Single Edge Notched Tensile Specimens Research Articles

Experimental studies on the extent of 3D and 2D stress–strain states at sharp V-notches

A sharp V-notched plate under plane stress loading conditions is always accompanied by a strong three-dimensional (3D) stress–strain region close to the notch tip, followed by a 3D-2D transition state and a region dominated by the 2D stress–strain field. Experimental studies on planar V-notched bodies constitute an important part of notch fracture mechanics and therefore it is important to know the extents of the above fields for accurate measurement of field variables. In the present work, experimental verification of the extent of the 3D stress–strain field, 3D-2D transition zone and plane stress zone ahead of the tip of a sharp V-notch has been carried out in order to suggest a minimum radius required for identification of the plane stress dominant zone. Knowing this value apriori is vital for accurate sampling of the 2D field variables in any experimental study on the sharp V-notched configurations. In order to achieve this, strain gage experiments on the single-edge notched tensile (SENT) specimens have been conducted. The above zones have been identified by studying the error in the measured mode I NSIF of the SENT specimen. The experimental results of the current study agree well with the results of published numerical works. The results clearly show that the minimum radius of the plane stress dominant zone has been found to be 1.25 times the thickness of the plate. Results also show that 1.5 times the thickness is the most conservative and dependable value for this radius. The results of the present investigation also indicate that not only the identification of the plane stress region, but also the locations of the sampling points within the plane stress region are important for achieving the required accuracy.

Fracture toughness of freestanding plasma sprayed yttria stabilized zirconia coatings via in situ tensile experiments

Tensile response and fracture toughness of the freestanding atmospheric plasma spray 8YSZ coatings was investigated using in situ high resolution optical full-field strain measurement. The elastic properties obtained directly from the tensile experiments were employed in computing the critical mode-I stress intensity factor. Comparison between the fracture toughness values computed from specimen geometry and J-integral via displacement field around the crack tip showed small differences on single edge notched tensile (SENT) specimens to extract their intrinsic mode-I fracture toughness. Two kinds of coatings having different porosity were studied to obtain stress-strain response and compute effect of porosity on mode-I fracture toughness. The samples were fabricated by depositing the coatings on, un-notched and pre-notched substrate templates, and then carefully separating the coatings by acidic dissolution of the substrate. High resolution in situ deformation of the coatings was captured for both tensile and SENT specimens using digital image correlation method combined with an in-house designed microtensile stage. The deformation from the specimen surface was used to obtain elastic material properties under uniaxial tension, which are then used to compute the energy release rate by the path independent J-integral. The critical stress intensity factors calculated from SENT geometry for the low and high porosity coatings were found to be 1.23 ± 0.21 MPa-m0.5 and 0.67 ± 0.12 MPa-m0.5, respectively. In addition, considering the experimental observation of nucleation and propagation of a crack from the notch tip before catastrophic failure, the J-integral based calculation of the critical fracture toughness values were 1.37 ± 0.19 MPa-m0.5 and 0.5 ± 0.1 MPa-m0.5, for the low and high porosity coatings, respectively. As the crack always nucleates from the notch before complete fracture, it can be concluded that the J-integral based method is accurate and suitable for finding the fracture toughness in porous coatings.

Pin-loaded SENT specimen for constraint-matched fracture testing of radially propagating longitudinal cracks in thin-walled pipelines

There is considerable interest in the fracture community for representative measurements of the resistance of pipe structures to fatigue crack propagation and fracture. However, to date there has been a lack of standard test methods available for the accurate fracture testing of radially propagating longitudinal flaws in thin-walled piping. A pin-loaded single edge-notched tensile (SENT) specimen was developed, denoted the CSR SENT specimen. This test geometry has been shown to reproduce the plastic constraint for longitudinal cracks in thin-walled piping, resulting in transferable fracture test data to that application beyond the conventional limits of single-parameter J-dominated fracture mechanics. The CSR SENT was simulated with finite element analysis (FEA) to yield the parameters necessary for the experimental determination of the J-integral using standard fracture test methods. The FEA results showed that the stress intensity geometry factor, F, for the CSR SENT matched literature solutions for most crack lengths, but that the CMOD-Eta factor – ηCMOD, used to calculate the plastic component of the J-integral from the applied load and crack mouth opening displacement (CMOD) – differed from literature solutions and is specimen size-dependent. The error associated with the calculation of the J-integral is estimated to be < 5% with the obtained ηCMOD, while the use of literature solutions for ηCMOD result in significant non-conservative overestimates of the experimental J-integral. It is recommended that fracture testing be limited to a specific specimen geometry associated with a FEA-derived ηCMOD solution. The crack growth correction to the J-integral measurement was also obtained, and is recommended to be derived from the ηpin (also referred to as ηLLD, derived from the load-displacement of the pin boundary), which can be used alongside the ηCMOD for experimental calculation of the J-integral in standard single-specimen fracture testing. Friction at the pin-specimen interface results in significant non-conservative bias error in the J-integral, and a rolling pin-clevis contact is recommended with a pin diameter twice that of the specimen width to mitigate pin contact plasticity and damage.

Evaluation of ductile tearing resistance of an interstitial free steel sheet using SENT specimens

Ductile tearing resistance of a 1 mm thick interstitial free steel sheet has been characterized using single edge notched tensile (SENT) specimens with or without fatigue precracks at quasistatic strain rates. In this study, the critical Crack Tip Opening Angle (CTOA), and also crack initiation parameters, namely the initiation energy per unit ligament area (wi), critical value for the J-integral (Jc), and critical crack tip opening displacement (δc) have been examined and compared with the corresponding parameters from the Essential Work of Fracture (EWF) method. Dependence of these parameters on specimen geometry has also been examined. The EWF paradigm has been extended to the initial mixed-constraint regime of crack extension. 3D cohesive zone modelling for flat fracture becomes reasonably successful in simulating crack growth in the SENT specimens.

A study on the matching of constraint between steam turbine blade and laboratory specimens

The matching of constraint between laboratory specimens and actual cracked structures is a key problem of the accurate structure integrity assessment. Different laboratory specimens and the steam turbine blade with different constraints were selected, the matching of constraint between steam turbine blade and laboratory specimens was investigated. The results shown that the steam turbine blade with 2 c = 50 mm, a/2 c = 0.20 has a matching constraint with single edge-notched bend specimen with a/ W = 0.6 and single edge-notched tensile specimen with a/ W = 0.3. The steam turbine blade with 2 c = 50 mm, a/2 c = 0.25 has a matching constraint with single edge-notched bend specimen with a/ W = 0.7. The steam turbine blade with 2 c = 50 mm, a/2 c = 0.30 has a matching constraint with single edge-notched bend specimen with a/ W = 0.5 and single edge-notched tensile specimen with a/ W = 0.1. The steam turbine blade with 2 c = 50 mm, a/2 c = 0.35 has a matching constraint with single edge-notched bend specimen with a/ W = 0.4, compact tension specimen with a/ W = 0.3 and central-cracked tension specimen with a/ W = 0.7. The steam turbine blade with a = 15 mm, a/2 c = 0.30 has a matching constraint with compact tension specimen with a/ W = 0.7 and single edge-notched tensile specimen with a/ W = 0.5. The steam turbine blade with a = 15 mm, a/2 c = 0.40 has a matching constraint with compact tension specimen with a/ W = 0.4. The steam turbine blade with a = 15 mm, a/2 c = 0.50 has a matching constraint with single edge-notched bend specimen with a/ W = 0.5.

FATIGUE CRACK GROWTH AND FATIGUE FRACTURE MORPHOLOGY OF RECYCLED RUBBER POWDER–FILLED NR/BR BLEND COMPOUND

ABSTRACTThe effect of two different types and particle sizes (micronized cryo-ground 74 μm or ambient-ground 400 μm) of recycled rubber powder (RRP) was studied during fatigue crack growth (FCG) in a natural rubber/butadiene rubber (NR/BR) compound using a fracture mechanics approach. Absolute and relative hysteresis losses using single-edge notch tensile specimens were determined with a displacement-controlled strain compensating for the permanent set of the samples throughout the FCG experiments. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates. At lower tearing energy, the larger RRP-filled compound showed slower, but not significant, different crack growth rates than the NR/BR control compound. Fracture morphologies for NR/BR and RRP-filled compound were associated with different fracture surface topographies at various tearing energies, which revealed the dependency of the crack growth microstructure on the tearing energies.

Mitigation of Intergranular Cracking in Al-Mg Alloys via Zn-Based Electrode Potential Control in Sodium Chloride Solution

Mitigation of the initiation and propagation of intergranular stress corrosion cracking (IG-SCC) in a single edge notch tensile specimen is achieved through use of various Zn-based anodes on highly...

Influence of grain size on the small fatigue crack initiation and propagation behaviors of a nickel-based superalloy at 650 °C

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

An experimentally validated computational model for progressive damage analysis of notched oxide/oxide woven ceramic matrix composites

The focus of this paper is to examine the effect of a notch on the failure response of an oxide/oxide ceramic matrix composite (CMC) subjected to room-temperature tensile loading. The CMC is made of aluminosilicate matrix reinforced by 3M Nextel™ 610 alumina fibers (AS/N610) that are woven into an eight-harness satin weave (8HSW) preform. Uniaxial tensile tests were carried out on both un-notched and single-edge-notched tensile (SENT) specimens of two different lay-ups, (0/90)S and (45/0/-45/0)S , using a hydraulically activated load frame. The digital image correlation (DIC) technique was utilized to map the deformation histories and identify the locations of strain concentration for both un-notched and SENT specimens. The experimental results were subsequently used to develop a lamina-level constitutive model for a single 8HSW ply that incorporates damage and fracture mechanics implemented through a finite element (FE) framework. A secant-modulus approach was employed to model the pre-peak nonlinear damage evolution, while the post-peak softening was modeled using the smeared crack approach (SCA). Utilizing the ply properties measured from the un-notched (0/90)S and (45/0/-45/0)S tension tests, the proposed computational model is able to predict the failure strength and strain localization in the SENT specimens. Such a predictive model can aid in the design of CMC structures and accelerate the development of CMCs for engineering applications.

A SFEM-based evaluation of mode-I Stress Intensity Factor in composite structures

The Strong Formulation Finite Element Method (SFEM) based on the Generalized Differential Quadrature (GDQ) method is applied in the present work to estimate numerically the Stress Intensity Factor (SIF) for a single edge-notched tensile specimen (SENT) with different length-to-width ratios and composite materials, as well as for a Center Cracked Tension (CCT) and a Double Edge-Notched Tensile (DENT) specimens, under a mode-I loading condition. The basis notions of the fracture mechanics are herein analyzed numerically in order to evaluate the influence of possible cracks within materials and to relate the dimensions of cracks and the applied loading to the varying stress distribution. The numerical results, in terms of stresses and SIFs, are straightforwardly compared to those ones given by the standard Finite Element Method (FEM) and theoretical predictions available from the literature. This work presents a consistent approach for the computation of the SIF using a strong form methodology. The main aim is to demonstrate the accuracy and efficiency of the proposed methodology when treating classical plane stress problems with cracks. We show and discuss results from several numerical examples, including different composite materials and varying geometries for a mode-I SENT, CCT and DENT specimens.

Determination of Fatigue Properties Using Miniaturized Specimens

It is very important to determine Paris’ power law constants to know the fatigue crack growth rate (FCGR) of a structure containing crack. These constants are generally determined by FCGR test using conventional specimens as specified by ASTM-E647 standard. This standard FCGR test procedure has placed strict size requirements on the conventional specimens and hence a large amount of material is needed to fabricate the specimens. However, sometimes, like in the case of testing of irradiated materials or nuclear pressure vessel surveillance, the material available for testing is not sufficient to make standard size specimens. Therefore, miniature specimens should be used to minimize the amount of material. In this work, constant load amplitude FCGR behavior of nuclear pressure vessel steel (20MnMoNi55) has been studied using miniature Single Edge Notched Tensile (SENT) specimens. The fatigue tests have been performed according to the procedure described in ASTM-E647 standard. The Paris’ power law coefficient and exponent obtained by FCGR test of miniature SENT specimens are in the range of 10−12 to 10−13 and 2.8–3.5 respectively, when the FCGR (da/dN) is expressed in m/cycle and stress intensity range (ΔK) in MPa√m. These values of Paris’ power law constants of miniature specimens are nearly of same order as that of the standard specimens where the results of standard specimens are taken from the literature.

Limit Load Equations for Miniature Single Edge Notched Tensile Specimens

While carrying out fatigue tests on miniature Single Edge Notched Tensile (SENT) specimens, one needs to know the limit load of the specimen to determine the maximum and minimum loads of fatigue cycle. In the literature, several limit load equations, for example, EPRI, Graba, Miller were available. There were large differences in the predicted values of these equations. Moreover, these equations were developed for conventional size of the SENT specimens. Although, limit load equations should be ideally size independent, it was not clear whether boundary effect of the miniature specimens is there or not. Therefore, it was important to investigate the accuracy of these equations and check whether they are applicable for miniature specimens or not. Consequently, detailed non-linear finite element analyses were carried out on a number of miniature SENT specimens to determine the limit load by twice elastic slope method. Elastic-perfectly plastic material behavior was assumed as is the convention. Based on these results, new limit load equations have been proposed for miniature SENT specimens. It was found that the present results match very closely with the prediction of Miller equation. However, they widely differ with respect to EPRI and Graba equations. Consequently, it is concluded that Miller equation is the most accurate to predict the limit load of SENT specimens and is applicable for miniature specimens as well without any boundary effect.

Study of SENT specimens with a tilted notch to evaluate ductile tearing in spiral welded pipeline applications

There is an increasing interest for the use of spiral welded pipelines in strain based design applications. Environmentally imposed loads are able to plastically deform the pipelines, meaning that their structural response is of the utmost importance. However, since the influence of the spiral weld is not fully grasped, further investigation is necessary. The mechanical response of the pipeline is not only influenced by its material properties, but also by the angular position of the welds. Subsequently, the effect of mixed mode loading is a crucial aspect when assessing the helical welds. To evaluate the ductile tearing of the pipeline material, multiple single edge notched tensile (SENT) tests - each with a tilted notch of 25° with respect to the transverse direction - were executed. The extension of the crack is assessed by means of potential drop measurements and finite element simulations. Resistance curves were realized by combining the crack opening displacement with the associated crack extension. This is an ongoing investigation and in this paper a first set of five tests are evaluated.

Damage progression and notched strength recovery of fiber-reinforced polymers encompassing self-healing of interfacial debonding

This study examines the interfacial debonding behavior of self-healing fiber-reinforced polymers and the performance of repair of interfacial debonding. Self-healing is accomplished by incorporating a microencapsulated healing agent and a catalyst within a coating layer on the surface of the fiber strand. Single edge-notched tensile specimens of unidirectional fiber-reinforced polymers with coated fiber strands were prepared and tested. The healing efficiency was evaluated by the critical fracture loads of virgin and healed specimens. To investigate the release of the healing agent from microcapsules during interfacial debonding, an ultraviolet fluorescent dye was added to the healing agent, and post-fracture specimens were examined by an optical microscope under ultraviolet light. In addition to conducting experiments, finite-element analyses were performed using a three-dimensional model to predict the damage progression in single edge-notched tensile specimens.

A crack propagation criterion based on ΔCTOD measured with 2D‐digital image correlation technique

ABSTRACTThe fatigue cracks growth rate of a forged HSLA steel (AISI 4130) was investigated using thin single edge notch tensile specimen to simulate the crack development on a diesel train crankshafts. The effect of load ratio, R, was investigated at room temperature. Fatigue fracture surfaces were examined by scanning electron microscopy. An approach based on the crack tip opening displacement range (ΔCTOD) was proposed as fatigue crack propagation criterion. ΔCTOD measurements were carried out using 2D‐digital image correlation techniques. J‐integral values were estimated using ΔCTOD. Under test conditions investigated, it was found that the use of ΔCTOD as a fatigue crack growth driving force parameter is relevant and could describe the crack propagation behaviour, under different load ratio R.

Evaluation and interpretation of ductile crack extension in SENT specimens using unloading compliance technique

Resistance curves obtained from Single Edge Notch Tensile (SENT) specimens gain interest for defect assessment in pipelines. To determine these resistance curves, the unloading compliance technique is a commonly applied single specimen technique. A frequently reported problem is the occurrence of an initial decrease in the compliance, resulting in an apparent negative crack growth. To overcome this problem an offset in combination with a blunting correction is proposed. This approach is evaluated by finite element simulations. Then the proposed approach is validated experimentally. The presented approach is simple in use and provides an acceptable estimation error when it comes to constructing resistance curves.

Considerations in selecting laboratory scale test specimens for evaluation of fracture toughness

The assessment of defects in large steel structures requires a trustworthy evaluation of the material’s toughness. This toughness is not only a material property but is also influenced by the loading conditions and geometry; the so-called constraint. The resulting representative value is referred to as the apparent toughness. The evaluation of apparent fracture toughness in a flawed structure is preferentially performed through laboratory scale testing, as full scale tests are both expensive and often challenging to perform. Several laboratory scale test specimens are available, among which a Single Edge Notch Bending specimen, Single Edge Notch Tensile specimen, Double Edge Notch Tensile specimen and Centre Cracked Tensile specimen. Each of these specimens has its own specific constraint. Therefore, the selection of an appropriate test specimen is of primary importance for limiting the conservatism and avoiding potential unconservatism with respect to full scale behaviour. This paper provides a general framework to select an appropriate test specimen based on detailed finite element simulations of both the full scale structure and the laboratory scale test specimens. These finite element calculations allow for a characterization of the crack tip stress fields in both situations. Different theoretical frameworks are available for this characterization; theQ -parameter is considered in this paper. To demonstrate the applicability of this procedure, an example case is presented for circumferentially oriented defects in pressurized pipelines under longitudinal tension. It is concluded that the presented framework allows for efficiently selecting a laboratory scale test specimen, which enables to evaluate the apparent fracture toughness for a given large scale structure. The obtained toughness can thus be incorporated in analytical flaw assessment procedures, reducing the degree of conservatism. This in turn allows an economically effective design.

Ductile crack growth and constraint in pipelines subject to combined loadings

An important failure mode of offshore pipelines is ductile fracture of the pipe wall triggered by a hypothetical welding defect. In this study, pipelines having an external part-through semi-circumferential crack of various sizes, subject to combined internal pressure and inelastic bending are considered. This is done to assess the response of pipelines during both their installation and operational conditions. Detailed 3D nonlinear finite element (FE) models of pipelines are developed. A row of elements ahead of the initial crack front are modeled using a voided plasticity material model, which enables simulation of crack growth and the subsequent fracture failure mode (denoted by the critical curvature, κ crit ). After discussing the typical response characteristics of such pipelines, the FE model is used to parametrically investigate the influence of varying pipe and crack dimensions, and also the internal pressure levels, on κ crit . In the second part of this paper, the crack tip constraint ahead of a growing crack in such pipes is evaluated and systematically compared to the crack tip constraint of both the traditionally used deeply cracked Single Edge Notch Bend (SENB) specimens and the constraint-matched Single Edge Notch Tensile (SENT) specimens. This is achieved by comparing the crack resistance curves ( R-curves) along with stress triaxiality and equivalent plastic strain fields evaluated ahead of a growing crack of the three systems. The results present grounds for justification of usage of SENT specimens in fracture assessment of such pipes as an alternative to the traditional overly conservative SENB specimens.

A method for determining the stress intensity factor of a single edge-notched tensile specimen

A straightforward method for determining the stress intensity factor of a single edge-notched tensile specimen with unknown end conditions is presented. It involves the use of measured crack mouth opening displacements, coupled with existing solutions for stress intensity factors and crack opening displacements for uniform tensile loading and for pure bending. The use of the method is demonstrated through experiments on a porous matrix ceramic composite.

INVESTIGATION OF STATIC AND DYNAMIC FRACTURE TOUGHNESS OF SHORT CERAMIC FIBER REINFORCED POLYPROPYLENE COMPOSITES

Short ceramic fiber reinforced polypropylene composites have been investigated to determine their static and dynamic fracture toughness for different reinforcing fiber contents. The composites were reinforced with fibers produced by a carding technique combined with needle-punching. Static fracture toughness (K c) was measured on single-edge notched tensile (SEN-T) specimens, while dynamic fracture toughness (K d) was tested by impact strength Charpy specimens. Specimens in both cases were cut transverse (T) and in longitudinal (L) directions. Test results show that dynamic fracture toughness is larger than the static one. During loading of SEN-T specimens the burst-type acoustic emission (AE) signals were monitored. From AE signals it can be concluded that the main damage form is the pull-out in the T specimens, and debonding in L ones. These results were supported by scanning electron microscopy micrographs taken from fracture surfaces.