Crack Growth Essay

1.4 Literature Review

Fatigue crack growth life prediction is a hot topic of current research. Crack growth curve under service loading is one of the two inputs required for damage tolerance design and evaluation of aircraft structures.

A variety of factors affect the fatigue crack growth. Among the many factors that affect crack propagation, the following should be taken into consideration for crack growth prediction.

i) Thickness ii) Type of product iii) Heat treatment iv) Cold deformation

v) Temperature vi) Manufacturer vii) Batch to batch variation viii) Environment and frequency.

For the factors lower in the list, it is less likely that they can be properly accounted for. Most of these effects cannot be accounted properly in the analysis

1.4.2 DSTO experiments on FCG

Since 1970, Defence Science and Technology Organization (DSTO) in Melbourne have conducted many full aircraft fatigue tests under various fighter spectra. The full scale fatigue test of F/A-18 aircraft aft fuselage and empennage assembly are reported in [9 & 10]. Similar data on F-111 wing structure is in [11 & 12].

A highly reliable QF technique developed in DSTO has made it possible to collect full scale fatigue flight-by-flight crack growth data for a number of load spectra applied to several airframe materials. Analysis of such flight-by-flight crack growth data indicates that the log (crack depth) is linearly related to the number of flights [13 & 14]. The slope of this straight line appears to depend on the peak load level of the spectrum. The slope increase with increase in the peak load level [15]. Such relationship can be expressed as

ln(a)= ln(ai)+ λ B Where a = crack length at number ‘B’ of blocks of flight-by-flight loading. ai = initial crack