Single Edge Notch Research Articles

Effect of specimen thickness on fatigue crack shape evolution for aluminum alloy

The investigation of fatigue crack propagation and fracture mechanisms in high-thickness metal plates holds pivotal importance in the advancement of a three-dimensional damage tolerance assessment methodology. This research also bears significant implications for ensuring the safe operation of large-scale mechanical equipment. In this study, we examine the influence of specimen thickness on the evolution of fatigue crack shapes in an aluminum alloy. Fatigue crack growth tests were conducted on single-edge notch tension specimens to explore the impact of specimen thickness on the shape of fatigue crack growth. Additionally, we propose a calculation model for crack growth shape based on an energy model in this paper. Both experimental and analytical findings indicate that specimen thickness does indeed exert an influence on fatigue crack shape. Specifically, as the specimen thickness increases, the crack shape transitions from resembling a “fingernail” to a “saddle” shape.

Effect of stacking direction and raster angle on the fracture properties of Onyx 3D printed components: A mesoscale analysis

3D printing is a technology that has gained increasing importance both in academia and industry for the possibilities offered. Among the many, one of the most appealing is the possibility to choose different printing configurations, tailoring stacking direction, and raster angle. The choice of such parameters deeply influences the structural response, as already shown in the literature. This study aims at providing a deep understanding of the phenomena taking place at the mesoscale level which justify such differences in the fracture behavior. An experimental campaign is carried out with a Single Edge Notch Bending (SENB) specimen printed in Onyx using four different combinations of raster angles and stacking directions. The results are compared in terms of mechanical response, fracture toughness and surface roughness to identify the main driving mechanisms during the fracture process. The same bending test is replicated numerically, aiming at comparing the fracture toughness values obtained experimentally with the ones used in the simulation to match the experimental curves. The study shows that the stacking direction and the raster angle deeply influence the fracture behavior and the mechanical properties of the specimen, along with the fracture toughness and surface roughness. In particular, it is highlighted how improved mechanical behavior can be achieved by printing the specimen in the vertical direction and with a raster angle of 45°/−45°. Moreover, useful fracture toughness values are identified for different combinations of printing parameters by means of a Cohesive Zone Model formulation, providing useful input values for numerical simulations.

A study on fracture behavior of SEN natural hybrid composite specimen under tensile load

In this work, two reinforcements (natural jute fiber and glass fiber) hybridized with different volume fractions, are reinforced with a constant volume epoxy resin through hand layup method. The ASTM standard tensile specimen of jute natural epoxy composite of different volume fractions and orientations of fibers, with varied single edge notch (SEN) size were subjected to uniaxial tensile load in universal testing machine. Effect of jute volume fraction, notch size and fiber orientation on tensile strength and fracture toughness has been studied through experimental results. Increased percent of jute fibre showed decrease in tensile strength and fracture toughness. Also, with the increase in notch size, the tensile strength decreased and the fracture toughness increased. Further, the tensile strength and fracture toughness were superior in 0°/90° fiber orientation specimen than those with ±45° fiber orientation. Furthermore, experimental results were validated by conducting statistical and fractographic analysis. The jute fiber percentage was ranked as best level factor affecting the fracture behavior as per taguchi method. Morphological features of fractured surfaces were analyzed through scanning electron microscopic (SEM) images with respect to nature of jute fiber failure under uniaxial loading conditions.

Experimental and Numerical Analysis of Fracture Mechanics Behavior of Heterogeneous Zones in S690QL1 Grade High Strength Steel (HSS) Welded Joint.

The heterogeneity of welded joints' microstructure affects their mechanical properties, which can vary significantly in relation to specific weld zones. Given the dimensional limitations of the available test volumes of such material zones, the determination of mechanical properties presents a certain challenge. The paper investigates X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The experimental work is focused on tensile testing to obtain stress-strain properties, as well as fracture mechanics testing. Considering the aforementioned limitations of the material test volume, tensile testing is carried out with mini tensile specimens (MTS), determining stress-strain curves for each characteristic weld zone. Fracture mechanical testing is carried out to determine the fracture toughness using the characteristic parameters. The experimental investigation is carried out using the single edge notch bend (SENB) specimens located in several characteristic welded joint zones: base metal (BM), heat affected zone (HAZ), and weld metal (WM). Fractographic analysis provides deeper insight into crack behavior in relation to specific weld zones. The numerical simulations are carried out in order to describe the fracture behavior of SENB specimens. Damage initiation and evolution is simulated using the ductile damage material behavior. This paper demonstrates the possibility of experimental and numerical determination of fracture mechanics behavior of characteristic heterogeneous welded joint zones and their influence on crack path growth.

Stress state analysis of root notch for X80 girth welds with variable wall thickness and misalignment geometric features

Notches formed due to weld geometric features seriously affect the structural safety of the pipeline. In this paper, the stress states of girth welds under various forming conditions and internal pressures are thoroughly evaluated via numerical simulation analysis. Controlled studies are carried out between crack-free and cracked welds. The susceptibilities of stress state to variations in internal pressure under different forming conditions are elucidated. Also, newly modified single-edge notch tensile specimens (MSENT) are manufactured, and the strain characteristic and fracture behavior analysis are performed. The calculation results reveal that the stress states of either the root notch or the crack tip are significantly intensified by welds' geometric features, and the increased brittle fracture sensitivity can be qualitatively demonstrated by MSENT analysis. Simultaneously, the misalignment continuously enhances the sensitivity of stress states to internal pressure variation. However, opposite patterns are observed between crack-free and cracked welds characterizing variable-wall-thickness features. The research results can provide a strong reference for the structural design, service evaluation, and related research of pipelines.

Delayed fracture caused by time-dependent damage in PDMS

An experimental and theoretical study of delayed fracture of polydimethlsiloxane (PDMS) is presented. Previous works have demonstrated that delayed fracture in single edge notch specimens is caused by time dependent damage due to chain scission. Here we study the interactions between damage and the elastic field using different specimens and crack geometries with blunt and sharp cracks. Our experiments show that initial toughness is not well defined, as stable slow crack growth can occur over a range of applied loads. Our experiments demonstrate that there is a universal relation between crack growth rate and applied energy release rate. A model coupling the nonlinear elastic deformation and rate dependent bond scission is proposed and is in good agreement with experimental data.

Impact-dynamic properties of aromatic hyperbranched polyester/RTM6 epoxy nanocomposites

Present study investigates the influence of aromatic hyperbranched polyester (AHBP) nanofillers on the impact-dynamic properties of RTM6 epoxy. The strain rate dependence of the energy absorption capacity of neat RTM6 epoxy and RTM6 nanocomposites with filler weight ratios of 0.1%, 1%, and 5% was determined through compression testing. Additionally, static and dynamic tensile tests were performed on these materials. Universal testing machines were used for the quasi-static tests, split Hopkinson bar setups for the high-strain rate experiments. Furthermore, dynamic single-edge notch bending (SENB) tests were conducted to measure fracture toughness using an adapted split Hopkinson compression bar setup. In addition to global deformation measurements, full-field deformation measurements were also performed in all experiments using the digital image correlation (DIC) technique. The results indicate that the energy absorption capacity, tensile strength, fracture strain, and toughness are significantly influenced by both strain rate and filler content of the RTM6 epoxy.

Composition optimization of PMMA base denture reinforced with different percentages of Nano Hydroxyapatite and alumina particles to obtain highest KIc and KIIc values using hybrid IWO/PSO algorithm

Composition of three-phase dental denture (Polymethyl methacrylate or PMMA + Nano Hydroxyapatite + Nano Alumina) was optimized using hybrid IWO/PSO algorithm to obtain the highest KIc and KIIc. First, initial fracture toughness tests were performed on nine groups of PMMA base dentures with different percentages of ingredients. Symmetric single edge notch beam and asymmetric three-point bend beam with inclined crack were used for KIc and KIIc testing, respectively. The initial design parameters were considered as: {75–100% PMMA, 0–10% Nano Hydroxyapatite and, 0–15% Nano Alumina}. As the initial guess values for starting the hybrid optimization algorithm, B-spline curves (as the cost function) were fitted to the experimental results of both modes I and II. Then the IWO/PSO algorithm was run to find the optimum composition and maximum fracture toughness value of each mode. The best KIc was found for the composition of {78.5% PMMA + 9% Nano Hydroxyapatide + 12.5% Nano Alumina}. Similarly, using the hybrid IWO/PSO algorithm the optimum composition of {83.4% PMMA + 8.7% Nano Hydroxyapatide + 7.9 % Nano Alumina} was obtained to gain the highest KIIc. The experimental fracture toughness values obtained from the proposed compositions via employing the hybrid algorithm were approximately 40% higher than the all tested nine groups of PMMA base dentures under both modes I and II. The fracture toughness ratio KIIc/KIc of the optimum mix-designs predicted by the hybrid algorithm was equal to 0.78 and showed good agreement with the experimental KIIc/KIc ratio. The fracture toughness for any desired mixed mode condition can also be predicted in-terms of the KIc and stress intensity factors and T-stress of the desired mode mixity.

Fracture Behavior of Concrete under Chlorine Salt Attack Exposed to Freeze-Thaw Cycles Environment.

Fracture behavior is one of the key properties to study concrete cracking under sodium chloride attack exposed to the freeze-thaw cycles environment, which is frequently neglected. In this paper, 24 single edge notch beam specimens and 24 cubes were poured. The corresponding freeze-thaw cycles test in sodium chloride solution, standard cube compressive strength of concrete test, and three-point-bending tests were carried out. The research revealed that the fracture toughness, fracture energy, relative dynamic modulus of elasticity, and standard cube compressive strength were decreased by increasing freeze-thaw cycles under sodium chloride attack, and the damage degree of concrete caused by sodium chloride solution was deeper than that of pure water. In particular, there existed good linear correlation between the fracture behavior and imposed freeze-thaw damage for various solution. Accordingly, a more reliable damage model using fracture control parameters as damage factors was proposed.

A Comparison of the Damage Tolerance of AA7075-T6, AA2024-T3, and Boeing Space, Intelligence, and Weapons Systems AM-Built LPBF Scalmalloy

This paper first presents the results of an experimental study into the damage tolerance of AA7075-T6, which is widely used in both fixed- and rotary-wing aircraft, space structures, and laser bed powder fusion (LBPF) Scalmalloy specimens built by Boeing Space, Intelligence, and Weapons Systems. To this end, four single edge notch AA7075-T6 specimens and four identical single edge notch LBPF Scalmalloy specimens were tested. The resultant crack growth curves reveal that Boeing Space, Intelligence, and Weapons Systems AM-built Scalmalloy is more damage tolerant than conventionally built AA7075-T6. This finding leads to the observation that the da/dN versus ΔK curves associated with Scalmalloy and conventionally manufactured AA2024-T3 are similar. These findings highlight the potential for Boeing Space, Intelligence, and Weapons Systems AM-built Scalmalloy to be used to extend the operational lives of military aircraft by the on-demand printing of limited-life Scalmalloy replacement parts.

ProCrackPlast: a finite element tool to simulate 3D fatigue crack growth under large plastic deformations

Many structural components and devices in combustion and automotive engineering undergo highly intensive cyclic thermal and mechanical loading during their operation, which leads to low cycle (LCF) or thermomechanical (TMF) fatigue crack growth. This behavior is often characterized by large scale plastic deformations and creep around the crack, so that concepts of linear-elastic fracture mechanics fail. The finite element software ProCrackPlast has been developed at TU Bergakademie Freiberg for the automated simulation of fatigue crack growth in arbitrarily loaded three-dimensional components with large scale plastic deformations, in particular under cyclic thermomechanical loading. ProCrackPlast consists of a bundle of Python routines, which manage finite element pre-processing, crack analysis, and post-processing in combination with the commercial software Abaqus . ProCrackPlast is based on a crack growth procedure which adaptively updates the crack size in finite increments. Crack growth is controlled by the cyclic crack tip opening displacement varDelta CTOD, which is considered as the appropriate fracture-mechanical parameter in case of large scale yielding. The three-dimensional varDelta CTOD concept and its effective numerical calculation by means of special crack-tip elements are introduced at first. Next, the program structure, the underlying numerical algorithms and calculation schemes of ProCrackPlast are outlined in detail, which capture the plastic deformation history along with the moving crack. In all simulations, a viscoplastic cyclic material law is used within a large strain setting. The numerical performance of this software is studied for a single edge notch tension (SENT) specimen under isothermal cyclic loading and compared to common finite element techniques for fatigue crack simulation. The capability of this software is featured in two application examples showing crack growth under mixed-mode LCF and TMF in a typical austenite cast steel, Ni-Resist. In combination with a crack growth law identified in terms of varDelta CTOD for a specific material, the tool ProCrackPlast is able to predict the crack evolution in a 3D component for a given thermomechanical loading scenario.

Flexural and fracture behavior of additively manufactured carbon fiber reinforced polymer composites with bioinspired Bouligand meso-structures

Carbon fiber reinforced polymer (CFRP) composites are strong and lightweight materials extensively used across aerospace, automotive, and construction industries. Bioinspired designs such as Bouligand structures have the potential to improve the mechanical properties of CFRP composites. In this study, we examine the effects of the single and double Bouligand meso-structures with varying twist angles on the flexural and fracture properties of additively manufactured continuous CFRP composites. 3-point flexural tests are conducted to characterize the flexural properties such as flexural modulus, strength, and energy absorption. Single edge notch bend (SENB) tests are performed to quantitively characterize the mode I fracture toughness and effective fracture energy. The elasto-plastic fracture theory is used to quantify the contributions of elastic and plastic energies to the effective fracture energy. Experimental results indicate that twist angles significantly affect the flexural behavior of CFRP composites. The Bouligand meso-structures with a twist angle of 10° increase the flexural strain energy absorption by 2-3 times. The single and double Bouligand layup configurations with the same twist angle result in different flexural properties. In addition, the twist angle affects the magnitude of fracture toughness. A twist angle of 10° results in an improvement of up to 60% in fracture energy dissipation compared to the quasi-isotropic meso-structure. The single and double Bouligand meso-structures with the same twist angle exhibit almost the same fracture toughness and absorbed fracture energy. The Bouligand meso-structures do not increase fracture toughness and elastic fracture energy for crack initiation compared to the quasi-isotropic meso-structure.

Experimental evaluation of J-integral in elastic and elastic–plastic polymers by means of digital image correlation and higher-order eigenfields under mode-I

This study examines the capability of the digital image correlation method to determine the J-integral for polymers with elastic and elastic–plastic behavior. To achieve this, the required analytical concepts are discussed in detail and single-edge notch tension specimens manufactured out of PMMA and HDPE are tested under pure mode I loading conditions. Three specimens of each material are tested under different loads, and the displacement field at the surface of each specimen is obtained using DIC. Using the least square method, the displacement field derived by DIC is used to determine the higher-order singular and non-singular terms of the Williams expansion. The J-integral for each specimen is then computed by combining these terms. In addition, the J-integral obtained by DIC is compared to that derived by finite element analysis, and an excellent correlation is observed. Lastly, the influence of different DIC parameters on the computed results is discussed.

Prediction of SIF range for plain API 5L Grade X65 steel under corrosion using AI & ML models

Corrosion is one of the significant aspects of science & engineering and the effects of corrosion on various infrastructures due to fatigue loading are paramount. Experimental investigations to evaluate fracture parameters and fatigue crack growth life take a considerable amount of time and effort. Recently, the computational boon in the form of Artificial Intelligence (AI) and Machine Learning (ML) has paved the way for predicting the required fracture parameters through training with the available limited experimental data. In this paper, the fracture parameter, Stress Intensity Factor (SIF) of plain API 5L X65 grade steel is predicted using the experimental data of crack growth studies performed on the specimen with single edge notch, eccentrically loaded, subjected to inert and corrosive environments (2.0% and 3.5% NaCl). From the experimental data, the fracture parameter, SIF range is determined for various crack lengths corresponding to certain loading cycles. Multivariate Adaptive regression splines (MARS), Artificial Neural Network (ANN), and relevance vector machine (RVM) based models are developed by using the experimental data to predict the SIF range. The efficiencies of the developed models are verified through several statistical parameters. The studies show that the proposed models are capable of predicting the SIF range for plain API 5L X65 grade steel effectively with the available experimental data. The predicted SIF range is highly useful for the prediction of crack growth life and the evaluation of the residual strength of structural components.

Fracture Resistance and Crack Growth Mechanisms in Functionally Graded Ti–TiB

This paper presents the results of fracture tests and crack path observations for a layered functionally graded material (FGM) consisting of Ti and TiB phases. The composition varied in a nearly linear manner from a TiB-rich layer at the bottom to commercially pure (CP) Ti at the top. Elastic properties of the mixed phase interlayers were measured using nanoindentation testing, demonstrating a linear variation with composition. These results differ significantly from approximations calculated in previous studies using a non-linear rule-of-mixtures approach. Fracture tests were conducted on single edge notch bend [SEN(B)] specimens with the notch aligned orthogonal to the direction of the composition gradient. For this crack orientation, "average" R-curve behavior based on the J-integral was investigated to understand the mechanics of crack growth. The value of J was found to be minimal (less than 1 N/mm) below 47 pct volume fraction of TiB compared to a reported value of approximately 150 N/mm for pure Ti. These results indicate that a steeper transition to high concentrations of the metallic phase is necessary to achieve adequate fracture resistance in this metal/ceramic FGM. Observations on the specimen surface indicate crack path toughening mechanisms of this functionally graded material include crack bridging, branching, and deflection.

Phase field model for mixed mode fracture in concrete

Concrete is a quasi-brittle material and generally fails in mixed mode I-II fracture. The fracture energy release in mode I and mode II during crack propagation is usually different. However, the tensile test fracture toughness has generally been considered in most of the numerical models to study fracture in concrete. In this paper, a phase field model is presented which considers the fracture energy of mode I and mode II for mixed mode I-II fracture simulation in concrete. In addition, it uses a fracture mode controlling parameter χ which, in a sense, controls the governing failure criterion. The post-peak material response is modeled using a cohesive zone approach. The effect of the length scale parameter, mesh size and χ is investigated. For validation, a single-edge notch beam with a notch at various eccentricities is considered. It is shown that the model is mesh independent for a sufficiently fine mesh in cracking region. Also, the effect of the length scale parameter on global response is insignificant. It is observed that χ≥1 gives satisfactory results both for mode I and mixed mode I-II fracture in single edge notch concrete beam. The numerical results for the load–displacement curve are compared with the experimental data both qualitatively and quantitatively. It is concluded that the proposed phase field model can simulate mode I and mixed mode I-II fracture in concrete with sufficient accuracy.

Optimized Strength of Chopped Strand Mat Glass Fiber-Reinforced Polyester Laminates with Single-Edge Notch under the Effects of Notch Shape/Size and Strain Rate

Optimized Strength of Chopped Strand Mat Glass Fiber-Reinforced Polyester Laminates with Single-Edge Notch under the Effects of Notch Shape/Size and Strain Rate

Study on Numerical Simulation Method of Fracture Behavior of Pipeline Girth Weld

Abstract The failure accidents in girth weld of pipelines occur frequently due to the combination of internal defects and external loads. However, the research on the fracture behavior of girth weld defects is relatively poor at present. To solve this problem, the cracking behavior and strain evolution law of the inner wall defects of the pipe girth weld are studied in combination with full-scale tests (FST). The constitutive and Gurson-Tvergaard-Needleman damage parameters of the pipe base metal zone, weld zone and heat-affected zone (HAZ) are calibrated through the small punch test (SPT) and single edge notch bending (SENB) test. On this basis, the welded pipe model with inner wall defects is established, and a numerical simulation method for dynamic fracture behavior based on damage mechanics is formed. The numerical simulation method is verified by FST data and theoretical calculation. The results show that the numerical results are consistent with the FST and theoretical calculation in the elastic stage, plastic stage, and fracture stage, and the error is within 10%. The novel numerical simulation method is provided as a means for the fracture behavior research of pipeline girth weld.

Numerical aspects of phase field models for low-temperature fracture in asphalt mixtures

Unlike the cohesive interface element model, the phase field model (PFM) is a newly developed computational model which provides a unified approach to predicting crack nucleation, propagation, coalescence and branching without any ad-hoc criterion. Over the years, several variants of the phase field model are presented. This paper aims to investigate the suitability of different phase field models and determine the optimal characteristic functions/model parameters for low-temperature fracture in asphalt mixtures. The analysis results of the phase field method (which belongs to a class of diffuse damage models) are also compared with the interface element model (which belongs to a class of discrete damage models). The models are compared by examining the governing equations and performing numerical simulations on single-edge notch specimens. Furthermore, various aspects of cohesive zone model (CZM) based PFM are discussed including the type of softening laws and damage growth under uniform/non-uniform stress states. It is concluded that brittle phase field models are not fully capable to predict fracture in asphalt mixtures even at low temperatures.

Simulation of low-temperature brittle fracture of asphalt mixtures based on phase-field cohesive zone model

In this study, a phase-field cohesive zone model (CZM) was presented to simulate the low-temperature brittle fracture of asphalt mixture by finite element method (FEM). The phase-field CZM was applied directly to simulate low-temperature homogeneous fracture of asphalt mixtures. Furthermore, a combination method was proposed for the heterogeneous fracture analysis of asphalt mixtures at low temperature. In this method, the phase-field CZM and the conventional interface CZM are used to simulate the cohesive fracture within the fine aggregate matrix (FAM) and the adhesive fracture at the FAM-aggregate interface, respectively. Then, the applicability of the phase-field CZM to simulate mixed-mode fracture in asphalt mixtures was analyzed by the single-edge notch beam tests with different notch offsets. Furthermore, the cohesive fracture simulation of asphalt mixture based on phase-field CZM and interface CZM was compared. The results indicate that the numerical results based on the phase-field CZM in both homogeneous and heterogeneous fracture analysis are in good agreement with the experimental results. The softening curve of the interface CZM gradually converges to that of the phase-field CZM as the adhesive stiffness increases. Under the same traction-separation law, the crack path predicted by the combination method is similar to that of the pure CZM method. However, the combination method shows a more pronounced brittle behavior of the material. In addition, the combination method has higher computational efficiency and requires less computational memory. In summary, this study provides an effective method to simulate homogeneous and heterogeneous brittle fracture of asphalt mixture at low temperatures.