The effect of sample thickness on the relative breakdown strength of epoxy systems

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Description/Abstract

It is long established that initial testing of new techniques or systems is performed using reduced quantities or loadings to lower costs and give a quick indication of performance, before more realistic large scale tests are undertaken. This is also often the case for initial material studies. One specific technique is the electrical breakdown strength of dielectric materials to be used as insulators. Since it is difficult to generate the very high voltages to give breakdown data on final-geometry samples, especially without causing excessive damage to equipment, the ASTM standard allows smaller samples to be tested to give an indication of the material’s properties. In recent work, 100 m thick samples were created to provide breakdown strength data for a range of epoxy-based systems; the quantitative effect of scaling up from the limited sample thickness to technologically realistic values needs to be considered. Volume and area effects are intrinsic to Weibull analysis since they affect the probability of a defect or impurity being in the breakdown path. Also, further size effects come into play in the case of filled materials, where anomalous effects can be introduced when filler dimension approach that of the sample being tested. This investigation aims to analyse the effect of sample dimensions on the experimental breakdown strength of epoxy systems with varying sample thickness. Using a proven sample production technique, thin epoxy films with thicknesses varying from 50 um up to 1 mm have been produced. These samples were then electrically tested using a specialised electrical breakdown instrument and data processed using Weibull statistics. This paper analyses the breakdown characteristics of the samples relative to their thicknesses in order to (a) test the validity of the Weibull distribution and (b) to provide estimates of the optimum sample dimensions for different material formulations.