Amir Nasrollahi

Multimodal structural health monitoring based on active and passive sensing

We present a structural health monitoring system based on the simultaneous use of passive and active sensing. The passive approach is based on acoustic emission, whereas the active approach uses the electromechanical impedance and the guided ultrasonic wave methods. As all these methods can be deployed with the use of wafer-type piezoelectric transducers bonded or embedded to the structure of interest, this article describes a unified structural health monitoring system where acoustic emission, electromechanical impedance, and guided ultrasonic wave are integrated in the same hardware/software unit. We assess the feasibility of this multimodal monitoring in a large flat aluminum plate instrumented with six transducers. Acoustic emission events are simulated by exciting a tone burst or by using the conventional pencil lead break test, and the detected signals are processed with a source localization algorithm to identify the position of the source. For the active sensing, damage is simulated by adding a small mass to the plate: the raw waveforms are processed with a delay-and-sum algorithm to create an image of the plate, whereas the electrical admittance of each transducer is analyzed using the statistical index of the root-mean-square deviation. The results presented in this article show that the proposed system is robust, mitigates the weaknesses of each method considered individually, and can be developed further to address the challenges associated with the structural health monitoring of complex structures.

Nondestructive testing of concrete using highly nonlinear solitary waves

Abstract In this article, we describe the capability of a nondestructive evaluation (NDE) method at determining the strength of concrete surfaces. The method uses highly non-linear solitary waves (HNSWs) propagating inside a metamaterial in contact with the concrete to be tested. The metamaterial consists of a chain of spherical particles, and is part of a built-in transducer designed to excite and detect HNSWs. The NDE method exploits the dynamic interaction between the metamaterial and the concrete, and the hypothesis is that this interaction depends on the stiffness of the specimen being inspected. In this study, we tested this hypothesis by assembling two kinds of transducer and by casting cylinders with different water-to-cement ratios. The results show that the time of flight of the HNSWs is inversely proportional to the modulus of elasticity of the concrete. Once fully developed, the proposed NDE method may easily assess concrete surfaces.

On the Reliability of a Solitary Wave Based Transducer to Determine the Characteristics of Some Materials

In the study presented in this article we investigated the feasibility and the reliability of a transducer design for the nondestructive evaluation (NDE) of the stiffness of structural materials. The NDE method is based on the propagation of highly nonlinear solitary waves (HNSWs) along a one-dimensional chain of spherical particles that is in contact with the material to be assessed. The chain is part of a built-in system designed and assembled to excite and detect HNSWs, and to exploit the dynamic interaction between the particles and the material to be inspected. This interaction influences the time-of-flight and the amplitude of the solitary pulses reflected at the transducer/material interface. The results of this study show that certain features of the waves are dependent on the modulus of elasticity of the material and that the built-in system is reliable. In the future the proposed NDE method may provide a cost-effective tool for the rapid assessment of materials’ modulus.

Nondestructive assessment of waveguides using an integrated electromechanical impedance and ultrasonic waves approach

In this paper we present a structural health monitoring (SHM) paradigm based on the simultaneous use of ultrasounds and electromechanical impedance (EMI) to monitor waveguides. The paradigm uses guided ultrasonic waves (GUWs) in pitch-catch mode and EMI simultaneously. The two methodologies are driven by the same sensing/hardware/software unit. To assess the feasibility of this unified system an aluminum plate was monitored for varying damage location. Damage was simulated by adding small masses to the plate. The results associated with pitch-catch GUW testing mode were used in ultrasonic tomography, and statistical analysis was used to detect the damages using the EMI measurements. The results of GUW and EMI monitoring show that the proposed system is robust and can be developed further to address the challenges associated with the SHM of complex structures.

Metamaterials made of granular chains for nondestructive evaluation applications

In the last two decades it has been demonstrated that highly nonlinear solitary waves (HNSWs) propagating in metamaterials made of chains of granular particles can be used in many physics and engineering applications, including acoustic lenses, impurity detectors, and nondestructive evaluation (NDE). HNSWs are compact nondispersive waves, and in this paper, we demonstrate their application for the nondestructive evaluation of civil, mechanical, and aerospace structures. In particular, this presentation delves with the non-invasive assessment of concrete strength and the evaluation of thermal stress in slender beams. For the concrete application, the NDE method exploits the dynamic interaction between the metamaterial and the concrete, and the hypothesis is that this interaction depends on the stiffness of the specimen being inspected. The results show that the time of flight of the HNSWs is inversely proportional to the modulus of elasticity of the concrete. For the buckling application, we present the co...

Non-Contact Smartphone-Based Monitoring of Thermally Stressed Structures

The in-situ measurement of thermal stress in beams or continuous welded rails may prevent structural anomalies such as buckling. This study proposed a non-contact monitoring/inspection approach based on the use of a smartphone and a computer vision algorithm to estimate the vibrating characteristics of beams subjected to thermal stress. It is hypothesized that the vibration of a beam can be captured using a smartphone operating at frame rates higher than conventional 30 Hz, and the first few natural frequencies of the beam can be extracted using a computer vision algorithm. In this study, the first mode of vibration was considered and compared to the information obtained with a conventional accelerometer attached to the two structures investigated, namely a thin beam and a thick beam. The results show excellent agreement between the conventional contact method and the non-contact sensing approach proposed here. In the future, these findings may be used to develop a monitoring/inspection smartphone application to assess the axial stress of slender structures, to predict the neutral temperature of continuous welded rails, or to prevent thermal buckling.

Recent developments on the generation and detection of highly nonlinear solitary waves for NDE applications (Conference Presentation)

We describe the feasibility and the repeatability of highly nonlinear solitary waves (HNSWs) in nondestructive evaluation (NDE) of different materials and structures. HNSWs formed by tapping the first particle of a one-dimensional chain of spherical particles in contact with the material or the structure being assessed. This way, one pulse forms and travel within the chain, and one or more pulses (depending on the mechanical properties of the contact structure) reflect back to the chain at the chain/structure interface. The hypothesis is that the reflected pulses features such as their amplitude or velocity depend on the stiffness of the material or the structure in contact with the chain. The results show that the time-of-flight (TOF) is an appropriate wave feature for NDE applications because it is highly repeatable and influenced by the mechanical properties of contact material/structure. In the future, the proposed NDE method may potentially serve as a cost-effective tool for the rapid evaluation of existing structures.

Axial stress determination using highly nonlinear solitary waves

This article presents a nondestructive evaluation (NDE) method to infer the neutral temperature and the axial stress in thick beams. The method relies on the propagation of highly nonlinear solitary waves generated at one end of a chain of spherical particles in a dry point contact with the beam to be evaluated. The waves are reflected back to the chain and the research hypothesis is that the axial stress, which influences the beams stiffness, affects the amplitude and speed of the reflected waves. To verify this hypothesis a general finite element model of thermally stressed beams was developed and coupled to a discrete particle model able to predict the propagation of the waves along an L-shaped granular medium. The models were validated experimentally to quantify the repeatability of the setup, the sensitivity of the wave features on the thermal stress, and the independence of the wave features on the neutral temperature of the beam. The hypothesis was proven valid by both the numerical and the experimental results. In the future, these findings may be used to refine a NDE method to assess stress in columns, to infer the neutral temperature of continuous welded rails, and to prevent thermal buckling of critical structures.

Integration of AE, GUWs, and EMI (Conference Presentation)

In this paper, an integrated nondestructive evaluation / structural health monitoring (NDE / SHM) system based on the use of acoustic emission (AE), electromechanical impedance (EMI) and guided ultrasonic waves (GUWs) is presented. The system is integrated into a single hardware/software unit and is driven by a few graphical user interfaces created in the laboratory. The feasibility of this multi-modal monitoring approach is assessed by monitoring an aluminum plate with an array of six wafer-type piezoelectric transducers. AE events are generated with the pencil-lead break technique whereas damage is simulated in the form of permanent magnets attached to the plate. The waveforms associated with the AE are processed using a source localization approach, whereas the GUWs and EMI data are processed using simple metrics based on cross-correlation. The results presented here show that the proposed system is robust and the three NDT methods complement each other very well.

Highly nonlinear solitary waves to estimate the modulus of concrete with different water-to-cement ratios

We describe the feasibility of a nondestructive evaluation (NDE) method for concrete based on the propagation of highly nonlinear solitary waves (HNSWs) along a one-dimensional chain of spherical particles placed in contact with the concrete to be tested. The chain is part of a built-in transducer designed and assembled to excite and detect HNSWs. The method exploits the dynamic interaction between the particles and the concrete. The hypothesis is that the interaction depends on the stiffness of the concrete and influences the time of flight and amplitude of the solitary pulses reflected at the transducer/concrete interface, and traveling within the chain. The results show that the time of flight is monotonically dependent upon the modulus of elasticity of the concrete and that the transducers designed and assembled in this study are reliable and repeatable. In the future, the proposed NDE method may potentially serve as a cost-effective tool for the rapid evaluation of existing concrete structures.