PSI - Issue 14

P R Krishna Mohan et al. / Procedia Structural Integrity 14 (2019) 176–183 P R Krishna Mohan/ StructuralIntegrity Procedia 00 (2018) 000–000

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1. Introduction

According to Schwartz (1997) for manufacturing the components such as flaps, aileron, landing- gear doors, etc. in aeronautical industry carbon-reinforced polymeric composites are used. Buehler et al. (2000) and Hayes et al. (2000) referred that apart from the aerospace, composite materials have gained importance in automotive, sport and construction industries. In the aerospace industry, carbon/epoxy composites are used extensively since the mechanical strength, chemical resistance and service temperatures requirements are fulfilled apart from higher fatigue strength and higher corrosion resistance. In recent years, prepreg material, which is an intermediate material has gained its importance in manufacturing structural composite components as these materials allow to adjust reinforcement positioning, thickness, number of layer and their orientations in the molding. Composites made by hand lay-up process can be seen in several works as shown in the literature of Elanchezhian et al. (2014), Paiva et al. (2006) and Paiva et al. (2009). However, composites fabricated by prepregs utilize hand lay-up process that leads to non-uniform deposition of epoxy. Parts manufactured by using prepregs are clean and offer ease in manufacturing of complex geometries with almost 60% fiber volume fraction. Hence, in this work, our objective was to make indigenous prepreg and prepreg based composites with optimized curing cycle. 2. Prepreg making and material preparation For making the unidirectional prepreg tape, an in-house prepreg making facility is used. In the present work, the constituents used for making unidirectional prepregs are diglycidyl ether of bisphenol- A (DGEBA) epoxy resin (LY556), methyl tetra hydraphthalic anhydride (MTHPA) the curing agent, 2, 4, 5-tris [(dimethylamino)methyl]- Phenol (DMP-30) as an accelerator and TC36s carbon fiber with 12k filaments. We have used a resin-hardener mixture in a weight ratio of 100:80 along with 0.5% accelerator. The silicone oil is applied to the drum of the prepreg machine uniformly. A silicone release film is wrapped onto the drum. The spool of carbon fiber is mounted on the spool holder. The fiber tow is passed through a dip roller immersed in an impregnation medium, and guide rollers for extra squeezing of resin mounted on resin bath frames. The impregnation medium contains a resin-hardener mixture. Retrofitting of a heating unit to the existing prepreg machine has allowed maintaining a uniform temperature of the resin-hardener mixture at 50-60°C. The impregnation unit comprising of resin bath along with its accessories are fitted on a carriage, the linear motion of the carriage and the speed of the drum is set at 508 mm/sec and 14 rpm, respectively. Fiber tow enters the impregnation zone, dipped into the resin-hardener mixture and finally guided by the guide rollers to the exit, winds on to the drum without any overlapping. Once the required size of the prepreg is laid onto the drum, a perforated polyethylene release film is wound on it. A unidirectional prepreg tape of 74 x 200 cm in size is obtained. Tackiness and drapability are the two essential factors to judge the quality of prepreg. By visual inspection, these properties are checked while B-staging the prepreg at room temperature for about 9 hours. These prepregs are cut and are stored at a controlled temperature of about -18°C. Several changes are made to the prepreg machine to increase the strength of the in-house prepreg. We have introduced two-squeeze rollers for removing the extra amount of resin. They also help in maintaining the uniform distribution of resin in the fiber and the thickness of the prepreg made. The two rollers are coated with silicon and are kept stationary. The factors such as the increase in the dipping time and number of dips of fiber in the resin bath have to be considered for better impregnation of the matrix with the fiber. These have motivated us in introducing an additional dip roller, therefore, increasing the distance between the dip roller and the squeeze roller (nip roller). The tension in the fiber has increased with the increasing distance between the two rollers. These aspects have led to fiber breakage while making prepreg sheet. To avoid this problem, we have to decrease the tension in the fiber. To reduce the tension of the fiber, we have introduced roller in the first squeeze roller, and guide rollers in between the 2.1. Prepreg Makings 2.2. Modifications incorporated in the prepreg machine