Issue 39

S. Doddamani et alii, Frattura ed Integrità Strutturale, 39 (2017) 274-281; DOI: 10.3221/IGF-ESIS.39.25 277 where, σ y is the yield strength, in N/mm 2 K IC is fracture toughness, MPa m ½ Also, the length of the specimen is taken here for the testing was 4 times the specimen diameter which was the requirement taken into consideration while preparing the CNT specimens. Tension test was performed at room temperature on the CNT specimens for the Al6061-Graphite particulate metal matrix composites of 3%, 6%, 9% and 12% graphite using a UTM following standard test procedures in agreement with the ASTM:E8M – 91 standards [14] . The CNT specimens are allowed for tensile test. The load at fracture (P f ) is noted for each specimen of above said weight fractions. The load at fracture (P f ) obtained are taken in consideration to calculate the fracture toughness (K IC ) using the empirical relation (1) . Evaluation of fracture toughness was investigated for Al6061-graphite MMC using CNT specimen testing. From the outcomes it was observed that the crack durability results assessed from the CNT test were justifiable (in plain strain condition) even though the samples were not fatigue pre-cracked. R ESULT AND DISCUSSIONS racture toughness for the various weight fractions of Al6061-Grphite particulate MMC are evaluated experimentally and results are shown in Tab. 1. Fracture toughness of the A6061-Grphite composite for 3, 6, 9, 12% Graphite is determined using CNT specimens on UTM. The plane strain fracture toughness condition was met with the CNT sample diameter of D=12.5 mm using Eq. (2) which validates the fracture toughness values obtained from experimentation. Sl. No Composite Fracture load (P f ) kN Fracture Toughness (K IC ) MPa m 1/2 1 Al6061-3%Gr 12.51 13.92 2 Al6061-6%Gr 13.70 15.25 3 Al6061-9%Gr 14.24 15.85 4 Al6061-12%Gr 14.10 15.69 Table 1: Experimental fracture toughness of Al6061-Graphite MMC. From the outcomes it is observed that the fracture toughness results assessed from the CNT test were substantial (in plain strain condition) even though the samples were not fatigue pre-cracked. Fracture toughness of the A6061-Grphite composite for 3, 6, 9, 12% Graphite is determined using CNT specimens on UTM. For each composition three specimens are prepared and tested for fracture toughness. Load – elongation curve for Al-graphite for different weight fractions of graphite is shown in the Fig. 3. It is observed that the elongation increases with increase in load. For all specimens there is small difference in change in elongation. Fracture load and the maximum elongation of Al-3% graphite is P f = 12.51kN and 4.21mm respectively. Fracture load and the maximum elongation of Al- 6% graphite is P f = 13.70kN and 4.31mm respectively. Fracture load and the maximum elongation of Al-9% graphite is P f = 14.24kN and 4.58mm respectively. Fracture load and the maximum elongation of Al-12% graphite is P f = 14.10kN and 4.55mm respectively. The increase in elnogation is because of the existence of the hard and higher modulus graphite particles embedded in the Al6061 matrix, which act as a barricade to oppose plastic flow when the MMC is subjected to an applied load. Also, the decreased interparticle spacing, due to the increased weight percent of graphite reinforcement, creates increased resistance to dislocation motion, which gives the improved strength to MMCs. F