Clamped SE Research Articles

Crack tunneling effects on the elastic unloading compliance of C(T), SE(B) and clamped SE(T) specimens and correction methodology

This paper covers the effects of crack tunneling on SE(B), C(T), and clamped SE(T) specimens and presents a correction methodology for this effect and is divided in two parts. Part one presents an investigation of how crack front curvature affects instantaneous crack size predictions based on the elastic unloading compliance technique. Relative crack depths (a/W) of 0.2, 0.5, and 0.7, were considered alongside five levels of crack curvature. Refined finite element models provided load-CMOD records in order to support compliance assessment. The crack front was modeled as a semi-ellipse, and the compliance results agreed with experimental data from the literature. It was shown that for the same equivalent physical straight crack standardized by ASTM, compliance generally decreases as tunneling increases. Since the maximum crack curvature allowed by the aforementioned standards is very restrictive, compliance did not meaningfully change within that limit, however, if violated, this paper shows that higher deviations may occur, leading to inaccurate crack depth estimations and invalid test results. These limits and deviations were clearly determined and, as a step to improve the techniques, this paper also presents – in part two – an exploration of a possible approach to mitigate this problem, which is based on the modification of how the equivalent straight crack of a curved crack front is determined. This new approach presents reduced errors in compliance-based crack size estimation as crack curvature increases when compared to current standardized protocols, and it can support further investigations in order to validate and standardize improved measuring techniques. Finally, it is important to state that even though the ASTM E1820 is used for the determination of crack driving forces, this study is based only on the study of the crack front curvature, the limit imposed by this standard and the deviations on crack size estimation when those limits are violated, while not focusing on determining errors directly on the J-integral. This paper is a further development on the studies published before by the research group.

A numerical study on the correlation of ductile crack extension in Ni-based clad pipe girth welds with circumferential surface cracks and clamped SE(T) specimens

This work describes a numerical investigation on the relationships between ductile crack extension in Ni-based clad pipe girth welds and an API X80 pipeline steel with circumferential surface cracks and SE(T) specimens under fixed grip conditions. A primary objective is to gain further insight and understanding of the effects of constraint on ductile tearing behavior in piping components with dissimilar girth welds and how well crack growth resistance curves measured using single edge notched tension fracture specimens correlate with ductile fracture behavior in circumferentially surface cracked pipes. The numerical description of CTOD−R curves employs a computational cell approach incorporating the Gurson–Tvergaard (GT) constitutive model to simulate ductile crack extension in the crack configurations analyzed. The numerical analyses consider predictions of ductile crack growth in 3-D models of circumferentially cracked pipes with varying crack depth over pipe thickness ratio, a/t, from measured J-resistance curves derived from fracture resistance testing of clamped SE(T) specimens. The analyses also focus on comparing predictions of ductile crack extension using two different definitions for the CTOD.

Effect of Mounting Orientation on Testing Equipment on Elastic Compliance of Clamped SE(T) Specimens: an exploration

The accurate evaluation of fracture mechanics properties is key to the safety of operation of high responsibility structures such as nuclear reactor components. This paper evaluates the impact of mounting orientation on the test machine of clamped SE(T) (SE(T)c) specimens on its compliance. The elastic unloading compliance is a commonly used technique to measure the crack depth of specimens within fracture mechanics tests. Inaccurate measurements affect reliability of resulting measured properties. Two mounting orientations are evaluated in this paper together with two SE(T)c specimens with different width-to-thickness ratios (W/B=2 and W/B=4). Conclusions show that machine stiffness is significantly different in both orientations, and such value has the potential to affect crack depth prediction. On the other hand, the analysis scope of this work was unable to detect significant differences between both orientations, probably because loading was limited to maintain specimen integrity. Further investigation is necessary to precisely adress the impact of this effect.

J-resistance curve testing of a pressure vessel steel and a clad pipe girth weld using clamped SE(T) specimens and the normalization method

This work addresses an exploratory application of the normalization method to determine crack growth resistance curves for a typical pressure vessel steel and a clad pipe girth weld using clamped SE(T) specimens. A primary objective is to verify the effectiveness of the methodology in providing accurate descriptions of the ductile properties for the tested materials using clamped SE(T) specimens. Fracture toughness testing conducted on an A285 Gr C pressure vessel steel and a dissimilar Nickel–Chromium girth weld provides the load–displacement data from which J-resistance curves are evaluated on the basis of the normalization procedure. For the tested pressure vessel steel and clad pipe girth weld, the normalization method provides experimental measurements of ductile crack extension in terms of J−Δa curves which are in good agreement with experimentally measured crack growth responses derived from the UC procedure.

New formulation for fracture toughness characterization using four-point bend specimens

The present work aims to present a new complete procedure to characterize fracture toughness in terms of stress intensity factor K or the J−integral for Mode I of loading using four-point bend specimens. Four-Point Bend (4PB) specimens are shown to be valid configurations for fracture toughness characterization tests with certain advantages when compared to traditional three-point bend geometries and single edge notch tension SE(T) specimens under clamped conditions. Fracture toughness is a key parameter in defining critical flaw sizes in a fitness-for-service (FFS) procedure based upon fracture mechanics principles. Accurate estimation of fracture toughness is therefore necessary to assess the integrity of critical engineering structures by following the failure assessment diagram concept. Extensive 3D finite element analyses were performed to obtain the analytical relationships necessary to calculate the fracture toughness parameters. New formulas are provided for the elastic compliance, stress intensity factor, and eta factor for calculating fracture toughness valid for four-point bend specimens. Using the distance between inner and outer roller supports as a normalization parameter, the stress intensity factor and the normalized elastic compliance equations are shown to be independent of the specimen supporting configuration. The eta factor based upon the CMOD is dependent on the distance between external and internal roller supports as well as a∕W. Experimental fracture tests were carried out to check the practicality of the methodology. J-R curves obtained by four-point bend specimens were compared to resistance curves derived from three-point SE(B) specimens and clamped SE(T) samples which are standardized specimens. The results show that the four-point bend specimens provide reliable fracture toughness values somewhat higher than three-point bend geometries.

Evaluation of CTOD resistance curves in clamped SE(T) specimens with weld centerline cracks

This work addresses the characterization of the crack tip opening displacement (CTOD or δ) for extending cracks in clamped single edge notched tension, SE(T), fracture specimens based upon fracture toughness tests and numerical analyses of stable crack growth conducted on specimen geometries extracted from pipe girth welds with different material properties. The study covers fracture resistance tests and crack growth analysis of plane strain and 3-D models for clamped SE(T) specimens with a weld centerline crack, in which a computational cell methodology to model Mode I crack extension in ductile materials is utilized to describe the evolution of J and CTOD with increased loading for the extending crack. Laboratory testing of girth welds made of different materials provides the measured crack growth resistance curves from which the key microstructural parameters utilized in the growth analysis are calibrated. The present investigation shows that CTOD resistance curves based on the double clip-gage (DCG) procedure are consistently higher than the corresponding fracture resistance data based on J-CTOD relationships, thereby potentially yielding nonconservative fracture toughness estimates.

Impacts of residual stresses on J-integral for clamped SE(T) specimens with weld centerline cracks

Three-dimensional (3D) finite element analyses (FEA) are performed on clamped single-edge notched tension (SE(T)) specimens to investigate the impact of residual stresses on the average J-integral (J) evaluated over the crack front. Both shallow- and deeply-cracked (i.e. a/W = 0.2 and 0.5) specimens are considered in the analysis. The residual stresses are introduced by different mechanical pre-loading techniques and consistent with those measured from pipeline girth welds and those specified in well-known structural integrity assessment standards. The analysis results indicate that the impact of the residual stress on J depends strongly on the pre-loading techniques employed for the generation of residual stresses and level of the primary loading applied to the SE(T) specimen.

A reversed η approach to estimate load-deformation curves for fracture specimens and surface-cracked pipes

This paper presents a reversed η approach to estimate the load-deformation curve for fracture specimens subjected to monotonic and reversed loadings based on the measured J-R curve and extends the same procedure to surface-cracked pipes under monotonic actions. The proposed method predicates on the original η approach to uncouple the load-deformation data from the potential energy at different crack sizes, through a load separation method. This approach estimates closely the load-deformation curve for SE(B), C(T) and clamped SE(T) specimens under monotonic loadings. Besides, the proposed procedure predicts closely the envelope of the load-deformation curve for C(T) and SE(T) specimens under low-cycle actions. For pipes with a circumferential surface crack, the reversed η approach also leads to a reasonably accurate estimation of the load-deformation response. The proposed method provides a basic approach to estimate the fracture dependent load-deformation curve for large scale structural components, which allows upscaling of the material fracture resistance into the load-deformation characteristics required in the integrity assessment of large-scale engineering structures.

Fracture resistance testing of pipeline girth welds with strength undermatch using low-constraint specimens

This work presents an exploratory experimental investigation of the ductile tearing properties for the girth weld of a typical CMn pipe internally clad with a nickel-chromium corrosion resistant alloy (CRA) using crack growth resistance curves (J - Δa curves). Here, the material of the external pipe is an API 5L Grade X65 pipeline steel whereas the inner clad layer is made of ASTM UNS N06625 Alloy 625 with different mechanical properties. Unloading compliance (UC) testing of the pipeline girth welds employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves. This exploratory experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining experimentally measured R-curves for dissimilar girth welds.

Validity limits of the one-parameter elastic-plastic fracture mechanics (J-integral) considering SE(B), C(T) and clamped SE(T) specimens

The increasing severity of current structural applications (stresses, strains, displacements and aggressive environments), combined to the use of high toughness materials and low constraint geometries, strongly affects the validity of fracture mechanics methods to predict the mechanical behavior and final fracture of such structures using data obtained from laboratory tests, motivating further research in the field. However, the most important aspect of the one-parameter fracture mechanics framework is to ensure that the stress-fields in a reduced laboratory specimen are comparable to those found in real structures to whose design the properties taken from the specimen will be employed. This is the similitude concept, in which a single-parameter can describe the stress-fields ahead of a specimen’s or structure’s crack tip. To establish objective criteria for assessing similitude, this work compared the stress-fields obtained from high-constraint reference models (MBL) with those obtained from laboratory scale fracture mechanics specimens. The extensive analysis matrix, considering computational simulations under plane-strain, complemented by 3-D analyses, allowed the determination of the deformation limits M for C(T), SE(B) and clamped SE(T) geometries considering a wide range of geometrical features and material properties characteristic of structural steels applicable to pressure vessels and pipelines. The results confirmed the low constraint response for short-cracked SE(B) and SE(T) specimens and clarified the effects of crack depth and thickness on M values. In addition, some unexpected behaviors were evidenced and explained, as the case in which (for some particular crack depths and loading modes) thin specimens seem to be more constrained than thick ones. Thus, this work provides insights and quantitative results that enable the development of an objective basis to guarantee similitude in structural integrity assessments based on elastic-plastic fracture mechanics supported by the J-integral either with its critical values (Jc) or J-R curves.

Low constraint fracture toughness testing using SE(T) and SE(B) specimens

This work presents a brief review of current progress in fracture resistance test procedures to measure crack growth properties of pipeline steels using clamped SE(T) and shallow-crack SE(B) specimens. A central objective is to assess recently developed practices for J and CTOD resistance curve test methods with particular emphasis on the estimation procedures for J and CTOD as well as the experimental evaluation of stable crack growth. An overview of the fracture resistance test methodology based on the unloading compliance (UC) procedure is introduced to provide the requisite background to determine fracture resistance data for these fracture specimens. A rather comprehensive presentation of the estimation formulas to evaluate J (and, equivalently, CTOD) and crack length from load-displacement (CMOD) records is also given. Constraint effects in SE(B) and clamped SE(T) specimens in terms of the J−Q methodology are then described, followed by the presentation of key results for crack growth resistance testing conducted on low constraint fracture specimens extracted from typical pipe girth welds.

Effective Thickness of Side-Grooved Clamped SE(T) Specimens for J-R Curve Testing

Abstract Three-dimensional (3D) finite element analyses (FEA) of clamped single-edge tension (SE(T)) specimens were performed to investigate the effective thickness used in the calculations of the stress intensity factor (K) and plastic eta factor-based J-integral (J). The SE(T) specimens with six relative crack lengths (a/W ratios from 0.2 to 0.7), one relative thickness (B/W = 1), and eleven depths of side groove (i.e., BN/B = 1, 0.94, 0.92, 0.9, 0.88, 0.86, 0.85, 0.84, 0.82, 0.8, and 0.75) were considered in the analyses. Based on the FEA results, new effective thickness expressions and values for K and J estimations for side-grooved SE(T) specimens were proposed and validated.

Fracture resistance testing of dissimilar nickel–chromium girth welds for clad line pipes

This work presents an experimental investigation of the ductile tearing properties for the girth weld of a typical C–Mn pipe internally clad with ASTM UNS N06625 Alloy 625 using measured crack growth resistance curves (\(J{-}\Delta a\) and \(\mathrm {CTOD}{-}\Delta a\) curves). Here, the material of the external pipe is a typical API 5L Grade X65 pipeline steel whereas the inner clad layer is made of a nickel–chromium corrosion resistant alloy steel. Testing of the girth weld employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance method using a single specimen technique. This experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining accurate experimentally measured R-curves for this class of material, including the effects of weld strength mismatch.

Numerical simulation of ductile crack growth in medium wide plate specimens using 3-D computational cells

This work examines the applicability of a local damage method based on the computational cell methodology, incorporating the Gurson-Tvergaard (GT) yielding model, to predict ductile crack extension of circumferential crack-like defects in Medium Wide Plates (MWP) specimens, extracted from girth welds of pipelines, using standard single edge bend SE(B) specimen and recently standardized clamped single edge tension SE(T) specimens to obtain model parameters. Laboratory testing of SE(B) and SE(T) specimens at room temperature provides necessary and sufficient information to calibrate the parameters of the GT yielding law. After the model parameters have been defined using small specimens, predictions of crack growth behavior can be carried out for other geometries and loading modes. Therefore, parameter transferability between different specimens can be investigated and comparative studies of predicted crack driving forces can be performed. In this study, the simulated tearing resistance curves are compared to experimental data of MWP specimens, where the former is obtained by the cell model calibrated using either SE(T) or SE(B) specimens and the latter is obtained by large-scale experiments. Overall, a good agreement between numerical and experimental δ-Δa curves was observed demonstrating that the cell methodology can be considered as a valid engineering tool for studying the integrity of structural components.

Low-Constraint Toughness Testing of Two Single-Edge Notched Tension Methods in a Single Specimen

Single-edge notched tension (SE(T) or SENT) specimens have been increasingly proposed as a low-constraint toughness test to measure toughness of line pipe materials, as the crack tip constraint approximates a circumferential surface flaw in a pipe under loading. The clamped SE(T) single-specimen procedures recently developed by Shen and Tyson (2008, “Fracture Toughness Evaluation of High Strength Steel Pipe,” ASME Paper No. PVP2008-61100; 2008, “Development of Procedure for Low-Constraint Toughness Testing Using a Single-Specimen Technique,” CANMET Materials Technology Laboratory, Technical Report No. 2008-18 (TR)) and Tang et al. (2010, “Development of the SENT Test of Strain-Based Design of Welded Pipelines,” 8th International Pipeline Conference, IPC 2010, Calgary, AB, Canada) have both used in common the use of a clamped single-specimen of similar geometry and relied on the unloading compliance technique for crack size estimation. In the former case, a single clip gauge is attached to the integral knife edge and the crack-tip opening displacement (CTOD) is estimated by means of a J-integral-to-CTOD conversion, similar to the procedure of ASTM E1820-11. The latter uses a pair of clip gauges mounted on an attachable raised set of knife edges to estimate CTOD at the original crack tip position by a triangulation rule. Consolidating these two sets of clip gauges in a specimen makes direct comparisons of two SE(T) methods under identical test conditions: material, specimen geometry, equipment, test temperature, and operator (Weeks et al., 2013, “Fracture Toughness Instrumentation Techniques for Single-Specimen Clamped SE(T) Tests on X100 Linepipe Steel: Experimental Setup,” 6th Pipeline Technology Conference, Ostend, Belgium). In this study, SE(T) testing employing these two SE(T) methods on a single specimen was conducted on B × B shallow-cracked (a/W ∼ 0.35) specimens of two X70 pipeline girth welds. This paper discusses details of the two SE(T) methods and techniques on the same specimen.

Fracture mechanics analysis of heterogeneous welds: Numerical case studies involving experimental heterogeneity patterns

Weld flaw assessments require a quantification of crack driving force, and procedures to do so are generally based on the assumption of homogeneity. However, welds generally have a heterogeneous microstructure. The authors have developed a method to account for continuous strength property variations in the estimation of crack driving force. It considers average material properties along slip lines originating from the crack tip, thus defining a homogenized weld which is expected to produce a similar crack driving force response. This paper presents a numerical validation of the proposed homogenization approach. Weld heterogeneity is characterized in three pipeline girth welds by means of automatic hardness testing. The resulting distributions are input into a finite element model of a clamped SE(T) specimen with element-specific material properties. Following, crack tip opening displacement (CTOD) responses of heterogeneous and idealized welds are compared. Two loading modes are considered: pure tension (mode I) and in-plane shear (mode II). The homogenization approach is found to strongly simplify complex heterogeneous welds into homogeneous structures and, in doing so, maintain an acceptable level of accuracy with respect to mode I crack driving force.

J–CTOD relations in clamped SE(T) fracture specimens including 3-D stationary and growth analysis

This work provides improved relationships between J and CTOD for clamped SE(T) specimens based upon extensive 3-D computations conducted for crack configurations with varying crack sizes and material properties incorporating stationary and crack growth analysis. A computational cell model for crack extension in ductile materials is utilized to describe the evolution of J and CTOD with increased loading. Laboratory testing of an API X70 steel provides the crack growth resistance curves for the material from which the cell parameters are calibrated. The study enables establishing accurate relationships between J and CTOD for use in testing protocols for toughness measurements.

Effects of crack front curvature on J–R curve testing using clamped SE(T) specimens of homogeneous materials

Three-dimensional (3D) finite element analyses (FEA) of clamped single-edge tension (SE(T)) specimens are performed to investigate the impact of the crack front curvature on the elastic compliance, compliance rotation correction factor and average J-integral evaluated over the crack front. Specimens with six average crack lengths (i.e. aave/W = 0.2–0.7) and three thickness-to-width ratios (i.e. B/W = 0.5, 1 and 2) are analyzed. The curved crack front is assumed to be bowed symmetrically and characterized by a power-law expression with a wide range of curvatures. Several crack front straightness requirements for SE(B) and C(T) specimens specified in BS7448, ISO and ASTM E1820 standards are reviewed. Based on results of the numerical investigation, new crack front straightness criteria for the SE(T) specimen are proposed in the context of the nine-point measurement by using as a criterion that the errors in the estimated compliance and average J values should be no more than five percent. The proposed criteria depend on both aave/W and B/W, and are more advantageous than those specified in the BS, ISO and ASTM standards in terms of controlling the differences in J and compliance between the specimens with curved and straight crack fronts.

Numerical investigation of compliance equations used in the R‐curve testing for clamped SE(T) specimens

AbstractThree‐dimensional finite element analysis of clamped single‐edge tension specimen is performed to examine the accuracy of the crack‐mouth opening displacement compliance equations reported in the literature. The analysis covered both plane‐sided and side‐grooved specimens with a wide range of specimen configurations including nine relative crack lengths (a/W) ranging from 0.1 to 0.9 and seven relative thicknesses (B/W) ranging from 0.25 to 4. Based on the finite element analysis results, the crack length/compliance modification factors (ρ(a/W) and ρ(BCE)) are developed to improve the accuracy of the compliance equations. The results of this study can improve the accuracy of the experimentally determined J‐R and CTOD–R curves using the single‐specimen technique and single‐edge tension specimens.

Delamination effects on fracture behavior of a pipeline steel: A numerical investigation of 3-D crack front fields and constraint

This work addresses a numerical investigation of the crack front fields and effects of crack-tip constraint in conventional fracture specimens with prescribed transverse delamination cracks. One purpose of this study is to conduct a systematic evaluation of delamination effects in side-grooved and plane-sided C(T) and clamped SE(T) fracture specimens, which are commonly utilized in fracture toughness testing of pipeline steels. Another is to quantify the potential coupling influence of specimen geometry and delamination size on crack-tip constraint by means of the J−Q theory thereby providing valuable insight into the effect of delamination cracks on macroscopic fracture behavior in conventional fracture specimens. Laboratory testing of an API 5L X70 steel at room temperature provides the mechanical properties used in the numerical analysis incorporating delamination cracks with varying sizes. Nonlinear finite element analyses of very detailed 3-D finite element models of C(T) and clamped SE(T) fracture specimens for the API X70 pipeline steel enable assessing the effects of prescribed delamination cracks on the crack front fields and constraint with increased deformation levels as characterized by the J-integral. Overall, the present analyses reveal important features of 3-D crack front fields in fracture specimens with a crack-divider delamination that have a direct bearing on the often observed toughness increase in fracture testing of materials with through-thickness anisotropy in mechanical properties.