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Early detection of developing structural cracks is extremely important in improving safety and reducing maintenance costs in structural, aerospace, chemical, petrochemical, gas, and oil industries. Nonlinear structural health monitoring methods, such as wave modulation spectroscopy (NWMS), can detect the cracks in their early stages of development. Such methods usually rely on the combination of a high and a low-frequency, vibro-acoustic, excitation for defect detection. However, a priori knowledge of the characteristics of the crack is required for selection of the appropriate frequency combination. In this study, the Comprehensive Heterodyne Effect Based Inspection (CHEBI) method is proposed to address this issue. The CHEBI method applies ascending and descending broadband frequency sweeps simultaneously to study the response of the structure in wide ranges of frequencies in the time-frequency domain. Therefore, it eliminates the need for multiple experimental tests to find appropriate combinations of the high and the low-frequency components. The proposed method detected cracks from 1 mm to 25 mm in length on a dog-bone shaped aluminum specimen. These results confirm that the CHEBI can be successfully applied for detection of cracks with varying severities without the need for adjustment of the excitation frequencies.

With increasing utilization of the composite materials in commercial and military applications, the longevity of composite bonds has become a crucial concern of the aerospace industry. In spite of the previous studies in the literature for characterization of bonding condition in composite materials, implementation of more advanced structural health monitoring (SHM) techniques are still required to ensure flight safety and quick inspections of structures. In this study, two different SHM methods were used to evaluate the strength of composite bonds by using two separate experimental setups. First, the heterodyne effect method was used for assessing the separation in the composite joint. Then, the surface response to excitation (SuRE) method was used for studying various simulated contamination levels. Results of the experimental studies showed that the proposed methods could be efficiently utilized for evaluating bonding strength of composite materials.

Composite materials are a preferred choice when high strength/weight ratio and resistance to corrosion are needed. For assembly, composite parts are joined by using adhesives and/or fasteners. Due to the increased use of composites, there is a need for reliable and affordable structural health monitoring (SHM) methods for the detection of weakened bonds and loosened fasteners. Heterodyne effect may be utilized for the evaluation of debonded area when the linear characteristics of the system changes to nonlinear as a result of light contact in the bonding zone and this nonlinear system responds to appropriate bitonal excitations with new frequencies. Nonlinear elastic wave spectroscopy (NEWS) methods are using the same concept although they are limited to the combination of a high and a low frequency. Heterodyne method allows the engineers to have control over the new output frequencies as indicators of nonlinearity in the target structure. In this study, implementation of the heterodyne method is proposed for identification of the debonded region and evaluation of the compressive forces applied to facing plates. The proposed SHM method proved to be effective in both scenarios.

Array systems of transducers have found numerous applications in detection and localization of defects in structural health monitoring (SHM) of plate-like structures. Different types of array configurations and analysis algorithms have been used to improve the process of localization of defects. For accurate and reliable monitoring of large structures by array systems, a high number of actuator and sensor elements are often required. In this study, a compact phased array system consisting of only three piezoelectric elements is used in conjunction with an updated total focusing method (TFM) for localization of single and multiple defects in an aluminum plate. The accuracy of the localization process was greatly improved by including wave propagation information in TFM. Results indicated that the proposed CPA approach can locate single and multiple defects with high accuracy while decreasing the processing costs and the number of required transducers. This method can be utilized in critical applications such as aerospace structures where the use of a large number of transducers is not desirable. (C) 2017 Elsevier Ltd. All rights reserved.

The Surface Response to Excitation (SuRE) method is a guided-wave based Structural Health Monitoring (SHM) technique. Up to date, no analytical model has been developed and validated for the SuRE method. This paper experimentally and analytically investigates the delamination between two plates using the SuRE method in conjunction with the COMSOL Multiphysics software. Simulation results are validated by experimental results. The results showed that the findings from the analytical approach correspond with the experimental results and can be effectively used for studying delamination. This approach can be utilized for different types of structures with similar conditions.

Composite is one of the most widely used industrial materials because of high strength, low weight, and high corrosion resistance properties. Different parts of composite structures are normally joined using adhesives or fasteners that are prone to defects and damages. A reliable method for prediction of the defect location is needed for an efficient structural health monitoring (SHM) process. Heterodyne effect is recently utilized for damage detection in the bonding zone of composite structures where debonding is expected to change the linear characteristics of the system into nonlinear characteristics. This paper briefly introduces this novel defect locating approach in composite plates using the heterodyne effect. For the first time, an Artificial Neural Network methodology is utilized with heterodyne effect method to find the defect location in composite plates. The main objective of this article is to develop a neural network based methodology for prediction of damage location, particularly for the bond inspection of composite plates.

Surface response to excitation (SuRE) method is a low-cost alternative to electromechanical impedance based structural health monitoring (SHM) technique. The SuRE method uses one piezoelectric transducer to excite the surface of a structure with a sweep sine wave. Piezoelectric sensors or scanning laser vibrometer can be used to monitor the dynamic response of structure. In this study, the performance of the SuRE method was evaluated with the conventional piezoelectric elements and scanning laser vibrometer used as contact and non-contact sensors, respectively, for monitoring the presence of loads on the surface. In order to determine the accuracy and reliability of both monitoring approaches in detecting changes in level of applied load, three different experimental setups were studied. Response of a system in the presence of a single ​load applying and multiple loads applying and its performance in detecting tightness in a nut and bolt system were investigated. The spectrum of the dynamic response is collected at the optimal operating condition. Any significant change of the spectral characteristics may indicate defects, improper loading or loose fasteners. The performance of the SuRE method using contact and non-contact sensors indicated that both variations of the method could be successfully used in load monitoring applications.

Surface response to excitation (SuRE) method was originally developed for structural health monitoring (SHM) applications. SuRE was used to evaluate the performance of completed milling operations. The method generates surface waves on the plate and studies the spectrum changes at selected points to detect defects and change of compressive forces. In this study, the length, depth, and width of a slot were changed step by step. The surface of the aluminum plate was excited in the 20–400 kHz range with a piezoelectric element. A laser scanning vibrometer was used to monitor the vibrations at the predetermined grid points after the dimensions of the slot were changed methodically. The frequency spectrums of measured vibrations were calculated by using the Fast Fourier Transformation (FFT). The sums of the squares of the differences (SSD) of the spectrums were calculated to evaluate the change of the spectrums. The SuRE method was able to determine if the dimensions were changed in each case at all the selected points. The scanning laser vibrometer is not feasible to be used at the shop floor. However, the study demonstrated that a piezoelectric element attached to any of the grid points would be able to evaluate the completed machining process.

With daily increase of application composite materials in commercial and military aircrafts, longevity of composite bonds has become a substantial factor in aerospace industry. Although tremendous amount of research has been conducted on monitoring the integrity of composite bonds, developing new structural health monitoring (SHM) techniques for evaluating bond strength could be essential. In this study, heterodyning method is implemented for exploring bond strength and detecting delamination in composite materials. Heterodyne effect may be utilized for the evaluation of debonded area when the linear characteristics of the system changes to nonlinear. The light contact in the bonding zone can result in this conversion from linear to nonlinear behavior. The nonlinear system responds to two appropriate excitation frequencies with new frequencies. For some nonlinear systems, the output has these new frequencies at the subtraction and summation of excitation frequencies. This is called heterodyne effect. This experimental setup was designed to detect debonding in a composite coupon. In the experiment, two composite coupons are manufactured and bonded together by using film adhesive. To create a 2-inch-long debonding, no adhesive was applied in the left side of the bonded coupons. Three piezoelectric disks were attached to the bonded coupons while the middle disk acting as the exciter. The objective of the experiment was to study the alteration of the signals received by the piezoelectric disks attached to the opposite ends due to nonlinear behavior of the structure. The experimental setup was designed for the investigation of undesirable bonding conditions. Since the method evaluates the bond strength by detection of the changes of linear characteristics of the system to nonlinear, there is no need for baseline data. Results show that the proposed method can be used effectively for the composite bond inspection purposes.

Active structural health monitoring methods are becoming increasingly popular for defect detection in hollow cylindrical structures utilized in aerospace applications. Electro mechanical impedance (EMI) based methods are commonly used to study the health of hollow cylinders in broad frequency ranges. However, the EMI analyzers are bulky and costly. In addition, EMI based methods require high quantity of sensors for monitoring a large area. The Surface Response to Excitation (SuRE) method was developed to eliminate the need for EMI analyzers in broad frequency range inspections and to decrease the number of sensors in monitoring of large structures. In this study, the performance of the SuRE method in detection of internal defects is evaluated theoretically and experimentally. Different internal damages were created in an aluminum pipe specimen. The data collected from four bonded piezoelectric elements were used for damage detection and quantification. The test results showed the high sensitivity and robustness of the SuRE method in internal damage detection.

Manufacturing the raw materials for aerospace structures with embedded sensors to be connected them to the central monitoring station of the structural health monitoring (SHM) systems has been considered by many researchers and industries. These structures can be inspected in very short time by connecting the sensors to a monitoring system. The quality of the part may be evaluated while geometric features such as holes and slots are created during the machining process. Surface response to excitation (SuRE) method is a low-cost alternative to the electromechanical impedance (EMI) method. The SuRE method uses one piezoelectric transducer to excite the surface of a structure with a sweep sine wave, and one or more piezoelectric sensors or scanning laser vibrometer monitor the dynamic response of the system. Once a change which may lead to a structural failure is introduced to the structure, some mechanical properties are altered. The frequency spectrum of the signal received by the sensors will change after the dimensions of the geometric features are modified. In this study, feasibility of the SuRE method was evaluated for inspection of the quality of the machined features on aluminum plates. The SuRE method was used to characterize different geometric features created by the milling operation on identical aluminum plates. One piezoelectric transducer was used as an exciter and propagation of the surface waves were monitored by a scanning laser vibrometer. The Fast Fourier Transformation (FFT) was used for the analysis of the sensory data after each data collection. By increasing the size of milling in two different dimensions, the sum of the squares of the differences (SSD) of the spectrums drastically changed. Also in order to get rid of the need for baseline (reference) data collected at the initial conditions, some advanced techniques such as Geometric mean, Harmonic mean and 5th momentum were introduced.

Surface response to excitation (SuRE) method is a low-cost alternative to the electromechanical impedance method developed for the structural health monitoring (SHM) applications. This method uses one piezoelectric transducer to excite the surface of a structure with a sweep sine wave. One or more piezoelectric sensors or scanning laser vibrometer monitors the dynamic response of the surface to the excitation. The spectrum of the dynamic response is collected at the optimal operating conditions. Any significant change of the spectral characteristics may indicate defects, improper loading or loose fasteners.

Surface Response to Excitation Method (SuRE) is an active structural health monitoring (SHM) method. SuRE method was implemented by exciting the surface of a plate with a piezoelectric element. The propagation of the created surface waves were monitored with one or more sensors. These sensors could be piezoelectric elements or noncontact sensors such as laser scanning vibrometer. During the experiments, one of the piezoelectric elements excited the block at the 20-400 KHz range while the signals of other piezoelectric elements were recorded and their spectrums were calculated. The reference spectrums of sensors were obtained when the block had no defect. However, in the presence of defect the spectrum deviates from reference response. SuRE method quantifies these changes by calculating sum of square of differences (SSD). It is shown that SuRE method is able to effectively detect the defects. doi: 10.12783/SHM2015/233

Rapid screening and monitoring of hollow cylindrical structures using active guided-waves based structural health monitoring (SHM) techniques are important in chemical, petro-chemical, oil and gas industries. Successful implementation of the majority of these techniques in the SHM of pipes depends on the identification of the appropriate guided-waves modes and their frequencies for each application. The highly dispersive nature of the guided-waves and presence of multi modes at each frequency makes the mode selection and the interpretation of signals a challenging task. The surface response to excitation (SuRE) method was developed to detect the defects and loading condition changes on plates with minimum dependence on the excitation of particular modes at certain frequencies. In the present study, the SuRE method is proposed for quantification of longitudinal and circumferential defects, with varying severities, as common examples of axisymmetric and nonaxisymmetric defects in pipes. The results indicate that the SuRE method can be used effectively for damage quantification in hollow cylinders.

Detection of defects in the early stages of growth and nucleation is very important in structural health monitoring (SHM) applications. Defects, such as delamination, debonding, loose bolts and cracks create nonlinear behavior at the surface response. Different techniques have been developed to identify the transformation of a structure from the linear to the nonlinear state in presence of defect(s). Nonlinear wave modulation spectroscopy (NWMS) based methods were developed to detect the nonlinearity created due to combination of low and high excitation frequencies. The excitation signal of the NWMS method is created by modulating the ultrasonic probe signal with a low frequency vibration. In this study, SHM and sensor-less SHM (SSHM) methods are proposed for detection of loose bolts. The proposed methods rely on the heterodyne effect. The frequencies of the two harmonic excitation signals are not limited to the combination of low and high frequencies. In the presence of loose bolts the structure behaves like a nonlinear system and new frequencies are generated. The new frequencies can be observed at the summation, subtraction, and other harmonics of the excitation frequencies. The new frequencies may be monitored with sensors, similar to conventional SHM systems. It is also possible to select the difference of the excitation frequencies to be lower than 20 kHz (audible frequency range) to create audible alarms. The new SSHM eliminates the need for the sensor(s) and processor(s). The SSHM may be used to make very low cost SHM systems or it can be used as back up when the sensor(s) or the processor(s) of the SHM system fail.