Single Edge Notch Tension Research Articles

Influence of weld porosity on crack size estimations in Single Edge Notched Tension (SENT) testing of steels: A numerical study

The acceptance of weld volumetric imperfections such as porosity from the perspective of structural integrity is guided by workmanship criteria. However, there is a lack of knowledge on how and to what extent volumetric imperfections can affect the outcome of fracture toughness tests. The small size of laboratory test specimens potentializes the relative impact of such imperfections. This study numerically investigates the influence of porosity on two crack sizing techniques commonly applied to single specimen fracture toughness testing, namely Unloading Compliance (UC) and Direct Current Potential Drop (DCPD). The investigation focuses on Single Edge Notched Tension (SENT) specimens, commonly used to characterize fracture behavior in low crack tip constraint conditions. An extensive finite element parametric study was performed where single pores and random porosity distributions were generated by element deletion. The results show that both UC and DCPD crack size estimations are influenced by pores of sizes within workmanship criteria acceptable limits, which in some cases might induce (or contribute to) crack size errors superior to validity limits set by fracture toughness test standards.

Structural Integrity Assessment of Defected Gas Pipelines Using a Simplified Ductile Damage Model

Abstract The finite element method using the damage model has been increasingly used to predict the failure of various structures. Thus, various damage models were presented, and recently, a phenomenological model called the local fracture strain model was presented, making it easy and accurate to predict the damage of the structure. This model has the advantage of defining fracture strain as a function of stress triaxiality with only a few notched tensile tests but has a limitation because it does not consider the damage evolution because of the void growth. This study presents an enhanced damage model that improves the accuracy of the failure simulation of defected structures by adding a parameter that considers stiffness degradation according to void growth to the damage model based on the fracture strain. Therefore, loading–unloading tests were conducted and the damage index of fracture was identified using a three-dimensional digital image correlation system. The failure simulation results using the proposed damage model were compared with experimental, such as notched tensile, single edge notched tension (SENT), and full-scale burst tests.

Fracture toughness of thermal aged 16MND5 bainitic forging steel under varying 3D constraint conditions: An experimental study using SENT specimens

This paper presents the experimental investigations of the effects of thermal aging on the embrittlement failure mechanism and fracture toughness of 16MND5 reactor pressure vessel (RPV) steel using clamped single edge notched tension (SENT) specimens with different sizes. In order to study the embrittlement induced by thermal aging, the 16MND5 steel was subjected to accelerated thermal aging for 1000 h, 3000 h and 5000 h at 425 °C, and then the Vickers hardness and tensile properties were determined. To investigate the combined effects of thermal aging and specimen geometric sizes, the fracture toughness of various thermal aged materials was studied using SENT specimens with different specimen thickness ratios, B/W, and crack depths, a/W. A total of 59 SENT specimens were tested covering a wide range of thermal aging time and geometric size combinations. Results showed that as the aging time increases, the material’s micro hardness and tensile properties change slightly, but the R-curves (J-R and CTOD-R) decrease significantly. The ductile fracture properties of thermal aged RPV steels depend markedly on the geometric dimensions (crack depth, a/W and specimen thickness, B/W) of tested specimens. Furthermore, the effects of geometric sizes and thermal aging on fracture behavior are shown to have a strong interaction. Specifically, as the aging time increases, the influence of the geometric sizes on the fracture toughness decreases, especially with the variations of a/W. Also, as the geometric dimensions increase, the effect of thermal aging on fracture toughness decreases. Finally, combined with fractographic observations, the influence of thermal aging on the embrittlement mechanism and ductile fracture properties of 16MND5 steel was discussed.

Suitability Of Standard Fracture Test Specimens For Low Constraint Conditions

Test specimen selection plays a vital role in successful transformation of laboratory fracture parameter findings to the real fracture behavior of any component. The ASTM and BSI Standard test methods for measurement of fracture toughness are dominated by high constraint specimens like Compact Tension (C(T)) and Single Edge Notch Bend (SEN(B)). However, most cracks in real time structure are shallow and surrounded by low constraint. Hence, the standard specimen like Middle Crack Tension (M(T)) in ASTM and Clamped Single Edge Notch Tension (Clamped SEN(T)) specimen in BSI, yield fracture toughness value to be test specimen dependent. The Clamped SEN(T) specimen suitable for various real pipeline (low constraint conditions) application has been widely reported in literature, that however in not the case for M(T) specimen which has scarcely reported. The 3D numerical analysis of SEN(T) and M(T) specimens with an identical flaw size for fracture toughness and constraint parameters (T-stress) under linear-elastic conditions are dealt in this article. The standard specimen that emulates low constraint fracture behaviour forms the basis for recommending suitable specimen for defined real time conditions. The conservatism associated with usage of standard specimen to find fracture parameters for an application has been quantified in this work for field implementation by practitioners in fracture mechanics.

Modelling the Variability and the Anisotropic Behaviour of Crack Growth in SLM Ti-6Al-4V.

The United States Air Force (USAF) Guidelines for the Durability and Damage Tolerance (DADT) certification of Additive Manufactured (AM) parts states that the most difficult challenge for the certification of an AM part is to establish an accurate prediction of its DADT. How to address this challenge is the focus of the present paper. To this end this paper examines the variability in crack growth in tests on additively manufactured (AM) Ti-6Al-4V specimens built using selective layer melting (SLM). One series of tests analysed involves thirty single edge notch tension specimens with five build orientations and two different post heat treatments. The other test program analysed involved ASTM standard single edge notch specimens with three different build directions. The results of this study highlight the ability of the Hartman–Schijve crack growth equation to capture the variability and the anisotropic behaviour of crack growth in SLM Ti-6Al-4V. It is thus shown that, despite the large variability in crack growth, the intrinsic crack growth equation remains unchanged and that the variability and the anisotropic nature of crack growth in this test program is captured by allowing for changes in both the fatigue threshold and the cyclic fracture toughness.

A constraint correction method based on use of a single test specimen

Commonly, fracture toughness tests on deeply cracked specimens are used to assess defects in large-scale components. The paper presents a method for selection of test specimen type, size and crack length in order to obtain fracture toughness estimates relevant to defects in cracked pipes. The method uses available closed-form T-stress, stress intensity factor and limit load solutions to determine the required specimen dimensions. The paper reports elastic–plastic finite element analyses for single edge notched bend (SENB) and single edge notched tension (SENT) specimens and cracked pipes which demonstrate good agreement of the matching approach, although care is needed in selecting the appropriate limit load solution for SENT geometries.

Effect of NbC precipitation on toughness of X12CrMoWNbVN10-1-1 martensitic heat resistant steel for steam turbine blade

In the present study, the microstructure, toughness and cracking behavior of a batch of X12CrMoWNbVN10-1-1 martensitic heat resistant steels used for manufacturing steam turbine blades in power plant were investigated. The manufacturing route of the X12CrMoWNbVN10-1-1 martensitic steels was casting, forging and heat treatment. These martensitic steels were taken from different positions in the same ingot, but they exhibited different toughness, which were marked as two groups: (i) the matrix located in the core of the ingot, hereinafter referred to sample A; (ii) the matrix located in the edge of the ingot, hereinafter referred to sample B. Impact toughness was characterized by Instrumented Charpy impact test, fracture toughness was determined by single edge notch tension (SENT) test in terms of double-clip gauge method, and crack propagation behavior was observed by in-situ single edge notch bend (SENB) test. Optical microscopy (OM), scanning electron microscopy (SEM) and high-resolution energy dispersive X-ray spectrometry (EDX) were used to examine the microstructure and cracking features. The impact toughness of sample A was much lower than that of sample B, which was proved to be attributed to the large amount of NbC precipitation in sample A. The clustered NbC particles could induce brittle cracking and thus significantly reduced absorbed energy for stable crack propagation, resulting in the poor impact toughness of sample A. It was believed that the large amount of NbC in sample A was precipitated directly from liquid metal during casting process, which might be related to the high niobium content of the core of the ingot due to the low cooling rate. The results revealed that the large number of clustered NbC particles formed in the casting process not only cannot promote precipitation-induced reinforcement, instead might be detrimental to toughness.

Influence of cold rolling on the fatigue crack growth behavior of tungsten

The influence of cold rolling on the fatigue crack growth (FCG) behavior of pure tungsten was investigated. For that tungsten sheets with three different thicknesses (2 mm, 1 mm and 100 μm) corresponding to different degrees of deformation were selected to prepare compact-tension (CT) and single edge notched tension (SENT) specimens. To account for orientation influences the crack propagation direction parallel, perpendicular and under 45° to the rolling direction was investigated. It could be shown that a pronounced cyclic R-curve behavior evolves, leading to relatively high thresholds of stress intensity factor range ΔK th , even at high load ratios. Furthermore, for the smallest grain size a pronounced anisotropy of the FCG behavior was observed where the crack growth direction perpendicular to the rolling direction exhibits the highest ΔK th and the lowest crack growth-rates. Especially the tungsten foils with 100 μm thickness showed stable crack growth with a pronounced Paris-regime. As a consequence, cold rolled tungsten qualifies as a fatigue resistant laminate material applicable for future structural applications in harsh environments.

Modeling of dynamic mode I crack growth in glass fiber-reinforced polymer composites: fracture energy and failure mechanism

The mode-I dynamic fracture energy and failure mechanisms of glass fiber-reinforced polymer composites are investigated with an embedded cell model of the single-edge-notched-tension (SENT) geometry. Under an applied dynamic loading, a crack may propagate in the embedded microstructure, accompanied by the development of a fracture process zone in which fiber/matrix debonding, matrix cracking and ductile matrix tearing are observed. Reaching a maximum nominal strain rate of 250/s, a series of SENT tests are performed for different loading velocities and specimen sizes while the dynamic energy release rate is evaluated using the dynamic version of the J-integral. The influence and interaction of loading rate, time-dependent material nonlinearity, structural inertia and matrix ligament bridging on the fracture toughness and failure mechanisms of composites are evaluated. It is found that with the given material parameters and studied loading rate range, the failure type is brittle with many microcracks but limited plasticity in the fracture process zone and a trend of increasing brittleness for larger strain rates is observed. The inertia effect is evident for larger strain rates but it is not dominating. An R-curve in the average sense is found to be strain-rate independent before the fracture process zone is fully developed and afterwards a velocity–toughness mechanism is dictating the crack growth.

2D characterization at submicron scale of crack propagation of 17-4PH parts produced by Atomic Diffusion Additive Manufacturing (ADAM) process

In this study, a Surface Mechanical Attrition Treatment (SMAT) is applied to a 17-4PH single edge notched tension (SENT) sample made by Atomic Diffusion Additive Manufacturing (ADAM). Grating of gold nanoparticles allowing multiscale characterizations is deposited on the surface of specimen by electron beam lithography. In-situ tensile test is carried out and images of the surface are recorded. The crack propagation is followed and the local parameters influencing the propagation are analyzed. The evolutions at the microstructure scale are compared with the macroscopic behavior of the specimen.

Elastic and fracture behavior of three-dimensional ply-to-ply angle interlock woven composites: Through-thickness, size effect, and multiaxial tests

This work presents a comprehensive investigation of the elastic and fracture behavior of ply-to-ply angle interlock three-dimensional woven composites. The research investigated novel splitting and wedge-driven out-of-plane fracture tests to shed light on the tensile fracture behavior in the thickness direction and to provide estimates of the out-of-plane tensile strength and fracture energy. In addition, size effect tests on geometrically-scaled Single Edge Notch Tension (SENT) specimens were performed to fully characterize the intra-laminar fracture energy of the material and to study the scaling of structural strength in this type of three-dimensional composites. The results confirmed that size effect in the structural strength of these materials is significant. In fact, even if the range of sizes investigated was broader than in any previous size effect study on traditional laminated composites and two-dimensional textile composites, all the experimental data fell in the transition zone between quasi-ductile and brittle behavior. This implies strong damage tolerance of the investigated three-dimensional composites. The analysis of the data via Bažant’s Type II Size Effect Law (SEL) enabled the objective characterization of the intra-laminar fracture energy of three-dimensional composites for the first time. Finally, Arcan rig tests combined with X-ray micro-computed tomography allowed unprecedented insights on the different damage mechanisms under multi-axial nominal loading conditions, particularly tension-dominated and shear-dominated conditions.

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.

Constraint analysis of thickness effects on fracture resistance behavior of clamped single-edge notch tension specimen

The correlation between the out-of-plane stress and the in-plane stress is established based on the bending modified J-Q theory and the out-of-plane constraint parameter TZ. Accordingly, an improved J-QZ-M constraint theory is developed to characterize the crack-tip fields by taking into account opening- and bending-dominated deformations in the ligament of the clamped single edge notch tension (SE(T)) specimens. Three-dimensional elastic-plastic finite element analyses (FEA) with a wide range of in-plane and out-of-plane constraints are performed to validate the proposed approach. It is found that the proposed J-QZ-M constraint theory can effectively quantify the crack-tip fields of SE(T) specimens, and the constraint parameter QZ exhibits approximately load- and distance-independence until large scale yielding (LSY).

Fatigue Crack Growth Prediction under Spectrum Load in an Aluminium Alloy using Crack Driving Force K*eff Approach

Fatigue Crack Growth (FCG) behaviour in a Single-Edge-Notched Tension (SENT) specimen of 2024-T3 aluminium alloy under a standard mini-FALSTAFF spectrum load sequence was experimentally determined. Further, the FCG behaviour was predicted using cycle-by-cycle method and compared with experimental results. Prediction procedure involved are rain-flow counting of fatigue load cycles, estimation of crack driving force for each of the counted cycle and prediction of crack extension per cycle from constant amplitude crack growth rate equation. In the present work, a new crack driving force (CDF) K*eff involving Kujawski’s crack driving force K* in conjunction with Elber’s crack closure concept was used to account for load interaction effects. FCG prediction was also made using conventional CDF ΔKeff (Elber’s) approach. A good correlation was observed between experimental and predicted FCG behaviour under spectrum loads by the proposed K*eff approach. Also, this prediction was observed to be better than that predicted by conventional ΔKeff approach.

Ductile fracture properties of 16MND5 bainitic forging steel under different in-plane and out-of-plane constraint conditions: Experiments and predictions

Bridging the gap between small-scale laboratory specimens and engineering full size engineering structures plays a critical role in integrity assessment of reactor pressure vessels (RPVs). In the present work, in-plane and out-of-plane constraint effects are both considered to determine ductile fracture properties under various low constraint conditions. The fracture behavior of 16MND5, a bainitic forging steel was investigated using clamped single edge notched tension (SENT) specimens with different crack sizes, specimen thicknesses and span lengths. Various fracture properties, including R-curves (J-integral, CTOD) and initiation fracture toughness (JSZW, J0.2, CSZW and C0.2) were determined. The results showed that the fracture properties are highly dependent on the in-plane (crack depth) and out-of-plane (specimen thickness) constraint conditions. The lower in-plane and out-of-plane constraint levels will introduce higher fracture properties. To further understand and quantify the impact of in-plane and out-of-plane constraint effects, the finite element method (FEM) and fracture surface analysis were conducted. Constraint dependent R-curves and initiation fracture toughness are developed for 16MND5 steel using a newly developed three-parameter fracture mechanics methodology. In particular, two independent constraint parameters, one for in-plane and one for out-of-plane, were used to quantify the 3D constraint effect. Finally, the present method was used to predict fracture properties for other specimens or low-constraint RPV cracked structures with different in-plane and out-of-plane constraint conditions.

Experimental and numerical investigation of the influence of temperature on the fracture behavior of high impact polystyrene evaluated by the J-integral approach using multiple specimen method

The aim of the present work is to study the mechanical behavior in fracture of high impact polystyrene (HIPS) subjected to tensile tests under the effect of temperature to determine the R-curves. The theory of the J-integral contour has been used for the development of a characterization method of the fracture strength appropriate to the case of this non-linear elastoplastic polymer material. To this end, we used the method of multiple specimens (Single edge notch tension SENT) of thin thickness; we used several identical test pieces containing cracks of the same lengths. The tensile tests were conducted under different temperatures, ranging from 25°C to 100°C. The results suggested a very temperature-dependent behavior due to significant increase in crack resistance as demonstrated by a comparison of JIC values related to initiation of crack propagation. The fracture energy absorbed as a function of the temperature suggested that the increase of the temperature is marked by the brittle-ductile transition in the material, which we confirmed numerically via a CASTEM software simulation.

Virtual crack closure technique in peridynamic theory

In this paper, the virtual crack closure technique (VCCT), commonly used in the numerical finite element method (FEM), is for the first time reformulated in nonlocal peridynamic theory, and a new peridynamics-based virtual crack closure technique (PD_VCCT) is proposed for the energy release rate calculation in the framework of peridynamics. The mode I, mode II, and their mixed mode fracture cases are considered. Two typical tests using the single edge-notched tension (SENT) and asymmetric double cantilever beams (ADCB) specimens are considered for implementation and verification, and the numerical energy release rates calculated by the present PD_VCCT are compared with those by the FEM-based VCCT. The close comparisons demonstrate that the proposed PD_VCCT can successfully predict the energy release rates for the cases of mode I, mode II and their mixed mode fracture and it can be used as a viable and effective tool for fracture analysis.

Effect of ferrite/pearlite banded structure on the local deformation and crack initiation at notches in pipeline steel

The effect of ferrite/pearlite (F/P) banded structure on the local deformation and crack initiation of single-edge notched tension (SENT) pipeline steel specimen was investigated by digital image correlation (DIC) and uniaxial tensile tests under the microscope. The results show that the banded distribution of the deformation field caused by F/P banded structure gradually disappears as the force increases and the strain concentration occurs at the shear band near the notch for each specimen. Micro cracks are prone to occur at the interface of F/P banded structure in 45° and TD specimens and the crack propagation path of the LD specimen is more tortuous. The effect of F/P banded structure on the maximum force and extension of SENT specimens are not obvious.

Quantitative multi-scale characterization of single basalt fibres: Insights into strength loss mechanisms after thermal conditioning

This article presents an experimental investigation to quantify the effects of high temperature exposure (400–600 °C) on the mechanical properties of single basalt fibres. To this purpose, a combination of single edge notch tension and nanoindentation micro-pillar splitting methods was used to provide an assessment of the fracture toughness of as-received and thermally treated basalt fibres. Similar values were obtained by the two different methods, and interestingly both highlighted an increase in KIc after heat treatment, up to 22% after exposure at 600 °C for 1h (1.59±0.06MPam). The increase in KIc suggests that microstructural changes occur in the fibres, as confirmed by high-speed nanoindentation mapping. Local radial heterogeneity in the fibre structure and elastic modulus and, possibly, the loss of defect orientation originally induced during the fibre drawing process are envisaged to control the decay of basalt fibres tensile strength during high temperature exposure, mimicking a thermal recycling process for composites.

Fatigue crack threshold analysis of TiAl SENT and CC specimens – Influence of starter notch and precracking

Due to their low density, excellent high temperature strength and good oxidation resistance, TiAl alloys are being used for components in commercial aero-engines. Fatigue crack threshold analysis is a safety critical input to the component design. Fatigue properties of a TiAl alloy with a near fully lamellar microstructure were investigated in detail by means of fatigue crack growth testing. Two different types of standard fatigue specimens (SENT – single edge notch tension and CC – corner crack) were machined and subjected to cyclic loading at temperatures between room temperature and 700 °C with a stress ratio R = 0.1. The present work examines the influences of sample geometry, starter notch width, starter notch radius and the effect of compression precracking on the measured threshold stress intensity factor range ΔKth. The results show to which extent the different specimen geometries and starter notch preparations yield the same fatigue crack growth (FCG) threshold results and which setup yields in the most conservative results, i.e. lowest crack growth thresholds for the same material and load condition.