Sample Preparation for Directional Breathing Tests

To evaluate the effect of the gas transport direction on porosity, samples with different sealing configurations were made from MTM45-1/5HS-Thick prepreg (Figure 5-1). 5HS laminates were made of 8 layers of 12.7 cm × 12.7 cm prepreg, cured ply thickness (CPT ≈ 0.39 mm). The laminates were prepared based on the sealing configurations shown in Figure 5-1. After edge or surface sealing, a room temperature debulk (laminate held under full vacuum under a vacuum bag at room temperature) was performed for 0.23 to 0.5 hours.

Figure 5-1 Sealing configurations: a) surface and edge breathing, b) edge breathing only, c) surface breathing only.

Configuration (a) is the manufacturer’s recommended bagging procedure in which the laminate is free to breathe from all sides except the surface that is in contact with mold. In configuration (b) the laminate is sealed at top and bottom surfaces and is only able to breathe in the in-plane direction. In configuration (c) the laminate is sealed at all edges and breathing is limited to the through-thickness direction. After sealing the laminates with standard sealant tape, bagging materials were placed sequentially according to the manufacturer’s recommendations (Figure 5-2) [52]. Edge breathing dams were used at the edges of the laminates with configurations (a) and

54

(b). All laminates were covered with non-perforated release film and a larger breather layer on top of that. Strips of peel ply were placed around the edges of the laminates to allow air paths under the release film into the breather (to provide an air path between the breather and the top surface of the laminate). A vacuum bag covered the whole arrangement and was sealed with sealant tape to the Aluminum tool. Figure 5-2 shows the laminate preparation steps. Once the samples were prepared they were cured in a Thermotron oven (Holland, Michigan, USA) with 80

°C cure cycle. Details of the 80 °C cure cycle are presented in section 5.2.2. In edge breathing tests, samples were taken out after 0 (as laid up), 0.23 (debulk), 1.23, 2.23, 4.23 and 22.23 hours.

In surface breathing test samples were taken out after 0 (as laid up), 0.23 (debulk), 1.23, 2.23, 6.23, 8.23 and 22.23 hours. The times were chosen based on the in-plane and through-thickness permeability graphs (Figure 5-15) to enable evaluation of the relationship between porosity and permeability. Three repeats were done for samples with “edge and surface breathing” and all the fully cured samples (22.23 hr). One repeat was done for partially-cured samples in “edge breathing only” and “surface breathing only” tests.

55

Figure 5-2 Laminate preparation steps in edge breathing and surface breathing tests.

56

5.2.4 Void Characterization

It is necessary to measure and characterize voids during the process to gain a thorough understanding of void evolution and underlying mechanisms. In this study several methods including µCT, density and optical microscopy were evaluated but optical microscopy (OM) was chosen as the primary characterization method due to the level of detail required from the

images. Optical microscopy was used to characterize porosity and tow geometry in partially and fully cured laminates. A Nikon stereo microscope (SMZ745T) in reflection and transmission light modes was used to investigate the surface morphology of uncured prepreg. X-ray micro computed tomography (µCT35-Scanco Medical) was employed for studying the three

dimensional micro structure of the laminate before processing. An X-ray voltage of 70 kV and intensity of 154 µA were used to achieve a 3µm/pixel resolution. A 7.62 cm × 6.35 cm 8 layer laminate of MTM45-1/5HS-Thin laminate was laid up at room temperature and a 5 mm × 28 mm section of this laminate was cut and scanned. The scanned data was processed and reconstructed using the µCT35 scanner software creating two dimensional slices and three-dimensional information about the porous structure of the laminate.

Optical microscopy and image analysis were the main tools used to characterize porosity and tow geometry of semi-cured and cured laminates. The first step was surface preparation of the

samples. Fully cured laminates were prepared according to conventional procedures [83]. The preparation of semi-cured samples for optical microscopy requires special attention, as the partially cured matrix is still soft (see section 4.2.3). The samples were therefore mounted with a special low viscosity, room temperature curing resin (Epo-color Buehler) that filled the pores and vacant spaces of the laminate. The cured mounting resin supported the soft composite material

57

during the subsequent grinding and polishing steps and reduced surface damage [83,84]. Surface preparation and image acquisition steps are described in section 4.2.3. Two to three replicate samples were analyzed for each data point. To characterize voids and the geometrical features of fiber tows, the mosaic images were analyzed using the Image – J software [85]. The area fraction of voids is used as a measure of porosity. The voids are categorized into three groups: inter-laminar, fiber tow and resin voids. Details of different void types and their origins, are presented in section 5.3.1.1. The area fraction of each type of voids is measured individually and their sum is reported as void content or porosity.

𝜑_𝑇 = 𝜑_𝐼+ 𝜑_𝐹+ 𝜑_𝑅

Where: φT (%): Total porosity, φI (%): Inter-laminar porosity, φF(%): Fiber tow porosity, φR

(%): Resin void porosity.

In partially cured laminates, direct area measurement of all voids located inside fiber tows is impractical because many of the smaller voids are located between the individual fibres and are of the size of the individual fibres or smaller. In previous work on this prepreg system, only the area fraction of larger size fiber tow voids were reported, which gives an underestimate of the total fibre tow porosity in the laminate [39, 96]. In the current work, the total fiber tow porosity of partially cured laminates is calculated based on measurements of the total area and subtracting the inter-laminar and resin void areas and the area of the final cured porosity free laminate. The location and coordinates of different voids were also measured and used to study their

distribution within the laminate. The geometry of individual elliptically shaped tows were studied and characterized by measuring their minor and major axes.

58