· · · Aerospace Engineering reports

Results are presented of microscopical investigation on the initiation and growth of cracks in a ductile material under static load. Crack propagation appears to be primarily a slip mechanism. The consequences of this conclusion for establishing a criterion for ductile fracture are briefly discussed

Residual strength tests were performed on 2024-T3 and 7075-T6 aluminium alloy sheet specimens of 600 mm width. After the cracks had shown some stable slow growth the specimens were unloaded and then reloaded. Unloading appeared to have no effect on the residual strength

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.

This report will become part of an AGARD Survey on the application of fracture mechanics in aircraft design. It consists of two parts: PART A BASIC INFORMATION PART B FATIGUE CRACK PROPAGATION IN STRUCTURES. Part C on residual strength of structures will appear as a separate report. Other parts of the fracture mechanics survey will be written by various authors from different countries.

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.

A brief review is given of the literature dealing with the effect of a stiff ener on the stress distribution in a cracked sheet, Exipting theories for the prediction of the residual static strength of cracked stiffened panels are analyzed. A new approach to this problem is developed, using the residual strength characteristics of the unstiffened panel. Results are given of residual strength tests carried out on panels with two simple strap stiffeners to check the validity of the proposed method.

Residual strength tests with a duration ranging from 30 minutes to about 0.4 seconds were carried out on 2024-T3 alclad sheet specimens containing a central transverse saw cut. It turned out that the stress to initiate slow crack growth decreased at increasing loading rate, but in the range of loading rates tested the residual strength did not depend upon the rate of loading. Some incidental results were obtained on the rate of crack propagation during fracture instability.

The topography of the stretched zone was measured from stereo electron micrographs. The stretched zone reflects crack tip blunting preceding fracture. The depth of the stretched zone of fourteen fracture toughness specimens of some Al-Zn-Mg alloys appeared to correlate reasonably with the critioal crack tip opening displacement. The width of the stretched zone appeared to be in the order of magnitude of the ligament spacing in Kraffts model.

This paper deals with crack propagation under programme-fatigue loading; the programme being derived from a gust spectrum. The validity of the Palmgren-Miner rule for crack propagation has been studied and its usefulness for design purposes is discussed.

Residual strength tests were performed on 2024-T3 and 7075 " T 6 aluminium alloy sheet specimens of 600 mm width. After the cracks had shown some stable slow growth the specimens were unloaded and then reloaded. Unloading appeared t o have no effect on the residual strength

Results are presented on the effect of thickness on the residual strength of aluminium alloy sheet specimens. It could be explained why a state of plane stress at the crack-tip results in a slant fracture and why a state of plane strain results in a square fracture (flat tensile mode). It could also be explained why a plane strain fracture occurs at a lover stress than a plane stress fracture. Explanations are based on the observation that crack growth is a mechanism of void ooalesoense and that void initiation and growth are governed by shear stresses, whereas void ooalesoense is governed by the tensile stress acting on the plane of maximum void concentration

Results are given of residual strength tests on specimens of 2024-T3 and 7075-T6 aluminium alloy sheet material. Three types of specimens were used, which had a width of 150 mm, 300 mm or 600 mm. A fracture criterion is developed for a sheet of finite size. This fracture criterion is in reasonable agreement with the test results.

Specimens of 2024-T3 and 7075-T6 aluminium alloy sheet containing fine saw cuts appear to have the same residual strength as specimens with fatigue cracks of the same length. The stress level at which the fatigue cracks were grown does not affect the residual strength properties. Stop holes drilled at the crack tip postpone the onset of slow crack growth to a higher stress. If slow crack growth proceeds fracture the residual strength is not affected by the acuteness of the initial crack, and there always is slow crack growth if the stress that initiates slow crack growth is not raised to or beyond the residual strength of a specimen with a fatigue crack of equal initial length.

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.