CTOA Measurement of Pipe Steels Using DWTT Specimen

2008 ◽  
Author(s):  
S. Xu ◽  
W. R. Tyson
Keyword(s):  
Crack Tip ◽  
High Strength Steels ◽  
High Strength ◽  
Measurement Methods ◽  
Surface Measurement ◽  
Opening Angle ◽  
Single Specimen ◽  
Pipe Steels ◽  
Mill Test ◽  
Using Data

A major issue of applying the crack-tip opening angle (CTOA) approach to fast ductile fracture control is to characterize material toughness (CTOAc) using laboratory-scale specimens and to recommend a mill test. In this report, existing direct and indirect CTOA measurement methods are comprehendsively reviewed. Direct surface measurement methods as standardized in ASTM E2472-06 are mainly based on experience with sheet materials, and the measurement distance from the crack tip recommended in the ASTM standard should be increased for thicknesses typical of pipe steels. Methods using data on load (P) vs. load-line displacement (LLD) from instrumented drop-weight tear test (DWTT) specimens are given special attention, including original and modified two-specimen methods, original and modified single-specimen methods (SSM) and a recent simplified single-specimen method (S-SSM). The results of DWTT/CTOA tests at CANMET-MTL on six pipe steels (five modern high-strength steels and one older X52 steel) are briefly presented. The simplified single-specimen method (S-SSM) is repeatable and may be suitable for a mill test. However, there remains a discrepancy in CTOA values between surface measurements and those from the S-SSM, and it was found that the CTOA decreased somewhat with increasing ligament size from 66 mm to 95 mm. Transferability of results from lab to field will be discussed briefly.

Measurement of CTOA of Pipe Steels Using MDCB and DWTT Specimens

2010 ◽  
Author(s):  
S. Xu ◽  
W. R. Tyson ◽  
R. Eagleson ◽  
C. N. McCowan ◽  
E. S. Drexler ◽  
...  
Keyword(s):  
Crack Tip ◽  
High Strength Steels ◽  
High Strength ◽  
Surface Measurement ◽  
Opening Angle ◽  
Electric Discharge Machining ◽  
Loading Rates ◽  
Crack Tip Opening Angle ◽  
Drop Weight Tear Test ◽  
Crack Tip Opening

Two types of specimen for crack tip opening angle (CTOA) measurement have been investigated for pipeline applications, i.e., the modified double cantilever beam (MDCB) (at NIST) and the drop-weight tear test (DWTT) specimen (at CANMET). Results of effects of specimen types, thicknesses and loading rates on CTOA are summarized and discussed. The main observations include: (i) For both MDCB and DWTT specimens tested at quasi-static loading rate, crack front tunnelling (i.e., with a deep triangular crack-tip shape) was present in high-strength steels; (ii) For DWTT specimens, CTOA values measured optically at the surface were significantly higher than those from the simplified single-specimen method (S-SSM) and those measured at mid-thickness [on sections cut using electric discharge machining (EDM)]; and (iii) CTOA values from surface measurement of MDCB specimens were comparable to those derived from S-SSM of DWTT specimens, but the surface values of DWTT were higher than those of MDCB specimens.

A Single Specimen CTOA Test Method for Evaluating the Crack Tip Opening Angle in Gas Pipeline Steels

2004 ◽  
Author(s):  
Sayyed H. Hashemi ◽  
Ian C. Howard ◽  
John R. Yates ◽  
Robert M. Andrews ◽  
Alan M. Edwards
Keyword(s):  
Steady State ◽  
Crack Growth ◽  
Crack Tip ◽  
Gas Pipeline ◽  
Test Method ◽  
Opening Angle ◽  
Single Specimen ◽  
Data Set ◽  
Test Technique ◽  
Pipeline Steels

Failure information from recent full-scale burst experiments on modern TMCP gas pipeline steels having a yield strength level of 690MPa and higher has shown that the CTOA fracture criterion can be effectively used to predict the arrest/propagation behaviour of the pipe against possible axial ductile fractures. The use of CTOA as an alternative or an addition to the Charpy V-notch and DWTT fracture energy in pipelines is currently under review. A significant difficulty currently limiting the more extensive use of CTOA in pipeline assessment is its practical evaluation either in the real structure or in a laboratory scale test. Different combinations of experimental and finite element analyses have been proposed for the measurement of the CTOA of a material. Although most of these models are able to predict the CTOA effectively, their implementation requires extensive calibration processes using the test load-deflection data. The authors have recently developed a novel test technique for direct measurement of the steady state CTOA using a modified double cantilever beam geometry. The technique uses optical imaging to register the uniform deformation of a fine square grid scored on the sides of the specimen. The slope of the deformed gridlines near the crack tip is measured during crack growth from captured images. Its value is a representative of the material CTOA. This paper presents recent results from the implementation of the technique to determine the steady state CTOA (steady state in this work refers to regions of ductile crack growth where CTOA values are constant and independent of crack length) of API X80 and X100 grade gas pipeline steels. In each case the approach was able to produce large amounts of highly consistent CTOA data from both sides of the test sample even from a single specimen. This extensive data set allowed an evaluation of the variance of the stable CTOA as the crack grew through the microstructure. The test method generated a steady CTOA value of 11.1° for X80 and 8.5° for X100 steels tested, respectively.

Crack Tip Opening Angle: Measurement and Modeling of Fracture Resistance in Low and High Strength Pipeline Steels

2006 ◽  
Author(s):  
Ph. P. Darcis ◽  
G. Kohn ◽  
A. Bussiba ◽  
J. D. McColskey ◽  
C. N. McCowan ◽  
...  
Keyword(s):  
Steady State ◽  
Fracture Resistance ◽  
Fracture Process ◽  
Pipeline Steel ◽  
Crack Tip ◽  
High Strength ◽  
Opening Angle ◽  
Test Technique ◽  
Crack Tip Opening Angle ◽  
Crack Tip Opening

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture, in cases where there is a large degree of stable-tearing crack extension during the fracture process. This type of steady-state fracture resistance takes place when the CTOA in a material reaches a critical value, as typically occurs in low-constraint configurations. Our current research has applied the CTOA concept as an alternative or an addition to the Charpy V-notch and the drop weight tear test (DWTT) fracture energy in pipeline characterization. A test technique for direct measurement of CTOA was developed, using a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software are used to record the progression of the crack tip and to estimate CTOA using the crack edges adjacent to the crack tip. A steady-state CTOA has been successfully measured on five different strength grades of gas pipeline steel (four low strength grades and one high strength grade: X100). In addition, two-dimensional finite element models (2D FEMs) are used to demonstrate the sequence of the fracture process and the deformation mechanisms involved. The CTOA measurements and models are correlated and agree well.

Hydrogen Environment Assisted Cracking of a Modern Ultra-High Strength Martensitic Stainless Steel

CORROSION ◽  
10.5006/2437 ◽  
2017 ◽  
Vol 73 (9) ◽  
pp. 1132-1156 ◽  
Author(s):  
Greger L. Pioszak ◽  
Richard P. Gangloff
Keyword(s):  
Stainless Steel ◽  
Martensitic Stainless Steel ◽  
Crack Tip ◽  
High Strength Steels ◽  
Coincident Site Lattice ◽  
High Strength ◽  
High Yield ◽  
Large Area ◽  
Hydrogen Environment ◽  
Ultra High Strength Steels

A modern martensitic stainless steel (Ferrium® PH48S™) resists hydrogen environment assisted cracking (HEAC) in aqueous NaCl at ultra-high yield strengths (1,400 MPa to 1,600 MPa). HEAC is transgranular, because of increased steel purity and La addition, compared to severe intergranular HEAC in Custom® 465-H900 without rare earth elements. Minimum threshold for HEAC (KTH) is low (8 MPa√m to 17 MPa√m) for each steel under substantial cathodic polarization. Transgranular HEAC occurs along martensite packet and {110}α′-block interfaces in PH48S, likely a result of H decohesion enabled by localized plasticity. Martensite transformation produces a large area of coincident site lattice interfaces in the refined microstructure of PH48S. However, a susceptible network of random packet/block interfaces is connected in 3D to limit interface engineering. Nanoscale strengthening precipitates in PH48S reduce effective H diffusivity to the mid-10−10 cm2/s range, because of reversible H trapping with a binding energy of 12 kJ/mol. This diffusivity reduces the Stage II growth rate by 1 to 3 orders of magnitude compared to C465 and carbide strengthened ultra-high strength steels. PH48S and C465 are nearly immune to HEAC when cathodically polarized by 50 mV to 500 mV, attributed to a minimum in occluded-crack tip overpotential for H production. The breadth of this protective-potential window increases with decreasing steel strength. Increased Cr does not degrade HEAC resistance, suggesting that crack passivity dominates cation acidification to reduce H production and/or uptake. A quantitative decohesion model effectively predicts the potential dependence of da/dtII using crack tip H solubility reverse calculated from a KTH model.

Microscopic Mechanism of Hydrogen Embrittlement in Fatigue and Fracture

2013 ◽  
Vol 592-593 ◽  
pp. 3-13
Author(s):  
Yukitaka Murakami ◽  
Junichiro Yamabe ◽  
Hisao Matsunaga
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Hydrogen Embrittlement ◽  
Crack Growth ◽  
Crack Problem ◽  
Crack Tip ◽  
High Strength Steels ◽  
High Strength ◽  
Deformation Mechanics ◽  
Low Strength

The microscope mechanism of hydrogen embrittlement (HE) is overviewed from the viewpoint of Mechanics-Microstructure-Environment Interactions. The plastic deformation (Mechanics) at crack tip for low strength steel is controlled by hydrogen concentration (Environment) to crack tip, eventually resulting in very strong time dependent phenomenon in static fracture and fatigue crack growth. Various typical phenomena in low strength steels which can be understood from the viewpoint of Mechanics-Environment Interactions will be presented. Fracture and fatigue of high strength steels (Microstructure) are strongly influenced by hydrogen. Especially, fatigue crack growth is remarkably accelerated by hydrogen-induced deformation twins. The HE phenomemon of the high-strength steels was applied to a newly inclusion rating method. Hydrogen trapped by nonmetalliec inclusions causes the elimination of fatigue limit at very high cycle fatigue. The values of threshold stress intensity factor KTH in hydrogen for small cracks are much smaller than those for long cracks measured by the standard WOL or CT specimens, which are eventually unconservative for the design of hydrogen components. This phenomenon is similar to the small crack problem in fatigue.

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