Issue 48

S. E. Oliveira et alii, Frattura ed Integrità Strutturale, 48 (2019Y) 249-256; DOI: 10.3221/IGF-ESIS.48.26 254 displacement curves. The stiffness modulus was calculated using the linear elastic Hooke’s law in the linear region of load versus displacement plot. The collected values presented a correlation coefficient at least 0.99. Contrary to the reported on scientific literature [3], a decreasing of the tensile strength with the increasing of both fiber content and length was observed. These results are justified by the poor fiber distribution, poor fiber/matrix adhesion and increasing porosity for higher fiber dosage composites. On the other side, and despite high dispersion of the results, rotation-cutting speed seems to play a reduced effect on composite tensile strength, for both materials. Figure 7 : Maximum temperature around carbon tool against drilling speed. Figure 8 : Tensile strength against rotation-cutting speed. The analysis of the stiffness modulus, shown in Fig. 9, indicates that in this case the results are in accordance with the reported on literature [3], as it was observed an increasing of the stiffness with the increasing of both fiber content and length. The results mean that fibber distribution and porosity play a less significant role on composite stiffness than in tensile strength. The analysis of the figure also shows significant dispersion (with standard deviations from 0.25 to 2.65 GPa), but an important aspect must be stated for both materials: above 3000 rpm there is a significant loss of stiffness. This loss of rigidity may be associated with the temperature developed in the machining process and consequent degradation of the matrix.