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The capabilities of most non-destructive testing methods have been combined with some degree of automation in recent years, to enhance data acquisition speed, part coverage and inspection reliability. A plethora of automated or semi-automated inspection systems have been engineered to enable the robotic manipulation of specific types of sensors. Robotic inspection systems are usually operated through off-line programmed tool-paths. This approach works well when an accurate model of the part is available and the robotic inspection takes place in a well-structured environment, where the part position is precisely registered with respect to the robot reference system. However, it makes the inspection setup for each new part very time-consuming and dependent on the skills and experience of the robot programmer. Moreover, the real geometry of a part may significantly deviate from its digital counterpart, resulting in inaccurate tool paths. This work introduces a new approach capable of conferring full autonomy to robotic sensing applications, providing a breakthrough in the stateof-the-art. As a result of this work, fully autonomous single-pass geometric and volumetric inspection of complex parts, using one single robotized sensor, becomes possible. This concept can find wide applicability to the open problems of structural health monitoring of the modern age.

The spinal column is the load-bearing structure of the human being along with its components, which together build a strong, resistant, and stable structure, but there are a few different pathologies from which it can suffer, such as herniated discs. The intervertebral disc acts as a shock absorber and ensures the spine’s great capacity to support high loads and different states of stress, thanks to its viscoelastic properties. Some studies have attempted to describe the viscoelastic behaviour of the intervertebral disc using classical rheological models, such as the Kelvin-Voigt, or multi-parameter models. Even if these models partially describe the viscoelastic response of disc, all viscoelastic characteristics are not fully captured. This article aims to present the current studies on the biomechanics of intervertebral disc and to introduce a new approach using the powerful mathematical tool of fractional calculus. With fractional rheological models, it could be possible to formulate a fractional law that can fully describe the viscoelastic behaviour of the intervertebral disc. This new approach could lead to a breakthrough in the study of herniated pathologies by understanding how the intervertebral disc is damaged and identifying strategies to deal with these pathological problems.

Ultrasonic testing has been used for material analysis and inspection since 1930's. Nevertheless, the applicability of ultrasonic waves to new complex cases is still growing, thanks to the availability of powerful electronics and software. However, the complication that slows down the deployment of ultrasonic inspection to geometric complex parts and structures arises from the wave refraction phenomenon. A clear understanding of the ultrasound wave propagation, impacted by refractions, is crucial to interpret the data obtained from the inspection of multi-layered/multi-medium test subjects as it is not always possible to assume that mechanical waves travel in straight lines. This work presents suitable approaches for solving the ray-tracing problem in multi-layered structures. Accurate benchmarking shows that the use of the Newton-Raphson root-finding method allows a threefold reduction of the computation time, when compared to the bisection-based root-finding methods. An effective combination of the Newton-Raphson methods with bisection-type iterations is also proposed and discussed. Although the work repeatedly refers to the field of ultrasonic inspection, the presented findings are relevant and applicable to areas beyond material inspection.

Many industrial sectors face increasing production demands and need to reduce costs, without compromising the quality. Whereas mass production relies on well-established protocols, small production facilities with small lot sizes struggle to update their highly changeable production at reasonable costs. The use of robotics and automation has grown significantly in recent years, but extremely versatile robotic manipulators are still not commonly used in small factories. Beside of the investments required to enable efficient and profitable use of robot technology, the efforts needed to program robots are only economically viable in case of large lot sizes. Generating robot programs for specific manufacturing tasks still relies on programming trajectory waypoints by hand. The use of virtual simulation software and the availability of the specimen digital models can facilitate robot programming. Nevertheless, in many cases, the virtual models are not available or there are unavoidable differences between virtual and real setups, leading to inaccurate robot programs and time-consuming manual corrections. This could be avoided by measuring the real-geometry and the position of the specimen, which creates the paradox of having to plan robot paths for surface mapping purposes, before the originally intended robot task can be approached. Previous works have demonstrated the use of robotically manipulated optical sensors to map the geometry of samples. However, the use of simple user-defined robot paths, which are not optimized to the part geometry, typically causes some areas of the samples to not be mapped with the required level of accuracy or to not be sampled at all by the optical sensor. This work presents an autonomous framework to enable adaptive surface mapping, without any previous knowledge of the part geometry being transferred to the system. The article gives an overview of the related work in the field, a detailed description of the proposed framework and a proof of its functionality through both simulated and experimental evidences.

In the railway field, the safety of passengers and the service life of train components are a crucial issue. For this reason, continuous periodic inspections by non-destructive techniques are required. Among these, ultrasonic tests are widely used in this field, even though the conventional ultrasound techniques have the disadvantage of requiring the disassembly of the wheels and of putting the train out of service. This procedure is expensive and time-consuming and can be neglected if non-contact ultrasonic techniques are used. In this work, the authors present an experimental research on some defects, artificially obtained on a railway wheel supplied by Trenitalia Spa, by adopting three different experimental setups and comparing the results. The proposed method uses laser equipment in generation and reception of the ultrasonic waves. The receiving unit combines a continuous wave laser and an interferometer in order to detect the surface displacements. In addition, when used in service conditions, the receiving by the interferometer is facilitated, because the tread of the wheel is very reflective (thanks to wheel – rail contact). The research aims at optimizing the experimental setup through a number of measurements carried out by varying some experimental parameters to evaluate the robustness and reliability of the technique.

Metal additive manufacturing is nowadays a well-established technology for cutting edge applications in the automotive, aerospace, defense and medical sectors. Since additive metal deposition is basically a welding method, which creates parts by successively adding layers of material, there is a chance for defects like pores, cracks, inclusions and lack of fusion to develop. As a matter of fact, interlayer and intralayer defects are often observed in additive manufactured components. However, if one considers the typical end applications along with the high costs involved in metal additive manufactured components, a “zero defect” target is close to mandatory for this technology. Planning an inclusion of the integrity assessment right into the additive manufacturing process would allow for quick corrective actions to be performed before the component is completed. Some effort has been spent in the quest of an efficient in-process flaw inspection, however, no conventional nondestructive testing (NDT) approach has been fully satisfying yet. This work suggests an experimental evaluation of the effectiveness of flying laser scanning thermography, when detecting flaws on an Additively Manufactured acetabular cup prosthesis made in titanium alloy, where some defects have been artificially created. The rough surface scanned is what’s typically left by the additive manufacturing process, and has been left so in order to prove the efficacy of the NDT inspection in real conditions. Potential benefits and limitations of the technique are discussed.

The present paper proposes a new procedure to analyze the temperature field distribution during Flying Laser Spot and Laser Line Thermographic scanning (FLST, FLLT) of metallic components, in order to detect vertical surface cracks. The methodology exploits the changes in the temperature field produced by a vertical crack, acting as a barrier towards heat diffusion, when the laser approaches the defect. A number of small regions of interests (ROIs) is placed nearby and around the laser source. The average temperature from each ROI is then monitored during the laser scanning. Vertical cracks can be detected by analyzing and comparing the temperature fluctuations from each ROI when the laser crosses a crack. The paper, in particular, illustrates how the use of multiple ROIs, placed at different locations, may provide additional information that can be used to characterize the defect, and to identify the crack tip location. The approach is validated on plates made of steel and aluminum alloy, where natural cracks have been introduced by fatigue loading, and whose surface has been painted to enhance emissivity. Scratches in the paint have been artificially made in order to analyze their influence on the defect signature. The proposed experimental setup is further simplified by moving the plate samples, mounted on slits, in front of a still laser source and camera head.

Ultrasonic testing has been used for many decades, proving itself very efficient for detecting defects in many industrial sectors. The desire to apply ultrasonic testing to geometrically complex structures, and to anisotropic, inhomogeneous materials, together with the advent of more powerful electronics and software, is constantly pushing the applicability of ultrasonic waves to their limits. General ray tracing models, suitable for calculating the proper incident angle of single element probes and the proper time delay of phased array, are currently required. They can support the development of new imaging techniques, as Full Matrix Capture and Total Focusing Method, and the execution of very challenging ultrasonic inspections. This paper introduces a generalized iterative method for the computation of ultrasonic ray paths, when ultrasonic source and target are separated by multiple complex material interfaces in the two dimensional and three dimensional domains. The manuscript starts with a review of the well-known bisection method, and extends the applicability of the method to cases with increasing complexity. An application example, in the field of in-process weld inspection, shows that the introduced generalised bisection method can enable the computation of optimum incidence angles and focal delays for accurate ultrasonic focusing. There is no restriction on the analytical interfaces to be surjective. Interface folding is permitted. It is not necessary to know, a priori, with what sequence the interfaces are crossed by the rays. The presented implementation of the method completes each iteration of the bisection method in 4 ms, for a case with a single interface, and in 960 ms for the case with 52 interfaces.

In this work a novel data collection and processing is proposed for the Infrared Non-Destructive Testing (IR-NDT) of interlaminar disbonds on Fibre Metal Laminate (FML) hybrid composites. The adopted active IR-NDT scheme uses a pointwise laser heat source that is moved along a raster scanning trajectory over the object surface. A Focal Plane Array IR camera is employed to acquire the thermal field generated by the moving heat source. Disbonds defect signatures are then searched by analysing the perturbations of the temperature distribution over a reference area following the heat source. The proposed methodology has been implemented on a GLARE sample, since this class of FMLs has gained extensive use in aerospace structures. In particular, a sample of GLARE 1 3/2–0.3 was manufactured in-house, containing triangular shaped artificial disbonds at different interlayers. The novel inspection approach was able to detect the position, size and to some extent the shape of interlaminar defects by recording the changes in standard deviation of the temperature over the monitored area. The sensitivity found in detecting disbonds proposes the presented methodology as a potential alternative to more conventional inspection routes for FMLs.

Fibre Metal Laminates (FMLs) describe a number of aerospace-grade layered structural materials where thin metal alloy foils are interleaved with Fibre Reinforced Polymer (FRP) plies. The evaluation of bonding integrity in multilayered primary structures is of great importance, as is generally the case with the specific performance requirements of aeronautical applications. This work describes the successful implementation of an Infrared-NDT approach to evaluate debonds in FMLs panel made of Titanium/Graphite (Ti-Gr). The proposed approach uses a flying laser heat-source, raster scanning the surface, in search of defect signatures arising on the temperature distribution profile left by the moving laser, and acquired from the component surface by an IR camera. A numerical simulation of the above NDT approach is proposed and used to investigate the mechanisms allowing to correlate defects signatures to defect features and material thermal response. The outcomes of the model are also evaluated in order to optimize the experimental setup.

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

A wide use of advanced carbon nanotube polymer composites can be boosted by new non-destructive evaluation (NDE) techniques that can test the quality of the products to ensure that their specifications are met. It is well known in literature that the parameter that far more than others can affect the enhancing capabilities of the carbon nanotubes is their dispersion. Here we have presented a novel NDE technique based on infrared thermography able to evaluate the dispersion of the added nanoparticles in polymer nanocomposites. The NDE technique was used to compare pairs of samples whose difference is represented only by the level of dispersion. It was found a significant difference in the thermal response to heat transfer transients. Thus, the thermal response of a nanocomposite allows one to identify consistently good levels of dispersion with respect to lower levels of dispersion. A reference product, which has the expected dispersion level and achieves the desired design performance, can be used to test the thermal behaviour of other products coming out of the production process and those with poor dispersion can be identified. The physical phenomena that can explain the effects of multi-walled carbon nanotubes (MWCNTs) dispersion on the thermal response of the nanocomposites to the heat transfer transients were also identified.

Passengers’ safety and in-service life of wheelset axles play an important role in railway vehicles. For this reason, periodic inspections are necessary. Among non-destructive techniques, ultrasonic ones are widely applied in this field. The main disadvantage of conventional ultrasonic techniques is that the overall inspection of wheels requires the train to be put out-of-service and disassembly each part, which is time-consuming and expensive. In this paper, a non-conventional non-contact laser ultrasonic inspection for train wheels is proposed. The proposed method uses a laser interferometer to receive the ultrasonic wave without contact. The receiving system allows choosing the distance between the surface to be inspected and the interferometer, overcoming any encumbrance issue. The experimental investigation is carried out on standard-reproduced defects in order to evaluate the reliability and the accuracy of the technique and to verify its applicability for railway components, as wheels, which have a complex geometry. The experimental setup consists of a pulsed laser for the ultrasonic wave generation. The receiving unit combines a continuous-wave laser and an interferometer in order to acquire the surface out-of-plane displacements. Surface and internal standard defects are detected by collecting all the A-scans in a B-scan map. The results are promising for the application of the laser technique to detect both surface and internal defects on in-service components.

Laser powder deposition (LPD) is a rapid additive manufacturing process to produce, layer upon layer, 3D geometries or to repair high-value components. Currently there is no nondestructive technique that can guarantee absence of flaws in LPD products during manufacturing. In this paper a laser ultrasonic technique for in-line inspection of LPD components is proposed. Reference samples were manufactured from Inconel and machined flaws were created to establish the sensitivity of the technique. Numerical models of laser-generated ultrasonic waves have been created to gain a deeper understanding of physics, to optimize the set-up and to verify the experimental measurements. Results obtained on two sets of reference samples are shown. A proof-of-concept prototype has been demonstrated on some specific deposition samples with induced flaws, that were confirmed by an ultra-high sensitivity X-ray technique. Experimental outcomes prove that typical micro-defects due to the layer-by-layer deposition process, such as near-surface and surface flaws in a single layer deposit, can be detected.

Definition of size, shape and location of defects into a mechanical component is of extreme importance in the manufacturing industry in general and particularly in high-tech applications, and in applications that can become dangerous due to the structural failure of mechanical components. In this paper, a laser-UT system has been used to define position and shape of internal defects in aluminum plates. An infrared pulsed laser is used to generate ultrasonic waves in a point of the plate and a CW laser interferometer is used as receiver to acquire the out-of-plane displacements due to the ultrasonic waves in another point of the plate. The method consists of acquiring a B-Scan map on which some information on the defects in the mechanical component are visible. Storing the characteristics of the wave reflected by the defect and acquired in the B-Scan, the detection and the drawing of the defect is possible. The acquisition of the times of arrival of the waves reflected by the defect from the B-scan allows defining large parts of the shape of the defect. The times of arrival are acquired from the B-scan by analyzing the colour variations due to the wave reflected by the defect. The experiments operated from both sides of the plate allow drawing the defect in a virtual image of the plate section, from which the definition of defect shape and position can be determined.

This work describes a numerical study on non-destructive evaluation of interlayer disbond defects in aerospace grade Fibre Metal Laminate sheets (FMLs). A recently proposed infrared non-destructive testing setup is considered, where a continuous laser is moved over the material surface, while the thermal footprint of the moving heat source is acquired, e.g. by an infrared thermal camera. Interlayer disbonds are then detected by analysing the features of the acquired thermograms. The experimental feasibility of this approach has been recently proved. The present work proposes a numerical simulation of the NDT approach, where the material thermal response is analysed and correlated to defects signatures. The numerical study has in particular investigated the influence of a number of different features on the defect detectability, and on the accuracy of defect edges and position identification. Such features comprise different FML materials (GLARE, CARAL, Ti-Gr), laser heat deposition and regions of data analyses.

Fibre Metal Laminates (FMLs) describe a number of aerospace-grade layered structural materials where thin metal alloy foils are interleaved with Fibre Reinforced Polymer (FRP) plies. Such material hybridisation at the layer meso-scale poses a challenge for non-destructive testing (NDT). On the other hand, the need for NDT inspection of FMLs arises due to the potential development of interlayers disbonds, both during fabrication or in-service. This work describes the successful implementation of two Infrared-NDT approaches to evaluate a defected FML panel made of Titanium/Graphite (Ti-Gr). One technique is based on the modulation of the deployed heat from a low-cost halogen lamp, to retrieve defects as phase-contrast signatures from harmonics in the frequency domain. The second technique uses a flying laser heat-source, raster scanning the surface, searching for defect signatures on the heat distribution trail of the moving laser. The work explores also the opportunity to collimate the laser heat source as a round spot or a linear distribution. The proposed non-conventional IR-NDT approaches are also compared with a well-established Pulsed Phase Thermography (PPT) algorithm, demonstrating a similar ability of defects evaluation, but offering some favourable features, which are highlighted in the work.

Despite continuous technological advances in additive manufacturing, the lack of non-destructive inspection techniques during the manufacturing process is a limit for the industrial breakthroughs. Additive manufacturing is mainly used in industrial sectors where the zero defect target is crucial. The inclusion of the integrity assessment into the additive manufacturing process would allow corrective actions to be performed before the component is completed. To this end, the development of in-process monitoring and processing techniques is of great interest.This work proposes and compares two remote non-destructive inspection techniques: laser ultrasound and laser thermography. The two techniques are evaluated on Inconel samples with laser drilling holes to establish their sensitivity. Experimental results show that those discontinuities are efficiently detected with both techniques. The remote inspection by optical methods would allow the integration of the evaluation system into the additive manufacturing equipment, thus allowing continuous monitoring throughout the entire production process. Potential benefits and limitations of the two techniques are discussed. Copyright (C) 2018 The Authors. Published by Elsevier B.V.

Additive manufacturing has been recently employed in industrial sectors with the fundamental requirement for zero defect parts. Technological developments in additive manufacturing notwithstanding, there continues to be a scarcity of non-destructive inspection techniques to be exploited during the manufacturing process itself, thus limiting industrial advancements and extensive applications. Therefore, being able to integrate the defect inspection phase within the additive manufacturing process would open the way to enabling corrective actions on the component in itinere, that is, before reaching the final product. For this reason, new methods of in-process monitoring are gaining more and more attention nowadays. In this work, a remote laser thermographic methodology is employed as a mean to detect micrometric defects in additive manufactured samples. Beforehand, a preliminary Finite Element Analysis was carried out in order to optimize the sensitivity of the technique to the micrometric defects. Our results indicate that the technique is proved to be quite successful in detecting flaws, with the added plus of being suitable for integration in the additive manufacturing equipment, thus allowing a continuous in-line inspection.

The present work investigates the features of an active Infrared-NDT Thermography technique derived from a Flying Laser Spot set-up for the analysis of interlaminar disbonds in layered structures in general and Fibre Metal Laminates in particular. The presented technique uses a laser-spot heat source, which moves at a constant speed, raster scanning the object surface. Interlaminar defects parallel to the surfaces act as barriers towards through-the-thickness heat diffusion. This produces some modifications over the surface thermal field which are well identified in the Standard Deviation calculated over a Reference Area following the heat source. The mechanisms leading to such defect signature are investigated in this work by means of Finite Element Analyses which model the dynamic thermal problem on a GLARE sample with triangular disbond defects at inner interfaces. An extended number of parameters is changed to study their influence on the defect signature, and the FEA analysis is also compared to an analytical and an experimental case studies for further validation.

Understanding guided wave propagation in multi-layered plates and interaction with discontinuities can be difficult, as well as the interpretation of the ultrasonic signals. Propagation of guided waves can be studied analytically solving the equations of motion with the proper boundary conditions; nevertheless analytical models can be difficult to solve for complex multi-layered structures or having inner discontinuities. The problem can be efficiently studied using numerical techniques. Simulation of guided wave propagation in multi-layered structures, for ultrasonic waves in the MHz range, is solved here with the finite element analysis based on an explicit integration rule to solve the equations of motion in a dynamic analysis. Simulation allows a better understanding of propagation and interference phenomena by creating a window of observation in the multi-layered plate. Numerical results determined for a three-layer Al plate, without or with discontinuities, matched very well with experiments, providing an efficient tool to visualize and extract significant information in the transmitted waves and to optimize wave mode and configuration for a rigorous ultrasonic inspection.

Ultrasonic methods are well known as powerful and reliable tool for defect detection. In the previous decades focus and interest have been directed to non-contact sensors and methods, showing many advantages over contact techniques where inspection depends on contact conditions (pressure, coupling medium, contact area). The non-contact hybrid ultrasonic method described here is of interest for many applications, requiring periodic inspection in service or after manufacturing. Despite the potential impact of laser-generated ultrasound in many areas of industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. Here a specific numerical method is presented to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range traveling in relatively large bodies and through air. This work improves a previous numerical model where propagation of the acoustic waves through air had not been considered, allowing us to simulate the presence of a non-contact transducer in reception in order to simulate numerically the complete experimental setup. It is very important to limit the amount of air to be considered in the FE analyses; otherwise the computational cost would often exceed the resources available. A way to solve the problem is to implement non-reflecting boundary conditions. A non-reflecting boundary condition allows all outgoing waves to exit the domain at the boundary where they have been imposed without reflection; thus, it is possible to model only the portion of air between the non-contact transducer and the solid under testing. Several numerical and experimental analyses were conducted on a 136 lb AREMA rail; here we study in detail two fully non-contact testing configurations for the rail head and web. The information that can be acquired is very valuable for choosing the right setup and configuration when performing non-contact hybrid ultrasonic inspection.

Laser generated ultrasound holds substantial promise for use as a tool for defect detection in remote inspection thanks to its ability to produce frequencies in the MHz range, enabling fine spatial resolution of defects. Despite the potential impact of laser generated ultrasound in many areas of science and industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. The laser generated ultrasound propagation in complex structures is an intricate phenomenon and is extremely hard to analyze. Only simple geometries can be studied analytically. Numerical techniques found in the literature have proved to be limited in their applicability, by the frequencies in the MHz range and very short wavelengths. The objective of this research is to prove that by using an explicit integration rule together with diagonal element mass matrices, instead of the almost universally adopted implicit integration rule to integrate the equations of motion in a dynamic analysis, it is possible to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range travelling in relatively large bodies. Presented results on NDE testing of rails demonstrate that the proposed FE technique can provide a valuable tool for studying the laser generated ultrasound propagation.

Pressure vessels operating in sour service conditions in refinery environments can be subject to the risk of H₂S cracking resulting from the hydrogen entering into the material. This risk, which is related to the specific working conditions and to the quality of the steel used, shall be properly managed in order to maintain the highest safety at a cost-effective level. Nowadays the typical management strategy is based on a risk based inspection (RBI) evaluation to define the inspection plan used in conjunction with a fitness for service (FFS) approach in defining if the vessel, although presenting dangerous defects such as cracks, can still be considered “fit for purpose” for a given time window based on specific fracture mechanics analysis. These vessels are periodically subject to non-destructive evaluation, typically ultrasonic testing. Phased Array (PA) ultrasonic is the latest technology more and more used for this type of application. This paper presents the design and development of an optimized Phased Array ultrasonic inspection technique for the detection and sizing of hydrogen induced cracking (HIC) type flaws used as reference for comparison. Materials used, containing natural operational defects, were inspected in “as-service” conditions. Samples have then been inspected by means of a “full matrix capture” (FMC) acquisition process followed by “total focusing method” (TFM) data post processing. FCM-TFM data have been further post-processed and then used to create a 3D geometrical reconstruction of the volume inspected. Results obtained show the significant improvement that FMC/TFM has over traditional PA inspection techniques both in terms of sensitivity and resolution for this specific type of defect. Moreover, since the FMC allows for the complete time domain signal to be captured from every element of a linear array probe, the full set of data is available for post-processing. Finally, the possibility to reconstruct the geometry of the component from the scans, including the defects present in its volume, represents the ideal solution for a reliable data transferring process to the engineering function for the subsequent FFS analysis.

The non-destructive evaluation of defects by automatic procedures is of great importance for structural components. Thanks to the developments of the non-contact ultrasonic techniques, the automation of the inspections is gaining a progressively important role. In this work, an automatic inspection technique for the evaluation of defects by the analysis of B-scan images obtained by a laser ultrasonic system is presented. The data are extracted directly from a B-scan map obtained for a panel with internal defects, and are used to build an image of the cross section of the panel. The proposed automatic procedure allows the definition of size, position and shape of defects in panels of known thickness.

The use of air-coupled sensors to detect flaws in Al welded joints and the advantage of Lamb wave for their characterization has been proved. Ultrasonic waves were generated by a pulsed laser and by contact angle-beam transducer, and received by an air-coupled capacitive transducer. Laser-based configuration was used to quickly locate the defect, whereas the angle-beam contact probe configuration to characterize it thanks to the dispersive behavior of Lamb waves. Guided waves allow inspection of the complete thickness with only one scan, permitting to detect and to size both internal and surface defects; their high flexibility in the measurement is due to the various parameters characterizing their behavior when compared to the bulk waves. Moreover, L-scan configuration can be used to inspect single-side access structures, and compared to the conventional A, B, C and P-scan allows to reduce inspection time, and therefore the related costs.

Non-destructive inspection of railroad wheels is performed in maintenance shops, where wheels are removed and inspected individually. No technique is yet available to the railroad industry to perform wayside inspections of wheels on a moving train. The characteristics of laser and air-coupled ultrasound are discussed to justify the use of a laser-air hybrid ultrasonic technique. Laser generation of ultrasound is combined with air-coupled detection to provide a flexible non-contact and remote technique that would enable the railroad industry to perform wayside inspections of moving railroad wheels. The present paper describes Proof of Concept set-up and results of the experiments performed at the Transportation Technology Center facilities, in Pueblo, Colorado. Sensors are used to detect the position of the wheel, drive a two-position mirror to direct the laser beam to one of two test stations and trigger the laser when the wheel is in position for detection. Wheels are inspected for subsurface shattered rim cracks, thermal fatigue cracks along the tread and the flange areas of the wheel. Analysis is based on the time domain radio frequency (RF) signal and time-frequency wavelet transforms. The experimental results are promising, indicating that dynamic wayside inspections are possible with the laser-air hybrid ultrasonic technique.

Train loads and travel speeds have increased over time, requiring more efficient non-destructive inspection methods. Railway axles are critical elements; despite being designed to last more than 20 years several cases of premature failure have been recorded. Train axles are inspected regularly, but the limits associated to the traditional inspection technologies create a growing interest towards new solutions. Here a novel non-destructive inspection method of in-service axles based on non-contact data collection is presented. The propagation of surface waves, generated by a thermo-elastic laser source, is investigated using a finite element method based on dynamic explicit integration. Coupled thermo-mechanical simulations allow visualization of the ultrasonic field guiding the definition of the optimal NDT setup. The geometry of the axle and of the elements mounted on it is accurately reproduced; moreover the press fit effect caused by the wheel and the bearing rings is implemented. The current NDT techniques for railway axles require removing wheels and other components from the axle. The presented scheme uses non-contact ultrasonic generation and detection allowing non-contact in-service inspection of railway axles at trackside station. The numerical results are promising and encourage us to test the new approach experimentally.

I controlli non distruttivi basati sull’impiego di ultrasuoni sono ampiamente usati per la loro efficacia e affidabilità nel rilevamento di difetti. La generazione di onde ultrasonore e la propagazione in strutture di forma non regolare sono difficili da analizzare, soprattutto se la sorgente impiegata è un laser. Le tecniche numeriche per la simulazione del fenomeno reperibili in letteratura mostrano limiti di applicabilità per frequenze nel campo dei MHz e lunghezze d’onda molto corte. In questo lavoro presentiamo un metodo numerico in grado di risolvere accuratamente ed efficientemente problemi di generazione di onde ultrasonore tramite laser, con frequenze nel range dei MHz, e di propagazione in corpi relativamente estesi. La ricezione viene simulata con la propagazione degli ultrasuoni in aria, al fine di poter ottimizzare la configurazione completa per controlli non distruttivi con ultrasuoni senza contatto. Diverse configurazioni di ispezione sono state prima simulate tramite l’analisi numerica e poi riprodotte sperimentalmente per confrontare i risultati. Non-destructive testings (NDT) based on ultrasonic methods are extensively used for their effectiveness and reliability in detecting defects. The generation of ultrasonic waves and propagation in non-regular geometries are difficult to analyze, especially if the source used is a laser. Numerical techniques to simulate the phenomenon found in literature have proved to be limited in their applicability by the frequencies in the MHz range and very short wavelengths. In this paper we present a numerical method to accurately and efficiently solve problems of laser-generated ultrasounds, with frequencies in the MHz range, and propagation in relatively large bodies. Detection is simulated with the propagation of ultrasounds in air, to optimize the complete configuration for non-contact ultrasonic NDT. Different configurations of inspections have been first simulated using numerical analysis and then reproduced experimentally to compare the results.

Laser Powder Deposition (LDP) techniques are being adopted within aerospace and automotive manufacturing to produce innovative precision components. Non-destructive techniques (NDT) for detecting and quantifying flaws within these components enables performance and acceptance criteria to be verified, improving product safety and reducing ongoing maintenance and product repair costs. In this work, software enabled techniques are presented for in-process analysis of NDT laser ultrasonic signals and pulsed laser thermography images of sequential metallic LPD layers. LPD tracks can be as thin as 200μm while deposited at a rate of 500 mm/min, requiring ultrafast inspection and processing times. The research developed analysis algorithms that allow senior engineers to develop inspection templates and profiles for in-process inspection, as well as an end-to-end, user friendly interface for engineers to perform complete manual Laser Ultrasonic or Laser Thermographic inspections. Several algorithms are offered to quantify the flaw size. location and severity. The identified defects can be imported into a sentencing engine which then automatically compares analysis results against the user defined acceptance criteria so that the manufacturing products can be verified. Where both laser ultrasonic and laser thermographic NDT data is available further statistical tools could increase the confidence level of the inspection decision.

A computationally-effective approach for calculating the electromechanical behavior of SWNTs and MWNTs of the dimensions used in nano-electronic devices has been developed. It is a mixed finite element-tight-binding code carefully designed to realize significant time saving in calculating deformation-induced changes in electrical transport properties of the nanotubes. The effect of the MWNT diameter and chirality on the conductance after mechanical deformation was investigated. In case of torsional deformation results revealed the conductance of MWNTs to depend strongly on the diameter, since bigger MWNTs reach much earlier the buckling load under torsion their electrical conductivity changes more easily than in small diameter ones. For the same outer diameter, zig-zag MWNTs are more sensible to twisting with respect to armchair MWNTs since the hexagonal cells are oriented in such a way that they oppose less resistance to the buckling deformations due to torsion. Thus small diameter armchair MWNTs should work better if used as conductor, while big diameter zig-zag MWNTs are more indicated for building sensors.

The use of Rayleigh waves enables the solution of several important inspection problems. Propagation of surface waves along straight boundaries has been properly studied but investigations about their propagation on cylindrical surfaces are not sufficient, despite they can be still of interest for NDE applications. It has been proved experimentally that a surface wave pulse suffers a phase shift during its propagation along a cylindrical surface. A numerical approach has been developed to efficiently study these effects for different materials, curvatures and frequencies. This study can help the scientific community to better understand the phenomenon, quite complex and not yet fully explored.

The aim of this work is the analysis of the contact area in a knee prosthesis using two different approaches. In particular, the interface between the femoral component and the polyethylene insert has been studied both numerically and experimentally. The interest in studying the contact area is related to the fact that the wear of the polyethylene insert, due to the high contact pressures, represents one of the major causes of failure of the total knee prosthesis. The possibility to evaluate the contact area at different loads and mutual position between femur and tibia is, therefore, of fundamental importance to study the service life of a prosthesis and to improve its performance. The finite element numerical approach has required the acquisition, through reverse engineering, and CAD modelling of the prosthetic components. Then the FEM simulations have been developed considering two different load conditions. In order to compare the calculated data, the same load configurations have been used for experimental tests based on ultrasonic method. In this case, some preliminary tests were required to calibrate the system depending on the particular characteristics of materials, geometries and surface finish of the prosthesis. The results show a good correlation between the data obtained with the two different approaches and, consequently, a good level of reliability of the procedures developed for the numerical and experimental evaluation of the contact area. The numerical procedure can be used to determine the area for different angles and loads, but especially in the design phase. The ultrasonic technique can be used to validate the numerical data.

The analysis of guided wave propagation in a plate edge can be useful for some applications, such as for the inspection of the rail base. Here guided waves generated by a laser beam hitting, on the inclined surface, a solid media with trapezoidal cross-section have been studied. We have found that, for the geometry investigated, two guided waves propagate along both free surfaces of the plate, with same velocity and frequency. The analysis of the trajectories of the two material points, due to waves propagation, shows that the wave packets can be identified as Rayleigh waves. The presence of S0 Lamb mode, with low frequency and low amplitude, has been revealed too. As an example of a possible application, the detection of pitting corrosion in the rail base was investigated experimentally and numerically. Results prove that even if it is not possible to access the bottom of the rail, a remote NDE setup working on the upper surface may potentially be used to identify pitting. The numerical method was useful for a better understanding of the phenomenon, thus allowing the optimization of the experimental setup.

Adhesively bonded multi-layer structures are frequently used, mostly in the aerospace industry, for their structural efficiency. Nondestructive evaluation of bond integrity in these types of structures, both after manufacturing and for periodic inspection during service, is extremely important. A laser-based ultrasonic technique has been evaluated for non-contact detection of disbonds in aluminum multi-layer structures. Two configurations have been used to detect disbonded areas: pitch-catch with unidirectional guided wave scan and through-transmission with bidirectional scan. Guided wave scanning was done with a laser line source and air-coupled transducer sensing at 500 kHz, 1 ;MHz, and 2 MHz. Signals showed attenuation of the main frequency component and frequency shift on disbonded areas, whereas, a regular and standard waveform is seen outside disbonds. In through-transmission the longitudinal wave at normal incidence was monitored with a 1 MHz probe. One sample showed, besides the introduced inserts, other disbonded areas. After the ultrasonic measurements the sample was cut to visually check adhesive and interfaces. The guided wave pitch-catch scan allowed fast inspection and quick indication of disbonded zones, while the through-transmission C-Scan provided better definition of defects but was slower and required access from both sides of the test part.

Laser-Air Hybrid Ultrasonic Technique (LAHUT) combines laser generation with air-coupled detection of ultrasound. The technique is non-contact and has the characteristic of operating from remote distances. Acoustic wave laser-generation apparatus can be metres away from the interrogated surface while air-coupled detection standoff can be on the order of several centimetres. The technique has the unique capability of interrogating structural materials in their true industrial environment. Dynamic tests are performed on parts with complex geometry, limited accessibility and curved surfaces. Also, dark and rough finish surfaces, which significantly reduce the efficiency of optical detection techniques, can be interrogated successfully. These characteristics make the LAHUT ideal for many industrial applications including the railroad industry. It was developed for railroad inspections targeting the most critical cracks in rails and wheels. State-of-the-art inspection techniques available to the railroad industry often miss Vertical Split Head (VSH) and Transverse Detail Defect (TDD), which lie in unfavourable positions and orientations in the rail head. No method exists to perform dynamic inspections of the rail base or any part of the railroad wheel. Laboratory experiments were performed for the detection of TDD and Proof Of Concept (POC)field tests were performed for VSH, rail base cracks, thermal fatigue cracks along the wheel flange and tread and subsurface Shattered Rim Cracks (SRC) along the wheel tread. The results were successful and highly repeatable. The technique lends itself for digital collection and automated processing of data making the LAHUT a very strong candidate,for next-generation railroad inspection technique.

The Laser-Air Hybrid Ultrasonic Technique (LAHUT) combines laser generation with air-coupled detection of ultrasound. The technique is non-contact and has the characteristic of operating from remote distances. Acoustic wave laser-generation apparatus can be metres away from the interrogated surface, while air-coupled detection stand-off can be on the order of several centimetres. The technique has the unique capability of interrogating structural materials in their true industrial environment. Dynamic tests are performed on parts with complex geometry, limited accessibility and curved surfaces. Also, dark and rough finish surfaces, which significantly reduce the efficiency of optical detection techniques, can be interrogated successfully. These characteristics make the LAHUT ideal for many industrial applications including the rail industry. It was developed for railroad inspections targeting the most critical cracks in rails and wheels. State-of-the-art inspection techniques available to the rail industry often miss Vertical Split Head (VSH) and Transverse Detail Defect (TDD), which lie in unfavourable positions and orientations in the rail head. No method exists to perform dynamic inspections of the rail base or any part of the railroad wheel. Laboratory experiments were performed for the detection of TDD and Proof Of Concept (POC) field tests were performed for VSH, rail base cracks, thermal fatigue cracks along the wheel flange and tread and subsurface Shattered Rim Cracks (SRC) along the wheel tread. The results were successful and highly repeatable. The technique lennds itself for digital collection anal automated processing of data, making the LAHUT a very strong candidate for next-generation rail inspection technique.

Long- and short-range acoustic-signal interaction with surface-breaking cracks and geometric boundaries of the rail-wheel tread area are presented in this article. Ultrasonic signals are generated using a laser-line source of varying length and distance from a crack and are detected with a 1-MHz contact transducer to map the sound field behind the crack in the near, intermediate, and far fields of the insonified region. The factors that affect the behavior of a laser-generated surface acoustic wave propagating along the tread area of a rail wheel are discussed. A signal normalization method is proposed to help in sensing the presence of a crack from the transmitted signal unaffected by the boundary effect of the complicated wheel geometry, diffraction of the acoustic wave around the crack tip, and source-to-crack length ratio and separation distance.

Two non-contact remote ultrasonic detection systems are compared. The first consists of a continuous wave (CW) Nd:YVO4 laser operating at 532 nm used in conjunction with a photo-induced electromotive-force (EMF) sensor. The second system employs an air-coupled capacitance transducer that has a bandwidth from 100 kHz up to 2 MHz. Both systems exhibit higher sensitivity to out-of-plane than in-plane measurements of stress wave signals. Ultrasound generation is carried out by an Nd:YAG pulsed laser at 532 nm in the thermoelastic regime in a 1.6 mm aluminum plate. A0 Lamb mode is generated at frequency-thickness product with strong out-of-plane displacements. Ultrasonic waveforms acquired are reported and the performance advantages of the two remote non-contact detectors are compared with each other. For lower frequency tests (up to 2 MHz), better results have been achieved with the air-coupled transducer that also has superior overall performance and benefits. The tests enabled direct comparison of air-coupl...

The assessment of skin/core bond in honeycomb samples was carried out by a bistatic ultrasonic non-contact system. Lamb waves were generated by a Nd:YAG pulsed laser and detected by an air-coupled capacitance transducer located in pitch-catch configuration. Periodically spaced line sources, generated on the sample surface using a four element lenticular array, allowed generation of narrow-band directional Lamb waves. Changes in amplitude of the transmitted signal permitted detection of skin/core disbonds.

The results of an experimental study conducted to detect subsurface defects in a thick Gr/PPS composite test sample using a noncontact ultrasonic system are presented. Surface waves are generated by a pulsed laser and detected by an air-coupled capacitance transducer. By controlling the surface wave wavelength through a shadow mask, it is possible to control surface wave penetration depth in the sample. Surface wave peak-to-peak amplitude is related to the near-surface material condition. Results indicate that signal amplitude decreases as the width of the defect increases and an approximately linear relation can be deduced.