Single Edge Notched Tension Specimens Research Articles

Experimental and numerical assessment of fracture toughness variations in steam boiler components after a long period of operation

This study presents an experimental and numerical assessment of the fracture toughness of steam boiler components, which have been in operation for more than 100,000 h. The materials tested are components of two different parts of a steam turbine unit: the pipeline section that connects the headers with turbines, which is made of 13HMF steel, and the valve chamber section, which is made of L17HMF steel grades. Full-size arc-shaped tension specimens (AT) are employed for the experimental evaluation. AT and single-edge notch tension (SENT) specimens are used for numerical assessment. The finite element method (FEM) formulations are developed and examined in Abaqus, which is set up as a shell model with the specimen thickness designated as the mid-surface for the analysis. The notch is modeled using a contour integral definition based on the maximum energy release rate criterion. The fracture toughness is quantified using the stress intensity factor (KI), and the results are compared. The valve chamber is found to have a higher fracture toughness than the pipeline section. The effect of specimen geometry is also noticeable, and the fracture toughness of SENT and AT specimens are compared. The results are useful in assessing the component’s durability after long operation.

On the validity of the Tada stress intensity factor solution for the single edge notch tension specimen with pinned ends

The validity of the Tada stress intensity factor (KTada) for pinned-ends single edge notch tension [SEN(T)] specimens is assessed via a combined experimental-modeling approach. Analysis of fatigue crack growth rate reductions during constant-ΔKTada loading demonstrates that specific combinations of alloy stiffness, geometry, and loading result in the true K deviating below KTada. Geometrically non-linear, 3-dimensional finite element calculations confirm mild-to-strong influences of these parameters, which are not captured by KTada. Most existing pinned SEN(T) data are found to use parameters where KTada is not significantly reduced, but the present results underscore the need for a more broadly applicable K solution.

The effect of hydrogen and notch orientation in SENT specimens on the fracture toughness of an API 5L X70 pipeline steel

Transporting hydrogen gas through pipelines is an important topic within the context of energy transition. An essential challenge hereto is the phenomenon of hydrogen embrittlement of pipeline steels which includes a reduction in fracture toughness. As the theories accounting for hydrogen embrittlement are still debated, improving fundamental insights into the underlying mechanisms is crucial. Fracture toughness tests on single edge notched tension (SENT) specimens have been performed to examine the influence of hydrogen on the fracture behavior of a pipeline steel. An API 5L X70 steel was selected for this purpose, characterized by a distinctive banded microstructure commonly seen in pipeline steels. The test specimens were extracted from the steel pipe, and a notch in two different orientations was machined. By using a combination of fractography and X-ray micro-CT (Computed Tomography), the fracture mechanisms are revealed. The tearing resistance characteristics of the steel, with and without the presence of hydrogen, are demonstrated through tearing resistance curves. The experiments show that the banded microstructure plays a key role in the results through the development of splits at the microstructural bands. While these splits are also present in uncharged toughness test specimens, they are increasingly present in hydrogen charged specimens suggesting a hydrogen related weakening of the banded microstructure. In addition, it is demonstrated that hydrogen induces an increasingly zigzagged crack path. This is attributed to a potential combination of accelerated void nucleation at the microstructural bands, and accelerated shear-driven fracture.

Determination of notch stress intensity factors under mode I loading using the 3D-DIC and finite element over-deterministic methods

In this paper, the fracture behaviour of Single Edge Notched Tension Specimens (SENT) is investigated experimentally and numerically using the three-dimensional digital image correlation (3D-DIC) experimental method and the Finite Element Analysis (FEA) technique, respectively. The mode I notch stress intensity factors (NSIFs) for the SENT specimens are derived from the measured and simulated displacement fields using the Finite Element Over-Deterministic (FEOD) method. The step-by-step process of a 3D-DIC experimental system for the SENT specimens is described. The SENT specimens were tested with pinned and clamped boundary conditions (BCs) and different notch initial opening angles. There are three main contributions of the paper: (1) the derivation of the NSIFs under Mode I Loading for thin plates subjected not only to the common pinned BCs, but also to the rare, clamped BCs; (2) the application of the FEOD method to derive the NSIFs from the displacement fields measured using the 3D DIC technique and predicted using FEA; (3) the validation of the NSIFs derived numerically via experimentally-determined NSIFs for notched thin plates subjected to both pinned and clamped BCs. Thus, four sets of NSIF data are provided in the paper and enable confident application of the NSIF values for the conservative (pinned BCs) and light-weight (clamped BCs) design of notched thin plates.

Effect of specimen thickness on fatigue crack shape evolution for aluminum alloy

The investigation of fatigue crack propagation and fracture mechanisms in high-thickness metal plates holds pivotal importance in the advancement of a three-dimensional damage tolerance assessment methodology. This research also bears significant implications for ensuring the safe operation of large-scale mechanical equipment. In this study, we examine the influence of specimen thickness on the evolution of fatigue crack shapes in an aluminum alloy. Fatigue crack growth tests were conducted on single-edge notch tension specimens to explore the impact of specimen thickness on the shape of fatigue crack growth. Additionally, we propose a calculation model for crack growth shape based on an energy model in this paper. Both experimental and analytical findings indicate that specimen thickness does indeed exert an influence on fatigue crack shape. Specifically, as the specimen thickness increases, the crack shape transitions from resembling a “fingernail” to a “saddle” shape.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

ProCrackPlast: a finite element tool to simulate 3D fatigue crack growth under large plastic deformations

Many structural components and devices in combustion and automotive engineering undergo highly intensive cyclic thermal and mechanical loading during their operation, which leads to low cycle (LCF) or thermomechanical (TMF) fatigue crack growth. This behavior is often characterized by large scale plastic deformations and creep around the crack, so that concepts of linear-elastic fracture mechanics fail. The finite element software ProCrackPlast has been developed at TU Bergakademie Freiberg for the automated simulation of fatigue crack growth in arbitrarily loaded three-dimensional components with large scale plastic deformations, in particular under cyclic thermomechanical loading. ProCrackPlast consists of a bundle of Python routines, which manage finite element pre-processing, crack analysis, and post-processing in combination with the commercial software Abaqus . ProCrackPlast is based on a crack growth procedure which adaptively updates the crack size in finite increments. Crack growth is controlled by the cyclic crack tip opening displacement varDelta CTOD, which is considered as the appropriate fracture-mechanical parameter in case of large scale yielding. The three-dimensional varDelta CTOD concept and its effective numerical calculation by means of special crack-tip elements are introduced at first. Next, the program structure, the underlying numerical algorithms and calculation schemes of ProCrackPlast are outlined in detail, which capture the plastic deformation history along with the moving crack. In all simulations, a viscoplastic cyclic material law is used within a large strain setting. The numerical performance of this software is studied for a single edge notch tension (SENT) specimen under isothermal cyclic loading and compared to common finite element techniques for fatigue crack simulation. The capability of this software is featured in two application examples showing crack growth under mixed-mode LCF and TMF in a typical austenite cast steel, Ni-Resist. In combination with a crack growth law identified in terms of varDelta CTOD for a specific material, the tool ProCrackPlast is able to predict the crack evolution in a 3D component for a given thermomechanical loading scenario.

Experimental evaluation of J-integral in elastic and elastic–plastic polymers by means of digital image correlation and higher-order eigenfields under mode-I

This study examines the capability of the digital image correlation method to determine the J-integral for polymers with elastic and elastic–plastic behavior. To achieve this, the required analytical concepts are discussed in detail and single-edge notch tension specimens manufactured out of PMMA and HDPE are tested under pure mode I loading conditions. Three specimens of each material are tested under different loads, and the displacement field at the surface of each specimen is obtained using DIC. Using the least square method, the displacement field derived by DIC is used to determine the higher-order singular and non-singular terms of the Williams expansion. The J-integral for each specimen is then computed by combining these terms. In addition, the J-integral obtained by DIC is compared to that derived by finite element analysis, and an excellent correlation is observed. Lastly, the influence of different DIC parameters on the computed results is discussed.

Dynamic fracture toughness of cellular materials with different microstructures

Ultra-lightweight cellular materials have been widely utilized owing to their excellent specific stiffness, strength and fracture resistance ability. However, current understanding of the fracture resistance of cellular materials is mostly limited to quasi-static loading conditions, and their resistance to dynamic fracture is comparatively less understood. In this study, we investigate the dynamic fracture behaviors of the triangular, kagome, and hexagonal honeycomb cellular materials using numerical methods. Both single-edge notched bend and single-edge notched tension specimens under dynamic loading are considered, and the J values of dynamic fracture toughness of the three configurations are determined. Simulation results demonstrate that the fracture resistance of the triangular honeycomb increases with increasing loading velocity, whereas the fracture toughness of the two other structures exhibits slight variation with respect to the loading rates. After examining the fracture response by changing the strain-rate-dependence of the constituent material, we concluded that the microstructure has a more significant effect on the rate-dependence of fracture toughness than the constituent material. A mechanism based on crack deviation is proposed to account for the distinct rate-dependence of fracture toughness in specimens with different microstructures.

Numerical calculation of [formula omitted] to simulate fatigue crack growth under large scale viscoplastic deformations

Crack propagation under low cycle fatigue and thermomechanical fatigue is characterized by high plastic and creep strains that extend over large regions around the crack, so that concepts of linear-elastic fracture mechanics cannot be applied. In these cases, the cyclic crack tip opening displacement ΔCTOD is a promising loading parameter to quantify crack growth. In this work, suitable definitions and Finite Element techniques are investigated and compared for an accurate calculation of ΔCTOD under cyclic mechanical and/or thermal loading. A viscoplastic temperature dependent material model of Chaboche-type is used along with large strain settings, specified for the austenitic cast iron Ni-resist. Extensive two-dimensional analyses of Single Edge Notch Tension specimens revealed that collapsed special crack tip elements are superior compared with commonly used regular quadrilateral 8-node elements. At the same level of accuracy of ΔCTOD, they require an about ten times coarser mesh and show less sensitivity w.r.t. element size for both stationary and propagating cracks. In order to simulate fatigue crack growth, an efficient, fully automated FE-technique is developed for an incremental crack propagation by successive remeshing, whereby the deformations and internal state variables are mapped from the old mesh onto the new one. Recommendations are made regarding important numerical control parameters like optimal size of crack tip elements, length of crack growth increment in relation to plastic zone size and ΔCTOD value.

Estimating the mode I through-thickness intralaminar [formula omitted]-curve of unidirectional carbon fibre-reinforced polymers using a micromechanics framework combined with the size effect method

A three-dimensional micromechanics framework is developed to estimate the mode I through-thickness intralaminar crack resistance curve of unidirectional carbon fibre-reinforced polymers. Finite element models of geometrically-scaled single edge notch tension specimens were generated. These were modelled following a combined micro-/meso-scale approach, where the region at the vicinity of the crack tip describes the microstructure of the material, while the regions far from the crack tip represent the mesoscopic linear-elastic behaviour of the composite. This work presents a novel methodology to estimate fracture properties of composite materials by combining computational micromechanics with the size effect method. The size effect law of the material, and consequently the crack resistance curve, are estimated through the numerically calculated peak stresses. In-depth parametric analyses, which are hard to conduct empirically, are undertaken, allowing for quantitative and qualitative comparisons to be successfully made with experimental and numerical observations taken from literature.

Effects of aging on the mechanical and fracture properties of chopped fiber composites made from repurposed aerospace prepreg scrap and waste

In this study, the effects of prepreg out time on the mechanical and fracture properties of Discontinuous Fiber Composites (DFCs) are investigated. Carbon fiber prepregs are aged to 0×, 1×, 2×, and 3× the out life (28 days) in an environmental chamber at 23∘C and 50%RH.From tension, compression, and shear experiments it is seen that the out time has no effect on the elastic moduli. However, the strength increases with increasing age of the specimens for all the loading conditions. The percent increase compared to the unaged material ranges from 15% to 33%. This is likely caused by plasticization of the matrix with age, allowing for higher energy absorption.More complex trends are reported for the fracture tests on geometrically-scaled Single Edge Notch Tension (SENT) specimens. It is found that the fracture energy and characteristic length initially decrease with age, and then finally increase for the longest out time. This trend is owed to two factors with countering effects on the fracture behavior: 1) the increase of the number of platelets through the thickness with aging due to increase in resin viscosity, and 2) the plasticization of the matrix with aging.The results of this study suggest that the mechanical performance of Discontinuous Fiber Composites (DFCs) made from reused materials is not negatively affected by aging of the starting material. On the contrary, repurposed DFCs can exhibit equal, if not better, performance than unaged DFCs. The results of this work further corroborate the feasibility of using DFCs as a mean to mitigate the environmental impact of thermoset composite waste. The experimental data presented in this work can be used as a baseline to design DFC composite components made from repurposed prepreg scrap and waste.

Solutions and applications of 3D elastic–plastic constraint parameters for clamped single edge notched tension (SENT) specimens

Single edge notched tension (SENT) specimen is a common fracture toughness test specimen now widely used in the pipeline and pressure vessels industries. In this paper, an extensive study has been conducted by three-dimensional (3D) finite element analyses (FEAs) to investigate the in-plane constraint parameters (namely A and QSSY) and out-of-plane constraint parameter, that is, Eε33 for clamped SENT specimens with side-grooves. A wide range of crack depth to specimen width ratios a/W = 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7, specimen thickness to specimen width ratios B/W = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 were included. The various strain hardening exponent (n = 4, 7, 10) and applied loading levels (from small-scale yielding to large-scale yielding) were also investigated. The results show that the effects of crack depth, specimen thickness, loading level and strain hardening index on the 3D constraint parameters show a significant coupling effect. With the increase of load level, A value increases monotonically, while QSSY and the normalized Eε33 (called V33) decrease, and the influence of specimen geometry on the constraint levels will become greater. The values of A and V33 obviously decrease with the increase of hardening exponent, but the change of QSSY is relatively small. Moreover, it can be observed that both A and QSSY can well describe the in-plane constraint effect, and V33 can better quantify the out-of-plane constraint effect under varying loading levels and n. In addition, elastic–plastic constraint parameters were also calculated for various experimental test specimens made of 16MND5 steel, with n = 9.2. It was demonstrated these constraint parameters can be applied to quantify and predict the J-R curves of 16MND5 steel with good agreements for specimens with different in-plane and out-of-plane constraint conditions.

On elastic anisotropy of 3D printed acrylonitrile butadiene styrene structures

Material extrusion is one of the most widespread additive manufacturing processes because of its low-cost implementation and its flexibility in design. In this study, integrated digital image correlation is used for the identification of constitutive parameters of two configurations of acrylonitrile butadiene styrene (ABS) samples. It is shown that an isotropic elastoplastic model calibrated from one single edge notched tension specimen described correctly the behavior of 3D printed ABS parts independently of the internal infill strategy. • Material parameters were calibrated from single SENT specimen via I-DIC in anisotropic elasticity. • The impact of elastic anisotropy of printed layers on the mechanical response is quantified. • Isotropic elastoplastic model calibrated from one single SENT specimen described the behavior of 3 printed ABS parts.

Fracture of Honeycombs Produced by Additive Manufacturing

Lattice materials, such as honeycombs, are remarkable in their ability to combine high stiffness, strength and toughness at low density. In addition, the recent and pervasive development of additive manufacturing technologies makes it easier to produce these cellular materials and opens new possibilities to improve their properties by implementing small modifications to their microstructure. Such developments open new opportunities towards the design of new classes of architectured materials. For example, recent computational studies have shown that honeycombs with lattice density gradients have a fracture energy under tensile loading up to 50% higher than their uniform counterparts. The aim of this study is to provide experimental evidence for these promising numerical results. To achieve this, single-edge notched tension specimens, with a honeycomb lattice structures, were manufactured by stereolithography using a ductile polymer resin. The performances of three different honeycombs were compared: (i) a uniform sparse lattice, (ii) a uniform dense lattice, and (iii) a gradient lattice with alternating bands of sparse and dense lattices. The results indicated that specimens with a density gradient may achieve a work of fracture per unit volume that is up to 79% higher than that of a uniform lattice.

Multi-physics FE-analysis and measurements for thermo-mechanical fatigue crack growth rate testing applications

The presented methodology in this study is addressed to in-phase (IP) and out-of-phase (OOP) loading cycles in stationary and transient thermo-mechanical fields. The subject of the numerical and experimental study is a single edge notch tension (SENT) specimen produced from a high-temperature nickel-based alloy ХН73М. In order to determination a local thermo-mechanical stress-strain rate and displacement fields a new algorithm for the multi-physics numerical calculations developed and implemented incorporates Maxwell 3D, Fluent and Transient Structural modules of ANSYS 2021R1. The employment of proposed algorithm to represent the cyclic history associated with the TMF conditions in the experiments, multi-physics finite element (FE) modelling of the stress, strain and displacement fields in the SENT specimen was performed. Additionally, time dependent non-uniform temperature fields were determined with the same cyclic variations and magnitudes as in the experimental OOP and IP cycling. As a complement to the FEM computations, the infra-red thermography temperature distribution measurements was implemented for the TMF state in the experiments in the SENT specimen. The comparison multi-physics FE-analysis and direct measurements shown in the present study is intended to contribute to a better understanding of the different mechanisms driving TMF crack growth and the address the outstanding questions associated with basic methodology.

Crack tip position evaluation and Paris’ law assessment of a propagating crack by means of temperature-based approaches

This paper investigates the possibility to evaluate the crack tip position, crack growth rate and stress intensity factor (SIF), in order to derive the Paris law by starting from the temperature distribution measured by means of an infrared camera, with high geometrical resolution. In more detail, constant amplitude, fully reversed, stress controlled, crack propagation fatigue tests were carried out on two single edge notch tension specimens machined from 4-mm-thick, hot-rolled AISI 304L stainless steel sheets.The temperature maps were then processed by using a signal reconstruction algorithm based on the least square method in order to extract the maps of the amplitude and phase signal components of the first and second harmonics. In particular, the amplitude maps of the thermoelastic signal allowed the estimation of the stress intensity factor (SIF) using well-established methods, while the thermoelastic phase and second harmonic phase maps were used for estimating the crack-tip position. As for the determination of the crack tip using the phase maps, an experimental approach based on the evaluation of the inversion point of a phase profile (parallel to the crack propagation direction ) was adopted. Such a point would represent the beginning of the reverse plasticity zone in the first and second harmonics phase maps.The crack tip position based on temperature maps was systematically compared to that measured by means of a digital microscope. Similarly, experimental SIF values derived from the thermoelastic signals were compared with the relevant numerical values from linear elastic FE analyses. The resulting Paris laws were critically compared. The good agreement between the preliminary data suggest further investigations regarding the applicability of these methods in all load conditions.

Crack propagation velocity and fracture toughness of hydroxyl-terminated polybutadiene propellants: Experiments and simulations

A critical part of failure analysis in solid rocket motors is understanding the fracture behavior of solid propellants. For single-edge notched tension (SENT) specimens of hydroxyl-terminated polybutadiene (HTPB) propellant, self-developed in-situ video imaging was employed to characterize the crack propagation and the crack-path morphology. Combining this in-situ video recording system with image digitization, then the crack propagation velocity can be obtained. Due to the viscoelastic properties of the propellant, the fracture properties were studied under different tensile rates (5-500 mm/min). A fracture mechanics method based on the J-integral was used to evaluate the fracture behavior of propellants, and a fracture criterion considering the burning rate of the propellant was proposed. Experimental research revealed that the HTPB propellant had a pronounced blunt crack tip during the fracture process. The plastic region in front of the crack tip where voids form and coalesce could cause crack propagation. The distribution of voids in front of the crack tip mainly determined the direction of crack propagation. The fracture characteristics of the HTPB propellant had evident rate dependence, and the J-integral changed significantly under different tensile rates. A simulation study was carried out in this paper. The 1/r singular elements were used to predict the J-integral of the SENT specimen during the tensile process. Two numerical simulation methods (XFEM: extended finite element method and CZM: cohesive zone model) were used to obtain crack propagation velocity and load–displacement curve. Furthermore, the simulation results were compared with experimental data.

Determination of R-curves for thermal aged 16MND5 bainitic forging steel using 3D constraint-based fracture mechanics

The current paper presents the application of three-dimensional (3D) constraint-based fracture mechanics for the determination of R-curves (J-R curves and CTOD-R curves) of thermal aged 16MND5 bainitic forging steel. First, the fracture toughness test results of 16MND5 steel using different sized clamped single edge notched tension (SENT) specimens under various thermal aging durations were summarized. Then, 3D finite element method (FEM) was applied to determine constraint parameters for the tested clamped SENT specimens. From the analysis results, it is shown that the T11 and the independent portion of T33 normalized by the nominal stress, named F11 and V33, can accurately quantify the in-plane and out-of-plane constraint levels, respectively. In addition, for all thermal aging condition, the thickness strain and V33 have a good linear relationship, which proves the applicability of V33 to quantify out-of-plane constraint effect from the experimental point of view. Next, the in-plane and out-of-plane constraint dependent R-curves were developed. Results showed that the developed R-curves can reproduce the experimental J-R curves and CTOD-R curves of thermal aged RPV steel obtained by clamped SENT specimens with generally good agreements. The combined effects of in-plane constraint, out-of-plane constraint and thermal aging on the fracture toughness were revealed and quantified. Finally, the R-curves of 16MND5 steel with various in-plane and out-of-plane constraint levels, and thermal aging durations were predicted, and compared to available test data.

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.