Funciones de un líquido de corte

Figure 4.21 shows SEM pictures of fracture surface for the non-interleaved specimens. For the satin weave Ep1 system laminate, the fracture surface is covered with the matrix on the both fracture surfaces. It is found that there are many hackles on

Chapter 4 Mode-II Interlaminar Toughness

the surface, as illustrated in Figure 4.21 (a). These hackles would contribute to improving the Mode-II initial value [19]. The crack propagation tends to pass through the matrix region, same as the Mode-I fracture behaviour. It is thought that the adhesion between woven fabric and matrix is good. The crack, therefore, can propagate only in the matrix area. In the VE system composite, the matrix and exposed woven fibres appear on the fracture surface, as illustrated in Figure 4.21 (b). The exposed woven fibres are unravelled. The hackles are not seen on the surface. The adhesion between the carbon woven fabric and VE resin may not be as good as the Ep1 resin system. Consequently, the woven fibres are unravelled on the surface. This poor adhesion of the VE resin would lead to the crack propagating easily. Therefore, the initial values are significantly smaller than the Ep1 case.

The fracture surface in the UD fabric Ep1 system laminate is similar to the satin weave Ep1 system specimens. Both fracture surfaces are covered with the matrix. Although the fracture surface reveals trace of the UD fibres, they are not unravelled on the surface, as shown in Figure 4.21 (c). It can be seen that the hackles appear on the fracture surface. It seems that these hackles are less than the satin weave samples. Hence, the Mode-II interlaminar toughness may be low, compared with the satin weave case. The fracture surface in the VE system is similar to the satin with VE system. The UD carbon fibres are exposed and unravelled (see in Figure 4.21 (d)). However, these frayed fabrics would not work as fibre-bridging reinforcements significantly. In the UD fabric VE system laminate, the poor adhesion and flat surface would lead to be poor

GIIC values, compared with the satin weave materials. Therefore, the Mode-II interlaminar toughness could not improve considerably.

In the plain weave Ep2 system laminate, the fracture surface is covered with the matrix, as shown in Figure 4.21 (e). The woven fibres are not unravelled completely. In Ep2 system, the hackles also appear on the fracture surface. Moreover, the fracture surface seems to be coarser than samples made with the other fabrics. On the other hand, the fracture surface for the VE system is the same as the 5-harness satin weave cases. Some of woven fibres are frayed over a partial area, as shown in Figure 4.21 (f). These fibres would work for bridging during crack propagation.

Chapter 4 Mode-II Interlaminar Toughness

Figure 4.21 Micrographs of fracture surface taken by SEM for non-interleaved specimens: (a) Satin weave Ep1 system, (b) Satin weave VE system, (c) UD fabric Ep1 system, (d) UD fabric VE system, (e) Plain weave Ep2 system, (f) Plain weave VE system

The cross-sections taken by the optical microscope are illustrated in Figure 4.22. From Figure 4.22 (a), it can be seen that the crack path is not straight, unlike the DCB

Hackles 100um Unravelled carbon woven fibres 1mm

(c) (d)

1mm

Unravelled carbon woven fibres

(e)

100um

(f)

1mm

Exposed carbon woven fibres

(a) (b)

Hackles

Hackles

Chapter 4 Mode-II Interlaminar Toughness

tested specimen (see in Figure 3.30 (a)). The crack propagates in the matrix region. For the VE system, the crack would propagate along one side weave surface and then some of the carbon woven fibres seem to work as fibre-bridging. The fracture surface is almost the same as the Mode-I fracture surface (see in Figure 3.29(b)). It can be seen that the woven fibres are unravelled, as shown in Figure 4.22 (b). The crack paths in UD specimens with both resin systems are straight, as shown in Figure 4.22 (c) and (d). Therefore, it is thought that the crack growth would be easier than the satin weave case. The crack paths in the plain weave with both composites of either resin are not straight in the interlaminar region, as shown in Figure 4.22 (e) and (f). This is because the surface of the plain weave is rougher than that of the 5-harness satin weave and UD fabric samples. The fibre-bridging does not contribute to the improvement of the Mode-II interlaminar toughness unlike the Mode-I interlaminar toughness. The behaviour of the crack propagation in the Ep2 system laminate is almost the same as the satin weave Ep1 system case. Compared to the areal weight of the fabrics, the HAW specimen has higher GIIC values than the LAW sample. In the Mode-II interlaminar toughness, surface of the fabric would influence crack growth strongly. The non-interleaved specimen in the VE system has a crack path that is a mixture of intralaminar and interlaminar regions. The crack propagation in the plain weave VE system laminate was significantly slow. Therefore, bending fracture was initiated at an early stage. It is thought that the roughness of the fabric surface is a cause.

Figure 4.22 Micrographs of cross-section taken by optical microscope for non-interleaved specimens: (a) Satin weave Ep1 system, (b) Satin weave VE system, (c) UD fabric Ep1 system, (d) UD fabric VE system, (e) Plain weave Ep2 system, (f) Plain weave VE system Propagation direction (a) (b) (c) 1mm

Chapter 4 Mode-II Interlaminar Toughness

Figure 4.22 (Continued)

4.4.2 Mechanisms of Mode-II Interlaminar Toughness for Interleaved Specimens