Single Edge Notch Bend Specimens Research Articles

Effects of support friction on mode I stress intensity factor and fracture toughness in SENB testing

Effect of support friction on the values of stress intensity factor (SIF) and fracture toughness obtained from the single edge notch bending (SENB) specimen is evaluated both numerically and experimentally. In the first step, a comprehensive finite element (FE) study is performed to assess the friction effects for different dimensions of SENB specimen and various friction coefficients. The variations of stress intensity factor against the friction coefficient between the specimen and the bottom supports are evaluated. On the other hand, to investigate the friction effects experimentally, two different sizes of SENB specimens made of polymethyl methacrylate (PMMA) with three different types of contact between the supports and the specimens are considered. The digital image correlation (DIC) method is employed to assess the effect of support friction on the mode I SIF. Fracture tests are also performed on the PMMA specimens to obtain the critical stress intensity factor for each case separately. Both FE and experimental results reveal that the support friction affects the results significantly, and in order to extract the SIF and fracture toughness of PMMA accurately, it is mandatory to use the freely rotating roller contact between the supports and the SENB specimen.

Fracture Toughness Testing of 6061Al–Graphite Composites Using SENB Specimens

Aluminum-based metal matrix composites have been increasingly utilized as engineering materials over the last three decades replacing the conventional engineering materials. The effects on the KIc by addition of reinforcements have been comprehensively studied for aluminum-based metal matrix composites. Hence, engineers have been working on enhancing the fracture toughness (KIc) of metal matrix composites in recent years. Aim of the present work is to evaluate the fracture toughness (KIc) of aluminum (6061)–graphite particulate-reinforced composites produced using a stir casting method. 6061Al–graphite particulate composites for 3, 6, 9 and 12 wt% of graphite were produced. Single edge notch bend specimens are used for bending test to investigate the fracture toughness of the composites. Two-dimensional finite element analyses using ANSYS were carried out, and the outcomes are judged against with the experimental data. It has been shown that the predictions are in reasonable agreement with the experimental data. From the results, it is found that the aluminum–graphite MMC has more fracture toughness then aluminum-silicon carbide metal matrix composites for all compositions .

Enhancement of fracture toughness under mixed mode loading of ABS specimens produced by 3D printing

Purpose The purpose of this paper is the application and the improvement of a previous method based on an acrylonitrile butadiene styrene thread deposition in fused deposition modeling. To gain up to 20 per cent of mechanical strength in comparison with a classical deposition, this method suggests a smart threads deposition in the principal stresses direction. Design/methodology/approach In this work, the authors use single edge notched bend specimens with mixed mode I+II loading cases to study the influence of the thread deposition on the fracture toughness of the specimens. For this purpose, finite elements simulations have been used to evaluate the fracture toughness of the specimens through the calculation of the J integral. The study presents a method to compare the optimized and classical specimens and also to gather data and suggest a numerical model for this optimized deposition. For this reason, tensile tests are carried out to characterize the mechanical behavior of the printed samples with respect to the raster angle. Extra attention has been paid to 45 per cent samples behavior that shows a pronounced plasticity before the fracture. This interprets partially the improvement in the fracture behavior of the single edge notched bend samples. Findings The results show an enhancement through this optimization which leads to an increase of the maximal force in fracture up to 20 per cent and the fracture toughness of the specimens with stress intensity factors KI and KII increases about 30 per cent. Originality/value Additive manufacturing is increasingly gaining importance not only in prototyping but also in industrial production. For this reason, the characterization and the optimization of these technologies and their materials are fundamental. An adaptive deposition through a smart material based on specific mechanical behaviors would be an advance.

Experimental determination of the notch stress intensity factor for sharp V-notched specimens by using the digital image correlation method

Digital image correlation (DIC) method is used to experimentally determine the notch stress intensity factor for sharp V-notched samples under mode I loading conditions. First, two well-known test specimens, namely the single edge notch tension (SENT) and the single edge notch bending (SENB) specimens are chosen. The specimens are made of Polymethyl-methacrylate (PMMA) and for each specimen, five different notch opening angles are considered. Then, the experimental displacement fields in the specimens are derived by using DIC method. Afterward, the over-deterministic method is employed to derive the mode I notch stress intensity factors for the samples tested. According to the over-deterministic method, the analytical displacement field is fitted to a large amount of experimentally obtained local displacement values. Finally, the results obtained from DIC are compared with the results available in the literature. The good accuracy of the results obtained demonstrates the ability of DIC method in derivation of the stress field for sharp V-notched structures.

A simplified method for evaluation of brittle crack arrest toughness of steels in scaled-down bending tests

The wide-plate crack arrest test under tensile load for characterizing the brittle crack arrest toughness of high strength shipbuilding steel requires high economical cost and long lead-time, so there is still a substantial industry need for the simplified evaluation using the scaled-down specimen. The alternative method using a single edge-notched bending specimen, which assumes the complete load redistribution during the crack propagation, was recently proposed. However, it was confirmed that the results obtained using this method exhibited a low correlation with the arrest toughness obtained using the wide-plate tests. A new scaled-down version of the crack arrest test under bending load and a series of simplified evaluation based on the dynamic elasto-plastic FEA were introduced to characterize the crack arrest performance of the two types of steel plates in this investigation. The specimen is a single edge-notched tapered plate subjected to a three-point bending load in an isothermal environment. The geometry was selected so that the dynamic crack driving force calculated using FEA decreases monotonically with the crack propagation. For adopting a simplistic analytical approach considering the dynamic effect, the dynamic FEA simulation assumed the constant crack velocity and flat crack front and output the opening stress distributed ahead of the growing crack. The experimentally obtained values of the arrested crack length and the analytically obtained transition curves of the dynamic SIF were employed to characterize the crack arrest performance of the two steels. The results obtained using the simplified method developed in this investigation exhibited a high correlation with the arrest toughness obtained using the wide-plate crack arrest test.

A M-theta Method on Mode I and II Failure of Two Cameroonian Hardwoods in Bending

This work deals with the numerical simulation on bending test to characterize two Cameroonian hardwoods under mode I and II loading for different crack lengths. The finite element analysis for fracture in orthotropic medium is developed. The algorithm of fracture is introduced in a finite element software Cast3M. According to the Mtheta method, the calculation of the stress intensity factors and the energy release rate for pure mode I and II fracture are deduced using a SENB (Single Edge Notch Bending) specimen. The path independence of Mθ-method on the specimen is confirmed.

M-theta Method on Mode I and II Failure of Two Cameroonian Hardwoods in Bending

This work deals with the numerical simulation on bending test to characterize two Cameroonian hardwoods under mode I and II loading for different crack lengths. The finite element analysis for fracture in orthotropic medium is developed. The algorithm of fracture is introduced in a finite element software Cast3M. According to the Mtheta method, the calculation of the stress intensity factors and the energy release rate for pure mode I and II fracture are deduced using a SENB (Single Edge Notch Bending) specimen. The path independence of M?-method on the specimen is confirmed.

Study of the fracturing behavior of thermoset polymer nanocomposites via cohesive zone modeling

This work proposes an investigation of the fracturing behavior of polymer nanocomposites. Towards this end, the study leverages the analysis of a large bulk of fracture tests from the literature with the goal of critically investigating the effects of the nonlinear Fracture Process Zone (FPZ).It is shown that for most of the fracture tests the effects of the nonlinear FPZ are not negligible, leading to significant deviations from Linear Elastic Fracture Mechanics (LEFM) sometimes exceeding 150% depending on the specimen size and nanofiller content.To get a deeper understanding of the characteristics of the FPZ, fracture tests on geometrically-scaled Single Edge Notch Bending (SENB) specimens are analyzed leveraging a cohesive zone model. It is found that the FPZ cannot be neglected and a bi-linear cohesive crack law generally provides the best match of the experimental data.

Effect of graphite on fracture toughness of 6061Al-graphite

This paper exhibits the investigative work conducted on the fracture toughness and microstructure of 6061Al-graphite particulate composites. The requisite specimens were prepared using stir casting technique with graphite proportions ranging from 3 to 12% by weight. The microstructure and interfaci al bonding of 6061Al-graphite particulate composites were studied. Fracture toughness investigations were carried out on 6061Al-graphite for 3%, 6%, 9% and 12% of graphite by using single-edge notch bend (SENB) specimens. The maximum fracture toughness was found for 6061Al-9%Gr for a∕W = 0.45 and the value is 17.94 MPa √m. Examination of specimen surface of 6061Al-graphite was done using scanning electron microscope (SEM). Due to good interface bonding and uniformity in distribution, improved fracture toughness properties are achieved.

Prediction of the ultimate strength of quasi-isotropic TP-based laminates structures from tensile and compressive fracture toughness at high temperature

This paper is intended to test the capacity of a simple model based on fracture mechanics concepts to predict the ultimate strength of notched hybrid carbon and glass fibers woven-ply reinforced PolyEther Ether Ketone (PEEK) thermoplastic (TP) quasi-isotropic (QI) laminates under different temperature conditions. In such materials, translaminar failure is the primary failure mode driven by the breakage of 0° and 45° oriented fibers in tension as well as the formation of kink-band in compression. Single-Edge-Notched Bending (SENB), Open-Hole-Tensile (OHT) and Open-Hole-Compression (OHC) specimens have been conducted at room temperature (RT) and at a temperature higher than the glass transition temperature (Tg). The Critical Damage Growth model derived from the Average Stress Criterion and Linear Elastic Fracture Mechanics (LEFM) have been applied to open-hole specimens to determine the critical damage zone from which the fracture toughness in tension (0° and 45° fibers breakage) KIctension and in compression (kink-band formation) KIccomp. are estimated. In Single Edge Notched Bending (SENB) specimens experience simultaneous tension/compression. From the estimation of KIctension and KIccomp., the ultimate strength of SENB specimens can be predicted. LEFM equations combined with the critical fracture toughness in tension give relatively accurate results, suggesting that failure is driven by fibers bundles breakage in tension.

Variation of Magnetic Memory Signals in Fatigue Crack Initiation and Propagation Behavior

To monitor fatigue crack initiation and propagation, and to judge the fatigue damage status of ferromagnetic material, fatigue bending tests of four-point single edge notch bend (SENB4) specimens were carried out. Metal magnetic memory signals were measured during the whole fatigue process. The results showed that the fatigue process could be divided into four stages by observing the morphology of the fracture surface. With the increase of fatigue loading cycles, the tangential component of the magnetic field (Hx) and the normal component of the magnetic field (Hy) increased. At the notch Hx presented a “trough” shape and had a maximum value at the midpoint, while Hy at the notch rotated clockwise around the midpoint. Compared with the tangential characteristic parameters, the variation of normal characteristic parameters (i.e., maximum gradient value of Hy (Ky-max) and the variation range of Hy at the notch (∆Hyn), with the fatigue loading cycles are more similar to the variation of fatigue crack length with loading cycles), both Ky-max and ∆Hyn had a good linear relationship with fatigue crack length. Plastic deformation accumulated on both sides of the fatigue crack, and metal magnetic memory (MMM) signals measured from the specimens were able to indicate the location of the fatigue crack and the variation of the fatigue crack length. Furthermore, the distribution of magnetic signals was analyzed according to the theories of stress magnetization and magnetic flux leakage.

Effect of the free surface on the fatigue crack front curvature at high stress asymmetry

The purpose of this paper is to investigate the effect of the vertex singularity on the fatigue crack front behaviour. Single edge notch bend specimens of steel EA4T and aluminium alloy 7075 of various thicknesses are subjected to the cyclic loading and the angles of the fatigue crack front curvature was measured. The experimental procedure is simulated using finite element analysis. Two methodologies used for the fatigue crack front shape estimation are compared. One is using stress singularity exponent as a controlling parameter and the second one is using stress intensity factor. Both methodologies provide comparable results of crack front formation process and they are in very good agreement with experimental results.

Experimental Measurements of J c-Values for a Low Alloy Structural Steel Using Nonstandard Bend Specimens and Evaluation of the Reference Temperature, T

Abstract This work experimentally investigates the cleavage fracture behavior of a high strength, low alloy structural steel using standard and nonstandard single-edge notched bend specimens, including nonstandard precracked Charpy-type configurations tested under three-point bending. The primary purpose of this study is to investigate the effects of geometry and loading mode on fracture toughness using nonstandard bend specimens under three-point and four-point loading. Fracture toughness testing conducted on various bend specimen geometries extracted from an ASTM A572, Standard Specification for High-Strength Low Alloy Columbium-Vanadium Structural Steel, Grade 50 steel plate provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc. The experimental results show potential effects of specimen geometry and loading mode on Jc-values that can help to mitigate the effects of constraint loss often observed in smaller fracture specimens. An exploratory application derived from the master curve methodology to determine the reference temperature, T0, also provides additional support for using nonstandard bend specimens in routine fracture applications.

An experimental-finite element method based on beach marks to determine fatigue crack growth rate in thick plates with varying stress states

The relation between the fatigue crack growth rate and stress state is significant for fatigue life estimation, especially for thick-welded structures with complex stress states. This paper proposes a hybrid Experimental-Finite Element Method aimed to obtain the fatigue crack growth rate corresponding to plane stress and plane strain states. A static load marking method is proposed to mark a series of crack fronts in a single edge notched bending specimen at known numbers of cycles, which is analysed by nonlinear finite element method to evaluate the stress state by a local constraint factor. Stress intensity factor at the center of the thickness and the surface position where plane strain and plane stress states approximately exist and remain unchanged with crack propagation, is calculated by the 1/4-node displacement method with consideration of non-orthogonal mesh influence. The J-integral method and the virtual crack closure technique are also applied to check the performance of the methods in dealing with the actual cracked body problem. Crack length increments are determined based on the crack front profiles. The da/dN-ΔK curves of plane strain and plane stress states are determined and compared with the results from normal fatigue crack growth rate tests according to Chinese standard: GB/T 6398-2000. The fatigue crack growth behavior of a semi-elliptical surface crack located at the weld toe of a T-plate welded joint subjected to cyclic tension loading is assessed based on the curves and the analytical results are compared with the experimental results. The asymptotic crack shape observed in experiments is successfully predicted by using the curves determined by the hybrid method.

The matching of crack-tip constraint between standard and non-standard specimen

The matching of crack-tip constraint is a key issue for the developing of accurately structure integrity assessment method. For more and more non-standard specimens with different constraint states were used in the scientific research, it is especially significant to establish the matching of constraint between standard specimen and non-standard specimen. In this article, the single edge-notched bend specimens with various constraint states were selected, and the unified constraint parameter Ap under different J-integrals was calculated by finite element analysis. And then, based on the J-Ap curves, the matching of crack-tip constraint between different specimens was studied, and the matching specimens were given. In order to confirm the reliability of results, some experiments focused on the matching specimens were performed. The experiment results showed that the matching is valid. For the fracture properties of matching specimens are matching also, it is significant to the accurate assessment of structural integrity.

Fatigue crack growth anisotropy, texture and residual stress in austenitic steel made by wire and arc additive manufacturing

Wire based additive manufacturing of metals is a novel and cost-effective method for the production of large-scale metallic parts in a wide range of engineering applications. While these methods display excellent tensile properties, relatively little is known of the microstructure-to-property relationships of wire-based additively manufactured stainless steel builds as they relate to fatigue and fracture behavior. Stainless steel alloy 304L walls were fabricated using wire and arc additive manufacturing and subjected to mechanical tests to characterize location and orientation dependant properties and microstructural features affecting crack growth. Fatigue crack growth rate analysis in the high cycle fatigue regime was undertaken on horizontally- and vertically-oriented single-edge notch bend specimens extracted at several positions from the wall. The R ratio was 0.1 and the test frequency was 10 Hz. Paris Law behavior similar to that observed wrought steel alloys has been achieved with vertical orientations showing the greatest crack growth resistance. In conjunction with mechanical testing, scanning electron microscopy and electron backscatter detection were used to assess microstructural effects on crack growth within the build.

Effect of hydrogen on fracture toughness properties of a pipeline steel under simulated sour service conditions

The effect of hydrogen on the fracture toughness properties of an API X65 pipeline steel is studied under simulated H2S in-service conditions. The fracture toughness properties are measured in LT and SL directions (perpendicular and parallel to the pipeline wall thickness, respectively), following ASTM E1820. Due to size restrictions of standard single edge notch bending (SEB) specimens at the direction parallel to the thickness of the pipeline wall, an experimental protocol (see the patent) was developed to carry out the fracture toughness tests, while complying with ASTM standard 1820. This approach is especially useful in situations where hydrogen induced cracking (HIC) and in a broader sense, stepwise cracking takes place, since these cracks initiate and grow primarily in planes parallel to the pipeline rolling plane. Such values of fracture toughness are often different from those commonly measured in planes perpendicular to the rolling plane. Hydrogen might not have the same effect on fracture toughness properties as measured in different directions, due to microstructural features which are inherent from steel manufacturing process. The steady state H2S in-service conditions are simulated by electrolytically charging the specimen, for 48 h and then testing (ex-situ) the specimen for evaluating the fracture toughness properties. The steady state H2S environment charging was obtained by measuring the hydrogen concentration in the bulk of the specimen through thermal desorption spectroscopy (TDS) at three levels of hydrogen concentration. It was observed that the KQ was moderately decreased with increasing hydrogen concentration in the bulk of the steel, while CTOD0 showed a significant reduction with increasing hydrogen concentration.

Evaluation of Fracture Toughness of Red Mud Reinforced Aluminium Matrix Composite

Metal matrix composites are formed by combination of two or more materials (at least one of the materials is metal) having dissimilar characteristics. In the present investigation Red Mud Particle of average 90μm size was used as reinforcement and aluminum 8011 alloy was used as matrix material. Conventional stir casting method was employed to reinforce with the matrix. Metallographic studies carried out using optical microscope, reveal fair distribution of reinforcement in the matrix material. Prepared composite are then machined as per the ASTM standers to test Compact Tension (CT) and Single Edge Notched Bend specimen(SENB) . Fracture toughness values are the compared against the pure matrix aluminum and reinforcement weight percentage. For Compact tension specimen, the value of Maximum load (Pmax) increases by 35% at 12wt% red mud composite and gradually decreases at 16wt% compared to the base alloy, whereas the stress intensity factor K1c increases by 6% at 12wt% and decreases 16wt% of reinforcement. For Single Edge Notch Bend specimen, the value of Maximum load(Pmax) increases and stress intensity factor K1c increases with the weight percent of composite, the Maximum load id increased by 45% and stress intensity factor is increased by 11 % at 16wt% of reinforcement .

Mark tracking technique for experimental determination of fracture parameters

The Mark Tracking method represents a modern and simple optical method to determine the displacement and strain fields. The principle of the marks tracking method is based on the comparison between two images acquired before and after sample deformation. The algorithm of method track the local displacement of the marks in two directions. Having measured displacements in certain points around the crack it is relative simple to estimate the Stress Intensity Factor (SIF) by displacement correlation methods. The students can compare different data processing algorithms for extracting the SIF’s. Basically, a simple geometry like Single Edge Notch Bend specimen loaded in Three Point Bending was tested, for which the exact solution of SIF is well known. The reliability of experimental technique is assessed by comparing the results with the exact solution.

Numerical simulation of crack propagation in high-strength low-alloyed welded steel

The industrial application of high-strength low-alloyed steel (HSLA) in welded structures has increased the demand for understanding fracture behavior and structural integrity assessment of this type of steel and produced welded joints. The aim of this paper is to simulate the experimental evaluation of the fracture mechanics specimens by using the micromechanical model. The investigation is performed on two standard single edge notch bend (SENB) specimens with imposed crack in the central region. Numerical analysis was carried out by using Simulia Abaqus software package on 2D models used to simulate the damage development on the local level. The comparison between numerical and experimental results is presented through measured values of J-integral, load-line displacement νLL and crack growth resistance (J-Δа) curves. This paper shows that numerical simulations are promising in respect to their accuracy. The application of this model enables to decrease the amount of expensive experiments for determination of the load level that causes crack propagation.