Key Engineering Materials Vols. 569-570

Abstract: As the complicate structure and unstable working condition, the vibration signal of engines always be non-stationary and drown in strong noise, especially for the case of slight quality defection. Such case directly results in difficulties for inspection and diagnosis of engine incipient defection. A new inspecting method based on empirical mode decomposition (EMD) and envelope spectrum is proposed in this paper. In the new method, the intrinsic mode functions (IMF) of de-noised angular vibration signals are firstly obtained; then the envelope spectrums of IMFs are obtained through envelope analysis, lastly the discrepancy of the envelope spectrums between normal and abnormal case is extracted to be the feature indicator for the engine quality inspection. This method is validated experimentally. The results show that the proposed method has an excellent performance in the quality inspecting in the presence of abnormal clearance between crankshaft and connecting rod of the engine.

Abstract: Real-time structural health monitoring is becoming increasingly tractable on commodity wireless sensor devices and platforms. Such algorithmic implementations must be realised in as efficient a manner as possible, in terms of memory, computation, radio communications and power efficiency. This work describes an efficient, real-time, structural change detection algorithm implemented on constrained, commodity wireless sensor nodes. The algorithm, based on the Hilbert Huang transform, initially characterises the structure and reliably signals subsequent change using a hierarchical monitoring and alert infrastructure. The system operates entirely autonomously, and algorithmic parameterisations, such as sensitivity and training period duration, can be dynamically and remotely adjusted across the air interface. The system has been evaluated on two different structures which were subject to structural change during the experiments; a Single-Degree-of-Freedom discrete dynamical system, and a 5kW wind turbine blade. The system demonstrated a highly reliable capacity to promptly detect and actuate response to structural change.

Abstract: Belt conveyors are one of the most popular methods of material transport in many branches of industry, especially in mining. The average length of mining belt conveyor is about 1000 m. Taking into account that total length of transportation ways in averaged mine can approach several dozen of kilometers and network of several conveyors may cover large area, maintenance of such specific transportation system is very difficult. In this work we propose an automatic multi-channel system for data acquisition and processing for damage detection in belts. Belts with steel cords are considered here, they consist of top and bottom rubber covers and steel cords in between them. Due to many reasons (mainly sharpness of transported materials) covers may be damaged and it may initiate degradation process or straight damages in steel cords. Properties of steel cords are therefore crucial for overall strength of belts, if they are damaged, it may cause catastrophic failure of the whole conveyor. So, monitoring of belts conditions is a crucial issue. We proposed a monitoring system that measure and process data from array of magnetic sensors. The system allows to acquire up to 24 channels of NDT signals and uses automatic algorithms to process them in order to get information about begin of the belt loop, location of joints of particular belt segments and the final location and size of damages related to corrosion or cuts of steel cords inside belts. These techniques will be presented in the paper. Our approach has been validated in a lignite mine for several conveyor belts.

Abstract: The flexibility can be approximately synthesized with the first several measured modal parameters, i.e. the so called modal flexibility. The modal flexibility matrix will change with damage in a structure, and the change of modal flexibility should contain the information of damage. It is important to find a damage index that can pick up damage from the change of modal flexibility. To address this issue, the mathematical tool LU decomposition is introduced to deal with the modal flexibility matrix in order to find damage clearly. After decomposition, the modal flexibility is decomposed into a lower triangular matrix L and an upper triangular matrix U. Numerical results of both single and multiple damage cases under white noise excitation indicate that the matrix U has enough information of damage; and the proposed new technique can be utilized to locate the damage accurately. The present numerical study will lay a foundation for the application of real-time structural health monitoring in experiments and engineering.

Abstract: Hearing loss is determined by the diapason change of perceived ear sound frequencies and intensity. Cochlear tonotopicity represents the relationship between stimulation frequency f and place ℓ along the cochlea by the equation of the acoustic-wave hearing model at before-receptors stage ℓ(f) = Lo.22log(f/fmo), where Lo = 32 mm – the cochlear duct length, fmo = 20 kHz – maxima frequency of audible sound. Age-related frequencies standards can be represented by f(t)=fmoe–rt, where r=0.01 year–1 – high-frequency loss factor sound. Using both relations together, we get the length of the cochlear duct for T years LT = Lo.22log(fT/fmo). Destruction of the cochlear duct is exposed apex experiencing cyclical exposure to sound, which represents a decline of frequency fmo. The real length of the cochlear duct LR determined by the same ratio at a frequency fT = fmaxR, established audiometric. Treatment (pharmacological or physical therapy) causes a change in the physical and audiometric properties of inner ear structures. Daily monitoring of the upper frequency sound (fmaxX) determines the effective (useful) the length of the cochlear duct LX. Value LX/LR can be a criterion of treatment effectiveness: LX/LR →1 when the treatment is effective.

Abstract: Guided wave tomography is an attractive tool for the detection and monitoring of the critical area in a structure. Using signal difference coefficient (SDC) as the tomographic feature, RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective and flexible tomography algorithm. In this algorithm, signal changes are exclusively attributed to the structural variation. However, external environment factors like water loading or oil loading also change signals significantly. The presence of anti-symmetric mode with a predominant out of plane displacement makes it very sensitive to these interferences and leads to false alarms. In this paper, Lamb wave is excited in the low-frequency domain, where only the fundamental modes A0 and S0 exist. The significant difference in group velocity between the two modes makes it possible to separate them in time domain. A new method is proposed to extract pure S0 mode signal as valid measurement data to improve the algorithm in addressing false alarm caused by water loading. The results of the experiment demonstrate that the improved algorithm has the capability of providing accurate identification of damage in the presence of water loading.

Abstract: This paper is concerned with vibration based non-destructive evaluation of structures, with a focus on quantitative assessment of damage. In previous works, a reliable method to locate open cracks in beams has been proposed and tested using both data from numerical simulations and laboratory experiments. It bases on the fact the natural frequency of a bending vibrations mode attend different changes, depending on the loss of stored energy for the slice on which the damage is located. As bigger the mode shape curvature value on that location, so bigger the loss of stored energy and consequently the natural frequency decrease in that mode. Analyzing the natural frequency changes for a larger series of vibration modes, it’s possible to precisely locate damages. The authors succeed to find a single mathematical relation describing the frequency changes for all bending vibration modes, involving one term defining damage’s location and one defining its depth. While the first term changes for different modes, being defined by the mode shape curvature, the second maintain its value for all modes, being affected just by damage depth. This finding permits decoupling the location issue with that of quantitative assessment of damage. Latest researches, presented in this paper, succeed by finding the relation between the second term of the relation and some mechanical characteristics of the beam, i.e. extending the proposed method by including evaluation of damage severity. The approach is illustrated on a cantilever beam, modeled with 3D elements.

Abstract: This paper applies a methodology for damage detection in beams proposed by the authors. The methodology is based on a continuous wavelet analysis of the difference of mode shapes between a damaged state and a reference state. The wavelet transform is used to detect changes in the mode shapes induced by damage. The wavelet coefficients for each mode are added up and normalized to unity in order to obtain a clear and precise damage assessment. A curve fitting approach reduces the effect of experimental noise in the mode shapes. When only a small number of measuring points are available, a cubic spline interpolation technique provides additional “virtual” measuring points. The interpolation technique may also be used when measuring points are not equally spaced. It also serves as a softening technique of the mode shapes when applied, and no curve fitting approach is used in that case. An antisymmetric extension at both ends of the mode shapes is used to avoid the edge effect in the wavelet transform. The paper presents the results obtained for steel beams with an induced crack. Several sizes and locations of the crack have been considered. The paper addresses several issues affecting the accuracy of the proposed methodology, such as the number of measuring points and the effect of the extension, curve fitting and interpolation techniques.

Abstract: In this paper, Continuum Damage Mechanics (CDM) theory is applied to low cycle and high cycle fatigue problems. Damage evolution laws are derived from thermodynamic principles and the fatigue number of cycles to crack initiation is expressed in terms of the range of applied stresses, triaxiality function and material constants termed as damage parameters. Low cycle fatigue damage evolution law is applied to adhesively bonded single lap joint. Damage parameters as function of stress are extracted from the fatigue tests and the damage model. High cycle fatigue damage model is applied to fretting fatigue test specimens and is integrated within a Finite Element Analysis (FEA) code in order to predict the number of cycles to crack initiation. Fretting fatigue problems involve two types of analyses; namely contact mechanics and damage/fracture mechanics. The high cycle fatigue damage evolution law takes into account the effect of different parameters such as contact geometry, axial stress, normal load and tangential load.

Abstract: The inspection of austenitic and dissimilar welds using ultrasound demands for sophisticated testing techniques. The application of reconstruction methods like the Synthetic Aperture Focusing Technique (SAFT) on the measurement results provides an appropriate approach for defect characterization and sizing. Nevertheless, the reconstruction algorithm has to consider the aniso-tropic wave propagation inside the inhomogeneous weld material. In recent years the detection of transverse cracks has become increasingly important for ensuring the structural integrity of pipes in the primary circuit of nuclear power plants or longitudinally welded, cladded pipes. However, relia-ble inspection techniques are hardly available. In this particular case, it is expected that the compar-atively long propagation path of the ultrasonic wave field inside the inhomogeneous weld material enhances the effect of anisotropy and influences the accuracy and the signal-to-noise-ratio of the reconstruction result. In this contribution we suggest an advanced ultrasonic testing technique for detecting and sizing of transversal cracks in austenitic and dissimilar welds. The method applies a SAFT reconstruction algorithm considering the anisotropy and the inhomogeneity. A V-arrangement of the transducers in pitch-catch technique is chosen to avoid a direct coupling on the weld face. The reconstruction algo-rithm is based on an extended 3-dimensional weld model and uses a ray-tracing approach for de-termining the wave propagation paths. Along with the reconstruction algorithm the transducer set-up and experimental results of different specimens with artificial transverse flaws are presented. The availability of the proposed method for crack sizing is assessed in comparison to conventional testing techniques.