MATERIAL CHARACTERISATION AND VALIDATION EXPERIMENTS Material characterisation tests were conducted on the GRP and glue materials These tests

and to establish the tensile strength of the GRP laminate, critical to the failure criteria and correct implementation of the elasticity matrix for the GRP in the numerical models.

4.4.1 TENSILE RESIN TESTS

Four moulds were manufactured for the casting of Norpol Dion 9500-501 vinyl-ester resin samples for tensile tests. The moulds were created using a British Standard steel test specimen as a template and were cast using a silicone-based rubber moulding compound. The silicone moulding compound was chosen due to its ease of use being a cold curing material. The resulting moulds can be used at temperatures up to 70ºC which is greater than the heat expected from the exothermic reaction of the resin cure cycle. In addition, the silicone compound allows for easy removal of the resin casting and its durability allows it to be repeatedly used. Figure 4.7 shows the moulds used for the resin casting.

The resin was mixed with the appropriate catalyst as per the manufacturers instructions. Any foaming of the resin/catalyst mixture was allowed to subside. The resin was not degassed under vacuum as it was felt that this would provide an over ideal material property for the resin that would not be representative of the resin as found in both the DLHC and HSC†. The resin was then poured carefully into the moulds to prevent any additional air entrapment. Three castings were made from the same batch of catalysed resin. The castings were allowed to cure for 24 hours. As with the DLHC and HSC the resin castings were post cured at 60°C for 12 hours. The resin specimens were tested in tension to failure; an extensometer was used to measure strain. The results obtained are presented in Table 4.1. A typical stress-strain plot is given in Figure 4.8. The resin response has a large linear region to the stress-strain plot with a small amount of non-linearity just before maximum stress. In a number of the experiments there was a further increase in strain post maximum stress indicating a continuing absorption of energy to cause complete rupture. This is an important aspect to the numerical modelling in terms of the post initial failure material degradation model as discussed in Chapter 5

.

Table 4.1 Results of resin bulk tests

Specimen Young’s Mod (MPa) Failure Load (N) CSA (mm2) Ultimate strength (MPa) 1 2774.9 1484.9 31.53 47.1 2 2780.5 1791.9 32.08 55.9 3 2775.6 1692.9 30.54 55.4 4 2804.1 1662.6 30.48 54.6 Average 2783.8 1658.1 31.16 53.25 Coef. Of Var (%) 0.5 7.7 2.5 7.8

4.4.2 RESIN FRACTURE TOUGHNESS TESTS

Double cantilever beam (DCB) tests were conducted to obtain the Mode I fracture toughness (GIC) for the resin material Norpol Dion 9500-501. In order to obtain correct

values of fracture toughness, the two substrates used for a DCB specimen must have the same flexural stiffness. As the hybrid connection has dissimilar materials the flexural stiffness of the substrates is not equal. To ensure cohesive failure two GRP substrates were used to obtain the fracture toughness of the resin. It could not be guaranteed that cohesive failure would occur if steel specimens were used and the facilities for the replication of the surface preparation of the steel were not available. Pre-cured laminates of glass-reinforced vinyl-ester were manufactured using the resin infusion technique for the DCB tests.

The DCB specimens were manufactured by bonding two GRP substrates with a nominal width of 25 mm and nominal thickness of 4 mm using the resin system. As with the HSC and DLHC the bondline thickness is very small, therefore no adhesive bondline thickness spacers were used. The resultant specimens had an average bondline thickness of 0.18 mm, which compared relatively well with the approximate bondline thickness of the HSC and DLHC specimens. A Teflon insert was placed at the end of the bondline to act as a crack pre-starter. Aluminium blocks were bonded to the ends of the DCB specimens containing the Teflon strips to enable loading. The specimens were loaded in tension at a rate of 1.0 mm/min. The tests were conducted according to BS 7991 ‘Determination of the Mode I adhesive fracture energy GIC of structure adhesives using double cantilever beam (DCB)

and tapered double cantilever beam (TDCB) specimens’. The specimens manufactured

† _{For infused laminates the resin is initially held at room pressure, any entrapped air caused by over vigorous}

corresponded to the ‘type a’ specimens as described in the above standard. The principal dimensions for the specimens are given in Table 4.2. The average fracture energy obtained was 241.8 J/m2 (7.8 % coefficient of variation).

Table 4.2 DCB test specimen dimensions

Dimension Value Units

Length 290 mm

Width 25 mm

Substrate thickness 4 mm Adhesive thickness 0.2 mm End block height 10 mm End block length 25 mm Loading rate 1 mm/min 4.4.3 GRP STIFFNESS TESTS

Two GRP laminates were manufactured using the two different E-glass reinforcements found in the DLHC and HSC. The first laminate representing the HSC contained the 3x1 twill woven roving. The second laminate, representing the DLHC contained the tri-axial cloth. Both panels were manufactured using the resin infusion method. Both panels were cut to provide tensile specimens of the dimensions 250x50 mm lengths. The thicknesses of the two panels varied slightly and were measured as 3.5 mm for the woven roving and 4.9 mm for the tri-axial. The specimens were tested in the Instron A test machine described in Chapter 4. As with the resin tests, strain was measured via the use of an extensometer. The average result for Young’s modulus provided a value of 20.6 GPa for the woven roving and 19.7 GPa for the tri-axial cloth. These values can be said to be typical for GRP laminates [6].

Chapter 5