Double Edge Notched Tension Specimens Research Articles

Nanoscale printed tunable specimen geometry enables high-throughput miniaturized fracture testing

Two-photon lithography (TPL) enables the design of novel micromechanical specimens, down to sub-micron resolution, thus extending the possibilities for device and material characterisation and pushing the boundaries of a broad range of miniaturized technologies such as optics, analytics, and medicine. Employing a push-to-pull geometry, incorporating double edge notched tension specimens loaded in mode I, the specimen manufacturing and testing can be automated to a large extent. This allows for the use of large parameter space characterisation methods as the essential work of fracture, with an experimentally simpler to realize compression testing setup. Within this work, a methodology is outlined for automated specimen direct laser writing with a TPL-device and subsequent testing via a nanoindenter. In total, 2100 specimens were manufactured, of which 1997 could be used for evaluation. Estimations for the essential work of fracture of the used photoresist is presented, with regards to influencing parameters such as testing displacement rate and laser writing power. A discussion of its statistical robustness and validity considerations is included. This will act as a basis framework for further statistical fracture evaluation schemes for other resin materials, as well as for probing thin film systems.

Fracture toughness examination of dual‐phase and interstitial free steel using essential work of fracture method

AbstractFracture toughness of automotive‐grade dual‐phase steel (DP450) and interstitial free (IF) steel is determined using essential work of fracture (EWF) methodology. Geometrical constraints of standard fracture toughness tests made the EWF method an alternative for thin sheets. Double edge notched tension (DENT) specimens are used for the EWF tests. The two steels have different mechanical and microstructural properties, which helps test the applicability of the EWF methodology to different materials. The effect of notch‐tip radius on the EWF test results is analyzed for both the steels. The sensitivity of fracture toughness to notch‐tip radius is not the same for both steels. During the EWF test, the digital image correlation (DIC) technique is used to determine the local strain variation of DENT specimens. The hole expansion ratio is determined for the two steels to compare against the EWF test results. The IF steel, despite lower strength, has better fracture toughness and hole expansion capability against the DP450 steel.

The role of intermetallic particles on mode I crack propagation mechanisms in metal plates

In metal plates, the crack propagation mechanism sets the amount of the plastic deformation before failure: a slanted or a cup–cone crack typically yields limited plate thinning within the fracture process zone, while large deformation precedes cup–cup crack propagation. The present work investigates the effect of intermetallic particles on the propagation mechanisms and the associated fracture surface morphologies when tearing Al 1050 plates under far-field mode I loading. Both single edge notched and double edge notched tension specimens, with thicknesses ranging from 0.5 to 5 mm, were tested. The chemical compositions of intermetallic particles were determined by performing energy dispersive X-ray measurements, and their morphological features were characterized by Scanning Electron Microscopy (SEM). Likewise, SEM images were taken to display the fracture surfaces, and the details of the surface morphology were visualized in three dimensions by using X-ray Tomography scanning. The experimental results indicate that an increase in the volume fraction, size, and aspect ratio of the intermetallic particles all promote slanted/cup–cone cracks, while a low amount of small, circular particles leads to cup–cup cracks. Furthermore, two-dimensional finite element simulations for mode I crack propagation support the experimental findings.

Characterization of Ductile Crack Growth Resistance Behaviour of Interstitial-Free Steel Sheet using Energy Dissipation Rate Parameter

The present study aims at charactering the crack growth resistance parameter in an 1 mm thick interstitial-free (IF) steel sheet using the energy dissipation rate parameter, R. The R-Δa plots of single-edge and double-edge notched tension specimens have been determined with only additional computations of mechanical and optical test data already generated in the course of determining the variation of crack tip opening angle with Δa following the optical method of ASTM E 2472. A new method has been adopted for determining the stationary value of energy dissipation rate, R∞, This has been done by analyzing mechanical test data generated in the course of essential work fracture (EWF) testing, with additional inputs on crack initiation points identified in those EWF tests. An attempt has been made to normalize the R-Δa plots in order to reduce the specimen geometry dependence of the energy dissipation results.

Essential work of fracture assessment of acrylonitrile butadiene styrene (ABS) processed via fused filament fabrication additive manufacturing

Experiments and finite element (FE) calculations were performed to study the raster angle–dependent fracture behaviour of acrylonitrile butadiene styrene (ABS) thermoplastic processed via fused filament fabrication (FFF) additive manufacturing (AM). The fracture properties of 3D-printed ABS were characterized based on the concept of essential work of fracture (EWF), utilizing double-edge-notched tension (DENT) specimens considering rectilinear infill patterns with different raster angles (0°, 90° and + 45/− 45°). The measurements showed that the resistance to fracture initiation of 3D-printed ABS specimens is substantially higher for the printing direction perpendicular to the crack plane (0° raster angle) as compared to that of the samples wherein the printing direction is parallel to the crack (90° raster angle), reporting EWF values of 7.24 kJ m−2 and 3.61 kJ m−2, respectively. A relatively high EWF value was also reported for the specimens with + 45/− 45° raster angle (7.40 kJ m−2). Strain field analysis performed via digital image correlation showed that connected plastic zones existed in the ligaments of the DENT specimens prior to the onset of fracture, and this was corroborated by SEM fractography which showed that fracture proceeded by a ductile mechanism involving void growth and coalescence followed by drawing and ductile tearing of fibrils. It was further shown that the raster angle–dependent strength and fracture properties of 3D-printed ABS can be predicted with an acceptable accuracy by a relatively simple FE model considering the anisotropic elasticity and failure properties of FFF specimens. The findings of this study offer guidelines for fracture-resistant design of AM-enabled thermoplastics.Graphical abstract

On the sensitivity of fracture mechanism to stress concentration configuration in a two-step quenching and partitioning steel

The effect of stress concentration geometry on the fracture behavior and fracture mechanisms of a two-step quenching and partitioning steel consisting of tempered martensite and retained austenite (RA) is investigated using double edge notched tension (DENT) specimens with different notch root radii. The initial notch opening is varied from a fatigue pre-crack to 15 mm radius. The fracture surface consists in flat triangular regions and slant zones. The fracture mode evolves from intergranular brittle fracture to ductile fracture with dimples. The ductile fracture process further changes from internal necking coalescence to a void sheet shear type coalescence when increasing the notch root radius. A similar transition of the fracture behavior is observed along the ligament of the DENT specimen. This change in the fracture mode results from a decrease of the stress triaxiality. The essential work of fracture is equal to $$40\hbox { kJ/m}^{{2}}$$. This is small when looking at the large value of the product of ultimate tensile strength and total elongation ($$\sim 25, 000\, \hbox {MPa}{\cdot }$$%). The small fracture toughness is ascribed to an intergranular fracture mode, which results from the distribution of blocky RA islands along the boundaries of martensitic packets and from the large stress concentration near the notch tip. 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, and to avoid the relatively low toughness and more brittle failure observed when stress triaxiality increases.

An improvement to the single specimen test method for fracture characterization of elastomer materials using DENT specimen

Fracture behavior of elastomer materials is investigated based upon single specimen and multiple specimen test methods using the practically recommended double edge notched tension (DENT) specimen. Combining analytical, experimental, and numerical analysis, the critical J integral is determined based upon η factor concept. The main challenge with this approach is how to extract the η (or η*) factor. For DENT specimen, previous studies have proposed that η* factor, unlike η, could be considered approximately constant, and even estimated from its homologous LEFM-based formula (ηel). In this paper, it is shown that the values of coefficients η and η* may significantly depend on the material type and the constitutive model and even on the load level. Moreover, η* values deviate significantly from ηel predictions. Consequently, the claim of a constant coefficient η* for DENT specimen is not correct, though numerical estimations of this variable factor by using the appropriate constitutive parameters could still be provided.

Crack-Tip Opening Displacement Behavior Based on Energy Partitioned Work of Fracture Technique on Ultraviolet-Irradiated Mordenite Zeolite-High Density Polyethylene Composites

Crack-tip opening displacement (CTOD) has been widely used to measure the fracture toughness of materials. Based on the energy partitioned work of fracture concept, CTOD behavior is able to describe in more depth the characteristics of mechanical fracture of materials. This study aims to investigate the CTOD behavior in the ultraviolet (UV) irradiated zeolite-HDPE composites. The annealed double-edge notched tension (DENT) shape composite was exposed in UV radiation environment for 50, 100, 150 and 200 hours at a constant temperature of 60oC and atmospheric pressure. The DENT specimens are subjected tensile loaded according to ESIS protocol. CTOD was analyzed based on the concept of partitioned work of fracture. The results show that at the yielding and crack initiation stage, the CTOD value begins to increase when the displacement has passed the 0.5 mm value. At the crack propagation stage, the range of CTOD and displacement values are shorter with increasing duration of UV radiation. At the same crack extension, the CTOD value decreases by increasing the duration of UV radiation.

A morphology based constitutive model for high density polyethylene

High density polyethylene (HDPE) has been widely used in pipe systems for water delivery. Over the course of service, chlorine diffuses into HDPE and causes changes in the material morphology. This process has a significant influence on the deformation and failure mechanisms of HDPE. It has been demonstrated that the failure of HDPE exhibits a ductile-to-brittle transition as the corrosion level increases. Capturing this transition in the material failure mode is essential for predicting the long-term behavior of HDPE structures exposed to a chlorinated environment. In this study, a morphology based constitutive model of HDPE is developed. The model takes into account several essential deformation and failure mechanisms, which include viscoplastic deformation due to intermolecular resistance and homogeneous void growth in the crystalline and amorphous phases, as well as entangled network resistance and craze damage in the amorphous phase. The constitutive parameters are explicitly related to the molecular weight, which varies with the corrosion level. The proposed model is calibrated using uniaxial tensile tests on HDPE samples performed at different deformation rates, crystallinities, and corrosion levels. The model is then used to simulate the mechanical behavior of double-edge notched tension specimens exposed to different corrosion levels. It is shown that the model is capable of capturing the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment.

Determination of the crack resistance curve for intralaminar fiber tensile failure mode in polymer composites under high rate loading

This paper presents the determination of the crack resistance curve of the unidirectional carbon-epoxy composite material IM7-8552 for intralaminar fiber tensile failure under dynamic loading. The methodology, proposed by Catalanotti et al. (2014) for quasi-static loading conditions, was enhanced to high rate loading in the order of about 60 s−1. Dynamic tests were performed using a split-Hopkinson tension bar, while quasi-static reference tests were conducted on a standard electromechanical testing machine. Double-edge notched tension specimens of different sizes were tested to obtain the size effect law, which in combination with the concepts of the energy release rate is used to measure the entire crack resistance curve for the fiber tensile failure mode. Digital image correlation is applied to further verify the validity of the experiments performed at both static and dynamic loading. The data reduction methodology applied in this paper is suitable for intralaminar fiber failure modes without significant delamination. Sufficient proof is given that quasi-static fracture mechanics theory can also be used for the data reduction of the dynamic tests. It is shown, that the intralaminar fracture toughness for fiber tensile failure of UD IM7-8552 increases with increasing rate of loading.

A strain-based approach to study interaction between non-coplanar through-thickness edge notches

Structural integrity assessment procedures to assess the effect of interaction between multiple adjacent flaws normally consist of two stages. First, alignment rules categorize non-coplanar flaws as aligned or non-aligned. Second, combination rules classify aligned flaws as interacting or non-interacting. Although these criteria are applied to different failure modes like brittle fracture, elastic–plastic fracture and plastic collapse, most of them were developed based on linear elastic fracture mechanics for the sake of simplicity. However, there are very limited references available for the technical background of these criteria. This lack of justifying backgrounds becomes more critical when applying these procedures to any other failure modes other than brittle fracture. This article studies the interaction between non-coplanar edge notches in scenarios of large deformation. Hereto, strain patterns are studied through full-field deformation measurements utilizing both experimental and numerical tools. Digital image correlation is used to measure strain during experiments and to verify the finite element analyses. The results show that in addition to the crack driving force, which represents a local response of notches, the global strain distribution within the specimen in terms of strain patterns can be used to probe the interaction between non-coplanar flaws. The authors suggest a novel criterion based on the trajectory of maximum equivalent strain to distinguish between aligned and non-aligned flaws. This study is based on double-edge notched tension specimens and gives a fundamental insight into flaw interaction in failure modes other than brittle fracture.

Notch effect on the fracture of a polymeric film

The fracture behavior of a bio-based thermoplastic copolyester film has been studied by combining the essential work of fracture, the J-integral, and the crack tip opening displacement characterization methods on double edge notched tension specimens, in an attempt to provide a better understanding of the details that play an important role in the repeatability and the reproducibility of the essential work of fracture test, with particular attention to the effect of the quality of the notches generated by two different notch sharpening techniques. Specifically, the femtosecond pulsed laser ablation and the classic razor blade sliding techniques have been applied. The equivalence, reported and discussed in the literature, between the specific essential work of fracture and the J value at crack initiation, as well as their relationship with the crack tip opening displacement, have been confirmed. It is also identified the parabolic shape of the stress-displacement curves, which modeled relates the specific non-essential work of fracture with the stress at initiation and the extension ratio during the crack growth.

The fracture testing of ductile polymer films: Effect of the specimen notching

The fracture of a ductile polymer film, a heterophase ethylene-propylene block copolymer, has been studied, combining a range of characterisation methods in an attempt to provide a better understanding of the intricate details that play an important role in the repeatability and reproducibility of the essential work of fracture test. The experimental factors that have a strong influence on the resulting parameters are clearly explained, with particular attention to the effect of the quality of the notches, the non-collinearity of the two edge notches in double edge notched tension specimens, and the lack of alignment of the specimen with the load axis once it is mounted on the load train. Furthermore, the influence of these experimental factors on the registered stress-displacement curves is also studied, and a criterion and the method for separating non-valid specimens are established.

Effect of specimen notch quality on the essential work of fracture of ductile polymer films

The essential work of fracture approach has been employed to analyse the effect of notch sharpening on the fracture toughness of a semicrystalline multiphase ethylene-propylene block copolymer. Double edge notched tension specimens were sharpened using different techniques: femtolaser ablation, razor blade sliding at room and liquid nitrogen temperatures, saw cutting, plastically deformed saw cutting, and scalpel sliding. The notch sharpening techniques provide notches of different quality in relation to both the notch tip radius and the plastic deformation in front of the crack tip. The best quality notches were produced by the femtolaser ablation technique, which provides very sharp notches without plastic deformation ahead of the crack tip. The effects of the non-collinearity of the notches and tilted specimens on the testing machine grips were also analysed. The shape of the registered stress–displacement curves shows differences, but only in the range comprised between the displacements corresponding to the maximum stress and the onset of crack initiation. A larger crack tip radius and/or larger extent of plastic deformation in front of the notch root leads to larger values of the displacement at the onset of crack initiation, resulting in higher values for the specific essential work of fracture. Yet on the other hand, the values of the slope of the essential work of fracture plot remain unchanged. A lower value for the specific essential work of fracture was obtained for the specimens sharpened using the femtolaser ablation technique.

Elastic-plastic defect interaction in (a)symmetrical double edge notched tension specimens

Interaction of defects tends to intensify their crack driving force response compared to the situation where these defects act independently. The interaction between multiple defects is addressed in engineering critical assessment standards like BS7910 and ASME B&PV Section XI. Nonetheless, the accuracy of these rules is open to debate since all of them are based on re-characterization procedures which in essence introduce conservativeness. The authors have developed a fully parametric finite element (FE) model able to generate multiple notches in different topologies, in order to investigate their interaction effect. An experimental validation study is conducted to verify the FE model in terms of CTOD response and surface strain distribution. To that end, symmetrically and asymmetrically double edge notched tension specimens are tensile tested and their deformation monitored by means of 3D digital image correlation. In this study the CTOD is opted as a local criterion to evaluate the interaction between notches. These results are compared with an evaluation of strain patterns on a specimen’s surface, as a global interaction evaluation. Through this comparison a deeper understanding is gained to allow us to develop a novel approach to address flaw interaction. Moreover, the validation of the FE model allows future studies of interaction between other defect types (e.g., semi-elliptical, surface breaking) in plate-like geometries.

Essential work of fracture, crack tip opening displacement, and J-integral relationship for a ductile polymer film

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modified film was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic specific work of fracture, we, is the specific energy just up to crack initiation, which is an initiation value. we has both a coincident value and the same conceptual meaning as Jo, the J-integral at the onset of crack initiation. The relationship between Jo and CTOD is also determined. The influence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.

A coupled FE–EFG approach for modelling crack growth in ductile materials

AbstractIn this work, a coupled finite element–element free Galerkin approach has been used to model crack growth in ductile materials under monotonic and cyclic loads. In this approach, a small discontinuous domain near crack is modelled by EFG method, whereas the rest of the domain is modelled by FEM to exploit the advantages of both the methods. A ramp function has been used in the transition region to maintain the continuity between FE and EFG domains. Two plasticity models (GTN and von‐Mises) and three hardening rules (isotropic, kinematic and mixed) have been used to model the nonlinear material behaviour. Four different problems, i.e. single edge notched tension specimen, double edge notched tension specimen, compact tension specimen and three‐point bend specimen, are solved under plane strain condition using J–R curve approach. Finally, a CT specimen problem is also solved by coupled approach using three hardening rules and two plasticity models under cyclic loading.

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.

Fracture Toughness of PC and PC/ABS Alloy

As a direct and simple method, essential work of fracture has been widely used for fracture measurement of ductile polymers. In this paper, fracture toughness of PC and PC/ABS alloy is experimentally investigated. A series of double edge-notched tension (DENT) specimens and essential work method are employed to measure the fracture toughness of PC and PC/ABS alloy. By the way, the critical crack tip opening displacement is obtained. Moreover, the fracture surfaces of DENT specimens are examined by using a scanning electron microscope (SEM).

Effect of annealing on fracture behavior of poly(propylene‐block‐ethylene) using essential work of fracture analysis

AbstractThe influence of annealing conditions on the fracture behavior of poly(propylene‐block‐ethylene) sheets was investigated by means of the essential work of fracture method, and was complemented by the study of the effect of annealing on crystal structure, using differential scanning calorimetry. It was shown that both the crystal perfection degree and crystallinity could be improved substantially as annealing temperature (Ta) increased, while the prolonged annealing time at 80°C mainly resulted in the improvement of crystallinity. The reasons for an increase in the specific essential work of fracture and a decrease in the specific plastic work item as crystal perfection degree and crystallinity grew are discussed. The displacement to failure of double edge notched tension specimens decreased gradually with increasing Ta, and the double‐plastic zone could be observed in all specimens. In addition, a novel method to aid the accurate measurement of intense stress‐whitening outer plastic zone height by adjustment of illumination conditions is proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3438–3446, 2007