Zubaidah Ismail

Combined modal parameters-based index for damage identification in a beamlike structure: theoretical development and verification

A new index for detecting the damage severity in structural elements by combining modal parameters is proposed in this study. The index is based on the combined effect of both the natural frequencies and mode shapes when a change in stiffness of the structural element occurs. In order to demonstrate the significance and capability of this new algorithm, the magnitude of damage was calculated from a finite element model of a beam-like structure model and comparisons with previous algorithms were carried out. The new index called Combined Parameter Index (CPI) compares the factor of reduction in stiffness according to reduction in natural frequencies and also the factor of reduction in stiffness according to change in mode shape. Various damage levels starting from reduction in stiffness of 1% were adopted to validate the sensitivity of the new index to detect the damage severity at various deterioration levels. Mid-span and quarter-span damage positions were adopted to verify the capability of the new damage index to detect the damage severity at various locations. Moreover, damage in support condition was investigated in order to ascertain that the new damage index can also identify support damage cases. The results indicate that the new index has better ability and higher sensitivity to determine the severity of the damage due to stiffness changes in the element or support. In addition, the CPI exhibits sensitivity to detect lower level of damage occurring at earlier stage by having the ability to detect a damage of 1% reduction in the structural element stiffness or elastic bearing stiffness.

Optimization of the Geometries of Biconical Tapered Fiber Sensors for Monitoring the Early-Age Curing Temperatures of Concrete Specimens

Structural health monitoring has received significant attention in research and involves the integration of sensor techniques, smart materials, data interrogation and transmission, computational power and processing ability inside the structures. A geometrical optimization procedure using biconical tapered fiber sensors is proposed for monitoring the early-age curing temperatures of concrete specimens in this article. The geometries of the sensors are theoretically optimized by the ray-tracing theory. The results of the theoretical analysis show that the performance of the sensors is heavily influenced by Evanescent Waves, which are due to the tunneling rays and are fully escaped by tapering the fiber. The effects of the geometrical parameters, which include the taper ratios, taper lengths, and ray launch angles, as well as the surrounding temperatures, on the behavior of the sensors are studied numerically. The numerical results demonstrate that higher performance of the proposed optimized sensors can be achieved by a longer taper length and smaller taper ratio combined with an initial ray launching angle of 0.01 rad. The findings in this article prove that the proposed sensor has the ability to determine the setting time of concrete since the setting process is accompanied by a temperature change in the concrete.

Approach to Reduce the Limitations of Modal Identification in Damage Detection Using Limited Field Data for Nondestructive Structural Health Monitoring of a Cable-Stayed Concrete Bridge

The objective of the study was to propose a technique to reduce the limitations of modal identification in damage detection using reduced field data for nondestructive structural health monitoring of a cable-stayed concrete bridge. Simply supported bridge models were constructed with predetermined damage at the midspan of the bridge. The technique necessitated the performance of linear and eigen analyses on the control beam and nonlinear analysis on the bridge with damage. Residuals from regression of the mode shape using the Chebyshev rational series on the modal frequencies and transformation and application into the fourth-order centered finite-divided-difference formula were shown. The use of the regressed-mode shapes for the RC bridge model showed very large residuals around the areas of the damage. The results showed that the method was successful in assisting to reduce the limitations of modal identification in locating damage on a bridge model with limited field data and was comparable to other techniques proposed by other researchers in terms of its simplicity.

Determination of damage severity on rotor shaft due to crack using damage index derived from experimental modal data

After long duration of an operation, a shaft under high spin speeds and heavy loadings may develop fatigue cracks. This could lead to catastrophic failure and could be difficult to detect. An accurate prediction of dynamic characteristics of rotor system is fundamentally important. Hence, a practical method to nondestructively locate and estimate the severity of a crack in terms of its damage index by measuring the changes in natural frequencies of the rotor shaft is presented. In this study, experimental modal analysis (EMA) data were utilized. A crack detection algorithm to locate and identify cracks in the rotor system using the first and second natural frequencies was outlined. Subsequently, a crack-locating model was formulated by relating the fractional changes in modal energy to the changes in natural frequencies as a result of cracks based on the experimentally obtained natural frequencies and mode shapes’ functions. The feasibility and practicality of the crack detection scheme were evaluated for several damage scenarios by locating and sizing cracks in test rotor shafts for which the first two natural frequencies were available. It was observed that crack could be confidently located with a relatively small localization error. It was also observed that crack severity could be estimated in terms of its damage index.

Use of Tapered Optical Fiber Sensors in Study of the Hydration Process of Cement Paste

In this paper, the tapered multimode plastic optical fiber sensor is used for the experimental study of the early-age hydration process of cement paste. A high reactive power is mixed as a specimen, in which a tapered fiber sensor is embedded to measure the liberated heat (temperature). The sensor characteristics are determined and calibrated by an embedded thermocouple, which have a sensitivity of 0.0293 mV/°C and resolution of ±0.34°C. The experimental studies are carried out for the host specimen with different sizes, various water/cement (w/c) ratios, as well as different ambient temperatures. From the experimental results, the curing temperature and setting time are determined by monitoring the curing temperature curves as the curing progressed. The curing temperature rose with increasing mould sizes, increasing w/c ratio, and increasing ambient temperature. The setting time could be shortened by a smaller size of a specimen, lower w/c ratio, and higher ambient temperature.

Study of open crack in rotor shaft using changes in frequency response function phase

In recent years, significant efforts have been devoted to developing non-destructive techniques for damage identification in structures. This study investigated the effects of cracks and damages on the integrity of structures, with a view to detect, quantify, and determine their extents and locations. Previous works have used parameters, such as, changes in natural frequencies and mode shapes, as detectors. However, such parameters are not sensitive enough to detect early defects. In this paper, phase measurements are sought. Measurements of the acceleration frequency responses at different points on each rotor shaft were taken using a multi-channel frequency analyzer. The damage detection schemes used in this study depended on the changes in the phase of the measured acceleration frequency response functions. To study the changes of phases, it was interpreted in phase spectrum and Nyquist plot. Nyquist plot was used as it includes both real and imaginary parts of the amplitude and this was used to study phase shifts due to the presence of crack. The changes in phase depended on crack depth and how close the crack is to that mode shape node. Meanwhile, the changes in phase of lower eigenvectors were observed clearly. Thus, first mode shape was helpful in identifying the location of the crack. The vibration behavior of the rotor shaft was shown to be very sensitive to the crack depth, crack location and mode number. It is concluded that changes in phase as a function of crack depths and locations can be effective in crack detection methodology.

Identification of material properties of composite materials using nondestructive vibrational evaluation approaches: A review

ABSTRACT Destructive identification approaches are no longer in favor since the advent of nondestructive evaluation approaches, as they are accurate, rapid, and cheap. Researchers are devoted to improving the accuracy, rate of convergence, and cost of such approaches, which depend greatly on the types of vibrational experiments conducted and the types of forward and inverse methods used in numerical section. Therefore, this article presents a review on the development of nondestructive vibrational evaluation approaches in identifying the elastic constants of composite plates, in experimental and numerical manners in order to enlighten researchers with the current trends of nondestructive vibrational approaches.

Enhancement of Impact-synchronous Modal Analysis with number of averages

A new method, namely Impact-synchronous Modal Analysis (ISMA), utilizing the modal extraction technique commonly used in Experimental Modal Analysis performed in the presence of the ambient forces, is proposed. In ISMA, the extraction is performed while the machine is running, utilized Impact-synchronous Time Averaging prior to performing the Fast Fourier Transform. The number of averages had a very important effect when applying ISMA on structures with dominant periodic responses of cyclic loads and ambient excitation. With a sufficient number of impacts, all the unaccounted forces were diminished, leaving only the response due to the impacts. This study demonstrated the effectiveness of averages taken in the determination of dynamic characteristics of a machine while in different rotating speeds. At low operating speeds that coincided with the lower natural modes, ISMA with a high number of impacts determined the dynamic characteristics of the system successfully. Meanwhile, at operating speeds that were away from any natural modes, ISMA with a moderate number of averages taken was sufficient to extract the modal parameters. Finally for high-speed machines, ISMA with a high number of impacts taken has limitations in extracting natural modes close to the operating speed.

Artificial neural networks for vibration based inverse parametric identifications

Display Omitted ANNs-solved vibration based parametric identification studies are reviewed.Factors which affect identification result are discussed.Pros and cons of ANN approaches are mentioned.Suggestions are given to potential researchers based on the discussion.Analysis with experimental results is provided to justify some point of view. Vibration behavior of any solid structure reveals certain dynamic characteristics and property parameters of that structure. Inverse problems dealing with vibration response utilize the response signals to find out input factors and/or certain structural properties. Due to certain drawbacks of traditional solutions to inverse problems, ANNs have gained a major popularity in this field. This paper reviews some earlier researches where ANNs were applied to solve different vibration-based inverse parametric identification problems. The adoption of different ANN algorithms, input-output schemes and required signal processing were denoted in considerable detail. In addition, a number of issues have been reported, including the factors that affect ANNs prediction, as well as the advantage and disadvantage of ANN approaches with respect to general inverse methods Based on the critical analysis, suggestions to potential researchers have also been provided for future scopes.

Effect of Irrigation Regimes and Nitrogen Levels on the Growth and Yield of Wheat

A field experiment was carried out to evaluate the effect of irrigation regimes and nitrogen levels on the growth and yield of wheat cv. Kanchan (Triticum aestivum L.). The experiment includes two factors such as four irrigation regimes and four nitrogen levels. Three farmer’s fields were selected for experimentation as replication. Yield and yield contributing factors were significantly affected by irrigation regimes and different doses of nitrogen. Maximum grain yield of 2.27 t ha−1 by the application of 200 mm irrigation treatment. Interaction between 200 mm irrigation and 120 kg N ha−1 was the best combination treatment.