Paulownia wood for nuclear waste transport buffers, its preparation, and methods for determining compressive strength.

Abstract

The present invention relates to the field of nuclear waste transportation technology, specifically to a paulownia wood for nuclear waste transportation buffer and its preparation and compression index determination method, wherein the preparation method includes the steps of basic data acquisition, quality screening, side splicing, lamination and gluing of paulownia wood units, and can solve the problem of quality stability, safety and reliability and coordination of manufacturing economic cost of paulownia glued wood for nuclear waste transportation buffer, fill the gap in the manufacturing technology of paulownia glued wood for nuclear waste transportation buffer, promote the high value-added application of artificial forest paulownia as the main energy-absorbing material in the field of nuclear waste transportation technology, and promote the localization of buffers for nuclear waste transportation devices. Among them, the method for determining the compression index of paulownia glued wood for nuclear waste transportation buffer includes the preparation of test specimens of paulownia glued wood and the determination of compression index, and can realize the rapid and accurate evaluation of the compression index of paulownia glued wood for nuclear waste transportation buffer.

Description

Technical Field

[0001] The invention relates to the technical field of nuclear waste transportation, in particular to a paulownia wood used as a nuclear waste transportation buffer and a preparation method thereof and a method for determining a compression index thereof. Background Art

[0002] According to the "China Nuclear Energy Development Report 2022" released by the China Nuclear Energy Industry Association, my country's total installed capacity of nuclear power in operation and under construction ranks second in the world. According to statistics, my country has accumulated about 14,000 tons of irradiated nuclear fuel, with an annual increase of about 1,300 tons. These nuclear wastes often need to be transported to the back-end processing plant for recycling through transportation devices. In order to ensure the safety of nuclear waste transportation, one of the key indicators is that the transportation device should meet the 9m drop impact test. Therefore, in order to ensure the drop impact performance of the transportation device, buffers (also called shock absorbers) are generally required at both ends of the transportation device. The filling materials of the buffer are mainly wood, foam aluminum, etc.

[0003] Due to the lack of systematic research on buffers for nuclear waste transport devices, China currently relies mainly on imported light wood and foam aluminum series buffer products, which are very expensive. It is urgent to realize the localization of buffers for nuclear waste transport devices. In the path of realizing the localization of buffers for nuclear waste transport devices, my country's rich artificial forests of Paulownia can be used as the main tree species to provide guarantee for the demand for buffer filling materials. It has good impact resistance and energy absorption performance, and generally matures in about 5 years. At present, my country can harvest about 24 million m3 of timber each year. 3 . However, due to the fact that paulownia is currently mainly used in the furniture field, the technical requirements for its application as a buffer filling material for nuclear waste transportation devices are basically blank. How to further control its manufacturing economic costs while ensuring quality and safety needs to be further resolved, involving the entire optimization of the preparation method of paulownia glued wood for nuclear waste transportation buffers. In addition, the traditional method of using a small cross-section of 20mm×20mm and traditional strain gauges is difficult to accurately obtain the compressive properties of paulownia glued wood for nuclear waste transportation buffers, and there is a lack of a method for obtaining the value of compressive performance indicators. These existing problems restrict the application of domestic artificial forest paulownia as a buffer filling material, and seriously hinder the localization process of buffers for nuclear waste transportation devices.

[0004] Such as Chinese standards GB / T 1935, GB / T 1939, GB / T 1943, GB / T50329, GB 50005 and other standards specify the compression test methods for wood materials used in building houses and furniture, but do not involve specific compression tests on paulownia glued laminated wood used as nuclear waste transport buffers and methods for determining their compression indexes; such as patents CN202111305622.1-"Furniture production process and intelligent production line based on paulownia wood", CN201510025237.X-"A preparation method for ecological paulownia wood wallboard", CN201710629514.7-"A weather-resistant paulownia wood hardening treatment method", CN202211155104.0-"Buffer device for spent fuel transport container and spent fuel transport container", CN201721383115.9-"Anti-overturning spent fuel storage device" and "Forest Products Industry 2016 The paulownia wood involved in the “Discussion on the Physical and Mechanical Properties of Paulownia Particleboard with Homogeneous Structure” in Volume 43, Issue 7, pp. 24-27 of 2006, “Study on the Process of Paulownia Veneer Strip Laminated Timber” in Forestry Science and Technology, Volume 31, Issue 1, pp. 35-37 of 2006, and “Study on the Hot Pressing Process of Paulownia Particleboard” in Forestry Machinery and Woodworking Equipment, Volume 44, Issue 3, pp. 16-19 of 2016 only stipulates its relevant production methods in the field of furniture, and the nuclear waste transportation buffer involved does not involve the manufacturing process of the buffer filling material. None of them solves the problem of the lack of preparation method of paulownia glued wood for nuclear waste transportation buffer and evaluation method of its compressive performance index, which restricts the application of domestic artificial forest paulownia as buffer filling material and seriously hinders the localization process of buffers for nuclear waste transportation devices.

[0005] It can be seen from the public contents of the above-mentioned documents that there is currently a lack of corresponding technologies for paulownia glued laminated wood used as a nuclear waste transport buffer, its preparation method, and a method for determining the compressive strength index, which cannot meet the needs of manufacturing and safe use in this field.

[0006] Therefore, by proposing paulownia glued laminated wood for nuclear waste transport buffer, its preparation method and method for determining the compressive resistance index, it is possible to prepare filling material products for nuclear waste transport buffers from domestic artificial forest paulownia, and accurately evaluate the compressive resistance index of paulownia glued laminated wood for nuclear waste transport buffers, so as to ensure the quality and safe application of paulownia glued laminated wood products for nuclear waste transport buffers. This is of great significance for promoting the high value-added application of artificial forest paulownia as the main energy-absorbing material in the field of nuclear waste transportation technology and promoting the localization of buffers for nuclear waste transportation devices. Summary of the invention

[0007] The purpose of the present invention is to provide a paulownia glued laminated wood for nuclear waste transport buffer, a preparation method and a method for determining the compressive index. Through the steps of basic data acquisition, quality screening, side splicing, lamination and gluing of paulownia wood units, the problem of quality stability, safety and reliability and coordination of manufacturing economic cost of paulownia glued laminated wood for nuclear waste transport buffer is solved, the manufacturing technology gap of paulownia glued laminated wood for nuclear waste transport buffer is filled, and an evaluation method for the compressive index of paulownia glued laminated wood for nuclear waste transport buffer is proposed, which promotes the high added value application of artificial forest paulownia as the main energy-absorbing material in the field of nuclear waste transport technology and promotes the localization of buffers for nuclear waste transport devices.

[0008] In order to solve the above technical problems, this application provides the following technical solutions:

[0009] A method for preparing paulownia wood for nuclear waste transport buffer comprises the following steps:

[0010] (1) Obtain basic data of Paulownia wood units, including:

[0011] (1-1) The paulownia logs are sawn, dried and finely planed to produce paulownia wood veneer units with a length of 1000 mm to 2000 mm, a width of 80 mm to 200 mm and a thickness of 40 mm to 100 mm;

[0012] (1-2) For the paulownia wood veneer unit, the density of each paulownia wood veneer unit is measured and recorded as ρ i ; The maximum annual ring width of each paulownia wood veneer unit is measured and recorded as W i ; The dynamic elastic modulus of each paulownia wood veneer unit is measured and recorded as E i ; Measure the length of the knot of each paulownia wood veneer unit on the wide surface of the veneer, recorded as D i ;

[0013] (2) Based on the basic data of the paulownia wood unit, quality screening of the paulownia wood unit is performed, specifically including:

[0014] (2-1) Material screening is performed according to the density, maximum annual ring width, dynamic elastic modulus and wood knots of the paulownia wood veneer unit. The specific method of material screening is as follows:

[0015] Density requirement: ρ i The range is 0.28±0.04g / cm 3 ;

[0016] Maximum annual ring width requirement: W i ≤30mm;

[0017] Dynamic elastic modulus requirement: E i ≥3800m / s;

[0018] Wood knot requirement: D i The ratio of the paulownia wood veneer unit is not more than 15%, and the number of wood knots allowed within any 1m length of the paulownia wood veneer unit shall not exceed 1;

[0019] (2-2) The paulownia wood veneer unit prepared according to step (1) and the density ρ obtained by testing i 、Maximum annual ring width W i , dynamic elastic modulus E i and the length of the knot D i According to the material screening method of step (2-1), the ratio of the number of paulownia wood veneer units that meet the material requirements to the total number of paulownia wood veneer units is not less than 30%;

[0020] (3) The paulownia wood veneer units are spliced ​​sideways along the width direction to form a paulownia wood target veneer, and finger jointing is not allowed in the length direction, and the width types of the paulownia wood veneer units spliced ​​sideways are no more than 2;

[0021] (4) laminating and gluing the target paulownia wood plywood along the thickness direction to form paulownia glued laminated timber of corresponding target size, specifically comprising:

[0022] (4-1) applying glue to one side of the paulownia wood target veneer;

[0023] (4-2) assembling the gluing target paulownia wood plywood along the height direction of the target paulownia glulam, wherein the horizontal spacing between the glue layers of adjacent paulownia wood target plywood in the height direction is not less than 20 mm;

[0024] (4-3) laminating and gluing the assembled target paulownia wood plywood to obtain target paulownia glued laminated timber;

[0025] (4-4) The target paulownia glued laminated timber is placed for curing.

[0026] Furthermore, in the step (1-1), the target average moisture content of the paulownia wood veneer unit after drying is 12%, and the paulownia wood veneer unit should be made of paulownia wood logs that are more than 500 mm away from the root and have uniform width.

[0027] Furthermore, in the step (1-2), a non-destructive dynamic tester is used to measure the dynamic elastic modulus of each paulownia wood veneer unit.

[0028] Furthermore, in the step (4-1), the adhesive is a two-component isocyanate adhesive, and the amount of adhesive applied is 280 to 330 g / m 2 The mass ratio of the main agent to the curing agent in the two-component isocyanate adhesive is 8 to 10.

[0029] Furthermore, in the step (4-3), the lamination and gluing pressure of the target paulownia wood veneer is 0.2-0.4 MPa, the cold pressing time is more than 8 hours, and the pressure compensation is performed 2-3 times within 15 minutes of the initial cold pressing time.

[0030] Furthermore, in the step (4-4), the target paulownia glued timber obtained by lamination and gluing is placed in an environment of 20-40° C. and 40%-70% humidity for curing for at least one week.

[0031] A method for determining the compressive strength index of paulownia wood used as a nuclear waste transport buffer comprises the following steps:

[0032] (1) Preparation of test specimens of paulownia glued laminated wood, specifically including:

[0033] (1-1) Processing a nuclear waste transport buffer with paulownia wood into a test specimen with a width of b, a thickness of t, and a length of L, wherein b of the test specimen is ≥ 100 mm, t is ≥ 100 mm, and L is ≥ 150 mm;

[0034] (1-2) The number of test specimens for evaluating each compressive strength of paulownia glued laminated timber should not be less than 30;

[0035] (2) Determine the compressive strength of paulownia glulam, including:

[0036] (2-1) The length direction of the test specimen in step (1) is set as the compressive loading direction, and the test loading speed is 1 to 5 mm / min;

[0037] (2-2) When determining the compressive strength, for the compressive strength along the grain, the maximum load in the first stage elastic section and the second stage platform section of the compressive load-displacement curve is taken to calculate the compressive strength along the grain of the test specimen, and for the compressive strength across the grain, the intersection of the straight line I of the first stage elastic section and the straight line II of the second stage platform section of the compressive load-displacement curve is taken to determine the compressive strength across the grain of the test specimen;

[0038] (2-3) For the compressive elastic modulus, a digital speckle pattern measurement method is used to determine the longitudinal compressive elastic modulus and the transverse compressive elastic modulus of the test specimen, wherein the digital speckle pattern measurement area is the center of the test specimen, the length of the digital speckle pattern measurement area is 1 / 3 of the length of the test specimen, and the width of the digital speckle pattern measurement area is 1 / 3 to 1 / 2 of the width of the test specimen;

[0039] (2-4) According to the compressive strength and compressive elastic modulus measured for each test specimen, a statistical analysis is performed, and the median value at a 99% confidence level is taken as the compressive strength index and compressive elastic modulus index of the paulownia glued laminated timber.

[0040] When the paulownia glued laminated wood for nuclear waste transport buffer prepared by the preparation method of the present invention is used to determine the compressive performance according to the compressive index determination method of the present invention, the coefficients of variation of the compressive strength along the grain, the compressive elastic modulus along the grain, the compressive strength in the radial direction across the grain, the compressive elastic modulus in the radial direction across the grain, the compressive strength in the tangential direction across the grain, and the compressive elastic modulus in the tangential direction across the grain are all no more than 15%.

[0041] The paulownia glued laminated wood for nuclear waste transport buffer prepared by the preparation method of the present invention has a compressive strength index along the grain of not less than 20.0 MPa, a compressive elastic modulus index along the grain of not less than 5100 MPa, a compressive strength index along the radial cross grain of not less than 2.0 MPa, a compressive elastic modulus index along the radial cross grain of not less than 600 MPa, a compressive strength index along the tangential cross grain of not less than 1.6 MPa, and a compressive elastic modulus index along the tangential cross grain of not less than 300 MPa when the compressive performance is determined according to the compressive index determination method of the present invention.

[0042] Compared with the prior art, the paulownia wood for nuclear waste transport buffer and the method for preparing the same and determining the compressive index thereof of the present invention have at least the following beneficial effects:

[0043] According to the present invention, the number of paulownia wood veneer units that can be used for manufacturing accounts for no less than 30% of the number of the entire paulownia wood veneer units through the steps of basic data acquisition, quality screening, side splicing, lamination and gluing of paulownia wood units, and the coefficients of variation of the compressive strength and compressive elastic modulus of the paulownia glued wood are no more than 15%, the compressive strength indicators along the grain, radial cross grain, and tangential cross grain are no less than 20.0MPa, 2.0MPa, and 1.6MPa, respectively, and the compressive elastic modulus is no less than 5100MPa, 600MPa, and 300MPa, respectively.

[0044] The method of the present invention can solve the problem of coordinating the quality stability, safety and reliability and manufacturing economic cost of paulownia glued wood for nuclear waste transportation buffer, fill the gap in the manufacturing technology of paulownia glued wood for nuclear waste transportation buffer, and can accurately evaluate the compressive resistance index of paulownia glued wood for nuclear waste transportation buffer, thereby accelerating the high value-added application of artificial forest paulownia as the main energy-absorbing material in the field of nuclear waste transportation technology.

[0045] The paulownia wood for nuclear waste transport buffer of the present invention and its preparation and compression index determination method are further described below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a schematic structural diagram of the paulownia glued wood used for the nuclear waste transport buffer of the present invention.

[0047] Figure 2The diagram is a schematic diagram of a knot of a paulownia wood veneer unit in the process of preparing the paulownia glued wood for nuclear waste transport buffer of the present invention, wherein D is the length of the knot of the paulownia wood veneer unit on the wide surface of the veneer.

[0048] Figure 3 The present invention is a schematic diagram of the calculation of the compressive strength in the method for determining the compressive index of paulownia glued wood used for nuclear waste transport buffer.

[0049] Figure 4 This is a schematic diagram of the digital speckle measurement area in the method for determining the compression index of paulownia glued wood used for nuclear waste transport buffer of the present invention.

[0050] Among them, 1-Paulownia wood veneer unit, 2-glue layer, 3-veneer with edge knots, 4-veneer with center knots, 5-wood knot. DETAILED DESCRIPTION

[0051] Example 1

[0052] like Figure 1 As shown, a method for preparing paulownia glued wood for nuclear waste transport buffer comprises the following steps:

[0053] (1) Obtain basic data of Paulownia wood units, including:

[0054] (1-1) The paulownia logs are sawn, dried and finely planed to produce paulownia wood veneer units with a length of 1000 mm, a width of 150 mm and a thickness of 60 mm. The target average moisture content of the paulownia wood veneer units after drying is 12%. The paulownia wood veneer units should be made from paulownia logs at least 500 mm from the root and have equal width.

[0055] (1-2) For the paulownia wood veneer unit, the density of each paulownia wood veneer unit is measured and recorded as ρ i , the maximum annual ring width of each paulownia wood veneer unit was measured and recorded as W i The dynamic elastic modulus of each paulownia wood veneer unit was measured using a non-destructive dynamic tester and recorded as E i , the length of the knot of each paulownia wood veneer unit on the veneer width is measured and recorded as D i ,like Figure 2 As shown;

[0056] (2) Based on the basic data of the paulownia wood unit, quality screening of the paulownia wood unit is performed, specifically including:

[0057] (2-1) Material screening is performed according to the density, maximum annual ring width, dynamic elastic modulus and wood knots of the paulownia wood veneer unit. The specific method of material screening is as follows:

[0058] Density requirement: ρi The range is 0.28±0.04g / cm 3 ;

[0059] Maximum annual ring width requirement: W i ≤30mm;

[0060] Dynamic elastic modulus requirement: E i ≥3800m / s;

[0061] Wood knot requirement: D i The ratio to the paulownia wood veneer unit is not greater than 15%, and the number of wood knots allowed within any 1m length of the paulownia wood veneer unit is not more than 1.

[0062] (2-2) The paulownia wood veneer unit prepared according to step (1) and the density ρ obtained by testing i 、Maximum annual ring width W i , dynamic elastic modulus E i and the length of the knot D i According to the material screening method of step (2-1), the ratio of the number of paulownia wood veneer units that meet the material requirements to the total number of paulownia wood veneer units is 38%;

[0063] (3) The paulownia wood veneer unit is assembled sideways along the width direction to form a paulownia wood target veneer, and finger jointing is not allowed in the length direction;

[0064] (4) laminating and gluing the target paulownia wood plywood along the thickness direction to form paulownia glued laminated timber of corresponding target size, specifically comprising:

[0065] (4-1) The target paulownia wood veneer is coated with glue on one side, and the type of glue is a two-component isocyanate adhesive, and the amount of glue is 300g / m 2 , the mass ratio of the main agent to the curing agent in the two-component isocyanate adhesive is 10;

[0066] (4-2) assembling the glue-coated paulownia wood target plywood along the height direction of the target paulownia glulam, wherein the horizontal spacing between the glue layers of adjacent paulownia wood target plywood in the height direction is 75 mm;

[0067] (4-3) laminating and gluing the assembled paulownia wood target plywood to obtain target paulownia glued wood, wherein the laminating and gluing pressure of the paulownia wood target plywood is 0.25 MPa, the cold pressing time is more than 8 hours, and the pressure compensation is performed 2 to 3 times within 15 minutes of the initial cold pressing time;

[0068] (4-4) The target paulownia glued laminated timber obtained by lamination was placed in an environment of 24° C. and 60% humidity for curing for 1 week.

[0069] Example 2

[0070] The method for determining the compressive strength index of the paulownia glued wood for nuclear waste transport buffer prepared in Example 1 comprises the following steps:

[0071] (1) Preparation of paulownia glulam test specimens, specifically including:

[0072] (1-1) The target paulownia glued laminated wood prepared in the preparation method of Example 1 was processed into a test specimen with a width of b=100 mm, a thickness of t=100 mm, and a length of L=200 mm;

[0073] (1-2) The number of test specimens for evaluating each compressive strength of paulownia glued laminated wood is 30;

[0074] (2) Determine the compressive strength of paulownia glulam, including:

[0075] (2-1) The length direction of the test specimen in step (1) is set as the compressive loading direction, and the test loading speed is 2 mm / min;

[0076] (2-2) When determining the compressive strength, for the compressive strength along the grain, the maximum load in the first stage elastic section and the second stage platform section of the compressive load-displacement curve is taken to calculate the compressive strength along the grain of the test specimen. For the compressive strength across the grain, the intersection of the first stage elastic section straight line I and the second stage platform section straight line II of the compressive load-displacement curve is taken to determine the compressive strength across the grain of the test specimen, such as Figure 3 As shown;

[0077] (2-3) For the compressive elastic modulus, the digital speckle measurement method is used to determine the longitudinal compressive elastic modulus and the transverse compressive elastic modulus of the test specimen. The digital speckle measurement area is the center of the test specimen, the length of the digital speckle measurement area is 1 / 3 of the length of the test specimen, and the width of the digital speckle measurement area is 1 / 2 of the width of the test specimen. Figure 4 As shown;

[0078] (2-4) According to the compressive strength and compressive elastic modulus measured for each test specimen, a statistical analysis was performed and the median value at a confidence level of 99% was taken as the compressive strength index and compressive elastic modulus index of paulownia glued laminated timber. The compressive strength indexes along the grain, radial grain, and tangential grain were 24.57 MPa, 2.56 MPa, and 2.29 MPa, respectively, and the compressive elastic modulus were 5548 MPa, 740 MPa, and 453 MPa, respectively.

[0079] The compressive properties of paulownia glued laminated wood used for nuclear waste transport buffer were determined by the above method and compared with the test results of the traditional test method (small sample + adhesive strain gauge). The results are shown in Table 1.

[0080] Table 1 Compressive properties of paulownia glued laminated wood used as nuclear waste transport buffer

[0081]

[0082] The results show that the coefficient of variation of each compressive performance measured by Example 2 is less than 15%, which can significantly overcome the problem of large fluctuation and high coefficient of variation of compressive performance test data caused by small sample size and difficult strain gauge pasting in traditional testing methods, and can accurately evaluate the compressive performance of paulownia glued laminated wood for nuclear waste transportation buffer. In addition, while ensuring that the ratio of the number of paulownia wood plywood units that can be used for manufacturing accounts for no less than 30% of the total number of paulownia wood plywood units, the compressive strength index of paulownia glued laminated wood along the grain, radial grain, and tangential grain are no less than 20.0MPa, 2.0MPa, and 1.6MPa, respectively, and the compressive elastic modulus is no less than 5100MPa, 600MPa, and 300MPa, respectively, which solves the problem of coordinating the quality stability, safety and reliability of paulownia glued laminated wood for nuclear waste transportation buffer and the economic cost of manufacturing.

[0083] The embodiments described above are merely descriptions of preferred implementation modes of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for determining the compressive strength index of paulownia wood used as a nuclear waste transport buffer, characterized in that: The following steps are involved: (1) Preparation of test specimens of paulownia glued laminated wood, specifically including: (1-1) Processing a nuclear waste transport buffer with paulownia wood into a test specimen with a width of b, a thickness of t, and a length of L, wherein b of the test specimen is ≥ 100 mm, t is ≥ 100 mm, and L is ≥ 150 mm; (1-2) The number of test specimens for evaluating each compressive strength of paulownia glued laminated timber should not be less than 30; (2) Determine the compressive strength of paulownia glulam, including: (2-1) The length direction of the test specimen in step (1) is set as the compressive loading direction, and the test loading speed is 1 to 5 mm / min; (2-2) When determining the compressive strength, for the compressive strength along the grain, the maximum load in the first stage elastic section and the second stage platform section of the compressive load-displacement curve is taken to calculate the compressive strength along the grain of the test specimen, and for the compressive strength across the grain, the intersection of the straight line I of the first stage elastic section and the straight line II of the second stage platform section of the compressive load-displacement curve is taken to determine the compressive strength across the grain of the test specimen; (2-3) For the compressive elastic modulus, a digital speckle pattern measurement method is used to determine the longitudinal compressive elastic modulus and the transverse compressive elastic modulus of the test specimen. The digital speckle pattern measurement area is the center of the test specimen, the length of the digital speckle pattern measurement area is 1 / 3 of the length of the test specimen, and the width of the digital speckle pattern measurement area is 1 / 3 to 1 / 2 of the width of the test specimen. (2-4) According to the compressive strength and compressive elastic modulus measured for each test specimen, a statistical analysis was performed and the median value at a 99% confidence level was taken as the compressive strength index and compressive elastic modulus index of paulownia glued laminated timber.

2. The method for determining the compressive strength index of paulownia wood for nuclear waste transport buffer according to claim 1, characterized in that: The coefficients of variation of the compressive strength along the grain, the compressive elastic modulus along the grain, the compressive strength along the radial direction, the compressive elastic modulus along the radial direction, the compressive strength along the tangent direction and the compressive elastic modulus along the tangent direction are all not more than 15%.

3. The method for determining the compressive strength index of paulownia wood for nuclear waste transport buffer according to claim 2, characterized in that: The compressive strength index along the grain shall not be less than 20.0MPa, the compressive elastic modulus index along the grain shall not be less than 5100MPa, the compressive strength index along the radial direction shall not be less than 2.0MPa, the compressive elastic modulus index along the radial direction shall not be less than 600MPa, the compressive strength index along the tangential direction shall not be less than 1.6MPa, and the compressive elastic modulus index along the tangential direction shall not be less than 300MPa.

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