Strength and failure analysis of cenosphere foam cored glass skin sandwiches

Abstract

In weight specific applications, sandwich structures provide a better solution as they can withstand larger bending loads with a lesser net weight of the structure. These structural members consist of two stiff and strong skins and a light weight core. In this case E-glass bidirectional woven fabric as skin and cores made of cenospheres filled in epoxy matrix. Cenospheres used are hollow spherical particles of fly ash separated using a low cost procedure. The size of these particles is in the range 20-200 μm with an average size of 108 μm. The cenosphere concentrations are varied from 25-58%, 58% being the maximum that could be filled in epoxy with dough flowing just enough to fill the mould. The standard three-point bend test was performed on the sandwich samples. Specific strength in bending of sandwiches increased about 17% with an increase in volume fraction initially (35%) and then it declined. This could be attributed to occurrence of debonds in larger particles and poor wetting of the particles by the resin especially at higher volume fractions. In order to reduce the severity of debonds and improve the compatibility, a surface treatment involving coating of a silane agent on the filler is attempted. The effect of the treatment reflected in the improvement of sandwich properties such as maximum increase in specific strength of about 26% is noticed at lower volume fractions. But at higher volume fraction agglomeration of the particles and the resulting poor wetting could have been a dominant phenomenon for reduction in strength. The samples tested under bending were observed in SEM to analyze and account for failure modes. Chiefly, two types of failures could be observed in the samples. Firstly, core failing well before the skin, implying a significant difference in the modulii of skin and core materials. Secondly, delamination observed at the skin-core interface, which could be due to reduced wetting on account of smooth surfaces of skin and core in contact. With compatible values achieved for core properties and with proper sanding of the cores, cenosphere foam cores can be used for realizing a significant cost reduction.