Showing papers by "Prabhu Rajagopal published in 2019"

Laser ultrasonic inspection of additive manufactured components

Abstract: Additively manufactured components are gaining popularity in aerospace, automotive and medical engineering applications. Additive manufacturing (AM) offers tremendous cost advantages over traditional manufacturing methods. However, inter- and intra-layer defects are observed in AM components. Moreover, the lack of appropriate testing methods for assessing the integrity of AM components deters its use, despite the several functional advantages it has to offer. Non-destructive testing (NDT) forms the most common and convenient way of inspecting parts. In this paper, a laser ultrasonic technique for the inspection of AM components is proposed. The results demonstrate laser ultrasonic testing (LUT) as a promising method for the non-contact inspection of additive manufactured components. Furthermore, the results were validated using X-ray computed tomography (CT) and ultrasonic immersion testing (UIT). The sample used in this study was manufactured through selective laser melting (SLM) AM process with built-in holes representing defects.

A transformation elasticity based device for wavefront manipulation

Abstract: The authors propose a metamaterial plate made of gradient refractive index phononic crystals for manipulation of the wavefront of propagating elastic waves in solid media. A square unit cell with through holes is considered as the basis for the proposed metematerial. Guidance on the design of the hole pattern and the choice of materials is obtained with the aid of transformation elasticity principles. Manipulation of a plane wavefront into a cylindrical wavefront is first observed through Finite Element simulations at low frequency (100 kHz), and then practically demonstrated through experiments. Wave propagation with and without the holey region is compared and possible applications at higher frequencies are discussed.

Lensing in the Ultrasonic Domain using Negative Refraction Induced by Material Contrast

Abstract: The focusing of ultrasound using topographic lenses, typically made of plates with step changes that cause an interaction between forward- and backward-propagating guided waves, has been widely studied in recent years. However, such ‘step-change’ lenses require precise machining and moreover, the thick-thin structure can be unstable during deployment in practical inspection applications. The work reported here follows from the insight that perhaps any approach to induce a mismatch in acoustical impedance as achieved by the step-change can also lead to focusing of ultrasonic guided waves. By carefully choosing the impedance pairing, a novel material contrast lens stacking Aluminium and Molybdenum plates in series is shown to achieve focusing of ultrasound through negative refraction. The interface between the two metals causes the interaction of the forward-propagating second symmetric Lamb mode S2 into the backward- propagating first symmetric S2b. The focusing of Lamb waves is demonstrated using numerical simulations validated by experiments. Comparison with a simple Aluminium-Aluminium plate combination brings out the underlying physics of focusing using the proposed material contrast lens. Simulation results showing super-resolution imaging using the proposed material contrast lens are also presented, demonstrating the power of the proposed approach. This report opens up the possibilities of developing new lensing devices for use in medical imaging and nondestructive evaluation, among other possible applications.

Fiber bragg grating based detection of part-thickness cracks in bent composite laminates using feature-guided waves

Abstract: Composite structures with bends are widely used in aerospace and industrial sectors. However health monitoring of such structures is challenging due to their complex topographical features. Recent literature shows that bends in composite laminates can confine and guide ultrasonic energy along their length, known as feature-guided waves (FGW). This article demonstrates a fiber Bragg grating (FBG) based technique using FGW modes for defect detection and identification in bent composite laminates. In addition, the effects of defect depth and excitation frequency on the FGW mode reflection coefficient are reported using 3D finite element simulations. Physical insight into the reflection behavior is discussed based on an analysis of mode interaction with part-thickness cracks.

Characterization of Deep Sub-wavelength Sized Horizontal Cracks Using Holey-Structured Metamaterials

Abstract: Image resolution in classical wave applications is limited by the diffraction limit which corresponds to half the operating wavelength (λ). To achieve higher resolution, ways of overcoming the natural diffraction limits are of interest. In this paper, an experimental demonstration of deep sub-wavelength resolution in the ultrasonic regime using a metamaterial lens is presented. Metamaterial lenses effectively transfer the evanescent waves to the imaging plane, which carry the details of the sub-wavelength features. The successful transmission of the decaying evanescent waves which contain much larger wave vectors than the propagating waves enables to overcome the diffraction limit set by the operating wavelength. We report an imaging technique using optimized holey-structured metamaterial lens to characterize a horizontal crack of size λ/25 in a layered aluminium sample.

Laser generation of narrowband lamb waves for in-situ inspection of additively manufactured metal components

Abstract: Recent developments in metal additive manufacturing (AM) has created a lot of interest in sectors including automotive, aerospace and biomedical engineering. It is imperative that the components manufactured additively be inspected for flaws, mechanical properties and dimensional accuracy. Several non-destructive testing (NDT) techniques such as X-ray computed tomography and conventional ultrasonic testing have been implemented to evaluate the quality of these components. Recently, research has been focused on techniques that can perform non-contact testing and carry out an online inspection layer by layer while the component is being fabricated. Laser based ultrasonic technique has been found to be a promising method owing to its non-contact nature and ability to operate in harsh environments. In our study, narrow band lamb waves were generated using a pulsed Nd-YAG laser system consisting of a spatial array illumination source. The generated wave modes were detected using a two-wave mixing based laser interferometer. The wavelength-matched method enabled generation of specific lamb wave modes for in-situ inspection of additively manufactured components.

Interfacial adhesion (kissing bond) detection using shear horizontal (SH) waves

Abstract: Horizontally polarized shear (SH) ultrasonic guided wave modes are considered to infer adhesion changes at different interfaces. In this study, the SH0 interaction with different lap joints are investigated. Experiments were performed on aluminium lap joint samples. Aluminium surfaces were prepared in a specific way to vary the adhesion. SH modes are generated and received using periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs), the periodicity of which is equal to the wavelength of the transducer. SH0 is generated in the plate that comprises the lap joint and received in transmission mode. Distinct signatures were observed for different samples. To investigate the physics of interaction with tri-layer and to corroborate the experiment results, three dimensional finite element (FE) models were developed. Different cases of interfacial adhesion were simulated by changing the boundary condition at both aluminium-epoxy-aluminium interfaces. This work shows the potential of SH modes for quantifying properties at adhesive interfaces.

Numerical Study on Swimming Performance Based on Flapping Orientations of Caudal Fins for Bio-robotic Systems

Abstract: Set in the context of the development of bioinspired robotics systems, this paper seeks to understand the influence of the choice of the flapping orientation of fins on the propulsive performance of small underwater vehicles. In particular, the thunniform mode of Body and/or Caudal Fin (BCF) propelled systems is studied. This research is motivated by the fact that not much literature is available on the influence of flapping orientation of marine organisms and a number of mechanisms are found in nature. Dorso-ventral flapping with a positive metacentric height is shown to yield better self-stabilizing effects and lesser energy consumption compared to sideways flapping. Moreover, with dorso-ventral flapping, the choice of metacentric height could lead to the possibility of adjusting the body's rotational oscillation amplitudes to positively affect the downstream fluid interactions for the caudal fin. This is not possible with sideways flapping where the designer would be forced to change the flapping kinematics or the body shape in the sagittal plane, to adjust the body oscillation amplitudes. While the main body of results are obtained using simulations for underwater vehicle dynamics with coefficients of the REMUS underwater vehicle, stability analysis for a generalised case is also presented.

Development and Analysis of an Autonomous Underwater Inspection Vehicle with Enhanced Maneuverability

Abstract: There is much interest in robots for underwater inspection and survey purposes. Typically, sensors for such applications are located in the robot’s main body or hull. Conventional underwater robot design uses a single hull carrying all the components with thrusters attached on it for propulsion. As the number of components (especially sensors, batteries) inside the robot hull increases, the designer has to choose a longer hull design which can accommodate them. But a long single hull shape leads to longer turning radius, making it difficult to operate the robot in constrained spaces. Here, we describe the design and working of a surface & underwater robot with split hull mechanism which will provide a lower turning radius. This provides better maneuverability than a single hull torpedo robot with similar overall dimensions. Kinematic/dynamic analysis of the same robot is also carried out later.

Rapid terahertz imaging for non-destructive evaluation applications using Schottky receivers and spatial adaptive sampling

Abstract: Terahertz (THz) technology is a competent non-destructive evaluation (NDE) technique, particularly for advanced materials such as Fibre Reinforced Polymer (FRP) composites due to its ability to penetrate most non-metallic and nonpolar substances. Typically, THz NDE studies are carried out using expensive and broadband pulsed THz systems limiting their widespread use in practical applications. In contrast, Continuous wave (CW) THz systems can potentially be a narrowband, cost-effective and scalable solution for NDE applications. However, conventional CW THz systems employ a coherent detection scheme which results in large acquisition time per pixel thus limiting their real-time applicability. In this paper, a CW THz system with incoherent detection scheme using Schottky receiver along with spatial adaptive sampling technique is employed to achieve rapid THz imaging of Glass Fibre Reinforced Polymer (GFRP) composite with artificial defects. Here, an initial coarse scan of 2 mm step size has been done, and gradient based thresholding criterion is used for identifying the regions of interest to progressively scan the sample with finer resolution down to a step size of 0.5 mm. Results demonstrate a total reduction in the image acquisition time by a factor of 50 compared to the coherent CW THz imaging. Further, the THz image acquired through adaptive sampling shows excellent correlation with that of the traditional uniformly sampled THz image with 0.5 mm step size.

Numerical model and experimental validation for online monitoring of cold metal transfer joining of aluminium to galvanized steel

Abstract: The effect of zinc coating on the weld brazability of dissimilar Al–steel joint made by the cold metal transfer (CMT) process using the passive infrared (IR) thermography sensing technique was investigated using a numerical model. The numerical model was developed to understand the heat transfer phenomena during the CMT process. In the developed model, a thin thermally resistive layer (TTRL) was introduced at the interface of the two joining metals in order to manipulate the absence of zinc deposition on the steel sheet. The model could clearly identify the lack of weld deposition. The results were compared with experimental observations. Two cases are considered in this paper. The first study identifies the sensitivity of the IR thermography technique and the second one reveals the resolution in defect detection. The developed 3D model can be used as a tool to identify the defects caused during welding and can provide insights into the online monitoring of cold metal transfer joining process.

Design and Simulation of a Tank Floor Cleaning Mechanism for Mobile Robots used in Storage Tanks

Abstract: Oily sludge on the floor of the tank is a significant problem for petrochemical industries and floor inspection robots. Oily sludge is a hazardous material containing a complex mixture of hydrocarbon, water, sand, and minerals deposited on the floor of the oil storage tanks. Sludge accelerates corrosion, reduces storage capacity, sticks to floor inspection robots and disrupts further tank operations Industries have started deploying robots in a tank to automate and replace the hazardous manual tank tasks. This paper presents the design of a screw conveyor based sludge cleaning mechanism to clean the sticky sludge from the floor of aboveground oil storage tanks and interface effectively with tank inspection robots to perform cleaning and inspection synchronously. The cleaning mechanism consists of a screw conveyor mounted on a 'C' shaped case with a bearing on both sides, a waterproof motor connected to the screw conveyor with a worm-wheel gear. A Rheometer is used for measuring sludge properties to understand its flow behavior. Computational fluid dynamics (CFD) based numerical simulation is performed to visualize the flow of oily sludge through the proposed cleaning mechanism.

Structural health monitoring of reinforced concrete structures using guided waves

Abstract: Ultrasonic guided waves provide a highly efficient method for non-destructive evaluation and the structural health monitoring (SHM) of solids with finite-cross-sectional dimensions (waveguides). Guided waves are widely used for the inspection of structures such as pipelines, annular tank plates, aircraft wing assemblies, composite radius fillers and wind turbines. They are also attractive for long-term structural health monitoring due to their ability to provide long-range and feature-rich through-structure information from a single transducer location. The use of guided waves in reinforced concrete structures is limited to the monitoring of setting and curing of concrete due to the heterogeneous nature of concrete and its high acoustic impedance which is comparable to the steel reinforcement bars. This leads to a significantly high energy leakage into the embedding concrete making it impossible to use guided waves for the inspection of reinforced concrete structures. Corrosion resistant epoxy coatings on reinforcement coatings are widely used for structure exposed to harsh environmental conditions such as high temperatures, humidity, water, acids, solvents, salts and other chemicals. In this work, the attenuation of the L (0,1) guided waves modes propagating along a 20 mm diameter mild steel is monitored using finite element simulation for a fusion bonded epoxy (FBE)rebar and for plain rebar both embedded in an infinite concrete medium. The attenuation in the epoxy coated rebar is shown to be substantially reduced compared to the plain rebar embedded in concrete. This reduction in guided wave attenuation can make it possible to use guided waves to conduct structural health monitoring of concrete structures.

Manipulating curvature of plane wavefront using holey plates

Abstract: Ultrasonic inspection is commonly used for Non-Destructive Evaluation (NDE). Phased array ultrasonic transducers (PAUT) enable beamforming and beam steering by implementing electronic time delays. By providing appropriate delays in exciting individual elements, PAUTs can generate a curved wavefront. The authors propose a metamaterial device made of Gradient Refractive Index Phononic Crystals to transform curvature of a wavefront generated from a single element transducer. The manipulation of the wavefront as it passes through holey region, is simulated using the finite element method. Results from experimental studies carried out on a holey plate, designed using this approach are comparable to simulation results. The authors suggest that add-on devices can be designed using this approach for tailoring wave incidence in ultrasonic inspection.