Notched Tension Specimens Research Articles

Characterization of slip lines in single edge notched tension specimens

The application of slip line analysis in weld failure assessment has not gained much attention to date. The presented research aims to predict slip line patterns taking into account the complex heterogeneous structure of the weld. A preliminary study based on Single Edge Notched Tension (SENT) test results sampling pure base material, i.e. not containing any welds is conducted to assess the impact of side grooves on slip line behaviour and to validate slip line theory and finite element analysis.

Critical mechanical properties and FEA simulation for crashworthiness assessment of a coarse-grained cast AM50 alloy

Abstract A coarse-grained AM50 alloy was used as a model alloy for investigation of constitutive behaviour, Charpy toughness and effect of stress state on deformation and failure of cast Mg alloys. The results provide critical mechanical properties of a cast AM50 alloy for crashworthiness assessment and development of finite element simulation techniques. For cast Mg alloys, the effect of strain rate and temperature is larger on tensile strength than on compressive strength because twinning is more extensive in compression than in tension. The effect of strain rate on compressive strength is negligible because twinning activity of the cast Mg alloy is dominant. The load vs. deflection of Charpy specimens were measured for modelling, and the effect of loading rate and temperature on load of Charpy specimens is very small because part of the specimen is in compression. The equivalent strain to fracture of the cylindrical round notched tension specimen decreases with increasing stress triaxiality; though for the flat-grooved plane strain specimen, the equivalent fracture strain remains constant over the range of stress triaxiality investigated. Because the two different specimen geometries give rise to different Lode angle values, the test results show that the Lode angle parameter is an important parameter for deformation and fracture of Mg alloys. Finite element simulations of loading of the cylindrical notched-tension and Charpy specimens were carried out using a Lode-angle dependent von Mises model, and were found to provide a reasonable description of the load–displacement curves measured in the tests. For the flat-grooved plane strain specimens, the computations under-predicted the force–displacement response measured.

Weight functions and stress intensity factors for pin‐loaded single‐edge notch bend specimen

AbstractA recently developed pin‐loaded single‐edge notch bend specimen provides an alternative to the single‐edge notch tension specimen commonly used for small‐crack growth testing. In this paper, weight functions for pin‐loaded single‐edge notch bend specimen are derived by using two methods, the classical analytical weight function method and the newly developed numerical weight function complex variable Taylor series expansion method. Excellent agreement between the two methods is achieved. Based on these weight functions, accurate stress intensity factors for two load cases, that is, pin‐loading and Dugdale loading, which is required for plasticity‐induced crack‐closure analysis based on the strip‐yield model, are determined.

Cover Image, Volume 98, Issue 6

A fractured micrometer‐sized polycrystalline silicon notched tension specimen fabricated with Sandia National Labs' SUMMiT V™ process for microelectromechanical systems. The close‐up view of the specimen's sidewalls and the fracture surface underscores the complex role of the material microstructure and surface topography on fracture. Photo courtesy of Dr. Brad L. Boyce, Sandia National Laboratories. See also the feature article in this issue by Jia‐Liang Le et al. image

INFLUENCE OF STRAIN AMPLIFICATION NEAR CRACK TIP ON THE FRACTURE RESISTANCE OF CARBON BLACK–FILLED SBR

ABSTRACT Singularity of strain field at the crack tip of elastomeric material has recently attracted considerable attention. For SBR filled with four different types of carbon black, the strain distribution at the crack tip of the single-edge notched tension specimens are investigated using digital image correlation (DIC) and finite element analysis (FEA). Both DIC and FEA results demonstrate that the larger the carbon black particle size, the less strain amplification at the crack tip of SBR. However, the strain amplification region obtained from FEA simulation is much smaller than the strain amplification observed from DIC, and the reasons are discussed. Critical J-integral (JIC) and tearing modulus (TR) are calculated via J-integral method and are connected to crack initiation and propagation resistance of the SBR, respectively. With increasing carbon black particle size, both JIC and TR decrease. Similar trends also occur for tearing energy deduced from the dynamic mechanical thermal analysis. These results are in agreement with the prediction based on strain amplification at the crack tip.

Characterization and correlation of 3-D creep constraint between axially cracked pipelines and test specimens

Three-dimensional creep crack-tip constraints of axially cracked pipes and conventional test specimens have been characterized and analyzed by a load-independent creep constraint parameter R∗ proposed by authors. An average measure of constraint over the crack front is utilized to correlate the creep constraint between the 3-D axially cracked pipelines and test specimens. It was found that single-edge notched tension specimens provide a closely matched creep crack-tip constraint with axially cracked pipes. The investigation results provide a strong support for use of constraint-designed specimens in creep life assessment of pressurized pipes.

Characterisation of the surface damage of X38CrMoV5 (AISI H11) tool steel at room temperature and 600 °C

AbstractA novel approach is used to characterise the surface damage of AISI H11 hot work tool steel. The fatigue crack growth rate (FCGR) experiments on thin specimens, considered representative of the surface of tool steels, are carried out. Single edge notched tension specimens of 8 mm width and different thicknesses are used in the study. Initially, the effect of thickness (scale) on the FCGR is investigated to establish if there exists a difference between bulk and near surface properties of the tool steel. Then, the effect of R value on the thin specimens is investigated. All the experiments are carried out at room temperature and 600 °C. These temperatures represent the limits of use of this steel. Paris curves are established. Effect of R ratio on the threshold value of propagation at elevated temperatures is investigated in detail. A special ascending ΔK experiment for establishing the threshold of propagation at elevated temperatures in small specimens is proposed, and the results are presented. The increase in R ratio increases the FCGR at low temperature, while it has no effect at 600 °C. A reduction in thickness shows a reduction in the FCGR. Increase in temperature increases the FCGR and dramatically increases the threshold of crack propagation. The sharp increase in threshold value is studied in detail. Scanning electron microscopy of the specimens is performed to explain some of the characteristics observed in the specimen testing.

Failure assessment diagram analysis of high density polyethylene pipes

This paper presents an analysis of failure assessment diagram for PE4710 grade high density polyethylene pipes, which are recently concerned to use in the nuclear industry. Available test data of tensile, single edge notched tension specimens and pressurised pipes are summarised and suitably characterised for use in the failure assessment diagram in R6 procedure. Critical crack lengths and failure mode for the pressurised pipes and single edge notched tension specimens are determined under sustained load and compared with test results. Based on the concept of linear elastic fracture mechanics, a slow crack growth rate of PE4710 at 95°C is obtained excluding crack initiation time. To consider the effect of temperature on slow crack growth rate, a shift method is introduced. Remaining life is calculated against the crack margin and compared with the failure time.

Effect of initial damage level and patch configuration on the fatigue behaviour of reinforced steel plates

ABSTRACTThe application of carbon fibre reinforced polymer composites externally bonded on cracked steel plates is an effective system in extending the fatigue life of these structural elements. In particular, composite patches bonded on the crack tip region reduce the stress concentration and the crack opening displacement, leading to an extension of the fatigue life. In order to additionally show the effectiveness of this kind of reinforcing technique, experimental tests were performed at the laboratories of the Politecnico di Milano. Fatigue tests were executed on single edge notched tension specimens reinforced by pultruded strips bonded to a single side (non‐symmetric reinforcement). Different patch configurations (reinforcement stiffness and patch location) and initial damage levels were considered as parameters influencing the repair effectiveness in extending the fatigue life. The results showed that the use of carbon fibre reinforced polymer materials bonded around the tip region allows extending the fatigue life for different amount of initial damage level. Finally, this work provides some useful information for the more efficient repair configuration.

Crystallographic orientation and temperature effects on the fatigue crack growth rate and resulting fracture surface morphology in PWA1484 single crystal superalloy

ABSTRACTThis research looked at the effect of crystallographic orientation and temperature on the fatigue crack growth rate and the resulting fracture surface morphology in PWA1484 single crystal superalloy. Two groups of single edge notch tension specimens, one group with controlled secondary orientations and one group with uncontrolled secondary orientation, were tested at temperatures from 649°C to 982°C at R‐ratios of 0.1 and 0.7. It was found that the effect of temperature on the crack growth rate becomes more pronounced as the crack driving force increases while the secondary orientation and R‐ratio effects on the crack growth rate increase with increasing temperature. Two types of crack surface morphology were seen during fractography. The first was a precipitate avoidance (γ′ avoidance) morphology that was rolling but still predominantly flat when observed on a larger scale. In <001> primary oriented specimens, this fracture mode tended to follow the precipitate/matrix faces (microscopically cubic) while macroscopically staying essentially normal to the applied loads. The second mode was a form of cleavage (γ′ shearing) and occurred predominantly on octahedral crystallographic planes.

J-integral analysis of heterogeneous mismatched girth welds in clamped single-edge notched tension specimens

Flaw assessment procedures require a quantification of crack driving force, and such procedures are generally based on the assumption of weld homogeneity. However, welds generally have a heterogeneous microstructure, which will influence the crack driving force. This paper describes a stress-based methodology to assess complex heterogeneous welds using a J-based approach. Clamped single-edge notched tension specimens, representative of girth weld flaws, are analyzed and the influence of weld heterogeneity on crack driving force has been determined. The use of a modified limit load for heterogeneous welds is proposed, suitable for implementation in a ‘homogenized’ J-integral estimation scheme. It follows from an explicit modification of an existing solution for centre cracked tension specimens. The proposed solution provides a good estimate of crack driving force and any errors in the approximation may be accounted for by means of a small safety factor on load bearing capacity.

Fracture Modeling of Damaged Stiffened Panels under Lateral Pressure Load

Numerical modelling and simulations were carried out in order to analyze fracture mechanisms for stiffened panels, damaged by a single and multiple cracks, and subjected to lateral pressure. For that purpose finite element models for stiffened panel specimens were developed using shell elements. Material property data used in the simulation were obtained from the tensile test and fracture test on a centrally notched tension specimen. By implementing elastic plastic fracture mechanics concepts (EPFM) critical pressures associated with fracture onset in the specimens were assessed based on the critical J-integral and CTOD parameters. Simulated critical pressure loads for stiffened panel specimens were compared with experimental results and a good agreement was observed. The developed procedure can be applied to real thin-walled structures.

Assessment of Pop-in Significance in Heterogeneous Weld Heat-affected Zone Using Finite Element Analyses

Small crack initiation and arrest behavior called pop-in is often observed in fracture toughness tests of welded joints. The material toughness is extremely sensitive to the local brittle zone (LBZ) in heterogeneous heat-affected zone (HAZ). The avoidance of pop-in initiation is very difficult in toughness tests of welded joints using high strength steels. These pop-ins may not always significant events in structural integrity due to the material heterogeneity in HAZ where the material surrounding LBZ has high crack arrest capability. The pop-in behavior in heterogeneous HAZ has been investigated using 3-dimensional dynamic finite element analyses (FEA) with calibrated cohesive elements. The models used are single edge notch bend and single edge notch tension specimens having material heterogeneities to assess loading mode and material heterogeneity effects on pop-in behaviors. It has been found that toughness heterogeneity in HAZ has a large effect on pop-in significance. And the effect of loading mode such as bending or tension on pop-in significance can be observed in FEA.

Fractographic analysis of the crack resistance of styrene–acrylonitrile/polybutadiene‐g‐styrene–acrylonitrile blends as evaluated by the essential work of fracture method

ABSTRACTThe fracture surfaces and deformation micromechanisms of styrene‐co‐acrylonitrile (SAN)/polybutadiene‐g‐styrene‐co‐acrylonitrile (PB‐g‐SAN) blends with the compositions ranging from 65/35 to 0/100 were studied with a scanning electron microscopy technique. The results were compared to the essential work of fracture parameters obtained in a previous study conducted on double‐edge notched tension specimens. Different plastic damage mechanisms were observed, and they depended on the blend composition. For blends of 65/35 and 45/55, a high degree of rubber particle cavitation and multiple cracking followed by the massive shear yielding of the matrix were found to be the main source of energy dissipation during crack growth. Within this compositional range, more intense plastic damage in a larger volume of material, especially at the notched region, was observed as the concentration of the rubbery phase increased. For the 25/75 blend, the prevailing mechanism was pure shear yielding without any sign of cavitation inside the particles, and the fracture surface became relatively flat and was covered with aligned small microcracks. This sample showed the highest specific essential work (we) value among the blends examined in the previous study. For the samples containing concentrations of dispersed phase higher than 75%, the shear yielding process gradually became less important with the progressive importance of multiple crazing so that high‐magnification micrographs revealed extensive microcracking/crazing both inside and between the rubber particles, as the only active deformation micromechanism for neat PB‐g‐SAN. The variation we and specific plastic work of fracture with the PB‐g‐SAN phase content were successfully explained in terms of prevalent deformation mechanisms. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40072.

Plastic limit load and its application to the fracture toughness testing for heterogeneous single edge notch tension specimens

ABSTRACTThe current investigation pursues the confirmation of the applicability of the limit load solutions in determination of the η factors necessary for fracture toughness testing protocols. The procedure begins with the correct calculation of limit load values in welded single edge notch tension (SE(T)) fracture specimens containing centreline cracks. Hence, the η factor is inferred through the principle of potential energy. Additionally, such results are compared with those obtained from finite element analyses, including strain hardening effects available in the literature. SE(T) specimens subject to pin‐loading display that the η factors are insensitive to the configurational effects and hardening properties. On the other hand, in clamped SE(T) specimens, such effects become meaningful, making its usage in fracture toughness experiments questionable. This work provides an alternative methodology to compute fully plastic proportionality coefficients (η) based on limit load solutions for heterogeneous cracked SE(T) specimens. These analyses also consider the limitations and potentialities of such an approach in experimental measurements of ductile crack growth.

Fracture characterization of high-density polyethylene pipe materials using the $$J$$ J -integral and the essential work of fracture

In this paper, fracture mechanics concepts are reviewed and their relevance to examine the toughness of highly deformable materials such as high-density polyethylene (HDPE) pipe materials is discussed. Using two different specimen configurations (single edge notched bending and compact tension), it was found that the \(J-R\) approach is unable to give pertinent indications on fracture toughness of HDPE. Alternatively, applying the essential work of fracture approach to double edge notched tension specimen, seems a more appropriate way to measure the fracture strength of HDPE and therefore to analyze the fracture process of such materials. Nevertheless, the severe necking occurring at the crack tip and in the plastic zone makes difficult the crack growth measurement, which clearly depends on the strain state and on the stress triaxiality level.

3D cohesive modelling of hydrogen embrittlement in the heat affected zone of an X70 pipeline steel

A three-dimensional finite cohesive element approach has been developed and applied in order to simulate the crack initiation of hydrogen-induced fracture. A single edge notched tension specimen of an X70 weld heat affected zone was simulated. The results were compared to similar two-dimensional plane strain model and the cohesive parameters were calibrated to fit the experimental results. The three dimensional simulations gave higher values in terms of opening stress at the stress peak, plastic strain levels at the crack tip and hydrogen lattice concentration when compared with two-dimensional simulations under the same global net section stress levels. Nevertheless a higher cohesive strength was needed for the 2D model for the onset of crack propagation. The best fit to the experimental data were obtained for a cohesive strength of 1840 MPa and 1620 MPa for the 2D and 3D simulation respectively. The critical opening was assigned to 0.3 mm for both models. The threshold stress intensities KIC,HE were 142 MPa√m and 146 MPa√m for the 2D and 3D models, respectively.

A 2D finite element implementation of the Fleck–Willis strain-gradient flow theory

The lay-out of a numerical solution procedure for the strain gradient flow (rate-independent) theory by Fleck and Willis [A mathematical basis for strain-gradient theory – Part II: Tensorial plastic multiplier, 57:1045–1057; 2009, JMPS] has been an open issue, and its finite element implementation is yet to be completed. Only recently, a sound solution procedure that allows for elastic–plastic loading/unloading and the interaction of multiple plastic zones has been put forward, and demonstrated within a 1D model set-up. The aim of the present work is to extend this procedure to form a basis for 2D and 3D calculations, and thereby to broaden its use within engineering applications. Focus is on the numerical implementation that adopts image analysis techniques to identify individual plastic zones and to treat none-regular mesh configurations. The developed finite element model is demonstrated through; i) tensile loading of a finite homogeneous material slab that offers a simple interpretation of results, ii) tensile loading of a double edge notch tension specimen involving symmetry considerations, and iii) shearing of a periodically voided structure that displays the ability of the procedure to treat periodic boundary conditions. A comparison between the implemented flow theory model and the corresponding rate-dependent visco-plastic version of the model shows coinciding results in the limit of zero rate-sensitivity.

Fracture testing of welded single edge notch tensile specimens

The fracture behavior of welded single edge notched tension specimens was examined using constraints and loading conditions which approximated those of a flawed ship structural panel. The specimens were notched and fatigue pre-cracked in the microstructural region associated with a brittle coarse grained heat affected zone and dynamically fractured at the minimum design temperature. In all cases, the fracture event was preceded by some degree of plasticity. With the aid of validated finite element method companion models, a crack mouth to crack tip opening displacement transfer function was generated allowing the fracture toughness at failure to be determined.

Cohesive zone modeling of ductile tearing process in brazed joints

Ductile tearing process in low carbon steel brazed joints with copper filler metal is studied using the Cohesive Zone Model. The Cohesive Zone Model is characterized by two parameters related to the cohesive strength and cohesive energy. A new method is developed to estimate the Cohesive Zone Model parameters through direct experimental measurements and numerical modeling. The cohesive energy of the brazed joints is obtained from the fracture test results performed on the single-edge notched bend specimens. Based on the obtained cohesive energy parameter, the fracture test is simulated using a cohesive zone finite element model. A unique value for the joint cohesive strength is determined by best fitting the finite element results to the experimental load-crack mouth opening displacement curves. The merit of the characterized Cohesive Zone Model is explored by finite element modeling of the tensile test performed on the single-edge notched tension specimens. The load-crack mouth opening displacement curve obtained from the finite element modeling conforms to the corresponding tensile test results. Furthermore, the effect of the crack tip stress constraint on the plastic zone shape and size is successfully captured by the model. The good agreement between the finite element simulation results and the experimental data demonstrates the applicability of the Cohesive Zone Model for fracture analysis of the brazed joints.