Showing papers in "Journal of Mechanics of Materials and Structures in 2007"

In-Situ Optimized PWAS Phased Arrays for Lamb Wave Structural Health Monitoring

Abstract: The use of piezoelectric wafer active sensors (PWAS) phased arrays for Lamb wave damage detection in thin-wall structures is presented. The PWAS capability to tune into specific Lamb-wave modes (which is an enabling factor for our approach) is first reviewed. Then, a generic beamforming formulation that does not require the conventional parallel-ray approximation is developed for PWAS phased arrays in connection with the delay-and-sum beamforming principles. This generic formulation is applied to a 1-D linear PWAS phased array. Particularly, 1-D PWAS array beamforming reduces to the simplified parallel ray algorithm when the parallel ray approximation is invoked. The embedded ultrasonic structural radar (EUSR) algorithm is presented. A couple of simple experiments are used to show that the linear EUSR PWAS phased array system can successfully detect cracks in large aluminum thin plates. To improve the EUSR image quality, advanced signal processing is studied for possible integration into the EUSR system. The approaches include Hilbert transform for envelope detection, thresholding techniques for removing background noise, discrete wavelet transform for denoising, continuous wavelet transform for single frequency component extraction, and cross-correlation for time-of-flight detection. The optimization of linear PWAS arrays is studied next. First we consider the effect of several parameters affecting the phased-array beamforming: (1) number of elements M; (2) elementary spacing d; (3) steering angle 0; (4) location of the target r. Second, we examine the so-called nonuniform PWAS arrays which are generated by assigning different excitation weights to each of the array elements. The design of two nonuniform linear PWAS arrays, the binomial array and the Dolph‐Chebyshev array, is presented. Significant improvement of the EUSR image is observed when using these nonuniform arrays.

Deformation and fracture modes of sandwich structures subjected to underwater impulsive loads

Abstract: Sandwich panel structures with thin front faces and low relative density cores offer significant impulse mitigation possibilities provided panel fracture is avoided. Here steel square honeycomb and pyramidal truss core sandwich panels with core relative densities of 4% were made from a ductile stainless steel and tested under impulsive loads simulating underwater blasts. Fluid-structure interaction experiments were performed to (i) demonstrate the benefits of sandwich structures with respect to solid plates of equal weight per unit area, (ii) identify failure modes of such structures, and (iii) assess the accuracy of finite element models for simulating the dynamic structural response. Both sandwich structures showed a 30% reduction in the maximum panel deflection compared with a monolithic plate of identical mass per unit area. The failure modes consisted of core crushing, core node imprinting/punch through/tearing and stretching of the front face sheet for the pyramidal truss core panels. Finite element analyses, based on an orthotropic homogenized constitutive model, predict the overall structural response and in particular the maximum panel displacement.

Mechanics of polycarbonate during high-rate tension

Abstract: Polymeric materials often undergo large inhomogeneous deformations at high rates during their use in various impact-resistant energy-absorbing applications. For better design of such structures, a comprehensive understanding of high-rate deformation under various loading modes is essential. In this study, the behavior of polycarbonate was studied during tensile loading at high strain rates, using a splitcollar type split Hopkinson tension bar (SHTB). The effects of varying strain rate, overall imposed strain magnitude and specimen geometry on the mechanical response were examined. The chronological progression of deformation was captured with a high-speed rotating mirror CCD camera. The deformation mechanics were further studied via finite element simulations using the ABAQUS/Explicit code together with a recently developed constitutive model for high-rate behavior of glassy polymers. The mechanisms governing the phenomena of large inhomogeneous elongation, single and double necking, and the effects of material constitutive behavior on the characteristics of tensile deformation are presented.

A linear curved-beam model for the analysis of galloping in suspended cables

Abstract: A linear model of curved, prestressed, no-shear, elastic beam, loaded by wind forces, is formulated. The beam is assumed to be planar in its reference configuration, under its own weight and static wind forces. The incremental equilibrium equations around the prestressed state are derived, in which shear forces are condensed. By using a linear elastic constitutive law and accounting for damping and inertial effects, the complete equations of motion are obtained. They are then greatly simplified by estimating the order of magnitude of all their terms, under the hypotheses of small sag-to-span ratio, order-1 aspect ratio of the (compact) section, characteristic section radius much smaller than length (slender cable), small transversal-to-longitudinal and transversal-to-torsional wave velocity ratios. A system of two integrodifferential equations is drawn in the two transversal displacements only. A simplified model of aerodynamic forces is then developed according to a quasisteady formulation. The nonlinear, nontrivial equilibrium path of the cable subjected to increasing static wind forces is successively evaluated, and the influence of the angle of twist on the equilibrium is discussed. Then stability is studied by discretizing the equations of motion via a Galerkin approach and analyzing the small oscillations around the nontrivial equilibrium. Finally, the role of the angle of twist on the dynamic stability of the cable is discussed for some sample cables.

Microscale hydrogels for medicine and biology: Synthesis characteristics and applications

Abstract: Microscale hydrogels with dimensions of 200µm or less are powerful tools for various biomedical applications such as tissue engineering, drug delivery, and biosensors, due to their size, biocompatibility, and their controllable biological, chemical, and mechanical properties. In this review, we provide a broad overview of the approaches used to synthesize and characterize microgels, as well as their applications. We discuss the various methods used to fabricate microgels, such as emulsification, micromolding, microfluidics, and photolithography. Furthermore, we discuss the effects of porosity and crosslinking density on the mechanical and biological properties of hydrogels. In addition, we give specific examples of the use of hydrogels, such as scaffolds and cell encapsulation for tissue engineering, controlled release materials for drug delivery, and environmentally sensitive sensors for microdevices. Finally, we will discuss the future applications of this technology.

A damage index for structural health monitoring based on the empirical mode decomposition

Abstract: This paper presents two novel damage indices based on empirical mode decomposition (EMD) and fast Fourier integration for identifying structural damage caused by a change in structural stiffness. The paper also demonstrates the effectiveness of the proposed damage indices formulated based on a series of coupled mathematical/engineering approaches that are used to detect damage in pipes reliably and accurately. The main approach is based on monitoring the vibration response of pipes using piezoelectric sensors and the first intrinsic mode functions (IMFs). Finite element analysis is used to simulate the response of a healthy pipe, as well as pipes with various sizes of damage. Damages are meant to represent the outcome of local corrosion (damage) with varying reduction in areas around the circumference of the pipe. The evaluated damage indices could effectively establish the location of the defects. Moreover, the evaluated energy indices could also distinguish various size defects. To demonstrate further the effectiveness of our proposed damage indices, the results are compared with other effective indices based on wavelet packet and other statistical methods reported in the literature.

Numerical and experimental analysis of the static compliance of chiral truss-core airfoils

Abstract: This paper presents an innovative wing profile featuring an internal truss-like structure of chiral topology. The chiral design is selected because of its unique deformation characteristics, which produce a theoretical, in-plane Poisson's ratio of -1. Such a Poisson's ratio yields a very high shear modulus, which in principle does not require the wing profile to be defined by a closed section or stressed-skin configuration. In addition, the peculiar deformation mechanism of the chiral configuration allows large decambering deflections to occur, with all the members of assembly behaving within the linear range of the material. Hence the proposed design combines large chordwise compliance and large in-plane shear stiffness. Such conflicting mechanical properties can be achieved through the proper selection of a limited number of geometric parameters defining the core configuration. The objective of the paper is to investigate the compliance characteristics of the airfoil. Two-dimensional profiles, designed according to results from previous investigations, are manufactured and tested to assess compliance and evaluate decambering deflection limits. The experimental analysis is guided by numerical models that account for deviations from the ideal configuration due to manufacturing limitations. Numerical and experimental results demonstrate the influence of core geometry on the compliance and confirm the ability of chiral-core airfoils to sustain large deflections while not exceeding yield strain limits.

A marching procedure for form-finding for tensegrity structures

Abstract: We give an algorithm for solving the form-finding problem, that is, for finding stable placements of a given tensegrity structure. The method starts with a known stable placement and alters edge lengths in a way that preserves the equilibrium equations. We then characterize the manifold to which classical tensegrity systems belong, which gives insight into the form-finding process. After describing several special cases, we show the results of a successful test of our algorithm on a large system.

Symmetry conditions and finite mechanisms

Abstract: Using group representation theory, a simplifled criterion for the detection of flnite symmetric mechanisms is presented.

On the numerics and correlation of scratch testing

Abstract: A numerical strategy based on the finite element method and intended for an accurate analysis of the scratch test is presented. For simplicity, but not out of necessity, the material was described ...

Performance and failure of metal sandwich plates subjected to shock loading

Abstract: The deflection and fracture of metal sandwich plates subjected to intense uniform impulsive pressure loads are studied for plates made of four steels representing a wide range of strength, strain hardening and ductility. Sandwich plates with both square honeycomb cores and folded plate cores are considered. The primary fracture modes of the sandwich plates are necking and subsequent tearing of the face sheets and webs and shear delamination of the core webs from the faces. Plates with square honeycomb cores have higher damage tolerance than those with folded plate cores in that they can withstand much larger loads above those at which the first signs of fracture appear. The trade-off between strength and ductility in plate performance is illustrated.

Dynamic shear rupture of steel plates

Abstract: Metallic sandwich panels with prismatic cores offer the potential for superior blast resistance relative to monolithic plates of equivalent areal density. However, under sufficiently high impulse, severe plastic strains can occur at the junctions of the face sheets and the core members shortly after arrival of the pressure wave but prior to significant deformation elsewhere. The potential consequence is localized shear rupture with minimal plastic dissipation. To characterize this failure mode, a combined experimentalnumerical protocol has been used to ascertain the plastic strain for dynamic shear rupture of ductile metals. The experimental component involves firing cylindrical projectiles through plates of the targeted materials and monitoring changes in projectile velocity during penetration. With appropriate combinations of plate thickness and projectile velocity, penetration occurs through propagation of an annular shear crack. In parallel, a numerical model of dynamic deformation and rupture has been employed to infer the critical strain through comparisons with projectile velocity change measurements. Experiments and analyses have been performed on both 304 stainless steel and superaustenitic AL6XN. Effects of mesh size on the resolution of the predicted strain distribution and the plastic dissipation associated with penetration are addressed.

Truss waviness effects in cellular lattice structures

Abstract: Methods have emerged for making metallic lattice structures either by the lay up of collinear wire arrays or by stacking woven textile meshes. The two fabrication routes result in similar lattice topologies: the collinear lattice has straight struts while those in the textile lattice are wavy. Wire waviness in the textile lattice results in a knockdown in both the stiffness and strength compared to the collinear lattice. Analytical estimates and finite element (FE) predictions of the through thickness compressive responses of collinear and textile lattices indicate that the stiffness and strength of lattices oriented to form a diamond structure are specimen aspect ratio dependent. By contrast, the stiffness of the collinear and textile lattices oriented to form a square structure is independent of both specimen aspect ratio and height while the strength depends on the sandwich height. Experimental measurements on specimens fabricated from 304L stainless steel are in good qualitative agreement with the elastic ideally-plastic analytical estimates while FE predictions incorporating the full strain hardening response of the parent material give accurate quantitative predictions of the measurements.

Calculation of inertial properties of the malleus-incus complex from micro-CT imaging

Abstract: The middle ear bones are the smallest bones in the human body and are among the most complicated functionally. These bones are located within the temporal bone making them difficult to access and study. We use the micro-CT imaging modality to obtain quantitative inertial properties of the MIC (malleus-incus complex), which is a subcomponent of the middle ear. The principal moment of inertia of the malleus along the superior-inferior axis (17.3 ± 2.3 mg/mm3) is lower by about a factor of six in comparison to the anterior-posterior and lateral-medial axes. For the incus, the principal moment of inertia along the superior-inferior axis (35.3 ± 6.9 mg/mm3) is lower by about a factor of two than for the other two axes. With the two bones combined (MIC), the minimum principal moment of inertia (132.5 ± 18.5 mg/mm3) is still along the superior-inferior axis but is higher than for the individual bones. The superior-inferior axis inertia is lower by a factor of 1.3 than along the anterior-posterior axis and is lower by a factor 2 along the lateral-medial axis. Values for inertia of the MIC show significant individual differences in three human ears measured, suggesting that middle ear models should be based on individual anatomy. Imaging by micro-CT scanner is a nondestructive modality that provides three-dimensional volume information about middle ear bones at each stage of manipulation with resolution down to 10μm. In this work extraneous tissue is removed to obtain a sufficiently small specimen. However, advances in imaging hold promise that this capability will be available for in vivo measurements.

Tension buckling in multilayer elastomeric isolation bearings

Abstract: Seismic isolators are constructed from multiple layers of elastomer (usually natural rubber) reinforced with steel plates; they are, therefore, very stiff in the vertical direction, but soft in the horizontal direction. The buckling of these bearings under compression load is a well-understood phenomenon and has been widely studied. It is therefore unexpected that the buckling analysis for compression predicts that the isolator can buckle in tension at a load close to that for buckling in compression. The linear elastic model that leads to both compression and tension buckling is an extremely simple one, so it might be argued that the tensile buckling may be an artifact of the model itself rather than a property of the isolator. To test the simple theoretical model we have conducted a numerical simulation study using a finite element model of a multilayer elastomeric bearing. We find that the prediction of tensile buckling by the simple linear elastic theory is indeed accurate and not an artifact of the model.

Hierarchical chemo-nanomechanics of proteins: entropic elasticity, protein unfolding and molecular fracture

Abstract: Proteins are an integral part of nature’s material design. Here we apply multiscale modeling capable of providing a bottom-up description of the nanomechanics of chemically complex protein materials under large deformation and fracture. To describe the formation and breaking of chemical bonds of different character, we use a new reactive force field approach that enables us to describe the unfolding dynamics while considering the breaking and formation of chemical bonds in systems that are comprised of several thousand atoms. We particularly focus on the relationship between secondary and tertiary protein structures and the mechanical properties of molecules under large deformation and fracture. Our research strategy is to systematically investigate the nanomechanics of three protein structures with increasing complexity, involving alpha helices, random coils and beta sheets. The model systems include an alpha helical protein from human vimentin, a small protein -conotoxin PnIB from conus pennaceus, and lysozyme, an enzyme that catalyzes breaking of glycosidic bonds. We find that globular proteins can feature extremely long unfolding paths of several tens of nanometers, displaying a characteristic sawtooth shape of the force-displacement curve. Our results suggest that the presence of disulfide crosslinks can significantly influence the mechanics of unfolding. Fibrillar proteins show shorter unfolding paths and continuous increase of forces until molecular rupture occurs. In the last part of the article we outline how a mesoscale representation of the alpha helical protein structure can be developed within the framework of hierarchical multiscale modeling, utilizing the results of atomistic modeling, without relying on empirical parameters. We apply this model to describe the competition between entropic and energetic elasticity in the mechanics of a single alpha helical protein molecule, at long time scales reaching several microseconds. We conclude with a discussion of hybrid reactive-nonreactive modeling that could help to overcome some of the computational limitations of reactive force fields.

Buckling and nonlinear response of sandwich panels with a compliant core and temperature-dependent mechanical properties

Abstract: This paper deals with the buckling response and nonlinear behavior of sandwich panels with soft cores that have temperature-dependent mechanical properties and are subjected to thermally induced deformations and mechanical loads simultaneously. This study investigates the effects of the degradation of properties of the core as a result of rising temperature on the response of the sandwich panel. Analyses are carried out for cases of pure thermal loading, with either uniform or gradient temperature fields through the depth of the panel, as well as for thermal loading acting simultaneously with external mechanical loads. The formulation is based on variational principles along with the high-order sandwich panel approach. It takes into account the flexibility of the core in the vertical direction as well as the dependency of the mechanical core properties of the temperature distribution through the core depth. The stress and deformation fields of the core have been solved analytically, including the case where the temperature-dependent properties attain a complex pattern. The buckling equations are derived using the perturbation technique, yielding a set of nonlinear algebraic equations for the case of a simply-supported panel and a uniform temperature field. The critical temperatures and modes of wrinkling and global buckling are determined numerically for some foam types of core made by Rohacell and Divinycell. The nonlinear response caused by thermally induced deformations is presented for Divinycell foam core with different temperature distributions through the depth of the core. Finally, the nonlinear response caused by the simultaneous action of external mechanical loading and increased temperatures on the compressive or the tensile side of the panel, with a thermal gradient through the core depth, is presented. The interaction between elevated temperatures and mechanical loads changes the response from a linear into an unstable nonlinear one when the degradation of the mechanical properties due to temperature changes is considered and the panel is unrestrained. Moreover, the unstable nonlinear behavior becomes even more severe when the face, loaded in compression, is subjected to elevated temperatures. This study reveals that a reliable, realistic design of a sandwich panel that is subjected to elevated temperature (within working temperature range) and mechanical loads must take into account the degradation of the properties of the core as a result of the thermal field even at working temperature range, especially when cores made of foam are considered.

The mechanics of tip growth morphogenesis: what we have learned from rubber balloons

Abstract: Morphogenesis of plant, fungal, and bacterial cells depends heavily on surface mechanics and in particular on the stiff wall that surrounds these cells. In this paper, we show that tubular rubber balloons offer a useful physical model of tip growth morphogenesis. In particular, the balloons reproduce accurately the inhomogeneity and anisotropy of surface expansion observed during tip growth. Comparison between the two systems has led to a simple model of tip growth that assumes linear constitutive relations with inhomogeneous material properties. The strain rate profile predicted by the model is a surprisingly good fit to the data given the model’s simplicity. We suggest that a meridional gradient of compliance or extensibility is the key mechanical feature that explains the similar strain rate profiles in tip-growing cells across broad taxonomic groups as well as in rubber balloon analogs.