Key Engineering Materials Vols. 569-570

Abstract: Helicopters are very critical aircrafts from the point of view of fatigue loads. A structural damage could grow fast to critical size because of the wide range of manoeuvre loads. As a consequence, scheduled maintenance is a major cost during the operative life of the aircraft. The most advanced methodologies for Structural Health Monitoring and Prognostic Health Management available today are aimed to provide a real-time structural diagnosis, thus maximizing the availability of the helicopter and reducing the maintenance costs. However, on-board diagnostic systems might be gradually introduced in the current maintenance procedures, trying to minimize the risk associated to these newly developed technologies. The work presented inside this paper is about the simulation of the integration of real-time diagnosis and prognosis into a typical scheduled maintenance procedure. A diagnostic unit is capable for anomaly detection and damage quantification (in terms of crack length). It is trained with Finite Element simulated damages and tested with real experimental data. The diagnostic output is then processed in a particle filter algorithm, based on sequential importance sampling technique, aimed at refining the estimation of the structural condition as well as to update the inference on the residual useful life distribution. The coupling of real-time diagnosis with off-line measures (taken during scheduled maintenance stops) is analyzed and applied to a damage tolerant structure, trying to outline the advantages and drawbacks of the proposed approach.

Abstract: The subject of this research is the implementation of a Smart Structural Health Monitoring based on structural analysis and its validation on the plate under compressive loading. In the given context the structural analysis provides the most probable failure mode of the structure, e.g. buckling and the stress and deformation state at failure. With this information an algorithm analyses the onset of failure and infers from these analysis results the measurement values, which have to be monitored to identify the damage. By means of this, an appropriate sensor and the optimal sensor placement are chosen. As this pre-process supports the damage identification it is referred to as Smart Structural Health Monitoring (SSHM). The potential of SSHM to optimize the damage identification process is compared to a simple Structural Health Monitoring is demonstrated with a plate under compressive loading. In an experimental setup the plate is loaded up to buckling. After buckling the loading still increases up to failure of the plate caused by the deformations in the post buckling region. With a minimum of sensors and in combination with the analysis results of the SSHM the damage of the plate is detected, located and quantified.

Abstract: The bond between concrete and steel is the critical element of reinforced concrete (RC) structures, which directly affects their load carrying capacity and serviceability. Hence the evaluation of bond strength degradation is an essential parameter to predict the residual strength of RC structures affected by reinforcement corrosion. Existing research studies in this field mainly focus on numerical and experimental investigations. Few attempts have been made using analytical approach but there is still a need of reliable model which considers the critical mechanical factors affecting the bond strength of corroded RC structures. This paper presents a simple and realistic analytical model of bond strength degradation by using fracture mechanics combining the action of adhesion, confining pressure and corrosion pressure at steel concrete interface. Finally, the results obtained from the proposed model are examined with published experimental data. The study demonstrates that the proposed analytical model agrees with the experimental data of existing investigations.

Abstract: A railway bridge has been the object of investigation in the context of structural health monitoring (SHM). The current work is focused on utilization of experimental data for refining a numerical model of the structure as well as on tests of dynamic excitations using a controlled hydraulic shaker and passing trains. The numerical model has been matched to experimental measurements using experimental modal analysis - classical and operational. The tailored SHM system for monitoring of the bridge consists of 15 piezoelectric strain sensors taking advantage of wireless communication for data transfer. Experimental responses of the bridge collected by the SHM system are confronted with the ones produced by the FE numerical model of the bridge. The long-term objective of the investigation is to elaborate a method for assessment of structural condition and prediction of remaining lifetime of the bridge.

Abstract: This work proposes to study the effects of physical parameters and loading conditions on both dynamic and acoustic responses of a brake system subjected to squeal. A simplified brake system model composed of a disc and a pad is investigated. The friction interface is modeled by introducing linear and non-linear stiffnesses at several local nodes to model contact. The classical Coulomb law is applied to model friction and the friction coefficient is assumed to be constant. A stability analysis of this system is performed with respect to the friction coefficient and the hydraulic brake pressure. Then self-excited vibrations are investigated for two cases of loading conditions: static loading and ramp loading. Time responses for these cases are significantly different: the case with ramp loading presents higher amplitude of velocity than the static loading case. For the case with ramp loading, the spectrum analysis performed by the Continuous Wavelet Transform, shows the appearance of the fundamental frequencies of unstable modes but also their harmonics and combinations frequencies. Sound pressures radiated during squeal event present different peculiar patterns of directivity for both cases and for a progressive load, the levels are significantly higher.

Abstract: Crack detection in structures has been one of the active research areas for decades. Several crack detection methods have been suggested in the literature with their own advantages and limitations. In this paper, a very simplified method has been presented which assumes the vibration of entire structure can be scanned through a laser vibrometer. Therefore, measurement at a large number of locations in any structure is possible. Excitation of the cracked structure at a frequency always generates higher harmonic components of the exciting frequency due to the breathing of the crack. The deflection of the structure at higher harmonics of the exciting frequency is mapped by (a) amplitude of deflection (AOD) at higher harmonics as suggested in the literature, and (b) a new method based on the operational deflection shape (ODS) (both amplitude and phase) at higher harmonics for the crack detection. This paper presents the proposed method and the results through numerical examples.

Abstract: Cracking is a common type of failure in machines and structures. Cracks must be detected at an early stage before catastrophic failure. In structural health monitoring, changes in the vibration characteristics of the structure can be utilized in damage detection. A fatigue crack with alternating contact and non-contact phases results in a non-linear behaviour. This type of damage was simulated with a finite element model of a simply supported beam. The structure was monitored with a sensor array measuring transverse accelerations under random excitation. The objective was to determine the smallest crack length that can be detected. The effect of the sensor locations was also studied. Damage detection was performed using the generalized likelihood ratio test (GLRT) in time domain followed by principal component analysis (PCA). Extreme value statistics (EVS) were used for novelty detection. It was found that a crack in the bottom of the midspan could be detected once the crack length exceeded 10% of the beam height. The crack was correctly localized using the monitoring data.

Abstract: A combined approach to strength assessment of stay ropes in suspension bridges is considered. The rope working condition is being analyzed by following steps: in-situ magnetic testing of rope structure, computational assessment of ultimate breaking load and evaluation of residual margin of safety. The diagnostic parameters distributed and/or local faults in wires are used as input data for mechanical model of rope structure. The model enables to calculate the stresses in each wire and to simulate the step-wise degradation process thus estimating the rope residual breaking tensile load. The safety factor of deteriorated rope is considered as a generalized parameter that specifies the rope safe operation as an element of bridge stay arrangement. Examples of integrity analysis of stranded stay ropes and locked coil stay ropes are presented.

Abstract: The central target of this work is to provide an alternative to machine learning approaches to structural health monitoring with one of robust multivariate statistic novelty detection. Damage detection and identification is a procedure that is hierarchical in nature. At its most sophisticated, diagnosis of the damage could include localisation, classification and severity assessment and even go so far as to estimate the time-to-failure of the structure. In this paper, robust multivariate statistics were investigated focused mainly on a high level estimation of the outliers which determines only the presence or absence of novelty - something that is of fundamental interest. These methods allow a diagnosis of deviation from normality and the option of identifying the presence of masking effects caused by multiple outliers. This paper is trying to introduce a new scheme for damage detection by adopting simple measurements and exploiting robust multivariate statistics.

Abstract: Initially symmetric structural elements or units are widely used in machines and engineering objects (reinforced members of thin-walled structures, sections of shafts or pipelines, etc.). Damages, which can appear in such structures during operation, disturb their initial symmetry. This paper considers damage diagnostic procedures based on utilization of vibration effects caused by the distortions of systems initial symmetry due to appearance of defects. Object of the study is a uniform viscoelastic fixed beam (e.g. span of a pipe) which is initially symmetric relative to the central section. In order to find possible defects, forced vibrations of the beam are excited with the aid of test harmonic force Psinωt applied in the middle section. Damage is simulated as a local reduction of beams bending rigidity in corresponding cross-section. The goal of the research is to find such vibration diagnostic signs, which will make it possible to detect damage, its approximate size and location with the highest sensitivity. Dynamics of the system has been analyzed using two different methods: modeling on the specialized analogue-digital computer system developed in Riga Technical University; numerical simulation with program ANSYS. New diagnostic procedures based on distortions of vibration flexural modes and frequency spectrums due to arising of defect are proposed. The main advantage of this approach lies in the more high detection sensitivity in comparison with traditional resonant frequency methods. It is shown, that further rise of detection sensitivity can be achieved by insertion of additional nonlinear element into the structure of testing object.