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QUANTITATIVE FRACTOGRAPHIC ANALYSES OF FATIGUE CRACK GROWTH IN LONGERONS OF IN‐SERVICE HELICOPTER ROTOR‐BLADES
Author(s) -
Shaniavski A. A.
Publication year - 1996
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1996.tb01047.x
Subject(s) - striation , rotor (electric) , materials science , fractography , ultimate tensile strength , helicopter rotor , rotation (mathematics) , tension (geology) , stress (linguistics) , paris' law , structural engineering , alloy , composite material , fracture mechanics , geometry , mathematics , crack closure , mechanical engineering , engineering , linguistics , philosophy
— An analysis is made of striation spacing measurements as observed on fatigue crack surfaces of specimens and longerons of helicopter rotor‐blades made from the aluminium alloy AVT. It is shown that, for the combined cyclic tensile stresses at various R ‐ratios and static rotation angles, α, striation spacings were controlled by an equivalent intensity factor K e = K 1 F (α, R ). Values of the F (α) for R i = const were computed. The crack growth life in terms of the number of flights and stress equivalents were computed for various longeron sections of the helicopter rotor‐blades which had failed in service. SUMMARY AND CONCLUSIONS (1) A fatigue striation spacing analysis was made from measurements on AVT Al‐alloy specimens tested in air under combined cyclic tension and static‐rotation loads at various a‐angles of rotation, and stress ratios, R. Fractographic analyses have shown similarities in terms of striation spacing formation in the investigated range of α‐angles and R ratios. (2) The similarity in the fatigue fracture mechanisms attested to the possibility of characterizing the striation spacing formation by a unified kinetic diagram via a K e = K 1 F (α, R ) approach for different α‐angles and R ratios. The function F (α, R ) was determined and it can be used to calculate the tensile stress equivalent according to values of σ i measurements taken from components broken in tests or in service. (3) It was a longeron of the rotor‐blade of helicopters manufactured from AVT Al‐alloy broken in service and tested cyclically that were investigated fractographically. An analysis of the formation of striations and their spacing has shown similarities with the investigated longerons fatigued in tests and in service. The crack growth life versus number of flights and stress equivalent was calculated according to values of the σ i measurements. (4) It was shown that a fatigue crack will not nucleate in longerons in service if the depth of defects on the external surface have a size less than 0.3 mm. The stress for tested longerons is near to the calculated equivalent for longerons that failed in service.