Single Edge Notch Bend Specimens Research Articles

Experimental evaluation of the short and long fatigue crack growth rate of S355 structural steel offshore monopile weldments in air and synthetic seawater

Welded steel structures used in the offshore wind industry are exposed to harsh marine environments, which can result in corrosion-induced fatigue damage. Of particular concern is the heat affected zone (HAZ) of welded joints, a region known for its altered microstructure and mechanical properties, which can significantly influence the initiation and propagation of fatigue cracks. This study investigates the short and long fatigue crack growth rates, and the effect of seawater exposure, for the HAZ in S355 steel weldments. Single-edge notch bend (SENB) specimens are used, with a shallow notch in the HAZ. A series of specimens is immersed in synthetic seawater that is continuously circulated at a controlled temperature to assess the synergistic effects of corrosion and fatigue. The experimental method integrates a novel application of front face strain compliance for monitoring short cracks, alongside an extended back-face strain compliance approach for monitoring long crack propagation. It is concluded that the short fatigue crack growth rate of the HAZ is 2.7 to 3.5 times higher in seawater as compared to air. As the crack propagates and enters into the long crack regime, the ratio decreases to 2.2 times at the transition point of the two-stage crack growth curve and further decreases to 1.5 times when the notch advances towards fracture. The findings indicate that the fatigue crack growth rates documented in standards tend to be on the conservative side. This study significantly enriches the fatigue crack growth data available in literature, which will contribute to a more accurate lifetime assessment offshore wind turbine structures.

Quantification of crack tip constraint effect based on the critical distance

AbstractThe effect of crack tip constraint on the fracture properties of flawed components is a critical consideration in the safety assessment of engineering structures. In this work, an approximate expression of three‐dimensional von Mises stress fields ahead of the crack tip has been derived by referring to the classic Hutchinson‐Rice‐Rosengren (HRR) solution. The critical distance rc is chosen as the constraint parameter to quantify the level of the crack tip constraint. A theoretical relationship between the parameter rc and fracture toughness JQ has been proposed. The critical constraint parameter rc can be experimentally determined by using the critical fracture toughness JQ. The experimental results of compact tension (CT) and single edge‐notched bending (SENB) specimens with different crack tip constraint levels for SA‐508 and Q345B steels successfully validate the relationship between the critical fracture toughness JQ and the constraint parameter rc. According to the test results, the critical distance rc is feasible to quantify the crack tip constraint of CT and SENB specimens with different crack tip constraint levels.

A model for modifying the S‐N curve considering the effect of boundary conditions on the fatigue crack growth of welded components

AbstractThe present study proposes a novel model to modify master S‐N curves of components according to their load redistribution capability reflected in different boundary conditions (BCs) based on the fracture mechanics analysis. To that end, a comprehensive numerical study was conducted on a Single Edge Notch Bend (SENB) specimen constrained with different kinematic BCs using discrete fatigue crack growth (FCG) simulation. It was observed that BCs indeed can have a significant effect on the crack growth behavior and consequently on the resulting fatigue life under the same nominal loading conditions. The proposed model was applied to the S‐N curve of a T‐welded joint, and the predicted fatigue life was validated against 3D FCG simulations. Finally, FCG tests were conducted on SENB specimens to experimentally corroborate the effect of BCs on the FCG rate.

Modelling of crack propagation in miniaturized and normal SENB specimens based on local failure criterion

The use of miniaturized specimen testing methods is a promising way to solve the problem of limited materials in RPV monitoring programs. The use of miniature specimens allows the evaluation of fracture toughness from other specimen materials used. In particular, the small-size compact tensile test specimen (0.16T CT) is promising for the determination of fracture toughness, as it can be produced from the standard size Charpy specimen that has already been tested. However, if we have only 0.16T CT test, we cannot investigate the dimensional response and also have only one restricted deformation state, which may pose problems in verifying geometry independence and determining local parameters for state-of-the-art analyses. It is therefore recommended to prepare at least two tests with two different restricted deformation specimens. Therefore, the testing of mini single edge notched bending (SENB) is also required and can be worked out from the Charpy specimens. The paper presents the determination of fracture toughness for these miniaturized specimens by modifying the virtual crack closure technique (VCCT) simulation method using GTN parameters instead of energy release as the driving force. This allows the calculation of the J-integral to proceed in parallel with the crack propagation.

Effect of stacking direction and raster angle on the fracture properties of Onyx 3D printed components: A mesoscale analysis

3D printing is a technology that has gained increasing importance both in academia and industry for the possibilities offered. Among the many, one of the most appealing is the possibility to choose different printing configurations, tailoring stacking direction, and raster angle. The choice of such parameters deeply influences the structural response, as already shown in the literature. This study aims at providing a deep understanding of the phenomena taking place at the mesoscale level which justify such differences in the fracture behavior. An experimental campaign is carried out with a Single Edge Notch Bending (SENB) specimen printed in Onyx using four different combinations of raster angles and stacking directions. The results are compared in terms of mechanical response, fracture toughness and surface roughness to identify the main driving mechanisms during the fracture process. The same bending test is replicated numerically, aiming at comparing the fracture toughness values obtained experimentally with the ones used in the simulation to match the experimental curves. The study shows that the stacking direction and the raster angle deeply influence the fracture behavior and the mechanical properties of the specimen, along with the fracture toughness and surface roughness. In particular, it is highlighted how improved mechanical behavior can be achieved by printing the specimen in the vertical direction and with a raster angle of 45°/−45°. Moreover, useful fracture toughness values are identified for different combinations of printing parameters by means of a Cohesive Zone Model formulation, providing useful input values for numerical simulations.

Experimental and Numerical Analysis of Fracture Mechanics Behavior of Heterogeneous Zones in S690QL1 Grade High Strength Steel (HSS) Welded Joint.

The heterogeneity of welded joints' microstructure affects their mechanical properties, which can vary significantly in relation to specific weld zones. Given the dimensional limitations of the available test volumes of such material zones, the determination of mechanical properties presents a certain challenge. The paper investigates X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The experimental work is focused on tensile testing to obtain stress-strain properties, as well as fracture mechanics testing. Considering the aforementioned limitations of the material test volume, tensile testing is carried out with mini tensile specimens (MTS), determining stress-strain curves for each characteristic weld zone. Fracture mechanical testing is carried out to determine the fracture toughness using the characteristic parameters. The experimental investigation is carried out using the single edge notch bend (SENB) specimens located in several characteristic welded joint zones: base metal (BM), heat affected zone (HAZ), and weld metal (WM). Fractographic analysis provides deeper insight into crack behavior in relation to specific weld zones. The numerical simulations are carried out in order to describe the fracture behavior of SENB specimens. Damage initiation and evolution is simulated using the ductile damage material behavior. This paper demonstrates the possibility of experimental and numerical determination of fracture mechanics behavior of characteristic heterogeneous welded joint zones and their influence on crack path growth.

Crack penetration versus deflection in extrusion-based additive manufacturing – Impact of nozzle temperature and morphology

Two different modes of fracture propagation can occur when a crack encounters a weak interface in a fused filament fabricated (FFF) part: the crack either deflects into the interface, or penetrates the subsequent layers. The objective of this work is to verify the suitability of an energy- and a strength-based criterion for predicting which failure mode will occur in FFF printed parts. Four different materials, glycol-modified poly(ethylene terephthalate), polylactide acid and two different poly(methyl methacrylate) grades were examined. Fracture mechanical tests were performed on single edge-notched bending specimens for the energy-based approach and tensile tests performed on dumbbell specimens for the strength-based approach. Additionally, porosity measurements and thermal analysis were carried out to provide structural information. The energy-based approach proved unreliable for failure mode prediction. Potential problems include failure to meet the requirements of linear elastic fracture mechanics and issues with notch design. The strength-based approach, in contrast, correctly predicted the crack path for all tested materials and seems a promising candidate for failure mode prediction in FFF materials.

Analysis of fracture toughness and fatigue crack growth rates of electron beam-welded dissimilar nickel-based superalloys: Mode-I cracks oriented normal to the weld

This paper discusses the experimentally obtained results of fracture toughness and fatigue crack growth rates of dissimilar metal-welded joints of Inconel 718 to Nimonic 80A. The dissimilar metal-welded joints were prepared in butt joint configuration using electron beam welding. Mode-I cracks were oriented normal to the weld. The base metals have different yield strength values (plastic mismatch) and ultimate tensile strength values but have the same elastic modulus and thermal expansion coefficients. The experiments were conducted using single-edge notched bend specimen design. The fracture toughness ( J) and fatigue crack growth rates of dissimilar metal-welded joints were affected by the direction of crack growth, such as cracks moving from stronger metal to weaker metal or vice versa. In comparison to Nimonic 80A (base metal), the J value of dissimilar metal-welded joint with crack direction from Nimonic 80A (stronger) to Inconel 718 (weaker) was lower. Moreover, the J value of dissimilar metal-welded joint was higher than that of Nimonic 80A (base metal) when crack traveled from Inconel 718 (weaker) to Nimonic 80A (stronger). In the fatigue crack growth rate test, the crack grew faster as it approached the weld interface from Nimonic 80A toward Inconel 718 due to crack acceleration. Conversely, the crack grew slowly in the opposite direction due to crack shielding. The onset of these effects began sooner for higher loads than lower loads.

Fracture toughness of nano-zirconia filled aluminum matrix composites: an experimental and numerical analysis

During the service life of components, they encounter several cyclic loadings that consequently generate stress which encourages crack interaction ultimately deteriorating materials’ performance. The present study aims to investigate the fracture behaviour of aluminium 6061 reinforced with 0–15 wt% ZrO2 particles (at step of 5 wt%) fabricated using stir casting technique. Compositional analysis through x-ray diffraction was performed on the prepared composite samples. The fracture toughness of prepared composites is investigated through bending test by using single edge notch bend (SENB) specimens. In addition, the computation of fracture toughness was also performed by finite element analysis (FEA) approach, and the numerical results obtained from the FEA were compared with the experimental value. Furthermore, fractography and microstructural tests were carried out in order to investigate the influence of reinforcement weight percentage on the failure behaviour of the composites that had been prepared. The results show that with inclusion of ZrO2 (15 wt%) in aluminum 6061 matrix the maximum fracture toughness of 500.83 MPa.mm0.5 was observed. The study performed through FEA highlights the stress phenomena and result are in good agreement with the experimental results.

Investigating fracture toughness of graphene epoxy nanocomposites using single edge notched bending specimens

Abstract The purpose of this study was to examine the fracture toughness of epoxy-based graphene nanocomposites. The single edge notched bending (SENB) samples with precrack tips were created by either tapping or sliding methods. For the tapping method, a single tap was conducted on a razor blade such a crack with a length of 2 mm was instantly propagated. For the sliding method, a razor blade was slid forward and backward into the samples, and a crack was generated with the blade edge. The SENB samples with graphene loadings (0.15 and 0.3 wt%) were subjected to three-point bending tests to evaluate fracture toughness. Results indicated that the graphene loading did not considerably affect the fracture toughness of the samples with tapping-induced precrack tips. However, the fracture toughness of the samples with sliding-induced precrack tips decreased as the graphene loading increased. For the 0.3 wt% graphene nanocomposites, the fracture toughness levels of the samples with sliding- and tapping-induced precrack tips were equal. In order to explicate the experimental results, the plastic zone sizes around the crack tip were evaluated using finite element analysis. For the sharp precrack tips (tapping method), the plastic zone was comparatively small (brittle behavior); hence, the effect of graphene on the inception of crack extension was minimal. Nevertheless, for blunt crack tips (sliding method), the plastic zone was larger and the size decreased as graphene loading increased; thus, the fracture behavior measured from the sliding method was dramatically influenced by the graphene loading.

Analytical and finite element analysis of the concrete stress intensity factor with carbon fibers

Concrete is a material with low tensile strength and fracture resistance. One way to improve these properties is through the insertion of synthetic fibers such as carbon fibers into the concrete. This work determined the stress intensity factor for concrete with aligned carbon fibers (laminate) using an analytical model and finite element methods – macro-mechanical analysis of the composite laminate using the ABAQUS finite element package. Symmetrical laminate models were used with carbon fibers ranging from ± 15° to ± 75°. A single-edge notch bending specimen geometry was used to determine the KI of the laminates. The mechanical properties of the laminate were obtained through analytical models of the laminate theory. Results indicate that the fiber angles influence the stress intensity factor. The behavior is practically linear and a strong correlation was observed between the angle of the fibers and the KI through Pearson’s r. The laminate with a fiber angle of ± 75° showed a higher KI value compared to the others (x14 about ± 15°). Comparisons of the analytical and numerical results showed good agreement in the determined KI.

Effect of residual stress introduced by local loading on fracture behaviour of DMWJ under different constraints

Taking nuclear safety end dissimilar metal welded joint (DMWJ) as the research object, the single edge-notched bend (SENB) specimen with a central crack was selected. Different pre-loads were applied to three local regions of the SENB specimen respectively to generate stresses at crack tip, and these stresses were introduced into the SENB specimen as residual stresses by restart method. The fracture behaviour of DMWJ under various constraints considering residual stress was systematically studied and compared with that under whole loading. The results showed that when the pre-load was located on different local regions, the tensile or compressive stress was generated, and resulted in the decreasing or increasing of J-R curve. The local loading was more consistent with the engineering practice than the whole loading. Compared with the low constraint SENB specimen, the J-R curve of high constraint SENB specimen was more sensitive to the change of residual stress.

Influence of the bi-nonlinearity on the characterization of mode I fracture parameter [formula omitted] for a cracked giant magnetostrictive material in the coupled magneto-elastic field: An experimental and numerical study

Terfenol-D, a giant magnetostrictive material exhibits bi-nonlinear stress-strain relationship in coupled magneto-elastic field. A novel hyperbolic vector generalized magneto-elastic constitutive model is employed to describe this bi-nonlinear stress-strain behaviour. Some physics-based experiments for magnetization and magnetostriction hysteresis loops and mechanical compression tests under the applied magnetic field are conducted. The general HM and stress-strain relationships are programmed in C language to define the coupled field constitutive material model. Coupled magneto-elastic response from the performed finite element analyses are compared with experiments to optimize the material parameters. Fracture experiments employing single edge notch bend (SENB) specimens are conducted with and without external magnetic field as per ASTM E399. The digital image correlation technique is used to capture the load line and crack opening displacement variation with peak fracture load data. Two parameter Weibull statistical theory of strength has been utilised to forecast the mean peak fracture load. The Weibull modulus and goodness of fit are assessed using linear regression analysis (LIN2), biased and unbiased maximum likelihood estimation (MLE2-B & MLE2-U) approach. Finite element crack propagation model based on the optimized nonlinear constitutive relations is built concerning to magnetization and stress dependent elasticity problem of Terfenol-D fracture. Three-dimensional J-integral is formulated considering the effects of magneto-elastic fields, which is subsequently used to characterize the crack front singularity near the fracture process zone. Critical strain energy release rate JIc was calculated using the mean peak fracture load in both absence and presence of the external magnetic field. Finally, the influence of bi-nonlinear modularity and applied external magnetic field on the fracture behaviour of the Terfenol-D SENB fracture specimen were compared with the unimodular experimental results and substantial differences were reported. The stipulation and importance of considering the bi-nonlinear critical strain energy release rate JIc in the context of a coupled magneto-elastic field was discussed certifying the observations.

The study on the influences of residual stresses on fatigue crack propagation in titanium alloy specimens

Fatigue crack growth is a harmful physical phenomenon in engineering materials that can be intensified by the presence of tensile residual stresses. In the present study, the effect of tensile residual stresses on the fatigue crack growth in single-edge notched bending specimens of Ti- 6Al-4V is studied. Mechanical residual stresses were created by applying a 4-point bending process. The residual stresses were evaluated utilizing the hole drilling approach under the ASTM E-837 standard. Fatigue crack propagation was measured by experimental test in specimens with and without initial residual stresses. A finite element analysis was conducted using commercial finite element software to study the plastic zone at the crack tip and fracture mechanic parameters. It was observed that the residual stress field is redistributed after each step of crack propagation. The tensile residual stress in front of the crack tip decreased from near yield strength to approximately 30% of yield strength. The tensile residual stresses near the yield strength in Ti-6Al-4V increased the fatigue crack propagation rate by approximately 50%.

Characterization of the non-isotropic tensile and fracture behavior of unidirectional polylactic acid parts manufactured by material extrusion

This study presents a comprehensive characterization of the fracture and tensile behavior of unidirectional polylactic acid (PLA) parts manufactured by material extrusion (MEX). The effect of the crack tip to filament orientation angle on the fracture behavior was investigated through three-point bending on single-edge notch bending (SENB) specimens printed at seven different build angles. The fracture test results show a brittle behavior for inter-layer fracture and a more ductile behavior for cross-layer crack propagation. Specimens with a 20° build angle required the most energy for the crack to extend completely, exhibiting mixed cross- and inter-layer crack propagation. The elastic properties and the ultimate strength distribution were measured through tensile tests on dogbone specimens printed at five different build angles. The experimental stiffness and strength distributions show good agreement with the orthotropic material model and the Tsai–Hill strength criterion, with a maximum model error of ≈ 10%. The printed mesostructures and the fracture surfaces were observed with scanning electron microscopy (SEM). Finally, differential scanning calorimetry (DSC) was used to evaluate the level of crystallinity in the printed structures. The resulting data allows the development of fracture models able to capture the complete failure behavior of MEX fabricated parts.

Determination and influential factors of fracture toughness in unidirectional laminated bamboo-composite using double cantilever beams and single-edge notched bend tests

This paper studied two testing methods for determining the matrix-dominate fracture toughness in the unidirectional bamboo laminate panels, using double cantilever beam tests and single-edge notched bend tests. For single-edge notched bend tests, an isotropic solution was proposed to calculate the fracture toughness in which only the load-CMOD curve is required. In this isotropic solution, when calculating the energy release rate G Ic by G Ic = (K Ic ) 2 /E, the modulus of elasticity E can be directly obtained from the experimental load-CMOD curves, the pre-determination of Young’s modulus E x , E y , shear modulus G xy and Poisson’s ratio υ xy , υ xy is unnecessary for orthotropic materials. The results of modulus of elasticity E, the stress intensity factor K Ic , and energy release rate G Ic using the isotropic solution were compared to those using an orthotropic solution; it revealed that the isotropic solution could be well applied to unidirectional bamboo laminate panels with errors under 2.7%. The isotropic solution for single-edge notched bend tests was also verified by comparing the results to those obtained from double cantilever beam tests; it turned out that the energy release rate values determined by the two testing methods agreed well with each other. Moreover, the influence of initial notch width, initial notch to depth, and specimen depth on the fracture toughness was investigated to put forward a practical testing method to obtain the size-independent fracture toughness of unidirectional bamboo laminate panels by single-edge notched bend specimens.

Study on fracture behavior of pipeline girth weld based on CTOD-Am method and Gurson ductile damage model

The mechanical properties of the materials in different regions of the pipeline girth weld are sometimes quite different, which will produce a more significant material constraint effect and complicate the fracture behavior of the girth weld. Therefore, this work adopts the finite element method combined with the material damage model to study the ductile tearing process of the weld. The results of the single edge notched bending (SENB) specimens with the weld center crack show that for the welds with different mismatch conditions, with the increase of the weld mismatch coefficient, the crack tip constraint decreases gradually, and the fracture toughness increases gradually. The smaller weld width will make this effect more significant. When the ratio of the weld width to the specimen width is larger than or equal to 0.43, the BM will basically no longer affect the crack tip constraint and the fracture toughness of the weld. The results of the SENB specimens with the interface crack show that for the overmatched weld, with the increase of the weld mismatch coefficient, the crack first grows along the interface and then deflects into the heat affected zone (base metal). When the weld mismatch coefficients are 1.44 and 1.86, the corresponding deflection angles are 14.84° and 24.61°. For undermatched weld, with the decrease of the weld mismatch coefficient, the crack growth will deflect into the weld metal. When the weld mismatch coefficients are 0.69 and 0.6, the corresponding crack growth deflection angles are 14.04° and 25.36°. The larger the degree of the weld mismatch, the more significant the deflection of the distribution of the equivalent plastic strain and the stress triaxiality ahead of the crack tip. The effect of the specimen thickness on the deflection angle of the interface crack growth is small. The verification shows that the numerical analysis results of this study are valid.

Fracture toughness and fatigue crack growth rate studies on rotary friction weldments of nickel-based superalloys

In this paper, fracture toughness (FT) and fatigue crack growth rate (FCGR) studies were carried out on dissimilar metal welded joints (DMWJs) of Inconel 718 and Nimonic 80A. The DMWJs were made using rotary friction welding (RFW). The base metals (BMs) showed plastic mismatch. Single edge notched bend specimen was used for FT and FCGR studies. The crack was initiated and propagated along the weld. BMs showed higher toughness than the DMWJs. The crack growth resistance offered by the DMWJs were inferior to BMs. Post-weld heat-treated DMWJ showed improved FT and crack growth resistance than the untreated DMWJ.

A finite element analysis approach to predict constraint benefits to ductile tearing initiation toughness

This paper provides a novel approach to define the parameters to derive effective ductile tearing initiation toughness curves for a range of materials under low constraint conditions, using finite element analysis alone. This eliminates the need for extensive low constraint testing programmes. The curves are provided for four nuclear steam raising plant materials, and a generic, lower-bound curve is also defined. The lower bound curve would allow an increased fracture toughness to be used in assessments where defects could be shown to be at low in-plane constraint (Q < 0), and within the bounds of the materials examined here.The curves have been calculated by defining the loads, quantified by the J integral, at which areas of material ahead of the crack tip, in 2D Single Edge Notched Bend specimen finite element models, enclosed by contours defined by a critical void size, are equal. By determining the load at high constraint at which the contour encloses a specific area, and obtaining the load at which the area is equal at low constraint, it has been possible to determine the toughness curves by assuming failure occurs when the areas enclosed by the contours are equal.It is shown that the shape of the toughness curve is largely independent to the critical void size ratio, defined using the Rice and Tracey parameter, and the value used for the high constraint toughness. This provides confidence in the method, alongside comparison against relevant initiation toughness test data, and similar modelling incorporating the GTN local approach.

Effect of the manufacturing parameters on the tensile and fracture properties of FDM 3D-printed PLA specimens

Fused Deposition Modeling (FDM), also known as material extrusion, is currently one of the most popular Additive Manufacturing (AM) technology on the market. In this study, two commercial FDM printers (Prusa i3 MK3 and WN400 3D Platform) were used for layer-by-layer manufacturing of polylactic acid (PLA) Dog Bone (DB) and Single Edge Notched Bend (SENB) specimens, aimed at investigating the influence of the manufacturing parameters on the tensile and fracture properties of PLA elements obtained by FDM technology. The effects of growing direction (horizontal and vertical), building orientation (0°, 45° and 90°), printer type, layer thickness (0.15 and 0.40 mm), specimen thickness (4 and 10 mm) and filament color (purple, white, black, gray, red, orange) are discussed in detail. Tensile tests were performed on DB specimens, while fracture mechanics tests on SENB specimens. Both the tensile and fracture properties of FDM 3D-printed PLA specimens have been found to be dependent on the investigated manufacturing parameters. From the microstructural analyses of the SENB fracture surfaces, it has been observed that the fracture mechanisms and crack propagation is a step-wise process. Finally, the material properties charts (Young’s modulus and mode I fracture toughness versus tensile strength) are plotted.