Issue 41

S. Doddamani et alii, Frattura ed Integrità Strutturale, 41 (2017) 484-490; DOI: 10.3221/IGF-ESIS.41.60 486 preparation and experiments were carried out according to ASTM-E 399 standard for three different crack length to width (a/w) ratios i.e., 0.45, 0.47 & 0.50. Figure 1: Dimensions of cast CT Specimens (All dimensions are in mm). The load-displacement data are analyzed to find out fracture toughness (K Ic ) of the Al6061-graphite particulate MMCs. The value of K Ic is determined by crack length and critical load (P Q ). The value of P Q is determined by plotting a secant slope on the load and displacement record. Typical [10] load-versus-displacement record for the two types of K Ic testing is shown in Fig. 2. The calculation for P Q engages representing the loading slope of the load versus displacement record. A slope of 5% less than the available slope, referred as secant slope, is then drawn. In Type I, monotonically rising load, the P Q is taken where the 5% secant incline meets the load-versus displacement curve [10]. In Type II or Type III, instability or other maximum load is reached before the 5% secant. The maximum load reached up to and including the likely intersection of the 5% secant is the P Q . Figure 2: Three types of load-displacement behavior in a K Ic test. Source: T.L. Anderson [1]. In the case of slope of the load-versus-displacement record of Al6061-graphite, the curve obtained was Type III curve as shown in Fig. 3. The maximum value of the load itself will be the critical load (P Q ). This corresponds to about 2% ductile crack extension [18]; this may be an effective crack extension linked to plastic zone development. K Ic values were determined from the P q values as prescribed in ASTM E399 Standard [10]. Experiments were carried out and results of the experiment are listed in Table 1. Fig. 3 shows the load-displacement curves of Al6061-graphite obtained using CT specimens for different crack length to width ratios.