Experimental procedures

5.1 Realization of the laminates

Quasi-isotropic and reinforced aluminium specimens have been used in experimental tests (Vasek et al., 1997). The vacuum bag technique has been used to implement the laminates (Marannano & Virzi’ Mariotti, 2008) with a polyester resin, since this resin is very used in shipbuilding for its low cost.

The catalyst (2-3% in volume) is added to make the resin more viscous and to reach a state where it is not more liquid: this is the gel point. The resin hardens to obtain the hardness and final properties; the reaction is accompanied by heat generation that increases reaction time, the process is known as curing. The construction of reinforced aluminium laminates is quite similar to quasi-isotropic composite materials, except the curing treatment, that is done at room temperature due to the different coefficients of thermal expansion that characterize the composite and aluminium, with consequent formation of residual stresses and possible cracks inside the specimens.

5.2 Basic concepts

An appropriate acquisition system was made to monitor the parameters of material damage during the experimental tests.

Induced stresses are monitored with an appropriate acquisition system during the tests (continuously or not, as appropriate). This system is made of a load cell, a "strain indicator", a digital-analog connector and a computer. Dedicated software can record and show the values of the material resistance, and so it is possible to have the load-number of cycles curves in real time. The acquisition software (Labview) records input signal and it shows the evolution of signals peak with the number of cycles at regular intervals through the connector, so it is possible to study the yielding of the specimen during the test.

5.3 First campaign of experimental test

5.3.1 Quasi-isotropic laminates

The first campaign of experimental test has the aim to evidence the better quality of the Glare respect to the optimized composite laminates. Table 1 shows the characteristics of composite materials to perform the tests. It shows the calculation performed in Excel to determine the volume fraction of fibres. The value 69.21% was achieved using the vacuum bag technique; in fact it can not get these properties with the normal lay-up at room temperature.

Particular laminates are quasi-isotropic, where the stiffness is independent of considered direction. They must have: a total number of laminates n greater or equal to 3 and laminates of the same constitution and thickness; the angle Д0 between the two laminates must be constant (angularly equidistant laminates). The name does not depend on the fact that these laminates can have small variations of the stiffness with the direction, but on the fact that they have isotropic behaviour with respect to traction-compression and not respect to bending and torsion. Quasi-isotropic laminates can be obtained with an appropriated orientation of the fibres, for example a symmetric laminate that has 12 layers positioned according to the scheme [+30/+90/+30]s (total thickness 3 mm). Half of the laminate is made of 6 layers spaced 60° angularly (Van Paepegem & Degrieck, 2001; Marannano & Virzi’ Mariotti, 2008)).