· · · Aerospace Engineering reports

Some recent developments of the masts and standing rigging of racing-yachts and, on the other hand, the failures of a number of masts and rudderstocks led to the application of aeronautical engineering. Load spectra of the standing rigging of a 10.00 m racing yacht are given and the application of them in a lug. Some information is given on stress-raisers and on materials. Finally, some test-applications are mentioned.

The propagation has been studied of fatigue cracks in 2024-T3 alclad sheet under fluctuating tension in different atmospheres. Of the principal constituents of air water vapour had a much more severe effect than oxygen; in wet -pure argon the endurance was much shorter than in dry pure oxygen. In air, other circumstances remaining unaltered, the endurance increased continuously at a reduction of the water vapour content; the endurance, at a water content of the air of 20 ppm being about 6 times as long as in air saturated with water. Between the limits of 30°/° and 100 /o relative humidity, as occurring in normal air, the endurance was only slightly affected. The thermodynamics and the mechanisms of the reactions of oxygen and water vapouir with the metal are discussed.

Constant-amplitude tests and programme tests were carried out on 2024-T3 lug specimens with standard holes, slotted holes and expanded holes. The results give information on (1) programme loading versus constant-amplitude loading, (2) cumulative damage and (3) the improvement of the fatigue endurance obtained by the application of slotted and of expanded holes.

Static tests were carried out on 280 mildly notched specimens of various aluminium alloys. It appears that the nett section fracture stress as a percentage of the ultimate tensile strength is in general higher for materials of lower ductility. This influence of ductility can be reasonably explained on the basis of measured plastic strain distributions. The static strength of aluminium alloys is lower when the elastic stress concention factor is higher.

The size effect on crack propagation was studied with sheet specimens of 2024-T3 Alclad. Values of the width were 80, 160, 300 and 600 mm, sheet thickness 3 mm, mean stress 8 kg/mm2 and stress amplitudes 2.5, 4 and 6.5 kg/mm2. Results are compared with the theory of the stress intensity factor and the Weibull theory. Practical implications of the results are indicated

Fatigue tests have been carried out on light alloy lugs of 2024 and 7075 material in order to investigate the effects of some geometrical parameters (lug size, geometry of lug) and various means to reduce fretting, viz. lubrication of the pin, flattened pins, slotted holes and expanded holes. Complete S-N curves have been determined at a positive mean stress. A discussion of the results includes a comparison with data from the literature. Special attention is focussed on fretting corrosion in the hole of the lug.

Several single lap Joints in clad 2024-T3 and 7075-T6 alitainium alloy with two rows of rivets or Huclcbolts have been evaluated in respect of their fatigue strength properties, both in tests with constant load amplitude and in tests with a programmed load sequence based on a gust spectrum. The results showed that: 1) The replacement of aluminium alloy snap rivets type AD or DD by Avdel blind rivets of the same diameter decreased the endurance in the programme tests approximately 4 times. 2) The Palmgren-Miner rule in general gave a concervative estimate of the endurance in the programme tests. 3) The grading orders of the different types of riveted joints as determined from the results of the constant load amplitude tests and the programme tests showed moderate agreement.

The investigation on the propagation of fatigue cracks in aluminium alloy sheet under fluctuating tension comprised four parts. (1) Tests to determine the combined effects of the humidity of the air and the lead frequency (clad 2024-T3 and 7075-T6). (2) Tests to determine the effect of water. (3) Tests to determine the effect of wetting with a water displacing oil. (4) Tests to determine the effect of the humidity of the air in two step tests. The latter three types of tests were carried out at a high frequency of loading and were performed on 2024-T3 Alclad only. The effect of the environment on fatigue crack propagation is analysed.

Fatigue tests on 2024-T4 alclad double row riveted single lap joints at fluctuating tension with a mean stress in the nett section of 9.0 kg/mm2 and stress amplitudes ranging from 8.6 to 3.0 kg/mm^ did not show any significant difference in endurance of similar specimens with a size ratio li2,66.

The following text is given in the format required for presentation at the Conference and Exhibition "Fatigue Testing and Design", to be held in the City University, London, 5-9 April 1976. The report gives a comparison between the fatigue performances of several sheet and forging materials under gust spectrum loading representing the lower wing surfaces of commercial transport aircraft. The materials are 2024-T3 and 7075-T6 aluminum alloy sheet, annealed Ti-6A1-4V sheet, and 705O and AZ 74»61 aluminum alloy forgings. The significance of the results for the design of panel concepts and forged fittings is discussed, and proposals for further testing are made.