In above , L means longitudinal (sheet production or rolling direction) and LT or T means the transversal direction. For the two letters T-L or L-T assigned to the individual material properties, the first letter is indicating the loading direction of the samples, and the second letter indicates the crack growth direction. If a is the crack length in mm, and N the number of dynamic load cycles, a crack of a sheet which is dynamically loaded with a stress Δσ will grow as a function of N. The test samples to examine crack growth behaviour are typically sized 400 mm in width and 700 mm in length.
To compare the crack growth behaviour of different alloys, the derivative of the crack growth rate da/dN is plotted versus ΔK, the stress intensity factor. This factor can be calculated according to below Eq.
where Δσ is the applied stress delta, and a is the crack length. The comparison between the materials is then drawn by evaluating the stress intensity factor at a given crack growth rate. In case of the “Fatigue Crack Growth T-L” value shown in above Fig. , the crack growth rate was set to 5X10X3 mm/cycle. The stress intensity factor at this growth rate of Aluminium-Lithium and Aluminium-Scandium alloys is higher than that of the reference material 2524; this means that higher cyclic stress Δσ would have to be applied in order to reach the same crack growth rate as for the conventional aluminium alloy This is a clear benefit for fatigue crack growth sized parts, as for a given cyclic loading the wall thicknesses of Aluminium-Lithium and Aluminium Scandium parts can be reduced compared to 2524 parts.