Materials and Experiment Methods 1 Materials Information

To investigate interlaminar toughness, three types of carbon fabrics were selected as the base materials. The carbon fabrics were plain and 5-harness satin weaves, and a unidirectional fabric. The plain weave fabric (38399 and 38422) was supplied by Carr Reinforcement Limited, in two different areal weights, 300 and 375g/m2. A ply thickness of the plain weave fabrics is approximately 0.48 (38399) and 0.54mm (38422), respectively. The 5-harness satin weave fabric (P1726) was supplied by Cytec having an areal weight of 275g/m2 and a ply thickness of approximately 0.35mm. The unidirectional non-crimped fabric (CT300) was supplied by Saint-Gobain Technical Fabrics having an areal weight of 615g/m2 and a ply thickness of approximately 0.5mm. Figure 3.1 shows schematic of weave pattern for the carbon fabrics.

Chapter 3 Mode-I Interlaminar Toughness

Figure 3.1 Schematic of weave pattern for carbon fabrics: (a) Plain weave, (b) 5-harness satin weave, (c) Unidirectional non-crimped fabric (Chain lines are stitches)

Two types of resin were used for interlaminar toughness tests, epoxy and vinyl ester and there were two types of epoxy resins. One epoxy was CYCOM 823R, which was supplied by Cytec. The other epoxy was MVR444, which was supplied by Advanced Composites Group. The vinyl ester resin used DION9102-500, was supplied by Reichhold. Table 3.1 shows the characteristics of the resins.

Table 3.1 Characteristics of resins for experiments [from data sheets]

Property Epoxy 1+ Epoxy 2++ Vinyl ester*

Density (g/m3) 1.23 1.14 1.03

Tensile strength (MPa) 80 77.6 79

Tensile modulus (GPa) 2.9 3.1 3.4

Flexural strength (MPa) 114 - 130

Flexural modulus (GPa) 3.4 - 3.25

Strain energy release (kJ/m2) 0.9 0.3 -

+: CYCOM 823R. ++: MVR444, *: DION9102-500

Five types of non-woven veils were used: two variants of Polyester/Carbon fibre hybrid (Hyb1 and Hyb2), and veils based on Carbon, Polyester (PE), and Polyamide (PA) fibres. The two types of hybrid veils differ in the polyester fibre content. Two variants of the pure polyester veils were also used with different areal weights. The hybrid, carbon, and PE1 (10g/m2) veils were supplied by Technical Fibre Products. The PE2 (20g/m2) and PA veils were supplied by Japan Vilene. Table 3.2 gives

Chapter 3 Mode-I Interlaminar Toughness

characteristics of the interleaf veils. Figure 3.2 shows SEM pictures of the interleaf veils. The carbon veil is quite stiff. In contrast, the PE and the PA veils are soft materials. The hybrid veils are moderately stiff. For the structure of veils, the hybrid, carbon, and PE1 are that the individual fibres are predominantly straight. The PE2 and PA veils often exhibit bent and curved fibres. The hybrid and PE2 veils have a relatively high fibre density. Therefore, these veils are opaque. The carbon and PE1 veils have a lower areal weight and a more open structure that allows easy resin percolation. The PA veil is different from the other veils. The fibre diameter is larger than the other veil fibres. Nevertheless the areal weight of the PA veil is about 20g/m2, it is very porous with apertures below the fibres.

Table 3.2 Characteristics of interleaf veils [from data sheets]

Properties Hyb1+ Hyb2++ Carbon PE1 PE2 PA

Areal weight [g/m2] 20 20 10 13 23 21 Thickness [mm] 0.018 0.017 0.007 0.012 0.07 0.010 Tensile strength [N/Length] - $ 7* 8* 12* 19** -$

Fibre diameter [um] 9 - 13 13 7 14 12 59

Content Polyester 70% Polyester 80% Carbon 100%

Polyester 100% Copolymer Polyester Copolymer Polyamide

Chapter 3 Mode-I Interlaminar Toughness

Figure 3.2 SEM pictures of non-woven interleaf veils: (a) PE/C (70:30) hybrid, (b) PE/C (80:20) hybrid, (c) Carbon, (d) Polyester (10g/m2), (e) Polyester (20g/m2), (f) Polyamide

3.2.2 Fabrication Methods of Composites for Mode-I Test

A schematic diagram of the moulding process is illustrated in Figure. 3.3. A steel mould was wiped with three layers of release agent to make sure that the composite panels could be removed easily from the mould. A tacky tape was placed on the surround of the mould. For specimens for interlaminar toughness testing, the fabrics and

(a) (b) (c) (d) (e) (f) 2mm 2mm 2mm 2mm 2mm 2mm

Chapter 3 Mode-I Interlaminar Toughness

interleaf veils were cut into 400mm (in length) x 210mm (in width) rectangles. 5-harness satin and plain weave (2-D) and unidirectional (UD) fabrics were laid in stacks of 10 plies (2-D) or 6 plies (UD) on the rigid mould, respectively. A single interleaf veil (400mm x 210mm), except for pure polyester/pure carbon (PE/C) and carbon veils, and release film (A6000, supplied by Aerovac, 470mm x 100mm) were placed at the mid layer of the stack. PE/C veils were overlapped PE1 and carbon veils before lay-up. For the carbon veil, 2 plies of interleaf veils were used, in order to provide corresponding areal weights of the interleaf materials. A purpose of the release film was to provide the starter crack for specimens cut from panels for toughness tests. The starter crack length was set at 50mm.

The laminate fabrics were covered by a peel ply, which prevents the panel adhering to the bag film. A flow media, which is a knitted thermoplastic monofilament, was put on the peel ply. The flow media is to accelerate resin flow across the dry materials. Two spring coils were connected to PVC inlet/outlet pipes at each end. These coils were put on the mould, as shown in Figure 3.3.

During infusion processing, epoxy1 (Ep1) resin and mould were pre-heated to 45oC. The Ep1 was degassed for 30min using vacuum oven, thereafter the resin was infused into the mould. When impregnation was finished, the resin infused mould was put into the oven at 130oC for 1h. The epoxy2 (Ep2) resin was also heated and degassed before infusion. After infusion, the mould was put in an oven where the temperature was programmed to increase from 30oC to 120oC ramping at 2oC/min. When the temperature reached 120oC, the oven was held at 120oC for 4h. After curing, the infused panel was removed from mould and post-cured. The post-curing temperature was set to increase from 30oC to 180oC ramping at 7oC/min, followed by holding at 180oC for 2h. The vinyl ester (VE) resin was mixed with 2wt% methyl ethyl ketone peroxide (MEKP) beforehand. After the VE resin had infused completely, the mould was held at ambient temperature for 24h. The composite panel was removed from mould, and post-cured for 3h in the oven at 80oC.

Chapter 3 Mode-I Interlaminar Toughness