Single Edge Notch Bend Research Articles

A phase separation strategy for enhanced toughness self-assembly graphene-network composites

The toughness of three unsaturated polyester resins modified with different quantities of polycaprolactone (PCL) and few-layer graphene microsheets was evaluated by single-edge-notch bending (SENB). The addition of PCL to the resin systems created phase-separated microstructures with different critical compositions at phase inversion (7, 15 and 16.5 wt%PCL). The addition of PCL in quantities lower than the critical compositions showed increases up to 90% in toughness. A similar effect was observed by the addition of graphene to the neat resin increasing the toughness by up to 96% with 6 wt% of graphene. The incorporation of graphene to a phase separating system was found to form a self-assembly graphene-network at the unsaturated polyester-rich phase of the microstructure. When such network showed an appreciable effect of the toughness compared to the system without the PCL (114% increase at 6 wt% graphene and 2 wt% PCL), it also positively impacted the electrical conductivity. A shift of the percolation threshold towards a lower quantity of graphene was achieved by the creation of such graphene-rich network, reducing by 2 wt% the quantity of graphene needed for conduction.

Performance comparison of carbon fiber reinforced polyaryletherketone and epoxy composites: Mechanical properties, failure modes, cryo‐thermal behavior, and finite element analysis

AbstractHigh‐performance thermoplastic composites are gaining attention as potential replacement for thermoset composites in high‐end structural applications owing to their superior fracture toughness and recyclability. In this study, polyaryletherketone (PAEK) and carbon fabric (CF) reinforced PAEK are compared with epoxy and epoxy‐CF composites. PAEK‐CF composites are prepared by film stacking method followed by compression molding while hand lay‐up technique was used for epoxy‐CF composites. PAEK and PAEK‐CF composites possess superior tensile properties, flexural strength and fracture toughness compared to those prepared from epoxy. The incorporation of CF improved the wear resistance in both PAEK and epoxy and PAEK‐CF exhibited the lowest wear rate. The investigation of interlaminar shear strength of the composites was conducted and the highest value observed for PAEK‐CF composite. Wear and fracture surface analysis suggested ductile failure for PAEK and PAEK‐CF composites and brittle failure in epoxy composites, as indicated by interfacial bonding/debonding and fiber pullout, at the fracture surface. Cryo‐thermal cycling effects on the tensile properties, fracture toughness and fracture surface morphology of the composites are studied. The failure behavior obtained from experimental studies was corroborated with finite element method‐based simulation models of fracture toughness response in single edge notch bend test using Abaqus/Explicit.

Effect of the Lüders plateau on the relationship between fracture toughness and constraint for pipeline steels

The effect of Lüders plateau on the fracture response of pipeline steels is studied. The study is carried out by using the finite element analysis method combined with the material damage model. The single edge notched bending (SENB) specimens are used, and the complete Gurson model (CGM) is used to simulate the crack growth process. The results show that the Lüders plateau has an important effect on the relationship between the fracture toughness and the constraint. When the constraint level is low, the fracture toughness increases with the increase of the Lüders plateau length. When the constraint level is high, the fracture toughness decreases with the increase of the Lüders plateau length. With the increase of the Lüders plateau length, the fracture toughness is more sensitive to the change of the constraint, and the decrease of the fracture toughness with the increase of the constraint is more significant. A constraint-based fracture failure criterion considering the Lüders plateau is proposed and validated. The validation results show that the criterion has high accuracy and great application potential to the fracture analysis for pipeline steels with the Lüders plateau.

Fracture Load Predictions in Additively Manufactured ABS U-Notched Specimens Using Average Strain Energy Density Criteria.

This paper provides a methodology for the prediction of fracture loads in additively manufactured ABS material containing U-notches. The approach is based on the Average Strain Energy Density (ASED) criterion, which assumes that the material being analysed develops fully linear-elastic behaviour. Thus, in those cases where the material has a certain (non-negligible) amount of non-linear behaviour, the ASED criterion needs to be corrected. In this sense, in this paper, the ASED criterion is also combined with the Equivalent Material Concept (EMC) and the Fictitious Material Concept (FMC), both being corrections in which the non-linear real material is substituted by a linear equivalent or fictitious material, respectively. The resulting methodologies have been applied to additively manufactured ABS U-notched single-edge-notched bending (SENB) specimens combining five different notch radii (0, 0.25, 0.5, 1 and 2 mm) and three different raster orientations (0/90, 45/−45 and 30/−60). The results obtained demonstrate that both the ASED-EMC and the ASED-FMC combined criteria provide more accurate predictions than those obtained directly through the ASED criterion, with the ASED-EMC criterion generally providing safe more accurate predictions, with an average deviation from the experimental fracture loads between +1.0% (predicted loads higher than experimental loads) and −7.6% (predicted loads lower than experimental loads).

Validation of the Phase-Field Model for Brittle Fracture

The phase-field approach to fracture modelling has gained much attention in the field of computational fracture mechanics in the past decade. The phase-field approach eliminates the need for the numerical tracking of the sharp crack discontinuities by the smooth transition of a scalar damage field whose value differentiates between the broken and intact material states. Its variational based approach has been proven to be thermodynamically consistent and able to solve complex fracture processes. Consequently, many different phase-field fracture models have emerged. In this contribution, a few well-known phase-field models for brittle fracture modelling have been implemented within the staggered algorithm with stopping criterion based on the control of the residual norm, recently developed by the authors. The implementation has been conducted within the commercial finite element software Abaqus and expanded to the three-dimensional settings. The experimental validation of the numerical models is then conducted on the tensile, compact tension (CT) and single edge notched bend (SENB) specimens made of the thermoplastic polymer, polymethylmethacrylate (PMMA). It has been demonstrated that with a suitable choice of the length scale parameter, the developed staggered phase-field fracture models can provide valid prediction of the brittle crack initiation and propagation under quasi-static loading conditions.

Crack path direction in plane-strain fracture toughness assessment tests of quasi-brittle PLA polymer and ductile PLA-X composite

Plane-strain fracture toughness is one of the main parameters in linear elastic fracture mechanics and its purpose is to show the material capability to withstand load while having a defect. Main validity aspect for such an assessment is to provide a wide enough crack front to enable plane-strain condition. Nonetheless, in FDM (Fused Deposition Modeling), due to structural anisotropy caused by polymer material properties and Additive Manufacturing (AM) process parameters, more validity aspects must be met. During the plane-strain fracture toughness test a crack must follow a straight line from initiation up to the point of structural failure. Plane-strain fracture toughness assessment is conducted according to the ASTM D5045-14 standard for testing of polymer materials. Tests are performed on SENB (Single Edge Notched Bend) specimens, made from two similar polymer materials: quasi-brittle PLA and ductile PLA-X composite. Specimens are manufactured with four different AM process parameters, i.e., layer height, infill density, printing orientation and one specimen batch was dried before testing.

Parametrically-upscaled continuum damage mechanics (PUCDM) model for multiscale damage evolution in bending experiments of glass-epoxy composites

This paper demonstrates the effectiveness of parametrically-upscaled continuum damage mechanics (PUCDM) models for multiscale damage analysis of S-2 glass fiber/SC-15 epoxy matrix composite beams subjected to single-edge notched bending (SENB) tests. The SENB experiments with [012/9012] double-ply and [08/908/08] triple-ply composite beams use synchrotron X-ray phase-contrast imaging for in-situ damage characterization. PUCDM models represent a class of thermodynamically-consistent multiscale constitutive models that bridge length-scales through explicit incorporation of representative aggregated microstructural parameters (RAMPs) in constitutive coefficients. Functional forms of RAMPs are generated through machine learning tools, operating on a micromechanical analysis database. The efficient PUCDM-based FE simulations reveal multiscale damage phenomena in the SENB experiments.

Improvement of fracture toughness based on auxetic patterns fabricated by metallic extrusion in 3D printing

This study aims to evaluate the effect of auxetic patterns on the fracture toughness of 3D printed metallic specimens manufactured by metallic extrusion of 17-4PH stainless steel. When it is pulled in the horizontal direction, the auxetic material expands in the vertical axis. This occurs since it has a negative Poisson's ratio. In this work auxetic cell structures have been employed in fracture mechanics characterization of Single Edge Notch Bending SENB specimens in order to improve their fracture toughness dynamically during strain.SENB specimens were designed with different cell patterns for fracture toughness assessment. (i) Auxetic pattern at 0° parallel to the notch direction, (ii) at 90° perpendicular to the notch direction and (iii) a conventional honeycomb pattern is defined with equivalent cell density on the specimen surface. Three-point bending tests have been carried out to evaluate the fracture toughness of each specimen. The mechanical performance ratio of specimens was evaluated by dividing the absorbed energy till fracture over the specimen weight. The ratios were also compared to a solid specimen ratio taken as a reference in this study.The results show that all specimens with cell patterns absorb more strain energy than the solid specimen does. Although the solid specimen is the stiffest, both cell patterns (auxetic and honeycomb) have higher performance indices considering the important weight reduction. For around 35 % of reduction in the maximum measured load, the auxetic pattern at 0° can absorb up to 88% more energy compared to the solid specimen. This proves its capability of delaying the crack initiation and enhancing the fracture toughness.

Determination of fatigue crack growth in the near-threshold regime using small-scale specimens

A complete description of the experimental procedure for characterizing the intrinsic fatigue crack propagation threshold (ΔKth,eff) as well as the fatigue crack growth rate (FCGR) in the near-threshold regime using small-scale specimens is presented. A comparative study is carried out on the high strength steel S960QL considering different single edge notch bend (SENB) specimen geometries. On one hand, the reference dimensions of 6 mm thickness (B) and 19 mm width (W) are analysed and referred to as conventional specimens. On the other hand, small-scale specimens with B = 3 mm and W = 4 and 6 mm are also considered. Several loading configurations (3-, 4- and 8-point bending) are used and a load ratio R = 0.8 is applied to avoid crack closure effects. The direct current potential drop (DCPD) technique is used to monitor the crack length. The reduced dimensions of the small-scale specimens imply the necessity of implementing a modified testing procedure compared to the recommendations of current standards for the generation of FCGR data. The results show a good agreement between tests conducted on different specimen sizes, which opens new perspectives in the use of small-scale specimens for characterizing the fatigue crack growth properties in metallic materials. Recommendations and limitations of the procedure are provided and discussed.

In-situ optical approach to predict mixed mode fracture in a polymeric biomaterial

The present article promotes the usage of the digital image correlation (DIC) technique to determine the in‑situ stress and predict the onset of fracture in a cracked dental biomaterial sample. For this purpose, the elastic and fracture properties of the dental material are measured via the DIC method. To perform mixed mode fracture experiments, a modified single edge notched bend (SENB) specimen with varying crack length is introduced and utilised. From the theoretical point of view, we next implement a stress-based fracture criterion and combine it with two different critical distance models. In this regard, the in‑situ stress is formulated using the data from the DIC analysis conjugated with a supervised learning algorithm. Finally, the crack growth angle and fracture load for the tested biomaterial specimens are estimated, the comparison of which with the experimental measurements reveals good correlation.

Set-up of a Generalized Dataset for Crack-Closure-Mechanisms of Cast Steel

In components, crack propagation is subjected to crack-closure-mechanisms which affect the build-up of the relevant threshold stress intensity factor range during cyclic loading. As structural parts are exposed to service loads incorporating a variety of load ratios, a significant change of the long-crack threshold value occurs, leading to a severe stress ratio dependency of crack-closure-mechanisms. Thus, an extensive number of crack propagation experiments is required to gain statistically proven fracture mechanical parameters describing the build-up of closure effects as crack growth resistance curves.The article presents a generalized dataset to assess the formation of crack-closure-mechanisms of cast steel G21Mn5+N. Numerous crack propagation experiments utilizing single edge notched bending (SENB) sample geometries are conducted, incorporating alternate to tumescent stress ratios. The statistically derived, generalized crack growth resistance curve features the impact of closure effects on the crack propagation rate in a uniform manner. To extend the dataset to arbitrary load ratios, the long-crack threshold approach according to Newman is invoked. The generalized dataset for the cast steel G21Mn5+N is validated by analytical fracture mechanical calculations for the utilized SENB-sample geometries. Incorporating a modified NASGRO equation, a sound correlation of analytical and experimental crack propagation rates is observed. Moreover, the derived master crack propagation resistance curve is implemented as a user-defined script into a numerical crack growth calculation tool and supports a local, node--based numerical crack propagation study as demonstrated for a representative SENB-sample. Concluding, the derived dataset facilitates the calculation of fatigue life of crack-affected cast steel components subjected to arbitrary stress ratios.

On the fracture toughness testing for single-edge notched bend specimen of orthotropic materials

Majority of the fracture toughness tests on orthotropic materials reported in the literature are based on the testing methods developed for isotropic materials since there is no available testing standards specifically dealing with orthotropic materials. In this study, a systematic two-dimensional finite element analyses (2D FEA) of orthotropic single-edge notched bend (SENB) specimens are performed to evaluate the stress intensity factor (SIF) and crack mouth opening displacement (CMOD), which are two key parameters used in the fracture toughness tests. The analysis matrix covers a wide range of material orthotropy. Highly accurate regression solutions for SIF, CMOD compliance and its inverse function were developed as functions of orthotropy parameters. Applications of the proposed equations on experimental data are presented and further discussed. The research outcome is expected beneficial to the standardization of the fracture toughness testing on orthotropic composite materials such as wood, bone and unidirectional fiber-reinforced polymer composites.

Mode I fracture toughness of Carbon/glass hybrid comopsite

Mode I fracture toughness of carbon/ glass reinforced polyster hybrid composite was invesitigated expermrntally and numerically by using the cosmos)/m 2.6 finite element software (fms) by utulizing the hand laup techniqe)(HLU). The single edge notch bending (SENB) test was developed to evalute the mode l fracture toughness of carbon composites, glass composite and hybrid compodite materials at varuos fiber configurations. Scaning electron microscope (SEM) was used to examine the fracture surface of the hybrid composite material under the effect of mode l loading. the expermental result showed the maximum stress intensity (sif) factor 882mpa. mm1/2. obtained in the hybrid composite with stacking dequencess (c/g/g/c) during the mode l loading compared to other stacking sequences. this is attributed to the goid interfical between the carbon and glass fibers and matrix face. From the resukts, it was suggested that the expermental results is a goid a gremnent with the finite element modelling

Mechanical Characterization and Fracture Behavior of Extruded ZK60A Magnesium Alloy

The current study addresses the mechanical and fracture behaviors of extruded ZK60A magnesium alloy. This material has numerous potential structural applications, and its characteristics need to be determined for it to be used as a structural material. Tensile, hardness, and Charpy impact tests were performed as per American Society for Testing and Materials standards to determine the mechanical behavior of the material. Its fracture behavior was determined using plane-strain fracture toughness, stable crack growth, and fractographic tests. The fracture toughness was calculated from the fracture toughness–thickness curve obtained from fracture tests on single-edge notch bend specimens with fatigue pre-cracks. The stable crack growth studies were conducted using three-point bend specimens to determine the initiation and maximum loads and the range of stable crack growth for both mode I and mixed-mode loading cases. X-ray diffraction analysis of the alloy was also conducted. Scanning electron microscopy was used to observe the fracture surface morphologies of the specimens. This study addresses the structural analysis, advanced materials, and safety and reliability aspects of aerospace research.

Numerical and Experimental Evaluation of Mechanical and Ring Stiffness Properties of Preconditioning Underground Glass Fiber Composite Pipes

The mechanical and ring stiffness of glass fiber pipes are the most determining factors for their ability to perform their function, especially in a work environment with difficult and harmful conditions. Usually, these pipes serve in rough underground environments of desert and petroleum fields; therefore, they are subjected to multi-type deterioration and damage agents. In polymers and composite materials, corrosion is identified as the degradation in their properties. In this study, tension and compression tests were carried out before and after preconditioning in a corrosive agent for 60 full days to reveal corrosion influences. Moreover, the fracture toughness is measured using a standard single edge notch bending. Ring stiffness of such pipes which, are considered characteristic properties, is numerically evaluated using the extended finite element method before and after preconditioning. The results reported that both tensile and compressive strengths degraded nearly more than 20%. Besides the fracture toughness decrease, the stiffness ring strength is reduced, and the finite element results are in good agreement with the experimental findings.

The Numerical Modelling Approach with a Random Distribution of Mechanical Properties for a Mismatched Weld.

The aim of this work was to include a local variation in material properties to simulate the fracture behaviour in a multi-pass mis-matched X-weld joint. The base material was welded with an over and under-match strength material. The local variation was represented in a finite element model with five material groups in the weld and three layers in the heat-affected zone. The groups were assigned randomly to the elements within a region. A three-point single edge notch bending (SENB) fracture mechanics specimen was analysed for two different configurations where either the initial crack is in the over or under-matched material side to simulate experimentally obtained results. The used modelling approach shows comparable crack propagation and stiffness behaviour, as well as the expected, scatter and instabilities of measured fracture behaviour in inhomogeneous welds.

Mode Mixity in the Fracture Toughness Evaluation of Heat-Affected-Zone Material Using SEN(T) Experiment

Abstract Single-edge-notch-tension, SEN(T), specimens have been found to provide a good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than single-edge notch bend, SEN(B) (bend-bars), and compact-tension, C(T) specimens. This lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Additionally, the SEN(T) specimen eliminates concern of material anisotropy since the crack growth direction in the SEN(T) specimen is the same as in a surface-cracked pipe. While the existing recommended and industrial practices for SEN(T) have been developed based on assumption of homogenous or monomaterial across the crack, their applicability for the evaluation of fracture toughness of the heat-affected zone (HAZ) were evaluated in this investigation. When conducting experiments on SEN(T) specimens with a prescribed notch/crack in the HAZ, the asymmetric deformation around the crack causes the occurrence of a combination of Mode-I (crack opening) and mode-II (crack in-plane shearing) behavior. The extent of this mode mixity is dependent on the relative difference between the material properties of the adjacent girth weld and pipe base metals, as well as the amount of crack growth in the experiment. This mode mixity affects the measurement of the crack-tip-opening-displacement (CTOD) and evaluation of fracture mechanics parameter, J. The CTOD-R curve depicts the change in toughness with crack growth, in a manner similar to the J–R curve methodology. Observations also show a mismatch in the length of the crack growth that is measured on the fracture surface, attributable to the material deformation differences across the two adjacent materials (weld and base metals). This paper discusses the experimental observations of Mode I and Mode II behavior seen in experiments conducted on SEN(T) specimens with a notch/crack in the HAZ and as the crack propagates through the weld/HAZ thickness. The paper addresses the issues related to and the changes needed to account for such behavior toward the development of recommended practices or standards for SEN(T) experiments of weld/HAZ. The effects of mode mixity in HAZ SEN(T) experiments is critical to the development of crack growth resistance, CTOD-R and J–R curves employed in engineering critical assessment (ECA) of pipelines.

Modeling Brittle Fractures in Epoxy Nanocomposites Using Extended Finite Element and Cohesive Zone Surface Methods.

Linear elastic fracture modeling coupled with empirical material tensile data result in good quantitative agreement with the experimental determination of mode I fracture for both brittle and toughened epoxy nanocomposites. The nanocomposites are comprised of diglycidyl ether of bisphenol A cured with Jeffamine D-230 and some were filled with core-shell rubber nanoparticles of varying concentrations. The quasi-static single-edge notched bending (SENB) test is modeled using both the surface-based cohesive zone (CZS) and extended finite element methods (XFEM) implemented in the Abaqus software. For each material considered, the critical load predicted by the simulated SENB test is used to calculate the mode I fracture toughness. Damage initiates in these models when nodes at the simulated crack tip attain the experimentally measured yield stress. Prediction of fracture processes using a generalized truncated linear traction–separation law (TSL) was significantly improved by considering the case of a linear softening function. There are no adjustable parameters in the XFEM model. The CZS model requires only optimization of the element displacement at the fracture parameter. Thus, these continuum methods describe these materials in mode I fracture with a minimum number of independent parameters.

Ductile crack initiation and growth on a plasticized Polyvinylchloride during air bag deployment

With the goal of ensuring the security of passengers for automotive industry, the present work addresses the ductile fracture process of plasticized PVC. Dedicated clamped single edge notch bending (SENB) specimens were used to characterize the mechanisms of crack initiation and propagation for the studied material. The exploitation of the experimental database associated with finite element simulation of the crack propagation allowed, on the one hand, the calibration factor η<sub>p</sub> of this specific SENB specimen to be established, as a function of the crack depth ratio. On the other hand, the fracture toughness of the studied plasticized PVC was estimated to be 10.8 kJ/m<sup>2</sup>, value which was close to that reported in the literature for modified PVC. By using this fracture toughness value, a methodology aiming at the prediction of ductile crack initiation of the PVC skin integrated into a real dashboard (full scale test) was proposed.

Design optimization of lattice structures with stress constraints

This paper presents an experimentally validated framework used to perform topology and orientation (morphology) optimization of lattice structures subject to stress constraints. The effective stiffnesses and yield stresses of a unit cell are obtained using numerical homogenization and validated experimentally. Due to the orthotropic behavior of the unit cell, the modified Hill’s yield criterion is used to describe the lattice strength. The effective orthotropic properties are implemented via macrostructure topology optimization to further improve the lattice structure stiffness. Homogenization-based optimization is performed using a coarse mesh and the optimized design is projected onto a fine mesh. This reduces the computational cost significantly. Finally, the projected design is post-processed to ensure the fabrication feasibility of the optimized lattice structure. The framework is tested for two cases: an L-shaped bracket and a single-edge notched bend (SENB) problem. A comparison of the compliance-based and stress-constrained designs used in the two cases demonstrates that the changes in the optimal material distribution that occur upon implementing the stress constraint result in higher yield strength. The SENB lattice structures are additively manufactured and the stiffnesses and yield strength of the optimized designs are compared to those obtained numerically.