Damage Assessment of Structures VIII

Volumes 413-414

doi: 10.4028/www.scientific.net/KEM.413-414

Paper Title Page

Authors: Colum M. Holtam, David P. Baxter, Rachel C. Thomson, Ian A. Ashcroft
Abstract: Steel catenary risers (SCR) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGR) are expected, and this can have a significant effect on defect tolerance. The aim of this paper is to provide guidance on the current best practice methods for performing engineering critical assessments (ECA) on internal surface breaking defects in SCRs operating in a sour environment and subject to VIV fatigue loads. Example ECA calculations are presented for circumferential girth weld flaws, based on the failure assessment diagram (FAD) approach within the framework of BS 7910 [1]. The influence of certain key input variables is demonstrated, including the FCGR, determined from recent sour test data generated as part of this research.
Authors: Wen Bo Duan, Feng Shou Gu, Iain Dupère, Shan Zhong, Andrew Ball
Abstract: The free vibration of a fluid/structure system consisting of a cylindrical blockage submerged in a liquid enclosed by a cylindrical shell is investigated for the purpose of pipe line transportation monitoring. The wavenumbers are obtained and the reflection and transmission characteristics of these waves at the blockage interfaces are investigated theoretically. Reflection and transmission ratios are obtained in the axisymmetric mode, as functions of frequency. High order modes play an important role in the near field of the discontinuity and are taken into account.
Authors: Yong Huang, Hui Li, Xin Yan, Jin Ping Ou
Abstract: Acoustic emission (AE) test monitoring is an effective non-destructive technique. In the paper, a new damage assessing method which is damage acuteness index for AE signal of PZT patches based on fractal theory was proposed. The damage index was deduced by the character of signal analysed by fractal theory. It is deduced that both the curve length and the fractal dimension (FD) of signal are related with damage development. The AE test of Pseudo-static experiment of a concrete-filled GFRP tubes (CFFT) was performed for validation. The results show that the damage acuteness index can assess damage development process effectively. So the damage acuteness index is a promising method to apply in AE test monitoring.
Authors: Nicolae Constantin, Alexandrina Mihai, Viorel Anghel, Mircea Găvan, Ştefan Sorohan, Wolfgang Hillger, Michael Sheerer
Abstract: Polymeric layered composites exhibit a variety of damages following in service loading conditions, like delamination, matrix cracking or even fibre breaking. Detection of such damages and assessing their extension and severity is vital during maintenance cycle, in view of keeping the normal operational reliability. For local inspections, IR thermography and ultrasonic scanning are among the best valued NDT methods. The paper describes the inspections performed by IR active thermography, in different variants, and pulse-echo ultrasonic scanning on GFRP. A variety of layered composites and defects/damages were inspected and the results are evaluated independently, in some cases being compared each other, with valuable conclusions for the users of the mentioned NDI techniques.
Authors: Daniele Zonta, Matteo Pozzi, Hua Yong Wu, Daniele Inaudi
Abstract: This paper presents the laboratory validation of a prototype optic-fiber instrumented structural element. The element is a reduced-scale reinforced concrete beam, of dimensions 3.8×0.3×0.5m that can be pre-stressed by an internal Dywidag bar. The sensing technology is based on a multiplexed version of the SOFO strain sensor, prepared in the form of a 3-field smart composite bar; in-line multiplexing is obtained by separating each measurement field through broadband FBGs. The experiment aims to identify the response of the sensors to differing damage conditions artificially produced in the element, including cracking and loss of prestressing. A numerical algo-rithm, based on Bayesian logic, is applied to real-time diagnosis: by processing the sensor meas-urements and prior information, the method assigns a posterior probability to each assumed damage scenario, as well as the updated probability distributions for each relevant structural parameter. With respect to classical damage detection approaches, the merit of those based on Bayesian logic is to provide not only information on the damage, but also the degree of confidence in this informa-tion. The paper discusses the ability of the system to identify the differing damage conditions. The reported test clearly shows that an occurrence such as a loss of prestressing can be recognized early with a high degree of reliability based on the strain data acquired.
Authors: Lucio Della Sala, Rosario Cerone, Alberto Gennari Santori, Mauro Tommasini
Abstract: The paper describes the evaluation of the actual conditions, the repairing design criteria and procedures, and the final validation tests carried out on a severely damaged prestressed bridge in southern Italy. In the first phases several release tests have been performed on damaged and, for comparison, on sound beams, with the aim of evaluating the residual prestressing stress acting both on concrete and on tendons. In particular, the tests on concrete have been carried out by extracting cores which have been previously instrumented with strain-gages; in a similar way, the strands were tested by gluing a micro strain-gage on a singular wire of a strand and the cutting the wire itself. The test results were used for optimising the design of an adjunctive prestressing system, consisting in 6 external unbounded cables and in a continuous FRC reinforcing of the bottom wing. The tensioning procedure of the external cables of each beam was then monitored by means of 12 strain-gages installed on concrete at mid span and at the two support sections, and by measuring the vertical deflection using laser optical levels. The validation of the repairing works was finally performed by a static load acceptance test, performed using the same apparatus previously adopted for the tensioning monitoring.
Authors: Yong Hong, Gao Ping Wang, Seung Ho Hwang, Hyun Sik Kim, Dong Pyo Hong
Abstract: A piezoelectric ultrasonic sensing system based on an optical fiber has been developed for detection of various damages. The ultrasonic wave generated from a piezoelectric actuator is guided and propagated in the optical fiber and then sensed by a piezoelectric sensor located at the other end of the fiber. The sensed signal can be influenced by environmental and physical changes around the optical fiber. In this study, the sensitivity of the optical fiber sensor is experimentally studied. Various patterns of damage, such as cracks, loosen bolts, holes, are more common forms in real structures. In particular, detection method of the damage with respect to various depths of the crack is presented in this paper.
Authors: Krzysztof Mendrok, Jarosław Bednarz, Tadeusz Uhl
Abstract: The paper describes a modification of the method which uses the modal filtration for damage detection [11] to make it suitable for rotating machinery. Authors have formulated a diagnostic procedure which is based on the output only data measured during object operation. The measurement can be performed both by accelerometers placed on the shaft bearing cages and directly on the shaft with use of the laser vibrometer. The procedure was verified on the experimental data. For the experimental verification authors have used the laboratory stand which allows simulation of different faults.
Authors: Dong Sheng Li, Hong Nan Li, Claus Peter Fritzen
Abstract: A novel sensor placement criterion is proposed for structural health monitoring after five influencing criteria are critically reviewed. The objective of the proposed criterion is to achieve best identification of modal frequencies and mode shapes through almost unbiased estimation of modal coordinates. The proposed criterion derived by the Representative Least Squares method depends on both the characteristics and the actual loading situations of a structure. It selects sensor positions with the best subspace approximation of the vibration responses from the linear space spanned by the mode shapes. Furthermore, the connection between the Effective Independence and the approximate Representative Least Squares estimator is obtained through matrix perturbation analysis. It is found that the Effective Independence is a step-by-step approximation to that of the Representative Least Squares criterion.
Authors: Nurulakmar Abu Husain, Andy Snaylam, Hamed Haddad Khodaparast, S. James, Geoff Dearden, Hua Jiang Ouyang
Abstract: Finite Element (FE) model updating is initially developed to update numerical models of structures to match their experimentally measured modal properties (i.e., natural frequencies and modes). In FE model updating, uncertain physical parameters of a structure are modified so that the discrepancies between the numerically estimated and experimentally measured modal properties are minimized. The process of updating is employed not only in parameter identification; it can also be developed for structural damage identification. In this work, a welded structure that is intended to represent a common configuration used in automotive body construction is investigated. It is known that presence of any damage in the welds of such a structure could affect its dynamic behavior. So, in theory modal test data can allow damage to be assessed accurately. As a typical automotive body contains thousands of welds, the effects of damage in the welds could be influential. The FE model updating process using experimental data is presented. It is carried out using NASTRAN optimization code. The procedure aims to adjust the uncertain properties of the FE model (from the weld joints) by minimizing the differences between the measured modal properties and the corresponding numerical predictions. The initial parameter values used in the numerical model are the nominal values. The procedure brings the numerical results of the structure as close as possible to the experimental ones, according to an objective function, therefore altering some of the FE model parameters of the structure. It may be concluded that when the identified values of certain parameters deviates from the nominal values to certain extent, there is a fault or damage at that particular joint.

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