Using traditional circular X-ray CT, it is challenging and time-consuming to locate the RoI to perform time-lapse tracking of the damage evolution due to the limited FoV. Based on previous studies of this material by Jespersen et al. [
8,
14,
15], it is expected that fatigue damage is most likely to initiate from regions where the backing bundles cross-over; however, it is important to confirm whether this is generally true for this material system or whether there are other factors that might localise fatigue damage. Owing to the extended length that can be viewed at high resolution by helical X-ray CT, the overall damage distribution along the composite specimen can be monitored in relation to the composite microstructure (e.g., backing fibre bundle cross-over regions, stitching thread cross-over regions, resin-rich regions, fibre misalignment in UD fibre bundles) at different stages of its fatigue life. In the specimen studied, four main damage modes were observed, namely off-axis cracking in the matrix or backing fibre bundles, fibre fractures, sub-surface debonding and longitudinal splitting in the UD layers.
Figure 3 shows the extracted fatigue damage within the specimen after 500,000 cycles, where the fibre fractures and longitudinal splits are visualised with respect to the UD and backing fibre bundles (rendered green). The regions with fibre fractures were manually delineated (see red contours in
Figure 3d) to highlight the extent of UD fibre fracture damage in 3D. The zinc iodide dye penetrated most of the highlighted fibre fracture region; however, some fibre fractures in region (e) were not stained as they were not connected to the external surfaces. The increased visibility of thin cracks in the high-resolution RoI images confirms the presence of the unstained cracks as shown in
Figure 3e and this damage region was also included in the 3D damage visualisation. The effect of staining on damage detectability in this material was discussed in [
12]. In addition, its effect on the observed damage development in this material system has been assessed by comparing the damage status of S1 and S2 after 2 million cycles. For S1, the fatigue test was interrupted at 0.5 million, 1.5 million and 2 million cycles, for repeated staining and imaging at each step; while for S2 the fatigue test was interrupted only after 2 million cycles for staining and imaging. The same damage mechanisms, as observed in S3 here, have been seen in S1 and S2. Moreover, the severity of damage in S1 and S2 is on the same level, with the occurrence of a few small regions of UD fibre fractures near backing fibre bundle cross-over regions. This indicates that there is no obvious influence of staining on the observed damage scenario. The evolution, distribution and interaction of the observed damage mechanisms in S3 will be presented and discussed in the following sections.
3.1. Off-Axis Matrix and Interface Cracks
It has been challenging to observe off-axis cracks in this material by X-ray CT due to the closing of those cracks to below the resolution limit after load removal [
15]. With the aid of contrast agent, we are able to observe the development of off-axis matrix cracks from the specimen edges and also off-axis cracks between backing fibres.
Stained off-axis cracks in the matrix were found to have initiated from specimen edges after 50,000 cycles (N
1) as a result of the axial tensile stress (see
Figure 4b). After exposure to more fatigue cycles (N
2), the crack density increases dramatically. The observed increase in off-axis cracking has also been reported by Jespersen et al. [
15] in a similar material. It is difficult to see the polymer stitching threads as their X-ray absorption capability is similar to that of the matrix material, as can be seen in the magnified views in
Figure 4e,f. It has been observed in
Figure 4d,f that the off-axis cracks in the matrix tend to be deflected when propagating into the stitching threads. This highlights the effect of stitching threads on delaying the propagation of off-axis matrix cracks from the edges by debonding. Once a crack crosses the column of material containing the stitching threads, it joins the off-axis cracks between backing fibres, as shown in the highlighted ellipse in
Figure 4f.
The presence of off-axis cracks in between backing fibres has been reported in a number of fatigue studies on this material [
5,
15] and is observed here after 500,000 cycles (see
Figure 5). Only off-axis cracks that are stained were detectable with the current CT acquisition set-up. These stained off-axis cracks are distributed at different heights in the specimen and multiple ones sometimes occur within a single backing fibre bundle; these tend to be distributed vertically relative to one another due to stress shielding [
16]. The regions denoted by the upward arrows in
Figure 5a and the highlighted ellipses in the YZ sections shown in
Figure 5d,e highlight the joining of stained off-axis matrix cracks in the backing layer and stained fibre fractures in the UD bundles. This confirms the presence of a connecting path between the two damage modes. This observation supports the hypothesis [
5] that off-axis cracking in backing fibre bundles triggers fibre fractures in the adjacent UD fibre bundles. It is worth noting that the stained fibre fractures were not necessarily directly connected to the off-axis cracks in some cases, as highlighted by the downward-arrows in
Figure 5d,e. However, the fact that these fibre fractures were stained successfully means that they were connected to the free surface, either through cracks running into the specimen surfaces, or tunnel-cracks (through backing bundles) extending to the edges.
3.2. Fibre Fractures in UD Fibre Bundles
The nucleation of UD fibre fractures was found to correlate with two micro-structural features; regions where backing fibre bundles cross-over (near mid-thickness of the specimen, see
Figure 3a–c) and regions where stitching threads cross-over (near the specimen surface, see the red boxes in
Figure 5b and Figure 8c). The development and distribution of UD fibre fractures from these two regions were tracked in the time-lapse CT images, and the results are presented and discussed in this section.
As mentioned above, the regions of UD fibre fractures were manually delineated (see
Figure 3d) to help visualise the 3D morphology of this damage mode. Fibre fractures were observed in the UD fibres adjacent to the cross-over regions of backing fibre bundles after 50,000 cycles, which is consistent with the observations of previous studies [
5,
8]. The onset of UD fibre fracture appears to be localised next to backing bundle cross-over regions (see
Figure 6b,d). After 500,000 cycles, seven fibre fracture regions were observed on the right-hand side of the specimen width; these were approximately evenly distributed (~2–3 mm) along the 20 mm length. Out of the seven damage regions, six were next to the backing bundle cross-over regions, while one small region was located next to −80° backing fibres only (see
Figure 6c). As can be seen in
Figure 3c, it should be noted that most of the UD fibre fractures (rendered red) developed near the UD fibre fracture site (rendered blue) developing on the other side of the backing layer. This site is at the same height (in Z) as the site of damage initiation of fibre fractures after 50,000 cycles (see
Figure 6b–e). This is the location where UD fibre fractures propagated most widely within the specimen after 500,000 cycles.
Figure 6e,f shows the propagation of fibre fractures (shown red) between 50,000 and 500,000 cycles. The propagation here was seen to be mostly along the length and width directions rather than through thickness. In the case with backing fibres oriented at ±45°/90° [
8] fatigued at maximum strain of 1%, fibre fracture was observed across the full width of the cross-over region and then propagated in the thickness direction. This difference in propagation sequence indicates the necessity of choosing the appropriate arrangement of backing fibres in the composite design for different wind turbine blade requirements. As can be seen in
Figure 6e, this damage region has two elongated branches. A local variation in fibre orientation is evident in the 0° fibre bundle (see
Figure 7g), which could be the cause of this damage morphology. In other words, 0° fibre alignment could be important in controlling the propagation of UD fibre fractures.
The distribution of UD fibre fractures in relation to the local microstructure within one damage region is revealed by the raw X-ray CT images where individual fibre fractures can be seen (see
Figure 7). In the XZ sections near the backing fibre bundles, the distribution of fibre fractures tends to be aligned with the backing fibre bundle in contact with the UD fibres, as shown in the XZ-2 sections. This is also true for the UD fibre fractures at the edge of the UD bundle (see
Figure 7a). Further away from the backing bundles, the distribution of fibre fractures is less dependent on backing fibre orientation but more affected by local fibre orientation in the UD bundle, as in the case for the XZ-3 and XZ-4 sections in
Figure 7. It is noteworthy that the UD fibre fractures are seldom aligned in a continuous line; instead they tend to cluster in small numbers at different heights.
UD fibre fractures were also observed to occur near one specimen surface after 500,000 cycles (see
Figure 3a and
Figure 5).
Figure 8 shows sub-surface XZ sections in this region through the fatigue cycling. It is evident that sub-surface debonding of stitching threads (see
Figure 8b,c) is associated with fracture of the adjacent UD fibres in this region, presumably because of higher local stresses. It is worth noting that a number of longitudinal splits developed, connecting the UD fibre fractures that were relatively more distanced from each other (see
Figure 8h). As shown in
Figure 3a, this damage site is located on the opposite side of the backing fibre bundles to that where fibre fractures initiate near the cross-overs after 50,000 cycles; this means that the damage sites are in two UD fibre bundles. However, these two damage sites are at a similar height, and the extents of propagation through the bundle thickness are similar. It is worth mentioning that another damage region was found further above the debonded stitching threads, which was caused by attaching the extensometer during the fatigue test.
3.3. Sub-Surface Debonding of Stitching Threads and Longitudinal Splitting
Extensive longitudinal splitting has been observed in the UD bundle where sub-surface debonding of stitching threads at the cross-over region of the threads has occurred. The longitudinal splits in the UD fibre bundle are extracted and visualised in
Figure 9. Longitudinal splitting tends to initially occur in the edge UD bundles due to the edge effect (see
Figure 9d). After 500,000 cycles, a longitudinal split originated from a sub-surface debonded region (see
Figure 5a and
Figure 9e). A section of the UD fibre bundle is separated from the full bundle as shown in the XY-1 plane after 500,000 cycles, and also in the 3D view in
Figure 9a. Apart from the fact that these splits are closely correlated with the UD fibre fractures, the splits lower the lateral constraint of the originally closely packed UD fibres, which could potentially make the composite susceptible to compressive loads and moisture. It could be inferred that if the wind turbine blades experience bending fatigue, longitudinal splitting could be a detrimental damage mode.