Single Edge Notch Tension Research Articles

Corrosion Fatigue Crack Growth Studies on Pressure Vessel and Piping Steels in Water Environment

Abstract Corrosion fatigue crack growth studies were conducted on eccentrically-loaded single edge notch tension specimens made of SA 333 Gr. 6 and SA 516 Gr. 70 carbon steels in water environment. The experiments were conducted using a ±250 kN capacity Universal Testing Machine under constant amplitude sinusoidal loading at a test frequency of 0.50 Hz and stress ratio of 0.1. The fabrication of test specimens and the experiments were carried out based on ASTM E 647 and ASTM E 1820. The crack initiation and growth were monitored and images were captured by using a digital camera at regular intervals of fatigue cycles. By using these images, the length of crack was measured. The tests were terminated when the uncracked portion of the specimens was insufficient to take further load. Crack growth rate and stress intensity factor range values were evaluated at incremental values of loading cycles and crack length. Using the crack growth rate vs. stress intensity factor range plots, best fit curves following power law in the form of Paris’ equation were obtained.

Stress corrosion cracking and corrosion fatigue cracking behavior of A7N01P‐T4 aluminum alloy

A modified single edge notch tension (SENT) specimen exposed to saline environment was used to investigate the stress corrosion cracking and corrosion fatigue cracking behavior of A7N01P‐T4 aluminum alloy. The crack propagation rate was determined, the microstructure, crack growth path, and the fracture surfaces of specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that the stress corrosion cracking rate of A7N01P‐T4 alloy in 3.5% NaCl was three orders lower than that of corrosion fatigue crack propagation rate. Mg and Zn segregation at the grain boundaries increased the stress corrosion crack susceptibility. The grain orientation had a significant effect on the crack propagation performance, and the high angle grain boundaries were susceptible to cracking. Hydrogen embrittlement was the primary cause of stress corrosion crack and corrosion fatigue crack propagation.

Numerical verification of stress intensity factor solution for clamped single edge notched tension (SENT) specimens

AbstractStress intensity factor solutions for clamped single edge notched tension (SENT) specimens, including a closed‐form function recently proposed by Zhu along with a function by CanmetMATERIALS referenced in the British Standard BS 8571, have been assessed. Solutions for an SENT specimen with a daylight‐to‐width ratio of 10 have been compared with new finite element results generated in this work to assess their accuracies. The results of this study show that the polynomial proposed by Zhu differs by no more than 0.23% compared with the numerical results over the range of 0.2 ≤a/W ≤ 0.7. The CANMET function differs by no more than 0.69% over the same range.

Fatigue performance of bonded crack retarders in the presence of cold worked holes and interference-fit fasteners

Bonded crack retarders (BCR) are of particular interest in the aerospace applications for improving fatigue performance of airframe structural assemblies. In respect of structural integrity, a reinforcing strap will require additional fixing by means of riveting or bolting to ensure fail-safety. Cold expansion is currently the common practice of increasing the fatigue performance of fastener holes. Thermal residual stresses introduced during the adhesive curing process at 120°C for strap bonding are of potential concern as they may affect the cold expansion stresses and thereby the fatigue crack growth performance of the fastener hole and reinforced structural assembly. In this paper, Single-Edge-Notched Tension (SENT) specimens are made of aluminium alloy 2624-T351. Fibre metal laminate GLARE is used as BCR strap. SENT specimens with BCR and with BCR plus an interference fit fastener are used to investigate the fatigue crack growth performance. Residual stress was measured by neutron diffraction method on specimens with BCR plus fastener. It is found that the GLARE strap provides a 2.3×improvement in life comparing to a plain specimen, and a 1.75×life improvement when a fastener is installed in a cold expanded hole.

Study on Fatigue Crack Propagation Behavior in Corrosion Environment of a Cold-Rolled Austenitic Stainless Steel

Fatigue crack growth testing and measurement of fracture toughness with single edge notch tension (SENT) specimens of American Iron and Steel Institute (AISI) 301L-MT stainless steel were performed in the air, distilled water, and 3.5% and 9% NaCl solutions. Type 301L-MT steel exhibited the same resistance to corrosion for various environments in near-threshold region. The crack growth in the Paris region could be accelerated in a [Cl−] containing environment by decreasing the threshold of stress corrosion KISCC, but independent of [Cl−] concentration. When specimens were fatigue precracked in different conditions, it was found that the fracture toughness of SENT specimens in the aqueous environment decreased 23% to 30% compared with that in the air. The normalized formula was established to describe the behavior of fatigue crack propagation of type 301L-MT steel. Three parameters, ΔKthcf, β, and JIQ, were used to clarify the effect of H-embrittlement mechanism on fatigue crack propagation.

Corrosion–fatigue crack propagation of aluminum alloys for high-speed trains

A modified single-edge notch tension (SENT) specimen exposed to saline environment was utilized to investigate the corrosion–fatigue crack growth behaviors of 5083, 6005 and 7N01 aluminum alloys. The fatigue crack propagation life, corrosion–fatigue crack rate ([Formula: see text]) were tested. The microstructures and fracture surfaces of specimens were examined by optical microscopy and scanning electron microscopy (SEM). The results showed that fatigue crack propagation rate of 7N01 in 3.5% NaCl was significantly higher than 6005 and 5083 alloys. The mechanisms of anodic dissolution and hydrogen embrittlement are used to explain the results.

Measurement techniques and procedures for standardized SE(T) testing of linepipe steel

The single edge-notched tension (SENT or SE(T)) test uses a low-constraint specimen geometry to determine the elastic-plastic fracture toughness of relevant linepipe steels. Application of the results include, but are not limited to, design methods, material selection, structural integrity assessment, fitness for service (FFS) and engineering critical assessment (ECA). Until recently, industry and government researchers have developed and published recommended practices without consensus from standard development organizations. A test standard was recently issued in 2014 by the British Standards Institution as BS 8571, and there is an ongoing effort to publish a robust standard test method within the American Society of Testing and Materials (ASTM International). Standardization of any test method should consider the influence of physical measurements on the results of the test. Generically, all measurements have uncertainty, and a standardized test method endeavors to produce results with a minimum uncertainty, as well as known precision and bias so that the results can be intelligently used for their intended purpose. This paper reviews the measurement techniques and procedures from each of the published recommended practices and BS 8571 and provides further guidance on specific techniques and procedures with respect to uncertainty.

Progress in development of fracture toughness test methods for SENT specimens

Fracture toughness testing using clamped single edge notched tension (SENT) specimens has been receiving broad attention in the oil and gas industry for direct measurement of low-constraint fracture toughness of pipeline steels. In order to develop robust SENT fracture toughness test methods, this paper reviews and summarizes existing SENT test methods, including the DNV practice, CANMET procedure, ExxonMobil method and BS 8571 standard, and their recent progress. Typical results are analyzed and compared with regard to the stress intensity factor solution, J-integral estimation, CTOD calculation and crack size measurement. On this basis, recommendations are made. Further studies and applications of SENT testing are also discussed.

Stress intensity factor for clamped SENT specimen containing non-straight crack front and side grooves

Three-dimensional (3D) finite element analyses (FEA) of clamped single-edge notched tension (SENT) specimens are performed to evaluate the stress intensity factor (SIF). Both the average and local SIF values over the crack front are evaluated. Plane-sided (PS) specimens containing both straight and curved crack fronts and side-grooved (SG) specimens containing straight crack fronts are considered including six average crack lengths (i.e. aave/W=0.2–0.7). PS specimens with 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. For SG specimens, one relative thickness (B/W=1) and ten depths of side groove are considered. The 3D SIF are compared with the present two-dimensional (2D) SIF. It is observed that the 2D SIF are sufficient as the difference between the 2D and 3D SIFs is in general within 10% if the crack front straightness requirements reported in the literature is satisfied. An empirical 3D SIF solution is developed that can potentially serve as a driving force for an assessment in actual structures that has similar crack geometries and loading configurations.

Effect of low temperature tensile properties on crack driving force for Arctic applications

Many petroleum companies expand their activities further north towards the Arctic region, resulting in design temperatures down to −60°C, which is much lower than what is usual for most current petroleum installations. As properties of steels are temperature dependent, it is of great interest to evaluate the effects of low temperature on the crack driving force in steels. The present work investigates these effects numerically using a finite element model of a single-edge-notched-tension (SENT) specimen with crack depth a/W=0.5. The effects of Lüders strain and yield strength are studied for gross stress levels σG/σy≤0.5, and it is shown that an increase in yield strength and Lüders strain, as a result of Arctic temperature, intensifies the crack driving force. An approximate model that can be used to estimate the crack driving force based on yield strength, Lüders strain and loading is proposed.

Crack growth characterization in single-edge notched tension testing by means of direct current potential drop measurement

Single-edge notched tension (SENT) testing has gained attention for the characterization of crack growth resistance in scenarios of low crack tip constraint conditions. One challenge of SENT testing is the experimental characterization of crack extension during the test. Two common methods exist: unloading compliance and direct current potential drop (DCPD). Whereas the former method is fairly extensively described in existing procedures, applying the latter mostly depends on user experience and in-house developed analysis algorithms. This paper describes Ghent University's approach for DCPD measurements of ductile crack extension in SENT testing. A reference voltage drop measurement is advised, aiming to account for current leak and changes in electrical conductivity. Then, attention goes to the sensitivity of the procedure with respect to changes in crack size and its robustness with respect to probe positioning errors. Finally, DCPD is shown to have similar capabilities to unloading compliance with respect to defect sizing accuracy. The results of this paper are aimed to contribute to a more prescriptive description of DCPD measurements in revised SENT test procedures or standards.

Effects of lattice orientation and crack tip constraint on ductile fracture initiation in Mg single crystals

In this work, plane strain finite element simulations of cylindrical void growth ahead of a notch tip in Mg single crystals are conducted to understand the effects of lattice orientation and crack tip constraint on the ductile fracture processes. The constitutive response is represented through crystal plasticity theory accounting for both slip and twinning. Three specimens - shallow, deeply cracked single edge notch tension and deeply cracked four point bend specimens, which display increasing levels of elastic biaxiality ratio (or constraint) are considered. Also two lattice orientations, with c-axis along the thickness direction and normal to the flat surfaces of notch (referred to as A and B) are studied. It is found that void growth and coalescence in orientation A (where predominantly prismatic slip occurs) take place directly ahead of the tip and fracture resistance enhances with reduction in constraint. By contrast, orientation B shows void coalescence by inclined shear bands, displays retarded void growth and higher J at crack initiation. The ductile fracture behavior is analyzed by examining slip, porosity and mean stress development in the voided cells ahead of the notch tip.

SENT testing standard BS 8571 and its ongoing development

A standard for SENT (Single Edge Notched Tension) testing was published in 2014 as BS 8571:2014. Since then user feed-back was received, new research published, and editorial issues identified. The main proposed change is to the equations for calculating J to enable a single equation for specimens with W/B ratios from 0.5 to 2. The method for calculating CTOD remains the same. The range of the permitted a0/W ratio is reviewed, along with the limits for initial and final crack shape. Advice is also given on SENT testing at low temperatures. A new method to correct the value of J and CTOD where crack path deviation (i.e. out of plane ductile crack extension) occurs has been proposed which will increase the number of results that will be qualified. This paper summarises the status of the British Standard since its publication, and the changes planned for its ongoing development.

Revised wide range compliance solutions for selected standard and non-standard fracture test specimens based on crack mouth opening displacement

This work addresses the development of wide range compliance solutions for tensile-loaded and bend specimens based on CMOD. The study covers selected standard and non-standard fracture test specimens, including the compact tension C(T) configuration, the single edge notch tension SE(T) specimen with fixed-grip loading (clamped ends) and the single edge notch bend SE(B) geometry with varying specimen span over width ratio and loaded under 3-point and 4-point flexural configuration. Very detailed elastic finite element analysis in 2-D setting are conducted on fracture models with varying crack sizes to generate the evolution of load with displacement for those configurations from which the dependence of specimen compliance on crack length, specimen geometry and loading mode is determined. To examine 3-D effects, including the influence of side-grooves, on the specimen compliance response, numerical analyses are also performed on 3-D models of selected plane-sided and side-grooved fracture specimens. The extensive numerical analyses conducted here provide a large set of compliance solutions, which not only validate the compliance relationships available in existing standards, but also help consolidating elastic compliance functions of common specimen geometries for use in routine test procedures for experimental evaluations of crack size changes in fracture toughness and fatigue crack growth testing.

Comparative study of CTOD-resistance curve test methods for SENT specimens

A comparative study is performed in this paper on the crack-tip opening displacement (CTOD) resistance curve test methods developed for single edge-notched tension (SENT) specimens in the end-clamped conditions. Different J-integral conversion methods and double clip gage methods, including CANMET, ExxonMobil and BS 8571 test procedures, are evaluated experimentally using SENT test data for both low and high strain hardening steels. Results show that the J-conversion methods with different m factors determine different CTOD-R curves, while the double clip gage methods determine similar results. In particular, the two-type CTOD-R curves differ significantly for high strain hardening steels. The CTOD test methods are quantitatively compared, two typical methods are identified, physical meaning of CTODs is analyzed, and applications of CTOD toughness are discussed.

Probabilistic approach for predicting fatigue life improvement of cracked structure repaired by high interference fit bushing

The purpose of this paper consists in developing a probabilistic approach for predicting fatigue life improvement of 7075-T6 cracked Single Edge Notch Tension (SENT) specimen repaired by high interference fit bushing. The developed approach is carried out by coupling FE-analysis and Monte Carlo reliability method. In this context, a 2D- finite element analysis of the cracked SENT specimen using ABAQUS commercial software is established to: (i) simulate the interference fit process and (ii) to predict the fatigue life improvement after repair. The non-linear isotropic/kinematic hardening model embedded in ABAQUS is used to characterize the material behavior. Different stress levels, different interference fit sizes and different expanded hole diameters are investigated. As a result, crack arrest hole diameter equals to 6 mm with larger interference fit size (IFS = 0.2 mm) provides higher beneficial compressive residual stress distribution and higher fatigue life improvement than drilling hole near the crack tip. The iso-probabilistic S-N curves and the Reliability-Life curves after repair can be determined for different interference fit sizes and different expanded hole diameters. This probabilistic approach can be used with the interference fit process as an interesting and practical tool to ensure an optimal maintenance planning for cracked structures and to improve the fatigue lives of these cracked components that cannot be replaced as soon as the cracks are observed, especially in the aeronautical filed.

Retardation of fatigue crack growth in high strength steel S690 using a modified stop-hole technique

The purpose of this paper is to study the effects of a new stop-hole technique on the subsequent delay in fatigue crack propagation. Single edge notch tension specimens of S690 steel alloy are used to perform the fatigue crack initiation/growth tests under cyclic stress loading. A parametric study is conducted on different geometrical parameters of double stop-hole. The experimental procedure is simulated using finite element analysis and fatigue life models. The experiments including fatigue crack initiation/growth measurements and microscopic examinations beside the numerical results show that the fatigue life extension caused by the double stop-hole method is significantly more than that of the conventional single stop-hole method. The lower stress level around the double stop-hole plays an important role in increasing the fatigue initiation life from the stop-hole edge.

An investigation of the small fatigue crack growth behavior in Ti-6.5Al-2Zr-1Mo-1V alloy

In this paper, we describe the experimental generation of a small fatigue crack in an SENT (single-edge notched tension) specimen of Ti-6.5Al-2Zr-1Mo-1V alloy. The growth behavior of the crack was analyzed using optical microscopy, scanning electron microscopy, transmission electron microscopy and other methodologies. We determined that when the crack length was approximately equal to the size of the plastic deformation zone at the crack tip, a knee point appeared in the da/dN-ΔK curve. At the knee point, the crack fracture mode transitioned from a crystallographic-faceted fracture to a striation-type fracture. Before the knee point, the change in the crack growth mode caused a zigzag crack growth path. The phase boundary and crack closure resulted in an irregular FCGR (fatigue crack growth rate). After the knee point, the small crack propagated forward by sliding while retaining traces of slip bands at the crack tip. The directions of the slips in the plastic deformation zone could be affected by the crystal orientations. Therefore, the crack growth path transitioned from a zigzag to a linear path due to the similar crystal orientation of the parallel α lamellae. In addition, the crack driving force far exceeded the resistance of the microstructures, which resulted in a significant acceleration of the FCGR.

Full-range stress intensity factor solutions for clamped SENT specimens

Single edge notched tension (SENT) specimen with clamped ends has been receiving increasing attention worldwide as a low-constraint specimen to measure less-conservative fracture toughness of pipeline steels in the oil and gas industry. Several SENT test methods were developed, but the solutions of stress intensity factor K used are different. The existing K solutions are thus reviewed and evaluated in this paper for the clamped SENT specimens, and then a full-range analytical solution of K is developed as a function of full-range crack sizes (a/W) and specimen aspect ratios (H/W). From this result, a simple closed-form solution of K is obtained particularly for H/W = 10, as used by the current SENT toughness test methods. The proposed full-range K solutions are validated using different numerical results and error analyses, and thus can be used generally to meet the needs of different SENT testing for determination of fatigue crack growth rate or fracture toughness in the low-constraint conditions.

Prediction of crack initiation and propagation in X70 pipeline steels

The present work deals with the concept of post-initiation softening applied to ductile crack initiation and propagation problems in thick structural components such as pre-cracked pipes and fracture specimens. The methodology includes a set of calibration steps to determine model parameters of crack initiation and propagation under monotonic quasi static loading condition. The phenomenological Modified Mohr-Coulomb (MMC) fracture criterion is used for fracture initiation along with post-initiation softening for crack extension. A comprehensive experimental/numerical program provides access to the load histories of each specimen geometry at the critical point, where fracture is assumed to occur. This first step enables the construction of the limits of ductility of the material in terms of a weighted von Mises equivalent plastic strain, which strongly dependent on stress triaxiality and Lode angle evolution. Post-initiation softening parameters are then calibrated from tearing analysis built upon resistance curves of single edge notch tension (SENT) specimens. The calibrated parameters model along with detailed FE analyses shows that the present fracture criterion accurately predicts the onset of stable crack growth and propagation along the uncracked ligament of low constraint specimens.