a/W ratio. The present analysis infers that while converting the magnitude of the CTOD to J one needs to carefully evaluate the value of dn depending on the material rather than considering it to be unity. Further, the study was extended to experimental and 3D FEA wherein J-integral and CTOD were estimated using the CT specimen. Experimental results reveal that the crack length, the specimen thickness, and the loading configuration have an effect on the fracture toughness measurements. The error analysis between the results obtained by 3D FEA and experimentation were conducted and found to be within limits.

In-Situ Estimation of Surface Crack Shape From Crack Opening Displacement by Use of NCOD Database System

2010 ◽  
Author(s):  
Jingxia Yue ◽  
Zheng He ◽  
Yukio Fujimoto ◽  
Weiguo Wu
Keyword(s):  
Crack Opening Displacement ◽  
Surface Crack ◽  
Fatigue Cracks ◽  
Crack Opening ◽  
Crack Depth ◽  
Database System ◽  
Shape Estimation ◽  
Opening Displacement ◽  
Crack Shape

This paper proposes an in-situ estimation of crack shape from crack opening displacement (COD) by using of a visualized database system consisting of numerical calculation data of normalized crack opening displacement (NCOD) for some kinds of crack types. The relation between crack depth and corresponding NCOD is discussed based on FE analysis results, from which a crack shape estimation principle is deduced. Visualized software named NCOD Database System was developed to facilitate convenient in-situ estimation of crack shape. Shapes of three kinds of surface crack, partial circle crack in plate, fatigue cracks in gusset weld joint and in large-scale member, are successfully estimated by this system. The paper is supported by the Programme of Introducing Talents of Discipline to Universities (B08031).

Strain-Based CTOD and J-Integral Estimations for Pipelines With a Surface Crack Under Large Plastic Strain and Internal Pressure

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
Youn-Young Jang ◽  
Ju-Yeon Kang ◽  
Nam-Su Huh ◽  
Ik-Joong Kim ◽  
Young-Pyo Kim
Keyword(s):  
Internal Pressure ◽  
Surface Crack ◽  
Material Properties ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Large Plastic Strain ◽  
Opening Displacement ◽  
Definition Of ◽  
Crack Tip Opening ◽  
Tensile Strain Capacity

Abstract Engineering solutions for crack-tip opening displacement (CTOD) and J-integral estimations for pipelines with a surface crack are proposed based on parametric finite element (FE) analyses for various geometries, material properties, and internal pressure conditions. Two kinds of CTOD definitions are considered in relation to strain-based estimation solutions for dealing with confusion regarding the definition of CTOD and to extend the applicability of tensile strain capacity (TSC) assessment. Moreover, influence functions of internal pressure are also suggested to take account of the effect of internal pressure on TSC. Using the proposed solutions, TSCs for cracked X65 and X70 pipes were assessed based on initiation and ductile instability. Curved wide plate tests were performed to obtain experimental TSCs, which were compared with those from the proposed solutions. Moreover, TSCs from the proposed solutions were also compared with those from other TSC-predicted models in order to assess their validity.

Reliability Calculation of Welded Pressure Pipe with Circumferential Crack Based on 3-D Elastic-Plastic SFEM

2008 ◽  
Vol 575-578 ◽  
pp. 639-642
Author(s):  
Bo Lin He ◽  
Ying Xia Yu ◽  
Li Xing Huo ◽  
Yu Feng Zhang
Keyword(s):  
Surface Crack ◽  
Safety Assessment ◽  
Crack Depth ◽  
Three Dimensional ◽  
Bending Moment ◽  
Mean Value ◽  
Pressure Pipe ◽  
Circumferential Crack ◽  
The Mean ◽  
Welded Pipe

In this paper, the reliability of welded pressure pipe with circumferential surface crack was calculated by using three dimensional stochastic finite element method. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on three dimensional elastic-plastic stochastic finite element program which was developed by ourselves. The effects of variables such as fracture toughness, bending moment and the depth of the circumferential surface crack on the structure reliability were also discussed. The calculation results indicate that the crack depth has great effect on the reliability of the welded pipe. When the mean value of the crack depth is changed from 3mm to 7mm, the failure probability of the welded pipe will change from 10-8 to 10-2. The bending moment also has great effect on the reliability of the welded pipe. When the mean value of moment is changed from10000 N.m to 15000 N.m, the failure probability of the welded pipe increases dramatically for the same circumferential crack depth. Irrespective of the changing of moment, the pipe has higher reliability if the crack depth is less than 5mm(a/t<0.5, t is the thickness of the pipe). The method has put forward a new way for safety assessment of welded pipe with circumferential surface crack.

Nonidealized Surface to Through-Wall Crack Transition Model for Axial Cracks in Cylinders

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Do-Jun Shim ◽  
Jeong-Soon Park ◽  
David Rudland
Keyword(s):  
Crack Front ◽  
Surface Crack ◽  
Crack Opening ◽  
Crack Depth ◽  
Outer Diameter ◽  
Transition Model ◽  
Opening Displacement ◽  
Proposed Model ◽  
Primary Water ◽  
Crack Transition

Recent studies have shown that a subcritical surface crack, due to primary water stress corrosion cracking (PWSCC), can transition to a through-wall crack (TWC) with significant differences between the inner diameter (ID) and outer diameter (OD) crack lengths. This behavior has been observed for both circumferential and axial cracks. Recently, a surface to TWC transition model has been developed for circumferential cracks using existing K and COD (crack opening displacement) solutions for nonidealized circumferential TWCs. In this paper, a similar crack transition model (CTM) was developed for axial cracks. As a first step, a study was conducted to define the appropriate crack front shape for nonidealized axial TWCs. Then, elastic finite element analyses were carried out to develop K and COD solutions using these crack front shapes. The newly developed solutions were utilized for the CTM. The present CTM includes a criterion for transitioning the final surface crack to the initial nonidealized TWC. This criterion determines when the transition should occur (based on surface crack depth) and determines the two crack lengths (at ID and OD surfaces) of the initial nonidealized TWC. Furthermore, nonidealized TWC growth calculation can be conducted using the proposed model. Example results (crack length and COD) obtained from the proposed model were compared to those obtained from the natural crack growth simulations. Results presented in this paper demonstrated the applicability of the proposed model for simulating axial crack transition.

Novel Specimen Design for Measurement of In-Plane Fracture Toughness of Metals

2019 ◽  
Vol 142 (2) ◽  
Author(s):  
Abderrazak Traidia ◽  
Elias Chatzidouros ◽  
Mustapha Jouiad ◽  
Kaamil-Ur-Rahman Shibly
Keyword(s):  
Fracture Toughness ◽  
Pipeline Steel ◽  
Compact Tension ◽  
Experimental Results ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Steel Grades ◽  
Plane Direction ◽  
Crack Tip Opening

Abstract Standard-compliant measurement of the in-plane fracture toughness of metals is often challenging due to insufficient material in the through-thickness direction to extract a full single edge bending (SEB) or compact tension (CT) fracture specimen. In the present work, we propose a new specimen design methodology to overcome this challenge. A W-shaped SEB specimen (called W-SEB) was developed, and its topology was optimized using finite element simulations. The new specimen design was validated numerically and experimentally on a case study showing excellent agreement with standard ASTM E1820 actual SEB specimen geometry. In view assessing the anisotropy of the fracture toughness (KQ and crack tip opening displacement (CTOD)) of pipeline steels susceptible to hydrogen-induced cracking (HIC), the W-SEB specimen was tested on X65 and X42 pipeline steel samples taken from the field. Experimental results show an increase in the maximum CTOD along the in-plane direction as compared to the transverse direction for both steel grades. Such experimental results could lead to important considerations with respect to accurate fitness for service assessment of HIC-damaged assets.

scholarly journals The Influence of the Grain Structure Size on Microstructurally Short Cracks

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Igor Simonovski ◽  
Leon Cizelj
Keyword(s):  
Surface Crack ◽  
Voronoi Tessellation ◽  
Constitutive Models ◽  
Crack Tip ◽  
Anisotropic Elasticity ◽  
Grain Structure ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Short Cracks ◽  
Crack Tip Opening

The dominant processes in the initialization and propagation of microstructurally short cracks include microstructural features such as crystallographic orientations of grains, grain boundaries, inclusions, voids, material phases, etc. The influence of the microstructural features is expected to vanish with distance from the crack tip. Also, the influence of the nearby microstructural features is expected to be smaller for a long than for a small crack. Finally, a crack of sufficient length can be modeled using classical fracture mechanic methods. In this paper the approach to estimate the crack length with vanishing influence from the microstructural feature is proposed. To achieve this, a model containing a large number of randomly sized, shaped, and oriented grains is employed. The random grain structure is modeled using a Voronoi tessellation. A series of cracks of lengths from about 1 to 7 grain lengths is inserted into the model, extending from a grain at the surface toward the interior of the model. The crack tip opening displacements are estimated and statistically analyzed for a series of random crystallographic orientation sets assigned to the grains adjacent to the crack. Anisotropic elasticity and crystal plasticity constitutive models are employed at the grain size scale. It is shown that the standard deviation of the crack tip opening displacement decreases from about 20% for a short surface crack embedded within a single grain to about 7% for a surface crack extending through seven grains. From the engineering point of view, a crack extending through less than about ten grain sizes is therefore considered to strongly depend on the neighboring microstructural features.

Measuring Thin Film Fracture Toughness Using the Indentation Sinking-in Effect and Focused Ion Beam

1999 ◽  
Vol 594 ◽  
Author(s):  
Ting Y. Tsui ◽  
Young-Chang Joo
Keyword(s):  
Thin Film ◽  
Fracture Toughness ◽  
Focused Ion Beam ◽  
Ion Beam ◽  
Crack Tip ◽  
Bending Moment ◽  
Hard Metal ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Extension Force

AbstractA new experimental technique is developed to measure the fracture toughness of a hard metal thin film deposited on a soft substrate. A pre-crack was fabricated in the thin film by using the advanced focused ion beam (FIB) milling techniques. The crack extension force was generated by means of the indentation sinking-in effect. The effect creates a bending moment and tensile stress on the hard thin film near the indentation, which promotes crack growth. The amount of crack tip blunting prior to the critical failure was measured from the FIB cross-sectioned micrographs. By using the crack tip opening displacement model (CTOD), the fracture toughness of the thin film was calculated. The results show the nickel phosphorus (NiP) thin film fracture toughness is at least 15.0MPa√m. The finite element method (FEM) was used to understand the modes of mixity near the crack tip. The results indicate the crack tip modes of mixity are dominated by the Mode I opening, provided the indentation is sufficiently far from the pre-crack or the indentation depths is small when compared with the film thickness.

scholarly journals Application of the Line-Spring Model to a Cylindrical Shell Containing a Circumferential or Axial Part-Through Crack

1982 ◽  
Vol 49 (1) ◽  
pp. 97-102 ◽  
Author(s):  
F. Delale ◽  
F. Erdogan
Keyword(s):  
Cylindrical Shell ◽  
Stress Intensity ◽  
Surface Crack ◽  
Crack Depth ◽  
Crack Problem ◽  
Bending Moment ◽  
Circular Ring ◽  
Spring Model ◽  
Axial Part ◽  
Line Spring Model

In this paper the line-spring model developed by Rice and Levy is used to obtain an approximate solution for a cylindrical shell containing a part-through surface crack. It is assumed that the shell contains a circumferential or axial semi-elliptic internal or external surface crack and is subjected to a uniform membrane loading or a uniform bending moment away from the crack region. To formulate the shell problem, a Reissner type theory is used to account for the effects of the transverse shear deformations. The stress intensity factor at the deepest penetration point of the crack is tabulated for bending and membrane loading by varying three-dimensionless length parameters of the problem formed from the shell radius, the shell thickness, the crack length, and the crack depth. The upper bounds of the stress intensity factors are provided by the results of the elasticity solution obtained from the axisymmetric crack problem for the circumferential crack, and that were found from the plane strain problem for a circular ring having a radial crack for the axial crack. Qualitatively the line-spring model gives the expected results in comparison with the elasticity solutions. The results also compare well with the existing finite element solution of the pressurized cylinder containing an internal semi-elliptic surface crack.

Three-Dimensional Finite Element Analyses of Thin-Sliced Compact Tension Specimens of Irradiated Zr-2.5Nb Materials With Consideration of Split Circumferential Hydrides

2016 ◽  
Author(s):  
Shin-Jang Sung ◽  
Jwo Pan ◽  
Poh-Sang Lam ◽  
Douglas A. Scarth
Keyword(s):  
Finite Element ◽  
Failure Criterion ◽  
Three Dimensional ◽  
Compact Tension ◽  
Finite Element Analyses ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Unit Thickness ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element

In this paper, the low energy mode associated with split circumferential hydrides is examined by conducting three-dimensional finite element analyses of thin-sliced compact tension (CT) specimens of irradiated Zr-2.5Nb materials with split circumferential hydrides. Finite element models of thin-sliced CT specimens with split circumferential hydrides and various slice thicknesses are developed with the assumption of the plane strain condition in the thickness direction except in the split circumferential hydride regions. The computational results indicate that with split circumferential hydrides, the crack tip opening displacement (CTOD) can increase 50% for thinner thin-sliced specimens under the same load per unit thickness. With the use of a strain-based failure criterion with split circumferential hydrides, the load per unit thickness for thinner thin-sliced specimens can reduce by at most 70% to meet the failure criterion.

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