Notched Tension Tests Research Articles

Crack propagation and burst pressure of longitudinally cracked pipelines using extended finite element method

This paper presents an investigation of the potential of extended finite element method (XFEM) implemented in Abaqus standard software, with maximum principal strain and fracture energy as damage parameters for analysis of crack propagation and prediction of burst pressure in pipelines. Models of API 5L X60 pipe were calibrated first, using data from single edge notched tension test and then using burst pressures from 3 full scale tests. Both calibrations produced low fracture energy and marginally varied strains as XFEM damage parameters. The models were used to predict burst pressures of pipe specimens having varying sizes of longitudinal cracks that were rectangular in profile and located on the external surface of the pipes. Burst pressures were also predicted using analytical methods and the results compared with tests and XFEM predictions. All analytical methods accurately predicted burst pressures of specimens with shallow crack depth but were inaccurate for deeper cracks. XFEM predictions were satisfactory for all specimens. Although few cracks were analysed, the results show XFEM can be effective for analysis of crack propagation and prediction of burst pressure in pipelines, but investigation using more crack sizes, pipe grades and pipe geometries is recommended for a firm conclusion.

Effects of warm mix additives on the thermal stress and ductile resistance of asphalt binders

The application of warm mix additives to asphalt was studied to analyze their influence on the low-temperature fracture performance of asphalt materials. To this end, a set of warm mix asphalt binders (WMABs) were prepared by mixing a hot mix asphalt binder (HMAB), PG 64-28 with two different types of warm mix additives (one being Sasobit and the other ET-3100). A bending beam rheometer test and a double edge notched tension test were performed to characterize the thermal stress and ductile resistance of the HMAB and WMABs, respectively. Fourier transform infrared spectroscopy (FTIR) was employed to study the modification mechanism during the application of the warm mix additives. The test results showed that the addition of Sasobit deteriorates the low-temperature fracture properties of the asphalt, with higher thermal stress, linearly increasing critical cracking temperature, and lower crack tip opening displacement. ET-3100 showed little effect on the low-temperature fracture properties of the bitumen, regardless of the added amount. The FTIR results showed that there was basically no chemical reaction in the modification process and both types of warm mix additives were physically modified.

Effect of recycled asphalt binder on high and intermediate temperature performance of polymer modified asphalt binder

The present study aimed at investigating the effect of Recycled Asphalt Pavement (RAP) binder on high and intermediate temperature performance of Polymer Modified Binders (PMB) through advance laboratory characterization techniques. Different RAP binder proportions (0%, 15%, 25% and 40% by the weight of PMB) were initially blended with PMB. Different characterization techniques such as Superpave rutting parameter (G*/Sinδ), Shenoy parameter, Multiple Stress Creep Recovery (MSCR) and Zero Shear Viscosity (ZSV) tests were used for evaluating high temperature performance. Based on the different rheological parameters, improvement in rutting performance was observed with the addition of RAP binder. Likewise, intermediate temperature performance was evaluated using Linear Amplitude Sweep (LAS), Double Edge Notch Tension (DENT) and Elastic Recovery (ER) test. The improvement in high temperature performance and degradation in intermediate temperature performance shows that care must be taken while selecting the appropriate amount of RAP based on prevalent failure type in the field.

Influence of Basalt, Hydrated Lime, and Fly-Ash Fillers on Aging Behavior of Asphalt Mastic Using Essential and Plastic Work of Fracture

Abstract The present study was undertaken to investigate the influence of Basalt (B), Hydrated Lime (HL), and Fly-Ash (FA) fillers on aging behavior based on the fracture properties of asphalt mastic. The asphalt mastic samples were prepared in the laboratory using an AC-30 asphalt binder with various combinations of B-HL and B-FA fillers by maintaining a constant filler-binder ratio of 0.8:1 (by mass ratio and 22.5 % by volume). The HL and FA contents were selected as 0 %, 5 %, 10 %, 15 %, and 20 % by the weight of the asphalt binder, and the content of B filler was adjusted accordingly. Correspondingly, three types of asphalt mastic samples, namely, unaged, short-term aged, and long-term aged were prepared in the present study to understand the effects of oxidative aging. Fracture properties of asphalt mastic samples were evaluated at an intermediate temperature using a Double Edge Notched Tension test. The analysis of fracture behavior includes the load-displacement curve pattern, Total Work of Fracture (TWF), Essential Work of Fracture (EWF), Plastic Work of Fracture (PWF), and Critical Tip Opening Displacement (CTOD) values. The results showed that nature, surface texture, mineralogy, and interaction ability of HL and FA fillers strongly influence the aging and fracture resistance of the asphalt mastic. The load-displacement curve pattern, TWF, and PWF values signify a clear indication of an increase in ductile behavior with the incorporation of HL or FA fillers. Similarly, the EWF and CTOD values showed that the inclusion of HL or FA fillers in the asphalt mastic enhanced the aging and fracture resistance behavior. The inclusion of the HL filler has a predominant impact in comparison to the FA filler in the enhancement of both the aging and fracture resistance of asphalt mastic.

Characterization and Control of the Compromise Between Tensile Properties and Fracture Toughness in a Quenched and Partitioned Steel

The enhancement of the fracture toughness is essential for opening the possible range of applications of advanced high-strength steels, while the focus in the literature is primarily on the strength–ductility compromise. A high fracture toughness is indeed needed for energy absorbing components as well as to limit edge cracking sensitivity during part forming. This study investigates the tensile properties and the fracture toughness of various quenched and partitioned microstructures. The fracture resistance is evaluated using double-edge notched tension tests. While the uniform elongation continuously increases with the retained austenite (RA) fraction, the fracture toughness shows a maximum at intermediate RA content. For the highest amount of RA, the relatively low fracture toughness is mainly attributed to the formation of brittle necklace of fresh blocky martensite in the fracture process zone due to a high stress triaxiality, inducing an intergranular fracture mode. For intermediate RA fraction, the RA morphology evolves from blocky to film type, leading to a transition from intergranular to ductile fracture mode, and the RA-to-martensite transformation contributes to a higher total work of fracture compared to tempered martensitic steel. A proper control of both the amount and morphology of RA during microstructure design is thus essential to generate the best compromise between tensile properties and fracture toughness.

Consideration of a Stress-Based Criterion for Local Failure and Its Implementation in a Damage Mechanics Model

Analyses of the notched tension tests of carbon steel show that ductile failure is initiated when the sum of flow stress and mean stress reaches the limit for the material, regardless of stress triaxiality. Therefore, this explicit critical stress condition could be a candidate criterion for local failure. An equation expressing the relation between stress triaxiality and critical strain was derived from the critical stress condition, and it was found that the critical strain diagram obtained by the equation nearly overlapped with that obtained by the conventional empirical equation. This suggests that the critical stress condition can be approximately determined if the critical strain diagram was obtained for a particular steel. The critical stress condition was consistent with the classical void nucleation theory, and the theory was incorporated into the void nucleation term of stress control in the Gurson–Tvergaard (GT) model—a well-known damage mechanics model for ductile failure. Since only the strain-controlled term is used in the recent GT model, herein, a finite element method (FEM) code was newly developed to implement the GT model with the stress-controlled term. Notched tension tests were analyzed with the critical stress condition using the developed code, and the analyses reproduced the failure behaviors and critical strains of the tests considerably well. These results strongly support the practicality of the stress-based criterion and demonstrate that ductile failure could be appropriately predicted by combining the GT model using the void nucleation term of stress control with the critical stress condition.

Investigation on the fusion zone microstructures and mechanical integrity of AISI 904L and Inconel 625 weld joints

The effect of over-alloyed Ni-rich fillers on the weld mechanical properties of AISI 904L and Inconel 625 dissimilar joints achieved by pulsating current approach in gas tungsten arc welding (GTAW) is analysed in the present investigation. Micro-segregation and the migrated grain boundaries are noticed on the jointed zones of ERNiCrMo-4 and ERNiCrCoMo-1 respectively. The columnar dendrites near to the fusion line and the fine equiaxed dendritic structures at the centreline are witnessed in the weld regions of both the joints. Tension failures have occurred in the AISI 904L base metal. The notch tension test inferred that the weld strength is higher than the weak parent metal (AISI 904L). The impact toughness was considerably declined sharply compared to the parent metals. The notch tension and impact tests recommend the use of ERNiCrMo-4 filler for joining these dissimilar metals. The research findings based on the relationship between structure-property drawn from this investigation can be employed in the industries requiring the use of the novel joints.

Critical strain energy release rate for rubbers: single edge notch tension versus pure shear tests

In order to estimate mode I fracture strain energy release rate of a rubber upon monotonic loadings, the material is submitted to pure shear and single edge notch tension tests. Catastrophic failure happens suddenly for both tests, revealing mirror-like crack surfaces, assessing the fragile fracture. Nonetheless, Griffith failure analysis could be carried out on pure shear tests only. This analysis leads to an energy release rate value that allows challenging approximate expressions existing in the literature for pure shear and single edge notch tension tests. The pure shear approximate expression provides quantities that match the Griffith analysis. Meanwhile, the strain energy release rate values calculated directly from the single edge notch tension tests differ significantly from the values obtained in pure shear. This discrepancy is explored and possible explanations are discussed showing that pure shear tests should be favored.

Determination of CMOD-force curves and R-curves in side-grooved single edge notched tensile (SENT) specimens in welded X42 pipeline steel

A comprehensive experimental project including tension and single edge notch tension (SENT) tests was carried out on 22 inch OD X42 welded pipeline steel to determine the mechanical properties and fracture toughness of the base and weld materials. In addition to traditional clip gauge measurements, digital image correlation (DIC) methodology was applied. Crack tip opening displacement (CTOD)-crack growth and crack mouth opening displacement (CMOD)-force curves were generated for SENT specimens subjected to monotonic tension loading using a multiple specimen technique. Significant differences were observed in the behavior of base metal and weld specimens. The evidence of fracture resistance measured from the weld metal specimens being lower than that from base metal specimens is important with respect to avoiding overestimation of R-curves in integrity assessment procedures. Additionally, the weld metal tolerated more force when the CMOD was less than 0.9 mm but the base metal tolerated more force after this point, while both CMOD-force curves reached a similar value of maximum force.

Notch insensitive orientation-dispersed pseudo-ductile thin-ply carbon/glass hybrid laminates

Notch sensitivity, free edge delamination and brittle failure are limiting factors for the wider use of conventional composite laminates. In our previous study, a hybrid layup concept with the different materials blocked together but with dispersed orientations was successfully used to design pseudo-ductile hybrid composites with no free-edge delamination. This study introduces a comprehensive set of designed and characterised orientation-dispersed pseudo-ductile thin-ply hybrid composites to address notch sensitivity, another important limiting factor in conventional composite laminates. Un-notched, open-hole and sharp notched tension tests were performed on three different thin-ply carbon/glass hybrid configurations. The investigated laminates showed a successful pseudo-ductile un-notched behaviour with improved notch-insensitivity and suppression of free-edge delamination that was an undesirable damage mode in previously investigated hybrids with plies of the same orientation blocked together. This notch insensitivity results from subcritical damage in the laminates due to the pseudo-ductile damage mechanisms, i.e. dispersed delamination and fragmentation. These damage mechanisms can eliminate stress concentrations near the notch and suppress the conventional damage mechanisms that govern the notched response of the laminates.

Toward Utilization of ground tire rubber and reclaimed pavement materials with asphalt Binder: Performance evaluation using essential work of fracture

Utilization of ground tire rubber (GTR) binder in combination with reclaimed asphalt pavements (RAP) binder can provide added benefits in terms of material, cost and environmental saving, and hence a promising step toward sustainable construction of pavements. The present study utilized essential work of fracture (EWF) principal to evaluate performance of a GTR binder blended with two different types of RAPs: RAP-A and RAP-S, having different rheological properties. The GTR binder was mixed with different percentages (0%, 15%, 25% and 40% by weight of virgin binder) of RAP-A and RAP-S. First, Superpave performance grade (PG) of different combinations of asphalt binders was determined in the laboratory. Thereafter, the EWF of binders was determined using a newly developed double edge notch tension (DENT) test. Further, the data obtained from the DENT test were used to estimate critical crack tip opening displacement (CTOD). The results showed that addition of RAP decreased fracture resistance of GTR binder. Addition of RAP-A resulted in a faster decrease in performance of GTR binder compared to RAP-S. It can be concluded that low percentage of RAP may be utilized with GTR binder. Further, analyses indicated that RAP sources and its quantity could have significant impact on performance of asphalt mixes.

Influence of fibres on rheological properties and toughness of bituminous binder

Many studies have been made to examine different ways to modify bitumen and asphalt mixtures in response to increasing performance requirements. One of these is the use of additive materials and in this study, the potential of cellulose and glass fibres to modify the rheological properties of bitumen has been investigated. To achieve this, mixtures of bitumen with different contents of fibre were prepared and the properties of the bitumen and resulting modified binders were tested (penetration, softening point, viscosity and double edge notch tension test along with rheological testing in the dynamic shear rheometer). The experimental results demonstrated that adding fibres improves the rheological properties of bitumen across a range of loading frequencies and temperatures. Adding fibre reduced the penetration and increased the softening point and viscosity of bitumen implying improved rutting resistance of asphalt mixtures using these mastics. Finally this investigation established that adding fibre to bitumen improved its toughness, which could lead to improvement in asphalt fatigue performance. However, there are some limitations that are also discussed.

Anisotropic tensile failure of metals by the strain localization theory: An application to a high-strength aluminium alloy

This paper investigates the influence of plastic anisotropy on the tensile ductility of a high-strength aluminium alloy. To this end, finite element simulations of smooth and notched tension tests in different material directions are performed with an anisotropic plasticity model. The stress and strain histories from these simulations are then applied in localization analyses with the imperfection band approach, using the anisotropic plasticity model outside the band and an anisotropic version of the Gurson model inside the band. The imperfection within the band is represented by a volume fraction of void nucleating particles. The high-strength aluminium alloy AA7075-T651 is considered in this study. The results show that the directional dependency of the tensile ductility of the alloy found experimentally is predicted with good accuracy using the adopted approach. The numerical study indicates that plastic anisotropy plays an important role in determining the anisotropic tensile ductility of this high-strength aluminium alloy.

Failure prediction of AZ31B magnesium alloy sheet based on a micro-mechanical void model incorporating the asymmetric plasticity constitutive law

In this paper, a fracture criterion based on a micro-mechanics void model was applied to ductile fracture model of AZ31B magnesium alloy sheet. In particular, the fracture model was coupled with an advanced plasticity model in which the asymmetric cyclic response of magnesium alloy due to combined slip and twin deformation can be reproduced accurately. The extended McClintock fracture model adopted in this study as a fracture criterion for the magnesium alloy sheet takes into account the effect of shear deformation by adding Lode parameter dependency to the original model. Both the fracture model and the plasticity model were implemented in the explicit finite element code, LS-DYNA, via a user-defined material subroutine. The parameters of the proposed fracture model were identified using multi-directional mechanical tests, such as uniaxial tension, simple shear, and notched tension tests. As validation, the proposed model was compared with existing models for predicting the fracture patterns in test specimens and limit dome heights.

Improvement of toughness and shear properties of multiwalled carbon nanotubes/epoxy composites

In this work, Epocast 50‐A1/946 epoxy have been modified with different weight percentages (0.25, 0.5, 0.75, 0.1, 1.25, and 1.5 wt%) of multiwalled carbon nanotubes (MWCNTs). Because of Epocast epoxy was primarily developed for joining and repairing of composite aircraft structural components, the optimum weight percentage of MWCNTs must be determined based on trade‐offs between the optimum tensile, in‐plane shear and toughness properties of the developed MWCNTs‐composites, which is the objective of this study. The effect of MWCNTs on the toughness and in‐plane shear was studied using single‐edge notch tension and Iosipescu shear tests, respectively. The experimental results showed that the composite materials with 0.75 wt% MWCNTs has the highest improvement in the in‐plane shear properties compared to the neat epoxy. While the maximum improvements in the tensile and toughness properties were observed at 0.5 wt% MWCNTs. Eleven models were selected from an extensive literature survey, for predicting the in‐plane shear moduli of the developed MWCNTs‐composites. The modified rule of mixture can effectively predict the in‐plane shear moduli of the composites with MWCNTs up to 0.75 wt%. While the nonlinear behavior of the modified Halpin–Tsai model with exponential shape function is the closest to the experimental results up to 1.5 wt% MWCNTs. POLYM. COMPOS., 39:815–825, 2018. © 2016 Society of Plastics Engineers

Prediction of ductile failure using a phenomenological model calibrated from micromechanical simulations

Unit-cell models were in this study utilized to numerically determine the failure locus of a cast and homogenized AA6060 aluminum alloy. Simulations were conducted for moderate and high stress triaxiality ratios, and for various Lode parameters between generalized tension and generalized compression. We estimated the orientation of the localization band that minimizes the failure strain in the unit-cell models for all the imposed stress states. The energy based Cockcroft-Latham (CL) failure criterion was calibrated based on the numerically determined failure locus and used in finite element simulations that we evaluated against experimental tests. Test-specimen geometries included smooth tension tests, notched tension tests and plane strain tension tests. These were designed to cover a wide range of stress states. The points of failure in the experimental tests were predicted with reasonable accuracy by the numerical simulations. We see that the method used for numerically determining the failure locus can be improved by refining the micromechanical simulations. Better agreement between the simulations and the experiments can also be obtained, for instance by employing a different macroscopic failure criterion than the CL criterion.

片側ノッチ引張（SENT）試験とひずみベース設計

片側ノッチ引張（SENT）試験とひずみベース設計

Fatigue Performance of Re-Refined Engine Oil Bottom–Modified Asphalt: Laboratory Study

Re-refined engine oil bottom (REOB), one of several products obtained from refining recovered engine oil, has been used in the asphalt industry since the 1980s. Generally, REOBs are used to help soften the base asphalt binder and are commonly used from 3% to 10% by weight to achieve desired low temperature asphalt binder properties. Recently, blame for poor cracking performance in a number of Canadian and northern U.S. pavement sections has been laid on the use of REOBs. This issue has prompted state agencies in the northeast United States to ban its use, without necessarily understanding how REOB affects asphalt binder and mixture performance. A research effort was conducted to evaluate the laboratory performance of asphalt binders and mixtures modified with REOB. Performance grading, master stiffness curves, the double-edge notch tension test (DENT), and black space analysis were conducted on the asphalt binders at different levels of laboratory aging. The research study showed that while being able to achieve softer asphalt binder grades in accordance with AASHTO R29, the addition of REOB accelerated the age-hardening effects in the asphalt binder, with higher levels of age hardening occurring at higher REOB dosage rates. The study also indicated that while the stiffness properties at low temperatures were not affected by REOB, the relaxation properties were highly affected. The black space analysis, using the Glover–Rowe approach, and the DENT test showed promise in identifying the age-hardening effects and correlated well with mixture fatigue cracking.

Experimental and numerical slip line analysis of welded single-edge notched tension specimens

Flaw assessments assume homogeneous material properties, but welds are heterogeneous. A procedure to account for heterogeneity effects on crack driving force (developed at Ghent University and Limerick University) requires strength properties along slip lines originating from the defect tip. For uniaxial tension loading, these lines are assumed straight and oriented 45° to the axis of loading. The presented paper investigates the soundness of this assumption. Clamped single-edge notched tension tests and simulations have been performed on base metal and welded specimens. Extensive deformation analyses reveal the evolution of slip line trajectories as the specimen deforms. Observed slip lines in base metal specimens are initially close to the assumed 45° trajectories, but deviate towards the notch section as deformation is increased. The change in angle is roughly linear as a function of crack tip opening displacement. Deformation analyses of the welded specimens reveal that hard regions can serve as barriers, causing slip lines to deviate from linearity and split into different branches. The implications of these observations on the flaw assessment of heterogeneous welds are investigated in ongoing work.

Fracture failure in crack interaction of asphalt binder by using a phase field approach

Fracture failure in crack interaction of asphalt binder has always been one serious problem in the pavement industry. In the state of the art research of asphalt cracking, single mode cracking has been studied by many researchers but there lacks theoretical and experimental research on the crack interaction of asphalt binder, which is more reasonable and realistic. The traditional way is to use the Griffith’s theory which is complex and complicated. In this paper, the phase field method (PFM) is presented for modeling, which describes the whole cracking system using a phase-field variable that assumes negative one in the void region (crack) and positive one in the solid region (intact). The fracture toughness is then considered as a material property and modeled as the surface energy stored in the diffuse interface between the intact solid and crack void. The non-conserved Allen–Cahn equation is adopted as the system governing equation to evolve the phase field variable to account for the growth of cracks. The energy based formulation of the phase-field method handles the competition between the growth of surface energy and release of elastic energy of crack interaction in a natural way: the crack propagation is a result of the energy minimization in the direction of the steepest descent. Both the linear elasticity and phase-field equation are solved in a unified finite element frame work, which is implemented in the commercial software COMSOL. Two cracking experiments, namely, direct tension test and double edge notch tension test are then performed for validation. It is discovered that the critical load of crack interaction by PFM agrees very well with both experiment results.