· · · Aerospace Engineering reports

Full-scale fatigue tests were previously carried out on wing tension skins under random flight loading, program loading and constant-amplitude loading. Tension skins left from these tests were partly stripped. Tensile test revealed fatigue crack nuclei at various locations. Results have some meaning with respect to the frequency of occurrence of cracks that had not been detected previously, the possibility of indicating part-through cracks by X-raying, the influence of a tensile stress on the sensitivity of X-ray inspections and the occurrence of certain cracks as dependent of the maximum load in a full-scale fatigue test.

Constant-amplitude tests and simplified flight-simulation tests with 10 or 50 gust cycles per flight were performed. The effect of the ground-to-air cycle is compared with the Palmgren-Miner prediction. Optical and electron microscope observations of the fracture are presented.

Sheet specimens of 2024-T3 Alclad and 7075-T6 Clad were tested in a test hall and outside this hall. Each specimen contained five cracks. The crack growth data allow comparisons to be made (I) between indoors and outdoors (2) between random and program loading (3) between these loadings with and without ground-to-air cycles and (4) between the two alloys. Damage calculations are made.

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.

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 concepts of reliability, sensitivity and accuracy of non-destructive inspections are defined. Next is an explanation of the limitations of repetitive use of NDT-rellabi1ity specimens followed by a comparison between the inspection results of fluorescent and red penetrants. Subsequently the paper deals with reliability, sensitivity and accuracy of a number of inspection methods in frequent use. They are applied to one hundred aluminium specimens fabricated purposely by the NLR, as well as to more than 220 rejected steel components of real aircraft undercarriages. It will be shown that there exists a fair amount of scatter between the results of different techniques and inspectors and also between subsequent inspections by one inspector. It is obvious that corrosion has a significant influence on all three aspects of NDT mentioned in the title. The results of the ultrasonic inspections turn out to be rather disappointing, whereas the results of penetrant inspections tend to depend rather heavily on the specimen configuration. Fluorescent penetrant performs far better than red penetrant, Eddy current gives good moderate results. On the steel components fluorescent magnetic ink as well as magnetic rubber achieve by far the best results, at the penalty of a fair amount of spurious indications.

Fatigue tests were carried out on full-scale tension skins of a wing center section of 7075-T6 material. Variable-amplitude tests were carried out with the following load sequences: (a) random load, (b) program load, (c) random load with GTAC (ground-to-air cycles) and (d) program load with GTAC. Constant-amplitude tests were carried out with GTAC and gust amplitudes. The main objectives were to investigate (1) the equivalence of random and program loading, (2) the damaging effect of GTAC and (3) recommendations for full-scale testing. Results obtained are related to the indication of fatigue critical components, fatigue lives, crack propagation, residual strength, En/N-values, scatter and inspection methods. Relevant information of the literature is summarized. Recommendations for full scale testing are concerned with the indication of the loads to be incorporated into the test, the assessment of load spectra, the highest loads to be applied, the smallest load fluctuations to be included, the load sequence to be adopted, the duration of the test, and experimental conditions.