Single Edge Notch Research Articles

Off-axis tearing properties of the biaxial warp-knitted fabrics

This study presents an experimental and analytical investigation of the tearing properties of the biaxial warp-knitted fabrics with different off-axis angles. Firstly, the off-axis tensile tests, central tearing tests, single edge notch tearing tests, and trapezoidal tearing tests were carried out at seven off-axis angles. The strength, failure mode, and failure mechanism of the specimens under each test condition were studied. The results show that in the off-axis tensile tests, the lowest values of the tensile strength of the biaxial warp-knitted fabrics appear at the off-axis angles of 15° and 75° rather than the conventional 45°. Under the coupling effect of tensile strength and shearing strength, the yarn layers peeling occurs instead of yarn fracture, which is a unique failure mode of biaxial warp-knitted fabrics. The yarns of the biaxial warp-knitted fabrics can fully deform to balance the load due to the warp-knitting method, which improves the tearing strength and makes the failure brittle.

Harnessing the thermo-mechanical properties and toughness of MWCNT/TiO2 hybrid epoxy nanocomposites

One of the significant challenges of developing high-performance polymer nanocomposites is the best exploitation of the potential of nanofillers. In this study, the multi-walled carbon nanotubes (MWCNTs) are exfoliated by TiO2 nanoparticles using ultrasonic waves. The obtained MWCNT/TiO2 hybrid nanofiller, in different wt.%, is infused into an epoxy (EP) matrix using an ultrasonic dual mixing (UDM) technique to obtain multifunctional nanocomposites with enhanced thermo-mechanical properties and toughness. XRD crystallography, Raman spectroscopy, TEM analysis, TGA, DMA, tensile, and 3-P single edge notch bending tests are used to investigate the properties of the MWCNT/TiO2 hybrid nanofiller and resulting composites. Results reveal that the optimum loading of MWCNT/TiO2 hybrid nanofiller (1.0 wt.%) in EP matrix provides 23.5%, 39.8%, and 46.8% improvement in storage modulus, tensile strength, and fracture toughness, respectively, compared to those of neat EP. The FESEM images of the fractured surface of nanocomposite specimens confirm the cluster-free distribution of MWCNTs in the EP matrix ensuring enhanced thermo-mechanical properties and toughness.

Estimating the mode I through-thickness intralaminar [formula omitted]-curve of unidirectional carbon fibre-reinforced polymers using a micromechanics framework combined with the size effect method

A three-dimensional micromechanics framework is developed to estimate the mode I through-thickness intralaminar crack resistance curve of unidirectional carbon fibre-reinforced polymers. Finite element models of geometrically-scaled single edge notch tension specimens were generated. These were modelled following a combined micro-/meso-scale approach, where the region at the vicinity of the crack tip describes the microstructure of the material, while the regions far from the crack tip represent the mesoscopic linear-elastic behaviour of the composite. This work presents a novel methodology to estimate fracture properties of composite materials by combining computational micromechanics with the size effect method. The size effect law of the material, and consequently the crack resistance curve, are estimated through the numerically calculated peak stresses. In-depth parametric analyses, which are hard to conduct empirically, are undertaken, allowing for quantitative and qualitative comparisons to be successfully made with experimental and numerical observations taken from literature.

Effects of aging on the mechanical and fracture properties of chopped fiber composites made from repurposed aerospace prepreg scrap and waste

In this study, the effects of prepreg out time on the mechanical and fracture properties of Discontinuous Fiber Composites (DFCs) are investigated. Carbon fiber prepregs are aged to 0×, 1×, 2×, and 3× the out life (28 days) in an environmental chamber at 23∘C and 50%RH.From tension, compression, and shear experiments it is seen that the out time has no effect on the elastic moduli. However, the strength increases with increasing age of the specimens for all the loading conditions. The percent increase compared to the unaged material ranges from 15% to 33%. This is likely caused by plasticization of the matrix with age, allowing for higher energy absorption.More complex trends are reported for the fracture tests on geometrically-scaled Single Edge Notch Tension (SENT) specimens. It is found that the fracture energy and characteristic length initially decrease with age, and then finally increase for the longest out time. This trend is owed to two factors with countering effects on the fracture behavior: 1) the increase of the number of platelets through the thickness with aging due to increase in resin viscosity, and 2) the plasticization of the matrix with aging.The results of this study suggest that the mechanical performance of Discontinuous Fiber Composites (DFCs) made from reused materials is not negatively affected by aging of the starting material. On the contrary, repurposed DFCs can exhibit equal, if not better, performance than unaged DFCs. The results of this work further corroborate the feasibility of using DFCs as a mean to mitigate the environmental impact of thermoset composite waste. The experimental data presented in this work can be used as a baseline to design DFC composite components made from repurposed prepreg scrap and waste.

Micromechanical modelling of API X80 linepipe steel

Gurson–Tvergaard–Needleman (GTN) model parameters have been estimated for the API X80 linepipe steel; these parameters and mesh size are transferable from one geometry to another. Fracture data are non-transferable from the lab specimen to the component level. These issues are resolved by local approaches to fracture to predict damage. GTN model incorporates the local features of the fracture, i.e., nucleation, growth, and coalescence of voids. The void volume fraction of material at different stages of loading governs the load-carrying capacity. This research has explored the methodology to determine the parameters by metallographic examination, known as micromechanical modelling. Parametric study and calibration of the parameter are done to fit the load-displacement curve of the notched tensile bar. These parameters are validated on the single edge notch bend (SENB) test. Estimated GTN parameters can predict the damage behavior for moderate to low constraints; hence, these parameters and mesh size are transferable from one geometry to another.

Assessment of fracture toughness of cemented carbides by using a shallow notch produced by ultrashort pulsed laser ablation, and a comparative study with tests employing precracked specimens

The use of fracture mechanics for rationalizing the fracture behavior of cemented carbides is valid, as far as sharp cracks, free of residual stresses and subjected to a well-defined stress state are used for assessing fracture toughness. However, machining a very sharp notch on the surface of hardmetals for fracture toughness testing has been a critical issue during many years. Within this context, introduction of surface “through-thickness” micronotches (SEμVNB) by means of ultrashort pulsed laser ablation (UPLA) is here proposed, implemented and analyzed as an innovative precracking-like route within flexural testing procedures for appropiated evaluation of fracture toughness of cemented carbides. UPLA parameters used for introducing the micronotch are optimized in terms of induced damage ahead of the notch tip. For comparison purposes, fracture toughness is also determined by means of flexural testing of previously cracked single-edge notch beams (SENB-Cracked) as well as specimens with V-notch tips sharpened through diamond polishing using a razor blade, and Palmqvist indentation microfracture method. The satisfactory agreement found between values measured using UPLA-micronotched and SENB-Cracked (reference) specimens allows to conclude that flexural testing of SEμVNB samples is a valid methodology for reliable determination of fracture toughness of hardmetals. This is feasible because of the extremely short time of laser-matter interaction. It yields small and somehow controlled damage in front of the notch tip as a result of shock wave propagation during ablation, which translates into effective precracking of SEμVNB specimens.

On the applicability of digital image correlation method in extracting the higher order terms in stress field around blunt notches

Accurate and deep understanding of the stress field has a key role in designing and optimizing the engineering structures. The main objective of the present article is to experimentally investigate the stress field around various blunt notches by means of the digital image correlation (DIC) method. First, several mode I tests were conducted using two types of specimens, namely single edge notch tension (SENT) and single edge notch bending (SENB), each consisting of three different notches (U, VO, and Keyhole). Then, the optical method of DIC was used to determine the stress field around the notch tip by calculating the coefficients of singular and higher-order terms (HOTs) for each notch type. To do so, the output displacement field of the DIC measurements was considered as input data for a computer program, and the coefficients of the stress field were calculated by the least square method (LSM) and an over-deterministic approach. Moreover, the effects of several geometry parameters like notch opening angle and notch tip shape on the stress distribution around the notch were discussed in detail. Finally, the precision of the calculated notch stress intensity factors (NSIFs) and the coefficients of HOTs were studied by comparing the DIC results with those obtained from the finite element (FE) analysis. It is also shown that HOTs of stress field have considerable contribution in more precise determination of the stress field around notches.

Stress intensity factors, T-stresses and higher order coefficients of the Williams series expansion and their evaluation through molecular dynamics simulations

Molecular dynamics method and finite element analysis are applied for the analysis of the stress field in the neighborhood of the crack tip in a copper plate with a single edge notch. The molecular dynamics simulations implemented in a classical molecular dynamics code LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) are aimed at evaluating classical continuum linear elastic fracture mechanics parameters such as stress intensity factors, T-stresses and higher order coefficients of the Williams power series expansion of the near crack stress field for Mode I, Mode II and Mixed Mode (Mode I + Mode II) loadings of the cracked specimen in isotropic linear elastic materials. The key objective of the study is the comparison of continuum and atomistic approaches for the estimation of the near crack tip fields using the example of one of the most common cracked configurations. Stress intensity factors, T-stresses and higher order coefficients of the Williams series expansion for a copper plate with the single edge notch under Mode I and Mixed Mode loadings are evaluated by atomistic modeling and by finite element method. The wide class of the computations in LAMMPS is performed. The atomistic values of stress intensity factors and higher order terms of the Williams series expansion are compared with the values obtained from the numerical solutions given by finite element method. It is shown that the continuum fracture theory successfully describes fracture and the near crack tip fields even at extremely confined singular stress field of only several nanometers. The angular distributions of the stress components from atomistic modeling are retrieved and compared with the angular distributions of the stresses from continuum linear elastic fracture mechanics. The comparison shows good agreement between two approaches.

Mechanical Testing Methods for Evaluating Thermoplastic Permanent Pavement Markings

The durability of permanent pavement markings (PPMs) on roadways is important for drivers’ safety. There are two primary mechanical failure modes: cohesive failure that occurs internally in PPMs through small defects, such as internal pores, and adhesive failure that occurs along the interface between PPMs and road surfaces. Thus, it is critical to characterize the intrinsic mechanical properties of PPMs as well as the adhesion of PPMs on road surfaces to understand their mechanical performance and, ultimately, the durability of PPMs. In this study, the flexural modulus and strength of PPMs were characterized via three-point bend testing, while fracture toughness was determined with single edge notch bend testing. To analyze the adhesive performance of PPMs on asphalt, a shear adhesion testing approach was developed to measure the apparent debonding energy of PPM specimens on asphalt. The shear adhesion test was performed on asphalt road surfaces and cut surfaces to investigate the chemical and mechanical interfacial effects on adhesion. Two commercial thermoplastic PPMs with different mechanical properties were investigated to study how various factors directly affect the adhesion of PPMs on asphalt surfaces. Through mechanical tests, the relationships between the intrinsic materials properties and the mechanical performance of PPMs on asphalt were studied. A PPM material that had lower modulus and higher deformation energy exhibited greater adhesion performance on asphalt, especially when the PPM material was applied at higher asphalt surface temperatures on rough asphalt surfaces.

Aluminium plates with geometrical defects subjected to low-velocity impact: Experiments and simulations

In this paper, an experimental and numerical program investigating impact in the low-velocity regime (≤10 m/s) on 1.5 mm thick AA6016 aluminium plates in three different heat treatments (T4/T6/T7) is presented. The tests were carried out in a drop tower where the force was continuously monitored by a strain-gauge instrumented striker. Results from both intact plates and plates with geometrical defects in the form of pre-cut slits are presented. The heat treatments result in materials with different strength, work-hardening and ductility. It was observed that the intact plates in high-strength material are more resilient to low-velocity impact whereas high ductility is preferred for the plates with pre-cut slits as the crack growth is more restrained. Quasi-static tests on the same plates showed a significant reduction in force only for plates in temper T6 and T7. The experiments are complemented by nonlinear finite element simulations using linear brick elements in Abaqus/Explicit. A von Mises plasticity model was calibrated from notched tension tests. The global response from single edge notch tension (SENT) tests was used to calibrate the failure criterion. The numerical model was able to sufficiently predict the force response and deformation of the intact plates, and the global response and crack propagation of the SENT tests. While the force response in the dynamic simulation of low-velocity impact on plates with slits was in agreement with the observed quasi-static response for all tempers, it was in general underestimated for temper T6 and T7 when compared to the data from the dynamic experiments. For plates in temper T4, the predicted response also agreed with the dynamic experiments.

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.

On the analysis of plasticity induced crack closure in welded specimens: A mechanism controlled by the stress intensity factor resulting from residual stresses

The aim of this paper is to clarify how and to what extent varied welding residual stresses (WRSes) affect the plane stress plasticity induced crack closure (PICC) simulation. A well-characterized, representative WRS field with tensile and compressive reversals was imported and the predicted PICC results were evaluated against the relevant WRS-free ones. It turned out that instead of WRS typically reported in the literature, there is actually a Kres (stress intensity factor resulting from residual stresses) controlled crack closure mechanism when investigating the PICC behavior in the presence of a WRS field, i.e., WRS influences PICC in the form of Kres. Four major categories where the crack tip was located in different Kres zones were defined using MT (middle tension) and SENT (single edge notch tension) specimens to investigate the mechanism in detail. In summary, the Kres value determined the general steady-state PICC level in the WRS field, i.e., a diminished steady-state opening stress level was predicted in the WRS field with a positive Kres regardless of whether the crack is located in a tensile or a compressive WRS zone, and an elevated value was estimated when the Kres was negative. Besides, the initial transient period was scrutinized that occurred in some WRS scenarios and the concept of primary and secondary plastic wake evolution was proposed to explain its formation. It was found that the transient behavior could be eradicated by tailoring the secondary plastic wake employing appropriate constitutive models and mesh refinement levels. The results attained in the present work provide some guidelines for the WRS-PICC modeling and simulation in the fracture mechanics community.

The Effect of Sintering Temperature on the Phase Composition, Microstructure, and Mechanical Properties of Yttria-Stabilized Zirconia.

It is known that the yttria-stabilized zirconia (YSZ) material has superior thermal, mechanical, and electrical properties. This material is used for manufacturing products and components of air heaters, hydrogen reformers, cracking furnaces, fired heaters, etc. This work is aimed at searching for the optimal sintering mode of YSZ ceramics that provides a high crack growth resistance. Beam specimens of ZrO2 ceramics doped with 6, 7, and 8 mol% Y2O3 (hereinafter: 6YSZ, 7YSZ, and 8YSZ) were prepared using a conventional sintering technique. Four sintering temperatures (1450 °C, 1500 °C, 1550 °C, and 1600 °C) were used for the 6YSZ series and two sintering temperatures (1550 °C and 1600 °C) were used for the 7YSZ and 8YSZ series. The series of sintered specimens were ground and polished to reach a good surface quality. Several mechanical tests of the materials were performed, namely, the microhardness test, fracture toughness test by the indentation method, and single-edge notch beam (SENB) test under three-point bending. Based on XRD analysis, the phase balance (percentages of tetragonal, cubic, and monoclinic ZrO2 phases) of each composition was substantiated. The morphology of the fracture surfaces of specimens after both the fracture toughness tests was studied in relation to the mechanical behavior of the specimens and the microstructure of corresponding materials. SEM-EDX analysis was used for microstructural characterization. It was found that both the yttria percentage and sintering temperature affect the mechanical behavior of the ceramics. Optimal chemical composition and sintering temperature were determined for the studied series of ceramics. The maximum transformation toughening effect was revealed for ZrO2-6 mol% Y2O3 ceramics during indentation. However, in the case of a SENB test, the maximum transformation toughening effect in the crack tip vicinity was found in ZrO2-7 mol% Y2O3 ceramics. The conditions for obtaining YSZ ceramics with high fracture toughness are discussed.

THE EFFECT OF ULTRA-FINE ALLOYING ELEMENTS ON THE PHASE COMPOSITION, MICROSTRUCTURE, HIGH-TEMPERATURE STRENGTH AND FRACTURE TOUGHNESS OF Ti–Si–X AND Ti–Cr–X COMPOSITES

Advanced Ti-based composites are promising for applications in components of modern aircraft and rocket engines as well as other power equipment owing to their high strength-to-weight ratio and fracture toughness in a temperature range of 20 °C to 650 °C. However, there is a need to increase their operating temperature range up to 700−800 °C. In this work, mechanical behavior of Ti–Si–X composites (X=Al and/or Zr, Sn, C) has been studied. For comparison, mechanical behavior of Ti–Cr–X composite (X=Al and/or C) has been studied. As-cast and thermo-mechanically deformed series of beam specimens were examined. Strength tests of specimens were performed under three-point bending in a temperature range of 20 °C to 1000 °C. Single-edge notch beam (SENB) tests under three-point bending of specimen series were carried out in a temperature range of 20 °C to 900 °C for estimating fracture toughness of materials. Based on the constructed dependences of fracture toughness and strength on testing temperature for the specimen series as well as the microstructure and failure micromechanism analyses, the role of ultra-fine alloying elements in achieving good high-temperature strength and fracture toughness of the studied composites was substantiated.

Crack closure of Ni-based superalloy 718 at high negative stress ratios

Fatigue crack growth damage mechanisms for nickel-based superalloys at high, negative stress ratios challenge classical crack tip shielding and closure concepts. Fatigue crack growth rate and crack closure for single-edge notch Ni-based superalloy 718 specimens at 100°C were evaluated for several stress ratios (0.5, 0.1, −1, and −2). Multiple methods were used to calculate the effective driving force for crack growth rates in region II (i.e., Paris or power law regime). The relationships between the effective driving forces, the experimentally-measured crack closure levels, and the fractographic evidence of damage mechanisms are discussed.

Fracture analysis of single edge notched specimen using phase field approach

For the damage tolerant design of cracked structural components, two important parameters namely, residual strength and remaining life are very important. To evaluate these, a reliable fracture parameter such as stress intensity factor (SIF) is to be determined. Phase field (PF) based finite element formulation for a brittle fracture to determine crack propagation trajectory is presented in this paper. PF methodologies have been formulated by coupling partial differential equations (PDEs) after minimizing the total potential energy, which includes elastic strain energy and fracture energy, with respect to the displacement field and PF variable. Solutions of these PDEs for PF variable resulted in identifying crack path geometry, while PDEs displacement field obtained near the crack field is used to determine SIF. In the present study, extending PF formulation to fracture analysis of a structural component is first of its kind. To simulate PF phenomena, a four noded 2-D bilinear quadrilateral element with three degrees of freedom (DOF) at each node (two translations and one for phase field) has been developed to analyze single edge notched (SEN) specimen under tensile and shear loading. By combining the displacement results of PF method with displacement extrapolation technique, SIF has been determined for various crack lengths representing crack propagation in SEN specimens. It is then compared with the analytical SIF solution available in Handbook, and also with the contour integral method SIF results. It is found that the SIF obtained by PF method is in good agreement (±10%) with the other solutions.

Determination of Cracks using Multiple DC Potential Drop Measurements – Experimental Verification of an Advanced Model

The Direct Current Potential Drop Method (DCPDM) is a frequently used technique for crack detection and crack length measurement in fatigue experiments. Recent investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in single edge notch specimens (Wiehler and Bär, 2020), (Bär and Nahbein, 2022) and in round bars (Hartweg and Bär, 2019). In the present work a more detailed investigation of crack initiation and propagation is undertaken on round bars to advance the geometrical model by Hartweg and Bär (2019). Three potential probes were equipped on notched steel bars – at 0° (U1), at 120° (U2) and at 240° (U3). The three potentials were measured simultaneously during the fatigue tests using amplifiers of the control electronics. Moreover, the introduction of overloads was used to mark the crack front on the fracture surface in defined intervals to compare crack and size calculated from the potential drop measurements with the real crack size and geometry.In the case of an initiated single crack, the experimental set-up delivers different potentials depending on the distance of the probes to the crack. The geometrical model can be used not only for the determination of the crack angle like performed by Hartweg and Bär (2019) but also to calculate the crack size. On the measured data a function is fitted that allows the calculation of the crack length independent of the location of the crack initiation site. The experimental results have shown that it is possible to determine a crack with a size of less than 1 % of the specimen cross-sectional area.

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.

Multi-physics FE-analysis and measurements for thermo-mechanical fatigue crack growth rate testing applications

The presented methodology in this study is addressed to in-phase (IP) and out-of-phase (OOP) loading cycles in stationary and transient thermo-mechanical fields. The subject of the numerical and experimental study is a single edge notch tension (SENT) specimen produced from a high-temperature nickel-based alloy ХН73М. In order to determination a local thermo-mechanical stress-strain rate and displacement fields a new algorithm for the multi-physics numerical calculations developed and implemented incorporates Maxwell 3D, Fluent and Transient Structural modules of ANSYS 2021R1. The employment of proposed algorithm to represent the cyclic history associated with the TMF conditions in the experiments, multi-physics finite element (FE) modelling of the stress, strain and displacement fields in the SENT specimen was performed. Additionally, time dependent non-uniform temperature fields were determined with the same cyclic variations and magnitudes as in the experimental OOP and IP cycling. As a complement to the FEM computations, the infra-red thermography temperature distribution measurements was implemented for the TMF state in the experiments in the SENT specimen. The comparison multi-physics FE-analysis and direct measurements shown in the present study is intended to contribute to a better understanding of the different mechanisms driving TMF crack growth and the address the outstanding questions associated with basic methodology.

Numerical modelling of mode-I cracking in asphalt concrete

This paper presents a numerical model to simulate mode-I fracture in asphalt concrete. The phase field method is used to simulate diffuse damage in asphalt concrete. In addition to the displacement degrees of freedom, a scalar damage degree of freedom is added at each node of a finite element. Evolution of the damage variable is governed by a crack potential which regularizes a sharp crack over a finite volume. The phase field method does not suffer from mesh dependency problem like the continuum damage models and therefore preferred for damage analysis. A single edge notch beam is analysed to show the performance of the model. The damage pattern as well as load versus crack mouth opening displacement graph is compared with the experimental data. The numerical results show good agreement with the experimental observations.