Antonios Kontsos

Mechanical properties and biomineralization of multifunctional nanodiamond-PLLA composites for bone tissue engineering.

Multifunctional bone scaffold materials have been produced from a biodegradable polymer, poly(L-lactic acid) (PLLA), and 1-10% wt of octadecylamine-functionalized nanodiamond (ND-ODA) via solution casting followed by compression molding. By comparison to pure PLLA, the addition of 10% wt of ND-ODA resulted in a significant improvement of the mechanical properties of the composite matrix, including a 280% increase in the strain at failure and a 310% increase in fracture energy in tensile tests. The biomimetic process of bonelike apatite growth on the ND-ODA/PLLA scaffolds was studied using microscopic and spectroscopic techniques. The enhanced mechanical properties and the increased mineralization capability with higher ND-ODA concentration suggest that these biodegradable composites may potentially be useful for a variety of biomedical applications, including scaffolds for orthopedic regenerative engineering.

An integrated structural health monitoring approach for crack growth monitoring

A novel structural health monitoring approach consisting of guided ultrasonic waves, acoustic emission, and digital image correlation, as well as real-time and postmortem analyses, was implemented to monitor and quantify crack growth in Al 2024 compact tension specimens, designed, and precracked according to ASTM E647-08. Tensile loads were applied according to ASTM E1290-08. Guided ultrasonic waves were generated with pulses centered at three different frequencies and were recorded using piezoelectric transducers. Guided ultrasonic waves were also modeled using finite element wave propagation models. The same transducers were further used for online acoustic emission monitoring. A digital image correlation system continuously monitored the crack growth and provided full-field surface strains. The application of this integrated structural health monitoring approach resulted in reliable damage detection and quantified crack growth measurements. In addition, a novelty detector based on the Mahalanobis distance was implemented in a data fusion scheme to assess the extent of damage. The reported results constitute a proof-of-concept investigation of a novel structural health monitoring approach based on the combination of real-time optical and acoustic nondestructive testing.

Phase-Field Modeling of Domain Structure Energetics and Evolution in Ferroelectric Thin Films

A computational model developed based on the phase-field approach is used to model domain structures in ferroelectric thin films and to quantify the effects of strain and applied electric field on the microstructural evolution, and on the induced dielectric, electrostrictive, and piezoelectric film properties. Theoretically predicted vortex-like poly-domain and experimentally observed bidomain and monodomain film morphologies are modeled using the continuum phase-field approach. A nonlinear finite element method is used to solve the boundary value problems relevant to ferroelectric thin films. The computed results agree with the Kittel law for specific ranges of film strain. Simulations that track the domain structure evolution and compute ferroelectric thin film properties given the film dimensions and the imposed electromechanical boundary conditions are also reported.

A data fusion approach for progressive damage quantification in reinforced concrete masonry walls

This paper presents a data fusion approach based on digital image correlation (DIC) and acoustic emission (AE) to detect, monitor and quantify progressive damage development in reinforced concrete masonry walls (CMW) with varying types of reinforcements. CMW were tested to evaluate their structural behavior under cyclic loading. The combination of DIC with AE provided a framework for the cross-correlation of full field strain maps on the surface of CMW with volume-inspecting acoustic activity. AE allowed in situ monitoring of damage progression which was correlated with the DIC through quantification of strain concentrations and by tracking crack evolution, visually verified. The presented results further demonstrate the relationships between the onset and development of cracking with changes in energy dissipation at each loading cycle, measured principal strains and computed AE energy, providing a promising paradigm for structural health monitoring applications on full-scale concrete masonry buildings.

Acoustic Emission of Deformation Twinning in Magnesium

The Acoustic Emission of deformation twinning in Magnesium is investigated in this article. Single crystal testing with combined full field deformation measurements, as well as polycrystalline testing inside the scanning electron microscope with simultaneous monitoring of texture evolution and twin nucleation were compared to testing at the laboratory scale with respect to recordings of Acoustic Emission activity. Single crystal testing revealed the formation of layered twin boundaries in areas of strain localization which was accompanied by distinct changes in the acoustic data. Testing inside the microscope directly showed twin nucleation, proliferation and growth as well as associated crystallographic reorientations. A post processing approach of the Acoustic Emission activity revealed the existence of a class of signals that appears in a strain range in which twinning is profuse, as validated by the in situ and ex situ microscopy observations. Features extracted from such activity were cross-correlated both with the available mechanical and microscopy data, as well as with the Acoustic Emission activity recorded at the laboratory scale for similarly prepared specimens. The overall approach demonstrates that the method of Acoustic Emission could provide real time volumetric information related to the activation of deformation twinning in Magnesium alloys, in spite of the complexity of the propagation phenomena, the possible activation of several deformation modes and the challenges posed by the sensing approach itself when applied in this type of materials evaluation approach.

Energy dissipation via acoustic emission in ductile crack initiation

This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture.

Investigation on Bridge Assessment Using Unmanned Aerial Systems

The U.S. currently spends tens of billions of dollars annually to inspect infrastructures and collect subjective, qualitative data that can often be unreliable or largely irrelevant. Inspections also require adequate access to remote locations, for example, appropriate scaffolding, lifting and additional protective equipment, which might increase the associated personal safety risk and add costs. The use of unmanned vehicles has experienced a tremendous growth primarily in military and homeland security applications. However, it is a matter of time until Unmanned Aerial Systems (UAS) will be widely accepted as platforms for implementing monitoring and inspection procedures. Researchers at Drexel University are exploring the use of quadcopters as vehicles to carry a set of remote sensors with the ultimate goal to perform bridge condition assessment. While the accuracy of remote sensing systems is somewhat limited compared to the one of contact sensing systems, the ability to quickly and periodically scan/inspect a structure without the need for scaffolding, ropes, or cherry pickers currently used during bridge inspections could transform the way the industry performs periodic bridge inspections. The Drexel team owns a number of UAS with different payload, flight time and range capabilities. In this paper, recent results obtained from preliminary testing on small mock-up concrete bridge decks as well as on small/medium size bridges are presented. One of the main efforts is to explore how multispectral imaging can provide a preliminary assessment of the deck condition of common highway bridges. Among future goals, Drexel’s team plans to develop and validate computer vision approaches leveraging data collected using UAS to permit geometric characterization (quantification of bearing position, girder deformations) and condition assessment (e.g. quantification of spalling and corrosion areas).

Acoustics and temperature based NDT for damage assessment of concrete masonry system subjected to cyclic loading

This paper represents a hybrid non-destructive testing (HNDT) approach based on infrared thermography (IRT), acoustic emission (AE) and ultrasonic (UT) techniques for effective damage quantification of partially grouted concrete masonry walls (CMW). This integrated approach has the potential to be implemented for the health monitoring of concrete masonry systems. The implementation of this hybrid approach assists the cross validation of in situ recorded information for structural damage assessment. In this context, NDT was performed on a set of partially grouted CMW subjected to cyclic loading. Acoustic emission (AE) signals and Infrared thermography (IRT) images were recorded during each cycle of loading while the ultrasonic (UT) tests were performed in between each loading cycle. Four accelerometers, bonded at the toe of the wall, were used for recording waveforms for both passive (AE) and active (UT) acoustics. For the active approach, high frequency stress waves were generated by an instrumented hammer and the corresponding waveforms were recorded by the accelerometers. The obtained AE, IRT, and UT results were correlated to visually confirm accumulated progressive damage throughout the loading history. Detailed post-processing of these results was performed to characterize the defects at the region of interest. The obtained experimental results demonstrated the potential of the methods to detect flaws on monitored specimens; further experimental investigations are planned towards the quantitative use of these NDT methods.

Automated Detection of Damaged Areas after Hurricane Sandy using Aerial Color Images

Rapid detection of damaged buildings after natural disasters, such as earthquakes and hurricanes, is an urgent need for first response, rescue and recovery planning. In this context, post-event aerial images which could be collected right after disasters are valuable sources for damage detection. However, manual analysis process of the acquired imagery could be both time consuming and costly. To address this issue, a series of classification models for post-hurricane automated detection of damaged buildings is presented in this paper. First, five feature sets were generated through feature extraction and transformation. Then, several classifiers were trained using two groups of classification methods: (1) the Minimum-distance and (2) the Support Vector Machine (SVM) methods. The effectiveness of these classifiers was evaluated in terms of classification accuracies and testing time. The results demonstrated the combination of feature sets and classification methods can provide the best performance. Furthermore, optimal classifiers were selected for future automated real-time damaged building detection. The observed performances of these optimal classifiers indicate promising application for a wide variety of image-based classification tasks.

Acoustic Emission and Digital Image Correlation as Complementary Techniques for Laboratory and Field Research

This article presents the advantages of combining Acoustic Emission (AE) and Digital Image Correlation (DIC) in nondestructive testing (NDT) applications focusing on in situ damage monitoring. This data-fusion approach is used herein to characterize the mechanical and damage behavior of a fiber metal laminate (Glare 1A) tested in both tension and fatigue. Furthermore, the approach is used to investigate the structural behavior of partially grouted reinforced masonry walls. The obtained AE datasets were post-processed, in combination with DIC and mechanical information, using signal processing and pattern recognition techniques to investigate progressive failure of the Glare 1A. In the case of the masonry wall specimens, DIC clearly identified critical damage areas as a function of applied loading, while AE was capable to monitor the damage process and reveal changes in the overall behavior. The presented analysis demonstrates the potential of integrating AE and DIC in data-driven damage mechanics investigations at multiple time and length scales.