Notched Tension Tests Research Articles

Critical analysis of using recycled wax oils in asphalt binder considering exudation effects

The tendency of asphalt to exude oil under certain conditions is often overlooked as a form of aging in asphalt pavement. To better understand the issue of oil exudation in asphalt during long term cold storage, two different oil sources of asphalt binders with recycled wax oils were investigated using the extended bending beam rheometer test (Ex-BBR), double edge notched tension test (DENT), extended Hansen solubility model, and fluorescence and polarizing microscopy. The results of the Ex-BBR and DENT tests demonstrate that the addition of recycled wax oils significantly enhances the low-temperature performance of asphalt. However, the use of fluorescing markers in cooling media indicates that recycled wax oil exudation can lead to ethanol diffusion in the asphalt binder. To further elucidate the factors influencing exudation, a device for accelerating the exudation of asphalt was developed. The research results show that, in addition to the intrinsic properties of asphalt and recycled wax oils, pressure, time, and temperature significantly influence the acceleration of exudation. The primary changes in recycled wax oil exudation occur early on, with the exudation rate decreasing over time. The fluorescent microscope reveals the separation of recycled wax oils as a fluorescent ring. Recycled wax oils alter the internal stability of asphalt, as evidenced by an increase in polarity and hydrogen bonding. This change is expected to impact the compatibility of asphalt binders. The mechanism of these interactions varies significantly depending on the source of the asphalt binder. After accelerated exudation, the three interaction forces representing sensitivity to chemical environments are further reduced.

On the suitability of single-edge notch tension (SENT) testing for assessing hydrogen-assisted cracking susceptibility

Combined experiments and computational modelling are used to increase understanding of the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility. The SENT tests were designed to provide the mode I threshold stress intensity factor (Kth) for hydrogen-assisted cracking of a C110 steel in two corrosive environments. These were accompanied by hydrogen permeation experiments to relate the environments to the absorbed hydrogen concentrations. A coupled phase-field-based deformation–diffusion-fracture model is then employed to simulate the SENT tests, predicting Kth in good agreement with the experimental results and providing insights into the hydrogen absorption–diffusion–cracking interactions. The suitability of SENT testing and its optimal characteristics (e.g., test duration) are discussed in terms of the various simultaneous active time-dependent phenomena, triaxiality dependencies, and regimes of hydrogen embrittlement susceptibility.

Development of a strain-based fracture assessment approach for X80 steel pipe welded girth by modified Mohr-Coulomb model

In the study, a strain-based fracture assessment scheme with the Modified Mohr-Coulomb (MMC) model is established to numerically characterize the tensile strain capacity (TSC) of X80 steel pipe welded girth. The TSC at crack initiation and local instability are predicted based on a lode-dependent MMC model accounting for crack onset and propagation phases. The combined experimental-numerical calibration of the MMC criterion for crack onset is undertaken for X80 pipeline steel, where seven geometrical specimens are designated to represent various stress states from low to high triaxiality, and the parameter determination of the MMC criterion for crack growth is carried on by fitting the single-edge notched tension tests. Finally, the comparison between the present work and PRCI and ExxonMobil in terms of TSC is performed. A relatively good agreement can be achieved, whereas PRCI gives non-conservative predictions of TSC at crack initiation when crack depth a/t is longer than 0.4 and ExxonMobil gives conservative predictions of TSC at local instability when internal pressure is small.

Fracture toughness determination for the surface-modified layers of carburized 18CrNiMo7-6 alloy steel via the compact tension method

Anti-fatigue manufacturing technology is the latest manufacturing technology to control surface integrity and modification. Surface modification techniques have been used to construct surface-modified layer (SMLs) on the workpiece surface. In this study, to accurately characterize and measure the fracture toughness of SMLs, a characterization method was proposed for evaluating the J-integral of fracture toughness of SMLs. The elastic–plastic parameters of the SMLs, such as the strain hardening index and yield strength, were obtained via peeling-layer tensile testing. The effect of n was considered when deriving the plasticity factor (ηp). The J-integral of the SML of 18CrNiMo7-6 alloy steel was obtained from single edge notched tension tests and finite element simulations. The results show that the fracture toughness of the SMLs can be accurately characterized using the proposed method.

A phase-field model for thermo-elastic fracture in quasicrystals

In this paper, a thermo-elastic phase-field model for brittle fracture in quasicrystals is developed. The kind of quasicrystals is not preset, so this model is suitable for almost all thermo-elastic quasicrystal solids. The phonon force and thermal load trigger the variation of the phonon elastic energy, and then drive crack initiation and propagation. The thermal conductivity in the cracked region is assumed to be degraded, which means cracks are adiabatic. For the static penny-shaped crack problem, the results based on the present model agree well with the existing analytical solution. For the quasi-static crack problems, both the tensile/shear fracture in a thermal environment and the thermal shock cracking failure can be dealt with by the present model. Several examples of 1D hexagonal and 2D decagonal quasicrystals are performed to evaluate the effects of thermal load and phason elastic field. The simulation results show that the thermal load can influence the peak force and fracture displacement. For the single-edge notched tension test, the effect of the thermal load on the peak force is small, but it on the fracture displacement is obvious. For the single-edge notched shear test, the thermal load has an obvious influence on both the peak force and fracture displacement. For the thermal shock cracking problem, the phason elastic field has a significant effect on the cracking time and crack number. The present model can serve as a powerful tool to simulate various thermo-elastic fracture problems in quasicrystals.

A reformulated non-ordinary state-based peridynamic method for dynamic failure of ductile materials

A non-local correspondence peridynamic model is reformulated to describe the dynamic failure of ductile materials, under the framework of non-ordinary state-based peridynamics. To eliminate the zero-energy mode as well as to improve the numerical stability, the deformation gradient is updated on the bond level, which owns information including both the non-local deformation and the bond deviation. Peridynamic thermo-viscoplastic constitutive formulations are presented for the failure analyses of ductile materials. Moreover, a mixed strain-stress criterion describing the bond breaking and considering the temperature effect is presented on the bond level, in which the bonds can keep intact in calculations. Several comprehensive cases, including a notched tension test and Kalthoff-Winkler impact tests with different velocities, are investigated to verify the applicability of the proposed model for ductile fracture and impact failure. Numerical results, in terms of failure patterns, cracking switching and propagation, as well as adiabatic shear band propagation, show a reasonable agreement with corresponding experimental observations.

A new shape optimization approach for fracture propagation

AbstractWithin this work, we present a novel approach to fracture simulations based on shape optimization techniques. Contrary to widely‐used phase‐field approaches in literature the proposed method does not require a specified ‘length‐scale’ parameter defining the diffused interface region of the phase‐field. We provide the formulation and discuss the used solution approach. We conclude with some numerical comparisons with well‐established single‐edge notch tension and shear tests.

Validation of material models for puncture of 7075-T651 aluminum plate

Plate puncture simulations are challenging computational tasks that require advanced material models including high strain rate and thermal-mechanical effects on both deformation and failure, plus finite element techniques capable of representing large deformations and material failure. The focus of this work is on the material issues, which require large sets of experiments, flexible material models and challenging calibration procedures. This work considers the puncture of 12.7 mm thick, 7075-T651 aluminum alloy plates by a cylindrical punch with a hemispherical nose and diameter of 12.7 mm. The plasticity and ductile failure models were isotropic with calibration data obtained from uniaxial tension tests at different temperatures and strain rates plus quasi-static notched tension tests and shear-dominated tests described here. Sixteen puncture experiments were conducted to identify the threshold penetration energy, mode of puncture and punch acceleration during impact. The punch was mounted on a 139 kg mass and dropped on the plates with different impact speeds. Since the mass was the same in all tests, the quantity of interest was the impact speed. The axis and velocity of the punch were perpendicular to the plate surface. The mean threshold punch speed was 3.05 m/s, and the mode of failure was plugging by thermal-mechanical shear banding accompanied by scabbing fragments. Application of the material models in simulations of the tests yielded accurate estimates of the threshold puncture speed and of the mode of failure. Time histories of the punch acceleration compared well between simulation and test. Remarkably, the success of the simulations occurred in spite of even the smallest element used being larger than the width of the shear bands.

A Critical Evaluation of Forming Limit Curves Regarding Layout of Bending Processes

Forming limit curves (FLCs) are important to predict failure against biaxial deformation in sheet metal forming. Particularly crucial is the detection and evaluation of the stable/instable transition which indicates necking and ultimately leads to rupture. In the past, it has been observed that specific processes, in particular free-form bending processes, might not be predicted well by conventionally evaluated FLCs, i.e. the Nakajima experiment, where tapered sheet metal specimens are stretched over a hemispherical punch until material failure. In the present study, FLCs are evaluated from such Nakajima tests and from notched tension tests (NTT) highlighting large differences in between both results. The differences in between the evaluation methodologies are discussed with respect to contact and bending strain in the forming zone. The FLCs of three tested sheet metal materials are compared to fracture strain resulting from an incremental bending process with free forming zone demonstrating an increased failure prediction accuracy by the NTT FLCs than by the conventional ones. In the light of these results, it is therefore encouraged to critically assess the application of FLCs obtained from NTTs to various free-form bending processes.

Aluminium plates with geometrical defects subjected to low-velocity impact: Experiments and simulations

In this paper, an experimental and numerical program investigating impact in the low-velocity regime (≤10 m/s) on 1.5 mm thick AA6016 aluminium plates in three different heat treatments (T4/T6/T7) is presented. The tests were carried out in a drop tower where the force was continuously monitored by a strain-gauge instrumented striker. Results from both intact plates and plates with geometrical defects in the form of pre-cut slits are presented. The heat treatments result in materials with different strength, work-hardening and ductility. It was observed that the intact plates in high-strength material are more resilient to low-velocity impact whereas high ductility is preferred for the plates with pre-cut slits as the crack growth is more restrained. Quasi-static tests on the same plates showed a significant reduction in force only for plates in temper T6 and T7. The experiments are complemented by nonlinear finite element simulations using linear brick elements in Abaqus/Explicit. A von Mises plasticity model was calibrated from notched tension tests. The global response from single edge notch tension (SENT) tests was used to calibrate the failure criterion. The numerical model was able to sufficiently predict the force response and deformation of the intact plates, and the global response and crack propagation of the SENT tests. While the force response in the dynamic simulation of low-velocity impact on plates with slits was in agreement with the observed quasi-static response for all tempers, it was in general underestimated for temper T6 and T7 when compared to the data from the dynamic experiments. For plates in temper T4, the predicted response also agreed with the dynamic experiments.

Evaluating cracking resistance of nano-hydrated lime treated asphalt mastic using work of fracture

ABSTRACT The present study aimed to understand the impact of inert-active filler combinations on cracking and resistance of asphalt mastic using work of fracture approach. A control grade asphalt binder (VG-30) and three fillers (inert: basalt (B); active: hydrated lime (HL) Nano hydrated lime (NHL)) were considered. The HL and NHL were added from 0% to 20% at 5% increment rate by weight of binder. The dosages of B were adjusted with respect to specific filler to binder ratios (0.6 to 1.2). The study focused on the influence of NHL on cracking resistance of mastic by means of ductile and fracture behaviour, evaluated using double edge notched tension test (DENT) at intermediate temperature condition. Findings indicated a beneficial impact in improving the cracking and fracture resistance of mastic due to reduction in the size of HL (NHL). The results signify that both HL and NHL can enhance the resistance to withstand ductile failure and fracture. Also, mastic prepared with B-NHL can be less susceptible to fracture over B-HL and hence, possessed improved ductile properties. Consequently, a lower dosage of NHL found to be superior over a higher dosage of HL in improving the cracking and fracture resistance of asphalt mastic.

Assessment of four strain energy decomposition methods for phase field fracture models using quasi-static and dynamic benchmark cases

Strain energy decomposition methods in phase field fracture models separate strain energy that contributes to fracture from that which does not. However, various decomposition methods have been proposed in the literature, and it can be difficult to determine an appropriate method for a given problem. The goal of this work is to facilitate the choice of strain decomposition method by assessing the performance of three existing methods (spectral decomposition of the stress or the strain and deviatoric decomposition of the strain) and one new method (deviatoric decomposition of the stress) with several benchmark problems. In each benchmark problem, we compare the performance of the four methods using both qualitative and quantitative metrics. In the first benchmark, we compare the predicted mechanical behavior of cracked material. We then use four quasi-static benchmark cases: a single edge notched tension test, a single edge notched shear test, a three-point bending test, and a L-shaped panel test. Finally, we use two dynamic benchmark cases: a dynamic tensile fracture test and a dynamic shear fracture test. All four methods perform well in tension, the two spectral methods perform better in compression and with mixed mode (though the stress spectral method performs the best), and all the methods show minor issues in at least one of the shear cases. In general, whether the strain or the stress is decomposed does not have a significant impact on the predicted behavior.

Plasticity and fracture behavior of Inconel 625 manufactured by laser powder bed fusion: Comparison between as-built and stress relieved conditions

In this study, the influence of stress relief on the plasticity and fracture behavior of Inconel 625 fabricated through laser powder bed fusion additive manufacturing (AM) was investigated. The as-built versus stress relieved microstructures were compared, showing similar grain structures but the presence of ~10 vol % δ phase in the stress relieved condition, and no δ phase in the as-built condition. Mechanical tests under plane strain tension were performed on the stress relieved samples, and an anisotropic plasticity model was calibrated and validated using finite element simulations. Uniaxial and notched tension tests were performed on both as-built and stress relieved samples to probe the effect of stress relief on stress state- and direction-dependent fracture behavior. It was found that on average, the fracture strain of the stress relieved samples along the build direction was 30% higher than that along the perpendicular build direction in the stress state range studied, and the stress relief heat treatment resulted in a 45% decrease in fracture strain. The fracture strain in stress relieved samples was more strongly dependent on stress state than in as-built samples.

RETRACTED ARTICLE: Influence of crumb rubber particle sizes on rutting, low temperature cracking, fracture, and bond strength properties of asphalt binder

The present study aimed at evaluating the effect of Crumb Rubber (CR) particle size on rutting, low temperature cracking, fracture, and bond strength properties of asphalt binder. Three different CR particle sizes (#10–20, #30–40, and #60–80) were considered in this research work. Rutting performance was evaluated using Multiple Stress Creep Recovery (MSCR) tests. Although the addition of CR was found to improve the rutting performance, asphalt binder combination with higher CR particle size exhibited better performance compared to asphalt binder combination with lower CR particle size. Similarly, low temperature cracking properties were evaluated using Bending Beam Rheometer (BBR). It was found that the addition of CR has positive impact on the low-temperature performance of asphalt binder. Fracture property was evaluated using Double Edge Notch Tension (DENT) test and found to be improving with the increase in CR particle size. Finally, the effect of CR particle size on moisture damage resistivity potential was evaluated using the recently developed Bitumen bond strength (BBS) test. Although the addition of CR was found to have deteriorating impact on bond strength behavior, the degree of impact was found to be lower for asphalt binder containing the lowest considered CR particle size.

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.

Micromechanical modelling of ductile fracture in pipeline steel using a bifurcation-enriched porous plasticity model

In this paper, we investigate the possibility of predicting ductile fracture of pipeline steel by using the Gurson–Tvergaard–Needleman (GTN) model where the onset of void coalescence is determined based on in situ bifurcation analyses. To this end, three variants of the GTN model, one of which includes in situ bifurcation, are calibrated for a pipeline steel grade X65 using uniaxial and notch tension tests. Then plane-strain tension tests and Kahn tear tests of the same material are used for assessment of the credibility of the three models. Explicit finite element simulations are carried out for all tests using the three variants of the GTN model, and the results are compared to the experimental data. The capability of the simulation models to capture onset of fracture and crack propagation in the pipeline steel is evaluated. It is found that the use of in situ bifurcation as a criterion for onset of void coalescence in each element makes the GTN model easier to calibrate with less free parameters, all the while obtaining similar or even better predictions as other widely used formulations of the GTN model over a wide range of different stress states.

A stability-enhanced peridynamic element to couple non-ordinary state-based peridynamics with finite element method for fracture analysis

The non-ordinary state-based peridynamics (NSPD) is a promising method for fracture analysis, and it can incorporate the constitutive relationship of classical continuum mechanics in peridynamics. However, the high computational cost is one of the main reasons limiting its usage. To improve computational efficiency of NSPD, a stability-enhanced peridynamic (PD) element is proposed to couple NSPD with finite element method (FEM), primarily for fracture analysis. Only the areas where cracks may initiate or propagate are solved with the proposed PD element, while the rest of the model is composed of conventional finite elements. The main feature of the proposed PD element is attributed to that the bond interaction does not have to be parallel to the bond direction in NSPD, and the stability of the element is enhanced by introducing the improved hourglass method into the element to restrain the instability due to the zero-energy mode. Based on the equation of motion of NSPD and the principle of virtual work of the PD element, the stiffness matrix of the PD element is derived and the coupled NSPD and FEM method with a global stiffness matrix is thus established. Meanwhile, a two-step interface correction method is developed to improve the accuracy at the interface of the two domains. Subsequently, to verify the proposed coupling method, the uniaxial tension of a strip plate, the propagation of longitudinal wave, the compact tension test of a steel plate, and the single edge notched tension test of an orthotropic lamina are simulated. With the proposed coupling method, the fracture process is accurately simulated, while the computing time is significantly reduced by up to more than 80% when compared with the one using NSPD alone. Moreover, with the constitutive relationship represented by the elastic tensor, the orthotropic materials can be easily modeled in the proposed PD element, and the stress-based criteria can be used with minor modifications. The proposed coupled NSPD and FEM method has high accuracy, computational efficiency, and convenience, and it has the potential to be a useful tool for quantitative analysis of complex engineering fracture problems.

Calibration of the modified Mohr-Coulomb fracture model by use of localization analyses for three tempers of an AA6016 aluminium alloy

This paper presents a novel calibration procedure of the modified Mohr-Coulomb (MMC) fracture model by use of localization analyses and applies it for three tempers of an AA6016 aluminium alloy. The localization analyses employ the imperfection band approach, where metal plasticity is assigned outside the band and porous plasticity is assigned inside the band. Ductile failure is thus assumed to occur when the deformation localizes into a narrow band. The metal plasticity model is calibrated from notch tension tests using inverse finite element modelling. The porous plasticity model is calibrated by use of localization analyses where the deformation histories from finite element simulations of notch and plane-strain tension tests are prescribed as boundary conditions. Subsequently, localization analyses are used to establish the failure locus in stress space for proportional loading conditions and thus to determine the parameters of the MMC fracture model. Finite element simulations of notch tension and in-plane simple shear tests as well as two load cases of the modified Arcan test are used to validate the calibrated fracture model. The predictions by the simulations are in good agreement with the experiments, even though some deviations are seen for each temper. The results demonstrate that localization analyses are a cost-effective and reliable tool for predicting ductile failure, reducing the number of mechanical tests required to calibrate the MMC fracture model compared to the hybrid experimental-numerical approach usually applied.

Ductile fracture prediction of AA6061-T6 in roll forming process

The present study evaluates the fracture behavior of the AA6061-T6 aluminum alloy during the roll forming process using ductile fracture criteria. Three criteria including Ayada, Rice-Tracey, and normalized Cockroft-Latham were utilized to develop an accurate fracture model for predicting fracture onset during the roll forming process. To this end, the fracture criteria were calibrated using uniaxial, plane-strain, and notched tension tests. The model was developed by integrating the calibrated fracture criteria into the numerical analysis (commercial finite element code Abaqus/Explicit) through an appropriate user subroutine. Based on the results, the damage weighting function and calibration procedures had a significant effect on the accuracy of the fracture prediction during the process. When the uniaxial tension test was used as the calibration test, the normalized Cockroft-Latham and Rice-Tracey ductile fracture criteria were not able to predict the fracture onset during the roll forming process and the Ayada fracture criterion could only predict the bending angle of the forming station where the fracture initiated. On the other hand, calibrating ductile fracture criteria using the plane-strain tension test led to higher accuracy in predicting fracture. The most accurate prediction was obtained using the Ayada fracture criterion calibrated by the plane-strain tension test with an error of 7.85%. Overall, the Ayada fracture criterion and plane-strain tension test can be considered as the most appropriate ductile fracture criterion and calibration method for predicting fracture in the roll forming process, respectively.

Modeling plasticity of an aluminum 2024T351 thick rolled plate for cold forming applications

Sheet metals exhibits plastic anisotropy due to the rolling process. This article focuses on the particular case of an 2024T351 Aluminium alloy thick sheet which showed in addition to the anisotropy of mechanical properties an important evolution of the these properties along the thickness. A large series of tension tests performed is carried out using samples machined varying the position along the thickness and the orientation in the rolling plane. Strong variation in terms of elastic limit and ultimate tensile stress are observed. The heterogeneity of the miscrostructure is studied by EBSD along the plate thickness and correlated with the mechanical properties variations; 4 main zones are identified. The constitutive behaviour of the material is model using an anisotropic yield criterion combined to a non-linear hardening rule and one set of parameters is identified for each material layer. The predictive capabilities of the model are illustrated by simulations of notched tension tests with three notch radii. Finally a large structure panel bending test representative of cold forming industrial operations is simulated using the identified model parameters. A very good agreement between the FE results and the experimental strain gauge signals from a scale 1 experiment is observed. This shows the thickness-based identification accurately account for the property gradient for large structure panel simulations.