Calculations and strength tests in mechanical engineering METHODS OF MECHANICAL TESTING OF METALS
Fatigue Test Methods
Strength analysis and testing in machine GOST 23026-78
building. Methods of metals mechanical and GOST 2860-65
testing. Methods of fatigue testing in parts 6L and 6.2
MKS 77.040.10 OKP 00 2500
By Decree of the USSR State Committee on Standards of November 30, 1979 No. 4146, the introduction date was set
The validity period was lifted according to Protocol No. 2-92 of the Interstate Council for Standardization, Metrology and Certification (IUS 2-93)
This standard establishes methods for testing samples of metals and alloys for fatigue:
in tension - compression, bending and torsion;
with symmetrical and asymmetrical cycles of stress or strain, changing according to a simple periodic law with constant parameters;
in the presence and absence of stress concentration;
at normal, high and low temperatures;
in the presence or absence of an aggressive environment;
in the high- and low-cycle elastic and elastoplastic region.
Terms, definitions and designations used in the standard are in accordance with GOST 23207-78.
The standard does not establish special methods for testing samples used in testing the strength of high-stress structures.
Sections 2-4 of the standard and annex can be used for fatigue testing of machine components and structures.
1. SAMPLING METHODS
1.1. Fatigue testing of metals is carried out on smooth round samples of types I (Drawing 1, Table 1) and II (Drawing 2, Table 2), as well as rectangular samples of types III (Drawing 3, Table 3) and IV (Figure 4, Table 4).
Official publication
Reproduction is prohibited
★
Edition with Amendment No. 1, approved in December 1985 (IUS 3-86).
Type I sample working part
Table 1 mm
Type II sample working part
G-2
Table 2 mm
Type IV sample working part
Table 4mm
1.2. The sensitivity of the metal to stress concentration and the influence of absolute dimensions is determined on samples of the following types:
V - with a V-shaped ring groove (Fig. 5, Table 5-8);
Working part of sample type U
Table 5
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When bending |
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Table 6
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In tension-compression |
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Table 7
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Torsional |
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Table 8
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In tension-compression |
torsion |
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VI - with symmetrical side cuts of a V-shaped profile (Fig. 6, Table 9);
Type VI sample working part
Table 9
VIII - with an annular groove of a circular profile (Fig. 8, Table 11); Working part of type VIII sample
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When growing | ||||||
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torsion |
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IX - with two symmetrically located holes (Fig. 9, Table 12);
Type IX working part
X - with symmetrical side cuts of a V-shaped profile (Fig. 10, Table 13).
Working part of sample type X
The dimensions of the samples are chosen in such a way that the fatigue failure similarity parameter
(L is the perimeter of the working section of the sample or its part adjacent to the zone of increased stress; G is the relative gradient of the first principal stress).
When bending with rotation, torsion and tension - compression of samples of types I, II, V, VIII
L "d,
during bending in one plane of samples of types III, IV, VI, as well as during tension and compression of samples of type VI L = 2b;
in tension - compression of samples of types III, IV, VII, IX, X L = 2h.
1.3. For low-cycle fatigue testing, specimens of types II and IV are used if there is no danger of buckling.
It is allowed to use samples of types I and III.
1.4. The working part of the samples must be manufactured with an accuracy of at least 7th grade GOST 25347-82.
1.5. The surface roughness parameter of the working part of the samples Ra should be 0.32-0.16 microns according to GOST 2789-73.
The surface must be free of traces of corrosion, scale, casting crusts, tarnish, etc. etc., if this is not provided for by the objectives of the study.
1.6. The distance between the grips of the testing machine is chosen so as to exclude longitudinal bending of the sample and the influence of forces in the grips on the tension in its working part.
1.7. Cutting blanks, marking and making samples should not have a significant impact on the fatigue properties of the original material. Heating the sample during manufacturing should not cause structural changes or physical and chemical transformations in the metal; processing allowances, mode parameters and processing sequence should minimize work hardening and exclude local overheating of samples during grinding, as well as cracks and other defects. Removal of the last chips from the working part and sample heads is carried out from one sample installation; burrs on the side faces of the specimens and the edges of the cuts must be removed. Blanks are cut in places with a certain orientation in relation to the macrostructure and stress state of the products.
1.8. Within the intended series of tests, the technology for manufacturing samples from the same type of metals should be the same.
1.9. Measuring the dimensions of the working part of manufactured samples before testing should not cause damage to its surface.
1.10. The working part of the sample is measured with an error of no more than 0.01 mm.
2.1. Fatigue testing machines must ensure loading of samples according to one or more schemes shown in Fig. 11-16. Fatigue testing machines that also provide statistical tensile testing must comply with the requirements of GOST 1497-84.
2. EQUIPMENT
Pure bending during rotation of specimens of types I, II, V, VIII
Transverse bending during rotation of specimens of types I, II, V, VHI under cantilever loading
Pure bending in one plane of specimens of types I-VIII
Sample working section
Transverse bending in one Repeatedly variable stretching
planes of samples of types I-VIII compression of samples of types I-X
under cantilever loading
Working section
| Sample |
Crap. 14 Damn. 15
Repeatedly alternating torsion of samples of types I, II, U, VIII
2.2. The total loading error during testing of samples depends on the type of machine and loading frequency and should not exceed in the range of 0.2-1.0 of each loading range as a percentage of the measured value:
± 2% - at /< 0,5 Гц;
± 3% - at 0.5
± 5% - at/> 50 Hz.
When testing on hydropulsation and resonance machines without strain gauge force measurement in the range of 0-0.2 of each load range, the load measurement error should not exceed ± 5% of the specified voltages.
2.3. The error in measuring, maintaining and recording deformations during low-cycle tests should not exceed ± 3% of the measured value in the range of 0.2-1.0 of each loading range.
2.4. The absolute error in measuring, maintaining and recording loads and deformations in the range of 0-0.2 of each range should not exceed the absolute errors at the beginning of this loading range.
2.5. Loads (under soft loading) or deformations (under hard loading) must correspond to 0.2-0.8 of the applicable measuring range.
2.6. When testing for low-cycle tension or compression and tension-compression, additional bending deformations of the sample due to loading misalignment should not exceed 5% of the tensile or compression deformations.
2.7. When testing for low-cycle fatigue, continuous measurement must be ensured, as well as continuous or periodic recording of the deformation process of the working part of the sample.
2.8. It is allowed to calibrate testing equipment under static conditions (including loading misalignment) with assessment of the dynamic component of the error by calculation or indirect methods.
3. TESTING
3.1. When testing samples, soft and hard loading is allowed.
3.2. Within the intended series of tests, all samples are loaded in the same way and tested on the same type of machines.
3.3. Testing of samples is carried out continuously until a crack of a given size is formed, complete destruction, or until a basic number of cycles.
Breaks in testing are allowed, taking into account the conditions of their conduct and a mandatory assessment of the impact of breaks on the test results.
(Changed edition, Amendment No. 1).
3.4. During testing of samples, the stability of the specified loads (deformations) is monitored.
3.5. Testing a series of identical samples under asymmetric cycles is carried out:
or at the same average cycle stresses (strains) for all samples;
or with the same cycle asymmetry coefficient for all samples.
3.6. To construct a durability distribution curve and estimate the average value and standard deviation of the logarithm of durability at a given stress level, a series of at least 10 identical samples is tested until complete destruction or the formation of macrocracks.
3.7. High Cycle Fatigue Tests
3.7.1. The main failure criteria when determining endurance limits and constructing fatigue curves are complete destruction or the appearance of macrocracks of a given size.
3.7.2. To construct a fatigue curve and determine the endurance limit corresponding to a 50% probability of failure, at least 15 identical samples are tested.
In the stress range of 0.95-1.05 from the endurance limit, corresponding to the probability of destruction of 50%, at least three samples must be tested, and at least half of them must not fail before the test base.
3.7.3. The test base for determining endurance limits is accepted:
10 10 6 cycles - for metals and alloys that have an almost horizontal section on the fatigue curve;
100 10 6 cycles - for light alloys and other metals and alloys, the ordinates of the fatigue curves along the entire length continuously decrease with increasing number of cycles.
For comparative tests, the basis for determining the endurance limits is 3 10^ and 10 10^ cycles, respectively.
3.7.4. To construct a family of fatigue curves based on the fracture probability parameter, construct a distribution curve of the endurance limit, and estimate the average value and standard deviation of the endurance limit, a series of at least 10 identical samples is tested at each of 4-6 stress levels.
3.7.5. From 10 to 300 Hz, the cycle frequency is not regulated if the tests are carried out under normal atmospheric conditions (according to GOST 15150-69) and if the temperature of the working part of the sample during testing is not higher than 50 °C.
For samples made of low-melting alloys and other alloys that exhibit changes in mechanical properties up to a temperature of 50 °C, the permissible test temperature is set separately.
3.8. Low cycle fatigue tests (with durability up to 5 1(I cycles*)
3.8.1. The main type of loading during testing is tension-compression.
3.8.2. The upper level of test frequencies is limited to values that exclude self-heating of the sample above 50 °C for light alloys and above 100 °C for steels.
In all cases, the cycle frequency is indicated when reporting the test results.
To record strain diagrams, it is allowed during testing to switch to lower frequencies that correspond to the required resolution and accuracy of instruments for measuring and recording cyclic stresses and strains.
3.8.3 When testing tensile-compression samples of types II and IV, strain measurements should be carried out in the longitudinal direction.
When testing specimens of types I and III, it is allowed to measure deformations in the transverse direction.
Note: To approximately convert transverse deformation into longitudinal deformation, use the formula
E prod - ^ (e y) across ^ (E p) across’
where (Ey) trans is the elastic component of the transverse deformation;
(Ep)per - plastic component of transverse deformation.
3.9. Tests at elevated and reduced temperatures
3.9.1. Tests at elevated and reduced temperatures are carried out under the same types of deformation and the same samples as at normal temperature.
* The number of cycles 5 ■ 10 4 is the conventional limit of low- and high-cycle fatigue. This value for ductile steels and alloys characterizes the average number of cycles for the zone of transition from elastoplastic to elastic cyclic deformation. For highly plastic alloys, the transition zone shifts towards higher durability, for brittle alloys - towards lower durability.
3.9.3. The test temperature of the samples is controlled according to dynamic calibration of the temperature difference between the sample and the furnace space. Temperature calibration is carried out taking into account the influence of the test duration. During calibration, thermocouples are fixed to the sample.
3.9.4. Thermocouples are verified both before and after testing according to GOST 8.338-2002. When testing on bases of more than 10 7 cycles, intermediate verification of thermocouples is also carried out.
3.9.5. The unevenness of temperature distribution along the length of the working part when testing smooth samples of types II and IV should not exceed 1% per 10 mm of the specified test temperature. When testing smooth samples of types I, III and samples with stress concentrators, the unevenness of the temperature distribution is regulated at a distance of ± 5 mm from the minimum cross-section of the sample. The deviation from the set temperature should not exceed 2%.
3.9.6. During testing, the permissible temperature deviations on the working part of the sample in °C should not exceed the following limits:
up to 600 inclusive.........±6;
St. 601 to 900 "............±8;
» 901 » 1200 »............±12.
3.9.7. Loading of samples is carried out after the established thermal regime of the “sample-furnace” system has reached the specified temperature of the sample.
3.9.8. The test base is accepted in accordance with clause 3.7.3 of this standard.
3.9.9. For comparability of results, tests of a given series of samples are carried out at the same frequency and base, unless the purpose of the tests is to study the influence of loading frequency. The test reports indicate not only the number of cycles completed, but also the total testing time for each sample.
3.10. Tests in aggressive environments
3.10.1. Tests in aggressive environments are carried out under the same types of deformation and on the same samples as in the absence of an aggressive environment. Simultaneous testing of a group of samples with registration of the moment of destruction of each is allowed.
3.10.2. The sample must be continuously exposed to a gas or liquid aggressive environment.
3.10.3. When testing in an aggressive environment, the stability of the parameters of the aggressive environment and its interaction with the surface of the sample must be ensured. Requirements for the frequency of monitoring the composition of an aggressive environment are determined by the composition of the environment and the objectives of the study.
3.10.4. For comparability of results, tests of a given series of samples are carried out at the same frequency and base, unless the purpose of the tests is to study the influence of loading frequency.
3.9-3.9.9,3.10-3.10.4. (Introduced additionally, Amendment No. 1).
4. PROCESSING RESULTS
4.1. Based on the results of fatigue tests, the following is carried out:
constructing a fatigue curve and determining the endurance limit corresponding to a 50% probability of failure;
constructing diagrams of limiting stresses and limiting amplitudes;
constructing a fatigue curve in the low-cycle region;
constructing elasto-plastic deformation diagrams and determining their parameters;
construction of fatigue curves based on the probability of failure parameter;
determination of the endurance limit for a given level of probability of destruction;
determination of the average value and standard deviation of the logarithm of durability at a given level of stress or deformation;
determination of the average value and standard deviation of the endurance limit.
The specified characteristics of metal fatigue resistance are determined for various stages of development of macrocracks and (or) complete destruction.
4.2. Processing of high-cycle fatigue test results
4.2.1. The initial data and results of each sample test are recorded in the test report (Appendices 1 and 2), and the test results of a series of identical samples are recorded in the consolidated test report (Appendices 3 and 4).
4.2.2. Fatigue curves are plotted in semi-logarithmic coordinates (o max; lgN or o a; lg/V) or double logarithmic coordinates (lg o max; lg/V or lg o a; lg/V).
4.2.3. Fatigue curves for asymmetric cycles are plotted for a series of identical samples tested at the same average stresses or at the same asymmetry coefficients.
4.2.4. Fatigue curves based on the results of testing a limited volume of samples (clause 3.7.2) are constructed by graphical interpolation of experimental results or by the least squares method.
4.2.5. To construct durability distribution curves and endurance limits, estimate average values and standard deviations, as well as construct a family of fatigue curves based on the probability of failure parameter, the test results are subjected to statistical processing (Appendices 5-7).
4.2.6. Diagrams of limiting stresses and limiting amplitudes are constructed using a family of fatigue curves obtained from the results of testing at least three to four series of identical samples at different average stresses or stress cycle asymmetry coefficients for each series.
4.3. Processing of low-cycle fatigue test results
4.3.1. The results are processed as indicated in clause 4.2.4.
4.3.2. The initial data and test results of each sample are recorded in the test report, and the test results of a series of identical samples are recorded in the consolidated test report (Appendices 8 and 9).
4.3.3. Based on the results of testing samples under severe loading, fatigue curves are plotted in double logarithmic coordinates (Fig. 17):
amplitude of total deformation E a - the number of cycles until the formation of a crack N T or until destruction N;
amplitude of plastic deformation g ra - the number of cycles corresponding to half the number of cycles before the formation of a crack N T or before destruction N.
Notes:
1. The amplitude of plastic deformation E pa is determined as half the width of the elastoplastic hysteresis loop gr or as the difference between the specified amplitude of total deformation and the amplitude of elastic deformation determined from the measured load, the corresponding stress and the elastic modulus of the material.
2. The amplitude of plastic deformation E pa at a number of cycles corresponding to half the number of cycles, before the formation of a crack or before failure, is determined by interpolating the amplitude values at pre-selected numbers of cycles close to the expected ones.
Fatigue curves under hard loading Fatigue curve under soft loading
Che R t - 17 Damn. 18
4.3.4. Based on the test results under soft loading, the following is built:
fatigue curve in semi-logarithmic or double logarithmic coordinates: stress amplitude o a - number of cycles before crack formation N T or failure N (Fig. 18);
dependence of the amplitude of plastic deformations (half the width of the hysteresis loop) gr on the number of half-cycles of loading K in terms of the stress amplitude parameter at the selected stress cycle asymmetry coefficient (Fig. 19).
Dependence of the amplitude of plastic deformations on the number of half-cycles of loading
a - for cyclically softening material; b for a cyclically stabilized material; c - for cyclically hardening material
PROTOCOL
sample testing (appendix to consolidated protocol No.__)
Purpose of the test_
Machine: type_, No._
Cycle voltages:
maximum_, average_, amplitude_
Loads (number of divisions on the load scale):
maximum_, average_, amplitude_
Indications of instruments recording load axiality or sample runout:
at the beginning of the test_
at the end of the test_
Number of completed cycles_
Loading frequency_
Failure criterion_
Tests were carried out by _
Head of laboratory _
sample testing (appendix to consolidated protocol No._)
Purpose of the test_
Sample: code_, transverse dimensions_
Machine: type_, No._
Cycle deformation:
maximum_, average_, amplitude_
Number of divisions on the deformation indicator: maximum_
average_, amplitude_
Indications of instruments recording load axiality:_
device No. 1_, device No. 2_, device No. 3
Meter readings (date and time):
at the beginning of the test_
at the end of the test_
Number of completed cycles_
Loading frequency_
Failure criterion_
Conducted tests
Head of laboratory
Purpose of testing___
Material:
make and condition_
fiber direction_
Test conditions:
type of loading_
test base__
loading frequency_
Failure criterion_
Type of samples and nominal dimensions of their cross-section
Surface condition_
Testing machine:
Date of testing:
start of testing of the first sample_, end of testing
latest sample_
Head of laboratory
Purpose of testing___
Material:
make and condition_
fiber direction_
type of workpiece (for complex shapes, a sample cutting plan is included)
Test conditions:
type of deformation_
test base___
loading frequency_
Destruction criteria_
type of specimens and nominal cross-sectional dimensions_
surface condition_
Testing machine:
Date of testing:
start of testing of the first sample_, end of testing of the last sample
Responsible for testing this series of samples
Head of laboratory
CONSTRUCTION OF A DURABILITY DISTRIBUTION CURVE AND ESTIMATION OF THE AVERAGE VALUE AND RMSE SQUARE DEVIATION OF THE DURABILITY LOGARITHM
The test results of a series of n samples at a constant voltage level are arranged in a variation series in order of increasing durability
N l Similar series for samples made of aluminum alloy grade B95, tested under cantilever bending with rotation until complete destruction at six stress levels as an example, are given in Table. 1. Durability distribution curves (P-N) are plotted on probability paper corresponding to a lognormal or other distribution law. The values of sample durability N are plotted along the abscissa axis, and the values of the probability of destruction of samples (accumulated frequencies) are plotted along the ordinate axis, calculated by the formula p i - 0.5 p ’ where i is the sample number in the variation series; n is the number of tested samples. If not all samples of the series fail at the stress level under consideration, then only the lower part of the distribution curve is plotted to the base durability. The drawing on logarithmically normal probability paper shows a family of P-N distribution curves constructed according to the data in Table. 1. Table 1 Variation series of the number of cycles before failure of samples made of B95 alloy at max, kgf/mm 2 (MPa) * The samples did not collapse. Durability distribution curves for samples made of B95 alloy 10*2 3 8 6810 s 2 38 6810 e 2 38 6810 9 2 3 8 6810 e N 1 - a max = 33 kgf/mm 2 (330 MPa); 2- a max = 28.5 kgf/mm 2 (285 MPa); 3- a max = 25.4 kgf/mm 2 (254 MPa); 4- a max = 22.8 kgf/mm 2 (228 MPa); 5- a max = 21 kgf/mm 2 (210 MPa); 6- a max = 19 kgf/mm 2 (190 MPa) An assessment of the average value a and the standard deviation o of the logarithm of durability is carried out for the stress levels at which all samples of the series failed. The sample average value of lg N and the sample standard deviation of the logarithm of the durability of samples (S lg d,) are calculated using the formulas: In table As an example, Figure 2 shows the calculation of log N and 5j g d for samples made of alloy grade V95, tested at a stress of max = 28.5 kgf/mm 2 (285 MPa) (see Table 1). table 2 X (lg^) 2 = 526.70. 526,70 - ^ ■ 10524,75 The volume of a series of samples n is calculated using the formula n>^-Z\_o-A 2 2 where y is the coefficient of variation of the value x = log/V; D a and D a - maximum relative errors for the confidence probability P - 1-a when estimating the average value and standard deviation of the value x = log/V, respectively; a is the probability of a type I error; Z | _ and - quantile of the normalized normal distribution, the corresponding probability P = 1 - tg 2 2 (the values of the most commonly used quantiles are given in Table 3). Error values are chosen within the range D a = 0.02-0.10 and D a = 0.1-0.5, the probability of a type I error a is taken to be 0.05-0.1. Table 3 CONSTRUCTION OF A FAMILY OF FATIGUE CURVES ACCORDING TO THE PROBABILITY OF DESTRUCTION PARAMETER To construct a family of fatigue curves, it is advisable to conduct tests at four to six stress levels. The minimum level should be selected so that approximately 5% to 15% of specimens tested at that stress level fail before the base number of cycles. At the next (in ascending order) stress level, 40%-60% of the samples should fail. The maximum stress level is selected taking into account the requirement for the length of the left branch of the fatigue curve (N > 5 ■ 10 4 cycles). The remaining levels are distributed evenly between the maximum and minimum voltage levels. The test results for each stress level are arranged in variation series, on the basis of which a family of durability distribution curves in P-N coordinates is constructed (Appendix 7). The values of the probability of destruction are set and, based on the durability distribution curves, a family of fatigue curves of equal probability is constructed. The drawing shows the fatigue curves of samples made of alloy grade V95 for the probability of failure P = 0.5; 0.10; 0.01, based on graphs. The minimum required number of samples to construct a family of fatigue curves is determined depending on the confidence probability P l = 1-a and the maximum relative error A p when estimating the endurance limit for a given probability P based on the formula ■ Zj-a ■ f(r) , where y is the coefficient of variation of the endurance limit; Z-quantile of the normalized normal distribution; Ф (р) is a function depending on probability, for which the endurance limit is determined. The values of this function, found by statistical modeling, are given in the table. Fatigue curves of samples made of B95 alloy CONSTRUCTION OF THE DISTRIBUTION CURVE OF ENDURANCE LIMIT AND ESTIMATION OF ITS AVERAGE VALUE AND RMSE SQUARE DEVIATION To construct the endurance limit distribution curve, samples are tested at six stress levels. The highest voltage level is chosen so that all samples at this voltage fail to a basic number of cycles. The maximum voltage value is taken (1.3-1.5) from the endurance limit value for P-0.5. The remaining five levels are distributed in such a way that at the middle level about 50% is destroyed, at two high levels - 70%-80% and at least 90%, and at two low levels - no more than 10% and 20%-30%, respectively. The stress value in accordance with a given probability of failure is selected based on an analysis of available data for similar materials or using preliminary tests. After testing, the results are presented in the form of variation series, on the basis of which durability distribution curves are constructed according to the method outlined in Appendix 5. Based on the durability distribution curves, a family of fatigue curves is constructed for a number of failure probabilities (Appendix 8). For this purpose, it is advisable to use probabilities of 0.01, 0.10, 0.30, 0.50, 0.70, 0.90 and 0.99. From these fatigue curves the corresponding endurance limit values are determined. The endurance limit for the probability of failure P = 0.01 is found by graphically extrapolating the corresponding fatigue curve to the base number of cycles. The found values of the endurance limits are plotted on a graph with the coordinates: probability of failure on a scale corresponding to the normal distribution - endurance limit in kgf/mm 2 (MPa). A line is drawn through the constructed points, which represents a graphical assessment of the endurance limit distribution function. The range of variation of the endurance limit is divided into 8-12 intervals, the average values of the endurance limit and its standard deviation are determined using the formulas: X AR g st y. ; where a R is the average value of the endurance limit; S„ - standard deviation of the endurance limit; Std - the value of the endurance limit in the middle of the interval; I - number of intervals; A Pi - probability increment within one interval. As an example, based on the results of cantilever bending tests with rotation of 100 samples of aluminum alloy grade AB, presented in Table. 1, build a distribution function of endurance limits for a base of 5 ■ 10 7 cycles and determine the average value and standard deviation. Based on the variation series (Table 1), durability distribution curves are constructed (Figure 1). Durability values for AB alloy samples Table 1 at max, kgf/mm 2 (MPa) * The samples did not collapse. By making horizontal sections of the durability distribution curves (Fig. 1) for probability levels P = 0.01, 0.10, 0.30, 0.50, 0.70, 0.90, 0.99 (or 1.10, 30 , 50, 70, 90, 99%), find the corresponding durability at given stress values, on the basis of which fatigue curves are constructed according to the probability of failure parameter (Fig. 2). Durability distribution curves for samples made of AB alloy 1 - Box, = 16.5 kgf/mm 2 (165 MPa); 2 - = 13.5 kgf/mm 2 (135 MPa); 3- a max = 12.5 kgf/mm 2 (125 MPa); 4- amax = 12.0 kgf/mm 2 (120 MPa); 5- Box = 11.5 kgf/mm 2 (115 MPa); 6- = 11.0 kgf/mm 2 (110 MPa) Fatigue curves for samples made of AB alloy for different probabilities of failure 1 - P = 1%; 2- P = 10%; 3-Р= 30%; 4-P= 50%; 5-P= 70%; 6-P= 90%; 7- P= 99% The values of endurance limits for a base of 5 ■ 10 7 cycles are taken from the graphs (Fig. 2). The values of endurance limits are given in table. 2. According to the results given in table. 2, build an endurance distribution curve (Fig. 3). table 2 Values of limited endurance limits for samples made of AB grade alloy (base 5 - 10 7 cycles) Distribution curve of the limited endurance limit of samples made of AB alloy (base 5 - 10 7 cycles) To determine the average value of the endurance limit and its standard deviation, the range of variation in the endurance limit is divided into 10 intervals of 0.5 kgf/mm 2 (5 MPa). The calculation of these characteristics in accordance with the given formulas is presented in table. 3. The required volume of fatigue tests to construct the endurance limit distribution curve is determined using the formula in Appendix 6. Table 3 Calculation of the average value and standard deviation of the limited endurance limit of samples from an AB alloy Interval boundaries, Middle of the interval Probability value (4_l) ,■ ■ Oh.! [(h_1> ,■ - 4_ll 2 (a /, kgf/mm 2 (MPa) at the boundaries of the interval 12.106 kgf/mm 2 (121.06 MPa); ^ D P i [(st_ 1) g - - o_ 1 ] 2 = 0.851; S„ = ^Gp5G = 0.922 kgf/mm 2 (9.22 MPa) PROTOCOL No. sample testing (appendix to consolidated protocol No. Purpose of the test_ Sample: cipher material_ hardness_ Machine: type Cycle voltages: maximum_ Cycle deformations: maximum_ average_ Meter readings (date and time): at the beginning of the test_ at the end of the test_ transverse dimensions Heat treatment_ Microhardness_ Recording scale: deformation (mm/%) load (mm/MN)_ minimum amplitude minimum amplitude The number of cycles passed before the formation of a microcrack of length Number of cycles completed before failure Loading frequency_ Meter readings at the beginning of the shift at the end of the shift Number of cycles (time) completed by the sample per shift Signature and date who passed his shift who took over the shift Note The tests were carried out by_ Head of laboratory CONSOLIDATED PROTOCOL No._ Purpose of testing___ Material: make and condition_ fiber direction_ type of workpiece (for complex shapes, a sample cutting plan is included) Mechanical characteristics_ Test conditions: loading type_ type of loading_ test temperature_ loading frequency_ specimen type and nominal cross-sectional dimensions surface condition_ Testing machine: Date of testing: start of testing of the first sample_ end of testing of the last sample Responsible for testing this series of samples Head of laboratory Methods for determining the mechanical properties of metals are divided into: When testing tensile strength, tensile strength (σ in), yield strength (σ t), relative elongation (δ) and relative contraction (ψ) are determined. Tests are carried out on tensile testing machines using standard samples with cross-sectional area Fo and working (calculated) length lo. As a result of the tests, a tensile diagram is obtained (Fig. 1). The abscissa axis indicates the value of the deformation, and the ordinate axis indicates the value of the load that is applied to the sample. Rice. 1. Tension diagram It should be noted that when stretched, the sample elongates, and its cross-section continuously decreases. The true stress is determined by dividing the load acting at a certain moment by the area that the sample has at that moment. In everyday practice, true stresses are not determined, but conditional stresses are used, assuming that the cross section Fo of the sample remains unchanged. The yield strength (σ t) is the load at which plastic deformation occurs, related to the initial cross-sectional area of the sample (Рт/Fo). However, during tensile tests, most alloys do not have yield plateaus on the diagrams. Therefore, the conditional yield strength (σ 0.2) is determined - the stress to which a plastic deformation of 0.2% corresponds. The selected value of 0.2% quite accurately characterizes the transition from elastic to plastic deformations. The characteristics of the material also include the elastic limit (σ pr), which means the stress at which plastic deformation reaches a given value. Typically, residual strain values of 0.005 are used; 0.02; 0.05%. Thus, σ 0.05 = Ppr / Fo (Ppr is the load at which the residual elongation is 0.05%). Limit of proportionality σ pc = Ppc / Fo (Ppc is the maximum load, under the action of which Hooke’s law is still satisfied). Plasticity is characterized by relative elongation (δ) and relative contraction (ψ): δ = [(lk - lo)/lo]∙100% ψ = [(Fo – Fk)/Fo]∙100%, where lk is the final length of the sample; lo and Fo are the initial length and cross-sectional area of the sample; Fk is the cross-sectional area at the rupture site. For low-plasticity materials, tensile tests are difficult, since minor distortions during installation of the sample introduce a significant error in determining the breaking load. Such materials are usually subjected to bending testing. Regulations: Hardness is the ability of a material to resist the penetration of another, harder body, an indenter. The hardness of the material is determined by the Brinell, Rockwell, Vickers, and Shore methods (Fig. 2). Rice. 2. Schemes for determining hardness according to Brinell (a), Rockwell (b) and Vickers (c) The Brinell hardness of a metal is indicated by the letters HB and a number. To convert the hardness number to the SI system, use the coefficient K = 9.8 106, by which the Brinell hardness value is multiplied: HB = HB K, Pa. The Brinell hardness method is not recommended for use for steels with a hardness of more than HB 450 and non-ferrous metals with a hardness of more than 200 HB. For various materials, a correlation has been established between the ultimate strength (in MPa) and the hardness number HB: σ in ≈ 3.4 HB - for hot-rolled carbon steels; σ in ≈ 4.5 HB - for copper alloys, σ in ≈ 3.5 HB - for aluminum alloys. Hardness determination by the Rockwell method is carried out by pressing a diamond cone or steel ball into the metal. The Rockwell device has three scales - A, B, C. The diamond cone is used to test hard materials (scales A and C), and the ball is used to test soft materials (scale B). Depending on the scale, hardness is designated by the letters HRB, HRC, HRA and is expressed in special units. When measuring hardness using the Vickers method, a tetrahedral diamond pyramid is pressed into the metal surface (being ground or polished). This method is used to determine the hardness of thin parts and thin surface layers that have high hardness (for example, after nitriding). Vickers hardness is designated HV. The conversion of the hardness number HV to the SI system is carried out similarly to the conversion of the hardness number HB. When measuring hardness using the Shore method, a ball with an indenter falls onto the sample, perpendicular to its surface, and the hardness is determined by the height of the ball’s rebound and is designated HS. Kuznetsov-Herbert-Rehbinder method - hardness is determined by the damping time of the oscillations of a pendulum, the support of which is the metal under study. Impact strength characterizes the ability of a material to resist dynamic loads and the resulting tendency to brittle fracture. For impact testing, special samples with a notch are made, which are then destroyed on a pendulum impact driver (Fig. 3). Using the pendulum pile driver scale, the work K spent on destruction is determined, and the main characteristic obtained as a result of these tests is calculated - impact strength. It is determined by the ratio of the work of destruction of the sample to its cross-sectional area and is measured in MJ/m 2. To designate impact strength, use the letters KS and add a third, which indicates the type of cut on the sample: U, V, T. The notation KCU means the impact strength of a sample with a U-like notch, KCV - with a V-like notch, and KCT - with a crack , created at the base of the cut. The work of destruction of a sample during impact tests contains two components: the work of crack initiation (Az) and the work of crack propagation (Ar). Determining impact strength is especially important for metals that operate at low temperatures and exhibit a tendency to cold brittleness, that is, a decrease in impact strength as the operating temperature decreases. Rice. 3. Scheme of a pendulum pile driver and impact sample When performing impact tests on notched samples at low temperatures, the cold brittleness threshold is determined, which characterizes the effect of a decrease in temperature on the tendency of the material to brittle fracture. During the transition from ductile to brittle fracture, a sharp decrease in impact strength is observed in the temperature range, which is called the temperature threshold of cold brittleness. In this case, the structure of the fracture changes from fibrous matte (ductile fracture) to crystalline shiny (brittle fracture). The cold brittleness threshold is designated by a temperature range (tb. – txr.) or one temperature t50, at which 50% of the fibrous component is observed in the fracture of the sample or the value of impact strength is reduced by half. The suitability of a material for operation at a given temperature is judged by the temperature margin of viscosity, which is determined by the difference between the operating temperature and the transition temperature of cold brittleness, and the larger it is, the more reliable the material. Fatigue is the process of gradual accumulation of damage to a material under the influence of repeated alternating stresses, which lead to the formation of cracks and destruction. Metal fatigue is caused by the concentration of stress in its individual volumes (in places of accumulation of non-metallic and gas inclusions, structural defects). The ability of a metal to resist fatigue is called endurance. Fatigue tests are carried out on machines for repeated-alternating bending of a rotating sample, fixed at one or both ends, or on machines for testing tension-compression, or for repeated-alternating torsion. As a result of the tests, the endurance limit is determined, which characterizes the material’s resistance to fatigue. Fatigue limit is the maximum stress under which fatigue failure does not occur after a basic number of loading cycles. The endurance limit is denoted by σ R, where R is the cycle asymmetry coefficient. To determine the endurance limit, at least ten samples are tested. Each specimen is tested at only one stress to failure or at a base number of cycles. The basic number of cycles must be at least 107 loads (for steel) and 108 (for non-ferrous metals). An important characteristic of structural strength is survivability under cyclic loading, which is understood as the duration of operation of a part from the moment of initiation of the first macroscopic fatigue crack of 0.5...1 mm in size until final destruction. Survivability is of particular importance for the operational reliability of products, the trouble-free operation of which is maintained through early detection and prevention of further development of fatigue cracks. 1. Tensile test These tests determine such characteristics as the limits of proportionality, elasticity, strength and ductility of metals. In a tensile test, the sample is stretched under the action of a gradually increasing load and brought to failure. The tensile diagram in the coordinates “load P - elongation?l” reflects characteristic sections and points. In the section 0 - P pts, the elongation of the sample increases in direct proportion to the increase in load. When the load increases above Pc, in the section Pc - P control, direct proportionality is violated, but the deformation remains reversible. In the area above the point P vpr, noticeable residual deformations occur, and the tensile curve significantly deviates from the straight line. Under load Pt, a horizontal section of the diagram appears - the yield plateau T-T 1. There is no yield plateau on the tensile curves of brittle metals. Above point Pt, the load increases to point A, corresponding to the maximum load P in, after which it begins to fall, associated with the formation of local thinning of the sample. Then the load drops to point B, where the sample fails. Tensile diagram of a sample made of plastic material The elastic limit at control is the stress at which plastic deformations first reach a certain small value, characterized by a certain tolerance: where P control is the stress corresponding to the elastic limit, N. Ultimate strength y - stress equal to the ratio of the greatest load preceding the destruction of the sample to its original cross-sectional area: where Pv is the stress corresponding to the tensile strength, N. Relative elongation d is found as the ratio of the increase in the length of the sample after rupture to its original calculated length, expressed as a percentage: where l k is the length of the sample after rupture, mm; l 0 - estimated (initial) length of the sample, mm. 2. Methods for determining hardness The most common method for determining the hardness of metallic materials is the indentation method, in which another, harder body is pressed into the test surface under the action of a constant static load. An imprint remains on the surface of the material, the size of which determines the hardness of the material. The hardness index characterizes the material’s resistance to plastic deformation, usually large, under local contact load application. Brinell hardness test. The essence of this method is that a hardened steel ball with a diameter of 10, 5 or 2.5 mm is pressed into the surface of the test metal, depending on the thickness of the sample under the influence of a load, which is selected depending on the expected hardness of the test material and the diameter of the tip according to the formulas: P = 30D 2 ; P = 10D 2 ; P = 2.5D 2 . An imprint is left on the surface of the sample, the diameter of which is used to determine the hardness. The diameter of the print is measured with a special magnifying glass with divisions. Hardness is calculated using the formula where HB is Brinell hardness, kgf/mm 2; F - area of the resulting print, mm 2; D - tip diameter, mm; d - imprint diameter, mm. Hardness measurement by Brinell (a), Rockwell (b), Vickers (c) methods Rockwell hardness measurement. The measurement is carried out by pressing a steel ball with a diameter of 1.588 mm or a diamond cone with an apex angle of 120° into the test metal. Rockwell hardness is determined by the depth of indentation of the tip. Indentation is carried out under the action of two sequentially applied loads - preliminary, equal? 100 N, and the final (total) load equal to 1400, 500 and 900 N. Hardness is determined by the difference in indentation depths of the prints. Different types and grades of metals and alloys are used for different products. The choice is usually based on the characteristics of the materials. When designing any structure, the properties and testing of the metals to which it has been subjected are taken into account. Tests performed on various types of metals help determine the mechanical, thermal, and chemical properties of metals. Accordingly, depending on the revealed properties of the metal, certain types of tests are carried out. We will talk further about what properties and tests of metals are of great importance, and what they are. Each type of metal has a certain set of properties - mechanical, technological and operational, which characterize its ability to heat and cool, weld, resistance to heavy loads, etc. The most important of them are the following: All these properties are revealed during tests: mechanical, chemical and others. When carrying out such tests, different loads are applied to the metal - dynamic (impact increase in stress in the metal) or static (gradual increase in stress). During loads, different types of stress can arise in the metal: For example, when twisting a metal, shear stress occurs in the material, while extension or bending simultaneously leads to compressive and tensile stress. According to these loads and the resulting stress, the following types of mechanical tests can be carried out: In addition, mechanical tests involve checking for material fatigue (usually during bending), deep drawing and creep. Hardness tests are also carried out, which are carried out using the indentation method and the dynamic method (a striker with a diamond tip is dropped onto the metal). Chemical testing methods are used to determine the composition of the metal, its quality, etc. During such tests, the presence of unnecessary and unwanted impurities, as well as the amount of alloying impurities, is usually revealed. Chemical tests also help to assess the metal's resistance to various reagents. One type of such test is selective exposure to certain chemical solutions. This helps to determine indicators such as porosity, number of inclusions, segregation, etc. Contact fingerprint testing is necessary to determine the level of phosphorus and sulfur in a metal. Seasonal cracking of metal is determined using special solutions to which the material is exposed. A number of other tests are also being carried out. During the tests, the metal is not only subjected to various types of influences, but also carefully examined under a microscope. Such studies make it possible to evaluate the quality of the metal, its suitability, structural characteristics, etc. In addition, metals are subjected to radiographic testing. These studies are carried out using gamma radiation and hard x-rays. Such control allows you to determine existing defects in the metal. Welded seams are often subjected to radiographic examination. There are also a number of other control methods to which the metal is subjected. Among them: All these tests, including control ones, are very important: they help determine which metals are suitable for different structures, what treatments the material can be subjected to, what welding modes to use, etc. Textbook for vocational schools. - M.: Mechanical Engineering, 1990. - 256 p.: ill. — ISBN 5-217-00830-X. The fundamentals of the theory of strength and ductility of metals and alloys are presented in an accessible form. The device, principle of operation, rules of operation of instruments and equipment for testing and flaw detection are considered. The mathematical foundations for processing measurement results are presented. The textbook can be used to train workers in production. Safety, fire safety and industrial sanitation
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- static, when the load increases slowly and smoothly (tensile, compression, bending, torsion, hardness tests);
- dynamic, when the load grows at high speed (impact bending tests);
- cyclic, when the load changes repeatedly in magnitude and direction (fatigue tests).Tensile test
Ultimate strength (σ in) is the maximum load that the material can withstand without destruction, related to the initial cross-sectional area of the sample (Pmax/Fo).Hardness test
A
b
V
Impact test
Fatigue test
Properties of metals.
Mechanical testing of metals.
Chemical testing of metals.
Optical and physical tests.
Basic safety information.
Fire safety.
Industrial sanitation.
Basic properties of materials
Raw metal materials. Basic information about the production of metals and alloys.
Basic properties of metals and alloys.
Non-metallic materials, their properties and applications.
Fundamentals of the theory of elastic and plastic deformation and fracture
General characteristics and atomic-crystalline structure of metals and alloys.
The concept of stress-strain state.
Elastic and plastic deformation.
The influence of temperature on the strength and ductility of metals and alloys.
Information about the destruction process.
Mechanical testing of metals and alloys
Classification of test methods.
Static tests.
Impact bending tests.
Fatigue tests.
Long-term strength and creep tests.
Hardness measurement.
Equipment and instruments for mechanical testing
Classification of equipment and instruments for mechanical testing.
Design and principle of operation of machines for static tests.
Design and principle of operation of impact testing machines.
Design and principle of operation of machines for repeatedly variable loads (fatigue tests).
Design and principle of operation of machines for special tests.
Instruments for measuring hardness.
Control and measuring equipment used during testing.
Non-destructive testing methods. Determination of physical properties of metals and alloys
Classification of non-destructive testing methods.
Defects in metals and alloys, the causes of their occurrence.
Thermal methods for detecting defects.
Thermal analysis of phase transformations in metals and alloys.
Thermal analysis at high temperatures.
Thermal analysis at high heating and cooling rates.
Calorimetric analysis.
Dilatometric method.
Magnetic methods.
Electrical methods.
Parametric eddy current method.
Acoustic methods.
Penetrant control methods.
Leak detection methods.
Radiographic and radioscopic methods.
Testing of non-metallic materials
Testing of building materials and products.
Testing of textile materials.
Testing of plastics.
Special types of tests
Tests on the machinability of metals by cutting.
Technological tests.
Testing of metalworking tools.
Basic information about standardization, metrology and product quality control
State standards and metrology.
Standardization and product quality.
Standards for testing materials and finished products.
Requirements for test samples and methods for processing test results
Samples and production of test specimens from them.
Statistical processing of test results.
Registration of test results.
Bibliography
