Notched Tension Tests Research Articles

Fracture-induced activation in mechanophore-linked, rubber toughened PMMA

Fracture-induced mechanochemical activation is achieved for the first time in a structural engineering polymer. Rubber toughened PMMA is lightly cross-linked (1.0 mol%) with the mechanophore spiropyran by free radical polymerization. Single Edge Notch Tension tests are performed on the spiropyran-linked material and a distinct change in color and fluorescence is detected at the crack tip, indicating the mechanochemical transformation of the spiropyran molecules. The degree of mechanophore activation is quantified via fluorescence imaging and is observed to increase with increasing crack length. The region of mechanophore activation correlates directly with the size of the plastic zone ahead of the crack tip.

Hydrogen embrittlement susceptibility of a weld simulated X70 heat affected zone under H2 pressure

The present paper deals with hydrogen embrittlement (HE) susceptibility of a weld thermal simulated heat affected zone of X70 structural steel in high pressure hydrogen gas at 20°C. Fracture mechanics Single Edge Notched Tension tests at various hydrogen pressures (0.1, 0.6, 10 and 40MPa H2) have been carried out. The HE susceptibility was quantified through the measurement of the fracture toughness KQ and J (the effect of hydrogen pressure was addressed through linear load increase conditions till failure was obtained). The results show that hydrogen causes a strong decrease in the fracture toughness with increasing hydrogen pressure. The critical hydrogen pressure for the onset of HE was observed to fall between 0.1MPa and 0.6MPa. These results were supported by Scanning Electron Microscope (SEM) investigations of the fracture surfaces which showed a clear shift in the fracture mode at 0.6MPa H2. Moreover, constant load tests were carried out in order to investigate the influence of hydrogen exposure time. The results imply that fracture always occurs within 8h and that longer time to failure is related to stronger toughness reduction. This effect is more pronounced for test at 40MPa than at 0.6MPa hydrogen pressure levels. 3D Finite Element calculations of hydrogen diffusion have been performed and the results are discussed in relation to the experiments, in order to attempt to identify the hydrogen populations (diffusible or trapped) which act predominantly on the embrittlement of the material.

Notch effect in clay-modified epoxy: a new perspective on nanocomposite properties

In this work, an experimental investigation of the notch effect on clay-modified epoxy resins is carried out, discussing the results from Single Edge Notch Bending tests and Double Edge Notch Tension tests on notched components. It is found that when the notch root radius is greater than a limit value, which depends on the clay content, the brittle failure of notched nanomodified specimens is controlled by the material strength. Under this circumstance nanomodification, while enhancing the polymer fracture toughness, might have a detrimental effect on the strength of notched components. This study brings to light a new feature of nanomodification according to which particular care should be used when using nanomodified resins for structural applications in the presence of notches or holes.

Evaluation of Fatigue Tests for Characterizing Asphalt Binders

AbstractThe current performance grade (PG) binder specification uses the parameter G*sin δ to quantify binder fatigue property, which has long been known to be the weakest part of the specification. The purpose of this study was to identify a better binder fatigue test. Six asphalt binders with a variety of modifiers were characterized using five asphalt binder tests, including the G*sin δ, elastic recovery test, the multiple stress creep recovery (MSCR) test, the linear amplitude sweep (LAS) test, and the double edge notch tension (DENT) test in this study. Additionally, the push-pull asphalt mix fatigue test was employed to verify these binder fatigue tests. Comparing the results of the binder tests and the asphalt mix push-pull test, this study further confirmed the poor performance of the parameter G*sin δ to differentiate binder fatigue resistance. It was also found that neither the MSCR nor the LAS test showed good correlation with asphalt mix fatigue resistance. Finally, it was identified that both...

Investigation of fracture in small-scale SENT tests of a welded X80 pipeline steel using Digital Image Correlation with node splitting

Single Edge Notch Tension (SENT) tests have been carried out on girth welded X80 pipeline steel using two different weld materials to investigate the effect of weld metal mismatch on ductile crack growth. In addition to the traditional clip gauge CMOD/CTOD measurements, information has been obtained by applying Digital Image Correlation (DIC). The DIC mesh has been modified to allow for node splitting in order to capture the discontinuous displacement field of the propagating crack. Parameters such as crack path, crack length and crack opening have been successfully measured using this modification to the standard DIC analysis. The paper contains a description of the material and the experimental setup, as well as results from the experimental tests, where the SENT specimens have been subjected to monotonic tension loading. Emphasis is laid on the DIC analysis and the additional information gained from this approach. Parallelization of the DIC algorithm and a discussion on the accuracies of 2D versus 3D DIC analyses are also included in this paper.

Anisotropic plasticity model coupled with Lode angle dependent strain-induced transformation kinetics law

A phenomenological macroscopic plasticity model is developed for steels that exhibit strain-induced austenite-to-martensite transformation. The model makes use of a stress-state dependent transformation kinetics law that accounts for both the effects of the stress triaxiality and the Lode angle on the rate of transformation. The macroscopic strain hardening is due to nonlinear kinematic hardening as well as isotropic hardening. The latter contribution is assumed to depend on the dislocation density as well as the current martensite volume fraction. The constitutive equations are embedded in the framework of finite strain isothermal rate-independent anisotropic plasticity. Experimental data for an anisotropic austenitic stainless steel 301LN is presented for uniaxial tension, uniaxial compression, transverse plane strain tension and pure shear. The model parameters are identified using a combined analytical–numerical approach. Numerical simulations are performed of all calibration experiments and excellent agreement is observed. Moreover, we make use of experimental data from ten combined tension and shear experiments to validate the proposed constitutive model. In addition, punch and notched tension tests are performed to evaluate the model performance in structural applications with heterogeneous stress and strain fields.

Damage and fracture behavior in dynamic tension tests: Modelling and numerical simulations

AbstractThe continuum damage model is based on a general thermodynamic framework for the modeling of rate and temperature dependent behavior of anisotropically damaged elastic‐plastic materials subjected to fast deformation. The introduction of damaged and fictitious undamaged configurations allows the definition of damage tensors and the corresponding free energy functions lead to material laws affected by damage and temperature. The damage condition and the corresponding damage rule strongly depend on stress triaxiality. Furthermore, the rate and temperature dependence is reflected in a multiplicative decomposition of the plastic hardening and damage softening functions. The macro crack behavior is characterized by a triaxiality dependent fracture criterion.The continuum damage model is implemented into LS‐DYNA as user defined material model. Corresponding numerical simulations of unnotched and notched tension tests with high strain rates demonstrate the plastic and damage processes during the deformation leading to final fracture numerically predicted by an element erosion technique. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Internal Pressure on Tensile Strain Capacity of X80 Pipeline

This paper examines the influence of internal pressure on the tensile strain capacity of pipelines. A pressurized full-pipe tension test and curved wide plate (CWP) test were conducted together with FE analyses in order to investigate the strain behavior of pipe under large axial loading with high internal pressure. The critical tensile strain drastically decreased under a high internal pressure condition. Single edge notch tension (SENT) tests with shallow notched specimens were also performed to obtain the material resistance curve (R-curve), and a series of FE analyses was conducted to obtain the crack driving force for ductile crack propagation. The R-curve and crack driving force curve were used in predicting the tensile strain limit of X80 girth welded pipe with a surface defect in the HAZ. The predicted critical tensile strain showed good agreement with that obtained in the pressurized full-pipe tension test. These results demonstrate that the tensile strain capacity of pressurized pipes can be predicted by using the R-curve from SENT tests and the crack driving force curve from FE analyses.

Asphalt pavement cracking: analysis of extraordinary life cycle variability in eastern and northeastern Ontario

This paper documents an investigation of the performance of 20 pavement contracts in eastern and northeastern Ontario, Canada. Eleven of these pavements showed little or no distress after 7–15 years in service. The remaining nine, aged between 7 and 13 years, all cracked prematurely and excessively. Creep testing of the recovered asphalt cements, according to an extended bending beam rheometer protocol, revealed that the long-life pavements were made with materials that suffered little from reversible ageing mechanisms during cold conditioning. Ductile failure testing in a double-edge notched tension test at 15°C revealed equally significant differentiation, with the superior performing materials possessing considerably higher strain tolerances. The newly developed methods were able to explain vast performance differences with 95% accuracy. This study further validates Ontario's interest in an improved asphalt cement grading approach and provides evidence that thin pavements can have long lives, provided the asphalt cement is selected properly.

Energy absorption and damage propagation in 2D triaxially braided carbon fiber composites: effects of in situ matrix properties

Results from an experimental program to investigate the propagation of damage and energy dissipation in 2D triaxially braided carbon fiber textile composites (2DTBC) under static conditions are reported. A methodology is presented in which classical concepts from fracture mechanics are generalized to address damage growth in an orthotropic and heterogeneous structural material. Along with results from the experimental program, a novel numerical technique that employs ideas from cohesive zone modeling, and implemented through the use of finite-element analysis, is also presented. The inputs that are required for the discrete cohesive zone model (DCZM) are identified. Compact tension specimen fracture tests and double notched tension tests were carried out to measure the fracture energy (GIc), and the maximum cohesive strength (σc), of the 2DTBC. The DCZM modeling strategy was independently verified by conducting single edge notched three-point bend tests using a modified three-point bend test fixture. The experimental and numerical analyses were carried out for two different types of 2DTBC made from the same textile architecture but infused with two different resin systems to validate the proposed methodology.

Characterizing the fracture resistance of proton exchange membranes

Pinhole defects that form in proton exchange membranes (PEMs) due to the cyclic hygrothermal stresses induced during the operation of a fuel cell and cause gas crossover may be interpreted as a result of crack formation and propagation. The goal of this study is to employ a fracture test to approach the intrinsic fracture energy of a perflourosulfonic acid proton exchange membrane. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric materials with minimal plastic dissipation and the in absence of viscous dissipation, and has been associated with the long-term durability of polymeric materials where subcritical crack growth occurs under slow time-dependent or cyclic loading conditions. Insights into this limiting value of fracture resistance may offer insights into the durability of PEMs, including the formation of pinhole defects. In order to achieve this goal, a knife slit test which significantly reduces the plastic deformation during the test by limiting the plastic zone size with a sharp blade is conducted. Additionally, double edge notched tension tests and trouser tear tests are conducted to obtain the essential work of fracture and tear energy, respectively. It has been found that although the fracture energy obtained with the knife slit test is still several times larger than the intrinsic fracture energy of regular polymer materials, it is several orders of magnitude lower than those obtained with the other two methods, where process-dependent viscous and plastic dissipation dominate over the intrinsic material property.

Characterization and Modeling of Aluminum Extrusion Damage Under Crash Loading

This paper studied the deformation and damage behaviors of aluminum-alloy under different loadings. A series of static and dynamic plate tension tests, notch tension tests and shear tests were carried out and were combined with the finite element model to acquire material parameters. This work demonstrated that damage behavior of aluminum-alloy depends strongly on stress triaxiality and cannot be modeled with simple damage models based on one constant fracture strain. A systematic investigation shows that the micromechanical Gurson damage model and phenomenological Johnson–Cook model can be used to simulate crash behavior of aluminum components. The tests on an aluminum component of extrusion under static loading were performed and simulated to validate the applicability of Gurson damage model. Since damage parameters depend on element size, the component calculation was performed with the calibrated parameters optimized with the element sizes.

9010 円周切欠引張試験による塑性加工限界の決定(GS-A2 一般セッション(製造関連解析技術))

9010 円周切欠引張試験による塑性加工限界の決定(GS-A2 一般セッション(製造関連解析技術))

Evaluation of Plastic Deformation Limit by Circumferentially Notched Tension Test

Circumferentially notched and smooth round tension tests were carried out at room temperature with and without compressive prestrain in a hot-rolled ferrite-pearlite SM490B steel, and the effects of the compressive prestrain and the notch on the plastic deformation limit (PDL) were investigated. The PDL was defined as a termination point in a true stress-true strain curve which was caused by a ductile or cleavage fracture. The notch caused variations in the average stress-true strain curve but did not affect the true stress-true strain curve. A true strain at the plastic instability was determined by a work hardening exponent, regardless of the notch. On the other hand, the true strain at the PDL differed according to the notch. An axial stress at the PDL was found to be about 1300 MPa, regardless of the ductile and cleavage fracture.

Hydrostatic Stress Effect on the Yield Behavior of Inconel 100

Classical metal plasticity theory assumes that hydrostatic stress has no effect on the yield and postyield behavior of metals. Recent reexaminations of classical theory have revealed a significant effect of hydrostatic stress on the yield behavior of notched geometries. New experiments and nonlinear finite element analyses (FEA) of Inconel 100 (IN 100) equal-arm bend and double-edge notch tension (DENT) test specimens have revealed the effect of internal hydrostatic tensile stresses on yielding. Nonlinear FEA using the von Mises (yielding is independent of hydrostatic stress) and the Drucker-Prager (yielding is linearly dependent on hydrostatic stress) yield functions was performed. In all test cases, the von Mises constitutive model, which is independent of hydrostatic pressure, overestimated the load for a given displacement or strain. Considering the failure displacements or strains, the Drucker-Prager FEMs predicted loads that were 3% to 5% lower than the von Mises values. For the failure loads, the Drucker Prager FEMs predicted strains that were 20% to 35% greater than the von Mises values. The Drucker-Prager yield function seems to more accurately predict the overall specimen response of geometries with significant internal hydrostatic stress influence.

Influence of stress state on cavitation during hot working of Ti-6Al-4V

Notched-tension tests were conducted on Ti-6Al-4V samples with a colony-alpha microstructure to determine the effect of the stress state on cavitation during hot working of alpha/beta titanium alloys. The experiments were complemented by finite-element-method analyses to establish the local stress state, strain, and damage factor for different areas within each sample. A critical damage factor, defined by a modified Cockcroft-and-Latham criterion, was found to be applicable for the prediction of cavity initiation for the different notch geometries. Measured cavity growth rates were also correlated to the stress state (i.e., ratio of mean-to-effective stress, σM/σe) and compared to predictions of prior models. Model predictions showed reasonable agreement with measurements at low levels of stress triaxiality, but exhibited some deviations at higher values of σM/σe. The differences were attributed to differences in the properties of the present material and those assumed in deriving the models as well as the neglect of cavity interaction in one of the models. The results were summarized in terms of a processing map, which delineates the initiation of cavities and their size as a function of the stress state and effective strain.

Evolution of dynamic fractures in poly(methyl methacrylate)

The results from single edge notched tension tests support the idea that the nucleation, growth, and coalescence of penny shaped microcracks govern the fracture evolution in poly(methyl methacrylate) (PMMA). The density of the microcracks (seen as parabolic markings on the fracture surface), and therefore the roughness of the fracture surface, increases with the crack velocity. The damage beneath the main fracture surface also increases with crack velocity and the crack length. This damage is formed by attempted and successful crack branches originating from the main fracture and other cracks propagating alongside the main fracture. Microscopy of the virgin material and fractured surfaces showed no consistent evidence of pre-existing flaws, dust particles, or other impurities that would provide nucleation sites for the microcracks. It can therefore be argued that the material microstructure, chain length in particular, must play a significant role in the nucleation of microcracks. This can be supported by an earlier work in which strong dependence of the microcrack density on the molecular weight of PMMA was reported. Crack velocity measurements show rapid initial acceleration of the crack followed by nearly constant mean velocity. The mean velocity is found to increase with decreasing initial notch depth. Oscillations in the crack velocities were also observed and they were more pronounced at higher crack velocities. To a large extent the degree of crack velocity oscillation is dependent on the filtering technique applied to process the raw data. Therefore, no conclusive correlation between the fracture history and surface was obtained.

Fracture behaviour of thin polyimide films

The deformation and fracture behaviour of aromatic polyimide films (BPDA-PDA, PMDA-ODA, PMDA-BAPB and PMDA-BAPP) was investigated and related with local molecular order. It was found that local molecular order, i.e. bundles of parallel-packed main chains dominated the deformation and fracture behaviour of polyimide films. The fracture behaviour of respective polyimides was heterogeneous over thickness in films with low degree of local molecular order, while this phenomenon disappeared in films with high degree of local molecular order. The tear energy showed maximum value as the degree of local molecular order increased. Also, the deformation behaviour of PMDA-BAPP films with low degree of local molecular order showed a combination of crazing and shear yielding. Other polyimide films with high degree of local molecular order showed only shear yielding in single-edge notched tension test.

Ductile fracture of A 508 Cl 3 steel in relation with inclusion content: The benefit of the local approach of fracture and continuum damage mechanics

A new methodology for ductile fracture analysis based on the local approach of fracture, through constitutive relations that take into account void growth and damage, has been applied to three heats of A 508 Cl 3 steel with different inclusion contents in the region of 10 −3–10 −4. The ductility of the three heats is well predicted by the ductile fracture model through its parameter f 0 : the initial volume fraction of voids. The model, first calibrated with the simple notched tension test, gives a good prediction of crack initiation and growth in a precracked specimen. Finally the statistical aspects in ductile fracture are briefly discussed.

Significance of the load shift in single-edge notch tension tests

When single-edge-notch (SEN) specimens are tested in tension, the load is usually applied at the extremities of the specimen through single pins aligned along its center line. Upon loading the specimen, an initial positive moment which tends to assist in opening the crack is created. This moment will continue as long as there is no appreciable plastic deformation ahead of the crack and no significant change in the initial specimen geometry. As the load increases, however, the load center line considerably shifts relative to the specimen center line giving rise to a negative bending moment which tends to close the crack. Furthermore, if the specimens are to be made of highly ductile materials (e.g., line-pipe steels), the shift will be much more pronounced; thus leading to substantial discrepancies between theoretically calculated and experimentally obtained crack-opening-displacement (COD) values. The ductile fracture analysis of the edge-cracked strip problem, in general, is based on the assumption of small displacements and rotations in which case reference to the underformed geometry for equilibrium is fully justified. In this paper, it is shown that if one uses small displacement theory to solve such a problem then extreme caution should be exercised in prescribing the nature of the external loads to account for the severe geometrical changes that actually occur during the tests.