Issue 48

S. E. Oliveira et alii, Frattura ed Integrità Strutturale, 48 (2019Y) 249-256; DOI: 10.3221/IGF-ESIS.48.26 250 high performance composites, with the increase of the fiber fraction leading to an increase in modulus and strength of the processed material [3]. However, for a high dosage of reinforcing the homogeneous distribution of the fiber becomes very difficult and the porosity drastically increases, which justifies more detailed studies of this subject. Machining of fiber-reinforced composites differs significantly in many aspects from the machining of conventional metals and their alloys. In the machining of fiber-reinforced composites the behavior of the material is not only inhomogeneous but it also depends on diverse fiber and matrix properties, fiber orientation, and the relative volume of matrix and fibers [4]. The cutting tool alternately cuts the matrix and the fibers which machining response may be entirely different [4]. In the manufacture of engineering components from composite material, it is sometimes necessary to perform drilling and milling operations. In the case of manufacturing components in a composite material with short fibers, milling operations may be performed to obtain the desired a local geometry of the components. The machinability of polymer matrix composites with fibers strongly depends on the type of fiber and composite dosage and their mechanical and physical (for instance, thermal) properties. However, the type of fiber used in processing of these materials has a major influence on the selection of tools and cutting parameters [5]. The cutting temperature is also a factor that has a significant influence on the quality of the machined surfaces and tool life. In turn, temperature around cutting point and the final surface roughness depend on rotation and feed cutting tools. During the milling of short-fibber-carbon-reinforced composites (SCFC), the temperature on cutting point can cause damage or degradation to the composite’s surface. The surface roughness of the machined area greatly influences the mechanical performance of the dimensional precision and manufacturing costs [5]. Therefore, surface roughness and degradation of the composite’s surface require study to ensure the use of an industrial process. This is the first step to study machinability involving a specific composite. Milling in composite materials is a rather complex task due to its heterogeneity and the emergence of some problems such as surface delamination appearing during the machining process, associated with the characteristics of the material and cutting parameters. Milling is the machining operation most frequently used in the manufacture of fiber-reinforced plastic parts, as a corrective operation to produce well-defined and high-quality surfaces that often require the removal of excess material to control tolerances. Therefore, the ability to predict the cutting forces is essential to select process parameters which are necessary for an optimal machining. Mechanical performance of SCFC depends mainly on the capacity to obtain good fiber dispersion, usually evaluated by nondestructive methods such as ultrasonic C scanning and acoustic emission [6,7]. This paper studied the effect of cutting conditions during milling machining on roughness, cutting temperature and mechanical properties, with the purpose of contributing for a better understanding of the interdependence of these parameters, especially in the case of composites with high dosage. M ATERIALS AND SAMPLES or this investigation two composite batches were manufactured: a) a 6mm length fiber, and using Biresin® CR120 as matrix, formulated by bisphenol A - epichlorhydrin epoxy resin 1,4 - bis (2,3-epoxypropoxy) butane, combined with the hardener CH120-3, with 40% (in volume fraction) of carbon fibers. b) 0.5 mm ± 0.15 in length and 7 µm ± 2 Ø monofilaments, using Biresin® CR83 matrix reinforced with 17.5% (in volume fraction) of carbon fibers. Resins were supplied by Sika, Stuttgart, Germany, while the company Sigrafil, SGL Group, Germany, supplied the fibers. Composite plates were manufactured by manual molding process, pressing the mold in a servo mechanical machine, Mazzola W15. The compression was applied such as it is shown schematically in Fig. 1, which also shows the mold and the positioning of the pressure transducer used to monitor inside pressure during the cure process. The desired amount of fibers previously subjected to the fiber separation treatment was added to the resin. Then, the materials were mixed and placed in the mold cavity. Afterwards, the mold was closed and placed in the mechanical compression load at 7600 daN, with corresponding pressure of about 50.7 bar. The processed plates were then subjected to cure and post cure processes. Fig. 2a) shows the mold design and a final composite plate. From this plate, test specimens were later machined according to the scheme and dimensions indicated in Fig. 2b). Composite plates were subjected to a cure process done at room temperature for 20 hours in the mold, and a post cure was carried out at 70º C for 12 hours. The pressure during the curing process was monitored by the pressure transducer shown in Fig. 1. As shown in Fig. 2c) the pressure remains nearly constant for the entire duration of the cure process. F