Crack Paths 2009

driving force in a unique mathematical relationship or equation since the stress intensity

factor range

K∆ should be an increasing function of crack length under a C A loading

independent of . In some cases, short cracks from notches also grow at the levels t K

below the threshold values determined by conventional L E F Mtests [10].

In general, depending on notch geometry, material and loading conditions, it is

believed that there are mainly two mechanisms dominating the growth behavior of

fatigue cracks from notches: (a) notch plasticity and (b) material microstructures [6].

Therefore, several fracture mechanics based approaches have been developed to

consider the notch plasticity effect. For example, by modifying stress intensity factor as

a function of not only applied stress and crack length but also notch stress concentration

factor and notch tip radius, Kujawski [11] successfully correlated F C G data from

notches experimentally obtained by Smith et al. [4]. However, more recently, for C A

loading, Ding et al. [12] found that the effect of the notch plasticity on F C Ghappened

only when . It was argued that when the R-ratio was negative, the contact of the R > 0

crack surfaces during a part of an unloading cycle reduced the cyclic plasticity of the

material near the crack tip. The combined effect of notch plasticity and possible contact

of the crack’s surface were claimed responsible for the observed crack growth

phenomenonnear a notch.

In addition to the modifiedK∆ approach, there are several other elastic-plastic

fracture mechanics (EPFM) approaches such as modified J-integral [13], crack tip

opening displacement (CTOD)[14] and cyclic notch plasticity [15] used to correlate the

experimental F C Gdata. To interpret the experimental notch F C Gdata, however, many

studies [7, 8, 16] indicated difficulties experienced by using these approaches.

Furthermore, it is very difficult to determine J-integral and C T O Dexperimentally and

numerically when the crack length is small.

F A T I G U EC R A C KG R O W TUHN D E RS P E C T R ULMO A D I N GF O R

D I F F E R E NST RESSC O N C E N T R A T I O N S

Fatigue crack growth tests of notched specimens under spectrum loading conditions

were conducted at DSTO.Full details of the test set-ups and results are given elsewhere

[17, 18]. The following sections contain only some highlights relevant to this study.

Geometries of Notched Specimens

Three groups of specimens with different net-section stress concentration factors ( ) t n K

were manufactured as shown in Figs 3-5. The net-section stress concentration factors

were calculated using finite element method (FEM).

Material Properties of 7050-T7451

All specimens were made from thick plate 7050-T7451 aluminium alloy. The general

mechanical properties of 7050-T7451 are listed in Table 1 [19].

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