Mohammad I. Albakri

Electromechanical impedance–based damage characterization using spectral element method

The high-frequency nature of impedance-based structural health monitoring makes the utilization of impedance signature for model-based damage characterization a challenging problem. In this study, a novel damage characterization approach that utilizes impedance signature measured with a single piezoelectric wafer is developed. Length-varying spectral elements are introduced to minimize the total number of elements required to describe the system, along with the number of damage characterization parameters. Several objective function definitions are studied and their behaviour with respect to each damage parameter is investigated. It has been found that an objective function definition based on the frequency shift in impedance peaks is the most effective definition compared to root mean square deviation and correlation-based objective functions. A novel damage localization method, referred to as sine-fit localization method, is developed based on the underlying periodic behaviour of impedance peak shifts as a function of damage location. The sine-fit localization method is integrated with gradient descend method in a two-stage optimization algorithm for damage characterization. The developed algorithm is capable of solving the ill-posed problem of damage characterization with few iterations and small number of objective function evaluations, which makes it computationally very efficient.

Dynamic analysis of a piezoelectric augmented beam system with adhesive bonding layer effects

Embedded and surface bonded piezoelectric wafers have been widely used for control and monitoring purposes. Several nondestructive evaluation and structural health monitoring techniques, such as electromechanical impedance and wave propagation–based techniques, utilize piezoelectric wafers in either active or passive manner to interrogate the host structure. The basis of all these techniques is the energy transfer between the piezoelectric wafer and the host structure which takes place through an adhesive bonding layer. In this article, the high-frequency dynamic response of a coupled piezoelectric-beam system is modeled including the adhesive bonding layer in between. A new three-layer spectral element is developed for this purpose. The formulation of this new element takes into account axial and shear deformations, in addition to rotary inertia effects in all three layers. The capabilities of the proposed model are demonstrated through several numerical examples, where the effects of bonding layer geometric and material characteristics on dispersion relations and damage detection capabilities are discussed. The results highlight the importance of accounting for the adhesive bonding layer in piezoelectric-structure interaction models, especially when the high-frequency dynamic response is of interest.

An experimental and theoretical study of two-dimensional traveling waves in plates:

The present work explores the generation of two-dimensional steady-state flexural waves that are non-reflective on a thin rectangular plate with free boundary conditions when excited by two macro-fiber composites (MFCs). The voltage signals to the MFCs have a frequency lying halfway between two adjacent resonant frequencies with a phase difference of 90 ∘ . Locations of MFCs and frequencies of actuation are varied to study the response of the plate due to these forces. A finite element plate model is developed and updated based on experimental modal tests on the plate. This model is able to predict up to the 40th damped eigenvalue with a maximum error of 2.5% and match mode shapes accurately, with a lowest Modal Assurance Criterion value of 0.92. Numerical simulations of traveling waves have been carried out and are compared with experimental results. Preliminary results show that the location of the MFCs and the frequency of excitation have an effect on the type and the quality of the traveling waves. These results shall lay the foundation for an exhaustive analysis of planar traveling waves in plates.

Reduced Plate Model Used for 2D Traveling Wave Propagation

The focus of this study is to understand traveling wave generation and propagation in reduced order 2D plate models. A plate with all clamped (C-C-C-C) boundary conditions was selected to be the medium through which the wave propagation occurs. The plate is excited at multiple locations by point forces which generates controlled oscillations resulting in net traveling waves. A finite element model is developed and the traveling wave response is simulated. The numerical model is complex with a large number of degrees-of-freedom making a parametric study computationally intensive. In order to overcome this computational burden, balanced truncation based and interpolation-based model reduction techniques are employed to reduce the total number of degrees-of-freedom. The capabilities of these reduction techniques to capture the steady-state frequency-domain characteristics and the steady-state time-domain response have been compared in this paper.Copyright

Impedance-based non-destructive evaluation of additively manufactured parts

Purpose This work proposes the utilization of electromechanical impedance measurements as a means of non-destructive evaluation (NDE) for additive manufacturing (AM). The effectiveness and sensitivity of the technique for a variety of defect types commonly encountered in AM are investigated. Design/methodology/approach To evaluate the feasibility of impedance-based NDE for AM, the authors first designed and fabricated a suite of test specimens with build errors typical of AM processes, including dimensional inaccuracies, positional inaccuracies and internal porosity. Two polymer AM processes were investigated in this work: material jetting and extrusion. An impedance-based analysis was then conducted on all parts and utilized, in a supervised learning context, for identifying defective parts. Findings The newly proposed impedance-based NDE technique has been proven to be an effective solution for detecting several types of print defects. Specifically, it was shown that the technique is capable of detecting print defects resulting in mass change (as small as 1 per cent) and in feature displacement (as small as 1 mm) in both extruded nylon parts and jetted VeroWhitePlus parts. Internal porosity defects were also found to be detectable; however, the impact of this defect type on the measured impedance was not as profound as that of dimensional and positional inaccuracies. Originality/value Compared to currently available NDE techniques, the newly proposed impedance-based NDE is a functional-based technique with the advantages of being cost-effective, sensitive and suitable for inspecting AM parts of complex geometry and deeply embedded flaws. This technique has the potential to bridge the existing gaps in current NDE practices, hence paving the road for a wider adoption of AM to produce mission-critical parts.

Investigation of propulsive characteristics due to traveling waves in continuous finite media

The present work generates steady-state traveling waves in fin-like continuous structures with the help of Macro-Fiber Composite (MFC) piezoelectric actuators. To produce traveling waves, two MFCs simultaneously excite a clamped-free beam at a common frequency with a preset phase difference. Previous research has shown that optimal traveling waves are developed in structures when the common frequency lies halfway between two adjacent resonant frequencies and the phase difference between the two inputs is 90°. These traveling waves closely replicate the undulatory patterns that propel aquatic animals but at a low amplitude linear regime. The present work studies the generation of underwater traveling waves and investigates the range of propulsive forces generated through such mechanism. The ability of continuous structures to produce thrust using undulations is the first step towards mimicking the bio-kinematics and biomechanics of aquatic animals.

A Novel Acoustoelastic-Based Technique for Stress Measurement in Structural Components

The acoustoelastic theory has been widely utilized for nondestructive stress measurement in structural components. Most of the currently available techniques operate at the high-frequency, weakly-dispersive portions of the dispersion curves, and rely on time-of-flight measurements to quantify the effects of stress state on wave speed. This adversely affects the sensitivity and accuracy of such techniques, and renders their accuracy limited by the precision within which time-of-flight can be determined.

Traveling Wave Generation on a Clamped, Thin Plate with Flush-Mounted Piezoelectric Actuators

Structural traveling waves have significant potential in applications such as propulsion and drag reduction. Using the two-mode excitation method, traveling waves which are both steady-state and open-loop controlled can be generated on various structures such as beams, plates, and cylinders. In order to develop this method further for applications such as drag reduction, more representative cases must be investigated. This work models and experimentally validates traveling waves generated on a thin, fully clamped plate with flush-mounted piezoelectric actuators. A two-step experimental modal analysis is conducted on a free and then clamped plated to validate and update a finite-element model of the plate. The finite element model, which has been developed in-house, accounts for piezoelectric actuators, pre-stresses in the plate, and non-ideal (elastic) clamped boundary conditions. The validated model is then used to compare experimental- and model-generated traveling waves.

NDE of Additively Manufactured Parts via Directly Bonded and Mechanically Attached Electromechanical Impedance Sensors

Additive Manufacturing (AM) allows increased complexity which poses challenges to quality-control (QC) and non-destructive evaluation (NDE) of manufactured parts. The lack of simple, reliable, and inexpensive methods for NDE of AM parts is a significant obstacle to wider adoption of AM parts.

In-field implementation of impedance-based structural health monitoring for insulated rail joints

Track defects are a major safety concern for the railroad industry. Among different track components, insulated rail joints, which are widely used for signaling purposes, are considered a weak link in the railroad track. Several joint-related defects have been identified by the railroad community, including rail wear, torque loss, and joint bar breakage. Current track inspection techniques rely on manual and visual inspection or on specially equipped testing carts, which are costly, timeconsuming, traffic disturbing, and prone to human error. To overcome the aforementioned limitations, the feasibility of utilizing impedance-based structural health monitoring for insulated rail joints is investigated in this work. For this purpose, an insulated joint, provided by Koppers Inc., is instrumented with piezoelectric transducers and assembled with 136 AREA rail plugs. The instrumented joint is then installed and tested at the Facility for Accelerated Service Testing, Transportation Technology Center Inc. The effects of environmental and operating conditions on the measured impedance signatures are investigated through a set of experiments conducted at different temperatures and loading conditions. The capabilities of impedance-based SHM to detect several joint-related damage types are also studied by introducing reversible mechanical defects to different joint components.