Determination of aircraft hydraulic system pipeline leak cause

. The purpose of this article is to determine the cause of leak of the aircraft hydraulic system pipeline detected during pre-flight flight preparation. Results of spectral analysis of material, mechanical testing, analysis of defect structure by optical fractography and metallographic analysis are presented.


Introduction
Despite the successes in the development of various technologies, the destruction of machine parts, accompanied by material losses and even human lives, is still happening. The accumulation of experience in determining the nature of damage and the causes of its occurrence subsequently contribute to the improvement of the manufacturing processes of parts and control methods, which in turn make it possible to prevent subsequent destruction of this type [1].
Elements of aircraft hydraulic system are subjected to considerable vibration loads during operation, as well as sharp pressure and temperature differences, as a result of which there is a high susceptibility of elements to the formation of various types of defects that lead to malfunctions and failures of the hydraulic system [2][3].
On a research the pipeline from steel 12X18H10T with thickness of a wall of 1.2 mm, for the purpose of definition of the reason of the leak found when carrying out preflight preparation of the aircraft arrived.
In order to achieve this goal, it is necessary to carry out: Material Characteristics Analysis; spectral analysis and mechanical testing to establish compliance with material standard requirement; analysis of defect structure by optical fractography method; metallographic analysis of material.

Material and methods of research
The material grade was determined using an optical emission spectrometer Q4 TASMAN.
Tensile strength is determined by stretching two longitudinal samples in the form of strips cut along the pipe axis (working part width 12.0 mm) on a test machine LFM-100 in accordance with the requirements of GOST 10006-80. The test machine automatically draws a stretch diagram showing the functional relationship between stress and strain during static stretching of the sample before it breaks.
Fractional analysis was performed using a Stemi 2000С stereomicroscope. The photography was done with Nexsys Image Expert.
Preparation of microslips for metallographic studies was carried out according to known techniques [4] on a polishing machine of the PHOENIX model. The metalgraphic research was conducted on the inverted Axio Vert.A1 microscope. Photographing is done using Thixomet PRO.
Cutting of samples of pipeline for examination is performed on abrasive-cutting machine of model AbrasiMet 250 using cooling emulsion, in order to prevent steel overheating.

Results and discussion of the study
Pipeline material -steel 12Х18Н10Т GOST 19277-73. Stainless steel 12Х18Н10Т belongs to the class of chromium-nickel austenitic steel not hardened by thermal treatment, which has a complex of valuable properties (high ductility, increased corrosion resistance, good processability), which in turn caused its use as a structural material subjected to the combined action of stresses, high temperatures and aggressive media [5][6].
In accordance with the requirements of GOST 19277-73, the chemical composition of steel should comply with the requirements of GOST 5632-72, the mass fraction of titanium should not exceed (С-0,02) 5% -0.7%. As a result of chemical analysis, it was found that the material corresponds to steel grade 12Х18Н10Т in terms of elemental composition. The chemical composition of steel obtained as a result of the analysis and the requirements of GOST 5632-72 are presented in Table 1. During visual-optical inspection of the external surface of the pipeline, at the place of leakage, no defects were found. After opening the pipeline along the generatrix, a defect in the form of a locally located cavity in the metal, accompanied by a dark-colored section, extended further along the generatrix, was visually detected in order to obtain a sufficient view for inspection of the inner surface. Length of defects ~ 20 mm, width ~ 8 mm. The appearance of the defect is shown in Fig.1a.
To analyze the structure of pipeline defects, the internal surface was examined by optical fractography. As a result of the pipeline study, it was established: the discovered cavity is directed mainly along the generatrix, spreads deep into the metal and has a "porous, spongy" structure; the dark area of the surface around the cavity, found during visual inspection, is pitting lesions with a diameter of up to 1 mm. The photo of defects obtained at magnification of 20x is shown in Fig. 1b The presence of damages with a porous, spongy structure is characteristic of cavitation wear that occurs when solids contact liquid. Cavitation is the process of formation of vaporgas bubbles in liquid media, followed by their collapse and the release of a large amount of energy, which is accompanied by hydraulic shocks. The impact of many such microshocks leads to the gradual destruction of nearby solids, the formation of rather deep and complex lesions in the relief. After the appearance of the initial surface damage, the cavitation process is accelerated [7].
Thus, in appearance, the defects are ulcerative corrosion lesions and traces of cavitation wear.
To clarify the nature of the defect, a metallographic study was carried out in transverse microslips. As a result, there are non-metallic inclusions near the defect. Assessment of metal contamination by non-metallic inclusions according to GOST 1778-70 is not possible due to insufficient pipe wall thickness (1.2 mm) under condition of minimum thickness of 6 mm. Also, traces of plastic deformation in the form of enveloped recesses of deformed austenite grains were found in the microslip of the examined section (Fig. 2), which indicates the presence of mechanical influence.