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Vacuum infusion processes have become more and more important over the past years, because of their environmental advantages and, above all, the improved performance and quality of the final products and the possibility to industrialize the moulding process for composite parts.
(Published on September 2007 – JEC Magazine #35)
GIUSEPPE COCCIA, PH. D. FIART MARE SPA
Glass impregnation coefficients
In most cases, the shipyard’s designer handles the structural design of a boat in accordance with naval register recommendations. For technological reasons, this limits the practicability of a structural project. Until very recently, there was no normative regulation to determine the dimensioning of a GFRP boat taking into account any moulding techniques other than wet lay-up or spray-up.
In this context, we propose to examine structural dimensioning according to the R.I.Na (Italian Naval Register), with particular reference to section B of the Register’s third chapter, from which we concluded that the procedures indicated below link dimensioning with manual moulding:
When examining the situation, we see that all of these assumptions are not valid for infusion technology. As a matter of fact, the impregnation coefficients associated with vacuum technology are completely different from those recommended by the certification register.
Using technical data from fibreglass manufacturers, the R&D laboratory of Fiart Mare conducted experiments that give the figures shown in the following table.
The values for manual fibre impregnation are similar to the corresponding coefficients suggested by the certification body while the values for vacuum infusion are about 20% higher. To avoid oversizing the laminate, a designer must take these new fibreglass impregnation relationships into account.
Optimizing the glass/resin ratio is very important in any composite material. If not enough resin is used, the fibres may not adhere well, while using too much will result in waste. The ratio also directly influences the macroscopic mechanical properties of the lamina. To better explain this concept, we will refer briefly to the Theory of the Lamina Micromechanics, from which we will deduce a functional relationship between the mechanical properties of the composite material and the relative percentages of its constituents.
We will examine the case where the elastic modulus E of a composite lamina is being determined. We assume that the lamina in question is composed of single directional fibres oriented according to a specific direction called di. Then, we assume that the lamina is subjected to traction in the “di” direction that exerts a constant load called Fcomp. This load, in turn, can be subdivided into two quotas: the first (Ff) represents the load supported by the fibres and the second (Fm) represents the load supported by the matrix. Then, we can write:
Fcomp = Ff + Ffm (1)
At this point, having the same load of the product of the tension for the surface of the resistant section, we obtain:
Fcomp = σc Sc (2)
Ff = σf Sf (3)
Fm = σm Sm (4)
where σi represents the tensions (appearing in the composite material, in the single fibre phase, and in the single matrix phase) and Si represents the resistant sections.
At this point, substituting (2), (3), (4) in (1), we obtain: σc Sc = σf Sf + σm Sm (5)
Now, applying Hooke’s Law, on the basis of which σ = E ε, we obtain:
Ec εc Sc = Ef εf Sf + Em εm Sm
Assuming perfect adhesion between the fibres and the matrices, the deformation (ΔL/L0)f undergone by the generic fibre will be equal in terms of the deformation (ΔL/L0)m undergone by the matrix.
Formula (6) can be simplified by eliminating the εi: Ec Sc = Ef Sf + Em Sm (7)
If we divide the resulting formula by Sc, we obtain the following comparison expressing the calculation of the longitudinal modulus of a composite lamina as a function of the volumetric fractions (Vf) of fibres and matrices:
Ec = Em Vf + EmVm (8)
This is called “Mix law” for composite materials.
Accordingly, when adapting to the theoretical model used to calculate the longitudinal deformation, we can deduce the law to calculate the tensile strength of the lamina as follows: σ1 = σf Vf + σm Vm (9)
Thus, (8) and (9) demonstrate that the mechanical properties of a composite lamina are closely connected to the volumetric fractions of fibres and matrices, i.e. to the fibre/resin ratio mentioned in the beginning of this paper.
We would like to elaborate on the preceding statement, using a very simple calculation example on the specific case of a single lamina of a fibreglass laminate used to produce an ordinary pleasure boat. To simplify the calculation, we use a lamina made of unidirectional fibreglass impregnated with polyester resin. The mechanical properties of the constituents are expressed in the following table.
In most cases, the infusion experiments produce a significant increase in mechanical properties compared to hand lay-up.
Production Time Analysis shows a decrease in the total working hours.
Widely documented experimental data show that for hand layup done correctly, with maximum Gc weight contents of the required glass reinforcement at 0.5, Vf is equal to 0.3. Using a vacuum-moulding process, a Gc of 0.65 can be obtained easily (Vf = 0.5). Based on all this, we know that we can deduce the following values from (8) and (9):
These few simple considerations should be sufficient to justify the application of vacuum-moulding technologies, if only to increase the specific strength of the lamina.