ELSEVIER SHORT COMMUNICATION A Note on Stretch Zones in an Al-Zn-Mg Alloy Agah U&IZ* and John W. Martin+ *Ulud@ University, Bursa, Turkey; +Depurfmenf of Materials, Oxford University, Oxford, England; UK The stretch zone is an intermediate fracture surface between the end of the fatigue pre- crack and the beginning of the crack pro- duced by monotonic loading (Fig. 1) in a frac- ture toughness test. The appearance of this zone, ductile like the fatigue precrack zone, is distingu:ished by a more hilly relief and more elongated dimples and its width is re- lated to the crack opening displacement (COD). Considering a blunt crack symmet- ric relative to the original line of the fatigue precrack, an equation relating stretch zone width (Wsz) to COD (6,) can easily be estab- lished [l, 21. In Fig. 1, the stretch zone is represented by arc AB and forms an angle 8 with the horizontal line. If AB is the criti- cal stretch zone width, we can write: 6, = 2Wsz x sin 8 (1) Under plane strain conditions, COD and Jlc are related by the equation [3]: Jlc = Mo,& (2) where o,, is the flow stress (average of yield stress and ultimate tensile stress). Taking M = 2: Jlc = 2006, (3) Inserting Eq. 1 in Eq. 3, we obtain: Jlr = 40, W,SZC sin 8 (4) The scanning electron microscope (SEM) is usually used to observe the stretch zone of fractured specimens. If the incident angle is a and the magnification of the image is G, then the stretch zone width can be evalu- ated by: W L 1 szc = cos (f3 - a) ’ G (5) where L is the measured length of the stretch zone on the micrographs. Substituting Eq. 5 in Eqs. 4 and 1, values of COD and Jlc can be evaluated directly from experimental data (L, a, and G): 6, = 2 x L x sin 8 x 1 cos (e - a) G In Eqs. 6 and 7, the only unknown in the right hand side is the angle 0 in which the magnitude depends on the capacity of the material to resist crack initiation. Although it is sometimes difficult to ob- serve the stretch zone (for example, in the case of the grain size being greater than the stretch zone), the relation between stretch zone and Jlc and COD has been studied by many workers [I, 4-101 and satisfactory results have been achieved. In the present work, applying the technique to a series of peak- aged Al-Zn-Mg alloys has been attempted. EXPERIMENTAL MATERIALS AND PROCEDURES The alloys studied in this work are based on the commercial aluminum alloy 7005 in MATERIALS CHARACTERIZATION 34:217-220 (1995) 217 0 Elsevier Science Inc., 1995 1044-5803/95:59.50 655 Avenue of the Americas, New York, NY 10010 SSDI 10445803(95)00017-S