GB/T 38534-2020Test method for tensile properties of oriented fibre reinforced polymer matrix composite materials at ultra-low temperature (English PDF)

4.1 Testing machine

The testing machine should meet the requirements of GB/T 1446.

4.2 Fixtures, tie rods, cryostat frame

Materials with high strength, good toughness and low thermal conductivity should be used to make fixtures, tie rods, and cryostat frames, such as austenitic stainless steel

Steel, martensitic steel, forged nickel-based superalloy and titanium alloy, etc.

4.3 Cooling device

4.3.1 Basic requirements

The cooling device should be able to cool the sample to the specified temperature. The specified temperature is obtained by cooling with a cooling medium (such as liquid helium, etc.) or a refrigeration device (such as

Refrigerator) is obtained by cooling.

4.3.2 Ultra-low temperature thermostat

4.3.2.1

The frame of the cryostat should match the testing machine and can accommodate the cryostat.

4.3.2.2

The ultra-low temperature thermostat should use a stainless steel vacuum Dewar with a multi-layer insulation structure, and can ensure the temperature deviation of the test area

Does not exceed ±3°C. The length of the isothermal zone should not be less than the length of the parallel section between the clamping ends of the specimen.

4.3.2.3

The cryostat shall not affect the coaxiality between the tensile force and the geometric axis of the specimen.

4.3.3 Auxiliary equipment

The cryostat and the cooling medium infusion pipe should be vacuum insulated. If a cooling medium is used for cooling, it should be equipped with a cooling medium infusion pipe,

Auxiliary equipment such as vacuum pumps, cooling medium Dewar flasks, nitrogen cylinders, etc.; if a refrigerator is used for cooling, auxiliary equipment such as circulating cooling water should be equipped.

Note. When operating the cooling medium, the test personnel shall take safety precautions in accordance with relevant regulations in advance to avoid personal injury and damage to the test equipment and test equipment.

Kind of damage.

4.4 Temperature measuring instrument

The temperature measuring instrument consists of a thermometer and an indicating device. Its resolution should be equal to or better than 1°C, and the thermometer should cover the working temperature range

And periodically calibrate.

4.5 Low temperature strain measuring device

4.5.1

Use extensometers or other low temperature strain measuring devices that can be used in low temperature environments to measure strain, and the measurement accuracy should meet

Requirements of GB/T 1446.

4.5.2

The extensometer should be calibrated regularly under room temperature and actual test temperature. Refer to Appendix A for the calibration of extensometer at low temperature.

4.5.3

When using strain gauges at low temperatures, the adhesive pasting the strain gauges should match the strain gauges and specimens.

5.1 Shape and size

The sample shape is shown in Figure 1, and the sample size is shown in Table 1.

7.1 Preparation before test

Check the appearance of the sample according to GB/T 1446, and number the sample. Measure and record the width of 3 different sections of the sample working section

Take the arithmetic mean of the degree and thickness, respectively, and measure the accuracy to 0.01mm.

7.2 Sample installation

7.2.1

When installing the sample in the cryostat, the signal wire of the instrument should be sufficiently loose.

7.2.2

The center line of the specimen shall be consistent with the center line of the fixture and the center line of the connecting rod.

7.2.3

After the installation of the sample is completed, the sample should be pre-loaded, and always keep the pre-load not exceeding 5% of the maximum breaking load.

7.3 Installation of temperature measuring device

7.3.1

When a gaseous cooling medium or refrigerator is used for cooling, a temperature measuring point should be installed at both ends and the middle of the sample working section, and

Keep the temperature measuring head in good contact with the sample surface.

7.3.2

When the liquid cooling medium is used for cooling, the sample is completely immersed in the cooling medium, and the temperature can be measured at any point in it.

7.4 Installation of low temperature strain measuring device

7.4.1

When measuring strain with one or more extensometers, install the extensometers as far as possible in the middle of the length of the specimen working section.

7.4.2

When using strain gauges to measure tensile strain, the temperature compensation sheet method shall be used for measurement.

Note. The temperature compensation sheet method is to attach the resistance strain gauge to a piece of material that is the same material as the sample but does not participate in the deformation, and is at the same temperature bar as the sample

Pieces. Correctly connect the compensation sheet to the Wheatstone bridge circuit to eliminate the influence of temperature changes.

7.5 Cooling process

7.5.1

Before cooling, use an air jet or hot blower to thoroughly dry the cryostat.

7.5.2

Install a cryostat to cool the sample.

a) When a cooling medium with a boiling point lower than liquid nitrogen is used for cooling, the ultra-low temperature thermostat should be vacuumed first (the relative vacuum degree should be less than

-0.097MPa), then use liquid nitrogen to pre-cool it, then empty the liquid nitrogen, and finally enter the low boiling point into the ultra-low temperature thermostat

Cooling medium for liquid nitrogen;

b) When the refrigerator is used for cooling, the ultra-low temperature thermostat should be kept in a vacuum state (the relative vacuum degree should be less than -0.097MPa) until the test

The test is over.

7.5.3

After the sample is cooled to the specified test temperature, it should be kept for a certain period of time.

a) When the cooling medium is liquid, the holding time is not less than 10 minutes;

b) When the cooling medium is gaseous, the holding time is not less than 20min;

c) When the refrigerator is cooling, the holding time is not less than 30min.

7.5.4

When the cooling medium is gaseous or refrigerating, it is allowed to be between the specified test temperature value and the measured temperature display value at the middle position of the sample

The temperature deviation does not exceed ±2°C. During the test, the absolute value of the temperature difference between the two ends of the working section of the sample should not exceed 3°C.

7.5.5

When the test temperature is -196°C or -269°C, the whole sample can be immersed in liquid nitrogen (-196°C) or liquid helium (-269°C) to cool

However, obtain the required test temperature.

7.6 Loading

7.6.1

When the sample reaches the test temperature, zero the extensometer or strain gauge. When the extensometer or strain gauge is stable, the speed is 1mm/min~

Load continuously at a loading speed of 2mm/min until the specimen is broken, and record the load-strain (or load-displacement) curve and maximum load of the specimen.

Load, failure load and failure strain or strain that may be close to the instant of failure.

7.6.2

The failure mode description after the sample is destroyed, according to the provisions of GB/T 3354.If the specimen is damaged in the clamp or at the end, it shall be invalidated.

8.1 Tensile strength

The tensile strength is calculated according to formula (1), and the result retains 3 significant figures.

sigmat=

Pmax

Wh (1)

Where.

sigmat --- Tensile strength, in megapascals (MPa);

Pmax---the maximum load that the sample bears before failure, in Newton (N);

W --- sample width, in millimeters (mm);

h ---The thickness of the sample, in millimeters (mm).

8.2 Tensile modulus of elasticity

The tensile modulus of 90° sample is calculated according to formula (2) or formula (3) within the longitudinal strain range of 0.0005 to 0.0015, and the tensile elasticity of other samples

The modulus is calculated according to formula (2) or formula (3) within the range of 0.001 to 0.003 longitudinal strain, and the result retains 3 significant digits.

Et=

deltaPl

Whdeltal (2)

Et=

deltasigma

deltaepsilon (3)

Where.

Et --- Tensile modulus of elasticity, in megapascals (MPa);

l --- The gauge length of the extensometer in the working section of the sample, in millimeters (mm);

deltaP --- load increment, in Newton (N);

deltal ---The deformation increment within the gauge length of the extensometer corresponding to deltaP, in millimeters (mm);

deltasigma ---The increase in tensile stress corresponding to deltaP, in megapascals (MPa);

deltaepsilon ---Strain increment corresponding to deltaP, dimensionless.

8.3 Poisson's ratio

Poisson's ratio is calculated according to formula (4) within the same strain range as the tensile elastic modulus, and the result retains 3 significant figures.

µ12=

deltaepsilon horizontal

deltaepsilon vertical (4)

epsilon vertical =

deltalL lL (5)

epsilonHorizontal=

deltalT lT (6)

Where.

µ12 ---Poisson's ratio;

deltaepsilonlongitudinal --- the longitudinal strain increment corresponding to the load increment deltaP, see formula (5), dimensionless;

deltaepsilonHorizontal --- the transverse strain increment corresponding to the load increment deltaP, see formula (6), dimensionless;

lL --- the gauge length of the longitudinal extensometer, in millimeters (mm);

lT --- the gauge length of the transverse extensometer, in millimeters (mm);

deltalL---corresponding to the longitudinal deformation increment of deltaP, in millimeters (mm);

deltalT---corresponds to deltaP lateral deformation increment, the unit is millimeter (mm).

8.4 Tensile failure strain

The longitudinal tensile failure strain measured by the extensometer is calculated according to formula (7), and the result retains 3 significant figures.

epsilonlt=

deltalb (7)

Where.

epsilonlt ---longitudinal tensile failure strain, dimensionless;

deltalb --- the longitudinal deformation within the gauge length of the extensometer when the sample is broken, in millimeters (mm).

8.5 Test results

For each set of tests, calculate the arithmetic mean, standard deviation and dispersion coefficient of each measurement performance in accordance with the provisions of GB/T 1446.