Quantification of material memory using high frequency ultrasonic and microcontinuum physics

After an initial reduction, the composite strength undergoes a dormant state that builds up inside the material, we call material memory. At that stage the distributed damage accumulates at the intrinsic length scales. However, such state does not affect the global strength of the materials. There is no experimental tool that can quantify such material state. Here, we introduce a precursor to damage quantification technique by bridging the gap between multiscale modeling and multiscale sensing. We propose a recently developed parameter called “damage entropy”. It is derived from the understanding of micromorphic behavior of materials where deformation is expressed as sum of macroscopic continuous deformation and internal microscopic deformation. The characteristic length scale evolves during the fatigue life of the composite and their progressive influence changes in a quantified sense thus increase the entropy. We obtained the quantified entropy using quantitative ultrasonic imaging and characterization technique, earlier formulated and applied by the authors. Growth of “damage entropy” is demonstrated in a material under fatigue with no visible damage

Elastic Wave Propagation in Sinusoidally Corrugated Waveguides

The ultrasonicwave propagation in sinusoidally corrugated waveguides is studied in this paper. Periodically corrugated waveguides are gaining popularity in the field of vibration control and for designing structures with desired acoustic band gaps. Currently only numerical method (Boundary Element Method or Finite Element Method) based packages (e.g., PZFlex) are in principle capable of modeling ultrasonic fields in complex structures with rapid change of curvatures at the interfaces and boundaries but no analyses have been reported. However, the packages are very CPU intensive; it requires a huge amount of computation memory and time for its execution. In this paper a new semi-analytical technique called Distributed Point Source Method (DPSM) is used to model the ultrasonic field in sinusoidally corrugated waveguides immersed in water where the interface curvature changes rapidly. DPSM results are compared with analytical solutions. It is found that when a narrow ultrasonic beam hits the corrugation peaks at an angle, the wave propagates in the backward direction in waveguides with high corrugation depth. However, in waveguides with small corrugation the wave propagates in the forward direction. The forward and backward propagation phenomenon is found to be independent of the signal frequency and depends on the degree of corrugation

Material State Awareness for Composites Part II: Precursor Damage Analysis and Quantification of Degraded Material Properties Using Quantitative Ultrasonic Image Correlation (QUIC)

Material state awareness of composites using conventional Nondestructive Evaluation (NDE) method is limited by finding the size and the locations of the cracks and the delamination in a composite structure. To aid the progressive failure models using the slow growth criteria, the awareness of the precursor damage state and quantification of the degraded material properties is necessary, which is challenging using the current NDE methods. To quantify the material state, a new offline NDE method is reported herein. The new method named Quantitative Ultrasonic Image Correlation (QUIC) is devised, where the concept of microcontinuum mechanics is hybrid with the experimentally measured Ultrasonic wave parameters. This unique combination resulted in a parameter called Nonlocal Damage Entropy for the precursor awareness. High frequency (more than 25 MHz) scanning acoustic microscopy is employed for the proposed QUIC. Eight woven carbon-fiber-reinforced-plastic composite specimens were tested under fatigue up to 70% of their remaining useful life. During the first 30% of the life, the proposed nonlocal damage entropy is plotted to demonstrate the degradation of the material properties via awareness of the precursor damage state. Visual proofs for the precursor damage states are provided with the digital images obtained from the micro-optical microscopy, the scanning acoustic microscopy and the scanning electron microscopy

Elastic Wave Field Computation in Multilayered Nonplanar Solid Structures: A Mesh-Free Semianalytical Approach

Multilayered solid structures made of isotropic, transversely isotropic, or general anisotropic materials are frequently used in aerospace, mechanical, and civil structures. Ultrasonic fields developed in such structures by finite size transducers simulating actual experiments in laboratories or in the field have not been rigorously studied. Several attempts to compute the ultrasonic field inside solid media have been made based on approximate paraxial methods like the classical ray tracing and multi-Gaussian beam models. These approximate methods have several limitations. A new semianalytical method is adopted in this article to model elastic wave field in multilayered solid structures with planar or nonplanar interfaces generated by finite size transducers. A general formulation good for both isotropic and anisotropicsolids is presented in this article. A variety of conditions have been incorporated in the formulation including irregularities at the interfaces. The method presented here requires frequency domain displacement and stress Green’s functions. Due to the presence of different materials in the problem geometry various elastodynamic Green’s functions for different materials are used in the formulation. Expressions of displacement and stress Green’s functions for isotropic and anisotropicsolids as well as for the fluid media are presented. Computed results are verified by checking the stress and displacement continuity conditions across the interface of two different solids of a bimetal plate and investigating if the results for a corrugated plate with very small corrugation match with the flat plate results

Material State Awareness for Composites Part I: Precursor Damage Analysis Using Ultrasonic Guided Coda Wave Interferometry (CWI)

Detection of precursor damage followed by the quantification of the degraded material properties could lead to more accurate progressive failure models for composite materials. However, such information is not readily available. In composite materials, the precursor damages—for example matrix cracking, microcracks, voids, interlaminar pre-delamination crack joining matrix cracks, fiber micro-buckling, local fiber breakage, local debonding, etc.—are insensitive to the low-frequency ultrasonic guided-wave-based online nondestructive evaluation (NDE) or Structural Health Monitoring (SHM) (~100–~500 kHz) systems. Overcoming this barrier, in this article, an online ultrasonic technique is proposed using the coda part of the guided wave signal, which is often neglected. Although the first-arrival wave packets that contain the fundamental guided Lamb wave modes are unaltered, the coda wave packets however carry significant information about the precursor events with predictable phase shifts. The Taylor-series-based modified Coda Wave Interferometry (CWI) technique is proposed to quantify the stretch parameter to compensate the phase shifts in the coda wave as a result of precursor damage in composites. The CWI analysis was performed on five woven composite-fiber-reinforced-laminate specimens, and the precursor events were identified. Next, the precursor damage states were verified using high-frequency Scanning Acoustic Microscopy (SAM) and optical microscopy imaging

Low Frequency Energy Scavenging using Sub-Wave Length Scale Acousto-Elastic Metamaterial

This letter presents the possibility of energy scavenging (ES) utilizing the physics of acousto-elastic metamaterial (AEMM) at low frequencies (\u3c∼3KHz). It is proposed to use the AEMM in a dual mode (Acoustic Filter and Energy Harvester), simultaneously. AEMM’s are typically reported for filtering acoustic waves by trapping or guiding the acoustic energy, whereas this letter shows that the dynamic energy trapped inside the soft constituent (matrix) ofmetamaterials can be significantly harvested by strategically embedding piezoelectric wafers in the matrix. With unit cell AEMM model, we experimentally asserted that at lower acoustic frequencies (\u3c ∼3 KHz), maximum power in the micro Watts (∼35µW) range can be generated, whereas, recently reported phononic crystal based metamaterials harvested only nano Watt (∼30nW) power against 10KΩ resistive load. Efficient energy scavengers at low acoustic frequencies are almost absent due to large required size relevant to the acoustic wavelength. Here we report sub wave length scale energy scavengers utilizing the coupled physics of local, structural and matrix resonances. Upon validation of the argument through analytical, numerical and experimental studies, a multi-frequency energy scavenger (ES) with multi-cellmodel is designed with varying geometrical properties capable of scavenging energy (power output from ∼10µW – ∼90µW) between 0.2 KHz and 1.5 KHz acoustic frequencies

Wave Propagation in Metamaterial using Multiscale Resonators by Creating Local Anisotropy

Directional guiding, passing or stopping of elastic waves through engineered materials have many applications to the engineering fields. Recently, such engineered composite materials received great attention by the broader research community. In elastic waves, the longitudinal and transverse motion of material particles are coupled, which exhibits richer physics and demands greater attention than electromagnetic waves and acoustic waves in fluids. Waves in periodic media exhibit the property of Bragg scattering and create frequency band gaps in which the energy propagation is prohibited. However, in addition to the Bragg scattering, it has been found that local resonance of artificially designed resonators can also play a critical role in the generation of low-frequency band gaps. It has been found that negative effective mass density and negative effective elastic modulus are created by virtue of the local resonators and are correlated with the creation of the frequency band gaps that can be artificially perturbed. In this paper, the authors present a novel anisotropic design of metamaterial using local split-ring resonators of multiple-length scales. Unlike traditional metamaterials, multiple split rings of different dimensions are embedded in a polymer matrix. Considering the complexity of the proposed material, it is extremely difficult to find the dynamic response of the material using analytical methods. Thus, a numerical simulation was performed in order to find frequency band gaps. Simultaneously, correlation between the band gaps and negative effective mass density and negative effective elastic modulus was verified. Both unidirectional split rings and bidirectional chiral split rings were studied. The effects of discontinuity in the rings at larger scales were compared with the dynamic characteristics of full rings in the proposed metamaterial. Application of such metamaterials will be primarily for vibration isolation and impact mitigation of structures. The proposed configuration is based on unit dimension and is, thus, dimensionless. The concept can be easily commutable between macro-scale structures for low-frequency applications and micro-scale MEMS devices for high-frequency applications

Phonon Confinement using Spirally Designed Elastic Resonators in Discrete Continuum

Periodic and chiral orientation of microstructures, here we call phononic crystals, have extraordinary capabilities to facilitate the innovative design of new generation metamaterials. Periodic arrangements of phononic crystals are capable of opening portals of non-passing, non-dispersive mechanical waves. Defying conventional design of regular periodicity, in this paper spirally periodic but chiral orientation of resonators are envisioned. Dynamics of the spirally connected resonators and the acoustic wave propagation through the spirally connected multiple local resonators are studied using fundamental physics. In present study the spiral systems with local resonators are assumed to be discrete media immersed in fluid. In this paper it is assumed that acoustic or ultrasonic waves in fluid are propagated along the plane of the spiral resonators and thus only the longitudinal wave mode exists due to nonexistence of shear stress in the fluid. Lagrangian formulation of the spiral systems were employed to obtain the governing Euler-Lagrange equation of the system. Discrete element method was employed to verify the equation assuming nearest neighboring effect