3D Modeling and Simulation of Guided (Lamb) Wave Propgation in Composite Plate-like Structures:

Elastic waves generated by foreign object impact and initiation or growth of fatigue cracks in structural component's carry useful information about the nature of damage associated with these events. A clear understanding of the quantitative relationship between the waves and their sources is essential in developing algorithms for detecting and characterizing the damage. Model-based analysis of the waveform signals recorded by surface mounted or embedded sensors located in the vicinity of the sources can lead to the development of an effective health monitoring system for a variety of structures.

This research is motivated by the need for an efficient and accurate tool to analyze the wave field produced by localized dynamic sources on the surface or the interior of anisotropic composite laminates. A semi-analytical method based on the wave number integral representation of the electrodynamics field is described that reduces the overall computational effort significantly over other available methods. This method is used to calculate the guided wave field produced in graphite/epoxy composite laminates by dynamic surface loads in an effort to understand the outstanding features of guided waves in simple plate-like structures. The results from an approximate shear deformation plate theory and a finite element simulation using LSDYNA are used for model verification. The modeling and simulation techniques are being extended to sandwich structures for the detection of disbonds. The approach can be used for rapid calculation of the elastic waves generated by PZT transducers, impact sources and fatigue damage initiation in plate-like structures, and should be useful in providing data for NDE and SHM of critical structural components.

Active Damage Monitoring of Structural Components Using Guided Waves:

The current research aims to develop novel algorithms for damage identification and localization in real-time based on ultrasonic techniques with and without based line signals. For example, an autonomous “damage index” approach is being developed that can provide approximate location and severity of the damaged zone without any visual inspection of the signals. The technique stipulates the use of a sparsely distributed high frequency ultrasonic sensor arrays located in critical areas to measure the local response of a structure. Using measurements performed on an undamaged or partially damaged structure as baseline, the damage indices are evaluated from the comparison of the frequency response of the monitored structure with an unknown damage under the same ambient conditions. Similarly, a baseline free algorithm is being developed using time-frequency analysis of the data collected from embedded PZT arrays. Simulations are performed to support experimental results and feature extractions for damage detection. These techniques are applied to metallic and composites structural components involving beams and plates with emerging defects in the forms of impact damage, fatigue cracks, corrosion, rivet holes etc. The relative effectiveness of the methods is examined for a variety of boundary conditions and sensor locations relative to the defects.

Impact-Acoustic Emission Monitoring of Structural Composites:

Acoustic Emission (AE) is the phenomenon in which elastic waves are emitted from sudden release of strain energy during the initiation and extension of cracks or other flaws in structural solids under (fatigue or other loading). Elastic waves are captured to extract information about the nature of damage, and the physical mechanisms responsible for generating the individual acoustic emission (AE) waveforms. Hidden damage caused by foreign object impact in a composite structure, if left undetected, can grow and lead to a catastrophic failure of the structure. Detection of impact events and characterization of the degree of damage caused by them, preferably in real time, would be extremely helpful in safe continued operation of composite structures.

The present study aims to develop a viable quantitative impact damage monitoring tool by combining experimental, theoretical and signal processing techniques. Low velocity impact experiments are carried out in composite plates using an customized impact tester and the surface motion at locations away from the impact point is recorded using an improved ultrasonic test-setup. It has been shown that the occurrence of an impact loading can be easily detected from the recorded signals. Delamination damage, if any, can also be determined through careful analysis of the recorded waveforms. The response of the plate due to localized sources is calculated using an exact plate theory providing detailed information on the relationship between the impact load and the signals generated by the load. Practical applications of the technique in structural health monitoring will require careful investigation and elimination of environmental noise.

Vibration Based Health Monitoring:

Development of efficient methodologies to determine the presence, location, and severity of hidden damage in critical structural components is an important task in the design and construction of structural health monitoring (SHM) systems in aging as well as new structures. The traditional non-destructive inspection (NDI) techniques such as ultrasonic pulse-echo, radiography, thermography, etc., can be time-consuming, costly, labour intensive and often require disassembly of the structure, which make them impractical and infeasible for large area inspections. Although several global damage identification techniques based on vibration approach are developed over the last few decades for SHM, their actual application poses many technical challenges. In particular, application of modal identification techniques considering changes in natural frequencies and mode shapes due to appearance of damage have been found to be challenging. As a result, methods based on the use of frequency response functions (FRFs) are developed due to their effectiveness for reliable damage monitoring in most applications. Thus, this research activity emphasizes the need of a unified procedure to improve the reliability of the defects detection capability based on FRF and aid in the development of autonomous health monitoring systems for defects-critical structures. The technique is being applied in both simple structural components such as beam and plates as well as structural frames.

Condition monitoring of concrete structures using Impact Echo simulation and measurements:

Reinforced concrete (RC) structures are widely used in civil infrastructure systems due to low construction cost and long service life under various conditions. Safety of these structures, in particular, is of paramount importance that requires periodic inspection and maintenance. Testing of thick concrete structures using ultrasonic pulse-echo technique is often difficult due to heavy scattering and attenuation of the sound energy in the medium. Impact-Echo Technique is therefore often utilized for inspections of thicker components. Research activities encompass development of techniques based on simulated and experimental data for rapid inspections of these structures to identify variety of defects.

Atomistic-continuum Modeling Approach:

A newly developed, numerically stable, and robust approach has been used to represent a wide variety of defects in three-dimensional space, spanning from the nanoscale to the macroscopic size. This approach bases the computational differential geometry of the defect on the so-called parametric dislocation dynamics (PDD), combined with local atomistic forces based on ab initio methods, to resolve the elastic field and the energies resulting from the defect. The method mainly accounts for a balance between the elastic forces due to long-range interactions among the dislocations (using elastostatic Green’s function), the curvature of the dislocation itself, applied force, and the local atomistic forces. The high displacement gradients (or discontinuities) across defect surfaces are represented by distributions of dislocations, whose shape is determined by differential geometry. The marriage of the continuum and atomistic treatments combines the strength of each component and represents an important step towards the grand goal of 'putting chemistry into mechanics'. The hybrid method deals with the long-range elastic interactions via the continuum PDD method, and retains the atomic details only at the dislocation core with the help of an ab initio-determined GSF energy (gamma) surface – thus offering a computationally effective method of minimal commitment to atomic details and removing the long-standing problem of singularities intrinsic to the classical continuum theory of dislocations.

This allows the prediction of nano-scale description of the dislocation structure in more realistic geometry and conditions (e.g. loop nucleation from crack tips or penetrating interfaces between anisotropic nono-layers, etc., which, in turn, determines the maximum strength) and offers a computationally effective method of minimal commitment to atomic details.