Key Engineering Materials Vols. 293-294

Abstract: This paper analyses the applicability of the Statistical Energy Analysis (SEA) for detecting incipient damages in a typical spacecraft structure, as a stiffened panel. The damage on attachment element is investigated by analyzing its influence on the system characteristics. Because of incipient damage affects mainly on highest modes, rather than on lowest, the coupling loss factor between sub-elements can be used to detect and localize the damage.

Abstract: The article is to show results of numerical and experimental examination of changes in wave propagation in a composite rod with additional mass. For numerical modelling the spectral element method is used. For experimental verification the IFFM PAS laboratory equipment was used. As actuators and sensors PZT elements were utilised. The results obtained via numerical and experimental simulations are compared and discussed.

Abstract: This paper concerns flexural and axial wave motion in a cracked beam. A combined finite element (FE) and spectral element (SE) model of a cracked beam is presented. A portion of the beam, which contains the crack, is modelled using FE analysis and combined with semi-infinite SEs. From the combined model the reflection and transmission coefficients of the crack are estimated. To determine the accuracy of this approach, a beam with a mass discontinuity is considered in the first instance. The reflection coefficients are estimated numerically and compared with experimental results. Secondly, a slot-type transverse crack is cut along the width of the beam. The experimental results are compared with both an FE model and a conventional lumpedparameter spring model. The purpose of this work is to investigate further the use of audiofrequency wave propagation as a basis for crack assessment and provide a valid model to use in the development of an assessment procedure.

Abstract: Ultrasonic surface waves provide a sensitive means to detect cracks in airplane structures. Until now several obstacles remained to use ultrasonic surface waves for on-line damage detection (i.e. in-flight). In this article a method will be proposed to detect a growing fatigue crack while the aircraft is operating. In contrast to classical ultrasonic measurement methods, that use a high voltage pulse, we applied an optimized multi-sine excitation signal with an amplitude of a few volts only (this agrees better with the applicable safety regulations for aircraft). Furthermore, an indicator quantifying the nonlinearity of the ultrasonic surface wave propagation is used. By using a nonlinearity index the influence of changing operation conditions that can be observed with most linear methods is eliminated. The proposed method is validated on a steel beam that is fatigue loaded with a force signal obtained from in-flight data.

Abstract: Cracks bring a serious threat to safety of structures. Most of the failures and fractures of engineering structures are due to initial cracks or fatigue cracks of materials. So it is very important to analyze the vibration characteristics and to identify the damage of cracked structures. A method for multi-crack identification based on wave propagation is proposed in this paper, which makes use of the driving-point mechanical impedance characteristics of the cracked beams stimulated by harmonic force. The proposed identification method is used to characterize the local discontinuity due to cracks, and a simplified rotational spring model is introduced to model cracks. Subsequently, the proposed method is verified by a numerical example of a simply supported steel beam with three cracks. The effect of crack depth on driving-point impedance is investigated. Combined with the first anti-resonances information, the proposed method can identify the presence of cracks, localize the multiple cracks, and qualitatively identify the extent of the crack damages.

Abstract: In the present paper, the Damage Location Assurance Criterion (DLAC) is extended to locate and assess damage in a circular cylindrical shell based on natural frequencies and mode shapes. Frequency sensitivities computed from a defect-free finite element model are applied to calculate the theoretical frequency changes. The axial position of the damage can be easily obtained by comparing the theoretical and measured frequency changes due to damage. For the shell is axisymmetric, additional information of mode shapes is introduced to locate the exact damage position. The damage extent can be estimated by the first order approximation method. The feasibility and practicality of the damage detection scheme are evaluated for several damage scenarios by locating and sizing damage in the free–free, simply-supported and free-clamped shells, respectively. Results from simulation examples show that the proposed detection scheme can confidently locate the single or multiple positions of damage. It is also observed that damage extent can be estimated with a relatively small error.

Abstract: Purpose of the paper is to present a new application of a Non Destructive Test based on vibrations measurements, developed by the authors and already tested for analysing damage of many structural elements. The proposed new method is based on the acquisition and comparison of Frequency Response Functions (FRFs) of the monitored structure before and after an occurred damage. Structural damage modify the dynamical behaviour of the structure such as mass, stiffened and damping, and consequently its FRFs, making possible to identify and quantify a structural damage. The activities, presented in the paper, mostly focused on a new FRFs processing technique based on a dedicated neural network algorithm aimed at obtaining a “recognition-based learning”; this kind of learning methodology permits to train the neural network in order to let it recognise only “positive” examples discarding, as a consequence, the “negative” ones. Within the structural NDT a “positive” example means “healthy” state of the analysed structural component and, obviously, a “negative” one means a “damaged” or perturbed state. The developed NDT has been tested for identifying and analysing damage on an aeronautical composite panel to validate the method and calibrate the neural network algorithm. These tests have permitted to understand the influence of environmental parameters on the neural network training capability. Thanks to these new techniques it is possible to carry out a smart Health Monitoring system which is going to lead to the reduction of time and maintenance cost and to the increase of the aeronautical structure safety and reliability.

Abstract: The application of a cyclic load on a composite material containing damage has the effect of heating due to the material viscoelasticity. This is exaggerated in the proximity of interlaminar failure because of friction between plies. Quantitatively studying a stressed component subject to these conditions using Thermoelastic Stress Analysis (TSA) has been inaccurate, as the localised heating has an effect on the thermoelastic response. Hence the thermoelastic signal from damaging composites will contain a stress-induced component and a temperature-induced component. In this paper a process is described that allows the thermoelastic signal to be de-coupled into a stress component and a temperature component. This is achieved using a combination of infra-red thermography and TSA. The process is based on the use of a special calibration device. The paper provides an experimental verification of the de-coupling using actual damaged composite components.

Abstract: A three-dimensional finite element model of a three-bolt, single-lap composite joint is constructed using the non-linear finite element code MSC.Marc. The model is validated against an experiment where the load distribution in the joint is measured using instrumented bolts. Two different joint configurations are examined, one with neat-fit clearances at each bolt-hole and another with a 240 µm clearance at one hole with neat-fits at the others. Bearing and by-pass stresses are extracted from the model and used in conjunction with published bearing/by-pass diagrams to predict the failure load, mode and location for the joints. It is shown that the proposed model accurately predicts the failure behaviour of the joints, as determined from experiments on three-bolt joints loaded to failure. It is also shown that introducing a clearance into one hole significantly changes the failure sequence, but does not affect the ultimate failure load, mode or location. The proposed method demonstrates a simple approach to predicting damage in complex multi-bolt composite joints.

Abstract: Simulations of damage scenarios were carried out using a finite element model of a newly constructed FRP composite footbridge, the Wilcott footbridge. This footbridge represents a new generation of suspension footbridges that have lightweight decks made of pultruded glass fibre reinforced polymer (GFRP) composite elements. It offers several advantages over conventional steel or concrete footbridges, e.g. speed of installation, high resistance to corrosion and saving in weight and foundations. On the other hand, its lightness and slenderness make it more sensitive to dynamic effects, both at serviceability and ultimate limit states. A finite element model using 3-D beam elements was constructed and damage scenarios were simulated and introduced in the model. The natural frequencies, mode shapes as well as time responses due to pedestrian loading were predicted. Different size of delamination in the composite deck was simulated at various locations along the bridge. The sensitivity of natural frequencies and mode shapes due to delamination were assessed by comparing the results of the damaged deck to those of the reference intact deck. The effect of changes in the cables’ initial strains on the modal parameters was also examined, and the sensitivity of modal parameters to cable degradation was assessed.