Detail of GB/T 41154-2021

Introduction of GB/T 41154-2021

1 Scope This document specifies the test apparatus, specimens, test procedures, test results and test reports for multi-axial thermo-mechanical fatigue testing of metallic materials under axial-torsional strain control. This document applies to thin-walled metal tube specimens under constant axial mechanical strain cycles, constant shear strain cycles and constant temperature cycles, corresponding to any constant cyclic strain ratio, constant axial mechanical strain-shear strain phase difference and constant axial mechanical strain-temperature phase difference. Note 1: The number of cycles is usually considered in the context of low cycle fatigue, i.e. fatigue life N ≤ 105. Note 2: The temperature of the test does not normally exceed 1 200 C. 2 Normative references The contents of the following documents constitute essential provisions of this document through the normative references in the text. Among them, note the date of the reference documents, only the corresponding version of the 8 issues apply to this document; do not note the date of the reference documents, the latest version (including all the revision of the list) applies to this document. 3 Terms and definitions The terms defined in GB/T 10623, GB/T 33812 and GB/T 40410 and the following terms and definitions are applicable to this document. 3.1 Axial-torsional thermo mechanical fatigueaxial-torsional thermo mechanical fatigue; AT-TMF Fatigue behaviour in which axial mechanical strain cycles, shear strain cycles and temperature cycles are simultaneously present. 4 Symbols and descriptions The symbols in Table 1 apply to this document. 5 Testing apparatus 5.1 Testing machines 5.1.1 General requirements 5.1.1.1 The test shall be carried out on an axial torsion test machine with a smooth start and no backlash when the force and torque are past zero. The tester shall be capable of controlling axial and shear strains according to a specified waveform, which may be hydraulically, electronically or mechanically loaded. 5.1.1.2 The load frame shall have a high lateral stiffness when the crossbeam is in the working position and the loading chain is accurately aligned (parallel and concentric). 5.1.1.3 The entire loading chain (including axial force transducer, torque transducer, tie rod/clamp and specimen) should have a high lateral stiffness to reduce bending of the specimen. 5.1.2 Axial force and torque measurement systems 5.1.2.1 The axial force transducer shall be suitable for the axial force applied during the test and the torque transducer shall be suitable for the torque applied during the test. 5.1.2.2 The axial force transducer and torque transducer shall be capable of temperature compensation, and the change in zero point drift or temperature sensitivity shall not exceed 0.002% of the full range of the transducer for every 1C change in temperature. 5.1.3 Specimen clamping device 5.1.3.1 The clamping device shall have no backlash when transmitting cyclic axial force and torque values to the specimen during the test, and its geometrical characteristics shall ensure that the coaxiality meets the requirements of 5.1.4. Note: The best design solution is to keep the number of mechanical connections to a minimum. 5.1.3.2 The clamping device shall ensure repeatability of the coaxiality of a series of subsequent specimens. 5.1.3.3 The material of the clamping device should be selected for proper operation over the entire range of test conditions. 5.1.4 Coaxiality of the loading chain The test machine frame (including the clamping device) shall be calibrated for coaxiality using a specimen whose geometry is as similar as possible to that of the specimen with the axial-torsional extensometer in the test. The maximum test bending strain permitted shall not be greater than 50 micro-axial strains at zero load or 5% of the added axial mechanical strain, whichever is greater. The above corrections shall be made every 12 months or when a) as part of the in-service testing of new machines; b) after accidental damage to the specimen, unless it can be shown that the coaxiality is not affected. c) if any adjustments have been made to the loading chain. 5.2 Strain measurement f on systems 5.21 The axial and shear strain of the specimen shall be measured using an axial-torsional extensometer. 5.22 Axial-torsional extensometers shall be suitable for measuring dynamic strains over long test cycles with minimal drift, slippage or hysteresis of the instrument. The axial deformation and torsional deformation should be measured directly over the specimen's standard length. 5.23 The sensor of the axial-torsional extensometer shall be protected against drift due to thermal fluctuations. 5.24 In view of the transient nature of the axial-torsional thermo-mechanical fatigue test temperature, active cooling of the axial-torsional extensometer is desirable to ensure that the sensor section of the axial-torsional extensometer is kept at a constant temperature during the test. 5.2.5 The dynamic design of the contact axial-torsional extensometer should ensure that the axial-torsional extensometer does not slip at the point of contact or at the cutter when the specimen area in contact with it moves laterally or at an angle. 5.2.6 The contact pressure and operating forces of the axial-torsional extensometer should not damage the surface of the specimen and should not cause cracks to develop in the contact or cutter area of the axial-torsional extensometer. 5.3 Heating system 5.3.1 The heating system should be capable of providing the maximum heating and cooling rates for a series of axial-torsional thermo-mechanical fatigue tests. 5.3.2 In order to reduce the radial temperature gradient of the specimen produced by the direct induction heating system, it is advisable to select a heating device with a sufficiently low frequency (generally in the range of a few hundred kilohertz or less). This will help to reduce the skinning effect during heating. 5.3.3 Thermocouples, pyrometers or other temperature measuring devices should be used during the test. An infrared thermometer or other temperature measuring device should be used to measure the temperature of the specimen. 5.3.4 For thermocouples, direct contact with the specimen should be ensured and no failure should occur at the contacts. Note: Common methods of attachment are spot welding or strapping and crimping of the thermocouple to the surface of the specimen outside the scale. 5.3.5 If an optical pyrometer is used to measure the temperature of the spaced section, steps should be taken to calibrate the possible changes in the thermal radiation of the specimen during the duration of the test. Possible methods include a two-colour pyrometer and a pre-oxidation treatment of the specimen surface. 5.4 Test monitoring equipment 5.4.1 An automated system capable of digital acquisition and processing of axial force, torque, axial deflection, torsional deflection, temperature and cyclic data is desirable. Data points should be sampled at a frequency that ensures the correct characterisation of the hysteresis return line and in particular the reversal region. Different data acquisition methods will affect the number of data points required per line. Typically 200 points per line are required. 5.4.2 Other analogue systems capable of measuring the same data can also be selected, but should include: Two X-Y-Y recorders, one for recording hysteresis returns for axial force, axial deformation and temperature, and the other for recording hysteresis returns for torque, torsional deformation and temperature; A continuous recorder for several time-dependent parameters: axial force, axial deformation, torque, torsional deformation and temperature; A peak detection device for each signal; a cycle counter, etc. 5.4.3 The recorder may be replaced by a storage device capable of reproducing the recorded signals in photographic or analogue form. These devices should record signals at a rate greater than the maximum rotation rate of the recorder and allow the temporary recordings to be played back later at a slower rate. 6 Specimens 6.1 General requirements This document is based on thin-walled tubes of circular cross-section as specimens for testing. In order to avoid instability under cyclic loading, the wall thickness of the specimen in the distance section should meet the thin-walled tube criterion of an average diameter to wall thickness ratio greater than or equal to 10:1. For polycrystalline materials, the presence of at least 10 grains in the wall thickness section should be ensured to maintain isotropy. 6.2 Material requirements The tensile and compressive properties of the specimen should be predicted. When designing specimens, tensile and compressive properties should be determined in accordance with GB/T 228.1, GB/T 228.2 and GB/T 7314. If the average grain size of the material needs to be determined, it should be determined in accordance with GB/T 6394. The recommended specimen shape is shown in Figure 1. The specimen should be designed to ensure concentricity and to avoid additional bending stresses. 7 Test procedure 7.1 Laboratory environment The accuracy of the axial-torsional thermomechanical fatigue test results is related to the test method and environment used. The test should be carried out under the following suitable conditions: . A constant ambient temperature; Minimal atmospheric contamination (e.g. dust, chemical vapours, etc.); No external electrical signals that could affect the control and data acquisition of the test machine; minimal external mechanical vibrations. 7.2 Specimen mounting The specimen should be clamped with minimal external force, taking care not to scratch the inner and outer surfaces of the tubular specimen scale section during the installation of the contact axial-torsional extension. 7.3 Temperature control 7.3.1 The temperature cycle should remain stable throughout the test. NOTE: See reference [2] for the importance of maintaining a steady temperature cycle throughout the test. 7.3.2 The hysteresis of the axial thermal strain (eu) shall not be greater than 5% of the corresponding axial thermal strain range (Oeu) at any given temperature point (T) of the temperature cycle under zero load conditions. 7.3.3 The temperature displayed by the temperature control device (e.g. thermocouple) at any given point in the cycle shall not deviate from the preset value by more than the greater of 5C or 1% of the stable temperature (i.e. established dynamic equilibrium of temperature) range throughout the test period. 7.3.4 The temperature displayed by the non-controlled temperature sensor at any given point in the cycle shall not exceed ±3C of the stable temperature value throughout the test period. 7.4 Temperature gradient 8 Test results 8.1 Initial data The modulus of elasticity and shear modulus of the specimen can be measured as a function of temperature according to Appendix A. The relationship between the axial thermal strain of the specimen and the temperature can be expressed in the form of a graph. 8.2 Processing of recorded data At a minimum, typical plots of axial stress, axial mechanical strain, shear stress, shear strain, temperature versus time, axial mechanical strain versus axial stress, temperature, and shear strain versus shear stress, temperature should be plotted. 8.3 Determination of life The same failure criterion shall be used for the same series of tests. 8.4 Presentation of test results 8.4.1 Expression of individual test results The expression of individual test results shall include plots of axial stress, axial total strain feedback, shear stress, shear strain feedback, temperature and axial thermal strain versus time, as well as hysteresis return lines for typical cycles, axial mechanical strain versus axial stress and temperature, and shear strain versus shear stress and temperature. Maximum and minimum axial stresses, shear stress, total axial strain feedback, shear strain feedback and temperature for each recorded cycle from start to failure should also be included. 9 Test report The test report shall include the following: a) The number of this document; b) Material grade and specification number, manufacturer, furnace batch number, material specification, heat treatment regime; c) Microstructure and grain size along the length and thickness of the thin-walled tube, chemical composition and general mechanical properties of the material (if applicable); d) shape, dimensions, surface condition of the specimen, etc; e) basic information on the test equipment; Appendix A (informative) Determination of material constants

Contents of GB/T 41154-2021

1 Scope 2 Normative references 3 Terms and definitions 4 Symbols and descriptions 5 Testing apparatus 6 Specimens 7 Test procedure 8 Test results 9 Test report Appendix A (informative) Determination of material constants