Issue 41

S. Doddamani et alii, Frattura ed Integrità Strutturale, 41 (2017) 484-490; DOI: 10.3221/IGF-ESIS.41.60 485 quantitative approach of stating a material's resistance to brittle fracture once a crack is present. If materials have high fracture toughness, it will possibly experience a ductile fracture whereas materials having low fracture toughness will possibly undergo brittle fracture. After studying the early work of Inglis, Griffith, and others, Irwin, head of the fracture mechanics research group at the Naval Research Laboratory, concluded that the basic tools needed to analyze fracture were already available [1]. Irwin’s initial major contribution was to reinforce the Griffith approach to metals as well as the energy dissipated by native plastic flow. Griffith, Irwin and others who worked on the conceptualization of fracture mechanics, studied the behavior of cracks in brittle materials. To investigate the fracture toughness, there are different methods. Basically an American Society for Testing and Materials (ASTM) Standard and recent advances in fracture toughness testing methods are used for testing of aluminium alloys and some of aluminium matrix particulate reinforced composites. ASTM standard testing methods include Fracture toughness testing by Single Edge Notch Bend (SENB) Specimen and Compact Tension (CT) Specimen. Other testing methods which are getting popular for their ease include Fracture toughness by Round Bar, Indentation techniques, Circumferential Notched Tensile (CNT) specimens [2]. Metal matrix composites (MMCs) are used in automobile and aerospace applications where they are required for weight savings, wear resistance, thermal management, etc. MMCs may be of continuously and discontinuously reinforced types [1]. By far the most commonly used MMCs are based on aluminum alloys reinforced with alumina (Al 2 O 3 ), silicon carbide (SiCp), Titanium carbide, graphite etc. Discontinuously reinforced MMCs are a great deal more affordable to fabricate than continuously reinforced composites. Discontinuously reinforced composites have isotropic properties [3], whereas continuous aligned reinforcements have highly anisotropic properties. In applications requiring isotropic properties, discontinuously reinforced composites can do better than continuous fiber reinforced composites. The literature survey presents a review of the published material available on the effect of various reinforcement types, their size and volume fraction, ageing behavior with Aluminium based Metal Matrix Composites (MMCs) [3] being a combination of two constituents, matrix and the reinforcement. Mechanical characterization such as tensile strength and elongation experiments by using Universal Testing Machine [4, 5] (UTM) of Al/SiC has been reported for a varying mass fraction of SiCp with Aluminium. Fracture toughness [6], tensile fracture behavior [7, 8, 9] on Circumferential Notched Tensile (CNT) specimens and Compact Tension (CT) test [10, 11] specimen of Al alloy with different reinforcements, fracture toughness by indentation techniques [12, 13] were studied by different researchers. Most of them compared their results with the unreinforced aluminium alloy. From the literature, it is identified that more work has been done on tensile and fracture characteristics of Al/SiC particulate [14, 15, 16, 17] MMCs. Research has to be carried on the aluminium matrix composites reinforced with graphite particles in the area of fracture and fatigue in order to improve the strength and fracture characteristics of the material to avoid cracking. The study of the tensile and fracture characteristics of aluminium matrix composites reinforced with graphite particles at varied weight fractions is here reported. P ROCESSING rocessing of MMCs was done by the stir casting technique [8]. Aluminium 6061-graphite specimens were prepared at varied weight fractions of graphite (3%, 6%, 9%, and 12%) using stir casting method. The aluminium blocks were melted in the furnace. After melting, molten aluminium was super-heated to desired temperature (about 750 0 C). The required amounts of graphite particles were added to the aluminium melts while stirring with stirrer at a speed of 550 RPM. The molten aluminium-graphite was poured into a split type permanent mould and it was allowed to solidify. The aluminium-graphite alloy bars were taken out from the mould. The specimens were prepared from as-cast alloys for determination of required properties. F RACTURE TOUGHNESS TESTING racture toughness of the aluminum-graphite particulate MMCs at varied weight fractions of graphite (3%, 6%, 9%, and 12%) was determined using a universal testing machine (UTM) as per ASTM E399 standard [10] testing procedure. In this method compact tension (CT) specimens are used as shown in Fig. 1. Compact tension (CT) specimens of various weight fractions of aluminium graphite are tested for their fracture toughness values. The specimen P F