Sharp Defects Research Articles

Assessment of qualitative and quantitative S0 guided wave tomography of sharp thickness loss defects in the isotropic membrane regime.

Progress in instrumentation, computer hardware, and inversion methods is encouraging the development of more advanced guided wave tomography techniques, especially for nondestructive testing of plate structures to characterize corrosion. An experimental S0 tomography performance assessment in the membrane regime is reported. One of the main interests of the fundamental membrane regime is that in this regime, waves are propagated over long distances. A 2 mm thick steel disk containing calibrated sharp artificial defects (flat bottom holes) is tested in both reflection and extinction modes. A reconstruction algorithm derived from the membrane approximation is presented. We expose a complete reflection mode inversion approach that includes beam inversion, waveform deconvolution, and thickness loss calibration. Non-linear correction factors are introduced and discussed for quantitative imaging. A width-regularity-depth description of defects is introduced to put the results into perspective with other defect geometries. The results show the relevance of the inversion method to enhance the imaging performance with regard to defect localization and sizing. Crucial points concerning instrumentation such as coupling, signal-to-noise ratio, excitation mode, coupling, selection of frequency, are also discussed.

Ultrasonic C-scan techniques for the evaluation of impact damage in CFRP

Abstract In the present work, different ultrasonic C-scan approaches were used to evaluate Carbon Fiber Reinforced Polymers (CFRP) submitted to impacts of low energy, in order to evaluate their effectiveness for the detection and characterization of small defects. In particular, as to the question how useful could be the air-coupled C-scan approach, using low frequencies, for in-service application. For that goal, several samples with different stacking sequences and thicknesses were impacted with 1.5 and 3 J. Then, ultrasonic C-scan images were produced by immersion pulse-echo (in amplitude and time-of-flight (TOF)) and immersion through-transmission, and also by air-coupling through-transmission. The immersion C-scan images were produced using 5, 10 and 20 MHz probes and the air-coupled C-scan was made using two 400 kHz probes. The obtained images for the considered samples show that all used methods are able to detect the defects and give acceptable information about their size and shape. However, if the way of delamination evolving over thickness is of interest, the images by TOF should be used. As expected, good image resolution with sharp contour defects require high frequencies. Nevertheless, the air-coupled C-scan demonstrated similar capabilities to detect defects, with the advantage that the coupling medium is air, thus widening the range of applications, such as real-time damage monitoring of composite structures. As a disadvantage, the air C-scan system requires high power emission signals, and also great amplification of the received signals, to face the considerable attenuation in the air.

Photonic topological Weyl degeneracies and ideal type-I Weyl points in the gyromagnetic metamaterials

As a new topological phase in three-dimensional momentum space, Weyl semimetals extend the repertoire of exotic topological states. In this paper, we find the existence of photonic Weyl points in electromagnetic continuous gyromagnetic metamaterials. The dispersion of the longitudinal mode of one of the two bands forming the Weyl point is flat; thus, the Weyl degeneracy point of the system is exactly at the critical transition between the type-I and type-II Weyl points. The Fermi arc connects the projections of the Weyl points with opposite chirality at the interface between the gyromagnetic metamaterial and vacuum. Full-wave simulation results show that surface waves can bypass sharp defects and achieve robust, unidirectional optical transmission without reflection. Furthermore, taking the spatial nonlocal effects into account, the ideal type-I Weyl points can exist in this class of metamaterials. This paper sheds light on fundamental understanding of topological physics, and the study of Weyl points in metamaterials will greatly enrich the field of topological photonics.

Structural, compositional, and functional effects of blunt and sharp cartilage damage on the joint: A 9-month equine groove model study.

This study aimed to quantify the long‐term progression of blunt and sharp cartilage defects and their effect on joint homeostasis and function of the equine carpus. In nine adult Shetland ponies, the cartilage in the radiocarpal and middle carpal joint of one front limb was grooved (blunt or sharp randomized). The ponies were subjected to an 8‐week exercise protocol and euthanized at 39 weeks. Structural and compositional alterations in joint tissues were evaluated in vivo using serial radiographs, synovial biopsies, and synovial fluid samples. Joint function was monitored by quantitative gait analysis. Macroscopic, microscopic, and biomechanical evaluation of the cartilage and assessment of subchondral bone parameters were performed ex vivo. Grooved cartilage showed higher OARSI microscopy scores than the contra‐lateral sham‐operated controls (p < 0.0001). Blunt‐grooved cartilage scored higher than sharp‐grooved cartilage (p = 0.007) and fixed charge density around these grooves was lower (p = 0.006). Equilibrium and instantaneous moduli trended lower in grooved cartilage than their controls (significant for radiocarpal joints). Changes in other tissues included a threefold to sevenfold change in interleukin‐6 expression in synovium from grooved joints at week 23 (p = 0.042) and an increased CPII/C2C ratio in synovial fluid extracted from blunt‐grooved joints at week 35 (p = 0.010). Gait analysis outcome revealed mild, gradually increasing lameness. In conclusion, blunt and, to a lesser extent, sharp grooves in combination with a period of moderate exercise, lead to mild degeneration in equine carpal cartilage over a 9‐month period, but the effect on overall joint health remains limited.

Reconfigurable topologically protected wave propagation in metastable structure

In this paper, topologically protected wave propagation in metastable structure has been investigated by mimicking the quantum valley Hall effect. Reconfigurable topological waveguides are achieved by switching the metastable states of the structure. First, a K-point Dirac cone, which is guaranteed by the C6 rotational symmetry of the metastable unit cell, is identified in the band structure. Then, by reconfiguring the bistable elements, the space inversion symmetry of the unit cell is broken and a topological bandgap opened. The topological non-triviality of this bandgap is verified by the values assumed by the Berry curvature and valley Chern number. Next, the topological edge modes localized at the interface are studied by analyzing the dispersion relation of a finite strip. The analysis results show that reconfigurable topologically protected waveguides, which are immune to backscattering at sharp corners and local defects, can be achieved utilizing the edge modes.

Topologically protected Mach–Zehnder interferometer

As photonic integration continues to evolve, compact and low loss Mach–Zehnder interferometers (MZI) have been an area of significant research in recent years. Here, we propose a valley-Hall effect based topologically protected Mach–Zehnder interferometer (TPMZI). Our simulations show that the TPMZI is efficient and stable even in complex situations. Sharp waveguide bending and defects such as obstructions and epitaxial disorders, do not affect the propagation of photons along the interface, which guarantees the TPMZI high output power. In addition, the TPMZI yields excellent sensitivity, extinction ratio and modulation depths, making it advantageous over existing systems when applied to optical communication, biochemical sensing, spectrometer analysis and measurement systems, etc.

Structure design and compression experiment of the supporting node for JUNO PMMA detector

Jiangmen Underground Neutrino Observation is going to build a huge polymethyl methacrylate (PMMA) detector to hold 20,000 tons of liquid scintillator to capture neutrinos in a 700-m underground cave. This PMMA detector has a spherical shape with an inner diameter of 35.4 m and is supported by an outside stainless steel structure through 590 supporting nodes. The maximum compression force applied to these supporting nodes would be about 150 kN when the detector is running. This paper focuses on the design and validation of the PMMA supporting node and compares the effects of two gaskets (round gasket and ring gasket) on the supporting node stress and ultimate compression load. An innovative PMMA supporting node structure is first proposed, and a 1/4 symmetric model with the material nonlinearity and frictional contact boundary is established in the finite element analysis (FEA). The FEA results show that the principal stress of the structure is less than 3.5 MPa and the Mises stress is less than 5.5 MPa. The stress and deformation at the groove of the supporting node using the ring gasket are smaller than that using the round gasket. The compression experiments of the supporting node using two types of gaskets were conducted to study the effect of gaskets on the ultimate compression load of supporting node. The ultimate compression load of the supporting node with ring gasket is larger than 900 kN, which is six times of design load. In a comparison of experimental results with FEA, the maximum difference is 15.78%, demonstrating the validity of FEA results. Through the material test of PMMA and experiment of the PMMA supporting node, it is known that PMMA is a brittle material and it is very sensitive to sharp corner defects that should be avoided in the design of PMMA structure.

Excitation of Single-Mode Shear-Horizontal Guided Waves and Evaluation of Their Sensitivity to Very Shallow Crack-Like Defects

Inspection is a key part of the asset management process of industrial plants and there are numerous plate-like structures that require inspection. Ultrasonic guided waves have been extensively used to detect various types of defect by monitoring reflected and transmitted signals because they enable faster screening of large areas. However, ultrasonic guided wave testing becomes difficult for very shallow, sharp defects as current inspection techniques suffer from a lack of sensitivity to such features. Previous studies, obtained by comparing various inspection techniques, suggest that the SH1 mode in particular, at around 3 MHz · mm, would be suitable when testing for shallow defects; however, it is clear that both the SH0 and SH1 modes can exist at this frequency-thickness product. This can complicate the inspection process and, therefore, limit defect detectability. This article investigates the possibility of a single-mode excitation of the SH1 mode at around 3 MHz · mm. The ability of this method toward detecting very shallow defects (<10% cross-sectional thickness loss) has also been studied. By means of analytical predictions and finite element, it is shown that a signal dominated by the SH1 mode can be generated using a single permanent periodic magnet (PPM) electromagnetic acoustic transducer (EMAT) (PPM EMAT). All predictions are then backed up by experimental measurements. It is also shown that, by studying the reflection coefficient of the SH1 mode, the pure SH1 mode can be used to detect defects as shallow as 5% thickness loss from a 500-mm stand-off. These defects would otherwise be missed by standard, lower frequency guided wave testing.

Reconfigurable Floquet elastodynamic topological insulator based on synthetic angular momentum bias.

Originating with the discovery of the quantum Hall effect (QHE) in condensed matter physics, topological order has been receiving increased attention also for classical wave phenomena. Topological protection enables efficient and robust signal transport; mechanical topological insulators (TIs), in particular, are easy to fabricate and exhibit interfacial wave transport with minimal dissipation, even in the presence of sharp edges, defects, or disorder. Here, we report the experimental demonstration of a phononic crystal Floquet TI (FTI). Hexagonal arrays of circular piezoelectric disks bonded to a PLA substrate, shunted through negative electrical capacitance, and manipulated by external integrated circuits, provide the required spatiotemporal modulation scheme to break time-reversal symmetry and impart a synthetic angular momentum bias that can induce strong topological protection on the lattice edges. Our proposed reconfigurable FTI may find applications for robust acoustic emitters and mechanical logic circuits, with distinct advantages over electronic equivalents in harsh operating conditions.

Experimental realization of a reconfigurable electroacoustic topological insulator

A substantial challenge in guiding elastic waves is the presence of reflection and scattering at sharp edges, defects, and disorder. Recently, mechanical topological insulators have sought to overcome this challenge by supporting back-scattering resistant wave transmission. In this paper, we propose and experimentally demonstrate a reconfigurable electroacoustic topological insulator exhibiting an analog to the quantum valley Hall effect (QVHE). Using programmable switches, this phononic structure allows for rapid reconfiguration of domain walls and thus the ability to control back-scattering resistant wave propagation along dynamic interfaces for phonons lying in static and finite-frequency regimes. Accordingly, a graphene-like polyactic acid (PLA) layer serves as the host medium, equipped with periodically arranged and bonded piezoelectric (PZT) patches, resulting in two Dirac cones at the K points. The PZT patches are then connected to negative capacitance external circuits to break inversion symmetry and create nontrivial topologically protected bandgaps. As such, topologically protected interface waves are demonstrated numerically and validated experimentally for different predefined trajectories over a broad frequency range.

Microwave applications of photonic topological insulators

This Perspective examines the emerging applications of photonic topological insulators (PTIs) in the microwave domain. The introduction of topological protection of light has revolutionized the traditional perspective of wave propagation through the demonstration of backscatter-free waveguides in the presence of sharp bending and strong structural defects. The pseudospin degree of freedom of light enables the invention of unprecedented topological photonic devices with useful functionalities. Our aim is to present a brief introduction of recent developments in microwave PTI demonstrations. We give a clear comparison of different PTI realizations, summarize the key features giving rise to topological protection, and present a discussion of the advantages and disadvantages of PTI technology compared to existing microwave device technology. We conclude with forward-looking perspectives of how the advantages of this technology can best be exploited.

Fracture mechanisms in largely strained solids due to surface instabilities

The effect of self-contacting surface defects generated by largely compressing metals on the ductile-to-brittle transition observed in metallic structures is investigated. In order to analyse such an effect, a finite element model of a half-space plane-strain material block with an imperfection was subjected to different levels of compression followed by reverse tensile straining. Experimentally validated associative J2 and porous plasticity models were used to describe the mechanical response of the pipeline steel employed as a baseline material for this investigation. Both models predicted onset of creasing at compressive strains of around 70%. To ascertain whether the creases created large and sharp enough defects to trigger the ductile-to-brittle transition during the tensile straining phase, a bifurcation analysis implemented within a user material subroutine was used as fracture initiation indicator. This confirmed that at compressive strains above 70% the self-contact defect acted as a crack during the tensile straining phase.

Fabrication of Edge Rounded Polylactic Acid Biomedical Stents by the Multi-Axis Micro-Milling Process

This paper presents the first try to fabricate degradable polylactic acid (PLA) biomedical stents with round edges by the multi-axis micro-milling process. Conventionally biomedical stents are produced by laser processing. Post-processing operations are usually required to handle sharp edges and thermal defects of the stent due to laser processing. A computer graphics software package was used to design the strut structures with round corners of the PLA stent. A PLA tube was first created using injection molding, and a degradable biomedical stent was then fabricated through micro-milling by using a five-axis computer numerical control (CNC) machine tool. This study investigated the error in the rotation center that can occur during five-axis micro-milling. Data obtained from experiments on center-of-rotation errors were substituted into homogeneous coordinate conversion formulas. Center-of-rotation errors in the five-axis machine tool were compensated for improving the milling precision (A and C axes) to be within 5 μ m. Furthermore, milling parameter optimization experiments were conducted, which determined the optimal conditions for milling PLA to be a spindle speed of 60,000 rpm, feed per tooth of 0.005 mm, and feed rate of 600 mm/min, and achieved the minimum burr 0.01 mm and the average surface roughness (Ra) 0.4 μ m. These optimal cutting parameters will be used in the following actual stent processing experiments. Finally, the error compensation and optimal parameters were combined in a CAM software package and layered spiral micromilling to machine the actual stent. The experimental results revealed that the combination of five-axis micro-milling led to the successful fabrication of a degradable biomedical stent (stent diameter = 6 mm, strut width = 0.3 mm, and radius at round corners = 0.1 mm). The machined actual stent had cross-section height and width errors within 0.01 mm, and arc depth of cut variation within 6 μ m. In addition, the PLA stent machining results indicated a rebound of approximately 33% at the strut round edge machining. This work may also open up future possibilities for complex three-dimensionally structured biomedical stents for better performance and special functionality.

Effect of small defects on the fatigue strength of martensitic stainless steels

The defect tolerance of three different martensitic stainless steels (17-4PH, X20Cr13 and AISI403) under cyclic loading was investigated. Fatigue tests were performed with specimens containing diverse artificial defects (corrosion pits, single and multiple drilled holes, sharp circumferential notches and pre-cracked holes). The measured cyclic strengths are compared to predictions based on the area parameter model proposed by Murakami and Endo. This model is well applicable to small sharp defects if the threshold condition for crack propagation determines the fatigue limit. The transition size from small to large defects, areatrans, – above which the threshold stress intensity factor range, ΔKth, becomes a constant value – is between 80 and 166 µm for the investigated steels. For larger defects, the threshold for long cracks, ΔKth,lc, serves well to predict the fatigue strength under fully reversed loading, i.e. at a load ratio of R = −1. Comprehensive tests were performed to study the influence of mean stress, and threshold conditions were determined that enable to predict the fatigue limit at different load ratios.A simple equation is introduced to estimate the critical defect size, areacritical, which is the size above which defects become detrimental under cyclic loading. It has been found that areacritical increases with increasing load ratio while areatrans is independent of R. Furthermore, higher values of the threshold stress intensity factor range were determined for large defects (with sizes larger than areatrans) compared to long cracks if the load ratio is higher than R = −1. This could be explained by the plane stress condition at the material surface which is prevalent for surface defects, while the condition for long cracks can be assumed as plane strain.It is further demonstrated that the effect of small defects on the fatigue strength of stainless steels is highly sensitive to the notch root radius, ρ. Small holes with diameters of 100 µm (ρ = 50 µm) or larger are less detrimental for the fatigue strength than defects with high stress concentrations – such as corrosion pits.

Prediction of fatigue limit in additively manufactured Ti-6Al-4V alloy at elevated temperature

Rotating bending fatigue tests were conducted at room temperature (RT) and at elevated temperature (ET) of 250 °C using a Ti-6Al-4V alloy fabricated by an additive manufacturing (AM) process, and the effect of defects on fatigue strength was investigated. Particularly, the applicability of Murakami’s model for predicting the fatigue limit at ET was examined. Microstructure observations revealed that the AM Ti-6Al-4V alloy had acicular α + β microstructures while the conventionally manufactured (CM) Ti-6Al-4V alloy had equiaxed α phases in β matrix. Round-shaped defects and crevice-liked sharp defects with the size of up to 50 μm were observed in the AM Ti-6Al-4V, which were not observed in the CM Ti-6Al-4V. The fatigue strengths at 107 cycles of the CM Ti-6Al-4V were 625 MPa at RT and 475 MPa at ET, and those of the AM Ti-6Al-4V were 300 MPa at RT and 250 MPa at ET. Scanning electron microscopy (SEM) on the fracture surfaces revealed that fatigue cracks in the AM Ti-6Al-4V specimens initiated from defects at ET as well as they did at RT. The fatigue strengths of the AM Ti-6Al-4V at RT and at ET were evaluated by Murakami’s model and the predicted value agreed well with the experimental value.

Mechanical Quantum Hall Effect in Time-Modulated Elastic Materials

Floquet topological insulators have inspired analogs in photonics, optics, and acoustics, in which nonreciprocal wave propagation in time-modulated materials is achieved by breaking time-reversal symmetry. This paper investigates a mechanical-wave analog of Thouless pumping and the quantum Hall effect respectively in one- and two-dimensional periodically time-modulated materials. These systems feature robust edge states that are immune to back-scattering by sharp corners, defects, and nonuniform phases. The work sheds light on the design and fabrication of quantum-Hall-type elastic topological insulators, to control elastic waves for broadband one-way transport in practical applications.

Influence of the Mechanical and Corrosion Defects on the Strength of Thermally Hardened Reinforcement of 35GS Steel

We study the influence of the shape and sizes of defects on the strength of thermally hardened steel reinforcement and construct the stress-strain diagrams for reinforcements with different types of defects. For the first time, the method of binarization of images is used for the evaluation of the areas of fracture surfaces of samples with an aim to establish true critical stresses. It is shown that isolated sharp defects are more dangerous than a system of defects of this kind or long isolated corrosion defects. We also demonstrate the advantage of application of true stresses for the evaluation of strength of the reinforcement.

The choice of ultrasonic inspection method for the detection of corrosion at inaccessible locations

Inspection for corrosion and pitting defects in the petrochemical industry is vital and forms a significant fraction of the operating expenditure. Low frequency guided wave inspection is frequently employed as it gives large area coverage from a single transducer position. However, detection becomes problematic at inaccessible regions such as pipe supports or beyond T-joints since the low frequency guided waves produce a significant reflection from the feature itself, hence limiting the defect detectability of the method. This suggests testing at higher frequencies which helps to minimise the reflection from the feature and also improves the sensitivity to smaller defects. There are a number of guided wave and related techniques implemented for corrosion inspection including the S0 mode (at ∼ 1 MHz-mm), SH0 and SH1 modes (at ∼ 3 MHz-mm), CHIME, M-skip and Higher Order Mode Cluster (A1 mode at ∼ 18 MHz-mm). This paper presents a systematic analysis of the defect detection performance of each method with sharp and gradual defects, as well as their sensitivity to attenuative coatings, liquid loading, surface roughness and ability to test beyond features such as T-joints. It is shown by finite element analysis backed up by experiments that the A1 mode provides the best overall performance when dealing with surface features such as T-joints and coatings because of its low surface motion. Additionally a combination of two or more methods is suggested for corrosion inspection at inaccessible locations: The A1 mode in reflection for severe, sharp, pitting type defects; long range guided waves in reflection for large-area thinning and the SH1 mode in transmission for shallow, gradual defects.

Uniform concentrating design and mold machining of Fresnel lens for photovoltaic systems

Fresnel lens arrays are widely applied as the core optical component in concentrated photovoltaic (CPV) systems for concentrating light on a solar cell. To improve the photoelectric conversion efficiency of CPV systems, a Fresnel lens design is proposed, in which both uniform concentration and machining feasibility are pursued by means of simpler structure. In this design, the concentrating facets of the Fresnel lens are conical ones with different inclined angles, and the light passing through these conical facets can be superposed on the same position. The optical performance of the lens is analyzed to optimize its geometrical parameters. Towards this uniform concentrating design with a simple structure, an ultra-precision diamond cutting process based on B axis rotation is also developed to machine the Fresnel lens mold. In this process, the V-shaped diamond tool is rotated from right to left to machine the micro grooves of the lens mold with a maximum cutting depth of the groove height, so that the grooves can be prevented from being cut by the tool tip with low strength. Cutting tests of the factors influencing the machining accuracy and surface quality are carried out to optimize the cutting trajectory planning and machining parameters. The Fresnel lens molds with no sharp corner defects and no surface scratches are machined, and dimensional accuracy of 1.0 μm and surface roughness of better than 13 nm are achieved. Then, a silicone rubber lens is cast based on this mold. Concentrating uniformity test of the lens is conducted, and a concentrating uniformity of better than 75% is achieved. The results show the effectiveness of the uniform concentrating design and the feasibility of the B axis rotating machining process.

Wounded to the bone: Digital microscopic analysis of traumas in a medieval mass grave assemblage (Sandbjerget, Denmark, AD 1300–1350)

Battle-related mass burials are considered the most unequivocal evidence of past violence. However, most published studies involve only macroscopic analysis of skeletal remains, commonly arriving only at broad conclusions regarding trauma interpretation. The current study considers a possible avenue for achieving both greater detail and accuracy through digital microscopy.Patterns of injury were investigated among 45 individuals from a Medieval Danish mass grave (Sandbjerget, AD 1300–1350). Injuries were recorded on every anatomical element, except hand and foot bones. Each was photographed and cast, facilitating remote evaluations. Macroscopic analysis was compared with digital microscopy in order to test the relative utility of the latter in characterizing skeletal injuries (mechanism, weapon class, direction, timing of injury).The location of 201 observed injuries, mainly sharp force defects, suggested that many lesions were probably not inflicted by face-to-face opponents. Some microscopic features were indicative of a specific lesion type and weapon class. Digital microscopy was therefore demonstrated to be a complementary tool to macroscopic assessment, enhancing feature observation and quantification and serving to compensate for many of the limitations of macroscopic assessment.