Page 382 - Biaxial Multiaxial Fatigue and Fracture

366 M. FONTE ET AL.

MATERIAL AND EXPERIMENTAL PROCEDURES

The chemical composition of A1 7049 alloy is shown in Table 1. This material is essentially
similar to the 7075 alloy, but is more susceptible to corrosion fatigue due to a higher Zn
content (7.1% instead of 5.1-6.1%). The specimens were obtained from a cold rolled sheet of
a 7049 aluminium alloy with 10 mm thickness.
The 10 mm thick sheets were underaged (UA) and overaged (OA) such that approximately
the same yield strengths resulted. The overaged heat treatment was the standard temper
treatment (T7351), and the UA treatment was carefully done to match the OA yield strength.
The heat treatments are shown in Table 2.

Table 1. Composition (wt pct) of 7049 aluminium alloy plate material

Zn Mg Cu Cr Fe Si Mn Ti Ga Zr A1
7.1 2.8 1.7 0.06 0.3 0.1 0.06 0.05 0.01 0.1 Bal.

Table 2. Heat treatment properties of the AI 7049 alloy

Solution heat treatment 470W 45mid water quench
Temper overage (OA) underage (UA)
liquid nitrogedl 5 min
T735 1 : 107 "C/8 hours +SO "C/lOmin
+ 163 "C I65 hours + 117 'CY90 min

Table 3. Room temperature mechanical properties of 7049 aluminium alloy
alloy/ yield strength UTS elongation area kc
temper (MPa) Rp 0..2 (MPa) % reduction (MPaJm)
7049-UA 445 578 17.2 I9 % ---
7049-OA 44 1 497 8.8 23 % 32.0

The mechanical properties of the two materials are listed in Table 3. Their yield strengths
are identical (440 MPa); the tensile strength of the UA material is 16% higher and its
ductility is 100% higher. The underaged material contains extremely fine GP zones and q'
intermetallic precipitates, whereas the overaged structure contains predominantly coarse q as
well as q' precipitates. The 7049-UA and 7049-OA are two materials exhibiting the same
crystallographic texture and grain morphology, but differing in precipitate microstructure.
Any difference in the mechanical behaviour of 7049-UA and 7049-OA can then be attributed
to these different precipitate microstructures. The UA alloy specimens were stored at -20 "C
prior to fatigue testing to prevent further room temperature aging.
After the heat treatments, 62.5~60~10 mm compact tension (CT) specimens were
machined, and notches were introduced parallel to the longitudinal direction so that crack
propagation took place in the rolling direction.