Single Edge Notch Bend Specimens Research Articles

Fracture toughness of epoxy/multi-walled carbon nanotube nano-composites under bending and shear loading conditions

The effects of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of epoxy/MWCNT nano-composites were studied with emphasis on fracture toughness under bending and shear loading conditions. Several finite element (FE) analyses were performed to determine appropriate shear loading boundary conditions for a single-edge notch bend specimen (SENB) and an equation was derived for calculating the shear loading fracture toughness from the fracture load. It was seen that the increase in fracture toughness of nano-composite depends on the type of loading. That is to say, the presence of MWCNTs had a greater effect on fracture toughness of nano-composites under shear loading compared with normal loading. To study the fracture mechanisms, several scanning electron microscopy (SEM) pictures were taken from the fracture surfaces. A correlation was found between the characteristics of fracture surface and the mechanical behaviors observed in the fracture tests.

Transformation of BS7448-CTOD to ASTM E1290-CTOD

Experimental and analytical investigations into crack tip opening displacement (CTOD) were conducted to demonstrate the relationship between BS7448-CTOD and ASTM E1290-CTOD. The CTOD test results showed that ASTM-CTOD was occasionally much lower than BS-CTOD both in single edge notch bend specimens and in compact tension (C(T)) specimens for low-strength structural steels, and this tended to be more remarkable in C(T) specimens. In addition, the analytical results of simplified elastic-plastic fracture parameter calculation using the Electric Power Research Institute scheme demonstrated that the ratio of ASTM-CTOD to BS-CTOD was not constant but varied according to CTOD changes. Material factors such as the yield stress, the strain hardening exponent, specimen size and configurations influenced the CTOD ratio, and low strain hardening exponents in the Ramberg–Osgood relation and C(T) specimen configuration significantly decreased the CTOD ratio. An equation that transforms BS-CTOD into ASTM-CTOD is proposed in this study. This equation gives a good estimate of ASTM-CTOD from BS-CTOD.

Pipe fittings in duplex stainless steel with deviation in quality caused by sigma phase precipitation

Pipe fittings in duplex stainless steel (grade 2205) are used both top-side and subsea by the oil industry offshore. Precipitation processes during heat treatment of such fittings may result in various detrimental intermetallic phases and carbides. Among these defects, the sigma phase has a particular interest since it already in small amounts may cause a considerable reduction in mechanical properties and corrosion resistance of the material. To study the influence of sigma phase content on the stress–strain behaviour and ductility under quasi-static and dynamic loading conditions, tensile tests at room temperature (+20 °C) and low temperatures (−46 °C, −10 °C) were carried out for duplex stainless steel with varying levels of sigma phase (∼0 vol.%, 1–5 vol.%, ∼10 vol.%). Specimens pre-charged with hydrogen were also tested to study the influence of sigma phase on cathodic protected subsea components. In addition, fracture toughness tests (CTOD) on single-edge notch bend specimens and Charpy impact tests on V-notched specimens at various temperatures were conducted to establish the influence of sigma phase on the toughness of the material.

A computational analysis to assess the influence of specimen geometry on cleavage fracture toughness of metallic materials

The fracture response of mild steel in the domain of brittle behavior, i.e., the cleavage range, has been carefully evaluated using a weakest link statistical model, assuming the existence of a distribution of cracked carbide particles in the microstructures. Experiments have provided an evidence of both scatter in test results and the existence of constraints. Statistical-based model to include micromechanics were developed in an attempt to study and analyze the problem. The Weibull stress micro-mechanical model was used in this study to quantify the constraint effects. This was done numerically using a constraint function (g(M)) derived from the Weibull stress model. The non-dimensional function (g(M)) describes the evolution of the effects of constraint loss on fracture toughness relative to the reference condition, i.e., plane-strain, small scale yielding (SSY) (T-stress = 0). Single-edge SE(B) notched bending specimens having different crack lengths, different cross-sections and side-grooves were modeled and the constraint function (g(M)) was calculated. In this paper, we compare the loss in constraint for both the deep notch and shallow notch specimens for a given cross-section of the single-edge notched bend specimen (SE(B)).

ICFPD method application for crack initiation determination for Charpy size 3-point bend specimens

Fracture mechanics has attained a great attention in establishing ultimate load limitations and assessing the integrity for a large number of engineering structures of multifarious types. Fracture mechanics based integrity assessment procedure of components requires fracture toughness properties of the materials that have to be independent of the geometry and the size of the component or the specimen. This prerequisite of transferability must be satisfied, as the component to be assessed usually differs in size and geometry from the small scale specimens that are used for testing according to the test standards [1, 2]. As a material characteristic, the physical crack initiation point can be considered, but its determination is complicated. New Induced Current Focused Potential Drop (ICFPD) method development for the Ji determination for Single Edge Notched Bend specimens (SENB) of Charpy size is presented in this paper. The results obtained with the ICFPD method are compared with the SZW based results.

More Accurate Approximation of J-Integral Equation for Evaluating Fracture Resistance Curves

Abstract Accurate estimate of the J integral is required in a valid experimental evaluation of J-based fracture resistance curves for ductile materials. The fracture toughness test standard ASTM E1820 allows a basic method and a resistance curve method to be used experimentally to evaluate the J values via standard specimens. The basic method obtains J estimates using the η factor method that was developed for a stationary crack. The resistance curve method obtains crack growth corrected J estimates using an incremental equation that was proposed by Ernst et al. (“Estimation on J-Integral and Tearing Modulus T from a Single Specimen Test Record,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743, 1981, pp. 476–502) for a growing crack and has been accepted as the most accurate equation available for about three decades. Recently, Neimitz (“The Jump-Like Crack Growth Model, the Estimation of Fracture Energy and JR Curve,” Eng. Fract. Mech., Vol. 75, 2008, pp. 236–252), and Kroon et al. (“A Probabilistic Model for Cleavage Fracture with a Length Scale-Parameter Estimation and Predictions of Growing Crack Experiments,” Eng. Fract. Mech., Vol. 75, 2008, pp. 2398–2417) presented two different approximate equations for the J-integral, which they proposed as more accurate than the Ernst equation. Therefore, further investigation is needed to determine a truly accurate approximation for the J-integral equation. With this objective, the present paper proposes different mathematical and physical models to approximate the J-integral equation. The physical models are developed in terms of the deformation theory and the jump-like crack growth assumption. Relations between the proposed models and the existing equations are identified. Systematic evaluations of the proposed models are then made using a theoretical procedure of J-R curves for both low and high strain hardening materials, and using experimental data from an actual single edge-notched bend specimen made of HY80 steel. Accuracy of the proposed models is determined, and a more accurate approximation of J-integral equation is thus suggested for J-R curve testing.

Effect of crack depth on fracture toughness of 20MnMoNi55 pressure vessel steel

Ductile fracture behaviour of 20MnMoNi55 steel under quasi-static loading has been studied using single-edge notched bend specimens. To understand the influence of crack depth on fracture behaviour, J– R curves were obtained from specimens pre-cracked to various a/ W in the range of 0.25–0.75, in steps of approximately 0.1. Stretch zone width dimensions were measured on the fractured surfaces of the broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J– R curves to obtain a value of the ductile fracture toughness parameter J SZW. The initiation toughness, J i, at the intersection of the blunting line and the power-law fit to the J– R curve, and the critical toughness, J c, following the ASTM standard, were estimated. Comparisons of J SZW with J i and J c have been made for 20MnMoNi55 steel as a function of the a/ W ratio. The J i and J c values are found to be higher at lower a/ W, i.e. under low constraint, and decrease with increasing a/ W. It is observed that J SZW is not affected by a/ W, and that its value is lower bound to J i and J c values of the material investigated.

Strength and fracture behavior of two-, three- and four-dimensionally reinforced carbon/carbon composites

A comparative study has been carried out on mechanical behavior of two dimensionally (2D), three dimensionally (3D) and four dimensionally (4D) reinforced carbon/carbon composites (C/Cs) having 50, 48 and 37 vol.% carbon fibers, respectively. The tensile and flexural strengths of 2D- and 3D-C/Cs, found as ∼25% higher than that of 4D-C/C, appear related to fiber volume fraction. The load–displacement plots obtained from three-point bend tests on single edge notch bend specimens appear non-linear, suggesting graceful failure. Scanning electron microscopy studies of crack paths have shown high tortuousity due to crack deflection, branching as well as fiber debonding and breakage. The results indicate that total fracture energy release rate ( J c) and displacement ( δ 0.6) at 60% of peak load are suitable parameters for evaluation of fracture resistance and damage tolerance, respectively of multi-directionally reinforced C/Cs. The values of J c and δ 0.6 have been found as maximum for 2D- and 4D-C/C, respectively.

The fracture behaviour of dental enamel

Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8–1.5 MPa*m 1/2 at the beginning of crack propagation up to 4.4 MPa*m 1/2 at 500 μm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m 1/2.

Application of normalization method to fracture resistance testing for storage tank A285 carbon steel

To determine the fracture resistance and its constraint effect for storage tank A285 carbon steel, a set of single edge notched bend (SENB) specimens with different crack lengths are tested under the guideline of ASTM standard E1820. Normalization method is adopted to develop J–R curves by directly using the load versus load-line displacement data without the need for online crack size measurement. For validation, the conventional electric potential drop method is also used to monitor crack extension, and to determine the J–R curves. The results indicate that the crack extension, J–R curves and fracture initiation toughness determined from both methods are nearly identical for the SENB specimens considered. The J–R curves obtained from the A285 SENB specimens show a strong dependence on crack size or fracture constraint. In reference to the J–A 2 fracture theory, the constraint effect is quantified by a constraint parameter A 2. Through experimental and numerical analyses of the specimens, a constraint-corrected J–R curve is obtained as a function of crack extension and the constraint parameter A 2. It is concluded that the normalization method is valid and cost-effective for the determination of J–R curves from both standard and nonstandard SENB specimens. In addition, the parameter A 2 can reasonably quantify the constraint effect on the experimental J–R curves.

An experimental and numerical investigation of fracture resistance behaviour of a dissimilar metal welded joint

Dissimilar welds impose a challenge to the engineers concerned with the structural integrity assessment of these joints. This is because of the highly inhomogeneous nature of these joints in terms of their microstructure, mechanical, thermal, and fracture properties. Fracture mechanics-based concepts cannot be directly used because of the problems associated with the definition of a suitable crack-tip loading parameter such as J-integral crack tip opening displacement (CTOD), etc. Again, depending upon the location of initial crack (i.e. base, weld, buttering, different interfaces, etc.), further crack propagation can occur in any material. The objective of the current work is to use micro-mechanical models of ductile fracture for initiation and propagation of cracks in the bimetallic welds. The authors have developed a finite element formulation that incorporates the porous plasticity yield function due to Gurson—Tvergaard—Needleman and utilized it here for the analysis. Experiments have been conducted at MPA Stuttgart using single edge-notched bend (SEB) specimens with cracks at different locations of the joint. The micro-mechanical (Gurson) parameters of four different materials (i.e. ferrite, austenite, buttering, and weld) have been determined individually by simulation of fracture resistance behaviour of SEB specimens and comparing the simulated results with those of the experiment. In order to demonstrate the effectiveness of the damage model in predicting the crack growth in the actual bimetallic-welded specimen, simulation of two SEB specimens (with initial crack at ferrite—buttering and buttering—weld interface) has been carried out. The simulated fracture resistance behaviour compares well with those of the experiment.

Fracture and fatigue of natural fiber-reinforced cementitious composites

This paper presents the results of an experimental study of resistance-curve behavior and fatigue crack growth in cementitious matrices reinforced with eco-friendly natural fibers obtained from agricultural by-products. The composites include: blast furnace slag cement reinforced with pulped fibers of sisal, banana and bleached eucalyptus pulp, and ordinary Portland cement composites reinforced with bleached eucalyptus pulp. Fracture resistance ( R -curve) and fatigue crack growth behavior were studied using single-edge notched bend specimens . The observed stable crack growth behavior was then related to crack/microstructure interactions that were elucidated via scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Fracture mechanics models were used to quantify the observed crack-tip shielding due to crack-bridging. The implications of the results are also discussed for the design of natural fiber-reinforced composite materials for affordable housing.

A Comparison of Two- and Three-Dimensional Fracture Assessments in the Presence of Residual Stresses

The influence of various assumptions on the modeling of cleavage fracture in the presence of residual stresses was investigated. Analyses were undertaken for modified single edge notched bend specimens, manufactured from A533B ferritic steel. The influence of residual stress fields, introduced by a method of in-plane compression, was investigated through the use of a modified J-integral, designed to retain path independence in the presence of initial stress and strain fields and nonproportional loading. Application of modified J values to predict fracture using probabilistic methods, and their use in a well-known structural integrity assessment code, showed that assumptions about levels of out-of-plane constraint, material hardening behavior, and the method of crack introduction have a significant influence on the conservatism of the resulting failure predictions. It was found that more realistic modeling of crack introduction had a major effect on the accuracy of failure predictions, with the effects of material hardening being of secondary importance.

Mechanical, Thermal and Oxidation Behaviour of Zirconium Diboride Based Ultra-High Temperature Ceramic Composites

A comparative study has been carried out on the mechanical properties at room temperature, thermal shock and ablation resistance as well as oxidation behaviour of ZrB2-20SiC, ZrB2-20SiC-5Si3N4 and ZrB2-20ZrC-20SiC-5Si3N4 (amounts represent volume percent) composites. Fracture toughness has been determined using either three-point bend tests on single edge notch bend specimens, or by indentation technique. Addition of Si3N4 as sintering aid leads to enhancement in flexural strength and fracture toughness in the composite without ZrC. The specimens were subjected to thermal shock by quenching from temperatures in the range of 800o- 1200oC to ice cold water, and to ablation by exposure to oxy-acetylene flame at 2200oC. The composite having ZrC as constituent, exhibits the highest resistance to damage due to thermal shock and ablation, while the ZrB2-SiC composite shows the least change in mass during ablation. On the other hand, thermogravimetric experiments from room temperature to 1300oC have shown that the presence of ZrC is detrimental for oxidation resistance. Hence, the constituents of the composites need to be selected on the basis of the nature of application. The results of this study show that the investigated ZrB2 based composites bear the potential for multiple use thermal protection of reentry type space vehicles.

Fitness-For-Service Assessment of the Pipelines with Localized Geometric Imperfections such as Buckles and Wrinkles

In this paper, FFS assessment procedure for the buckle damaged pipeline with cracks in the girth welds is presented. For FFS assessment the tensile and J R-curve data from a pre-strained pipeline material, API 5L X65 were obtained in the laboratory to study the influences of the large plastic strain on the material properties and the fracture resistance of the pipeline girth welds. Tensile and single edge notch bend specimens in as-received, 10% pre-strained and 20% prestrained conditions were tested. The test results show significant increase in yield and tensile strength in the pre-strained specimens. Generally, the elongation and fracture resistance decreased after pre-straining. In FFS material specific failure assessment diagrams (FADs) generated based on the stress-strain curves obtained from testing were used. The critical flaw sizes of the pipeline girth welds were calculated, and the influence of the large plastic strain on the FFS results was discussed.

On the relationship between J-integral and CTOD for CT and SENB specimens

In this investigation the relationship between J-integral and CTOD is studied considering a Compact Tensile (CT) and Single edge notched bend (SENB) specimens using finite element analysis. The magnitude of CTOD is estimated by 90o-intercept method and also by plastic hinge model. The results indicate that there exists a discrepancy in estimation of CTOD by 90o-intercept method and by plastic hinge model. The CTOD values obtained by both the methods are found to be linearly proportional to J-integral. The linear proportionality constant dn between CTOD and J is found to strongly depend on the method of estimation of CTOD, specimen geometry and a/W ratio of the specimens.

Fracture toughness of the nano-particle reinforced epoxy composite

Abstract Although thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites due to their good mechanical properties compared to those of thermoplastic polymers, they are usually brittle and vulnerable to crack. Therefore, ductile materials such as micro-sized rubber or nylon particles are added to thermoset polymers are used to increase their fracture toughness, which might decrease their strength if micro-sized particles act like defects. In this work, in order to improve the fracture toughness of epoxy adhesive, nano-particle additives such as carbon black and nanoclay were mixed with epoxy resin. The fracture toughness was measured using the single edge notched bend specimen at the room (25 °C) and cryogenic temperature (−150 °C). From the experimental results, it was found that reinforcement with nano-particles improved the fracture toughness at the room temperature, but decreased the fracture toughness at the cryogenic temperature in spite of their toughening effect.

Crack-growth behavior of epoxy adhesives modified with liquid rubber and cross-linked rubber particles under mode I loading

The crack-growth resistance ( R-curve) of bulk single-edge notch bend (SENB) and adhesively bonded double cantilever beam (DCB) specimens was investigated under mode I loading conditions using two types of rubber-modified epoxy adhesive: one was a liquid rubber (CTBN)-modified adhesive and the other was a cross-linked rubber particle (DCS)-modified adhesive. As a result, for both the SENB and DCB specimens, the gradient of the R-curve for the DCS-modified adhesive was steeper than that for the CTBN-modified one, however, the difference in fracture toughness between DCS- and CTBN-modified adhesives is smaller for DCB than for SENB specimens. To elucidate such behavior, crack-growth simulation based on Gurson's model was conducted, where the DCS- and CTBN-modified adhesives were characterized by both the initial void fraction and nucleation. The difference in the behavior of R-curves was also observed in simulations. Moreover, it was found that the difference in fracture surface roughness observed by SEM for both adhesives correspond to the variation in R-curves.

Effect of vibratory weld conditioning on welded valve properties

The approach to improve the quality of full welded valve by the application of vibratory weld conditioning (VWC) was investigated in this paper. The effect of VWC on the welded joint properties was studied. Cylinders with different thicknesses were welded by submerged arc welding (SAW). The experiment results show that the vibration applied during welding generally reduces the residual deformation and stress. The yield strength, as well as the tensile strength, does not change distinctively in vibratory SAW (V-SAW) when compared with that in normal SAW (N-SAW). The bend property has been improved in V-SAW. Fracture test was carried out using single edge notch bend (SENB) specimens. The fracture surfaces of failed weld metal (WM) specimens are characterized by dimple-like structures. The depth and the density of these dimples are increased after the vibration applied in welding. River marking, the characteristic of cleavage fracture, is observed in the heat-affected zone (HAZ) specimens. The possible solidification behaviour in VWC is discussed in this paper.

Characterization and Modeling of Fracture and Damage Behavior of Notched Woven Fabric GFRP Laminates under Three-point Bending at Cryogenic Temperatures

This study focuses on understanding the cryogenic fracture and damage behavior of woven fabric GFRP laminates. Fracture toughness tests were conducted in three-point bending using single-edge-notched bend (SENB) specimens at room, liquid nitrogen, and liquid helium temperatures. The effects of temperature and specimen size on the apparent fracture toughness were evaluated. A three-dimensional finite element analysis coupled with damage was also employed to study the nonlinear load-displacement response and the damage evolution of SENB specimens, and to examine the effects of temperature, specimen size and damage on the fracture mechanics parameter. The predictions were compared with the experimental data.