ROYAL AIRCRAFT ESTABLISHMENT FARNBOROUGH (UNITED KINGDOM)
As part of a comprehensive evaluation programme, direct stress fatigue tests have been made on three aluminium alloy sheet materials, DTD 5070A, Alclad 2024-T81, and BS L73. Tests were made with two types of fatigue stressing, at room temperature and at 150 C and with a range of stress concentrations. Where comparisons could be made it was found that DTD 5070A had the highest fatigue strength, followed by 2024-T81 and BS L73 in that order. Little difference was found between the fatigue properties in the longitudinal and transverse directions. Soaking unstressed for 1000 hours at 150 C had little effect on room temperature fatigue strength of any of the materials, but soaked DTD 5070A and 2024-T81 tested at 150 C showed a reduction in fatigue strength of about 20 compared with room temperature tests. Considering DTD 5070A, soaking holed test pieces for 1000 hours at 150 C with an applied tension stress of 17000 lbfsq in. resulted in a slight improvement in fatigue strength, but a tension stress of 23000 lbfsq in. resulted in an improvement of about 20 both at room temperature and 150 C. Stress concentrations generally were less detrimental at 150 C than at room temperature.