SU1758490A1 - Method of determining material fatigue characteristic - Google Patents

Abstract

The invention relates to the determination of the fatigue characteristics of composite materials used for the manufacture of plates, shells, gas turbine engine blades operating under high-frequency dynamic loading conditions and provides simple information on the fatigue strength of materials with stress-strain on samples of simple geometry. state, close to the implementation in the conditions of operation of full-scale products. The goal is to increase accuracy when testing composite materials by taking into account the anisotropy of their properties. The tests were carried out in three successive stages on rectangular plates of constant thickness. At each stage, samples of different reinforcement structures are tested, sizes and boundary conditions for fixing the samples are changed, thus creating a corresponding stress-strain state in the material, the limited endurance limits are determined at each stage, and the strength criteria components are calculated by their values the material endurance is found in a flat stress state with a different ratio of active stress components (normal and shear). 4 il.

Description

The invention relates to the study of the strength properties of anisotropic materials and can be used in various fields of mechanical engineering, aircraft engine building and turbine construction in determining the endurance of sheet materials in complex types of stress state.

A known method for determining the endurance of anisotropic materials on cylindrical specimens using a two-component apparatus for cyclic testing under complex loading.

The disadvantages of this method are the impossibility of determining the endurance of sheet materials, its low productivity in

connections with low test frequencies (10-30 Hz), as well as the difficulty of making samples.

The closest to the present invention is a method for studying the fatigue of sheet materials on samples in the form of annular plates hinged on an internal contour, in some of which excite oscillations in the second axisymmetric form of bending oscillations. The method is carried out on an installation containing a magnetostrictive exciter.

The disadvantage of this method is the impossibility of studying the endurance of materials at different ratios of normal stresses and shear. The purpose of the invention is to improve the accuracy in testing composite materials by taking into account the anisotropy of their properties.

This goal is achieved by conducting tests in three successive stages: at each stage, the sizes and boundary conditions of the fixation of the samples are changed, exciting the corresponding forms of their own flexural vibrations. Thus, at each stage, a corresponding stress-strain state (VAT) in the sample material is created. Based on the test results, limited (usually on the basis of N 107 loading cycles) fatigue strengths of materials are determined. After that, the components of the strength criterion are found, with the help of which the endurance of materials is determined at shear in the plane of the sheet.

The excitation of vibrations at all stages is carried out with the help of a modulated jet of compressed air on a KuAI-BB type vibration stand. The tests are carried out in the mode of constant relative deformations, which are measured using strain gauges glued to the samples. With the help of them, the frequency of oscillations of the test objects is also measured, the stresses are calculated through the values of these strains Ј according to Hooke's law. The tests are carried out in the mode of constant relative deformations in - const. The criterion for the destruction of samples is a decrease in the resonant frequency of oscillations f by 3-5% relative to the initial value. From Figures 1, 2, and 3, there are diagrams of fixing samples of constant thickness when tested at each stage and the place of gluing of resistance strain gages, where 1 - samples 2 - reinforcement direction, 3 - nodal lines, 4 - - Resistance gages, 5 - hinge support circuit . It also shows the types of VAT arising in the working area of the samples (places of the gages of the strain gauges) and shows normal (7ts, O22 and tangents in the plane of the voltage sheet acting on the material at each test stage, 6 - free edge of the samples in Fig. 1).

The method is carried out as follows.

Stage 1. Samples 1 (Fig. 1) in the form of plates of a constant thickness of the unidirectional structure of the reinforcement are secured with a cantilever. They excite the plate-like oscillations in them (with two nodal lines 3). In this case, in the center of the free edge of the samples, a uniaxial VAT is realized in the material. If the direction of reinforcement material - 2 coincides with the direction

the free edge 6 (p 0 °) of the sample, then the endurance limit in the direction of the material base, au, is determined (Fig. 1a). If the direction of the sample reinforcement is orthogonal (p 90 °) to the free edge

(Fig. 1b), uniaxial loading of the material in the weft -022 direction is realized, and the corresponding endurance limit is determined. Using the values of ac and o22 found, the components of the strength criteria (in this case, the Mises-Hill criteria) are determined by the formulas:

NSG1 1 / 011IG & 22 1 / C &. (one)

2 stage. The sample is hinged in the fixture along the contour 5 in a special fixture. In this case, the axes of the elastic symmetry of the material (the direction of reinforcement is 2) coincide with the direction of one of the sides of the sample (see Fig. 2).

The sample excites the main form of vibrations. At the same time the place of destruction (maximum normal stress)

is in the center of the plate, and the material is in a flat VAT condition, when it is simultaneously affected by the stresses ac and O22 Through the strain values measured by resistance strain gages 4, pasted

along the direction of reinforcement and in the orthogonal direction at the center of the plate, the values of these stresses and the corresponding fatigue limit are calculated. The ratio of the voltages ai / 022 is controlled by the choice of the ratio of the lengths of the sides of the sample a / b. Through the found values of voltages AU and 022, at this stage of testing, the mixed component of the criterion (Mises-Hill) is determined.

  (- Iflll 1 - G & 22 - A2) / 2 A.

(2) where A # 22 / cn 1.

3etap. The sample, as in step 2, is fixed with a contour (see Fig. 2), but

The axes of its elastic symmetry (the direction of reinforcement) are directed at an angle a to the side (or axis) of the plate. In the center, 3 strain gauges are glued (along side a, along side b and at an angle of 45 ° to them). In the sample, the main form of oscillations is excited (the place of the maximum normal (7ci Oii, as well as tangents in the plane of the C712 sheet stresses in the center) and the limits of material endurance under the influence of it simultaneously (7ts, 022I and 7i2 - stresses calculate the mixed component of the criterion.

P1212 {1/011 - Ifllll + G & 22 + + G $ 222 C + 2 rflm C / 4 V,

(3)

where С O22 ai, B 7ii are the components of the stresses in the center of the sample that act on the material at this stage of research. The limited fatigue limit of the material is found through this component according to the formula derived from the applied criterion (Mises-Hill).

(712 1/2 YTK212.

To calculate the fatigue limit 012, other criteria can be used (for example, Ashkenazi B.K. strength criterion), then expressions (1-4) will be somewhat different.

The accuracy and reliability of the obtained values of the proposed method can be improved by testing samples of the same material at different ratios of network voltages, O22 and U12) in experiments. The resulting experimental data field is then approximated (for example, by the least squares method) and then the fatigue limit is calculated.

Endurance tests have been carried out to determine the boroslumini of a unidirectional reinforcement structure with a 25% volume fraction of boron fibers. The thickness of the samples was 0.8 mm. The oscillation frequency of the samples, depending on the test stage, was in the range of 1000-3600 Hz, the deformations were measured using strain gages of type 2 FKPA-1-5-B with a base of 1 mm. The signal from the strain gauge station (strain level) was measured with a F564 voltmeter;

15

totomer 43-33. The shape of the oscillations was controlled by the shapes of Lissajous using an oscilloscope.

Figure 4 presents the results of tests of boroaluminium — dependence on Mc4 is calculated by the Mises-Hill criterion. Line 1 in this figure was obtained when defining 1 $ 122, when SP 1.267 (a / b 1.388). and line 2, when 711 / O22 3.535 (a / L 0.706). Point 3 in this figure corresponds to the endurance limit of the same boroaluminium based on N 107 cycles, obtained using the Ashkenazi criterion. The data presented indicate a small variation in the experimental results when determining (7i2 by the proposed method and

„N its practical independence from the applied criteria. This indicates the reliability of the experimental results obtained.

Claims (1)

Invention Formula "A method for determining the fatigue characteristics of a material, namely, that a sample of a material in the form of a plate is hinged in the form of a cantilever, is loaded by exciting resonant bending vibrations, and the fatigue parameter of the material is determined, characterized in that x of composite materials, by taking into account the anisotropy of their properties, three additional are subjected to loading by resonant bending vibrations with the direction of reinforcement at one angle inclined to its edges WITH. and the other three have parallel respective edges; two plates with parallel edges are reinforced with a cantilever: one along the edge parallel to the reinforcement, one end along the edge, perpendicular to the reinforcement, - the two other plates are fixed along the contour and define their limited endurance limits, on the basis of which the fatigue parameter of the material is judged. 35 4 4 t ff four lf /: / W J  a 6 yy ff 6tf g ///////) /////// I Si H) 10 10s 1Q6 JO7 Haze cycles immersion nts Fy Fig.Z