Page 378 - Biaxial Multiaxial Fatigue and Fracture

362 M. FONTE ET AL.

toughness is clearly seen in commercial alloys when inadequate heat treatments are applied
[12-141. The microstructure/aging condition is known to have a significant influence. The
underaged (UA) microstructure has the maximum susceptibility and the overaged (OA)
microstructure a susceptibility, which is decreasing with aging. The heat treatment clearly
influences many metallurgical parameters and since the 1960s it was hypothesised that
dislocation-precipitate interactions play an important role during stress corrosion cracking of
the AI-Zn-Mg-Cu alloys 1151. Stress corrosion cracking of aluminium alloys is a complex
phenomenon involving time-dependent interactions between alloy microstructure, mechanical
deformation, and local environment conditions [ 16,171. Environment effects are time-
dependent and K,, is recognised as the characterising parameter.
The mechanical behaviour of materials depends strongly on its microstructure and
environment effect [ 18-24]. It is well-known that an aluminium alloy exhibits very different
properties depending on whether it is cold rolled or heat treated under different temper
conditions. Although a combination of the local microstructural features and the applied stress
intensity range (AK) primarily governs the slip characteristics and the growth mechanisms,
the resulting cyclic crack advance can be substantially changed by the presence of an
environment. Kirby and Beevers [25], for example, demonstrated that even the seemingly
innocuous environment of laboratory air can lead to a marked increase of crack propagation
rates in the near-threshold fatigue regime of 7XXX series aluminium alloys, if compared to
vacuum.
Lin and Starke [26] showed that microstructure-environment interactions at low stress
intensities could be completely different from those at higher growth rate levels. It has been
recognised [27] that environment effects on slow fatigue crack growth in high-strength
aluminium alloys strongly depend on alloy composition, heat treatment, moisture content of
the surrounding air and the presence of certain embrittling species.
The aim of this study is to examine the mechanisms governing the fatigue behaviour of
commercial AI 7049 alloy under controlled microstructural and environment conditions,
specifically involving an underaged (VA) and overaged (OA) alloy, having the same chemical
composition, crystallographic texture and yield stress, but different precipitate features. The
experimental work was designed to obtain the fatigue crack growth thresholds. Then, several
mechanical tests were performed on both material conditions and both macroscopic and
microscopic responses are compared. Near-threshold fatigue behaviour in room temperature
environments is contrasted with that for vacuum for a range of load ratio values.
Micromechanisms of fatigue crack growth are discussed in terms of the specific role of
several concurrent processes involving crack closure, environmentally assisted crack growth,
and intrinsic microstructural effects. Results are discussed on the basis of the main
deformation mechanisms and microstructure, the embrittling influence of environment
(ambient air and vacuum) and the two intrinsic parameters of crack growth: A&,, Kmx.

ENVIRONMENT AND MICROSTRUCTURE INTERACTIONS

Crack closure efects

The near-threshold fatigue properties quite often are discussed in light of crack closure
mechanisms. The concept of plasticity-induced closure was introduced by Elber [28], to
explain decreasing crack growth rates with increasing crack length as a result of plastic
deformation at the crack tip. Later, roughness of fatigue surfaces, oxides, etc. have been
identified as additional reasons for reduced crack growth rates [29-321. The load ratio
dependence was considered not as an intrinsic material property, but arising from changes in