GB/T 46591.1-2025Metallic materials — In-situ testing method — Part 1: Tensile test (English PDF)

3.1 In-situ tensile test

A test method that combines microstructural characterization techniques while simultaneously applying tensile load to obtain data on the microstructural evolution and failure process of a material in real time.

3.2 Hold time

The time during which the specimen is held under a constant loading state.

NOTE. during this period, the specimen undergoes specific microstructural evolution, stress relaxation, creep, dislocation movement, etc., without further loading or unloading.

3.3 Interrupted test

In an in-situ tensile test, the specimen is held at a specific displacement (strain) or load (stress) level, and during the hold time, the microstructure of the specimen is observed, characterized, or otherwise analyzed.

3.4 Strain field

The spatial distribution of strain at various points in a material under external force due to deformation.

NOTE. it describes the degree of deformation of a material in space and is usually expressed by the strain tensor, including normal strain (tension or compression) and shear strain (shear deformation), etc. The distribution of the strain field is closely related to external loading conditions, the geometrical shape of the material, and its microstructure, and is one of the modes to study the deformation behavior and mechanical response of a material.

3.5 Deformation mechanism

The basic process and mechanism by which a material undergoes plastic deformation under external force.

NOTE. common deformation mechanisms include slip, twinning, and phase transformation. These deformation mechanisms may simultaneously or alternately occur under different materials and loading conditions, collectively determining the overall deformation behavior of the material.

3.6 Failure mechanism

In tensile test, the specific process and cause of material damage accumulation and fracture due to external force.

NOTE. these failure mechanisms are usually closely related to the material’s microstructure, the application of external force, temperature, and other factors, and are an important constituent part of the material failure process in tensile test. Common failure mechanisms include plastic failure, cleavage cracking, grain boundary cracking, and void evolution, etc.

5.1 Geometric Shape of Specimens

5.1.1 The geometric shape of specimens depends on the shape and size of the metal product under test and shall be compatible with the dedicated in-situ mechanical loading device and its matching strain acquisition device, environmental accessories, and microscopic characterization devices.

5.1.2 While meeting the requirements for microscopic observation, the shape and size of the specimens should preferably meet the requirements of GB/T 228.1, GB/T 228.2 and GB/T 228.3 Recommended tensile specimen forms include plate specimens, rod specimens, tubular specimens, and longitudinally arc-shaped specimens, etc. Specific shapes and sizes are given in Appendix A.

5.1.3 The size selection of the specimens shall meet the measuring range requirements of the in-situ mechanical loading device used (the maximum test force is recommended to be around 2/3 of the measuring range), and the center plane of the specimens shall be coplanar with the loading plane and the center plane of the force sensor.

5.2 Specimen Preparation

Rough specimens shall be cut, and specimens shall be prepared in accordance with the relevant product standard requirements of the material. The surface condition of the specimens shall meet the general requirements of the microscopic characterization instrument used.

5.3 Specimen Storage and Transportation

Specimens shall be protected from scratches, oxidation and bending deformation, etc. They should preferably be stored in individual boxes or pipes with sealed heads. Polished specimens shall be stored in vacuum bottles or desiccators containing silica gel.

6.1 General Requirements

Test equipment generally includes a dedicated in-situ mechanical loading device and auxiliary strain acquisition device, high and low temperature environmental loading device, and supporting microscopic characterization device. Typically, the loading device, acquisition device, and environmental loading device are installed inside or deeply integrated with the microscopic characterization device.

6.2 In-situ Mechanical Loading Device

6.2.1 In-situ mechanical loading devices typically employ bidirectional symmetrical loading to

fix the specimen’s center position relative to the microscopic characterization device, i.e., the observation point position remains unchanged, which facilitates the characterization within a fixed area.

6.2.2 If a single-end actuated mechanical loading device is used, it shall be used in conjunction with a two-dimensional platform that fixes the loading device. Before characterization begins, the position of the loading device or the detector of the microscopic characterization device shall be appropriately moved to ensure that the observation area on the specimen remains essentially unchanged. For special microscopic characterization devices, additional moving mechanisms can be added to the in-situ mechanical loading device as needed, for example, the Eulerian circulation mechanism used in in-situ neutron diffraction test.

6.2.3 The force measurement system of the in-situ mechanical loading device shall meet the accuracy requirements of Grade 0.5 in GB/T 16825.1.During high and low temperature tests, the force measurement system shall be capable of temperature compensation. During the test using loading devices with high and low temperature environmental accessories, it shall be ensured that the force sensor is kept within the specified allowable temperature range.

6.2.4 The in-situ mechanical loading device shall have sufficient stiffness and shall be able to perform uniform stretching on the specimen under displacement (or strain) and load control.

The loading device shall have the function of interrupting the test, and the fluctuation value of load or displacement (strain) during the hold time shall be within 0.5% of the target value.

6.2.5 The clamps of the in-situ mechanical loading device shall be able to smoothly transfer the applied load to the specimen axis. The adjustable distance between the two clamps shall meet the clamping requirements of the in-situ tensile specimens. In accordance with GB/T 34104, check the coaxiality of the loading to ensure good alignment in each loading axis. For specimens that cannot be directly clamped, secondary clamps can be used to avoid obstructing the microscopic observation process. For rotatable clamp structures, eccentricity shall be avoided.

For clamps that use grooves machined on the clamping surface to hitch and fix the specimen, the shape and position tolerance accuracies of the clamp grooves and the transition section of the specimen shall be ensured, and the hardness and strength of the clamp material shall be higher than that of the specimen under test.

6.2.6 The main body weight of the in-situ mechanical loading device shall meet the load-bearing capacity requirements of the objective table of the microscopic characterization device, and the shape and size of the main body shall ensure that it does not interfere with the accessories of the in-situ microscopic characterization device during the test. The in-situ mechanical loading device shall not produce oil stains during the test. The material selection and surface treatment process shall also take into account factors such as vacuum and irradiation to avoid affecting the vacuum system, electronic probe and other components of the microscopic characterization device, or causing absorption, background scattering, irradiation swelling or the generation of radioactive isotopes. Common in-situ mechanical loading devices are listed in Bibliography [7] ~ [10].

6.3 Strain Acquisition Device

6.3.1 General requirements

Where space and measurement permit, it is advisable to use a strain acquisition device to achieve accurate strain measurement. Common strain acquisition methods applied to in-situ tensile test include contact strain measurement devices (for example, mechanical extensometers) and non-contact strain measurement devices (such as video extensometers and full-field strain measurement devices). Where the test device permits, other strain measurement methods, such as strain gauges and laser strain sensors, can also be used.

6.3.2 Contact strain measurement devices

Mechanical extensometers shall meet the accuracy requirements of Grade 0.5 as specified in GB/T 12160.The selected extensometer shall have a relatively small volume, light weight, and low clamping force. The extensometer shall be fixed to the back of the microscopic characterization measurement surface, or away from the measurement optical path, to reduce the influence of the microscopic characterization process. During clamping, care shall be taken to adjust the contact force between the extensometer and the specimen to avoid scratching the specimen surface. The extensometer shall be able to avoid signal drift caused by temperature fluctuations.

6.3.3 Non-contact strain measurement devices

Video extensometers or full-field strain measurement devices shall comply with the verification and calibration procedures specified in JJG 762 or GB/T 12160.For video extensometers, it is recommended to use scribing, dot, or specimen surface texture identification methods to simplify the operation process and reduce the influence on the microscopic characterization process. The selection of the gauge length shall comply with the requirements of GB/T 228.1.

If the video extensometer requires speckle marking, or if a full-field strain measurement device is used to obtain a full-field strain cloud map, then attention shall be paid to spraying a uniform speckle pattern onto the gauge length of the specimen before the test. When the observation method is an optical microscope and a charge-coupled device (CCD) camera, the measurement method should refer to GB/T 38684, and speckle spraying should be performed with reference to GB/T 38719.For scanning electron microscope, the strain field can be obtained using surface texture, and special speckles can also be prepared to achieve higher resolution and more microscopic characterization information.

6.4 High and Low Temperature Environmental Loading Device

6.4.1 The heating mode shall be selected in accordance with the characteristics of the in-situ mechanical loading device, strain acquisition device, and microscopic characterization device.

Possible methods include resistance wire heating, contact heating, radiation heating, halogen lamp heating, laser heating, electromagnetic induction heating, etc. Since in-situ environmental loading devices are usually small in size, special attention shall be paid to temperature uniformity. The heating device shall ensure that the gauge length of the specimen can be uniformly heated to the specified temperature, and the temperature gradient of the material in all directions within the gauge length is not greater than 2 C or 1% of the nominal test temperature, whichever is greater. Care shall be taken to ensure that after startup, the heating device cannot damage the microscopic characterization device, nor shall it affect the normal operation of the microscopic characterization device.

6.4.2 The cooling mode shall be selected based on the characteristics of the in-situ mechanical loading device, strain acquisition device, and microscopic characterization device. Possible methods include contact cooling and liquid nitrogen cooling. The cooling source is liquid nitrogen, liquid helium, or Peltier semiconductor. The cooling device shall ensure that the gauge length of the specimen is uniformly cooled to the specified temperature. During the entire test, the temperature gradient of the gauge length shall not exceed 2 C or 1% of the nominal test temperature, whichever is greater. Care shall be taken to ensure that after startup, the cooling device cannot damage the microscopic characterization device, nor shall it affect the normal operation of the microscopic characterization device.

6.4.3 Temperature uniformity shall be ensured. When the gauge length is L, the length of the uniform-temperature zone shall not be less than 1.2L. The observation area shall be located at the center of the uniform-temperature zone, and the ratio of the volume of the uniform- temperature zone to the volume of the observation area shall not be less than 3.1 to reduce the influence of boundary effects. The temperature difference between any two points within the uniform-temperature zone shall not exceed 5 C. Thermocouples or other temperature measurement instruments are typically used to measure the temperature of the specimen’s uniform-temperature zone. If a non-contact temperature measurement device is selected, its reliability of temperature measurement shall be verified using a thermocouple as a reference under the same test conditions. On the premise of not affecting the test results (e.g., changes in stress-strain state or microstructure at the connection between the thermocouple and the miniature specimen shall be avoided), the specimen shall be in direct contact with the thermocouple. Thermocouples are usually connected to the specimen by spot welding. When using resistance wire heating, the thermocouple weld joints shall be protected from direct radiation. It shall be ensured that there is at least one sensor, independent of the control channel, ......