Key Engineering Materials Vol. 558

Abstract: Damage detection techniques are becoming more and more important issues for prolonging the lifetime of the old structures. In the paper the strain measurements on offshore oil platform leg model as well as the preliminary measurement system installed on jack-up rig is presented. During experiments performed on frame model of the platform leg different types of damage scenarios were tested. The Fiber Bragg Grating (FBG) strain sensors were capable to detect damage. After laboratory experiments, preliminary FBG system was installed on the real construction (jack-up rig) excited by environmental and operation-induced forces. It is evident that strain measurement based on FBG technology is very promising technique for use in truss/frame structures.

Abstract: The reported research concerns experimental investigation toward the monitoring of an aircraft panel. Guided wave propagation phenomena were used to obtain information about the state of the monitored structure. A curved aluminium panel with rivets was investigated. Piezoelectric transducer was used to excite guided waves in chosen structural element. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided waves. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This contactless measurement technique allowed to investigate phenomena related to wave propagation in the aircraft panel. In the first stage, due to high complexity of the element, baseline measurements were taken. Next, a discontinuity (additional mass) was introduced on the panel surface and the measurements were repeated. Signal processing methods for features extraction from signals were proposed. These features were applied in order to detect and localize the presence anomalies in the investigated panel. The signal processing was conducted in MATLAB with the procedures developed by the authors. The used measurement technology (vibrometer) allowed to register whole wavefield of the propagating guided waves. This allowed to visualize the interaction of the waves with rivets. After introducing the discontinuity on the panel surface wave interaction with it was investigated. Two positions of the additional mass were considered. One just before the riveted stiffener and second after the stiffener. Because of this the influence of the stiffener on the damage detection abilities could be investigated. It can be concluded that the guided wave can be used for monitoring of such complex structures. The vibrometer measurements allowed learn about the guided wave propagation phenomena and perform successful damage localization.

Abstract: This paper presents a hybrid finite element and analytical method to predict the 1-D guided wave propagation interaction with damage for nondestructive evaluation (NDE) and structural health monitoring (SHM) application. The finite element mesh is used to describe the region around the damage (defects or flaws). In contrast to other hybrid models developed elsewhere, the interaction between Lamb waves and defects is computed in the time domain using the explicit solver of the commercial finite element method (FEM) software ABAQUS. Analytical methods can perform efficient modeling of wave propagation but are limited to simple geometries. Realistic structures with complicated geometries are usually modeled with the FEM. However, to obtain an accurate wave propagation solution at ultrasonic frequencies is computationally intensive and may become prohibitive for realistic structures. In response to today's complex cases not covered by the simulation tools available, we aim to develop an efficient and accessible tool for SHM applications. This tool will be based on a hybrid coupling between analytical solutions and time domain numerical codes. Lamb wave interaction with a notch is investigated by using this method, and the results obtained are with respect to transmission, reflection and mode conversion. Because of the symmetric mode shape, S0 is more sensitive to the shallow notch than A0. By making use of the fact that the reflection increases with increase in notch depth and mode conversion are maximized when the notch is around half through the thickness of the plate, the reflection and conversion coefficients can be used to characterize the depth of the notch.

Abstract: FBGs are excellent strain sensors, because of its low size and multiplexing capability. Tens to hundred of sensors may be embedded into a structure, as it has already been demonstrated. Nevertheless, they only afford strain measurements at local points, so unless the damage affects the strain readings in a distinguishable manner, damage will go undetected. This paper show the experimental results obtained on the wing of a UAV, instrumented with 32 FBGs, before and after small damages were introduced. The PCA algorithm was able to distinguish the damage cases, even for small cracks. Principal Component Analysis (PCA) is a technique of multivariable analysis to reduce a complex data set to a lower dimension and reveal some hidden patterns that underlie.

Abstract: The interaction of the fundamental anti-symmetric Lamb wave (A₀) with debondings at structural features is investigated using experimental data and finite element (FE) simulations. In this study explicit three-dimensional (3D) FE simulations are employed, which allows the study of the scattered wave along different propagation directions. Good agreement between the FE predictions and the measurements are obtained that demonstrates that the 3D FE scattering model is able to accurately predict the Lamb wave scattering characteristics at debondings. The study show that the characteristics of Lamb wave reflected from the debondings at the structure feature is much more complicated than that from defects in flat composite laminates. Parameter studies show that the backward and forward scattering coefficient of Lamb wave is a function of debonding size to wavelength ratio and debonding location. This shows the potential of employing Lamb wave to identify the size and monitor the growth of the debondings. The findings of the study provide improved physical insights into the scattering phenomena, which are important to further advance damage detection techniques for complex structures made by composite laminates.

Abstract: A local damage detection method for shear structure based on substructure approach and sub-time series superposition is proposed, which is promising for application in parallel and distributed damage detection systems. This method requires only three sensors to identify localized damage in any story of a building and it doesnt require constructing the structure models. A substructure method was used in this method to divide a complete structure into several substructures. Each substructure has a considerably smaller number of degrees of freedom (DOFs) which makes fewer requirements on the computing power of the data processing system. The algorithm is simple, which involves only algebra calculation and does not need any iterative computation. Moreover the damage detection process can be independently conducted on each substructure. Thus, this method is easy, efficient and suitable for being embedded into parallel and distributed damage detection systems. To better assess the performance of the method proposed above, experimental verification of the proposed approach has been conducted, which shows the proposed method works quite well and stably.

Abstract: Designers of modern conventional submarines are constantly striving to improve the platforms capabilities in a time of diverse missions. The inherent space and weight restrictions imposed on submarine design dictate a high level of optimisation and the implementation of advanced technologies to combat these often conflicting requirements. The design of external stowages is an area where this design paradigm is especially evident. In order to provide the RAN with additional mission flexibility and meet rigorous submarine requirements a novel approach was adopted to develop a composite pressure proof stowage. The design develops the use of composite laminate construction with the aim of minimising weight and cost, maintaining buoyancy and maintaining impact resistance using glass/carbon reinforced plastic for the prototype stowage. The resultant prototypes require structural validation including strain gauge verification to ensure that the final items can demonstrate adherence to stringent component and ultimately ship safety criteria.

Abstract: In this paper, the health status of a full-scale four-story steel building is investigated using the data of collapse test at the E-defense in Hyogo Prefecture, Japan. We extracted the features such as hysteretic characteristics, story drift angle, natural frequency and shapes using only the acceleration data of the collapse test. We evaluated the reliability and the correlation of the features with structural damage. Moreover, we proposed the preferred index for the damage detection.

Abstract: Acoustic Emission (AE) is a sensitive technique which can be used to characterise damage in high strength composite plate. This paper describes an extension to an earlier piece of research work carried out by the ERC which resulted in the successful development of a novel source location methodology for the said material. The previous work concentrated on the source location in plate-like composite structures using acoustic emission. The work presented in this paper focuses on establishing the correlation between the different damage types suffered in the material namely de-lamination, matrix cracking, fibre rupture and stringer to skin debonding with key signal features of the AE activities. Controlled bending tests were initially carried out on laterally grooved slender composite specimens to progressively propagate damage in the weakened region of these specimens. The composite laminate plate itself is made from 16 plies of carbon fibre twill weaved in an epoxy matrix with bidirectional fibre alignments in the 0° and 90° directions with 60/40 fibre-matrix volume composition. These prepared samples were fully instrumented with broad band (100 kHz to 1MHz) Physical Acoustic AE sensors linked to the necessary signal conditioning hardware. The AE events were recorded using a high speed DAQ card accessed by customised software written in LabVIEW^TM. Gathered raw data were analysed off-line for key signal features including energy and frequency contents and subsequently correlated to actual damage types. It can be concluded from the empirical evidence that feature vectors are distinct to the type of damage. Results gathered from additional test on the progressive skin-stringer debonding of the same material to failure confirmed the uniqueness of the AE feature trends. An integrated system which is capable of both in-situ location of compromised sites and the diagnostic of flaw types in composite plate can potentially find engineering applications including the structural health monitoring of composite aircraft parts.

Abstract: Engineering structures under cyclic loads experience continuous accumulation of fatigue damage, deteriorating at an alarming rate. Most existing structural health monitoring (SHM) techniques use linear signal features, which may be unwieldy to the detection of fatigue damage in an initial stage. A dedicated finite element (FE) modeling technique for simulating nonlinear properties of ultrasonic Lamb waves under the modulation of fatigue cracks in metallic materials was established. Piezoelectric wafers were included in the model for exciting Lamb waves and capturing nonlinear characteristics. A nonlinearity parameter was constructed to calibrate the extracted wave nonlinear properties. Feasibility of the FE technique was experimentally validated, and the results showed satisfactory consistency in between, both revealing that (i) the developed FE modeling technique is able to faithfully simulate fatigue crack-induced nonlinear properties in Lamb waves, providing repeatable characterization for fatigue cracks; (ii) the defined nonlinear parameter decreases when the direct wave path offsets from the fatigue crack, nonlinearly subject to the offset distance from the crack to a sensing path; and (iii) a cumulative growth of the nonlinearity parameter against the wave propagation distance exists. All the observations enable quantitative characterization of micro-fatigue cracks using embeddable piezoelectric wafers, facilitating development of SHM technique with a capacity of quantitatively detecting damage small in dimension.