M. A. Salam Akanda

A new approach to instability testing of shells

Abstract In this paper, a new experimental technique for the buckling test of shells under external pressure is presented. This technique is simple and determines buckling load very efficiently. It has already been used extensively in the investigation of buckling load of electroformed sphericaltip conical shells under uniform external pressure. In this method, instead of the conventional monitoring of load and linear displacement of points on a shell, load and change in internal volume of the shell are recorded. Internal volumetric change of a shell is found to be a better indicator of its instability compared to the change in displacements of points on its surface. Volumetric change, a function of all the displacements and rotations contributing to the total change in its configuration, is observed to be a highly magnified indicator of shell deformation encompassing the whole shell, primarily the buckling zones and imperfection locations.

Accurate determination of the structural elasticity of human hair by a small-scale bending test

This paper reports on a small-scale bending method for human hair. The test sample, which is elliptical in cross-section, is fixed to a hollow steel needle using resin to form a cantilever. A loading probe is used to subject this to a lateral load, where the load is applied parallel to either the long or short axis of the elliptical cross-section. From these tests, load-displacement relationships for the hair were obtained. From the experimental data and analysis, we found that the structural elasticity determined is independent of the direction of bending, and precise measurements of the structural elasticity of human hair with scattering of less than 5% were realized using this test scheme. Finally, changes in the structural elasticity of hair due to hair treatments were detected and the changes are discussed based on a theoretical model of the multi-layered structure.

Sensitivity of acoustic microscopy for detecting three-dimensional nanometer gaps embedded in a silicon structure.

The sensitivity of acoustic microscopy for detecting three-dimensional defects in a Si structure is reported. Circular, nanometer gaps with diameters ranging from 5 to 1000 microm were embedded in Si disks by a direct bonding technique, and these were visualized using acoustic microscopy. The limits of detection for the gap thickness and diameter were observed simultaneously in samples with gaps of 4 and 140 nm. The behavior of the sensitivity in detecting the gaps can be explained by a simple analytical model. It is shown experimentally and theoretically that the gap thickness and diameter are not independent variables as regards detection. The sensitivity of acoustic microscopy is governed by the three-dimensional features of the embedded defects.

A finite difference solution of a steel plate with an internal hole using the displacement potential approach

The finite difference technique based on a displacement potential formulation is extended to solve an elastic plane stress problem with geometric perturbations such as holes, arbitrary defects, notches, etc. An elastic steel plate having an internal hole is constrained at one edge and a shear load is applied to the opposite edge. The present problem is solved by using the extended finite difference technique based on displacement potential approach. The different displacement and stress components at different sections of the plate are shown graphically and discussed; from the analysis of stress and displacement components at different sections of the plate, critical regions of the plate are identified. The superiority of the finite difference method (FDM) based on the displacement potential formulation over the finite element method (FEM) is verified by the comparison between the solutions of the present problem by the FDM and FEM. From the comparison between the solutions of the FEM and FDM, it is concluded that the present finite difference technique based on the displacement potential formulation is reliable and can be used to solve the structural elements having internal defects.The finite difference technique based on a displacement potential formulation is extended to solve an elastic plane stress problem with geometric perturbations such as holes, arbitrary defects, notches, etc. An elastic steel plate having an internal hole is constrained at one edge and a shear load is applied to the opposite edge. The present problem is solved by using the extended finite difference technique based on displacement potential approach. The different displacement and stress components at different sections of the plate are shown graphically and discussed; from the analysis of stress and displacement components at different sections of the plate, critical regions of the plate are identified. The superiority of the finite difference method (FDM) based on the displacement potential formulation over the finite element method (FEM) is verified by the comparison between the solutions of the present problem by the FDM and FEM. From the comparison between the solutions of the FEM and FDM, it is concl...

Ultrasonic NDE of Closed Cracks-Sizing and Evaluation of Crack Opening Stress Intensity Factor under a No Load Condition

A new ultrasonic evaluation method has been developed for the determination of crack opening stress intensity factor of small fatigue cracks under no load condition. The crack depth and the closure stress of an unknown closed crack are determined simultaneously from the measured shear wave response by solving an inverse problem, where a new modeling of the crack closure is used. From the evaluated crack depth and the crack closure stress, the crack opening stress intensity factor is determined. The method developed is proved to be a powerful tool for reliable nondestructive evaluation of small closed cracks under a no load condition. Introduction Fatigue crack propagation rate under cyclic loading is functionally related to the effective stress intensity range, which is primarily dependent on crack opening stress intensity factor (Kop). Therefore, nondestructive determination of the crack opening stress intensity factor under no load condition is of great practical importance from the fracture mechanics point of view. In order to determine Kop, quantitative nondestructive evaluation (NDE) of both crack depth (a) and closure stress (σp) is imperative. Closed cracks are always erroneously evaluated as being much smaller than their actual sizes if the effect of crack closure is not taken into account properly. In order to overcome the difficulties due to crack closure in the NDE of crack by ultrasonics, several models of crack closure have been proposed for the evaluation of closed cracks by using a normally incident longitudinal wave (LW) beams [1-3]. In an attempt to deal with smaller closed cracks quite recently, a new angle beam evaluation approach was developed [4], and thereafter the method has been extended to a number of cases of practical importance [5-9]. However, these studies have succeeded to provide information about the closure stress in a qualitative manner. Therefore, the quantitative evaluation of the closure stress under no load condition has still remained as a hurdle in the field of nondestructive evaluation of cracks. In the present study, the ultrasonic shear wave (SW) beam with 50 deg. incidence in solid [10] is used for characterizing the cracks in the depth range of 0.4 to 4.0 mm. The amplitude of the first crack corner echo is measured for specimens at several positions of the transducer. An empirical relation between the amplitude of the crack echo and the crack depth for open cracks in the proposed range is determined [11,12]. The initial closure stress in fatigue crack is estimated from the crack closure load measured by the standard compliance method [13-15]. An experimental relationship between the reflection coefficient, γ, and closure stress, σp, is established which is independent of crack depth. Finally, based on the aforesaid relations an empirical formula for the response of closed crack is developed for determining the crack depth and crack closure stress simultaneously by solving an inverse problem. The accuracy of the evaluation is verified by comparing the evaluated crack depth with the actual one obtained by destructive measurement. From the evaluated crack depth and closure stress, Kop is calculated and compared with KImax and KImin used for the preparation of specimen. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1277-1284 doi:10.4028/www.scientific.net/KEM.270-273.1277