J. Schijve

Some formulas for the crack opening stress level

Abstract Crack growth data for 2024-T3 sheet material were analysed with different formulas for ΔK eff as a function fo the stress ratio R . The data covered R values from −1.0 to 0.54. A good correlation was obtained for ΔK eff / ΔK = 0.55 + 0.33 R + 0.12 R 2 The relation between log d a/ d n and log ΔK eff was non-linear for high crack rates (> 1 μm/c).

Four lectures on fatigue crack growth

Abstract During the course year 1976–1977 the author presented a series of eight lectures on fatigue crack growth at the Department of Aerospace Engineering of the Delft University of Technology. Four of these lectures in a slighly modified version were presented in August 1977 as part of a Seminar on Fatigue, Fundamental and Applied Aspects, organized by the Lmkoping Institute of Technology (Prof. T. Ericsson). These lectures are reproduced here. Titles and summaries are given below.

The effects of environment and load frequency on the crack propagation law for macro fatigue crack growth in aluminium alloys

Abstract Data are presented for fatigue crack propagation of two aluminium alloys (2024-T3 Alclad-7075-T6 clad) in different environments and for several loading frequencies. Correlation of the data was attempted by the stress-intensity factor by application of the equation for crack growth proposed by Forman. The result was that the influence of the stress ratio on the crack growth was predicted by the equation in agreement with the test results but that the effects of the environment and the load frequency could not be coped with by changes in the constants of the equation. There may be a threshold level of the stress-intensity factor, strongly dependent on the environment and the alloy, in order to make the crack grow.

Fatigue crack growth in the aluminium alloy D16 under constant and variable amplitude loading

Fatigue crack growth experiments were carried out on sheet specimens of the Russian alloy D16Cz (Al-Cu-Mg alloy). Constant-amplitude tests were performed at different stress ratios. Variable-amplitude tests included experiments with a single overload, periodic overloads and underloads, and the flight-simulation load history miniFALSTAFF. Fractographic observations were made to obtain information from striations and the development of shear lips. Results are presented and compared to literature data for 2024-T3. Attention is paid to incompatible crack front orientations and consequences for crack growth prediction models.

Effect of load sequences on crack propagation under random and program loading

Abstract Crack propagation was studied in 2024-T3 Alclad sheet specimens under two types of random loading and under program loading with a very short period (40 cycles) and program loading with a longer period (40,000 cycles). In the program tests Lo-Hi, Lo-Hi-Lo and Hi-Lo sequences were employed. The loads were based on a gust spectrum. The crack rates were about the same under random loading and program loading with the short period. Under program loading with the longer period the crack rates were 2.5 times slower on the average, while a significant sequence effect was observed in these tests. Fractographic observations indicated different cracking mechanisms for the random tests and program tests with a short period on the one hand and the program tests with the longer period on the other hand. Implications for fatigue tests in practice are discussed.

Fatigue of aircraft materials and structures

Abstract The paper starts with a survey of some aircraft fatigue accidents, which have significantly affected ideas about fatigue requirements for aircraft structures and materials. Contributions of fatigue research are focused on the meaning of fatigue under variable-amplitude loading for full-scale fatigue tests, and fatigue of riveted joints in relation to ageing aircraft. The new fibre-metal laminates (ARALL and GLARE) are briefly introduced with reference to structural applications. The paper is completed with a discussion on different design options improve the fatique resistance of aircraft structures.

Fatigue damage accumulation and incompatible crack front orientation

Abstract Crack propagation tests were performed on 2024-T3 and 7075-T6 specimens. Crack growth was observed under variable amplitude loading, during high stress-amplitude cycles, associated with 45°-mode cracking, whereas the crack front orientation was still at an angle of 90°. Similarly it was also studied during low stress-amplitude cycles associated with 90°-mode cracking, whereas the crack front was already at a 45° angle. Crack growth curves, fractographic information and a brief discussion on interaction effects during fatigue damage accumulation are presented.

Fatigue properties of adhesive-bonded laminated sheet material of aluminum alloys

Abstract Comparative fatigue tests were carried out on centrally cracked specimens and lug type specimens, both made from solid sheet and laminated sheet, consisting of five 1 mm sheets of 2024-T3 Alclad material bonded by FM 123/5. Most tests were carried out under constant amplitude loading but growth delays due to peak loads were studied also. Observations are made for through cracks and part through cracks. The significance of the results for application in aircraft structures is analysed.

The effect of pre-strain on fatigue crack growth and crack closure

Abstract A single pre-strain ( ϵ = 0.03 ) applied to 2024-T3 material raised the static yield strength from 428 to 480 MN/m 2 . The growth rate of a fatigue crack in the pre-strained material was about twice as large as in the material that was not pre-strained. A pre-strain followed by 1000 high, but still elastic pre-stress cycles did not further increase the growth rate. It was shown by COD measurements that crack closure occurred to a lesser extent in the pre-strained material as compared to the original material. The significance of the increased yield strength for fatigue crack growth is briefly discussed.

Shear lips on fatigue fractures in aluminium alloy sheet material

Abstract An analysis is made of shear lip width measurements and the transition of tensile mode fatigue cracks to shear mode fatigue cracks, as observed on fatigue crack surfaces of aluminium alloy sheet material. It could be shown that these phenomena were controlled by ΔK eff , rather than K max or ΔK . For crack growth in air the shear lip width was approximately proportional to (ΔK eff ) 2 , but it was significantly larger than the estimated size of the reversed plastic zone. The initiation of shear lips, the transition from plane stress to plane strain along the crack front and the environmental effect on shear lips are briefly considered in the discussion.