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Chakrapani S.K.,Center for Nondestructive Evaluation | Barnard D.J.,Center for Nondestructive Evaluation | Dayal V.,Iowa State University
Journal of the Acoustical Society of America | Year: 2015

This paper presents the study of non-classical nonlinear response of fiber-reinforced composites. Nonlinear elastic wave methods such as nonlinear resonant ultrasound spectroscopy (NRUS) and nonlinear wave modulation spectroscopy have been used earlier to detect damages in several materials. It was observed that applying these techniques to composites materials becomes difficult due to the significant inherent baseline nonlinearity. Understanding the non-classical nonlinear nature of the composites plays a vital role in implementing nonlinear acoustic techniques for material characterization as well as qualitative nondestructive testing of composites. Since fiber reinforced composites are orthotropic in nature, the baseline response variation with fiber orientation is very important. This work explores the nature of the inherent nonlinearity by performing nonlinear resonant spectroscopy (NRS) in intact unidirectional carbon/epoxy samples with different fiber orientations with respect to major axis of the sample. Factors such as frequency shifts, modal damping ratio, and higher harmonics were analyzed to explore the non-classical nonlinear nature of these materials. Conclusions were drawn based on the experimental observations. © 2015 Acoustical Society of America. Source


Chakrapani S.K.,Center for Nondestructive Evaluation | Barnard D.J.,Center for Nondestructive Evaluation | Dayal V.,Iowa State University
Composites Part B: Engineering | Year: 2016

In the present work, a nonlinear forced vibration model for fiber reinforced composites was developed with varying fiber orientations and laminate sequences. A nonlinear viscoelastic beam model was developed using nonlinear von Kármán strains and Kelvin-Voigt stress-strain relationship to model viscoelasticity. The effect of fiber orientation and laminate sequence was included in the model using classic laminated plate theory. Method of multiple time scales was used to solve the resulting nonlinear equation and an inverse problem approach was used to extract the model parameters from experimental data. Theoretical model parameters were calculated and compared to experimentally determined values for different fiber orientations and laminate sequences, and a nominally good qualitative agreement was observed. Finally, the experimentally extracted model parameters were substituted in the analytical model, and the model predictions in terms of frequency shifts were compared against experimental observations. Nominally good agreement was observed for 45° and 90° fiber orientations, however the experimental observations didn't match well for 0°. © 2016 Elsevier Ltd. All rights reserved. Source


Chakrapani S.K.,Center for Nondestructive Evaluation | Barnard D.J.,Center for Nondestructive Evaluation | Dayal V.,Iowa State University
Journal of the Acoustical Society of America | Year: 2016

This paper presents the study of influence of laminate sequence and fabric type on the baseline acoustic nonlinearity of fiber-reinforced composites. Nonlinear elastic wave techniques are increasingly becoming popular in detecting damage in composite materials. It was earlier observed by the authors that the non-classical nonlinear response of fiber-reinforced composite is influenced by the fiber orientation [Chakrapani, Barnard, and Dayal, J. Acoust. Soc. Am. 137(2), 617-624 (2015)]. The current study expands this effort to investigate the effect of laminate sequence and fabric type on the non-classical nonlinear response. Two hypotheses were developed using the previous results, and the theory of interlaminar stresses to investigate the influence of laminate sequence and fabric type. Each hypothesis was tested by capturing the nonlinear response by performing nonlinear resonance spectroscopy and measuring frequency shifts, loss factors, and higher harmonics. It was observed that the laminate sequence can either increase or decrease the nonlinear response based on the stacking sequence. Similarly, tests were performed to compare unidirectional fabric and woven fabric and it was observed that woven fabric exhibited a lower nonlinear response compared to the unidirectional fabric. Conjectures based on the matrix properties and interlaminar stresses were used in an attempt to explain the observed nonlinear responses for different configurations. © 2016 Acoustical Society of America. Source


Li Y.,Iowa State University | Li Y.,Center for Nondestructive Evaluation | Bowler N.,Iowa State University
Studies in Applied Electromagnetics and Mechanics | Year: 2010

A microwave technique for nondestructive evaluation of radome sandwich structures is presented. The method employs a rectangular patch sensor for detecting defects of the radome core, such as water ingression, in the form of a change in resonant frequency of the sensor due to permittivity variations in the test-piece. Using well-established models that predict the resonant frequency of a rectangular patch antenna in the presence of a dielectric superstrate, the effects of different patch sensor parameters on sensor performance are analyzed. Based on this analysis, design of a half-wave patch sensor is optimized. A prototype half-wave patch sensor is fabricated and tested. Experimental results show that the sensitivity of the sensor to permittivity changes in the core of a laminar structure is better than Δε = 0.046. © 2010 The authors and IOS Press. All rights reserved. Source


Siegler J.,Iowa State University | Siegler J.,Center for Nondestructive Evaluation | Leifsson L.,Iowa State University | Leifsson L.,Reykjavik University | And 3 more authors.
Procedia Computer Science | Year: 2016

Ultrasonic testing (UT) is used to detect internal flaws in materials or to characterize material properties. Computational simulations are an important part of the UT process. Fast models are essential for UT applications such as inverse design or model-assisted probability of detection. This paper presents investigations of using surrogate modeling techniques to create fast approximate models of UT simulator responses. In particular, we propose to use data-driven surrogate modeling techniques (kriging interpolation), and physics-based surrogate modeling techniques (space mapping), as well a mixture of the two approaches. These techniques are investigated for two cases involving UT simulations of metal components immersed in a water bath during the inspection process. © The Authors. Published by Elsevier B.V. Source

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