Indirect (Statistical) Crack Growth Estimation Method in Stringers from the 01420 Material of the Mi-26(T) Helicopter

Aeronautical and Space-Rocket Engineering


Аuthors

Osipov D. N.*, Popov A. V.**

Federal Unitary Enterprise State Scientific Research Institute of Civil Aviation, Moscow, Russia

*e-mail: osipovdn@gosniiga.ru
**e-mail: 010775@mail.ru

Abstract

The Mi-26(T) heavy transport helicopter was released with a stated assigned resource of 12,000 hours. However, the assigned resource confirmed by tests thus far is 4,200 hours with the possibility of a stage-by-stage increase to 4,800 – 6,000 hours for helicopter samples, depending on the year of manufacture and technical condition. From the very beginning of the Mi-26(T) helicopter operation, a huge number of cracks in stringers (up to 1000 pieces per sample and about 10,000 per fleet) made of 01420 aluminum-lithium alloy have been detected. Since 1992, this alloy has not been applied, but more than 80% of the Mi-26T helicopter fleet of the civil aviation of the Russian Federation consists of helicopters produced in 1987-1992, and their airworthiness maintaining is an urgent need.
As of today, there is a vast number of mathematical models describing the fatigue cracks growth in various aviation materials. They may be both general, concerning a certain material, and accounting for the specifics of a particular structure, such as its geometry, operating conditions, etc. Fractography allows as well describing reliably the behavior of a crack in the section under study. However, as the experience of Mi-26(T) helicopters operating and fracture studies revealed, the 01420 material has extreme instability of fatigue resistance properties and a tendency to cracking. There is no clear dependence herewith of the number of cracks on the intensity of operation (cracks of stringers occur among other cases during prolonged demurrage of the sample), nor on the loading of the structure as a whole, nor on the climatic conditions.
Such a massive nature of the stringers cracks formation and growth process, if the crack occurrence is considered being an event from the point of view of probability theory, allows applying statistical methods to its description. GosNII GA, together with NCV Miles and Kamov, developed and implemented a methodology for assigning inspection intervals for a helicopter stringer set, based on a statistical analysis of the crack formation process at a certain stage of operation. The proposed as well a technique for evaluating the equivalence of operational loads reproducing during bench tests of the fuselage, based on the Kolmogorov criterion application to verify the hypotheses plausibility.
The article proposes a statistical method for estimating the rate of the crack growth in stringers made of 01420 material. The method is applicable for relatively small and stable helicopter operation intervals of 100-400 hours and is based on assumptions about the cracking process compliance with the conditions of the Poisson process, i.e. stationary state, a process without consequences and ordinariness. The proposed method supposes measuring lengths of the detected cracks of stringers. Based on the performed measurements, the initial detectable crack length and the critical length, after which the crack develops rapidly until the stringer profile is completely destroyed, are being determined. On the assumption of the sequential cracks formation, the rate of their propagation at the stage of stable growth is being estimated.
Application of the method is illustrated by the example of the Mi-26(T) RA-06018 helicopter currently in operation. The article demonstrates the obtained results correspondence to the previously performed direct measurements of the crack growth rate in operation and during bench tests of the “skin–stringer” elements.
The method can serve as an addition to the accepted techniques for determining the necessary intervals for the fuselage structure inspection at the next stage stage of confirming the resource and service life of a helicopter sample. It may be employed as well to assess the growth rate of fatigue cracks in other materials and for other elements of aircraft structures with defects comparable in mass to the cracks of stringers made of the 01420 material.
The proposed method does not require application of special equipment and high time costs.

Keywords:

critical fuselage zone, a crack in the stringer, fatigue crack, crack growth rate, fracture, Poisson process, crack length distribution, test sample, stable crack growth period

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