Asymmetric bending boundary layer: The λ-test

The problem of the experimental determination of elastic moduli of nanoscale objects is of present interest. The determination of the elastic moduli of thin macroscopic shells is usually based on experiments with plates. It is known that, when grown using certain techniques, nanoobjects are obtained only in the form of shells. Therefore, it is necessary to develop a method for determining the elastic moduli of nanoobjects on the basis of experiments with shells. Experimental determination of the bending stiffness of nanosize shells presents a serious problem, because for such widespread nanoobjects as nanotubes and fullerenes under arbitrary deformation, the material is subjected to both bending and tension. Therefore, all parameters (e.g., natural frequencies) that can be measured directly are complicated functions of both bending and tension stiffness. In recent years, together with nanotubes and fullerenes, nanoobjects of a more intricate configuration have been obtained [1‐4]. Nanosize cylindrical helices [1, 3] are of particular interest in connection with the possible experimental determination of bending stiffness. This is due to the fact that (1) in helical shells under arbitrary deformation, the material is mainly bent, so that the material tension effect can be neglected when interpreting experimental data; and (2) the natural oscillation shapes of helical shells are much more easily observed than those of cylindrical shells associated with pure bending of the material. The latter statement is illustrated in Fig. 1, which presents the first four helical shell oscillation shapes. The analysis of helical shell dynamics [5] presented below may be a theoretical foundation for experimental testing of the applicability of the continuum theory to (a) the calculation of mechanical characteristics of nanoobjects and (b) the experimental determination of the bending stiffness of nanoshells.

To enhance the prediction accuracy of forming simulations of carbon fiber unidirectional (UD) thermoset prepregs, particularly the wrinkle predictions, bending behavior of UD prepregs in the forming process needs to be accurately characterized and modelled. In this paper, an improved bending test system was proposed to characterize the bending behavior of UD prepregs at different forming temperatures and loading rates. It is found that both the temperature and the loading rate have significant impact on the bending stiffness of UD prepregs. An obvious nonlinear bending behavior can be noted in the fiber direction. The loading rate sensitivity of bending stiffness in the fiber direction decreases with temperature increasing. To simulate the bending deformation, a superimposed membrane-shell element model was employed to achieve the decoupling of tensile-bending deformation. Membrane element simulated the in-plane tensile and in-plane shear deformations of UD prepregs, while shell element primarily accounted for the bending deformation. Fiber orientation dependent nonlinear bending stiffness was accounted in the model. Deformation simulations were conducted for vertical cantilever bending, horizontal cantilever bending, and compression wrinkling of UD prepregs. Good agreement is noted between simulation and experiment, demonstrating efficiency of the model and validity of measured bending stiffness.

Nonlinear elastic behaviors and deformation mechanisms of nano-structured crosslinked biopolymer networks

Effect of obliquity on impingement heating characteristics of co-axial swirling flame jet - An experimental and numerical investigation

In this first part of a two-part study, the three-dimensional structure of the inner-core boundary layer (BL) is investigated in a full-physics simulation of Hurricane Irma (2017). The BL structure is highlighted during periods of intensity change, with focus on features and mechanisms associated with storm decay. The azimuthal structure of the BL is shown to be linked to the vertical wind shear and storm motion. The BL inflow becomes more asymmetric under increased shear. As BL inflow asymmetry amplifies, asymmetries in the low-level primary circulation and thermodynamic structure develop. A mechanism is identified to explain the onset of pronounced structural asymmetries in coincidence with external forcing (e.g., through shear) that would amplify BL inflow along limited azimuth. The mechanism assumes enhanced advection of absolute angular momentum along the path of the amplified inflow (e.g., amplified downshear), which results in local spinup of the vortex and development of strong supergradient flow downwind and along the BL top. The associated agradient force results in the outward acceleration of air immediately above the BL inflow, affecting fields including divergence, vertical motion, entropy advection, and inertial stability. In this simulation, descending inflow in coincidence with amplified shear is identified as the conduit through which low-entropy air enters the inner-core BL, thereby hampering convection downwind and resulting in storm decay.

An experimental apparatus was developed to study the three dimensional separated flow with spiral-foci. The internal decelerating flow was generated by the air suction from a side wall to produce the separation on an opposite-side wall. The relation between the upstream boundary layer and the generation of spiral-foci in the separation region was observed by a tuft method. As a result, it was clarified that the spiral-focus type separation could be produced on the side wall and its behavior was closely related to the vortices supplied into the separation region from the boundary layer developing along top wall or bottom one.

The aim of this investigation is to determine the effects of confinement on the stabilization of turbulent, lifted methane (CH4) jet flames. A confinement cylinder (stainless steel) separates the coflow from the ambient air and restricts excess room air from being entrained into the combustion chamber, and thus produces varying stabilization patterns. The experiments were executed using fully confined, semi-confined, and unconfined conditions, as well as by varying fuel flow rate and coflow velocity (ambient air flowing in the same direction as the fuel jet). Methane flames experience liftoff and blowout at well-known conditions for unconfined jets, however, it was determined that with semi-confined conditions the flame does not experience blowout. Instead of the conventional unconfined stabilization patterns, an intense, intermittent behavior of the flame was observed. This sporadic behavior of the flame, while under semi-confinement, was determined to be a result from the restricted oxidizer access as well as the asymmetrical boundary layer that forms due to the viewing window. While under full confinement the flame behaved in a similar method as while under no confinement (full ambient air access). The stable nature of the flame while fully confined lacked the expected change in leading edge fluctuations that normally occur in turbulent jet flames. These behaviors address the combustion chemistry (lack of oxygen), turbulent mixing, and heat release that combine to produce the observed phenomena.

Scramjet engine with no moving parts and extensive speed range is suitable for developing cheaper vehicles to outer space. The isolator, which isolates the inlet from the pressure buildup inside the combustor, is a critical component of the scramjet engine. In an actual scramjet, the combustor undergoes highly transient combustion giving rise to high amplitude fluctuating backpressure, which may cause the inlet to unstart. In general, combustion in a scramjet is a complex process that involves flows subjected to considerable pressure gradients, coexisting subsonic and supersonic flow regions in the presence of shock waves. Moreover, shock-induced boundary-layer separation patterns are different due to asymmetrical boundary layer development at the walls. Until now, no experimental study on the forced dynamic excitation in hypersonic flows is reported, mainly due to the limited available ground facilities that can reproduce such conditions. Such dynamic excitation can give to flow fields that are significantly different from that reported in other studies. The present study numerically addresses forced excitation inside the combustor generated by an elliptical-shaped rotor. During the unstart process, several stages were identified, such as separation and its growth in both the upper and lower walls of the combustor, isolator, inlet, and finally, shock excursion leading to unstart. The results provide ample insight into the upstream propagation of flow disturbances originating from the combustor, its interaction with the asymmetric boundary layers on top and bottom walls, and the dynamics of inlet unstart.

Subsurface damage in composite materials is difficult to detect using visual techniques, and other current inspection methods lack the ability to perform quick, wide-area inspections without the need for reference signatures or baseline measurements. This paper presents a method for detecting and locating subsurface damage in composite materials without historical reference measurements by considering the nonlinear behavior of the material in the vicinity of damage. Nonlinear behavior is identified by comparing frequency response functions measured at different input amplitudes. It will be shown that the nonlinear behavior of the material is most evident in the areas nearest to the damage. The proposed inspection method is demonstrated both analytically and experimentally. First, a finite element model of a sandwich beam is developed using Bernoulli–Euler beam elements to represent each layer of the beam and springs to represent the interface between the layers. A bilinear stiffness nonlinearity is simulated to represent disbond damage between the top and core layers of the beam. The simulated disbond damage is localized by identifying degrees of freedom which indicate significant nonlinear response through a comparison of frequency response functions measured at various input amplitudes. Next, the method is demonstrated experimentally by identifying disbond damage in a fiberglass sandwich panel. A three-dimensional scanning laser vibrometer is used to measure the forced frequency response of the panel in its damaged state as it is excited at two or more amplitudes of excitation by a piezoelectric actuator. Comparisons of the frequency response functions measured at different input amplitudes show that the subsurface damage introduces nonlinear behavior which resembles a bilinear stiffness nonlinearity, and the differences in the frequency response functions are largest in the vicinity of the damage location. In addition, it was found that improved localization of the damage is achieved by investigating the response at higher frequencies. This work has application as a nondestructive method for detecting and locating subsurface damage in composite materials and, by using a laser vibrometer for noncontact measurement, allows for quick, wide-area inspection of composite materials without the need for reference signatures or baseline measurements.

In order to analyze the effects on whole-bone behavior of aluminum accumulation, 14 rats, aged 90 days, received i.p. doses of 27 mg/day of elemental Al, as Al(OH)3, during 26 weeks, while 14 rats remained as controls. Their femur diaphyses were studied tomographically by peripheral quantitative computed tomography (pQCT) and mechanically tested in bending. The load/deformation curves obtained allowed distinction between effects observed during the linearly elastic (Hookean) and nonlinear (non-Hookean, or plastic) behaviors of bones before and after the yield point, respectively. Treatment reduced the cortical bone mineralization (volumetric bone mineral density [BMD], -2%; P < 0.01), with a negative impact on the bending stiffness (Young's elastic modulus) and the yield stress of cortical bone (-18% and -13%; P < 0.05). Despite the absence of any cortical mass increase (cross-sectional area), improved spatial distribution of cortical tissue concerning anterior-posterior bending stress (cross-sectional moment of inertia, 10%; P < 0.05) occurred through a modulation of modeling drifts. Up to the yield point, neither the structural strength (load supported), the strain, nor the structural stiffness (load/deformation ratio) of the diaphyses were affected. This suggests an adaptive response of bone geometry to the impairment of bone material stiffness. However, Al intoxication significantly reduced the ultimate load and the post-yield fraction of that load (-6% and -27%; P < 0.05). This suggests that the proposed, adaptive response could have improved bone design so as to make it adequate for maintaining a normal pre-yield diaphyseal stiffness and strength according to the bone mechanostat theory, but not so as to provide complete protection of the diaphyseal post-yield (and ultimate) strength. Although a relative inhibition of bone formation could not be discarded, an Al-induced impairment of the bone ability to resist loads beyond the yield point could have caused the unusual disparity observed between effects on bone (elastic) stiffness and (ultimate) strength. In addition, to explain the unusual finding, these results suggest that little-studied microstructural factors (spatial arrangement of elements within the mineralized tissue) affecting the post-yield behavior of cortical bone, regardless of bone mineralization in these and other conditions, ought to be further investigated in specifically designed studies as a novel, promising resource in skeletal research.

In this study, a theoretical analysis is presented for estimating the in-plane geometric nonlinear elastic stability behavior of steel members with tapered elements under dynamic loads. Beam-column approach is adopted for modeling the structural members as beam-column elements. The formulation is based on the Eulerian description taking into consideration the influence of axial force on bending stiffness. The changes in member chord length due to axial deformation and flexural bowing are also considered. In the dynamic analysis, the system mass properties have been represented using both lumped and consistent mass matrices. The consistent mass matrix is derived in three components: translational, rotational, and axial inertia. The formulation of the mass matrices in local and global coordinate systems of tapered members, which incorporates geometric nonlinearity, has been presented. A parametric study is conducted to examine the effects of number of tapered elements, tapering the prismatic members, time step size, and tapering ratio. The dynamic responses of tapered members have been shown to be significantly affected by these parameters.

Implementation of geometric nonlinearity in microelectromechanical systems (MEMS) resonators offers a flexible and efficient design to overcome the limitations of linear MEMS by utilizing beneficial nonlinear characteristics not attainable in a linear setting. Integration of nonlinear coupling elements into an otherwise purely linear microcantilever is one promising way to intentionally realize geometric nonlinearity. Here, we demonstrate that a nonlinear, heterogeneous microresonator system, consisting of a silicon microcantilever with a polymer attachment exhibits strong nonlinear hardening behavior not only in the first flexural mode but also in the higher modes (i.e., second and third flexural modes). In this design, we deliberately implement a drastic and reversed change in the axial vs bending stiffness between the Si and polymer components by varying the geometric and material properties. By doing so, the resonant oscillations induce the large axial stretching within the polymer component, which effectively introduces the geometric stiffness and damping nonlinearity. The efficacy of the design and the mechanism of geometric nonlinearity are corroborated through a comprehensive experimental, analytical, and numerical (finite element) analysis on the nonlinear dynamics of the proposed system.

Nonlinear dynamics of time-varying curvature balance rope coupled with time-varying length hoisting rope in friction hoisting system

Elevating stiffness without compromising compliance and agility is a key problem for soft finger applications, especially for articulate ones. Inspired by human finger, a multijoint soft finger with dual morphing through active/passive variable rigidity is proposed. The fabricated soft finger weighs 27.4 g. Conductive thermoplastic starch polymers (CTPSs) are embedded in a U-shape-joint pneumatic soft actuator segmentally like biological phalanges. Their stiffness can be independently adjusted utilizing customized thermomechanical property. Yoshimura origami imitating ligaments can passively match deformation and stiffness of the joints. Through electrothemal activation of CTPSs, the finger can realize dual independent articulate morphing: stiffened phalanges (mode 1) for dexterous manipulation and heavy load, softened phalanges (mode 2) for large deformation contact and light load. Comparative experiments of bending angle, output force, and stiffness are carried out between the active and passive stiffness adjustment of mode 1 and mode 2. The results show that the output force and stiffness of the finger adopting mode 1 can be improved more than two times and five times, and its compliance using mode 2 is almost similar, compared with the pure soft one. To further demonstrate performances of dual-mode morphing, a three-fingered gripper is assembled for grasping and manipulating targets with different shapes, sizes, rigidity, and weight, including playing card, unshelled raw egg, grapes, and unscrewing the bottle cap. It can successfully lift a dumbbell weighing 1460 g with a 7.6 load/weight ratio through a two-mode switch.

On fault induced nonlinear rotary response and instability