Crack Tip Opening Displacement Resistance Research Articles

Application of a semi-analytical method that accounts for constraint effects in the determination of resistance curves of mode I cracked specimens

In this study, a semi-analytical method concerning the constraint effectsoffracture specimens is successfully used to determine J resistance curves of mode I cracks on the basis of equivalent energy principle. Further, a constraint effective factor f is introduced in order to obtain the J resistance curves given by specimens with different constraints, which is determined by finite element analyses (FEA). In addition, the semi-analytical method can also be used to determine the crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) of specimens with different constraints according to the growing crack sizes of sharp cracked specimens determined by a semi-analytical expression. Taking compact tension (CT) specimens and single edge-notched bending (SEB) specimens as examples, a series of numerical simulations were conducted in order to determine the rotational radii R of specimens with different thicknesses, crack lengths and hardening exponents of materials based on a plastic hinge model. Further, the CTOD resistance curves of CT and SEB specimens with different thicknesses and CTOA resistance curves of thin CT specimens were determined by combining the equations of rotational radii and the semi-analytical method.

Numerical Investigation of Strength Mismatch Effect on Ductile Crack Growth Resistance in Welding Pipe

The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, My) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger My causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.

ExxonMobil SENT test method and application to Strain-Based Design

Strain-Based Design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Large ground movements may impose significant longitudinal strains to the pipelines. One of the key elements of the SBD approach is a Tensile Strain Capacity (TSC) prediction model for pipelines, which is usually provided in the form of an equation based on Finite Element Analysis (FEA). ExxonMobil's TSC prediction model has been validated against more than 100 full-scale tests. A key input to the prediction model is the tearing resistance of pipe and weld materials in terms of a Crack Tip Opening Displacement Resistance (CTOD-R) curve. Therefore, a test methodology is needed to appropriately characterize the ductile tearing resistance of pipes and girth welds. To this purpose, in recent years, ExxonMobil has developed a Single Edge Notched Tensile (SENT) test method, which has been shown to provide a level of crack tip constraint close to what is observed for defects in pipeline girth welds. In addition to providing input to TSC prediction, SENT testing is an integral part of Strain-Based Engineering Critical Assessment (SBECA) for pipelines.This paper describes the details of the ExxonMobil SENT test procedure and its application to carry out TSC predictions for SBD. Particularly, the paper covers the following aspects: (1) test procedure including analysis and interpretation of results; (2) validation of selected specimen geometry through comparison with data from pipe Full-Scale Tests (FST); (3) use of the CTOD-R curve as input to the ExxonMobil TSC prediction model; (4) finally, recent challenges related to crack path deviation are discussed with potential ways to address this issue.

CTOD‐R Curve Tests of API 5L X90 By SENT Specimen Using a Modified Normalization Method

AbstractA modified normalization method is presented for obtaining crack tip opening displacement (CTOD) resistance curves of single edge‐notched tension specimens. For the applied load mode, the normalization method should be based on force‐crack mouth opening displacement. A formula is proposed for CLL(i), the core parameter used in the normalization calculation. The modified method is validated using the unloading compliance method. The study of specimen geometry shows that side‐groove depth and crack depth have significant impacts on the obtained CTOD resistance curves, and the shape of the crack front affects the determination accuracy of crack length.

Determination of CTOD resistance curves in side-grooved Single-Edge Notched Tensile specimens using full field deformation measurements

The experimental evaluation of fracture toughness of linepipe steels is increasingly performed through Single Edge Notched Tensile (SENT) testing. The notch constraint in these specimens closely matches that in pipes. This article explores the possibilities of using full field deformation and strain measurements during SENT testing. Based on the obtained deformation fields, the Crack Tip Opening Displacement (CTOD) has successfully been evaluated. In addition, the deformation near the cracked ligament allows estimating the amount of ductile crack extension for shallow notched specimens. The combination of both allows constructing resistance curves for the tested materials.

Experimental Justification for Proposed Changes to the Measurement of KIc Using ASTM E 399

Abstract During the past five years considerable effort has been devoted to developing a fracture toughness measurement standard that combines the linear elastic stress intensity factor at the onset of crack extension, KIc, and the elastic-plastic fracture toughness measurement quantities, namely, JIc, the J resistance curve (J-R curve), and the corresponding crack tip opening displacement resistance curve (CTOD-R curve). The objective of this effort was to allow the engineer to start with one specimen geometry, conduct a specified test, develop either a KQ or a JQ−R curve, and then, after application of the relevant size requirements, obtain a valid fracture toughness result. At present, different specimens are required by E 399 and E 1737 (the combination and replacement of E 813 and E 1152), and if the engineer uses the E 399 specimen geometry and test procedure and subsequently finds that the measured KQ fails the E 399 size requirements, new specimens must be machined and tested according to E 1737 to obtain valid fracture toughness measurements. This paper describes experimental tests that have been conducted in parallel with the development of the new ASTM Standard E 1820-97, a combined fracture toughness test standard that is intended to address this problem. These experimental results show that the standard J integral specimen, incorporating load line crack mouth opening displacement measurements and side grooves, can be used to obtain KIc as presently defined by ASTM E 399. The use of larger a∕W ranges is also supported by these results as long as the basic size requirements of E 399 are met and applied to both the specimen crack length and remaining ligament as well as to the specimen gross thickness.

SOME IMPLICATIONS OF THE USE OF THE PD6493 LEVEL 3 FAILURE ASSESSMENT PROCEDURE

Abstract— Under a natural mapping between the standard R‐curve analysis diagram and the failure assessment diagrams of R6 and PD6493 Level 3 the R‐curve becomes the RCI (R‐curve image). It follows that whenever the assessment point moves along the failure assessment line during ductile crack growth, the implication is that the failure assessment line is the RCI. This result is used to test the conservatism of a specific PD6493 Level 3 analysis by two methods. The first calculates the variation during crack growth of the applied elastic‐plastic crack tip opening displacement (CTOD) parameter (or‘driving force') which is implied by the PD6493 analysis and then compares this variation with an independent estimate of it. The second uses an assumed driving force to deduce the CTOD resistance curve implied by the failure assessment line. It is shown by both methods and also by a direct R‐curve analysis that this particular PD6493 analysis is conservative relative to an R‐curve analysis which uses a crack driving force estimated by the EPRI (Electric Power Research Institute) procedures. However there is inconsistency between standard R‐curve analysis and PD6493 Level 3 analysis in that the latter implies a material resistance which, for a given material, depends on the geometry of the structure. A similar inconsistency arises in any failure assessment procedures like the Options 1 and 2 with Category 3 of R6, which require the assessment point to move on a geometry independent failure assessment line during crack growth; indeed, even when the failure assessment line is geometry dependent there is full agreement with R‐curve analysis only if the correct RCI is used as the failure assessment line. In a brief discussion it is noted that the new failure assessment diagram studies involving multi‐parameter fracture mechanics may help to ameliorate these problems.

Behaviour of A533B under biaxial loading at +70°C

Fracture analyses have generally assumed that biaxial loading can be assessed conservatively using uniaxial data. However, although there have been many analytical studies examining the response of cracks to biaxial loading, few large-scale tests have been carried out. This paper describes work carried out on a large biaxial test rig enabling the development of crack tip plasticity to be studied in 25 and 50 mm thick 500 mm square A533B plates. To date, tests under uniaxial (k = 0) and biaxial ( k = 1 2 and 1 ) conditions confirm that, in the linear elastic regime, the stress intensity factor K is not influenced by the loading ratio, k. However, the onset of plasticity in the wide plate tests occurs at different stresses as the loading ratio varies. Crack tip opening displacement resistance curves appear insensitive to loading ratio for the semi-elliptical part wall defects tested. However, once through-thickness cracks are developed, there are indications that the resistance curve is lower under equibiaxial conditions than for uniaxial loading.

Determination of Fracture Toughness by CTOD Resistance Curve Method

Abstract Fracture toughness δi of AISI 4340 steel in the hardened and tempered condition has been determined by the multiple-specimen CTOD (crack tip opening displacement) resistance curve method. This fracture toughness δi value has been compared with the JIc fracture toughness also obtained by the multiple-specimen JR curve technique. The investigation also examines the effect of specimen size, thickness, and width on δi and δR curve behavior. The effect of these parameters on the constraint factor m in the J-δ relationship has also been examined. Compact tension specimens with TL orientation were used. All specimens satisfied ASTM E 813 standard size requirements. This investigation demonstrates that Jδi values obtained by the CTOD resistance curve method are similar to JIc values obtained from the J-integral resistance curve method. The specimen thickness has no significant influence on δi or δR curve behavior. On the other hand, specimen width has a distinct influence on constraint factor m and δR curve behavior.