title: Characterisation of syntactic foams for marine applications
Creators: name: Salleh, Zulzamri
Publisher: name: University of Southern Queensland
Year: 2017
Subjects: subject: 091202 Composite and Hybrid Materials

description: Syntactic foams are light weight particulate composites that use hollow particles (microballoons) as reinforcement in a polymer resin matrix. High strength microballoons provide closed cell porosity, which helps in reducing the weight of the material. Due to their wide range of possible applications, such as in marine structures, it is desirable to modify the physical and mechanical properties of syntactic foams in particular to achieve both high specific compressive strength and high energy absorption with minimal or no increase in density. Based on a literature review, it was found that marine applications of syntactic foams mainly focus on mechanical properties, on light weight as buoyancy aid materials, and on enhanced thermal insulators in the deep water pipeline industry. In order to achieve all these characteristics, attention needs to be placed on the determination of the effects on wall thickness, on the radius ratio ( ) of glass microballons and on the presence of porosity in syntactic foams. The size of these parameters can be calculated and compared with observation by using SEM micrograph machine. In this study, the specific mechanical properties, particularly compressive and tensile properties with 2-10 weight percentages (wt.%) of glass microballoons, are investigated and discussed. It is shown that the mechanical properties, particularly compressive and tensile strength, decreased when glass microballoons with vinyl ester resin were added. The effect of porosity and voids content mainly contributed to a reduction of these results and this is discussed further in this study. Tensile and compressive characteristics of the vinyl ester matrix syntactic foam were investigated and it was revealed that tensile strength was 70-80 % higher than compressive strength when glass content was reduced. The fabrication of syntactic foam sandwich composites is also investigated and discussed in this study. It was found that mechanical properties such as compressive, tensile and flexural strength were varied with different amounts of glass microballoon content as core material. The compressive strength of the sandwich panels was significantly affected by a low density core foam, particularly 2 wt.% of glass microballoon, as well as their modulus of elasticity and maximum stress value. The tensile failure of the syntactic foam sandwich panels was also significantly affected by lower glass microballoon content (2 wt.%) and the core failure was clearly observed compared to other failure modes, such as cohesive and adhesive failure modes. The flexural shear testing or three-point bending (TPB) of the syntactic foam sandwich panels indicated a higher strength when the glass microballoon content was increased in the core materials compared to the un-symmetrical shear failure mode. The investigation into water absorption in room temperature (T: 25 oC) and a higher temperature (70 oC) have been investigated in this study to check the sustainability and reliability of syntactic foam for marine applications that were immersed in three different types of water (FW-Fresh water, DD-Double Distil water and SW-Salt water). Water absorption rates varied due to the effect of the density of syntactic foam as a result of the pores and void containment attributed to a higher glass microballoon content. The diffusion rate or coefficient D, could be estimated by using Fick’s law, which also predicted that the equilibrium stage could be achieved better at high temperature conditions when compared to room temperature. The diffusion rate also varied when immersed with different water conditions, for example SW being slower than FW and DD waters due to the effect of the pores’ activity. The mechanical properties of syntactic foam, when immersed in different types of waters at room temperature and under hygrothermal conditions, also varied with the duration at 30 days and at 60 days. It can be seen that the modulus of elasticity for both compressive and tensile properties showed decreases when more glass microballoon content was added, and when immersed for a long duration such as 60 days. The thermal stability of syntactic foam is also investigated in this study. The compressive and tensile specimens were subjected to a hygrothermal analysis to determine the glass transition temperature, Tg and thermal expansion, of syntactic foams. In this parametric thermogravimentric analysis (TGA) study, the results for Tg of syntactic foam with different (wt.%) of glass microballoon showed an increase after a hygrothermal process in which three different types of water were compared with dry specimens. Within the TGA/DTGA curve it was also found that Tonset, Tpeak, and Tend, showed varied temperatures when more glass microballoon content in syntactic foam was added. Moreover, their composition properties, such as their weight loss residue as well as their temperature residue, also decreased until all specimens changed properties in the ash coal type. The thermomechanical analysis (TMA) on kinetic energy was conducted according to the first-order reaction Broido method, which is commonly used in polymer composites that have been discovered. In this study, it is revealed that the parameter, such as activation energy (Ea), decreased when the degradation temperature increased. Within this finding, Ea was varied and depended on the (wt.%) of glass microballoon in syntactic foam. The lower activation energy was required to complete the decomposition process. A linear expansion study was done, especially with a focus on the thermal dimension stability of syntactic foam, and the result showed a decrease when more glass microballoon in syntactic foam was added. The lower thermal stability at a higher temperature could be very useful for an insulator product, particularly in marine and aerospace engineering applications. The linear dimension stability, also called coefficient of thermal expansion (CTE), decreased when the glass microballoon content increased. The modification of Turner’s model was applied in this study for a comparison of CTE in three different temperatures: 30 oC, 50 oC, and 70 oC for syntactic foam. The modification included parametric study involvement with the effect of radius ration, porosity and voids content in syntactic foam. As a result, the porosity content contributed much more to the CTE value, especially gap of ratio, which was different from the matrix porosity. In order to achieve a better quality of syntactic foams, the study also investigated the stress intensity factor (SCF) by modelling particularly from the tensile specimens, K around holes at the microballoons. The prediction of strain value between local strains from the experimental strain gauge was compared with the finite element analysis (FEA) simulation when their varied load in longitudinal and transverse axes was applied to a tensile and flexural sandwich panel’s syntactic foam. For the tensile specimen, the determination of the SCF used one strain gauge, which was attached near the hole in the middle of the extensometer length. The results show that the SCF values were comparable between experiments with extensometer and strain gage (SG) values, with percentages ranging from 0.40 % to 1.36 %. A comparison and a prediction were made between experimental values and the FEA analysis results. It could be estimated that the experimental values of around 90 % and 70 % followed the FEA values for SG1 and SG2, respectively. An investigation on the strain value for flexural sandwich panel syntactic foam was also carried out using the FEA approach to predict the properties’ behaviour in this study. It was found that the micro strain for SG1 in the FEA approach was 17% higher than the experimental value, even though they were at the same loading setting. However, the prediction for the micro strain of SG2 was only 2.7 % different, which was considered a good agreement to predict the properties of a syntactic foam core sandwich panel for different loading values.