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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.

Light alloy specimens consisting of a 2 mm 2024-T load sheet reinforced with doubler plates of 1 mm were loaded in fluctuating tension. Redux and Metlbond 4021 were used as adhesives with glueline thicknesses of about O.05 and 0,5 mm. The Metlbond bonded specimens had a higher endurance than the Redux bonded specimens indicating that in applications like doublers a low rigidity of the adhesive is favourable. The Reduz bonded specimens with a thiok glueline showed porosity and failed by loosening of the doublers. Thick Redux gluelines with gasbubbles have to be avoided.

Single row riveted double cover plate joints in alclad 2024-T aluminium alloy sheet and 2117-T snap rivets were loaded in fluctuating tension with cycle ratio's from 0.25 to 0,79. The specimens were then tested by a static rotation test to determine the effect of the pre-loading in fatigue on the friction between the sheets. The friction had increased by a factor of about three, which value had been attained already at a cycle ratio of 0.25

A comparison has been made between the fatigue strength at fluctuating tension of 2024-T3 alclad double row riveted single lap joints at room temperature in the as fabricated condition and at 150°C after stress-aging at this temperature for 100 hours. The tests showed an appreciable decrease in endurance of the specimens tested at 150°C, particularly at low stress amplitudes. However, both the fretting pattern of the specimens after dismantling and the location of the fatigue cracks indicated that a secondary effect, probably a decrease of the clamping force due to the aging treatment, was at least partially responsible for the decrease in fatigue strength.

A series of Redux-bonded single and double lap-joints with non-porous gluelayers of various glueline thicknesses were tested ultrasonically by means of the Fokker Bond Tester. Tensile shear tests were carried out after completion of the ultrasonic tests. Bond Tester indications were in good agreement with theoretical results , indicating the instrument's ability to detect glueline stiffness. The static strength of double lapjoints turned out to be nearly independent of glueline thickness} for single lapjoints a strong dependence on glueline thickness was found. The analysis of the tensile shear testof adhesive bonded Joints is discussed.

Electron microscopical investigation indicated that in Redux bonding of chromic acid anodized 2024-T alclad mechanical interlocking between the oxide and the adhesive occurred. Pickling in chromic sulfuric acid solutions improved the homogeneity of the aluminium surface and increased the roughness of the metal surface on a molecular scale.

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 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.

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.