Non-uniform load loading test device and method based on elastic-plastic partition of bearing coal pillar

Abstract

This invention provides a non-uniform load loading test device and method based on the elastoplastic zoning of a coal pillar, belonging to the technical field of non-uniform load loading tests. The technical problem to be solved is: to provide a non-uniform load loading test device and method based on the elastoplastic zoning of a coal pillar. The technical solution adopted is as follows: each rigid pressure bar is arranged according to the elastoplastic stress zoning pattern of the coal sample. The arranged rigid pressure bars are fixed by bolts through the front and rear side clamps and the left and right side clamps of the rigid pressure bar clamp. The sensing end of each rigid pressure bar is in contact with the coal sample; the detection end of each rigid pressure bar is in contact with the normal hydraulic transmission shaft and the lateral hydraulic transmission shaft; a rigid pressure bar clamp fixing shaft is installed on the outer side of the left and right side clamps of the rigid pressure bar clamp. This invention is applied to non-uniform load loading tests.

Description

Technical Field

[0001] This invention provides a non-uniform load loading test device and method based on the elastic-plastic partitioning of a bearing coal pillar, belonging to the field of non-uniform load loading test technology. Background Technology

[0002] During coal mining, the overlying coal and rock in the goaf will form an arching effect. This effect will cause changes in the stress on the coal pillar, resulting in different stresses in different areas of the coal pillar. During the mining process, the vertical stress distribution characteristics of the coal pillar will change, forming a plastic failure zone (stress reduction zone), an elastic zone (stress increase zone), and a pre-existing rock stress zone within the coal pillar. This change in stress distribution is the direct cause of the elastoplastic zoning of the coal pillar. The maximum width of the plastic zone of the coal pillar will increase with the increase of the working face burial depth and the width of the goaf. Under different goaf widths and working face burial depths, the stress distribution and plastic zone distribution patterns of the coal pillar will also be different, thus affecting the elastoplastic zoning of the coal pillar.

[0003] When a coal pillar exhibits elastoplastic zoning under non-uniformly distributed load conditions, its stability is significantly affected, leading to uneven distribution of internal stress and deformation. This reduces the pillar's load-bearing capacity, alters its failure mode, and affects the distribution of deviatoric stress in the floor, causing changes in the pillar's creep characteristics and energy evolution. Furthermore, non-uniformly distributed loads can also affect the instability mechanism of the coal pillar-roof system. Therefore, it is necessary to study the stability and mechanical behavior of the coal pillar under such load conditions through mechanical experimental models and parameter analysis to ensure safety and efficiency in coal mining.

[0004] The deformation of the surrounding rock in a roadway is a comprehensive reflection of mine pressure. The elastic-plastic zoning and fracture range of the surrounding rock are determined by the stress and strength of the surrounding rock, and can serve as an important basis for assessing roadway stability and determining the difficulty of support. Therefore, conducting non-uniformly distributed load loading tests based on the elastic-plastic zoning of the bearing coal pillar and studying the deformation and instability law of the roadway is of great significance for improving roadway stability, coal resource recovery rate, and support technology. However, the current test equipment is complex in structure and expensive, and the results of the test methods have limitations. When the gradient stress level increases or the non-uniformly distributed load is complex, the test equipment is difficult to implement and needs to be improved. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to solve the following technical problem: to provide a non-uniformly distributed load loading test device and method based on the elastic-plastic partitioning of a bearing coal pillar.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a non-uniformly distributed load loading test device based on the elastic-plastic partition of the bearing coal pillar, including multiple rigid pressure bars. Each rigid pressure bar is arranged according to the elastic-plastic stress partition of the coal sample. The arranged rigid pressure bars are fixed by bolts through the front and rear side clamps and the left and right side clamps of the rigid pressure bar clamp.

[0007] The sensing end of each rigid pressure bar is in contact with the coal sample;

[0008] A normal hydraulic transmission shaft is installed above the detection end of each rigid pressure bar;

[0009] The rigid pressure bar clamp has a rigid pressure bar clamp fixing shaft installed on the outer side of the left and right side clamps;

[0010] The coal sample is fixed by a coal sample side clamp and a coal sample rear baffle. The outer side of the coal sample side clamp is in contact with the hydraulic transmission shaft pressure plate. A lateral hydraulic transmission shaft is installed on the outer side of the hydraulic transmission shaft pressure plate. A rear baffle fixing shaft is installed on the outer side of the coal sample rear baffle.

[0011] The method for conducting tests based on a non-uniformly distributed load loading test device with elastic-plastic partitioning of a bearing coal pillar includes the following test steps:

[0012] Step 1: Determine the elastoplastic stress zoning pattern of the coal sample, as well as the stress distribution pattern of the applied load;

[0013] Step 2: Adjust the height of each rigid pressure bar according to the elastic-plastic stress zoning pattern of the coal sample, and select rigid pressure bars with different stiffnesses according to the stress distribution pattern. Arrange the rigid pressure bars according to the stiffness distribution sequence.

[0014] Step 3: Select rigid pressure strips with different stiffness coefficients according to the stiffness distribution, and arrange the rigid pressure strips according to the sequence of stiffness distribution;

[0015] Step 4: Determine the axial deformation of coal samples of the same specifications when they enter the elastic deformation stage, plastic deformation stage, and fracture development stage through preliminary experiments. Adjust the height and arrangement of each rigid pressure bar according to the deformation size of each stage, and control the coal sample to be divided into zones according to elastic-plastic stress after the loading equipment applies pressure.

[0016] Step 5: Using the sorted rigid pressure bars to bear the normal load, determine the normal load value applied by the normal hydraulic drive shaft at the detection end of each rigid pressure bar;

[0017] Step 6: Fix the coal sample with a coal sample clamp and apply lateral load to the left and right sides of the coal sample. When the predetermined horizontal pressure is reached, fix or remove the rear baffle. Observe the development of cracks and the outflow of coal dust during the coal sample failure process from the front end of the coal sample to simulate the pressure and failure of the coal pillar in the real roadway.

[0018] Step 7: After arranging the rigid pressure strips of different heights and stiffnesses, fix them with rigid pressure strip clamps, and place each fixed rigid pressure strip on the upper side of the coal sample through the fixing shaft of the rigid pressure strip clamps;

[0019] Step 8: Control the loading equipment to conduct an axial loading test and record the test data.

[0020] The specific method for adjusting the height of each rigid pressure strip in step two is as follows:

[0021] Calculate the height arrangement sequence of the rigid pressure strips:

[0022] Define the length of the long side of the rigid pressure strip section as y, and the length of the wide side as x;

[0023] The width of the elastic zone of the coal sample specimen used in the mechanical test is defined as a mm, the width of the plastic zone is b mm, and the width of the fracture zone is c mm. The number of rigid pressure strips arranged in each zone are n1, n2, and n3, respectively.

[0024] The parameters for obtaining the corresponding coal sample deformation are as follows: from the initial state to the end of the elastic deformation stage, the deformation is d mm; the deformation during the plastic deformation stage is e mm; and the deformation during the pre-peak failure stage is f mm.

[0025] The formulas for calculating the number of rigid strips in each zone are as follows:

[0026]

[0027] The calculation formulas for the height difference of the rigid strips in each zone are as follows:

[0028]

[0029] The height of the first rigid strip arranged in the elastic partition is defined as z1, and the heights of the subsequent rigid strips are z1+h1, z1+2h1, ..., z1+d.

[0030] The height of the first rigid strip arranged in the plastic zone is z1+d+h2, and the heights of the subsequent rigid strips are z1+d+2h2, z1+d+3h2, ..., z1+d+e.

[0031] The height of the first rigid strip arranged in the fracture zone is z1+d+e+h3, and the heights of the subsequent rigid strips are z1+d+e+2h3, z1+d+e+3h3, ..., z1+d+e+f.

[0032] The specific method for selecting rigid pressure strips with different stiffness coefficients in step three is as follows:

[0033] The stiffness of the rigid pressure strip is calculated:

[0034] Determine the load distribution pattern of the coal sample, and treat the distribution pattern as a discrete load, expressed as:

[0035] σ = F(x);

[0036] The loads at each discrete point are σ1, σ2, σ3, ..., σn, respectively.

[0037] Then, the stiffness distribution sequence of the rigid pressure strip is determined according to the load distribution pattern, where the relationship between the rigid pressure strip and the load it bears is:

[0038]

[0039] Where k1, k2, k3, ..., k n These are the stiffness coefficients of each rigid pressure strip, and they satisfy the following:

[0040] σ1:σ2:σ3:…:σ n =k1:k2:k3:…:k n .

[0041] The specific method for determining the normal load value applied to the normal hydraulic transmission shaft in step five is as follows: The formula for calculating the normal load value is:

[0042]

[0043] The normal input load value of the testing machine is calculated, and the specimen is preloaded.

[0044] The advantages of this invention over the prior art are as follows: The non-uniform load loading test scheme provided by this invention achieves the elastic-plastic partitioning of coal samples by arranging rigid pressure bars of different heights in sequence. Based on the fact that rigid pressure bars of different stiffness are subjected to different forces under the same deformation, non-uniform load is applied. The test device provided has the advantages of simple structure, convenient operation and easy processing. It effectively solves the problem of considering the damage of elastic-plastic partitioned coal pillars under non-uniform load, making the test effect of load loading more consistent with the actual field situation. Attached Figure Description

[0045] The present invention will be further described below with reference to the accompanying drawings:

[0046] Figure 1 This is a diagram showing the relationship between the zoning of the surrounding rock of the tunnel and the full stress-strain curve used in this invention;

[0047] Figure 2 This is a schematic diagram of stress zoning under three typical coal pillar loads according to the present invention;

[0048] Figure 3 This is a schematic diagram of the rigid pressure strip arrangement for achieving elastic-plastic zoning under three typical coal pillar loads according to the present invention;

[0049] Figure 4 These are front and side views of the front and rear clamping plates of the rigid pressure bar clamp of the present invention;

[0050] Figure 5 These are the front and top views of the left and right side clamping plates of the rigid pressure bar clamp of the present invention;

[0051] Figure 6 The images show the front and top views of the assembled rigid pressure bar clamp of the present invention (without the rigid pressure bar placed);

[0052] Figure 7 The images show the front and top views of the assembled rigid pressure bar fixture of the present invention (with the rigid pressure bar already in place);

[0053] Figure 8 This is a schematic diagram of the coal sample clamp of the present invention under loading conditions;

[0054] Figure 9 This is a schematic diagram of the coal sample loading test device of the present invention;

[0055] Figure 10 This is a statistical graph of the discretized load distribution of the coal sample in this invention;

[0056] In the diagram: 1 is the rigid pressure bar, 2 is the front and rear side clamps of the rigid pressure bar clamp, 3 is the bolt hole, 4 is the left and right side clamps of the rigid pressure bar clamp, 5 is the bolt, 6 is the coal sample, 7 is the side clamp of the coal sample, 8 is the rear baffle of the coal sample, 9 is the rear baffle fixing shaft, 10 is the lateral hydraulic transmission shaft, 11 is the hydraulic transmission shaft pressure plate, 12 is the fixing shaft of the rigid pressure bar clamp, and 13 is the normal hydraulic transmission shaft. Detailed Implementation

[0057] like Figures 1 to 10 As shown, the present invention provides a non-uniformly distributed load loading test device based on the elastic-plastic partitioning of a bearing coal pillar, including multiple rigid pressure bars 1. Each rigid pressure bar 1 is arranged according to the elastic-plastic stress partitioning form of the coal sample 6. The arranged rigid pressure bars 1 are fixed by the front and rear side clamping plates 2 and the left and right side clamping plates 4 of the rigid pressure bar clamping fixture through bolts 5.

[0058] The sensing end of each rigid pressure bar 1 is in contact with the coal sample 6;

[0059] A normal hydraulic transmission shaft 13 is provided above the detection end of each rigid pressure bar 1;

[0060] The rigid pressure bar clamp is equipped with a rigid pressure bar clamp fixing shaft 12 on the outer side of the left and right side clamping plates 4;

[0061] The coal sample 6 is fixed by a coal sample side clamp 7 and a coal sample rear baffle 8. The outer side of the coal sample side clamp 7 is in contact with the hydraulic transmission shaft pressure plate 11. A lateral hydraulic transmission shaft 10 is installed on the outer side of the hydraulic transmission shaft pressure plate 11. A rear baffle fixing shaft 9 is installed on the outer side of the coal sample rear baffle 8.

[0062] Based on this testing apparatus, the present invention also provides a non-uniformly distributed load loading test method based on the elastic-plastic partitioning of a bearing coal pillar, specifically including the following test steps:

[0063] Step 1: Refer to the correspondence between the surrounding rock zoning of the tunnel and the total stress-strain curve of the rock (see...) Figure 1 Based on the mechanical properties of the surrounding rock, the roadway surrounding rock is divided into four states: loose zone, fracture zone, plastic zone, and elastic zone. This determines the elastoplastic stress zoning pattern of the test coal sample and the stress distribution pattern under applied load (see...). Figure 2 );

[0064] Step 2: Adjust the height of each rigid pressure bar according to the elastoplastic stress zoning pattern of the test coal sample, and select rigid pressure bars of different stiffnesses according to the stress distribution pattern. Sort the rigid pressure bars according to the stiffness distribution sequence (see...). Figure 3 );

[0065] Step 3: Select rigid pressure strips with different stiffness coefficients based on the stiffness distribution, and sort the rigid pressure strips according to the stiffness distribution sequence (see...). Figure 3 );

[0066] Step 4: Determine the axial deformation magnitude of coal samples of the same specifications entering the elastic deformation stage, plastic deformation stage, and fracture development stage through preliminary experiments. Based on the deformation magnitude at each stage, adjust the height and arrangement of the rigid pressure strips to achieve elastoplastic stress zoning of the coal and rock samples after pressurization by the loading equipment.

[0067] Step 5: Determine the normal load value applied by the loading device to the upper end of all rigid pressure bars based on the normal load borne by each rigid pressure bar after sorting. Rigid pressure bars with different stiffnesses experience different forces under the same deformation, so the normal load applied by the loading device is converted into a non-uniformly distributed normal load on the specimen through each rigid pressure bar;

[0068] Step Six: Secure the test coal sample using a coal sample clamp. Lateral loading can be applied to the left and right sides to achieve the predetermined horizontal pressure. The rear baffle can be fixed or removed according to experimental requirements. There is no baffle restricting the front side (the free side of the bearing coal pillar), allowing observation of crack development and coal dust ejection during the coal sample failure process, simulating the pressure and failure of a real roadway bearing coal pillar (see...). Figure 8 );

[0069] Step 7: After arranging the rigid pressure strips of different heights and stiffnesses, fix them using rigid pressure strip clamps, and then fix them to the upper end of the test coal sample based on the fixing axis of the rigid pressure strip clamps (see...). Figures 4 to 7 );

[0070] Step 8: Conduct axial loading test (see...) Figure 9 ).

[0071] The calculation process involved in the above experimental steps is as follows:

[0072] Taking the stress zoning mechanical test under the first type of typical coal pillar load as an example, this is illustrated (see...). Figure 2 , Figure 3 ).

[0073] (1) Calculation of the height arrangement sequence of rigid pressure strips:

[0074] Let the length of the long side of the rigid pressure strip section be y, and the length of the wide side be x. Assume the mechanical test uses a 300mm × 100mm × 100mm (length × width × height) coal sample. The width of the elastic zone is a mm, the width of the plastic zone is b mm, and the width of the fracture zone is c mm. The original rock stress zone is not considered. The number of rigid pressure strips arranged in each zone is n1, n2, and n3, respectively. The deformation parameters of the coal sample obtained from the preliminary experiment are as follows: deformation from the initial state to the end of the elastic deformation stage is d mm, deformation in the plastic deformation stage is e mm, and deformation in the pre-peak failure stage is f mm. The formula for calculating the number of rigid pressure strips arranged in each zone is as follows:

[0075]

[0076] The height differences of the rigid strips in each zone are as follows:

[0077]

[0078] Let the height of the first rigid strip in the flexible partition arrangement be z1, and the heights of the subsequent rigid strips be z1+h1, z1+2h1, ..., z1+d;

[0079] The height of the first rigid strip in the plastic partition arrangement is z1+d+h2, and the heights of the subsequent strips are z1+d+2h2, z1+d+3h2, ..., z1+d+e.

[0080] The height of the first rigid strip in the fracture zone arrangement is z1+d+e+h3, and the subsequent ones are z1+d+e+2h3, z1+d+e+3h3, ..., z1+d+e+f.

[0081] (2) Calculation of the stiffness of the rigid pressure strip:

[0082] Determine the load distribution pattern of the test coal sample and approximate it as a discrete load (see...). Figure 10 The expression is: σ=F(x) (x is a positive integer), and the loads at each discrete point are σ1, σ2, σ3, ..., σn in sequence.

[0083] The stiffness distribution sequence of the rigid pressure strip is determined based on the load distribution pattern. The relationship between the rigid pressure strip and the load it bears is as follows:

[0084]

[0085] Where k1, k2, k3, ..., k n These are the stiffness coefficients of each rigid pressure strip, and they satisfy the following:

[0086] σ1∶σ2∶σ3∶…∶σ n =k1∶k2∶k3∶…∶k n .

[0087] The arrangement sequence of rigid pressure strips can be determined through the above calculations, including the high and low arrangement of rigid pressure strips and the arrangement of stiffness coefficients.

[0088] (3) Determine the normal preload of the testing machine. The calculation formula is as follows:

[0089]

[0090] This allows the determination of the normal input load of the testing machine, thus enabling the preloading of the specimen.

[0091] To prevent the rigid pressure bar from yielding during loading, the present invention requires that the compressive strength of the selected rigid pressure bar be no less than twice the required load; the roughness of the rigid pressure bar be no greater than Ra0.40, and the dimensional error be no greater than 0.5%; at the same time, the surface of the rigid pressure bar used needs to be polished to ensure that the contact surfaces are relatively smooth; lubricating oil needs to be applied between rigid pressure bars and between rigid pressure bars and rigid pressure bar clamps to reduce the influence of friction on the experimental error.

[0092] This invention further improves the practicality and accuracy of experiments: by simulating the elastoplastic partitioning behavior of actual coal pillars under non-uniform loads, this experimental method provides experimental data that more closely approximates actual mining conditions, thereby improving the practicality and accuracy of indoor experiments; the experiment can deepen the understanding of coal pillar failure mechanisms: this experimental method helps researchers to understand the failure mechanisms of coal pillars under different load conditions, especially the mechanical behavior during the formation and development of elastoplastic partitions; and based on the experimental results, the design parameters and support measures of coal pillars can be optimized to improve the stability and bearing capacity of the coal pillars and reduce safety accidents in coal mining; through indoor experimental research, the placement and mining processes of coal pillars can be optimized to improve resource recovery rates and reduce resource waste. In addition, the stability and failure risk of coal pillars can be predicted before practical application, avoiding economic losses caused by coal pillar failure in actual mining, and contributing to the development of more effective coal pillar stability assessment methods, providing a scientific basis for mine safety and reducing the incidence of mine accidents.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for conducting tests based on a non-uniformly distributed load loading test device for elastic-plastic partitioning of a coal pillar, characterized in that: The non-uniform load loading test device based on the elastic-plastic partition of the bearing coal pillar includes multiple rigid pressure bars (1). Each rigid pressure bar (1) is arranged according to the elastic-plastic stress partition of the coal sample (6). The arranged rigid pressure bars (1) are fixed by bolts (5) through the front and rear side clamps (2) and the left and right side clamps (4) of the rigid pressure bar clamp. The sensing end of each rigid pressure bar (1) is in contact with the coal sample (6); A normal hydraulic transmission shaft (13) is provided above the detection end of each rigid pressure bar (1); The rigid pressure bar clamp has a rigid pressure bar clamp fixing shaft (12) installed on the outer side of the left and right side clamping plates (4); The coal sample (6) is fixed by a coal sample side clamp (7) and a coal sample rear baffle (8). The outer side of the coal sample side clamp (7) is in contact with the hydraulic transmission shaft pressure plate (11). A lateral hydraulic transmission shaft (10) is installed on the outer side of the hydraulic transmission shaft pressure plate (11). A rear baffle fixing shaft (9) is installed on the outer side of the coal sample rear baffle (8). The steps for conducting the experiment are as follows: Step 1: Determine the elastoplastic stress zoning pattern of coal sample (6) and the stress distribution pattern of the applied load; Step 2: Adjust the height of each rigid pressure bar (1) according to the elastoplastic stress zoning pattern of the coal sample (6), and select rigid pressure bars (1) with different stiffnesses according to the stress distribution pattern. Arrange each rigid pressure bar (1) according to the stiffness distribution sequence. The specific method is as follows: Calculate the height arrangement sequence of the rigid pressure strips: Define the length of the long side of the rigid pressure strip section as y, and the length of the wide side as x; The width of the elastic zone of the coal sample specimen used in the mechanical test is defined as a mm, the width of the plastic zone is b mm, and the width of the fracture zone is c mm. The number of rigid pressure strips (1) arranged in each zone are n1, n2, and n3, respectively. The parameters for obtaining the corresponding coal sample deformation are as follows: from the initial state to the end of the elastic deformation stage, the deformation is d mm; the deformation during the plastic deformation stage is e mm; and the deformation during the pre-peak failure stage is f mm. The calculation formulas for the number of rigid pressure strips (1) arranged in each zone are as follows: The calculation formulas for the height difference of the rigid strips (1) in each zone are as follows: The height of the first rigid strip (1) arranged in the elastic partition is defined as z1, and the heights of the subsequent rigid strips (1) are z1+h1, z1+2h1, ..., z1+d. The height of the first rigid strip (1) arranged in the plastic zone is z1+d+h2, and the heights of the subsequent rigid strips (1) are z1+d+2h2, z1+d+3h2, ..., z1+d+e. The height of the first rigid strip (1) arranged in the fracture zone is z1+d+e+h3, and the heights of the subsequent rigid strips (1) are z1+d+e+2h3, z1+d+e+3h3, ..., z1+d+e+f; Step 3: Select rigid pressure strips (1) with different stiffness coefficients according to the stiffness distribution, and arrange the rigid pressure strips (1) according to the sequence of stiffness distribution. The specific method is as follows: The stiffness of the rigid pressure strip is calculated: Determine the load distribution pattern of the coal sample (6), and process the distribution pattern into discrete loads, expressed as: σ = F(x); The loads at each discrete point are σ1, σ2, σ3, ..., σn, respectively. Then, the stiffness distribution sequence of the rigid pressure strip (1) for loading is determined according to the load distribution pattern, wherein the relationship between the rigid pressure strip (1) and the load it bears is: Where k1, k2, k3, ..., k n These are the stiffness coefficients of each rigid pressure strip (1), and they satisfy: σ1:σ2:σ3:…:σ n =k1:k2:k3:…:k n； Step 4: Determine the axial deformation of coal samples (6) of the same specifications when they enter the elastic deformation stage, plastic deformation stage and crack development stage through preliminary experiments. Adjust the height and arrangement of each rigid pressure bar (1) according to the deformation size of each stage. Control the coal sample (6) to be divided into zones according to elastic-plastic stress after the loading equipment is pressurized. Step 5: Use the sorted rigid pressure strips (1) to bear the normal load and determine the normal load value applied by the normal hydraulic transmission shaft (13) at the detection end of each rigid pressure strip (1); Step 6: Fix the coal sample (6) with a coal sample clamp, apply lateral load to the left and right sides of the coal sample (6), and fix or remove the rear baffle when the predetermined horizontal pressure is reached. Observe the crack development and coal dust outflow during the coal sample failure process from the front end of the coal sample (6) to simulate the pressure and failure of the real roadway bearing coal pillar. Step 7: Arrange the rigid pressure strips (1) of different heights and stiffnesses, fix them with rigid pressure strip clamps, and place each fixed rigid pressure strip (1) on the upper side of the coal sample (6) through the rigid pressure strip clamp fixing shaft (12). Step 8: Control the loading equipment to conduct an axial loading test and record the test data.

2. The method for conducting tests using the non-uniformly distributed load loading test device based on the elastic-plastic partitioning of a bearing coal pillar according to claim 1, characterized in that: The specific method for determining the normal load value applied to the normal hydraulic transmission shaft in step five is as follows: The formula for calculating the normal load value is: The normal input load value of the testing machine is calculated, and the specimen is preloaded.

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