Crack Paths 2009

Figure 1. Schematic illustration of all load sequences used in testing program

The load sequence labelled as C Aconsisted of constant amplitude (CA) cycles with a

maximumload that produced a nominal net section stress of 200MPaand a stress

ratio (R=Rmin/Rmax) equal to 0.5.

The load sequences labelled as S1 and S2 consisted of single fully reversed

underloads separated by varying number of C A cycles (10, 20, 30, 50, 80 and 130).

Fully reversed compressive loads with stress ratio R=-1.0 were applied in sequence

S1 while reversed underloads with a stress ratio R=0 were applied in sequence S2

(low R cycles). Sequence S3 and S4 consisted of grouped underloads, increasing

subsequently in amount from 1 to 5, separated by 100 C A cycles. A similar logic was

adopted for these sequences as for S1 and S2: S3 contains fully reversed compressive

loads with stress ratio R=-1.0, while S4 contains reversed underloads with a stress

ratio R=0. The load sequence S5 consisted of single fully reversed underloads

progressively increasing in magnitude (stress ratio R: 0.45, 0.25, 0, -0.25, -0.5, -0.75

and -1.0) separated by 100 C A cycles. This testing sequence was modified by

selecting either only positive underloads, as in sequence 5B (stress ratio R=0.45, 0.25

and 0) or by selecting only negative underloads as in sequence 5C (stress ratio R=

0.25, -0.5, -0.75 and -1.0) separated by C Acycles like in sequence 5.

Test specimen

Flat, dogbone shaped specimens were machined from AA2024-T3sheets with a

length of 177mm, a thickness of 3.2 m mand a width of 40mm.The reduction in

width was introduced by a radius of 100mmmachined on both sides (to give net

width of 25mm). Additionally, the U-shaped notch of 0.1mmwide and 0.25mmdeep

with semicircular tip was machined at one of the sample’s side (using electro

discharge machining), tom propagate a crack in LT direction. This specimen

geometry gives an initial stress concentration (Kt) equal to 5.54. Figure 2 provides all

details on specimen’s geometry as used in test programme.

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