Notch Effect Research Articles

Analysis of fracture data from notched specimens can provide information on multiscale toughening mechanisms

The identification and quantification of toughening mechanisms in materials is a difficult task which is often made more difficult by the presence of several mechanisms operating at different scales. The hypothesis behind the present work was that the length scales at which these mechanisms operate might be deduced by analysing data from fracture tests conducted on samples containing notches of different root radii. A two stage approach was adopted. Firstly, theoretical models were created using imaginary model materials with well-known toughening mechanisms. In those models having toughening mechanisms on two different scales (known as Dual materials) it was found that the larger-scale mechanism controlled the measured fracture toughness (Kcm) for cracks and sharp notches, whilst the smaller-scale mechanism controlled the behaviour of blunter notches. This change in control created a distinct “kink” in the curve of Kcm as a function of notch root radius. Secondly, experimental data from the literature were examined. Similar kinks were found to be present in the results for some materials, implying Dual behaviour. A method of prediction known as the Theory of Critical Distances (TCD) was applied to the data to estimate the relevant length scales, which in some cases coincided with those of well-known toughening mechanisms. This type of analysis may prove useful for identifying the scale, and relative importance, of toughening mechanisms in a wide variety of materials. It also provides a more accurate way to use the TCD approach to predict the effect of cracks and notches when more than one toughening mechanism is present.

Fatigue assessment of as‐built and heat‐treated Inconel 718 specimens produced by additive manufacturing including notch effects

AbstractThe fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 were analysed in the as‐built and heat‐treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as‐built condition, fine subgrains were found, while in in the heat‐treated state, the subgrains were removed and the dislocation density recovered. SN‐curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the ‐method.

Non‐linear analytical modelling and optimisation of a 12/8 rotor excited flux‐switching machine

Rotor excited flux-switching permanent magnet machines (REFSPMs) have been recently introduced exhibiting high power density and low permanent magnet consumption. Despite the advantages, 12/8 configuration of REFSPM has an intrinsic unbalanced voltage waveform due to the existence of even-order harmonics. Aiming to improve the machine voltage THD, using notches on the rotor segments is proposed. To study the effects of the rotor notch on machine performance with low computational time, the non-linear magnetic equivalent circuit is developed taking into account the effects of fringing fluxes, iron saturation, and rotor transition. An optimisation procedure based on Taguchi method is applied to minimise the voltage THD value taking the effects of uncontrollable factors into account. Finally, the REFSPM is prototyped and the measured results are compared with the analytical and numerical results.

Fatigue of structural adhesives under stress concentrations: Notch effect on fatigue strength, crack initiation and damage evolution

The effect of stress concentrations on the fatigue behaviour of two structural adhesives, relevant materials in lightweight applications, was investigated regarding fatigue strength, crack initiation lifetime, damage evolution and fracture behaviour. Internally and externally notched adhesive bulk specimens were tested under constant amplitude loading with a stress ratio of 0.1. Majority of fatigue lifetime (60–95%) was spent on crack initiation, which was shortened by an increasing stress concentration factor. Crack initiation causes a sudden damage increase near to failure. A model was proposed correlating the fatigue strength of notched specimens with highly stressed region and stress concentration factor.

Study of the tensile behaviour of tungsten wires from ambient temperature to 320 °C in relation to their microstructure

The tensile test results have been interpreted in terms of the microstructure typical of drawn tungsten wires; the influence of test temperature between 25 °C and 320 °C is significant and is linked to the dependence of the mechanical behaviour of tungsten versus temperature and to the development with increasing temperature of a necking zone in which damage and rupture take place in two stages, a localized damage controlled first by the hoop stress associated to the notch effect linked to necking and rupture under the axial stress and the notch effect just mentioned.

The effects of double notches on the mechanical properties of a high-strength pipeline steel under hydrogen atmosphere

Tensile tests and fatigue life tests are performed on double-notched specimens in hydrogen and nitrogen atmospheres to investigate the effects of double notches on the mechanical properties of a high strength pipeline steel. The results show that the fracture occurs at the notch with a lower stress concentration factor (Kt), which is governed by the combination of the stress concentration and the strain hardening caused by plastic deformation in the tensile process. Hydrogen gas accelerates the crack initiation and growth, but it doesn't affect the competitive mechanism of stress concentration and strain hardening.

EFFECT OF FORCE ACTIVATION OF NOTCH SIGNALING ON MTOR

PURPOSE: Notch signaling is a prominent pathway necessary for repair of injured muscle. The interactions of Notch with other signaling pathways, specifically mechanistic/ mammalian target of rapamycin (mTOR), in regulating myogenesis is not well known. Studies have been conducted on Notch inhibition, but little research has been performed on activated Notch and the interactions with mTOR. This study was implemented to develop a Notch force activation protocol and to measure the effect of overexpressed Notch on C2C12 proliferation, differentiation and mTOR signaling. METHODS: Notch signaling was force activated via suspension or adhesion. For suspension, Notch-1 antibody was introduced to a 12-well plate with C2C12 cells at a concentration of 1:10. For adhesion, Notch ligand, Delta-like Ligand 1 (DLL1), was mixed with Extracellular Matrix (ECM) and coated on a 12-well plate at different concentrations (control, 2.5ug, 5ug, and 10 ug) for 12 hours. C2C12 cells were seeded at a concentration of 15,000 cells/well and differentiated for 96 hours. Following designated time period, lysates were collected for Western Blots. Primary antibodies probed for Notch (Hes1), total mTOR signaling (TmTOR), and β-actin. RESULTS: Preliminary data for suspension was analyzed via a two-way ANOVA (time x treatment). There is a significant difference in percent change of proliferation at 48 hours (p=0.02). Preliminary data of adhesion was analyzed via one-way ANOVA. There is a trend between Hes1 concentration in ligand concentration of 2.5 ug (p=0.0739). It shows no significant difference in TmTOR concentrations between the various Notch force activation concentrations in proliferating C2C12 cells (p=.4298). CONCLUSIONS: Preliminary data shows a stronger effect of suspension over adhesion for force activating Notch. A concentration of 2.5ug of ligand is possibly sufficient to force activate Notch, any higher appears too concentrated to activate Notch. Preliminary data also suggests that force activating Notch does not affect mTOR signaling. Experiments are currently testing the ideal DLL1 to activate Notch signaling, as well as examining the effect of force activating Notch on proliferation, differentiation and other markers of mTOR signaling. Different time points for differentiation should be tested in future research.

Effects of interlayer notch and shear stress on interlayer strength of 3D printed cement paste

In this paper for investigation of the effect of interlayer notch and shear stress on interlayer strength of 3D printed cement paste, experimental work is conducted with emphasis on the effects of nozzle shape, nozzle height and printing speed on the interlayer strength. It is observed experimentally that the cement paste printed by the circular nozzle generally achieves a higher interlayer strength, compared with that printed by the rectangular nozzle. A smaller nozzle height may impair the interlayer strength if the interlayer notch is not treated properly. In addition, a higher printing speed marginally increases the interlayer strength, showing the promise of a higher productivity. Theoretically, the experimental phenomena are innovatively explained by the interlayer notch coupled with shear stress on the contact surface, through finite-element and computational-fluid-dynamics simulations. Therefore, reducing the interlayer notch and increasing the interlayer shear stress become crucial in the cement paste for 3D printing process.

Effect of elliptical notches on mechanical response and progressive damage of FMLs under tensile loading

Abstract This paper aims to explore the residual strength and elucidate the failure mechanisms of elliptical notched FMLs with different design variables through experimental investigations and numerical simulations. Quasi-static tensile experiments are carried out to investigate the effects of off-axis angle, inclination angle and aspect ratio on the tension behaviors with the aid of digital image correlation technique (DIC). Subsequently, a progressive damage model integrated with a VUMAT subroutine is employed to study the progressive damage evolution and failure mechanism, considering thermal residual stress. Meanwhile, damage initiation and evolution as well as the finial damage patterns are explored systemically combining the strain distributions from DIC technique, equivalent plastic strain distributions of aluminum layers and final damage morphologies of specimens. Results show that the notch strength is more sensitive to off-axis angle and inclination angle as compared with the aspect ratio. The failure morphology exhibits a neat and straight crack for on-axis cases, whereas it presents a fracture along the fiber direction of adjacent layer with apparent pull out of fiber for off-axis cases. With the increase of off-axis angle, the ultimate failure mode gradually transfers from fiber-driven to aluminum-driven accompanied by the corresponding critical failure mechanism varying from tension to tension-shear.

Discrete numerical analyses of grain size influence on the fracture of notched rock beams

Abstract This paper studies the influence of the grain size and the notch effect on the fracture of U-shaped notched rock beams through the variation of the apparent fracture toughness. The research is based on an exhaustive campaign that comprises numerical simulations of 300 four-point bending tests, 30 uniaxial compression tests and 60 tensile splitting (Brazilian) tests. Non-porous, isotropic ideal and equivalent rocks with 5 different uniform grain sizes are modelled using the distinct element method, where the rocks are modelled as a discontinuous material, defining explicitly the grains and the boundary conditions. Several notch radii are simulated and the corresponding variation in the apparent fracture toughness is observed. The notch effect is interpreted using the Theory of Critical Distances (TCD), which uses a material intrinsic property called the critical distance ( L ) to evaluate the stress field around the notch tip. The paper shows the variation of the fundamental rock properties with the grain size, the applicability of the TCD to evaluate the notch effect and the correlation between the critical distance and the grain size.

Laser ultrasonics for defect evaluation on coated railway axles

This scientific paper focuses on the application of an advanced non-destructive technique for an effective inspection of railway axles. The method pertains to ultrasonic techniques, which are widely used in the railway field. The experimental investigation was carried out on simulated defects tooled near the cross section reduction of the axle, in order to simulate fatigue cracks which, due to notch effect, can trigger crack propagation and axle failure.The aim of this research activity is to evaluate how efficiently the proposed technique detects defects and to verify its applicability to axles with a black coating for protection. In view of the experimental setup, comprising a pulsed laser for ultrasonic waves generation and a continuous beam laser plus an interferometer as the receiving unit to measure surface displacements, the presence of a black coating represents a major challenge in terms of signal detection. Nonetheless, defects were detected by collecting all the waves in a B-scan map and data were processed by cross-reading A-scans and B-Scans in correspondence of each defect analysed. Results of the analysis show how very promising and robust the application of the proposed laser ultrasonics technique is in defect detection on painted railway axles.

Surface roughness effects on the fatigue behaviour of gyroid cellular structures obtained by additive manufacturing

Surface roughness effects on the fatigue behaviour of gyroid cellular Ti-6Al-4 V structures obtained by additive manufacturing were investigated. The normalized fatigue strength values for 10^6 cycles, obtained from the S-N curves, were in the range of 0.21–0.23. Finite element analyses showed that stress peak points were located on the stress concentrators due to surface defects, and scanning electron microscopy images showed that crack initiation was located at the surface. It is concluded that the expected good fatigue behaviour due to the curvature in the junctions of the struts, is diminished because of the superficial roughness contributing to the notch effect.

Research on three-phase four-wire active power filter based on LCLCL coupling structure

In this paper, the main circuit topology of the three-phase four-wire active power filter (APF) and its harmonic current detection and control algorithm are studied. An active power filter system combining LCLCL filter and a three-phase H-bridge structure is proposed. This paper has applied harmonic detection algorithm based on an improved synchronous phase-locked loop (SRF-PLL) and recursive discrete Fourier transform (RDFT) while using triangular carrier modulation control based on PI adjustment to realize the harmonic compensation of the power filter to the grid current. The system has a notch effect on the harmonic at the switching frequency. It can detect the fundamental component and each harmonic component of the load current separately with a small amount of calculation, good real-time performance. Finally, a 5KVA single-phase APF prototype was built to test the superiority of the main circuit topology and the algorithm of current detection and control.

Ageing condition of tensile specimens: fracture behavior of notched Al2024 sheet under tensile loading

The effect of different (T6 and T8) heat treatments on tensile properties and fracture behavior of Al2024 in presence and absence of notch by means of microhardness, tensile tests and scanning electron microscopy were investigated. Tensile and hardness specimens were subjected to two different artificial ageing (T6 and T8 heat treatment) for various times. These included under ageing (UA), peak ageing (PA) and over ageing (OA). T8 heat treatment, which has a cold rolling between solutionizing and ageing in its steps, showed a higher value of hardness and yield strength in comparison with common artificial ageing of T6 heat treatment. In notched-tensile specimens, yield stress was found to increase up to the peak ageing condition with a simultaneously decrease in elongation at fracture. This behavior was converse at OA condition. Although introducing of notch increase the yield stress of samples under T6 and T8 conditions in comparison with un-notched samples, the notch strengthening phenomenon was observed only under T8 treatment. Despite of an enhancement in strengthening by applying notch on tensile samples, the elongation to failure was notably lessen in both notched-heat treated samples in comparison with un-notches ones. Also, it was confirmed that the toughness of notched samples of both heat treatments at PA condition were significantly lower than un-notched ones. Consequently, toughness decrement was considerably dominated by the role of deformability compared to strengthening factor, however, the presence of cold rolling in the process of heat treatment (T8) could reduce the harmful effects of notch by increasing the stress bearing capacity in contrary with T6 heat treatment. Moreover, inserting the mechanical properties of peak aged samples from the un-notched tensile test in Abaqus finite element software; the V-shaped notch tensile test was simulated and confirmed the experimental results. It was shown in SEM results that the presence of notch enhanced the contribution of cracked particles, compared to particle/matrix deboning and matrix deformation, therefore, the non-homogeneous distribution of fracture features confirmed the harmful effect of notch. In the following, the distribution of three fracture micro-mechanisms were homogeneous in un-notched samples, which demonstrated the superior values of toughness in smooth samples. The present finding sheds light on development of processing techniques to optimize the mechanical properties of Al 2024 alloy.

Study on surface morphology and recast layer microstructure of medium thickness stainless steel sheets using high power laser cutting

With the rapid development of high-power lasers, efficiency of laser cutting and thickness of cutting material have been greatly increased. In this study, a 10 kW fiber laser was used to cut 18 mm thick 316L stainless steel sheets. The surfaces and cross sections of samples were observed by an optical microscope and a scanning electron microscope. It was found that the quality of laser cutting was closely related to its process parameters, and the most important defects during the laser cutting process included striation and dross. On this basis, effects of process parameters such as defocusing amount and cutting speed on the kerf morphology and microstructure of the recast layer were investigated. The cutting surface was clearly divided into four characteristic areas in the vertical direction; the surface morphology and the recast layer microstructure of each area were analyzed. The results of the nanoindentation test showed that microhardness of the recast layer increased significantly comparing with that of the substrate, which was due to the refined microstructure of the recast layer. The influence of the notch effect caused by the dross at the bottom of the cutting surface on the crack source is greater than that caused by the surface striation, so the crack source is more likely to be generated near the dross. The parameters for good cutting quality in this experiment are as follows: 10 bar N2 assist gas pressure, 9 kW of laser power, −15 mm of defocus, and 350 mm/min of cutting speed.

Characterization of an Extruded Mg-Dy-Nd Alloy during Stress Corrosion with C-Ring Tests

This study focuses on the characterization of the failure behavior of an extruded Mg10Dy1Nd alloy during stress corrosion. The microstructure, hardness, strength and corrosion behavior of binary alloys Mg10Dy and Mg1Nd are compared to those of the ternary alloy system. The ternary alloy Mg-Dy-Nd that is not fully recrystallized has the highest hardness but lacks ductility. The investigated alloys twin during plastic deformation. Static C-ring tests in Ringer solution were used to evaluate the stress corrosion properties, and stress corrosion could not be found. None of the alloys failed by fracturing, but corrosion pits formed to various extents. These corrosion pits were elliptical in shape and located below the surface. Some of the pits reduced the remaining wall thickness significantly, but the stress increased by the notch effect did not lead to crack initiation. Furthermore, the C-ring specimen was subjected to compressive loading until fracture. Whereas the Mg1Nd alloy showed ductile behavior, the alloys containing Dy fractured on the tensile side. The crack initiation and growth were mainly influenced by the twin boundaries. The Mg10Dy1Nd alloy had an inhomogeneous microstructure and low ductility, which resulted in a lower fracture toughness than that of the Mg10Dy alloy. There were features that indicate hydrogen-assisted fracture. Although adding Nd decreased the fracture toughness, it reduced the grain size and had a positive influence on the corrosion rate during immersion testing.

INFLUENCE OF ROUGH AS-BUILT SURFACES ON SMOOTH AND NOTCHED FATIGUE BEHAVIOR OF L-PBF AlSi10Mg

Near-net shape metal parts of great geometrical complexity are fabricated by the Laser Powder Bed Fusion (L-PBF) technology directly from a CAD model. Therefore, parts can be lightweight, less expensive in terms of material use and with shapes that may be impossible to produce by conventional technology. The fatigue behavior of L-PBF part in as-built condition is negatively affected by poor surface quality. Surface finishing after fabrication may be either unacceptably costly or impossible because the surface is inaccessible. Fatigue performance can be further reduced by the notch effect due to local geometrical variations. Among the Al-alloys, AlSi10Mg is readily processed with L-PBF and it is of interest for different industrial sectors.In this contribution two aspects, that is: i) the directional smooth fatigue behavior of as-built AlSi10Mg, and ii) the notch fatigue behavior with as-built surfaces are investigated. Eight sets of un-notched and notched miniature specimens of AlSi10Mg were produced as a single batch by L-PBF and tested in the as-build state under cyclic plane bending loading. The smooth fatigue behavior was determined as very sensitive to applied stress direction with respect to the build direction. The directional nature of the fatigue behavior was confirmed by notch fatigue data. Therefore, four notch fatigue factors that depend on the PBF technology were introduced and determined. The fatigue behavior of L-PBF AlSi10Mg obtained here was compared satisfactorily against recent data obtained with standard specimen geometries and test methods. The present methodology using mini specimens under cyclic bending efficiently determines the fatigue response of L-PBF metals.

The Fracture Behavior of 7085-T74 Al Alloy Ultra-Thick Plate During High Cycle Fatigue

As a new generation of Al-Zn-Mg-Cu alloy, 7085 Al alloy was first evaluated by high-cycle fatigue (HCF) tests. In the present work, HCF behaviors and the notch sensitivity of a 220-mm-thick 7085-T74 Al alloy ultra-thick plate were investigated. The fracture characteristics of the alloy were investigated, and the crack initiation mode of the alloy was proposed in the present study. The results show that the notch-sensitive coefficient is 0.367, which indicates that the 7085 Al alloy displays a sensitive notch effect. The fatigue crack initiation sites of 7085-T7452 Al alloy ultra-thick plate mainly occurred at the locations of coarse Fe-rich particles on the specimen surface. In addition, compared with smooth specimens, the notched specimens are prone to generating fatigue cracks and usually show more than one crack initiation site.

Elastic limit load resource when reaming holes in turbine rotor discs

The main topic of the article is the evaluation of load capacity by elastic stress analysis of impulse steam turbine rotors with bore drilled discs used for pressure equalization. Long-time operation of such turbines leads to fatigue fractures. The authors have analyzed stress distribution around the holes in turbine rotors in their nominal state and after reaming, which is the preferred method of removing fractures. Two approaches were used in analysis of bore drilled disc problem: continuous model and its mathematical solution and numerical based on finite element modeling. Both confirmed co-existence of stress concentration and crack initiation. Results show significant range for possible holes reaming - even with lowering stress level at critical area. These results were successfully used in overhaul practices and further turbine life time extension. It also allows to predict optimum hole layout and sizing at the design process for possible rotor disc notch effect reduction.

The effect of defects and notches in quasi-static and fatigue loading of Inconel 718 specimens produced by selective laser melting

Additively manufactured components are likely to contain defects deriving from the manufacturing process. They are also likely to be designed with high geometric complexity (including notches), due to very few design-limitations. In order to understand the influence of these two types of geometric features, we here investigate as-built Inconel 718 specimens produced by selective laser melting under quasi-static and cyclic loading. Notched and unnotched specimens are considered, and the main emphasis is set on the influence of the defects and on how they influence the failure locations at different load levels.