The main purpose of the present review paper is to illustrate the principal achievements of numerous researchers who have studied the non-linear effects caused by a closing crack in the most common ...types of structural elements such as beams, shafts and plates, the aim being to assess the potential and future prospects of using non-linear behaviour to detect damage. Indeed, for a wide range of practical applications, in order to avoid catastrophic failure, the development of diagnostics techniques which are sufficiently sensitive to incipient cracks in structures and machines is a crucial issue. The main potential advantage of using vibration diagnostics based on the use of non-linear effects is the relatively high sensitivity to the damage of the closing crack type, especially for application to beam-like structures and rotating shafts; instead the potential for application to plate-like structures has been found to be limited.
After analysing the state-of-art on this subject in detail, a discussion of the respective merits, drawbacks and prospects of a range of non-linear vibration methods for structural damage detection is presented. The general conclusion which can be drawn from the highly encouraging results of recent research is that further development of these techniques for non-destructive testing of structures with closing cracks would be highly worthwhile.
•Non-linear effects caused by a closing crack in structural elements are illustrated.•Single-degree-of-freedom oscillators simulating cracked elastic bodies are compared•Non-linear effects at free and forced vibrations of cracked beams are analysed.•The features of vibrations of damaged plates and rotating shafts are addressed.•Advantages, problems and prospects of non-linear vibration diagnostics are discussed.
•The theory for obtaining natural frequencies and mode shapes of a damaged multi-span beam is shown.•The model of crack-like damage in the hollow concrete beam is developed.•Frequencies ratios are ...proposed as damage characteristics in varying environmental conditions.•Analytical results are presented for a multi-span beam-like structure simulating a bridge.•Experimental results are presented for two real structures: the Z24 bridge - the Sanctuary of Vicoforte.
One of the main problems of damage vibration diagnostics based on the change of natural frequencies of structures is the influence of ambient and operational factors (temperature, humidity, wind, heavy traffic etc.) on natural frequencies. In some cases, this effect is comparable or even more significant than the effect of damage itself and reliable diagnostics of damage becomes problematic. In this paper, the use of the ratios between natural frequencies of different mode shapes is proposed as a characteristic of damage rather than the change of natural frequencies. The study has demonstrated that these ratios are fundamentally unaffected by the environmental and operational factors. Application of the proposed method is illustrated using the examples of the Z24 prestressed concrete bridge and the Sanctuary of Vicoforte. Firstly, an analytical study was performed to calculate the mode shapes and natural frequencies of damaged multi-span beam-like structure taking into account the model of crack-like damage in a hollow concrete beam. The results reveal that, in order to identify local damage of subcritical size, natural frequency data of at least four modes of vibration are necessary. Secondly, the natural frequencies acquired experimentally on the Z24 bridge and Sanctuary of Vicoforte have been used to prove that the ratios between natural frequencies can be a useful diagnostic tool in the field of structural health monitoring.
Optical measurements from high‐speed, high‐definition video recordings can be used to define the full‐field dynamics of a structure. By comparing the dynamic responses resulting from both damaged and ...undamaged elements, structural health monitoring can be carried out, similarly as with mounted transducers. Unlike the physical sensors, which provide point‐wise measurements and a limited number of output channels, high‐quality video recording allows very spatially dense information. Moreover, video acquisition is a noncontact technique. This guarantees that any anomaly in the dynamic behaviour can be more easily correlated to damage and not to added mass or stiffness due to the installed sensors.
However, in real‐life scenarios, the vibrations due to environmental input are often so small that they are indistinguishable from measurement noise if conventional image‐based techniques are applied. In order to improve the signal‐to‐noise ratio in low‐amplitude measurements, phase‐based motion magnification has been recently proposed.
This study intends to show that model‐based structural health monitoring can be performed on modal data and time histories processed with phase‐based motion magnification, whereas unamplified vibrations would be too small for being successfully exploited. All the experiments were performed on a multidamaged box beam with different damage sizes and angles.
•The theory for calculation lateral eigen frequencies of damaged multi-span bridge is developed.•The model of crack in the box-like concrete beam is developed.•The ratios of eigen frequencies for ...various mode shapes are proposed as ambient factors insensitive indicator of damage.•Verification of the proposed indicator of damage is carried out based on the operational database for the Z24.
Vibration based damage detection usually faces two fundamental problems, namely thelow sensitivity to the relatively small but potentially dangerous damage, and the dependence of vibration characteristics of structures on ambient factors (primarily temperature). Indeed, the effect of ambient factors on the vibration characteristics may be more significant than the influence of damage. In such situations, the reliable detection of damage is practically impossible. To solve this problem, the ratios of the natural frequencies for various mode shapes have been used. As the presented investigation reveals, these ratios are independent of the ambient factors and hence canimprovesubstantiallythe sensitivity and reliability of damage detection. To illustrate the higher efficiency of the proposed ratios applied to damage detection, the analytical model of the reinforced Z24 concrete bridge with the damage of crack type was developed. The natural frequencies of bending vibrations in vertical and horizontal planes at different crack depths and locations were calculated and compared with those measured on the Z24 bridge, demonstrating that the sensitivity of the natural frequencies change and ratios of frequencies for various mode shapes with respect to the size of damage is of the same order. At the same time, as opposed to the change of natural frequencies, the frequency ratios are dependent only on the parameters of damage, which makes them a much more efficient tool for health monitoring of structures operating in variable ambient conditions.
In this study, Computer Vision and Phase-Based Motion Magnification (PBMM) are validated for continuous Structural Health Monitoring (SHM) purposes. The aim is to identify the exact instant of ...occurrence for damage or abrupt structural changes from video-extracted, very low amplitude (barely visible) vibrations. The study presents three experimental datasets: a box beam with multiple saw cuts of different lengths and angles, a beam with a full rectangular cross section and a mass added at the tip, and the spar of a prototype High-Aspect-Ratio wing. Both mode-shape- and frequency-based approaches are considered, showing the potential to identify the severity and position of the damage as well A high-definition, high-speed camera and a low-cost commercial alternative have been successfully utilised for these video acquisitions. Finally, the technique is also preliminarily tested for outdoor applications with smartphone cameras.
The topic of non-destructively detecting localised damage in plates is addressed in this article. Since the presence of a crack or a delamination causes a discontinuity in the mode shape first ...derivatives, a numerical method for detecting discontinuities in smooth piecewise functions and their derivatives, based on a polynomial-annihilation technique is presented. The method, already proposed for beam-type structures, has been extended to enable the detection and localisation of damage in plate-like structures for which only post-damage mode shapes are available. Applying finite element analysis, the mode shapes of an isotropic plate with a saw-cut and a multi-layered composite plate with a delamination have been calculated and the performance of the approach evaluated for increasing amounts of noise. Encouraging results indicate that further development of the technique for non-destructive testing of plate-like structures would be highly worthwhile.
The experimental acquisition of large vibrations presents various technical difficulties. Especially in the case of geometric nonlinearities, dealing with very flexible, very light structures causes ...minimal variations in mass or stiffness to affect severely the dynamical response. Thus, sensors' added masses change the behaviour of the structure with respect to the unloaded condition. Moreover, the most common tools regularly employed for acquisition in vibration analysis - that is to say, laser vibrometers and accelerometers - are often designed with small amplitudes in mind. Their recordings are known to lack accuracy when the investigated structure undergoes large or very large motions, due to geometrical reasons. Image-based measurement techniques offer a valid solution to this problem. Here, an ensemble of three video processing techniques are benchmarked against each other and tested as viable options for the non-contact dynamic characterisation of slender beam-like structures. The methods have been applied to the case study of an aluminium spar for a highly-flexible airwing prototype and compared to the measurements recorded by a laser velocimeter and several Raspberry PI Inertial Measurement Units (IMUs), which also proved to be minimally invasive.
The interest in the properties of animal soft tissues is often related to the desire to find an animal model to replace human counterparts due to the unsteady availability of human tissues for ...experimental purposes. Once the most appropriate animal model is identified, it is possible to carry out ex-vivo and in-vivo studies for the repair of ligamentous tissues and performance testing of replacement and support healing devices. This work aims to present a systematic review of the mechanical properties of ligaments reported in the scientific literature by considering different anatomical regions in humans and several animal species. This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method. Moreover, considering the lack of a standard protocol for preconditioning of tissues, this aspect is also addressed. Ninety-six studies were selected for the systematic review and analysed. The mechanical properties of different animal species are reported and summarised in tables. Only results from studies reporting the strain rate parameter were considered for comparison with human ligaments, as they were deemed more reliable. Elastic modulus, ultimate tensile stress, and ultimate strain properties are graphically reported identifying the range of values for each animal species and to facilitate comparison between values reported in the scientific literature in animal and human ligaments. Useful similarities between the mechanical properties of swine, cow, and rat and human ligaments have been found.
A crack or a localized damage in a structure provokes a discontinuity in the rotation. Consequently, mode-shapes are nonsmooth at the damage position and the first derivative (strictly related to the ...rotation) presents a jump discontinuity. Based on this simple concept, a new approach has been developed in order to predict the location of the mode-shape derivative discontinuities, and therefore the location of damage, without the need to directly differentiate. This approach applies a Gaussian process regression to the mode-shape data, using a covariance function which allows for a nonsmooth point; this point, which indicates the crack position, can be determined by a maximum likelihood algorithm. Using a finite-element model of a cracked beam, the performance of this methodology has been analyzed for the case of single crack and multiple cracks, for increasing amounts of noise. The effects of several parameters (damage position, damage severity, number of measurement points and of considered mode-shapes) on the approach accuracy have also been investigated. Finally, the method has been verified using experimental data coming from a vibrating steel beam with a cut. Results are encouraging and indicate that further developments of the technique for nondestructive testing of beam-like structures would be highly worthwhile.
The electro-mobility is becoming an increasingly present reality in recent years. The most important drawback of this technology is known to be limited battery autonomy. In an attempt to overcome ...this problem, for specific studies and testing, a number of roads have been implemented with coil systems in order to transfer power to electric vehicles, as described in this article. While on the one hand this could solve the problem of charging, on the other hand the introduction of a technology within an existing infrastructure could result in further structural issues. Since little or no information on the possible structural effect of the introduction of a charging system in the road is currently available, this study has focused on the long-term fatigue analysis of an electric road infrastructure in which an inductive wireless charging system has been introduced into the road structure. To perform the fatigue analysis, a recursive procedure defined within a probabilistic framework was developed and applied to a benchmark case study. The results obtained from the analysis represent an initial database for the definition of strategies and protocols for the monitoring, maintenance and operations of future electric roads infrastructures.
PDF
