Optimisation of Cruciform Test Specimen for Biaxial Tensile Testing of Sheet Metal

One of the most restricting aspects of the biaxial tensile test for sheet metal is the design of the cruciform specimen. Although specimens of the cruciform type have previoussly been investigated quite extensively, no standard geometry for the cruciform specimen exists. Using a specifically designed pantograph apparatus for operation in a standard tensile testing machine, various cruciform specimens were analysed experimentally. Finite element modelling of the specimens was also conducted to establish optimum specimen geometry. Through a process of optimisation, a standard cruciform specimen was designed which can be used to accurately predict the mechanical behaviour of cold rolled low-carbon steel when formed in multiple directions simultaneously.

Dielectric Properties of Alumina Ceramics in the Microwave Frequency at High Temperature

The dielectric properties of alumina ceramics have been measured using a free-space time-domain technique from room temperature to 1400 oC in the frequency range 8.2 – 12.4 GHz. The effects of thickness and lateral size of specimen were investigated with comparing the measured values to the calculated ones based on the half-wavelength and the Gaussian beam focusing. From these results the optimum specimen dimension for the high temperature dielectric measurement was suggested with experimental verification.

Charge Contrast Imaging of Gibbsite Using the Variable Pressure SEM

The variable pressure scanning electron microscope (VP-SEM) allows imaging of insulators without the need for a conductive coating, due to charge neutralization at the surface from recombination of positive ions and surface electrons. Varying certain parameters such as pressure, bias, and working distance creates incomplete neutralization, and localized charging develops called charge contrast. Although the exact mechanism creating charge contrast imaging (CCI) is unknown, it is agreed that it is related to an optimum charge compensation. The behavior of the CCI is still vague, which presents a problem for determining the mechanisms. This article provides user-friendly methods of finding the optimum levels of charge contrast in the VP-SEM. We show that the CCI is obtained at optimum operating conditions where the specimen current is between 2.5 nA and 3.5 nA. The specimen current is a function of secondary electrons (SE) emission and ionization potential, producing an ion flux. Therefore an optimum specimen current represents the balanced conditions of SE emission and ion flux. Controlling the pressure, working distance, bias, scan rate, and beam current allows the microscopist to set the specimen current at this optimum level for charge contrast imaging. All the work was performed on gibbsite using the S3000N VP-SEM from Hitachi.