Detecting structural defects is one of the primary challenges engineers face. Consequently, the development of techniques and methods capable of detecting structural defects has always been critical. It should be emphasized that crack detection is only meaningful if it occurs before the final stages of structural failure. Accordingly, the early identification of structural defects has become a significant research challenge, motivating the development of techniques and diagnostic parameters that can effectively capture and reflect the structure’s nonlinearity or non-uniform behavior. This study aims to provide a more detailed examination of modulation phenomena observed in the measured response using the vibro-acoustic modulation (VAM) method, and propose a new model that simultaneously incorporates all three conventional modulation types (amplitude, frequency, and phase), which may offer a more accurate representation of the response signal behavior. Both theoretical and experimental results clearly confirm that the phase shifts of individual frequency components in the frequency domain vary throughout the lifetime of the tested specimen. This behavior, as anticipated by the proposed model, reveals a strong correlation between phase shifts and modulation indices (MIs). Furthermore, the relative sensitivity analysis indicates that the phase shift is more sensitive than the modulation index (MI), suggesting its strong potential as an indicator for early defect detection in structural components.

Welded joints suffer from reduced fatigue life due to geometric stress concentrations and metallurgical changes in the heat‐affected zone that promote crack initiation under cyclic loading. This study investigates a novel postweld treatment, utilizing a Cu/Ni nanometallic multilayer thin film deposited onto the welded butt joint. Deposition current densities and individual Cu/Ni layer thicknesses are systematically varied to optimize fatigue performance. A multiscale residual stress (RS) analysis characterizes stress states within individual multilayers and in the steel substrate, indicating all substrate RS are compressive in nature after thin film deposition. Results demonstrate a direct correlation between compressive RS magnitude and fatigue strength improvement. Tested at Δ σ R = 0.75 × f y , a more than a 300% increase in cycles to failure is seen compared to the as‐welded condition. This postweld treatment offers a promising approach for extending the operational life of welded structures across industrial applications.

Given the substantial environmental and economic consequences associated with deteriorating infrastructure, prolonging the service life of steel bridges is essential for advancing sustainability and economic viability. A critical factor in achieving this goal is enhancing the integrity of welded joints, which play a pivotal role in the bridge’s performance under dynamic loading conditions. Research indicates that the fatigue strength of welds can be increased by up to sixfold through the application of a nanometallic multilayer (NMM) composed of nickel and copper. To date, investigations into the fatigue enhancement of welded joints using NMM have predominantly been conducted on a laboratory scale. To enable practical application on existing infrastructure, a coating device is being developed to apply the NMM treatment directly onto surfaces without the need for full immersion in an electrolyte bath, thus supporting its implementation as a post-treatment process for steel bridges.

While traditional post weld treatments intend to reduce the fatigue failure potential by changing the weld seam geometry, introduction of compressive residual stresses and shielding off environmental impacts, the novel nanostructured metallic multilayer (NMM) post-weld treatment covers all three mechanisms simultaneously. NMM offer combined high strength and ductility and a significantly enhanced fatigue resistance. In a recent study a strong enhancement in fatigue resistance was detected. Utilizing energy-dispersive X-Ray diffraction techniques at the P61A-beamline at the German Synchrotron (DESY), it was found that residual stresses generated during the deposition process play a crucial role in this enhancement. Specifically, tensile stresses within the nanolaminate induce beneficial compressive stresses in the underlying substrate, effectively inhibiting fatigue crack initiation and resulting in an unprecedented increase in fatigue strength. NMM treatment of the double-V weld increases the fatigue strength from FAT class 80 to 225. This paper investigates which process parameters optimize the compressive stress profile in the steel base material, paving the way for the NMM post-weld treatment to reliably and economically contribute to longevity of cyclically loaded metal infrastructure.

Vibro-acoustic modulation (VAM) has been exploited over the last three decades to assess and monitor the integrity of structures. One major challenge is the separation of damage-induced and non-damage-induced modulation in the measured system response for reliable structural health monitoring (SHM). Most scientific works on VAM imply that the initiation and growth of structural damage is expected to cause modulation that adds up with non-damage-induced modulation increasing the total amount of modulation. This article unfolds why this assumption can be invalid for standard VAM applications: It is explained analytically why two nonlinearities working in opposite directions (one stiffening the structure under loading, one softening it) cause contrary modulations: The two nonlinear contributions can neutralize each other in the system response. Numerical simulations are then presented that investigate separately one damage-induced nonlinearity and two non-damage-induced nonlinearities in the same aluminum plate. The modulation caused by them individually is quantified and the subsequent comparison demonstrates the occurrence of contrary modulations in this representative VAM setup. It has to be concluded that damage-induced modulation does not necessarily increase the total modulation in the system response. This finding has potential to boost VAM-related research regarding its reliability and sensitivity.

Merging the macro-level design with the nano-level design in structural engineering, hence, using the superior properties of nanostructured metallic multilayers for protecting fatigue-critical joints of the macrostructure and ensuring the structural integrity of the steel infrastructure are the objectves of a research effort at TU Hamburg. Nanostructured metallic multilayers (NMM) have significantly higher strength, fatigue resistance and ductility than monolithic homogeneous metal cross sections. The superior structural properties of these nanostructured cross sections are known, and so it is surprising why no attempt has been made to date to use nanostructured cross sections in macro cross sections in structural engineering to improve the cross section properties. This paper links the advantages of nanostructured multilayers with the needs of homogeneous metallic macro-cross sections and examines the question to which extent the high-performance material nanolaminate can compensate for the structural weak parts of metallic infrastructure. The welded joint subjected to fatigue is addressed as vulnerable part of metallic infrastructure. The article provides insights on how nanostructured multilayer can potentially contribute to the future of steel construction, further, how nanostructured multilayer can potentially affect fatigue design. The design as well as the maintenance of cyclically loaded metallic infrastructures, such as bridges and offshore wind turbines, are discussed herein and it is shown how sustainability, resource conservation, reduction of CO2 footprint, readiness, security of supply and economic viability of steel infrastructure can potentially be achieved.

In recent years, the adoption of Wire Arc Additive Manufacturing (WAAM), now defined as Directed Energy Deposition based on Gas Metal Arc Welding (DED-Arc), in steel construction has increased significantly. However, the sequential layer deposition inevitably creates surface notches that cause high stress concentrations, leading to fatigue crack initiation. The current industrial standard for post-processing, CNC milling, is time-consuming and resource-intensive. A novel research approach focuses on directly controlling critical residual stresses of as-built specimens by introducing near-surface compressive residual stresses using a Cu/Ni nanostructured metallic multilayer (NMM). This study investigates the effect of NMM on DED-Arc structures and extends its application to metallic 3D-printed components. Optical microscopy provides detailed surface morphology and reliable roughness measurements, while X-ray diffraction (XRD) confirms the presence of residual tensile stresses in the NMM and the resulting residual compressive stresses in the steel substrate. Preliminary tension-tension fatigue testing provides insights into the fatigue strength increase due to NMM treatment of DED-Arc dogbone specimen.

Nanostructured metallic multilayers have significantly higher strength, fatigue resistance and ductility than monolithic homogeneous metal cross sections. The superior structural properties of these nanostructured cross sections are known, and so it is surprising why no attempt has been made to date to use nanostructured cross sections in macro cross sections in structural engineering to improve the cross section properties. This paper links the advantages of nanostructured multilayers with the needs of homogeneous metallic macro-cross sections and examines the question of the extent to which the high-performance material nanolaminate can compensate for the structural weak points of metallic infrastructure. The following weak points of metallic infrastructure are addressed: 1) the welded connection subjected to fatigue and corrosion, 2) additively printed metallic cross sections exposed to fatigue and corrosion. The article provides food for thought on how nanostructured multilayer can potentially contribute to the future of steel construction. As an example, the design and maintenance of cyclically loaded metallic infrastructure structures, such as bridges and offshore wind turbines, are discussed and it is shown how sustainability, resource conservation, reduction of the CO2 footprint, availability, security of supply and economic viability of steel infrastructure structures can potentially be achieved.

The potential of employing nanostructured metallic multilayer to increase the durability and extend the service life of welded joints of metal structures is analyzed. Using a nickel and copper-based multilayer nanocoating as an example, a notable increase in the durability of the welded joint of up to 300...600% is observed. Corrosion tests reveal that the use of nickel and copper nanostructured metallic multilayer leads to the localization of corrosion processes at the "base metal-nanocoating" boundary. Considering the significant improvement of fatigue characteristics of welded joints and lower corrosion rate compared to welds without lamination, Ni-Cu nanocoatings can be used on offshore structures, provided that the condition of the protective anti-corrosion coating is monitored to mitigate the risk of galvanic corrosion at the base metal-nanolamination boundary.

AbstractNanostrukturierte metallische Multilayer weisen eine wesentlich höhere Festigkeit, Ermüdungsfestigkeit und Duktilität als monolithische homogene Metallquerschnitte auf. Die überragenden strukturellen Eigenschaften nanostrukturierter Querschnitte sind bekannt, und so ist verwunderlich, warum bis heute kein Versuch unternommen wurde, nanostrukturierte Querschnitte in Makroquerschnitten im Konstruktiven Ingenieurbau zur Verbesserung der Querschnittseigenschaften einzusetzen. Dieser Aufsatz verknüpft die Vorteile der nanostrukturierten Multilayer mit den Bedürfnissen der homogenen metallischen Makroquerschnitte und geht der Frage nach, inwieweit das Hochleistungsmaterial Nanolaminat die konstruktiven Schwachstellen metallischer Infrastruktur ausgleichen kann. Die folgenden Schwachstellen der metallischen Konstruktion werden adressiert: 1) die Schweißnaht unter Ermüdungsbeanspruchung und Korrosion, 2) additiv gedruckte metallische Querschnitte unter Ermüdungsbeanspruchung und Korrosion. Der Aufsatz gibt Denkanstöße, wie nanostrukturierte Querschnitte möglicherweise die Zukunft im Stahlbau mitbestimmen können. Beispielhaft wird auf die Bemessung und Wartung von ermüdungsbeanspruchten metallischen Infrastrukturbauwerken, wie Brücken und Offshore‐Windenergieanlagen, eingegangen und aufgezeigt, wie Nachhaltigkeit, Ressourcenschonung, Senkung des CO2‐Fußabdrucks, Verfügbarkeit, Versorgungssicherheit und Wirtschaftlichkeit von Stahl‐Infrastrukturbauwerken potenziell erreichbar sind.

This paper reviews the state-of-the-art approaches in defect localization and specifies the remaining questions and challenges. Furthermore, this study presents a novel defect localization methodology using the nonlinear interaction of primary Lamb wave modes and vibroacoustic modulation (VAM), combined with damage imaging, to address the current shortcomings of defect localization. The study investigates this methodology experimentally with respect to defect interpretation, resolution, and applicability. Two Lamb waves with high and low frequencies, one being continuous and the other a tone burst, were excited using two different piezoelectric sensors. The amplitude of the measured signal at the first sideband frequency was evaluated with a short-time Fourier transform (STFT) and used for damage imaging via the delay and sum method. This study also includes a discussion on identifying the source of nonlinearity reflected in the first sideband. The experimental measurements prove that the localization of defect nonlinearity is possible with high accuracy, without the need for a baseline measurement, and with a minimum number of sensors. Sensitivity measurements with respect to the required length of the high-frequency tone burst and the sensor arrangement were also conducted.

The vibroacoustic modulation (VAM) is a nonlinear ultrasonic testing method that utilizes the modulation of a high-frequency/low-amplitude probe wave with a low-frequency/high-amplitude pumping vibration, resulting in high sensitivities to damages in the structure. However, applying the method outside of the laboratory as a structural health monitoring system for actual structures is challenging, since the amplitude and frequency of the ambient vibrations – which is ideally utilized as pumping vibration – fluctuates over time. To circumvent this, we present a synthetic generation of the VAM signal using only the probe measurements , acquired at two (or more) stress levels of the structure when a steady state is reached. We could show that only 16 values (8 measured values of only two stress levels with a sampling frequency of 1/20 of the Nyquist frequency) are required to generate the often calculated Modulation Index with a mean deviation of 0.97% to the expected dynamic measured values for glass fiber reinforced composites and 1.86% for the aluminum specimens, which is negligible compared to a typical increase of the Modulation Index of 10–30 dB in case of severe damage. Even undersampled measurements at each stress level can be used without sacrificing accuracy, which reduces the sensing requirements for the sensor nodes. Moreover, this method decouples VAM from the actual need for a constant recurrent frequency and amplitude of the natural vibration in order to reliably compare measurements throughout the lifetime. Hence, this work aims to open the possibility of ultimately applying VAM to assess the structural health of complex structures.

Abstract Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300% − 600% increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds. Material mechanisms responsible for the fatigue strength increase are introduced herein. A case study shows that the design of offshore wind turbine support structures applying NMM post-weld treatment enables a lifetime extension as well as a 28% weight reduction compared to the structure without post-weld treatment.

Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300–600% increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds. Material mechanisms responsible for the fatigue strength increase are introduced herein. A case study shows that the design of offshore wind turbine support structures applying NMM post-weld treatment enables a lifetime extension as well as a 28% weight reduction compared to the structure without post-weld treatment.

A comprehensive understanding of the linear/nonlinear dynamic behavior of wireless microresonators is essential for micro-electromechanical systems (MEMS) design optimization. This study investigates the dynamic behaviour of a magnetoelectric (ME) microresonator, using a finite element method (FEM) and machine learning algorithm. First, the linear/nonlinear behaviour of a fabricated thin-film ME microactuator is assessed in both the time domain and frequency spectrum. Next, a data driven system identification (DDSI) procedure and simulated annealing (SA) method are implemented to reconstruct differential equations from measured datasets. The Duffing equation is employed to replicate the dynamic behavior of the ME microactuator. The Duffing coefficients such as mass, stiffness, damping, force amplitude, and excitation frequency are considered as input parameters. Meanwhile, the microactuator displacement is taken as the output parameter, which is measured experimentally via a laser Doppler vibrometer (LDV) device. To determine the optimal range and step size for input parameters, the sensitivity analysis is conducted using Latin hypercube sampling (LHS). The peak index matching (PIM) and correlation coefficient (CC) are considered assessment criteria for the objective function. The data-driven developed models are subsequently employed to reconstruct/predict mode shapes and the vibration amplitude over the time domain. The effect of driving signal nonlinearity and total harmonic distortion (THD) is explored experimentally under resonance and sub-resonance conditions. The vibration measurements reveal that as excitation levels increase, hysteresis variations become more noticeable, which may result in a higher prediction error in the Duffing array model. The verification test indicates that the first bending mode reconstructs reasonably with a prediction accuracy of about 92 percent. This proof-of-concept study demonstrates that the simulated annealing approach is a promising tool for modeling the dynamic behavior of MEMS systems, making it a strong candidate for real-world applications.

Metal nanolaminate coatings are introduced as a new approach in post-weld treatment methods. A Cu/Ni nanolaminate coating is electrodeposited from a single Cu/Ni citrate bath onto a butt-welded tension-tension fatigue specimen. The nanolaminate coating consists of a Ni base layer and 160 alternating Cu and Ni layers. The specimen is tested in tension-tension fatigue with a stress range close to the yield strength of the specimen. This first study reveals surprisingly high lifetime extensions of welded joints. The tested specimens are examined using FIB/SEM and TEM. Local roughness measurements are carried out with AFM. This leads to observations on crack behavior of nanostructured Cu/Ni multilayers. The Cu layers show initial multi-crack formation, while the cracks arrest at the Cu/Ni interfaces. The Ni layers bridge those cracks and each Ni layer tears individually. Hypotheses are formed on the fatigue behaviour of Cu/Ni multilayers.

StahlbauVolume 91, Issue 1 JahresinhaltsverzeichnisFree Access Jahresinhaltsverzeichnis Stahlbau 2021 First published: 03 January 2022 https://doi.org/10.1002/stab.202270109AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume91, Issue1January 2022 RelatedInformation

This article introduces a simple physical explanation that demonstrates why a local or global nonlinearity in a structure causes amplitude modulation and phase modulation in the system response, when employing the vibro-acoustic modulation method for structural health monitoring (SHM) applications. The underlying major assumption and the explanation itself are validated experimentally and numerically. Afterwards, the explanation is used to demonstrate comprehensively why very different types of envelope functions can be observed in the system response, depending on the exciting ultrasonic carrier frequency ω. The beauty of the suggested explanation is that it does not only work for local nonlinearities, such as exhibited by defects, but also for any non-damage induced (local or global) nonlinearity in the system which can cause modulation as well. A physical understanding of cause and effect is of crucial importance here, because the modulation caused by non-damage induced nonlinearities might camouflage the effects of local damage that one is actually interested in during SHM applications.

Laminated metal composites are a promising design since the hybrid design enables superior and tailorable material properties compared with bulk material. The article introduces for the first time, laminated metal composites consisting of multiple bilayers of alternating layers of ductile and high-strength steel processed by wire arc additive manufacturing (WAAM). The layup of the laminated metal composites is built up by alternating deposits made of ductile steel and high-strength steel type wires. Governing parameters in the fabrication process affecting the material properties, such as dilution, are discussed. Enhanced material properties of the laminated metal composites fabricated by WAAM are investigated under static tensile, impact and tension-tension high-cycle-fatigue loading and compared to the relating homogenous weld metal. Potential reasons for the retardation of crack propagation in laminated metal composites fabricated by WAAM compared to findings in roll-bonded laminated metal composites are discussed. WAAM is conducted by a collaborative robot providing a high level of flexibility in respect to geometry and scalability. Tailorability of material properties through WAAM-fabricated laminated metal composites adds an important layer of flexibility which has not been explored yet.

Wartung und Inspektion von dynamisch beanspruchter stählerner Infrastruktur sind kostenintensiv. Die Frequenz der Inspektionen steigt zum Ende der Lebensdauer an. Schwachstellen einer Konstruktion sind meist die konstruktiven Verbindungen. Wirtschaftliche Sanierungsmaßnahmen von ermüdeten konstruktiven Verbindungen, die kostspielige Inspektionsintervalle unnötig machen, wurden bis jetzt nicht in die breite Praxis übertragen. In diesem Aufsatz wird mit der Nanolaminatbeschichtung, auch Nanolaminatpflaster genannt, ein neues Schweißnahtnachbehandlungsverfahren vorgestellt. Eine Ermüdungsstudie im Zugschwellbereich mit geschweißten und mit Nanolaminat beschichteten Stahlproben zeigt erstmals das Potenzial dieses Schweißnahtnachbehandlungsverfahrens hinsichtlich Lebensdauerverlängerung, auch gegenüber anderen bekannten Schweißnahtnachbehandlungsverfahren. Durch die Nanolaminatbeschichtung ergeben sich in dieser Vorstudie vielversprechende Einzelergebnisse: Bei einer Spannungsschwingbreite nahe der Streckgrenze wird die Lebensdauer im Mittel um das Drei‐ bzw. 4,5‐Fache erhöht. Die Nanolaminatbeschichtung wird im Vergleich mit anderen Schweißnahtnachbehandlungsverfahren diskutiert. Die Schweißnahtnachbehandlung mit Nanolaminatbeschichtungen erscheint industrieübergreifend von großem Interesse und ist bei neuen oder bestehenden Konstruktionen anwendbar.