GB/T 44030-2024Metallic materials - Compression test method at elevated temperature (English PDF)

1 Scope

The document specifies the principle, test pieces, test equipment, test conditions, determination of properties, rounding of test results, treatment of test results and test report for the compression testing of metallic materials at elevated temperature.

It applies to the determination, under uniaxial compression at a temperature above room temperature, of the proof strength for a non-proportional compressive extension, the proof strength for a total compressive extension, the upper compressive yield strength, the lower compressive yield strength, the compressive modulus of elasticity and the compressive strength.

2 Normative references

GB/T 7314 Metallic materials - Compression test method at room temperature; GB/T 8170 Rules of rounding off for numerical values and expression and judgement of limiting values; GB/T 10623 Metallic materials - Mechanical testing - Vocabulary; GB/T 12160 Metallic materials - Calibration of extensometer systems used in uniaxial testing; GB/T 16825.1 Metallic materials - Verification of static uniaxial testing machines - Part 1: Tension and/or compression testing machines - Verification and calibration of the force measuring system.

Dated references apply in the edition cited; undated references apply in their latest edition, including any amendments.

3 Terms and definitions

The terms and definitions of GB/T 7314 and GB/T 10623 apply, together with the following.

3.1 soaking time: the time for which the test temperature is held steady before the compressive force is applied.

4 Symbols and descriptions

Table 1 lists the symbols used in the document with their units and descriptions, grouped under test piece, deformation, force, proof and yield and compressive strength and modulus of elasticity, and temperature.

Test piece group, all in millimetres unless stated: original thickness of the test piece; original width of the test piece; original diameter of the test piece; height of the constraining device; length of the unconstrained part of a sheet test piece; length of the test piece; original gauge length of the test piece; and the original cross-sectional area of the test piece, in square millimetres.

Deformation group: the specified non-proportional compressive strain and the specified total compressive strain, both as percentages; the actual deformation of the original gauge length section under force and the deformation of the original gauge length section under force, both in millimetres; and the deformation magnification, dimensionless.

Force group, all in newtons: the actual compressive force, which for a test without lateral constraint equals the force at the upper end of the test piece; the force at the upper end of the test piece; the actual upper yield compressive force and the actual lower yield compressive force at yield; the friction force; the maximum actual compressive force during compression to failure for a brittle material, or the compressive force at the specified strain for a ductile material; and the actual compressive force at the specified non-proportional compressive deformation.

Strength and modulus group, all in newtons per square millimetre, with a note stating that one newton per square millimetre equals one megapascal: the compressive modulus of elasticity; the upper compressive yield strength; the lower compressive yield strength; the compressive strength of a brittle material or the compressive stress at the specified strain for a ductile material; the proof strength for a specified non-proportional compressive extension; and the proof strength for a specified total compressive extension.

Temperature group: the set or specified temperature at which the test is carried out, in degrees Celsius; the measured temperature at the surface of the test piece, in degrees Celsius; and the soaking time, in minutes.

5 Principle

An increasing uniaxial compressive force is applied along the axis of the test piece in an elevated temperature environment and the relevant compressive mechanical properties are determined.

6 Test pieces

6.1.1 The shape and dimensions of the test piece are to be designed so that: the compression within the gauge length is uniform and uniaxial throughout the test; the deformation measured by the extensometer equals the deformation of the gauge length section along the axis of the test piece; and the ends do not fail before the end of the test.

Cylindrical test pieces (Figure 1), square prism test pieces (Figure 2), sheet test pieces (Figure 3) and sheet test pieces with lugs (Figure 4) are recommended, and any other type of test piece meeting the requirements above may also be used. The key to Figure 1 gives the length of the test piece as 2.5 to 3.5 times, or 5 to 8 times, the original diameter, and the original diameter as 10 mm to 20 mm with a tolerance of plus or minus 0.05 mm. The key to Figure 4 gives the original gauge length as 50 mm with a tolerance of plus or minus 0.05 mm. All the figures are dimensioned in millimetres.

6.1.2 Figures 1 and 2 are test pieces without lateral constraint. Test pieces whose length is 2.5 to 3.5 times the original diameter, or 2.5 to 3.5 times the original width, are suitable for determining the proof strength for a non-proportional compressive extension, the proof strength for a total compressive extension, the upper and lower compressive yield strengths and the compressive strength. Test pieces whose length is 5 to 8 times the original diameter or the original width are suitable for determining the proof strength at a non-proportional compressive strain of 0.01 percent and the compressive modulus of elasticity. Test pieces whose length is 1 to 2 times the original diameter or the original width are suitable only for determining the compressive strength.

6.1.3 Figures 3 and 4 are sheet test pieces and are to be tested held in a constraining device.

6.1.4 The distance from either end of the original gauge length to the end face of the test piece is to be not less than half the diameter, or half the width, of the test piece.

6.1.5 The length of a sheet test piece is calculated by Formula (1) as the height of the constraining device plus the length of the unconstrained part.

6.1.6 The length of the unconstrained part of a sheet test piece is to be calculated from the relevant mechanical properties of the material under test and the height of the constraining device, and the platen is not to touch the constraining device during compression.

6.2.1 The number, position and orientation of the blanks taken are to comply with the relevant product standard or with the agreement between the parties concerned.

6.2.2 Cutting the blanks and machining the test pieces are to be done in a way that prevents the properties of the material being altered by cold working or by heat effects.

6.2.3 Where the thickness of a sheet test piece is the thickness of the product, the original surface is to be kept and is to be free of scratches and other damage; where the thickness is a machined thickness, the surface roughness is to be no worse than that of the original surface. The permitted deviation of the thickness, or of the diameter, within the gauge length is 1 percent or 0.05 mm, whichever is the smaller.

6.2.4 Cylindrical test pieces are machined according to Figure 1 and rectangular section test pieces according to Table 2, the edges being free of burrs.

6.2.5 Test pieces are to be straight; test pieces cut from coil or strip may carry a slight curvature that does not affect the determination of the properties.

Table 2 gives the dimensions and permitted deviations of rectangular section test pieces, in millimetres, in five columns: thickness, width, length, permitted deviation and figure number. Its three rows read: thickness from 0.2 up to but not including 2, width 12.5, length equal to the height of the constraining device plus the length of the unconstrained part, permitted deviation plus or minus 0.05, Figure 4; thickness from 2 up to but not including 10, width 12.5, the same length rule, permitted deviation plus or minus 0.05, Figure 3; thickness 10 and above, width 10 to 20, length 2.5 to 3.5 times, or 5 to 8 times, or 1 to 2 times the width, permitted deviation plus or minus 0.05, Figure 2. The note to Table 2 states that the head of a test piece less than 0.3 mm thick is generally bent, the printed shape of the bend being lost from the source text.

7 Test equipment

7.1.1 The accuracy of the testing machine is to be class 1 or better and is to comply with GB/T 16825.1.

7.1.2 The working surfaces of the upper and lower platens of the testing machine are to be parallel, the parallelism being not worse than 0.0002 to 1 mm/mm within the 100 mm of the mounting area. The platen material is to be hard enough and resistant enough to oxidation at the test temperature to prevent plastic indentation or eccentricity under the maximum load, and the hardness of the platens should be not less than 55 HRC.

7.1.3 Where the test piece is of high hardness, backing plates of a suitable hard material are to be placed at its two ends, and after the test the plate faces are to be free of permanent deformation. The parallelism of the two end faces of the backing plates is to be not worse than 0.0002 to 1 mm/mm, and the surface roughness Ra is to be not greater than 0.8 micrometres.

7.2.1 The additional devices are the force guiding device, the levelling block, the protective shield and the constraining device.

7.2.2 Testing machines that do not meet the platen parallelism requirement are to be fitted with a force guiding device; where the effect of eccentric compression by the testing machine is marked, a levelling block may be used; when brittle materials are tested, a protective shield or wire gauze able to withstand the test temperature and allowing the test piece to be observed is to be used to enclose the test piece and prevent fragments flying out or damaging the instruments.

7.2.3 A constraining device is to be used for compression tests on sheet test pieces. The note states that a constraining device is a device that supports the sheet test piece laterally, preventing it buckling during compression without affecting the axial deformation.

7.2.4 The additional devices are to comply with GB/T 7314 and are not to deform at elevated temperature.

7.3 The extensometer is to comply with GB/T 12160. An extensometer of class 0.5 or better is to be used for determining the compressive modulus of elasticity, and an extensometer of class 1 or better for determining the proof strength for a non-proportional compressive extension, the proof strength for a total compressive extension and the compressive yield strength. An averaging extensometer is recommended for determining the compressive modulus of elasticity and the proof strength at a specified non-proportional compressive strain of less than 0.05 percent.

7.4.1 The heating device is to be able to heat the test piece to the specified temperature; a radiant heating furnace is recommended in the document.

7.4.2 The constant temperature zone of the furnace is to be not less than twice the gauge length of the test piece, and the furnace chamber is to be large enough not to hinder the use of the measuring devices.

7.4.3 The permitted deviation between the measured temperature and the specified temperature, and the temperature gradient, are given in Table 3. The note states that the measured temperature is the temperature measured on the surface of the parallel length of the test piece, corrected for systematic error but without account being taken of the uncertainty of the temperature measuring device.

Table 3 has three columns, the specified temperature in degrees Celsius, the permitted deviation of the measured temperature from it in degrees Celsius, and the temperature gradient in degrees Celsius. Its four rows read: 600 and below, plus or minus 3, gradient 3; above 600 up to 800, plus or minus 4, gradient 4; above 800 up to 1000, plus or minus 5, gradient 5; above 1000 up to 1100, plus or minus 6, gradient 6.

7.5.1 The temperature measuring system is to have passed verification. If experience shows that the relative position of the heating device and the test piece keeps the variation of the test piece temperature within Table 3, the number of thermocouples may be reduced, but at least one measuring thermocouple is to keep good thermal contact with the surface of the test piece and direct thermal radiation from the heating body onto the thermocouple is to be avoided.

7.5.2 The resolution of the temperature measuring device is to be at least 1 degree Celsius and the permitted error is to be within 0.004 times the specified temperature or within 2 degrees Celsius, whichever is the larger. The note states that the temperature measuring system covers the whole chain of measuring components: sensor, leads, display device and junctions.

8 Test conditions

8.1.1 The thickness and the width of a rectangular section test piece are measured at the mid point of the original gauge length; the diameter of a cylindrical test piece is measured at the mid point of the original gauge length in two perpendicular directions and the arithmetic mean is taken. The resolution of the gauge or measuring device is chosen from Table 4, and the original cross-sectional area calculated from the measured dimensions is to keep at least four significant figures.

Table 4 gives the resolution of the gauge or measuring device in millimetres against the cross-sectional dimension of the test piece: 0.1 to 0.5, resolution 0.001; above 0.5 up to 2.0, resolution 0.002; above 2.0 up to 10, resolution 0.01; above 10, resolution 0.05. The note states that the cross-sectional dimension covers the thickness and width of a rectangular section test piece and the diameter of a circular section test piece.

8.1.2 When a sheet test piece with lugs is measured, the original gauge length is one quarter of the sum, over both side faces, of the inner distance and the outer distance between the two lugs along the axis of the test piece. The gauge is not to be brought close to the root of a lug.

8.1.3 All the geometrical dimensions of the test piece should be measured at room temperature; the gauge length of the extensometer may be measured at the test temperature.

8.2 When the test piece is installed, its two ends are to be coated with graphite lubricant and the test piece is to be aligned parallel and centred with the upper and lower grips. Where a constraining device is used with a sheet test piece, graphite lubricant is also to be applied to the two end faces of the test piece and to the grips.

8.3.1 Unless the product standard specifies otherwise, the test piece is to be heated to the specified temperature within 1 h and the soaking time is to be not less than 20 min.

8.3.2 During the test the permitted deviation of the test temperature and the temperature gradient are to comply with Table 3.

8.4.1 Where strain control is used, the strain rate is set to 0.005 per minute, with a tolerance of plus or minus 0.002 per minute; where load control or crosshead displacement control is used, a speed equivalent to a strain rate of 0.005 per minute should be set. If the material is sensitive to strain rate, a strain rate of 0.003 per minute may be used.

8.4.2 Where crosshead displacement rate control is used, the crosshead displacement rate is to be held constant so as to reach the mean strain rate needed during the test.

8.4.3 A constant crosshead displacement rate during the test does not necessarily ensure a constant strain rate during the test. Whichever method is used, a constant rate is to be used and sudden changes of strain rate are not permitted.

9 Determination of properties

9.1 Choice of the clamping force for sheet test pieces. The clamping force is chosen according to the proof strength at a non-proportional compressive strain of 0.2 percent, or the lower compressive yield strength, of the material and according to the thickness of the sheet. In general the friction force is kept to not more than 2 percent of the estimated value of the corresponding proof force; for very thin test pieces the friction force is allowed to reach 5 percent of that force. Provided the test can be carried out normally, the clamping force should be as small as possible. The note states that a test piece less than 0.3 mm thick is generally considered very thin.

9.2.1 Determination of the actual compressive force for sheet test pieces. During the test the force-deformation curve is plotted automatically, and its initial part is generally not linear because of the influence of friction. When the force is large enough the friction force reaches a fixed value and thereafter no longer affects the force-deformation curve. On the assumption that the friction force is evenly distributed over the surface of the test piece, the actual compressive force is calculated by Formula (2) as the force at the upper end of the test piece less one half of the friction force.

9.2.2 Where the actual compressive force is determined graphically, the straight elastic portion of the automatically plotted force-deformation curve should be extended backwards to meet the original horizontal axis at a point; a perpendicular is drawn at the mid point of the line joining the original origin to that point, meeting the extended straight line at a further point, which is the true origin of the force-deformation curve. Lines drawn through that origin parallel to the original axes form the corrected axes, on which the actual compressive force can be read directly, as shown in Figure 5. The original coordinate system is that of the force at the upper end against the deformation of the original gauge length section; the corrected system is that of the actual compressive force against the actual deformation.

9.3.1 The proof strength for a non-proportional compressive extension is generally determined graphically from the force-deformation diagram. The scale of the force axis is to place all the proof force points above the mid point of that axis, and the deformation magnification is to be chosen so that the length of the segment marked OC in Figure 6 is not less than 5 mm. On the automatically plotted force-deformation curve, a segment OC corresponding to the specified non-proportional deformation, that is the specified strain multiplied by the original gauge length and by the deformation magnification, is set off from the origin O, and a line CA parallel to the straight elastic portion is drawn through C to meet the curve at A. The corresponding force is the proof force sought, as shown in Figure 6, and the proof strength is calculated by Formula (3) as that force divided by the original cross-sectional area.

9.3.2 If the force-deformation curve has no clear straight elastic portion, the method of successive approximation is to be used. A point A0 is first estimated by eye on the curve, at about the force corresponding to a non-proportional compressive strain of 0.2 percent; two points G0 and Q0 are then taken on the slightly curved line, corresponding to 0.1 and 0.5 times that force, and the straight line G0Q0 is drawn; following the method of 9.3.1, a line CA1 parallel to G0Q0 is drawn through C to meet the curve at A1. If A1 coincides with A0, the force at A0 is the proof force sought, as shown in Figure 7. The slope of the line G0Q0 can generally be used as the basis for determining other proof strengths graphically. If A1 does not coincide with A0, the approximation is carried further: the force at A1 is used to fix the points G1 and Q1 corresponding to 0.1 and 0.5 times that force, and a parallel is again drawn through C to give the intersection A2. The same steps are repeated until the intersection obtained coincides with the one obtained before it. Figures 6 and 8 each have two parts, a) for a test without lateral constraint and b) for a test with lateral constraint.

9.4 The proof strength for a total compressive extension is generally determined graphically from the force-deformation diagram. The force axis follows 9.3.1 and the total compressive deformation should generally extend beyond the mid point of the deformation axis. On the automatically plotted curve a segment OD, equal to the specified total compressive strain multiplied by the original gauge length and by the deformation magnification, is set off from the origin O along the deformation axis, and a line DM parallel to the force axis is drawn through D to meet the curve at M. The corresponding force is the specified total compressive force, as shown in Figure 8, and the proof strength is calculated by Formula (4) as that force divided by the original cross-sectional area.

9.5 On the force-deformation diagram, the maximum actual compressive force before the first drop of the force and the minimum actual compressive force during the yield stage, discounting any initial transient effect, are identified and each divided by the original cross-sectional area of the test piece to give the upper compressive yield strength and the lower compressive yield strength.

9.6 The test piece is compressed to failure, the maximum compressive force is determined from the force-deformation diagram and the compressive strength is calculated by Formula (5) as that force divided by the original cross-sectional area.

9.7 The compressive modulus of elasticity is measured graphically from the force-deformation diagram, as shown in Figure 8: two points J and K as far apart as possible are taken on the straight elastic portion, the corresponding forces and deformations are read off, and the calculation follows Formula (6). The key to Formula (6) gives the forces at points K and J on the force-deformation curve in newtons, the original gauge length of the test piece in millimetres, the deformations at points K and J in millimetres, and the original cross-sectional area of the test piece in square millimetres.

10 Rounding of test results

The test results are to be rounded as required by the relevant product standard. Where nothing specific is laid down, the following applies: a) strength properties are rounded to 1 MPa; b) the result for the compressive modulus of elasticity keeps three significant figures, the rounding method following GB/T 8170.

11 Treatment of test results

11.1 The test result is invalid and the same number of tests is to be repeated in any of the following cases: a) the test piece buckles before the purpose of the test is reached; b) before the purpose of the test is reached, the end of the test piece is locally damaged, or the test piece fractures at a lug or outside the gauge length; c) the testing instruments or equipment fail during the test and affect the result.

11.2 Where metallurgical defects appear on the test piece, such as delamination, blisters, slag inclusions or shrinkage cavities, this is to be noted in the test record and in the report.

12 Test report

The test report is to include: a) the number of this document; b) the identification of the test piece; c) the name and grade of the material; d) the sampling direction and position of the test piece; e) the shape and dimensions of the test piece; f) the test piece fixture and the lubricant; g) the model and specification of the testing machine; h) the test conditions, that is the test speed and the control mode; i) the test temperature and the soaking time; j) the test results; k) any abnormal circumstance encountered during the test that might affect the results.