Abstract
Measurement of fatigue crack growth (FCG) in an Al 7075-T7351 alloy of a particularly high homogeneity was carried out as a part of the Proficiency Test Programme on fatigue crack growth measurement, organised by General Electric Aviation, USA, in collaboration with the company Exova in France. To pass the test programme successfully, a particular attention was paid to improve and verify DCPD (direct current potential drop) method being used for the crack length measurement. Additionally, an investigation of effects of overloads on fatigue crack growth and retardation was carried out. In the CT specimens used, the difference between actual and calculated crack length at the crack starting point was quite significant, approximately 4.3 mm. However, the difference as a function of crack length was almost constant. Results of FCG rates were very self consistent, likely due to the material homogeneity. Values of the parameters C and m measured at the SVĂM a.s. laboratory were evaluated as the third best precise from all the 16 worldwide participating laboratories. Retardation effects of overloads of the magnitudes 2.7-times and 3.0-times of the maximum load in the constant range fatigue loading were significant. The overloads resulted in substantial crack closure effects, which, however, did not occur immediately, but just after further fatigue crack extension, comparable with plastic zone size.
Highlights
Al 7075 alloy is a high strength material usually used for highly stressed components in lightweight structures, typically in aircraft fittings, gears and shafts, fuse parts, meter shafts and gears, missile parts, regulating valve parts, worm gears, keys, aircraft, aerospace and defence applications [1]
The test programme successfully, a particular attention was paid to improve and verify DCPD method being used for the crack length measurement
The most important point is some retardation of the crack closure reaction on the overloading
Summary
Al 7075 alloy is a high strength material usually used for highly stressed components in lightweight structures, typically in aircraft fittings, gears and shafts, fuse parts, meter shafts and gears, missile parts, regulating valve parts, worm gears, keys, aircraft, aerospace and defence applications [1] It can be applied in different heat treatment conditions, but the T7351 temper state is most widely used because of improved stress-corrosion cracking resistance. A rather problematic property of Al 7075 alloy may be fracture toughness, which vary from 22 MPa m1/2 to more than 30 MPa m1/2 according to the temper state and test orientation This may be a problem in structures using Damage Tolerance design philosophy having been recently more and more used in connection with the necessity to reduce total weight of structures and to exploit their potential service life to the maximum extent in such industrial branches like aircraft [2], and gradually in other fields, e.g. railway structures and components [3,4,5]. Results of this comprehensive programme are described and discussed in this article
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