Artificial Notch Research Articles

Crack Front Geometry and Stress Intensity Factor of Semi-circular Bend Specimens with Straight Through and Chevron Notches

The semi-circular bending (SCB) test is one of the most useful testing methods for determining the mode-I fracture toughness of rocks. An SCB specimen with an artificial notch is loaded at three points including two lower points and a single upper point during the test. In general, there are two types of geometries for artificial notches: straight through and chevron notches. The straight through notch is commonly adopted for the SCB (STNSCB) test as the suggested method for estimating mode-I fracture toughness of rocks in ISRM, while the cracked chevron notch SCB (CCNSCB) test using a specimen with a chevron notch has been performed by some researches. In this paper, by means of the commercial finite-element software ABAQUS, cracking behavior from the tip of an artificial notch during STNSCB and CCNSCB tests is analyzed with Extended Finite-Element Method (XFEM) to clarify crack front geometry in the process of cracking. The relationship between the crack length and the stress intensity factor can be obtained by analyzing stress intensity factors of the specimen with FEM, based on crack front geometries calculated with XFEM during the cracking process. Using this relationship, the minimum stress intensity factor at a critical crack length is determined for estimating mode-I fracture toughness of the rock for the CCNSCB test. Furthermore, by performing the SCB tests using Kimachi sandstone specimens with three different artificial notch geometries, fracture toughness of the sandstone is determined from the SCB test. Consequently, it is concluded that the values of fracture toughness from the test with three notch geometries are almost the same.

Defects Detection and Localization in Underwater Plates Using Laser Laterally Generated Pure Non-Dispersive S0 Mode

When working in humid environments, corrosion defects are easily produced in metallic plates. For defect detection in underwater plates, symmetric modes of Lamb waves are widely used because of their characteristics including long propagating distance and high sensitivity to defects. In this study, we extend our previous work by applying the laser laterally generated S0 mode to detection and localization of defects represented by artificial notches in an aluminum plate immersed in water. Pure non-dispersive S0 mode is generated in an underwater plate by lateral laser source irradiation and its fd (frequency·thickness) range is selected by theoretical calculation. Using this lateral excitation, the S0 mode is enhanced; meanwhile, the A0 mode is effectively suppressed. The mode-converted A0 mode from the incident S0 mode is used to detect and localize the defect. The results reveal a significantly improved capability to detect defects in an underwater plate using the laser laterally generated S0 mode, while that using A0 is limited due to its high attenuation. Furthermore, owing to the long propagating distance and the non-dispersion characteristics of the S0 generated by the lateral laser source, multiple defects can also be detected and localized according to the mode conversion at the defects.

A study on the efficiency of steel vs. synthetic vs. hybrid fibers on fracture behavior of concrete in flexure using acoustic emission

Synergistic effect of different fibers on the improvement of fracture behavior of concrete is a widely accepted phenomenon. However, the fracture mechanisms behind their improvement need to be clearly understood for optimizing the fiber dosage. In this work, the fracture behavior of synthetic fiber-reinforced concrete (SynFRC), steel fiber reinforced concrete (SFRC) and hybrid fiber reinforced concrete (HFRC) under flexural loading is studied using acoustic emission (AE) technique. Three different fiber dosages, by volume of concrete, of 0.50%, 0.75% and 1.0% and their hybrid combinations were considered as study parameters. Test results show that it is possible to retain sufficient workability in the fresh state and obtain good fracture resistance through a hybrid combination of hooked end steel fibers and macro- synthetic polyolefin fibers. AE parameters were investigated to acknowledge the efficiency of the fibers in improving the fracture behavior and toughness of SynFRC, SFRC, and HFRC. AE parameters such as hits, events, AE energy, and 3D-crack source locations were presented to illustrate the role of different fiber dosages on the Mode-I fracture response. The source locations of AE event above the artificial notches created at the mid-span of specimens were investigated and classified into tensile or shear cracks. AE and fracture energy increased with fiber dosage. SFRC demonstrated higher energy dissipation capacity followed by HFRC and SynFRC.

Maximum available tensile strength of carbon fibers

Development of carbon fibers from alternative precursory materials through new production processes is a recent topic of active research. In such a research, the maximum available tensile strength, i. e. the tensile strength which will be achieved after elaboration to suppress defect formation during production process, is the matter of great concern. We have developed a method for determining this strength through the tensile test on a single fiber after introducing an artificial notch. In the present paper, this method has been refined. By using the refined method, the distribution of the maximum available tensile strength at various radial positions has been measured for a polyacrylonitrile-based carbon fiber. The difference between the maximum available tensile strength and the strength predicted using other methods such as those based on the fracture toughness and the fiber-length dependence of the tensile strength has also been discussed.

Oxidation degradation and mechanical reduction on C/SiC composites with artificial notch defects

Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiCs) have been widely used in aeronautic and astronautic fields because of their more attractive high temperature properties with less structural weight. However, reinforcing carbon fibers are susceptible to oxidation especially when the notch defects (ND) expose them to air. Mechanical tests, microstructural characterization combined with computed tomography (CT) were performed to explore the effect of the ND on the oxidation behavior and residual properties of the C/SiCs. Results showed that, before oxidation, the remaining bending strengths of the C/SiCs with even 5 ND numbers maintained still above 93%, indicating that increase of the ND numbers had little effect on mechanical properties. However, after oxidation at 700 °C, weight loss ratio of the C/SiCs with the ND numbers of 0, 1, 3 and 5 increased from 1.01% to 3.73%. It suggested that the more the ND numbers, the greater the proportion of carbon fiber exposed to air, and the less the oxidation resistance. Meanwhile, the residual bending strength remaining ratio of the C/SiCs largely reduced from 83.7% to 60.7% with the increase of ND numbers. It pointed out that the ND induced oxidation degradation of the reinforcing fiber caused higher sensitivity to the mechanical strength of the C/SiCs, and with the increase in the ND numbers, the strengths decreased more obviously.

In Situ Scanning Electron Microscope (SEM) Observations of Damage and Crack Growth of Shale.

To better understand the formation and evolution of hierarchical crack networks in shales, observations of microscopic damage, and crack growth were conducted using an in situ tensile apparatus inside a scanning electron microscope. An arched specimen with an artificial notch incised into the curved edge was shown to afford effective observation of the damage and crack growth process that occurs during the brittle fracturing of shale. Because this arched specimen design can induce a squeezing effect, reducing the tensile stress concentration at the crack tip, and preventing the brittle shale from unstable fracturing to some extent. Both induced and natural pores and cracks were observed at different scales around the main crack path or on fractured surfaces. Observations indicate that the crack initiation zone develops around the crack tip where tensile stresses are concentrated and micro/nanoscale cracks nucleate. Crack advancement generally occurs by the continuous generation and coalescence of damage zones having intermittent en echelon microscopic cracks located ahead of the crack tips. Mineral anisotropy and pressure build-up around crack tips causes crack kinking, deflection, and branching. Crack growth is often accompanied by the cessation or closure of former branch cracks due to elastic recovery and induced compressive stress. The branching and interactions of cracks form a three-dimensional hierarchical network that includes induced branch cracks having similar paths, as well as natural structures such as nanopores, bedding planes, and microscopic cracks.

Analytic formula for depinning current of magnetic domain walls driven by spin–orbit torques from artificial notches

Here, we present an analytic formula for the domain-wall depinning current from artificial triangular notches driven by the spin–orbit torque combined with the Dzyaloshinskii–Moriya interaction. Interestingly, in contrast to the magnetic-field-driven depinning, the depinning current is governed solely by the notch slope angle, irrespective of the notch depth and wire width. An analytic formula is proposed to explain the present observation on the basis of the variational principle for minimum energy states. The validity of the formula is verified via micromagnetic simulation, confirming the detailed effects of the spin–orbit torque and Dzyaloshinskii–Moriya interaction strengths.

Dispersion of graphene nanoplatelets reinforcing type II cement paste

One of the most recent, affordable and at the same time cutting-edge class of carbon nanostructures are the graphene nanoplatelets (GnPs). They depict multifunctional capabilities such as improved mechanical properties in conjunction with high electrical and thermal conductivity making them suitable for the development of multifunctional materials. One of the most essential parameters, to successfully exploit the GnPs characteristics, is their homogeneous distribution inside the matrix. In this direction, the applicability of a method, similar to the one applied for the preparation of carbon nanofiber and carbon nanotube reinforced Type I cement nanocomposites, is here investigated. Ultrasonic processing and treatment with a 3rd generation superplasticizer was used to uniformly disperse the GnPs (of lateral size equal to 8 μm) in the mixing water. This admixture was exploited as a GnPs dispersing agent, as it is commonly used, to improve the workability of cementitious materials and is fully compatible with the matrix. The nanocomposites were prepared using Type II cement. The electrical resistivity of the nanocomposites developed was evaluated. Three-point bending tests were performed at prismatic beam specimens with an artificial notch at the age of 28 days. Both the effects of dispersing agent (superplasticizer) concentration and ultrasonication energy application were investigated. It was concluded that both the admixture concentration and ultrasonic energy application strongly affect the GnPs dispersion and reinforcing efficiency.

Experimental and numerical investigation of strain distribution of notched lead fatigue test specimen

The work here presented focuses on the test methodology related to effect of stress concentrators in strain controlled structures. Cable sheathing as used in subsea power cables are investigated by cyclic fatigue testing, Digital Image Correlation (DIC) and 3D Finite Element Analysis. Focus is put on the strain distribution in conventional specimen geometries and under the presence of artificial notches. It is evident that standardized fatigue testing provides limited input to the final fatigue life of strain controlled power cable sheathing both accounting for expected and unintended stress concentrators. The limitations can be explained by measured strain distribution inherent in most fatigue test specimens. The use of DIC and 3D FEM provides valuable insight into both the theoretical and practical stress and strain distribution. This can help in understanding and overcoming geometrical test constraints, when compared to the actual component loading mode.

A Christmas-Tree-Like Magnetic Field Sensor Based on Domain Wall Depinning in a Notched Nanowire

We propose a magnetic sensor with relatively high resolution that is based on domain wall (DW) pinning and depinning. In a nanowire patterned into a Christmas-tree-like geometry, asymmetric artificial notches serve as “bayonet locks”, which can pin a DW in one direction but allow it to pass in the other direction. The depinning field is linearly dependent on the inverse of the size of bayonet locks, and this dependence can be used to measure an external magnetic field. This concept is demonstrated with micromagnetic simulation, and the accuracy is experimentally estimated. At distances of nanometers, this type of sensor can be used to measure the magnetic fields of objects in biomedicine, such as cells labeled with immune-magnetic-nanoparticles.

Impact of defect extent on swept frequency eddy current responses in non-destructive evaluation

Novel approach using swept frequency eddy current technique is employed in non-destructive inspection of conductive materials. Possibility of defect extent evaluation from response signals is experimentally studied. Two plate specimens made of austenitic steel SUS 316L containing artificial electro-discharge machined notches are inspected. The notches vary in dimensions, and they are inspected by two eddy current probes. In particular, a length and a depth dimensions of the notches are concerned here. A wide frequency range is used for the experiments, while the probes containing a transmitting and a receiving coils are located at fixed positions over the plates’ surface during entire inspection. The voltage transfer frequency characteristics of the probes are recorded as the response signals. The frequency range is adjusted in such a way that the electrical resonance is clearly visible from the acquired characteristics. Influences of the defects’ length and depth on the response signals are evaluated in order to investigate possibilities of cracks’ evaluation. The presented results clearly show that the resonance frequency depends on a defect extent.

Static and Fatigue Behavior Investigation of Artificial Notched Steel Reinforcement.

Pitting corrosion is one of the most common forms of localized corrosion. Corrosion pit results in a stress concentration and fatigue cracks usually initiate and propagate from these corrosion pits. Aging structures may fracture when the fatigue crack reaches a critical size. This paper experimentally simulates the effects of pitting morphologies on the static and fatigue behavior of steel bars. Four artificial notch shapes are considered: radial ellipse, axial ellipse, triangle and length-variable triangle. Each shape notch includes six sizes to simulate a variety of pitting corrosion morphologies. The stress-strain curves of steel bars with different notch shape and depth are obtained based on static tensile testing, and the stress concentration coefficients for various conditions are determined. It was determined that the triangular notch has the highest stress concentration coefficient, followed by length-variable triangle, radial ellipse and axial ellipse shaped notches. Subsequently, the effects of notch depth and notch aspect ratios on the fatigue life under three stress levels are investigated by fatigue testing, and the equations for stress range-fatigue life-notch depth are obtained. Several conclusions are drawn based on the proposed study. The established relationships provide an experimental reference for evaluating the fatigue life of concrete bridges.

The effect of stress concentration on the bending behavior of a ZrCuNiAl bulk metallic glass

The bending behaviors of a Zr55Cu30Ni5Al10 bulk metallic glass (BMG) samples with different notch radii are studied. The artificial notch will introduce stress concentration within the BMG sample during bending. That means, smaller notch radius leads to greater stress concentration around the notch area, and thus results in earlier failure and smoother fracture surface. Then, the sample with smaller notch radius possesses smaller fracture toughness. The relationship between the stress concentration and deformation feature has been discussed in terms of grease model.

Behaviors of axially loaded square concrete-filled steel tube (CFST) Stub columns with notch in steel tube

Compressive tests of the CFST stub columns with artificial notches in the steel tubes were conducted in this paper to investigate the effects of the material imperfections of steel tubes on column performances. The load-strain responses, column strength and confining effects were discussed in detail. A parametric study, including the notch length, notch orientation, notch location and ductility, was also conducted. The results indicate that the notched CFST specimens have quite different failure modes from the intact CFST specimens. A notched CFST specimen may fail by bulging or closing of the notch, which was determined by the notch orientation. The results also show that the notched CFST specimens had lower mechanical performances than the intact CFST specimens because the notch steel tube could not offer sufficient confining effect on the concrete core inside it. Based on the experimental results, a practical calculation formula for the bearing capacity of notched CFST columns was proposed, which gave well agreement with the experimental results.

Fatigue behaviour of CFRP strengthened open-hole steel plates

Open-hole details have gained widely application in bolted connections subjected to fatigue loading. Although the use of carbon fibre reinforced polymer (CFRP) can be efficient in strengthening critically stressed tension area, its benefit in improving the fatigue life of open-hole details has not been extensively evaluated. This paper presents the results of an experimental and finite element analytical study on the fatigue behaviour of CFRP strengthened open-hole steel plates. The merits resulted from CFRP strengthening are investigated and compared with some others from referred open-hole fabrication methods and these with artificial initial cracks or notches as reported in the literature. Typical failure modes, local strain evolution and stiffness degradation behaviour of the test open-hole details are reported. The fatigue life results are evaluated and compared against the design curves of the AASHTO. The results show that the use of single and triple layered CFRP strengthening results in fatigue life enhancement of nearly 20% and 60% respectively in contrast to bare steel open-hole plates. Incorporating the stress ratios allowing for fibrous rupture and steel-adhesive interface failure, the fatigue life of CFRP strengthened specimens can be properly evaluated. Finally, the geometric and configuration parameters of CFRP on the stress concentration of the open-hole details are studied based on finite element modelling. It is demonstrated that the studied stress concentration can be reduced with the increase of the modulus of carbon fibre and the number of CFRP layers. The contribution of the effective width of CFRP to the decrease of stress concentration is also discussed.

Effects of surface roughness on magnetic flux leakage testing of micro-cracks

Magnetic flux leakage (MFL) testing owns the advantages of high inspection sensitivity and stability, but its testing results are always affected by surface roughness. The relationship between the surface roughness () and detection signals for surface-breaking cracks is mainly discussed. The existence of roughness magnetic compression effect (RMCE) in present MFL testing is specially pointed out and its relevant theory is also analyzed, which manifest themselves in the compression of MFL signal in its peak value and the baseline drifts mixed with noise. An experimental investigation on surface comparators with different arithmetic average height () and artificial notch size, is performed to analyze the effects of surface roughness on detection signals of cracks. The detection limit (DL) of micro-crack is analyzed by comparing the noise-signal ratio () and peak-peak signals of the cracks. Meanwhile, increases with the and , in this case, relatively shallow defects cannot be clearly distinguished at determined rough surface. Afterwards, a series of simulations are designed and performed to verify the effects of surface roughness on characteristic of the electromagnetic field, and a theoretical DL of micro-crack is presented as: . Furthermore, the optimal lift-off value is selected for the micro-cracks’ detection to weaken the negative magnetic compression effect. MFL signals cannot reflect the accurate sizes of the cracks on rough surface due to the RMCE and its relevant phenomenon. The discovery and results will benefit the quantitative evaluation of the MFL testing.

Helical comb magnetostrictive patch transducers for inspecting spiral welded pipes using flexural guided waves

A wavefront analysis indicates that a flexural wave propagates at a helix angle with respect to the pipe axis. The expression for calculation of the helix angle for each flexural mode is given, and the helix angle dispersion curves for flexural modes are calculated. According to the new understanding of flexural guided waves, a helical comb magnetostrictive patch transducer (HCMPT) is proposed for selectively exciting a single predominant flexural torsional guided wave in a pipe and inspecting spiral welded pipes using flexural waves. A HCMPT contains a pre-magnetized magnetostrictive patch that is helically coupled with the outer surface of a pipe, and a novel compound comb coil that is wrapped around the helical magnetostrictive patch. The proposed wideband HCMPT possesses the direction control ability. A verification experiment indicates that flexural torsional mode T(3,1) at center frequency f=64kHz is effectively actuated by a HCMPT with 13-degree helix angle. Flexural torsional modes T(N,1) with circumferential order N equals 1–5 are selected to inspect a seamless steel pipe, artificial defects are effectively detected by the proposed HCMPT. A 20-degree HCMPT is adopted to inspect a spiral welded pipe, an artificial notch with cross section loss CSL=2.7% is effectively detected by using flexural waves.

Correlation of Plastic Strain Energy and Acoustic Emission Energy in Reinforced Concrete Structures

This paper presents a comparison of the acoustic emission (AE) energy and the plastic strain energy released by some reinforced concrete (RC) specimens subjected to cyclic or seismic loadings. AE energy is calculated, after proper filtering procedures, using the signals recorded by several AE low frequency sensors (25–100 kHz) attached on the specimens. Plastic strain energy is obtained by integrating the load displacement curves drawn from the measurements recorded during the test. Presented are the results obtained for: (i) two beams (with and without an artificial notch) and a beam-column connection subjected to several cycles of imposed flexural deformations; (ii) a reinforced concrete slab supported by four steel columns, and a reinforced concrete frame structure, both of the latter are subjected to seismic simulations with a uniaxial shaking table. The main contribution of this paper, which is a review of some papers previously published by the authors, is to highlight that, in all cases, a very good correlation is found between AE energy and plastic strain energy, until the onset of yielding in the reinforcing steel. After yielding, the AE energy is consistently lower than the plastic strain energy. The reason is that the plastic strain energy is the sum of the contribution of concrete and steel, while the AE energy acquired with thresholds higher than 35 dBAE captures only the contribution of the concrete cracking, not the steel plastic deformation. This good correlation between the two energies before the yielding point also lends credibility to the use of AE energy as a parameter for concrete damage evaluation in the context of structural health monitoring.

Effect of artificial aging on the mechanical performance of (Al-Cu) 2024 and (Al-Cu-Li) 2198 aluminum alloys

Al-Cu-Li-Mg based alloys exhibit an excellent combination of low density, high elastic modulus and high specific strength and have already being used as structural materials for aerospace applications. The mechanical properties of these alloys are often associated with the addition of Li which enables the formation of several strengthening precipitates including δ΄ (Al3Li), θ΄ (Al2Cu), δ (AlLi) and T1 (Al2CuLi). Other precipitates have also been reported in these alloys that include GP zones, θ (Al2Cu), Ω (Al2Cu), S΄ (Al2CuMg), and β΄ (Al3Zr), e.g. [1-3]. Nevertheless, the quasi-static mechanical properties of AA2198 are scarcely reported in the open literature while quite few for the fracture toughness. For example, Chen et al. [4] performed tests on two different heat treated AA2198 (namely T351 and T851) and investigated their plastic and fracture behavior. Steglich et al. [5, 6] investigated experimentally and analytically the anisotropic deformation of AA2198-T8 occurred during mechanical loading with and without the presence of artificial notches. In a recent publication [7], a combination of transmission electron microscopy, atom probe tomography and high-energy X-ray diffraction was employed to investigate the influence of local microstructural changes on strengthening in AA2198 in different aging conditions.As AA2198 is supposed to replace AA2024 in aerostructures designed with the damage tolerance philosophy, the authors in the present work report and compare their tensile mechanical and fracture toughness behavior under different aging conditions to simulate the natural aging parameter. To this end, a comparison of both alloys of their tensile mechanical behavior is reported for different stages of aging, including conditions of under-aging (UA), peak-aging (PA) and over-aging (OA). Typical results of yield stress as well as elongation at fracture for the case of AA2024 can be seen in the diagrams of Figure 1. The effect of artificial aging on the fracture toughness is also assessed for both alloys. Although the peak-aged temper of AA2198 is of application interest, a detailed investigation of the structure-property relationships of other temper states is attempted since it provides the basis for understanding the influence of process-induced microstructural changes on the post-processing properties of the alloy. To this end, structural characterization as well as relationship between yield stress, elongation at fracture and fracture toughness is reported and discussed in the manuscript for all investigated aging conditions.

Fatigue and fracture behavior of laser clad repair of AerMet® 100 ultra-high strength steel

The effect of laser cladding on the fatigue and fracture behavior under variable amplitude loading is a major consideration for the development of laser cladding process to repair high value complex fatigue critical aerospace military components, that otherwise would be replaced. The selected material, AerMet®100, is a widely used ultra-high strength steel in current and next generation aerospace components, such as landing gears. Laser cladding was performed using AerMet® 100 powder on AerMet® 100 fatigue substrate specimens. No micro-cracking and very little porosity were observed in the clad layer. The fatigue tests were performed under variable amplitude loading with a maximum stress of 1000MPa. Residual stress, microstructure, and hardness, was also evaluated. Both the as-clad and post-heat treated (PHT) samples were compared to a baseline sample with an artificial notch to simulate damaged condition. Results show that laser cladding significantly improves fatigue life, as compared to the baseline sample with a notch. However, the fatigue life of the as-clad sample is lower as compared to a baseline sample without a notch. A compressive residual stress of 300–500MPa was observed in the clad region and HAZ. The fracture modes in the as-clad specimen consisted mainly of tearing topology surface and some regions of decohesive rupture through the columnar austenite grains. The PHT condition however was not effective in improving the fatigue life. The fracture modes showed mainly decohesive rupture, and as a consequence, reduced the fatigue life.