Download or read book Buckling of Spherical Shells with Large Axisymmetric Imperfection and Application in Soft Robotics written by Chuan Qiao and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "Spherical shells are ubiquitous in nature and engineering for their simple geometry and excellent load-bearing capacity. A long-standing topic of spherical shells subject to uniform external pressure is their highly unstable post-buckling response that leads to a catastrophic collapse. This type of failure fully exhausts the load-bearing capacity of the shell, and it is thus detrimental for traditional engineering applications. On the other hand, recent advances in soft robotics and metamaterials demonstrate that geometric nonlinearities and elastic instabilities can be harnessed to generate distinctive functions, such as shape morphing, fast motion, and logic operation.This thesis proposes the use of a large axisymmetric imperfection to control the buckling mode of spherical shells, and demonstrates its application when integrated into a soft bi-shell valve for the fast actuation of soft robots. On the first front, the proposed imperfection can escape the classical bifurcation of perfect spherical shells, and induce snap-through buckling followed by a stable post-buckling; this response provides increasing pressure resistance over a large change in volume. The buckling of the imperfect shell is investigated through a theoretical study that employs exact expressions of the middle surface strains, curvature changes, and live pressure loading along with validating experiments and numerical simulations. The post-buckling characteristics and transition between four buckling modes can be programmed by tuning the defect geometry and shell radius to thickness ratio. On the second front, a new concept for soft valves is proposed: a bi-shell valve consisting of an imperfect shell and a shallow spherical cap. The bi-shell valve harnesses the snap-through interaction of the constituent shells to convert a slowly imparted volume into a fast volume output. Upper bounds and performance metrics are presented to guide the design of the bi-shell valve, and its performance is illustrated through an application that demonstrates the fast actuation of a soft striker.Overall, the work in this thesis contributes to unveil original aspects of shell buckling with results that are promising for diverse applications. Geometric imperfections can be programmed into spherical shells to trigger new buckling modes that can be exploited for the design of soft robots, mechanical metamaterials, and soft matter. Furthermore, the bi-shell valve concept here introduced can inspire the design of future soft valves with unique actuation modes for soft pneumatic robots"--
Download or read book The Axisymmetric Buckling of Initially Imperfect Complete Spherical Shells written by Tatsuzo Koga and published by . This book was released on 1968 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt: A theoretical investigation of the axisymmetric buckling of complete thin-walled spherical shells under uniform external pressure is undertaken to determine the effect of axisymmetric initial imperfections. In the analysis, the complete spherical shell is divided into two parts; the shallow cap in which the initial imperfection exists, and the remaining portion of the shell most of which deforms in simple contraction. The rotation of the meridian of the cap is assumed in the form of a polynomial function. Four matching conditions are enforced at the juncture of the cap and remainder. The total potential energy of the complete spherical shell is minimized in accordance with the Rayleigh-Ritz approximation method. Two types of initial imperfections are studied; one is an axisymmetric dimple, and the other is a spherical region whose radius of curvature is greater than that of the perfect spherical shell. The maximal values of the pressure parameter rho in the equilibrium state before snap-through were calculated for various values of the geometric parameter lambda, which is proportional to the angular extent of the imperfection for a given radius-to-thickness ratio, and the imperfection amplitude parameter delta, which is the ratio of the initial deviation of the shell mid-surface at the axis of rotation to the wall-thickness. The results of numerical computations show that for all values of delta the lowest buckling pressure is reached when lambda is about 4. (Author).
Download or read book Newton Raphson written by Harry G. Schaeffer and published by . This book was released on 1970 with total page 36 pages. Available in PDF, EPUB and Kindle. Book excerpt: The feasibility of utilizing mathematical programing techniques for determining stable equilibrium states of nonlinear structural systems is explored. The physical problem chosen for evaluation of this methodology was the nonlinear behavior of a shallow spherical shell subjected to uniform pressure. The procedure found most useful was a hybrid method consisting principally of a generalized Newton-Raphson technique with intermittent application of the conjugate-gradient search technique. The variation of buckling pressure with changes in edge restraint and shell geometry is found for a uniformly loaded shell.
Download or read book Buckling of Conical Shell with Local Imperfections written by Paul A. Cooper and published by . This book was released on 1974 with total page 28 pages. Available in PDF, EPUB and Kindle. Book excerpt: Small geometric imperfections in thin-walled shell structures can cause large reductions in buckling strength. Most imperfections found in structures are neither axisymmetric nor have the shape of buckling modes but rather occur locally. This report presents the results of a study of the effect of local imperfections on the critical buckling load of a specific axially compressed thin-walled conical shell. The buckling calculations were performed by using a two-dimensional shell analysis program referred to as the STAGS (Structural Analysis of General Shells) computer code, which has no axisymmetry restrictions. Results show that the buckling load found from a bifurcation buckling analysis is highly dependent on the circumferential arc length of the imperfection type studied. As the circumferential arc length of the imperfection is increased, a reduction of up to 50 percent of the critical load of the perfect shell can occur. The buckling load of the cone with an axisymmetric imperfections is nearly equal to the buckling load of imperfections which extended 60 deg or more around the circumference, but would give a highly conservative estimate of the buckling load of a shell with an imperfection of a more local nature.
Download or read book Dynamic Plastic Buckling of Spherical Shells During Collapse written by and published by . This book was released on 1989 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Conditions are established that lead to the growth of initial displacement imperfections (buckling) of complete spherical shells during significant inward motion. This preliminary study is limited to axisymmetric buckling modes only. Shell thickening during inward motion is taken into account, and its effect is found to be important for the stability and maximum growth of a given initial imperfection mode. Upper and lower bounds on the initial, uniform impulsive velocity loading are found (in terms of spherical shell geometry and material parameters) such that growth of initial displacement imperfections remains less than a prescribed multiplier when initial inward velocity lies outside the bounded region. 10 refs., 8 figs.
Download or read book Some Recent Studies of the Buckling of Thin Shells written by Stanford University. Department of Aeronautics and Astronautics and published by . This book was released on 1969 with total page 130 pages. Available in PDF, EPUB and Kindle. Book excerpt: After a brief summary of the development of present knowledge of the buckling process of thin shells, the results of recent investigations are discussed in some detail. In particular, information is given on analyses of the maximum stress a circular cylindrical shell of infinite or finite length can carry in axial compression in the presence of an initial deviation of the shell wall from the perfect cylindrical shape, and on the number of circumferential waves into which finite cylindrical shells buckle. The maximum external pressure is calculated that can be supported by a complete spherical shell in the presence of known deviations of its wall from the perfect spherical shape. The results of recent experiments on cylindrical and complete spherical shells are also presented. Recommendations are made for the practical calculation of the buckling stresses of circular cylindrical shells and spherical shells. (Author).
Download or read book Finite Axisymmetric Deformation and Asymmetric Buckling of Truncated Spherical Shells written by M. T. Wu and published by . This book was released on 1969 with total page 142 pages. Available in PDF, EPUB and Kindle. Book excerpt: The paper is concerned with the theoretical study of buckling of truncated spherical shells. Sander's nonlinear equations for deep shells are used and equations of equilibrium are expressed in terms of displacements for spherical shells. Based on these equations, analyses are made for calculating prebuckling axisymmetric equilibrium positions and then examining these equilibrium states for points of bifurcation into asymmetric buckling deformations. An eigenvalue problem is formulated and the buckling loads for truncated spherical shells of different geometrical parameters are obtained numerically. The numerical results both for the prebuckling axisymmetric deformations and the points of bifurcation for the asymmetric buckling are plotted in figures. (Author).
Download or read book Buckling of Cylindrical Shells with Axisymmetric Toroidal Initial Imperfections written by Clark E. Beck and published by . This book was released on 1973 with total page 56 pages. Available in PDF, EPUB and Kindle. Book excerpt: Some recent advances in thin shell buckling theory which tend to explain the discrepancy between experiment and theory are reviewed. The results of a digital computer study to determine the effect of three discrete axisymmetric imperfections on the buckling load of two specific circular cylindrical shells are presented and discussed. The shells were 40 inches long with wall thicknesses of 0.02 inch and radii of five and ten inches. Initial imperfection amplitudes considered were 0.005, 0.01 and 0.02 inch. (Autho).
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1978 with total page 1018 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Axisymmetric and Bifurcation Creep Buckling of Externally Pressurised Spherical Shells written by Paul Christos Xirouchakis and published by . This book was released on 1978 with total page 55 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Buckling Under Axial Compression of Long Cylindrical Shells with Random Axisymmetric Imperfections written by John Chukwuemeka Amazigo and published by . This book was released on 1968 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Static and Dynamic Buckling of Shallow Spherical Shells Subjected to Axisymmetric and Nearly Axisymmetric Step Pressure Loads Using SATANS IIA A Modified Version of SATANS II written by Michael D. Shutt and published by . This book was released on 1976 with total page 168 pages. Available in PDF, EPUB and Kindle. Book excerpt: A digital computer program for the geometrically nonlinear analysis of totally arbitrarily loaded shells of revolution (SATANS-2) was modified to more accurately account for the conditions at the pole of the shell. This program was used to determine the buckling load of shallow spherical shells of various sizes when subjected to static axisymmetric, dynamic axisymmetric, and dynamic nearly axisymmetric step-pressure loads of infinite duration. A comparison was made between the new buckling results and previous results obtained without the new pole routine. The comparison revealed a significant change in the buckling pressures, due solely to the change in the pole routine. The new static axisymmetric, dynamic axisymmetric, and even the dynamic asymmetric critical buckling pressure loads appear to be fairly reliable results for perfect, shallow shells.
Download or read book Imperfection Insensitive Thin Shells written by Xin Ning and published by . This book was released on 2015 with total page 288 pages. Available in PDF, EPUB and Kindle. Book excerpt: The buckling of axially compressed cylindrical shells and externally pressurized spherical shells is extremely sensitive to even very small geometric imperfections. In practice this issue is addressed by either using overly conservative knockdown factors, while keeping perfect axial or spherical symmetry, or adding closely and equally spaced stiffeners on shell surface. The influence of imperfection-sensitivity is mitigated, but the shells designed from these approaches are either too heavy or very expensive and are still sensitive to imperfections. Despite their drawbacks, these approaches have been used for more than half a century. This thesis proposes a novel method to design imperfection-insensitive cylindrical shells subject to axial compression. Instead of following the classical paths, focused on axially symmetric or high-order rotationally symmetric cross-sections, the method in this thesis adopts optimal symmetry-breaking wavy cross-sections (wavy shells). The avoidance of imperfection sensitivity is achieved by searching with an evolutionary algorithm for smooth cross-sectional shapes that maximize the minimum among the buckling loads of geometrically perfect and imperfect wavy shells. It is found that the shells designed through this approach can achieve higher critical stresses and knockdown factors than any previously known monocoque cylindrical shells. It is also found that these shells have superior mass efficiency to almost all previously reported stiffened shells. Experimental studies on a design of composite wavy shell obtained through the proposed method are presented in this thesis. A method of making composite wavy shells and a photogrametry technique of measuring full-field geometric imperfections have been developed. Numerical predictions based on the measured geometric imperfections match remarkably well with the experiments. Experimental results confirm that the wavy shells are not sensitive to imperfections and can carry axial compression with superior mass efficiency. An efficient computational method for the buckling analysis of corrugated and stiffened cylindrical shells subject to axial compression has been developed in this thesis. This method modifies the traditional Bloch wave method based on the stiffness matrix method of rotationally periodic structures. A highly efficient algorithm has been developed to implement the modified Bloch wave method. This method is applied in buckling analyses of a series of corrugated composite cylindrical shells and a large-scale orthogonally stiffened aluminum cylindrical shell. Numerical examples show that the modified Bloch wave method can achieve very high accuracy and require much less computational time than linear and nonlinear analyses of detailed full finite element models. This thesis presents parametric studies on a series of externally pressurized pseudo-spherical shells, i.e., polyhedral shells, including icosahedron, geodesic shells, and triambic icosahedra. Several optimization methods have been developed to further improve the performance of pseudo-spherical shells under external pressure. It has been shown that the buckling pressures of the shell designs obtained from the optimizations are much higher than the spherical shells and not sensitive to imperfections.
Download or read book Applied Mechanics Reviews written by and published by . This book was released on 1989 with total page 410 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book On the Stability of Elastic Equilibrium written by Warner Tjardus Koiter and published by . This book was released on 1967 with total page 226 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Viscoelastic Materials written by Roderic S. Lakes and published by Cambridge University Press. This book was released on 2009-04-27 with total page 481 pages. Available in PDF, EPUB and Kindle. Book excerpt: This graduate text on viscoelastic materials addresses design applications as diverse as earplugs, computer disks and medical diagnostics.
Download or read book Resolution Of The Twentieth Century Conundrum In Elastic Stability written by Isaac E Elishakoff and published by World Scientific. This book was released on 2014-05-29 with total page 350 pages. Available in PDF, EPUB and Kindle. Book excerpt: There have been stability theories developed for beams, plates and shells — the most significant elements in mechanical, aerospace, ocean and marine engineering. For beams and plates, the theoretical and experimental values of buckling loads are in close vicinity. However for thin shells, the experimental predictions do not conform with the theory, due to presence of small geometric imperfections that are deviations from the ideal shape.This fact has been referred to in the literature as ‘embarrassing’, ‘paradoxical’ and ‘perplexing’. Indeed, the popular adage, “In theory there is no difference between theory and practice. In practice there is”, very much applies to thin shells whose experimental buckling loads may constitute a small fraction of the theoretical prediction based on classical linear theory; because in practice, engineers use knockdown factors that are not theoretically substantiated.This book presents a uniform approach that tames this prima-donna-like and capricious behavior of structures that has been dubbed the ‘imperfection sensitivity’ — thus resolving the conundrum that has occupied the best minds of elastic stability throughout the twentieth century.
