GOST 25.505-85

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  ГОСТ 25.505-85

GOST 25.505−85 Calculations and strength tests. Methods of mechanical testing of metals. Test method for low cycle fatigue under thermomechanical loading

GOST 25.505−85

Group B09

INTERSTATE STANDARD

Calculations and strength tests

METHODS OF MECHANICAL TESTING OF METALS

Test method for low cycle fatigue under thermomechanical loading

Design, calculation and strength testing. Methods of mechanical testing of metals. Method of testing on the low cycle tatigue at heat mechanical loading

ISS 77.040.10
AXTU 0809

Date of introduction 1986−01−01

Resolution of the USSR State Committee on standards dated March 22, 1985 N 686 date of introduction is established 01.01.86

Limitation of actions taken by Protocol No. 5−94 of the Interstate Council for standardization, METROLOGII and certification (I & C 11−12−94)

REISSUE


This standard establishes methods of testing the fatigue of metals and alloys under simple types of deformation (tension — compression) in the low-cycle elastic-plastic region up to 10cycles at low-cycle thermomechanical loading at high temperatures up to 1100 °C in air.

As the basic accepted test methods at the independent loading and heating (thermo-mechanical fatigue), and also at loading by the constraint of thermal deformation (termostati).

The essence of the methods consists in obtaining the main design characteristics and mechanical properties of resistance to mechanical deformation and fracture under loading before the formation of macro cracks detection.

The standard does not cover testing of materials under irradiation in aggressive environments, vacuum, and subassemblies (parts, models, assemblies, welded, riveted, press and other compounds).

Terms used in the standard — GOST 23207−78. Explanations of the terms given in Annex 1.

1. THE SHAPE AND DIMENSIONS OF SAMPLES

1.1. The main types of samples for testing under thermo-mechanical and thermostaatknop loading conditions in tension — compression are smooth samples with working part of circular cross-section:

tubular cylindrical (Fig.1and, table.1),

a solid cylindrical (Fig.1b, table.2),

tubular corset (damn.1inthat table.3),

solid corset (damn.1g, tabl.4).

Damn.1. The working part of the samples

The working part of the samples

Damn.1

Table 1

Table 2

Table 3

Table 4

1.2. The main type of sample for testing under alternating torsion — tubular cylindrical sample (Fig.1and, table. When 1 mm).

1.3. Allowed, if necessary, to apply geometrically similar specimens of other sizes. The diameter of the working part of the specimen in tension or compression shall not be less than 5 mm, torsion — not less than 18 mm.

1.4. When the tendency of the cylindrical specimens to the loss of stability, change and destruction in transition areas, it is recommended to use a corset samples. It is also allowed to use cylindrical specimens with a short working part (=2−5, damn.1b).

1.5. The shape and dimensions of the heads of the samples depend on the method of their fastening in the grips of the test machine.

1.6. The diameter of the transition part of the sample is selected based on achieving minimal stress concentrations and deformations in transition zones.

1.7. Allowed to use samples with a screw and welded heads in the manufacture of their parts or structural elements.

1.8. Samples are manufactured in accordance with GOST 25.502−79. For tubular specimen, the tolerance of alignment of the external and internal cylindrical surfaces of the working part is assigned to the 7th degree of accuracy (GOST 24643−81).

2. TESTING MACHINES AND APPARATUS

2.1. Machines and apparatus for testing under low-cycle thermomechanical loading, providing for the static burst tests shall meet the requirements of GOST and GOST 9651−84 28840−90 and play loading (deformation) and heat in the following conditions:

consistency from cycle to cycle maximum and minimum stress (soft loading), deformation (rigid loading) and temperature throughout the testing process (Fig.2, a-g);

given the variation of loads, deformations and temperatures in the cycle, including linear (Fig.2, a-d, damn it.3, a-d), with extracts and without extracts (Fig.2, d-h) and at different cycle asymmetry (Fig.2, b, g) in the frequency range, which allows to investigate the effects of long-and short-term cyclic loading;

synchronized mode the heating load for a given program, including a program independent of the loading with different phase cycles of loading and heating (Fig.3, a-d);

static loading with given speed of deformation and loading at a given temperature regime.

Damn.2. The loading regimes

The loading regimes

Damn.2

Damn.3. Examples of stress changes (deformation) and temperature under thermo-mechanical loading

Examples of stress changes (deformation) and temperature under thermo-mechanical loading

Damn.3

2.2. Machines for testing thermal fatigue needs to have variable stiffness in the range of 60−300 kN/mm.

2.3. Permissible error check stress and strain in time to meet the requirements of GOST 25.502−79.

2.4. For strain measurements using optical, strain gauge and other means of contact and contactless type. In the tests under tension — compression is allowed to measure a single component of deformation — longitudinal or transverse. The last recalculation in longitudinal performed in accordance with clause 3.9.

2.5. Select the basis and method of deformation measurement is determined by the requirements of PP.2.9 and 2.10 to the uniformity of heating and the type of the used sample. For samples corset type measure transverse strain.

2.6. With the destruction of the sample out of the base of strain measurement is necessary to ensure the requirements of the PP.2.9 and 2.10 according to the heating conditions of the sample on the basis of measurements of deformations in the area of education destruction.

2.7. For registration of deformations and loads in time and number of cycles are used in automatic recording instruments. Recording of the deformation diagrams, with the exception, as appropriate, free of thermal deformation of the sample is performed using a two-axis devices and other means of automatic registration.

To exclude the free thermal deformation of a sample using the system of automatic compensation of the photoelectric, capacitive, and other types that allows you to select for recording and controlling the mode of loading the actual mechanical deformation.

2.8. For heating samples using the heater resistance furnace, tube heaters, rod heaters of high-temperature refractory materials, directly passing a current through the sample, the induction method (high-frequency currents induced in the sample), etc. In this case the following conditions hold:

error of measurement, registration and maintenance of temperature shall not exceed ±1.0% of set maximum temperature during the testing process;

measuring and recording temperature are in the process of testing by automatic measuring devices;

the temperature of the working area of the sample is measured contact (thermocouple) or non-contact (pyrometric) method;

the diameter of thermoelectrodes (thermocouple) should be selected so as to achieve stable characteristics of the thermocouple at high temperatures and to avoid excessive inertia. It is recommended that the diameter of thermoelectrodes not more than 0.2 mm;

the cooling of the sample may be natural or forced conduction from the sample to the cooling system and purge air or gas.

2.9. The unevenness of the temperature distribution on the basis of the strain measurements and the difference of the maximum temperature in this area and in the area of education, the destruction of the sample should not exceed 1% of the maximum temperature.

2.10. The longitudinal temperature difference in the zones of deformation measurement and education of the destruction on the minimum length of the sample diameter should not exceed 3 °/mm; transverse differential in these zones must not exceed 1°/ mm.

2.11. May be applied on the sample cores, grooves, welding of thermocouples, if in testing it does not lead to preferential destruction of the sample in these zones.

3. TESTING

3.1. The main type tests on low-cycle fatigue under thermo-mechanical loading is tension — compression, the main type of loading — a hard loading.

3.2. The shape of the loading cycle and the heating is selected based on operating conditions, and the maximum test duration shall be not less than 10% of operating time. If you cannot fulfill this condition the testing duration is shortened by the application of reasonable methods of equivalent cyclic tests and extrapolating the results to the desired duration.

3.3. Appropriate, explore the possibility of combining modes of loading and heating, giving the greatest damaging effect when low-cycle thermomechanical loading. At the same time determine the effects of the sign of stress during high-temperature exposure (Fig.2, d-e) and the role of the phase of loading cycles and heat (damn.3, a-d). Allowed to test in other modes, for example, with different variable in the half-cycles in tension and compression with strain rate, with a step temperature change in a loop during the transition from stretching to compression, etc.

3.4. The test is performed at the operating temperatures of the actual operating mode, in case of necessity with the variation of the maximum and minimum temperatures of the cycle. Variation is determined by the type of material, the possible value of the casting temperature and unevenness of temperature field during operation.

3.5. Allowed testing with breaks. In this case it is necessary to assess the possible impact of interruptions determined by the characteristics of the material.*
____________________
* The text matches the original. — Note the manufacturer’s database.

3.6. The test is performed until the formation of surface cracks by 5% — 10% of the diameter of the sample (for the sample =10 mm crack 0.5−1.0 mm), determined by optical method or other ways.

Allowed to test before the final destruction without fixing the cracks, when the stage of crack propagation for a given mode does not exceed 10% of the total durability.

When tested in hard mode loading is allowed as rough estimates make the number of cycles until a macrocrack is equal to the number of cycles corresponding drop in the stress (load) in the loop by 50% compared to the steady-state value.

3.7. The number of samples to be tested depends on the variability. To build the fatigue curve is used at least 10−12 transcripts of results at different levels (four levels).

If you want to determine the statistical characteristics of the dispersion values of durability at each level of loading are experiencing 10−12 samples and determine the value of dispersion .

3.8. To study the kinetics of elastic-plastic deformation is carried out pazilova recording of the deformation diagrams (a) compensation of thermal deformation.

The periodicity of recording of parameters of loading and heating depends on the intensity changes of temperature, deformation and force characteristics in the process of loading (recommended to register, for example, in cycles 1, 2, 3, 4, 5, 10, 20, 30, 50, 100, 200, 300, 400, 500, 750, 1000, 1500, 2000, 2500, 3000, 4000, 5000, 7500, 10000, 12500, 15000, 20000, 25000, 35000, 50000).

3.9. If the tests under tension — compression measure the lateral deformation of the sample, the conversion of its longitudinal to isotropic materials perform according to the formula

,

where and plastic and elastic components of transverse strain; and the ratios of the transverse deformations in the plastic and elastic region. In the absence of appropriate experimental data can be taken

, .

In cases where the division of the whole of the lateral strain in elastic and plastic is difficult, the conversion can be carried out using a ratio of

,

where full shear deformation.

3.10. The results of the tests are excluded from further consideration:

with the destruction of the sample outside of its working parts or loss of stability;

if defects of material such as shells, inclusions, etc., identified in a fracture;

with a significant change in shape in the fracture zone of the specimen in the case of rigid or termostatico loading when unilaterally accumulated deformation is more than 0.1 magnitude have the ductility of material in tension obtained in the respective temperature and time conditions;

in breach of the precision of setting a maximum temperature and parameters of the loading cycle or in the event of overheating.

3.11. Allowed when testing in the field of large temporal databases (over 10h) to apply different from the recommended methods of loading and specimen types, and to determine the loading parameters calculation methods.

4. PROCESSING OF THE RESULTS

4.1. According to test results on the thermomechanical and thermal fatigue build:

curves in the parameters:

phase of loading cycles and heating

maximum and minimum temperature cycle

the frequency of loading,

the duration of single and double sided exposure

the asymmetry of cycle of loading on stresses and deformations ;

change curves of deformations and stresses in time and the number of cycles and charts of elastic-plastic deformation and determine their parameters.

4.2. The source data and test results of each sample record in the test report (Annex 2), and the results of testing a series of identical samples in a consolidated test report (Annex 3).

4.3. Curves of fatigue under severe loading built in double logarithmic coordinates: amplitude (p-p) total , plastic , elastic , irreversible deformation — number of cycles to formation of cracks .

4.4. The amplitude (magnitude) of the deformation is determined by the number of cycles by interpolation of the measured values in the cycles closest to .

4.5. Curves when soft loading built in semi-logarithmic or double logarithmic coordinates: amplitude (magnitude) of the stress  — number of cycles to formation of cracks .

4.6. Curves are built by the method of graphical interpolation of the experimental results or by the method of least squares.

4.7. Curves of change of the number of cycles and time width of the elastoplastic hysteresis loop, cyclic and one-sidedly accumulated creep strain, plastic and irreversible deformations, respectively , , , and , and stresses , and build in semi-logarithmic and double logarithmic coordinates

.

4.8. The strain diagram with initial static and cyclic low-cycle thermomechanical loading build in the coordinates and the parameters adopted in the tests of loading regimes and heat.

4.9. As auxiliary features in the process of non-isothermal tests receive the data on the thermal expansion of free sample depending on the heating temperature.

4.10. Methods of graphical representation the obtained characteristics according to GOST 25.502−79.

ANNEX 1 (mandatory). EXPLANATION OF TERMS USED IN STANDARD

ANNEX 1
Mandatory

Low cycle fatigue under thermomechanical loading — a fracture due to cyclic elastic-plastic deformation accompanied by temperature change.

Low-cycle thermal fatigue is a special case of low-cycle thermomechanical fatigue, in which the loading due to the constraint of thermal deformations during cyclic heating — cooling.

Long low-cycle loading — low cycle loading, when the duration of the cycle and the total time sufficient for the manifestation of the temperature-time effects (creep, dostavanie material, etc.).

Short-term low-cycle loading — low cycle loading, when the duration of the cycle and the total time, excluding the manifestation of the temperature-time effects.

 — sample temperature, °C or K;

 — the maximum temperature of the cycle;

 — minimum temperature of the cycle;

 — scale temperature cycle.

contraction ratio of the cross sectional area of the specimen under static tension, %;

% of the available ductility of the material, %;

 — the limit of elasticity determined at ambient temperature;

 — deformation corresponding to the elastic limit;

the amplitude of the voltage cycle;

2 — the magnitude of the voltage cycle;

,  — maximum and minimum stress cycle;

 — voltage in the half cycle when the count respectively from the points of zero crossing and the beginning of unloading;

 — secondary voltage cycle;

,  — the elastic limit -th half cycle when the count respectively from the points of zero crossing and the beginning of unloading;

the coefficient of cycle asymmetry on the stress;

 — the amplitude of total strain cycle;

, , ,  — amplitude of deformation of the loop, respectively, elastic, plastic, creep, and irreversible;

2, 2, 2, 2, 2 — swings above deformation cycle;

 — total deformation in the half cycle of loading;

, , respectively, the deformation of creep, plastic and irreversible deformations accumulated after -th half cycle loading;

, , respectively, the deformation of creep, plastic and reversible deformation accumulated at the time of formation of the macrocrack;

, respectively, the maximum and minimum total deformation cycle;

 — average deformation cycle;

the coefficient of cycle asymmetry in the deformations;

 — the number of half-cycles (=0, 1, 2, 3, …10, 20, 30, 100, 200, 300…);

 — the number of cycles before the formation of the macrocrack;

 — the number of loading cycles;

total time of loading, h;

 — deformation in the half cycle when counting from the beginning of discharge or point of zero crossing on the voltage;

 — the width of the hysteresis loop of the third half cycle;

 — cycle time;

 — loading frequency;

 — exposure time;

 — heating time;

 — cooling time;

 — modulus of elasticity.

The adopted coordinate system diagrams of deformation under static and cyclic loading are shown in the drawing.

The main parameters of the deformation diagram of cylindrical

APPENDIX 2 (recommended). TEST REPORT OF THE SAMPLE (ANNEX TO THE SUMMARY REPORT)

ANNEX 2
Recommended

PROTOCOL N___
the sample (Annex to the summary Protocol N____)

Treatment of hysteresis loops

APPENDIX 3 (recommended). SUMMARY REPORT

APPENDIX 3
Recommended

SUMMARY REPORT N_____