Curved Surface Structures Research Articles

Extracting modal parameters of ultrasonic bar horns from ESPI FRF data

This paper demonstrates the problems associated with determining the modal parameters of curved surface structures which exhibit high modal density and a predominant longitudinal response. The inadequacies of inferring modal parameters solely from conventional normal-to-surface measurements, typically using a laser Doppler vibrometer (LDV), are highlighted. A measurement solution is proposed which relies on extracting a grid of surface in-plane vibration amplitude and phase data, which is resolved from two laser speckle fringe patterns using electronic speckle pattern interferometry (ESPI). In order to obtain frequency spectrum data by in-plane ESPI, successive surface responses are captured through a swept-sine test and fringe processing algorithms are used to calculate a vibration amplitude and phase map at chosen grid positions on the structure's surface, at selected frequency steps. Further, by measuring an electrical signal input to the driving transducer, frequency response function data is obtainable such that modes of vibration estimated by modal analysis can be correlated with finite element model data. The technique is demonstrated by modal analysis of ultrasonic bar horns such that the longitudinal modes are successfully identified.

Collapse load analysis of prestressed concrete surface structures with unbonded tendons by the finite element method

The middle surfaces of curved surface structures (shells) and the axes of the tendons embedded between the two surfaces of a shell are assumed to be analytically describable. Essential features of the mathematical formulation are the exact consideration of the geometric form of the middle surface of a given shell by means of thin-shell finite elements and of the axes of the tendons. Prestress losses occuring during the tensioning operation as well as changes of the prestress forces resulting from loads applied after prestressing are taken into account. For prestressed concrete (PC) surface structures (panels, slabs and shells) with unbonded tendons, determination of these changes of forces is difficult if friction between the tendon and the duct is considered. The basis of the theoretical investigation is a formulation of the principle of virtual work which is suitable for incremental-iterative analysis of PC surface structures by the finite element method (FEM). It contains the expression for the virtual work of forces exerted by the tendons on the remaining part of the surface structure, treated as a free body. Geometric nonlinearity is considered on the basis of Koiter's shell theory of small displacements and moderately large rotations. Physical nonlinearity is taken into account by means of appropriate constitutive equations for intact and cracked concrete, respectively, reinforcing steel and prestressing steel. The numerical investigation consists of collapse load analysis of a PC slab with unbonded tendons. It demonstrates the usefulness of the theoretical concept.

Model Tests of Coastal Protective Structures in USSR

A variety of designs of underwater breakwaters, their process of wave destruction and dynamically balanced shore profile development were developed in extensive laboratory tests and comparative observations in nature, by the Soviet Institute of Engineers of the Maritime Fleet in Odessa. Twenty types of structures, under the influence of a variety of incidence waves and three stages of immersion, were tested and statistically evaluated as to their costal protective efficiency. Empirical equation and graph formulate the relationship of incidence wave parameters and breakwater dimensions for planned structures. Breakwaters with vertical surfaces generate turbulent destruction of waves with limited passage of sediment, whereas with both sides sloped 1 : 1 structures destroy the waves calmly and transfer one and a half times more beach material. In both cases decrease of the crest immersion produces a considerable decrease in the sediment transport capacity. Maximum disintegration occurs on the upper part of the vertical or sloped breakwater. Curved surface structures show considerably less abrasive effect, appearing only at the lower half of the front surface.