Defect Sensitivity Research Articles

A high-sensitivity and long-distance structural health monitoring system based on bidirectional SH wave phased array

When estimating a structural health monitoring (SHM) system, its defect sensitivity and area/distance coverage are most important factors. For commonly used guided wave sparse array system, it usually requires a reference state as the baseline which is not available in many cases. In comparison, phased array technique typically does not need the baseline in simple structures and it had been successfully used in nondestructive testing (NDT). However, currently developed phased array systems employed omni-directional transducers routinely, where the wave energy is distributed equally along all directions thus it is not favorable for long-distance detection. In this work, bidirectional piezoelectric transducers were used to form a linear phased array system, which can generate/receive shear horizontal (SH) wave with high energy concentration. Firstly, the configuration of the employed transducer composed by antiparallel d15 piezoelectric strips (APS) was presented. Then the total focusing method (TFM) employed for defect detection was introduced. After validating the radiation pattern of SH wave generated by the APS, the properties of beam steering for the proposed phased array was investigated. Finally, experiments were carried out to validate its performance in detection of various defects. Results indicated that even for a 1 mm through-thickness hole 700 mm away, the proposed phased array system can still detect it accurately, which is much better than previous SHM systems. Dual defects including a crack and a hole can also be clearly detected without baseline. The high-sensitivity of the proposed system was attributed to the employed bidirectional transducer which can generate non-dispersive SH0 wave with high energy concentration. This proposed SH wave phased array system will provide a high-performance SHM method for plate-like structures.

Correlation Between Dynamic Compressive Strength and Fracture Behaviors of Bulk Metallic Glasses

The mechanical behaviors of Zr43.5Cu43.5Ni4Al8Nb1, Zr55.4Cu31.6Ni4Al8Nb1, Ti32.8Zr30.2Ni5.3Cu9Be22.7 (at.%) metallic glass at different strain rates were studied. For all the present alloys, the dispersion over 700 MPa was observed on the strength in the repeated dynamic compressions, which was much stronger than that of the quasi-static compressive strength. Such the dispersion of the dynamic compressive strength was well correlated with the corresponding fracture behaviors. The area of fracture surface was calculated and also showed a strong dispersion for all the fractured specimens tested at the strain rate of 500 s−1 and 1000 s−1. All the specimens showed a linear relationship between the square of dynamic compressive strength and the area of fracture surface in the dynamic compression tests. This phenomenon was mainly thought to be related to the difference of mean initial free volume concentration of different samples, stress concentration caused by the split Hopkinson pressure bar experimental setup and high sensitivity of defects under dynamic deformation. These findings were beneficial to deeply understand the effect of strain rate on the mechanical properties of the metallic glass.

Eddy Current Probe With Three-Phase Excitation and Integrated Array Tunnel Magnetoresistance Sensors

Detecting defects in metal structures quickly and robustly is critical to maintaining safe operation in industry. This work discloses an eddy current (EC) testing probe with coils carrying three-phase currents as excitation and integrated array tunnel magnetoresistance (TMR) sensors measuring the magnetic field as receiver. The three-phase currents in the planar layout coils induce EC that migrates electrically in the material under test. This excitation method does not require multiplexing of the coils and is sensitive to defects of any orientations. The TMR array contains 64 sensors, which are microfabricated and packaged in a line. Within single probe pass, the sensors generate magnetic field image with high sensitivity ( $\text{1.99}\;\text{nT}/\sqrt {\text{Hz}} @\text{30}\;\text{kHz}$ ) and fine spatial resolution (0.5 mm pitch). By analyzing the output image, defects are identified and localized. The operating principle of the probe was investigated based on a finite-element method model. A prototype probe was developed and tested, with which machined defects in aluminum samples were inspected. Experimental results show that the probe has comparable sensitivity for horizontal and vertical defects, and it can recognize a small defect with length × width × depth = 1 × 0.2 × 1 mm3. The defects are located within 1 mm with an artificial neural network.

High sensitivity flexible double square winding eddy current array for surface micro-defects inspection

High sensitivity surface micro-defects nondestructive testing on the complex geometric sample is a critical filed for the scientific and industrial community. This paper proposes a novel flexible eddy current sensing system that incorporates double square winding excitation with a multi detection flexible array for cracks inspection. The new structure consists of two square drive traces located in the separated layers while the pick-up traces are sandwiched between the drive traces. In particular, two excitation coils are in parallel and multi-turns thin spiral rectangular coils are detectors while the whole probe constitutes a transmitting-receiving-transmitting structure. The new structure offers capability and sensitivity for micro defects as well as directional defects inspection. The theoretical derivation based on magnetic circuit principles has been developed for analysis and interpretation of the results. Both numerical simulation and experiments on defects detection with different defect sizes and orientations have been supplemented to validate the reliability and efficiency of the proposed system.

Surface Defects Sensitivity during the Unfolding of Corrugated Struts Made by Powder‐Bed Additive Manufacturing

Corrugated struts as part of lattice structures can lead to novel mechanical behavior by a combination of material and geometrical hardening. The unfolding behavior of such struts offers a potential of large macroscopic straining. However, their ability to be unfolded is impacted by the surface characteristics inherited from the additive manufacturing process. Herein, the unfolding sensitivity to these surface characteristics is evaluated. Corrugated struts with varying surface roughness are produced using a combination of electron beam powder‐bed fusion to produce corrugated samples with different nominal diameters and chemical etching assisted by micro‐computed tomography (CT) to achieve a given final diameter. In situ micro‐CT tensile tests are conducted to track the evolution of the struts morphology under loading. Surface defects play a significant role in the unfolding ability of such struts. They are characterized either by a global roughness or by a local notch depth. A quite broad unfolding dispersion remains for samples with the same level of roughness. A finer description of notch depth and location within the gauge length allows a more accurate prediction of the unfolding ability. A model for predicting the probability of failure during unfolding is presented.

Effects of building direction and defect sensitivity on the fatigue behavior of additively manufactured H13 tool steel

Selective laser melting (SLM) is a laser powder-bed fusion process, which produces near-net shaped components with layered microstructures leading to anisotropy in the mechanical properties. The intrinsic process-induced defects, particularly microporosity and lack of fusion, as well as their size and relative orientation with respect to the building direction, affect the fatigue performance of the SLM-processed components. Herein, the influence of building direction and defect sensitivity on the fatigue behavior of SLM-fabricated AISI H13 steel is investigated. Samples are tested after tempering and turning to find a compromise between tensile strength and toughness and minimize the effect of geometrical distortion. The fatigue strength is assessed by employing the Murakami model, and short-crack effect is evaluated by calculating the El Haddad–Smith–Topper length parameter. An inferior fatigue performance of the SLM parts is observed in comparison to that of the conventionally processed materials, owing to the presence of defects that act as origins of crack initiation. The fatigue strength is significantly influenced by the defect size, shape, and the distance from surface, which in turn are affected by the building direction. A slightly higher fatigue resistance is observed for the 90°-oriented samples, owing to the smaller defect size and the lower stress concentration factor. The data for the three building directions show a high scatter, attributed to the process-defect-induced stress concentration factors.

Thin-film hydrogels with superior stiffness, strength, and stretchability

Thin-film polymers and hydrogels show great potential for applications in biomedical and engineering fields. However, owing to their defect sensitivity and weak behavior at micrometer-scale thicknesses, the recently developed thin-film hydrogels have limitations in their mechanical properties that impede their use in actual applications. To overcome these drawbacks, we adopted the hydrophobic–hydrophilic copolymer strategy for fabricating thin-film hydrogels with good mechanical properties using a novel benzene-based bulky hydrophobic monomer. Strong hydrophobic associations and benzene–benzene​ interactions of the hydrophobic-rich hard-phase regions of the copolymer gel imparted high stiffness and strength, and the concomitantly formed hydrophilic-rich soft-phase regions were sacrificed reversibly to achieve high stretchability and toughness. The resulting hydrogel, with a thickness of 550μm and ∼52 wt% water content, exhibits an excellent Young’s modulus (18.03 ± 5.92 MPa), tensile strength (4.3 ± 0.21 MPa), and work of extension (11.85 ± 2.17 MJ/m3). Moreover, our hydrogels show biocompatibility and high stability in different media such as water, seawater, and acidic and physiological solutions. These properties further strengthen their potential for applications in biomedical and engineering fields.

Probing the compound effect of spatially varying intrinsic defects and doping on mechanical properties of hybrid graphene monolayers

Doping in pristine 2D materials brings about the advantage of modulating wide range of mechanical properties simultaneously. However, intrinsic defects (such as Stone-Wales and nanopore) in such hybrid materials are inevitable due to complex manufacturing and synthesis processes. Besides that, defects and irregularities can be intentionally induced in a pristine nanostructure for multi-synchronous modulation of various multi-functional properties. Whatever the case may be, in order to realistically analyse a doped graphene sheet, it is of utmost importance to investigate the compound effect of doping and defects in such 2D monolayers. Here we present a molecular dynamics based investigation for probing mechanical properties (such as Young’s modulus, post-elastic behaviour, failure strength and strain) of doped graphene (C14 and Si) coupling the effect of inevitable defects. Spatial sensitivity of defect and doping are systematically analyzed considering different rational instances. The study reveals the effects of individual defects and doping along with their possible compounded influences on the failure stress, failure strain, Young’s modulus and constitutive relations beyond the elastic regime. Such detailed mechanical characterization under the practically relevant compound effects would allow us to access the viability of adopting doped graphene in various multifunctional nanoelectromechanical devices and systems in a realistic situation.

Process-induced geometric defect sensitivity of Ti–6Al–4V lattice structures with different mesoscopic topologies fabricated by electron beam melting

Ti–6Al–4V lattice structures with different cell configurations are fabricated by electron beam melting (EBM) method. Two different cell topologies (octet truss and regular rhombic dodecahedron lattice) corresponding to stretching-dominated and bending-dominated respectively are taken into consideration. The mechanical properties of these specimens are examined by quasi-static compression experiments which are conducted by electronic universal machine, while the deformation evolutions of all the specimens are captured by a digital camera. The results indicate that the specimens with rhombic dodecahedron cells exhibit higher specific strength than the octet truss structures. This unusual phenomenon is proved to be caused by the geometric imperfections in the printed stretching-dominated structures. Then the X-ray computed tomography is adopted to capture the process-induced defects in the EBM-printed specimens, and virtual tests are performed by the X-ray tomography based 3D reconstruction model. The computational study explains that the collapse strength of the rhombic dodecahedron (RD) lattice is mainly affected by the surface roughness of the struts, while the weak connection at the joints of the struts leads to the significant reduction in the strength of the octet truss specimens. Meanwhile, the dynamic performance of the printed specimens is predicted by ideal shock wave model and validated by numerical simulation. The results indicate that the defects lead to the significant discrepancy between the theoretical prediction and numerical simulation of octet truss structure, but the failure mode transition of RD lattice structure can be well predicted by the analytical model.

Investigation of the sensitivity of the fatigue resistance to defect position in aluminium alloys obtained by Selective laser melting using artificial defects

This study focuses on the influence of defects on the fatigue behavior of AlSi7Mg0.6 alloy produced by Selective laser melting. Artificial defects were generated in fatigue specimens by introducing holes in the CAD files prior manufacturing. The introduction of such defects, whose sizes and positions are controlled, aims to provide a proper assessment of defect sensitivity, in particular regarding the influence of defect position. Fatigue tests were performed, and finite-element calculations accounting for real defect geometries were conducted. The criticalities of the simulated defects were then evaluated using a non-local Crossland criterion, and discussed with regard to the experimental results.

Failure in Two-Dimensional Materials: Defect Sensitivity and Failure Criteria

Abstract Two-dimensional (2D) materials have attracted a great deal of attention recently owing to their fascinating structural, mechanical, and electronic properties. The failure phenomena in 2D materials can be diverse and manifested in different forms due to the presence of defects. Here, we review the structural features of seven types of defects, including vacancies, dislocations, Stone-Wales (S-W) defects, chemical functionalization, grain boundary, holes, and cracks in 2D materials, as well as their diverse mechanical failure mechanisms. It is shown that in general, the failure behaviors of 2D materials are highly sensitive to the presence of defects, and their size, shape, and orientation also matter. It is also shown that the failure behaviors originated from these defects can be captured by the maximum bond-stretching criterion, where structural mechanics is suitable to describe the deformation and failure of 2D materials. While for a well-established crack, fracture mechanics-based failure criteria are still valid. It is expected that these findings may also hold for other nanomaterials. This overview presents a useful reference for the defect manipulation and design of 2D materials toward engineering applications.

Gingival inflammation, enamel defects, and tooth sensitivity in children with amelogenesis imperfecta: a case-control study.

Gingival conditions and tooth sensitivity of young patients with amelogenesis imperfecta lack in depth studies. This case-control study aimed to compare (1) the gingival inflammation, the presence of enamel defects, and tooth sensitivity in young patients with and without amelogenesis imperfecta and (2) to investigate if any difference exists between subtypes of amelogenesis imperfecta. Methodology We compared forty-two participants with amelogenesis imperfecta with forty-two controls matched for age, gender, and the number of examined sites. Based on interview, clinical examination, and intraoral photography, we collected data on periodontal conditions, enamel defects and the presence of tooth sensitivity. Comparison tests were performed to investigate if any difference existed between cases and controls; and among cases, between the different subtypes of amelogenesis imperfecta. We performed a post-hoc analysis for any significant difference observed.Results We observed more gingival inflammation, enamel defects and tooth sensitivity among cases (all p<0.05). Participants with hypocalcified amelogenesis imperfecta had more gingival inflammation, enamel defects, and tooth sensitivity than patients with the hypoplastic and hypomature subtypes (all p<0.05). After adjustment for dental plaque, gingival inflammation was associated with the presence of amelogenesis imperfecta (OR (95%CI) = 1.14 (1.05; 1.24). p<0.01).Conclusion Gingival inflammation, enamel defect and tooth sensitivity are more frequently observed among young patients with amelogenesis imperfecta, and more specifically among children with the hypocalcified subtype.

The Effect of Ti-6Al-4V Coating Structure Deposited by Warm Spray on Defect Sensitivity of Laser Ultrasonic Testing

The Effect of Ti-6Al-4V Coating Structure Deposited by Warm Spray on Defect Sensitivity of Laser Ultrasonic Testing

Guided wave sensitivity prediction for part and through-thickness crack-like defects

Guided waves allow for the efficient structural health monitoring of large structures using phased or distributed arrays of sensors. The sensitivity for specific defects can be improved by accounting for the angular scattering pattern. The scattering of the fundamental anti-symmetric guided wave mode (A0 Lamb mode) at through-thickness and part-through crack-like defects was studied experimentally and from three-dimensional finite element simulations. Experimentally, the scattered field around manufactured notches of different depths and lengths in an aluminium plate was measured using a laser interferometer. The scattered field was extracted by taking the complex difference in the frequency domain between baseline measurement and measurements around the defect. Good agreement was found between measurements and three-dimensional finite element simulations, and the amplitude and directionality pattern of the scattered field can be predicted accurately. The angular directionality pattern of the scattered field depending on the direction of the incident wave relative to the crack-like defect orientation, depth and length relative to the wavelength was investigated. For short and part-thickness defects, the main scattering effect is a reduction of the (forward) wave propagating past the defect with very limited backscattered amplitude. Significant energy scattered back towards the incident wave direction was only found for perpendicular incidence on long and deep defects. Even for large defects, almost no energy is scattered in certain directions from the defect, possibly complicating defect detection. Based on the predicted amplitude and angular dependency of the scattered guided waves, the sensitivity for defect detection using localized and distributed structural health monitoring sensor array systems can be quantified.

Periapical Microsurgery: Assessment of Different Types of Light-emitting Diode Transilluminators in Detection of Dentinal Defects

IntroductionThis study evaluated 4 different light-emitting diode (LED) transilluminators and the impact of operator experience in the detection of dentinal defects through an ex vivo TRUEJAW surgical model (Dental Engineering Laboratories, Santa Barbara, CA). MethodsForty-four extracted and endodontically treated mandibular premolar teeth were evaluated. Teeth were mounted in the models followed by surgical flaps and osteotomies to expose the apical third of the roots. After apical resection, the root-end surfaces were randomly inspected for the presence or absence of dentinal defects using a dental operating microscope (DOM) at ×19.4 magnification by experienced and novice LED evaluators. The assessment was made with the DOM light and 4 masked LED transilluminators of different diameters and luminous flux. The teeth were examined outside the models to establish the ground truth. The sensitivity, specificity, and kappa and McNemar test values of each light source by examiner were calculated. ResultsThe use of LED transilluminators improved the diagnostic sensitivity of dentinal defects when compared with the DOM light alone for both examiners. For the LED-experienced evaluator, the medium-low transilluminator had statistically significant higher sensitivity than the DOM light and the small-low and small-high transilluminators (P < .05). For the novice LED evaluator, the medium-high transilluminator had the highest sensitivity. There was a statistically significant difference between the sensitivities of the medium-low transilluminator between the examiners (P < .05). ConclusionsWithin the limitations of this ex vivo surgical study, dentinal defects were more often detected with the LED transilluminators with a larger diameter and increased lumens. The operator's LED transilluminator experience was found to have a positive effect on the detection of dentinal defects using transillumination.

Detection sensitivity analysis of pipe pile defects during low-strain integrity testing

The detection sensitivity of pipe pile defects during low-strain integrity testing is investigated with the consideration of soil plug, the soil sandwiched in neck defect and high-frequency interference at pile head. The additional mass model and fictitious ring-soil pile model (FRSPM) are introduced to simulate the soil plug effect and the dynamic interaction between the sandwiched soil and neck defect, respectively. The high-frequency interference is realized by considering the stress wave propagating both in the vertical and circumferential directions. An analytical solution for the dynamic velocity response of pipe pile is derived in the frequency domain through the transfer function method, and the corresponding semi-analytical solution in the time domain is then obtained by using the inverse Fourier transform. The reasonability and accuracy of the developed solution are verified by a series of field experiments. The dynamic interaction between the neck defect and the FRSPM is clarified via the parameter sensitivity analysis. The non-detection zone of pipe pile during low-strain integrity testing is analyzed. It is shown that the main factors to affect detection accuracy are the reduction degree and the location of defects, and there exists a location non-detection zone near the pipe pile toe.

Preparation and Properties of Pre-Stressed Ceramic Plate

Ceramics are widely used because of their excellent mechanical properties, electrical insulation properties, high temperature resistance and corrosion resistance. Ceramic plate is one of important branch of ceramic components. However, brittle fracture caused by low toughness and high sensitivity to surface defects limits the reliability of ceramics. Therefore, there are a lot of research on strengthening and reducing defects of ceramic plate. In this work, a pre-stressed ceramic plate was obtained by a simple method, which was impregnated with a layer of alumina slurry on the surface of zirconia substrate and sintered at 1500°C. Because the expansion coefficient of alumina is lower than that of zirconia, a compressive stress is formed on the surface of plate, which can reduce the defect sensitivity and improve the bending strength of ceramic plate. Compared with the pure zirconia plate, the strength was improved with about 40%.

Imperfect architected materials: Mechanics and topology optimization

Abstract

Nonlinear buckling analysis of arch structure to initial defect sensitivity

Due to the more and more prominent nonlinear buckling problems of large-span arch structures in engineering applications, and the lack of special design methods of large-span arch structures in existing specifications, this paper establishes a simplified mechanical model for the stability of arch structures with initial geometric defects. The finite element model of arch structure is established by using finite element software, and the characteristic value buckling analysis and geometric nonlinear analysis are carried out to find out the buckling mode of arch structure, and the sensitivity of defects to arch structure is studied. The results show that the existence of defects will reduce the bearing capacity of the structure, and the increase of defects will change the deformation form of the structure.

Design and Simulation of Sensor Based on One Dimensional Photonic Crystal Metallodielectric with Two Defect

In this paper we report characteristics of a 1D metallodielectric photonic crystal with a two defect rod. Simulations of the field propagation for the corresponding Transverse Electric (TE) modes were carried out using Finite Difference Time Domain (FDTD) technique on Maxwell’s Equation. The results show that for certain chosen parameters of the transmittance varies if the refractive index of the defect rod is changed and that approximate linier changes for an refractive index range of 1.3 – 1.5 with a slight increase for a second defect change and a relatively steep decrease for a first defect change. A second defect index change can be used as the first defect sensitivity control The characteristics change due to variation of the defect rod radius is also considered for sensor design optimization