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The Environment EfJect on Fatigue Crack Growth Rates in 7049 Aluminium Alloy at ...   315

           Sadananda [40,48]. The UA alloy shows an L-shape at positive R-ratios too, whereas a loss of
           fatigue crack propagation resistance may be recognised at negative R-ratios again. AK reaches
           a plateau for R<O  and Kmx therefore decreases with decreasing R to comply with constant AK
           required for crack growth. This means that the controlling mechanism switches from a K,,
           controlled behaviour for positive R-ratios to a AK controlled for negative R. This implies that
           reversed cyclic plasticity may become the governing factor for fatigue crack growth.
              Observing the experimental results, the role of environment and microstructure on  the
           threshold Mth-KmX curve for the  same 7049 alloy and  summarising the  contributions of
           several  investigators  [3,19,20,22,25,46,47],  it  is  clear that  the  introduction  of  moist  air
           environment reduces strongly the hK*th values of the UA and OA alloys. The wavy slip mode
           in the OA alloy probably is the reason for the reduced Math in comparison to the UA alloy in
           moist air  and  in  vacuum. The two microstructures likewise show different  fatigue crack
           growth behaviour in moist air, with higher thresholds of the UA structure than those of the
           OA alloy.
              Figures 8 (a) shows the better fatigue crack growth properties of both alloys in vacuum
           than in humid air which has been found in a similar extend by Kirby and Beevers [25,36]. The
           data of Kirby and  Beevers, resulting in  a AKth-Kmax curve with a slope of  1, point to  the
           prevailing influence of fatigue loading and microstructure. The different results of the present
           study probably are caused by  two  facts.  First, the studied alloy (AI  7049 alloy instead of
           7075) is more susceptible to corrosive influences, and  second, the vacuum  was not  a high
           vacuum (-2.6~10” Pa).
              Figures 9 (a) to (0 show the fracture surfaces typical for fatigue loading of the UA-OA
           alloy in air and vacuum at R=-1. The UA alloy shows planar slip in air in contrast to the rather
           ductile fracture surface in vacuum (Figs 9 (e) and (d)). In ambient air both the UA and OA
           alloy (Figs 9 (a) and  (c))  shows a brittle crystallographic fracture mode. As  an  additional
           example for the influence of the environment, Fig. 9 (d) shows that fatigue loading of the UA
           alloy in vacuum leads to a rather ductile fracture surface, whereas humid air causes some
           embritteling, as visible in Fig. 9 (c) and (e): The main influence seems to come from the load
           ratio, showing extensive crystallographic brittle fracture features at R  = -1 in air.  The UA
           alloy shows in addition crack branching, and the crack advance profile is zig-zag like. The
           main differences in FCGR behaviour of the OA and UA  microstructure indeed arise from
           different slip deformation behaviour: homogeneous and  wavy  slip in  the OA  alloy (more
           brittle in ambient air than in vacuum, probably induced by hydrogen) and localised planar slip
           in the UA microstructure.
              The present results for 7049 aluminium alloy tested in ambient air show a distinct trend of
           lower threshold hKth values and higher near threshold growth rates with  increasing aging
           treatment. These features can be  rationalised in  terms  of  several competing mechanistic
           processes: intrinsic and microstructural effects and microstructure environment interactions.
              In the absence of any environment effect, in vacuum, the crack propagation mechanism is
           governed only by  microstructural factors whose action in turn is governed by the loading
           conditions [54,55].  Crack propagation is intergranular, controlled by  slip in one or  many
           active planes. In the crack growth range where the Paris law is valid, Le., in stage 11, the crack
           tip loading conditions permit at least two slip systems to be active which in turn leads to a
           plane crack growth path affected only by the presence of large inter-metallic precipitates [14].
              The chemisorption phenomenon describes the formation of hydrogen by the dissociation
           of the absorbed water molecules. In such a case a hydrogen embrittlement mechanism can be
           brought into action [26,50,53]. Accordingly the thresholds uti, for both aging conditions are
           higher in vacuum than in humid air at all load ratios. Moreover, the differences between the
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