Rock steady failure test method based on stress-strain hybrid control

Abstract

The present application belongs to the field of geotechnical engineering and rock mechanics testing technology, and discloses a rock stable failure testing method based on stress-strain mixed control, aiming to solve the problem that the existing true triaxial test cannot simultaneously maintain the stress path and suppress brittle failure after the peak strength of rock. The core steps include: first, determining the target stress state (and) and the strain growth rate threshold; then, loading the hydrostatic pressure; then, loading and unloading at a constant proportion rate in the stress control mode; when the strain rate reaches the threshold, switching to the stress-strain mixed control mode, turning the maximum principal stress direction to strain control to stabilize the failure process, and dynamically coordinating the stress of the remaining two directions based on real-time feedback, so that the whole failure process always maintains the preset stress path. The present application can realize complete and stable testing of the pre-peak and post-peak mechanical behavior of rock under any complex stress state, and provides key technical support for accurately verifying the strength criterion and studying the failure mechanism.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering and rock mechanics testing technology, and specifically relates to a true triaxial test method for testing the mechanical properties of rocks. Specifically, it is a true triaxial loading and unloading control method that can controllably destroy rock samples under any stress state and fully capture their post-peak mechanical behavior. Background Technology

[0002] True triaxial testing is used to study rocks under anisotropic stress states (i.e., the three principal stresses). > > The key means of simulating the mechanical properties of rocks (which are not equal) is to simulate the actual stress environment of rock masses in strata in underground engineering, mining, oil and gas extraction, which is crucial for a deep understanding of the strength, deformation and failure mechanism of rocks.

[0003] Currently, conventional true triaxial rock testing methods mainly fall into two control modes: stress control and strain control. Stress control offers advantages such as a clear loading path and ease of implementation, typically applying loads independently in three orthogonal directions to the specimen using hydraulic or rigid loading plates. However, when the rock is loaded to near its peak strength, internal microcracks rapidly propagate and penetrate, leading to severe brittle fracture. The failure process cannot be stably controlled, resulting in the inability to obtain data on the post-peak softening phase and significant damage to testing equipment and sensors. Strain control, on the other hand, controls loading by adjusting displacement or strain rate, effectively suppressing sudden collapse after the peak and obtaining a complete stress-strain curve. However, with strain control, the stress state cannot be controlled, making it difficult to precisely maintain a preset stress state (such as a constant stress Lode angle). ).

[0004] The commonly used loading method for true triaxial testing is: first apply the minimum principal stress ( ) and intermediate principal stress ( ) until the specified state remains unchanged, then apply the maximum principal stress ( ) to rock sample failure. Under the maximum principal stress ( To achieve stable failure of rock, stress-controlled loading is first applied to 70% of the peak strength, followed by strain-controlled loading until failure, obtaining the full stress-strain curve including the post-peak state. This method can achieve stable failure of rock, but it is not suitable for controlling the stress state at failure. and It is difficult to achieve.

[0005] Stress control can precisely control the loading path but cannot stably control the failure process; while strain control can obtain post-peak curves but struggles to accurately maintain the target stress state under complex paths. This contradiction severely restricts the accurate characterization and study of the mechanical behavior of rocks under arbitrary true triaxial stress states, especially near failure.

[0006] Therefore, there is an urgent need for a novel test control method that integrates the advantages of both stress control and strain control. This method needs to precisely control the stress path to an arbitrary target stress state before failure, and smoothly and automatically switch to a strain control mode that can maintain a preset stress state ratio when the rock enters the accelerated deformation and failure stage. This will enable safe, stable, and accurate testing of the rock throughout the entire process from initial loading and peak strength to post-peak softening. Summary of the Invention

[0007] To address the problem that existing true triaxial tests cannot simultaneously achieve "precise control of stress path" and "suppression of brittle rock failure," this invention provides a rock stability failure testing method based on stress-strain hybrid control. This method enables complete and stable testing of the pre-peak and post-peak mechanical behavior of rocks under any preset stress state, providing reliable data for studying rock failure mechanisms and verifying strength criteria.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A rock stability failure testing method based on stress-strain hybrid control includes the following steps:

[0010] S1: Determine the target stress state for the test and set the maximum allowable strain rate threshold. The target stress state is determined by the target stress level. And the target stress Lode angle Common characterization, in which the target stress Lode angle The value range is -30° to 30°, and the target stress level is... Basic mechanical parameters of the rock were obtained through preliminary experiments, and calculations were performed based on strength criteria to ensure the expected minimum principal stress at rock failure. ;

[0011] S2: Perform hydrostatic pressure loading, simultaneously applying principal stresses in the three principal directions at the same stress loading and unloading rate until the target stress level is reached. The corresponding hydrostatic pressure state, at which point the stress state in the three principal directions satisfies ;

[0012] S3: In stress control mode, to maintain and A constant proportional rate is used to load and unload the principal stresses in the three directions until the strain rate increase in any principal direction reaches a threshold. ;

[0013] S4: Switch to stress-strain hybrid control mode to achieve strain control and stress control targets, and continue loading until the rock sample fails and is then unloaded;

[0014] The strain control objective is to ensure that the strain rate increase in all three directions does not exceed the threshold. The method of implementation is to use the maximum principal stress The direction switching is strain feedback control, which adjusts the input energy of the hydraulic servo system in that direction in real time to constrain the strain growth rate in all three directions to not exceed a threshold. ;

[0015] The stress control objective is to maintain the stress state of the specimen throughout the failure process. and Constant, achieved through intermediate principal stress. With minimum principal stress The direction remains in stress control mode, and a closed-loop predictive correction strategy based on discrete time steps is used for control.

[0016] As a further description of the above technical solution: the target stress state described in step S1 uses stress invariants in the stress space of the Haigh-Westergaard coordinate system ( , , ) indicates that, This is the first stress invariant, characterizing the (sphere) stress level; It is the square root of the second deviatoric stress invariant, representing the magnitude of the deviatoric stress; is the Lode angle of stress, which characterizes the angular position of the stress state on the π plane;

[0017] The stress invariants and stress tensors The transformation relationship for (i = 1, 2, 3) is as follows:

[0018] ,

[0019] ,

[0020] ;

[0021] And stress tensor Expressed in terms of stress invariants:

[0022] .

[0023] As a further description of the above technical solution, the constant proportional rate in step S3 is achieved by controlling the minimum principal stress. Directional loading and unloading rates To achieve a constant value;

[0024] The loading / unloading rate is defined as:

[0025] ;

[0026] in, For different measurement time points during the loading process, For fixed measurement time intervals;

[0027] Given the minimum principal stress Directional loading and unloading rates Under the condition of maintaining and Constant constraints, maximum principal stress With intermediate principal stress Directional loading and unloading rates and They are respectively:

[0028] ,

[0029] .

[0030] As a further description of the above technical solution, the control logic of the strain feedback control satisfies:

[0031] ;

[0032] At this point, the maximum principal stress The rate of change of stress in the direction is no longer a preset control quantity, but a dynamic feedback variable characterizing the real-time mechanical state of the specimen.

[0033] As a further description of the above technical solution, the method for achieving the stress control target specifically includes the following steps:

[0034] S41, Stress State Sensing: In At any given time, obtain the current stress values ​​in three directions ( , , ), of which the maximum principal stress The direction is in strain feedback control mode, and its stress change rate This is the system feedback value;

[0035] S42, Target prediction: based on the current rate of stress change As the rate of stress change at the next moment Forecast Stress value at time :

[0036] ;

[0037] S43, and Target prediction: based on and Constant constraints, according to Solve Target stress value at time and :

[0038] ,

[0039] ;

[0040] S44, Control Output: Calculate intermediate principal stress With minimum principal stress The next stress change rate in the direction and :

[0041] ,

[0042] ;

[0043] S45, Closed-loop verification: Measurement The actual stress state at time ( , , The actual measured value is used as the initial state of step S41 and the above process is repeated to achieve closed-loop tracking and dynamic correction of the stress path.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] This invention switches to a stress-strain hybrid control mode when the sample is near failure. While utilizing strain feedback to suppress runaway energy release and ensure a stable failure process, it maintains a preset stress state through a unique coordination algorithm. and By maintaining the constant stress level, this invention successfully obtains key parameters such as the post-peak stress-strain relationship and residual strength of rocks, which are unattainable by traditional methods. Furthermore, this invention can systematically and repeatedly achieve stable failure of rocks under any specified stress state, providing a reliable and standard experimental paradigm for accurately plotting the π-plane strength envelope, rigorously verifying the universality of various strength criteria, and studying the energy evolution law of the entire rock failure process. In addition, this method effectively prevents severe rock sample failure, improving the safety of experimental equipment and personnel. Attached Figure Description

[0046] Figure 1 This is a flowchart of the rock stability failure test method based on stress-strain hybrid control of the present invention.

[0047] Figure 2 This is a schematic diagram of true triaxial loading.

[0048] Figure 3 This is a schematic diagram of the stress path.

[0049] Figure 4 This is a schematic diagram of the loading stress path in stress space.

[0050] Figure 5 This is a schematic diagram of the stress-strain hybrid control mode.

[0051] Figure 6 These are the stress and strain test results from application examples.

[0052] Figure 7 These are the results of stress invariants and strain control in application examples. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures, but do not constitute any limitation on the present invention. Any limited modifications made by any person within the scope of protection of the claims of the present invention are still within the scope of protection of the claims of the present invention.

[0054] A rock stability failure testing method based on stress-strain hybrid control, the process of which is as follows: Figure 1 As shown, it includes the following steps:

[0055] S1. Determine the target stress state of the test, based on the target stress level. And the target stress Lode angle Common characterization, and setting a maximum allowable strain rate growth threshold. .

[0056] The target stress state is expressed in stress invariants in the Haigh-Westergaard coordinate system stress space. , , This indicates that it is related to the stress tensor. The transformation relationship for (i = 1, 2, 3) is as follows:

[0057] ,

[0058] ,

[0059] .

[0060] Stress Tensor Using stress invariants ( , , ) is represented as:

[0061] ;

[0062] Target stress Lode angle Select within the effective range of -30° to 30°;

[0063] Target stress level Basic mechanical parameters of the rock were obtained through preliminary experiments, and calculations were performed based on strength criteria to ensure that the target stress level was met. Under the condition of rock failure, the expected minimum principal stress .

[0064] S2. Perform hydrostatic pressure loading, simultaneously applying principal stresses in the three principal directions at the same stress loading and unloading rate, such as... Figure 2 As shown, until the target stress level is reached. The corresponding hydrostatic pressure state, such as Figure 3 During the t0-t1 segment, the stress in the three principal directions is: ,like Figure 4 The path OO' is shown in the middle.

[0065] S3: Under stress control mode ( Figure 3 (mid t1-t2 segment) to maintain and A constant proportional rate is used to load and unload the principal stresses in the three directions until the strain rate increase in any principal direction reaches a threshold. .

[0066] The stress loading and unloading rate is defined as:

[0067] ;

[0068] in, For different measurement time points during the loading process, For fixed measurement time intervals.

[0069] Controlling the minimum principal stress Directional loading and unloading rates Under the condition that -100 kPa / s is a constant value, based on maintaining and Constant constraints, maximum principal stress With intermediate principal stress Directional loading and unloading rates and They are respectively:

[0070] ,

[0071] .

[0072] S4: Switch to stress-strain hybrid control mode ( Figure 3 (Middle > t2 segment), its core control objectives include:

[0073] Strain control objective: To suppress strain instability and ensure that the strain growth rate in all three directions does not exceed the stated threshold. ;

[0074] Stress control objective: To maintain the stress state of the specimen throughout the failure process. and Constant.

[0075] The strain control objective is achieved by controlling the maximum principal stress. The direction switching is strain feedback control, which adjusts the input energy of the hydraulic servo system in that direction in real time to constrain the strain rate increase in all three directions to not exceed the maximum allowable strain rate increase threshold. Its control logic satisfies:

[0076] ;

[0077] In this mode, The rate of change of stress in the direction is no longer a preset control quantity, but a dynamic feedback variable characterizing the real-time mechanical state of the specimen.

[0078] The stress control objective is achieved by controlling the intermediate principal stresses. With minimum principal stress The direction is maintained in stress control mode, and a closed-loop predictive correction strategy based on discrete time steps is used for control, such as... Figure 5 As shown, the specific steps include:

[0079] S41, Stress State Sensing: In At any given time, obtain the current stress values ​​in three directions ( , , ),in The direction is in strain feedback control mode, and its stress change rate This is the system feedback value;

[0080] S42, σ1 target prediction: based on the current rate of stress change As the rate of stress change at the next moment Forecast Stress value at time :

[0081] ;

[0082] S43, and Target prediction: based on and Constant constraints, according to Solve Target stress value at time and :

[0083] ,

[0084] ;

[0085] S44, Control Output: Calculation and The next stress change rate in the direction and :

[0086] ,

[0087] ;

[0088] S45, Closed-loop verification: Measurement The actual stress state at time ( , , The actual measured value is used as the initial state of step S41, and the above process is repeated to achieve closed-loop tracking and dynamic correction of the stress path.

[0089] The load was continuously applied until the rock showed obvious signs of failure (such as a continuous decrease in stress and a sharp increase in strain), after which unloading was performed to complete the test.

[0090] Application examples:

[0091] The following describes the implementation process of the present invention in detail with reference to specific parameters and operations:

[0092] 1. Determine the experimental objectives

[0093] Target stress level =180 MPa (calculated through preliminary experiments and strength criteria to ensure rock failure) >0);

[0094] Target stress Lode angle Seven groups were selected at equal intervals within the effective range of -30° to 30°, namely -30°, -20°, -10°, 0°, 10°, 20°, and 30°.

[0095] Maximum allowable strain growth rate threshold =0.08 mm / min.

[0096] 2. Hydrostatic pressure loading

[0097] Stress was applied simultaneously to the rock in three principal directions at a stress loading rate of 0.5 MPa / s until... Complete hydrostatic pressure loading (corresponding to) Figure 4 (middle path OO').

[0098] 3. Stress-controlled loading

[0099] Set minimum principal stress The unloading rate in the direction is -100 kPa / s (constant), according to "maintaining and By applying the constraint of "constant", different Lode angles can be calculated. Down , Directional loading and unloading rates and The specific values ​​are shown in Table 1. Loading and unloading are performed at the rates shown in Table 1 until the strain growth rate in any principal direction reaches [a certain value]. =0.08 mm / min.

[0100] Table 1 Different Lode Angles Loading and unloading rates (kPa / s) in the next three principal stress directions

[0101]

[0102] 4. Stress-strain hybrid controlled loading

[0103] Strain control implementation: Direction switching is achieved through strain feedback control, which adjusts the input energy in real time via a hydraulic servo system to ensure that the strain growth rate in all three directions is ≤0.08 mm / min.

[0104] Stress control implementation: , Orientation-maintaining stress control, performing closed-loop predictive correction:

[0105] exist At any given time, obtain the current stress values ​​in three directions ( , , );

[0106] With the current Stress change rate Predict the next moment ( (Time) Stress Change Rate , combined =180 MPa, target ,calculate Target stress value at time and ;

[0107] Adjust according to the calculated rate , Load the load, measure the actual stress value at the next moment, and repeat the correction.

[0108] The load was continuously applied until the rock showed obvious signs of failure (such as a continuous decrease in stress and a sharp increase in strain), after which unloading was performed to complete the test.

[0109] Example Result Verification: When At -20°, the stress-strain curve obtained from the experiment is as follows: Figure 6 As shown, the complete pre-peak elastic segment, peak intensity point, and post-peak softening segment are visible. Figure 7 Display of the test process The pressure remained relatively stable at 180 MPa. The strain rate remained stable at -20°C and did not exceed the threshold of 0.08 mm / min, verifying the effectiveness of the proposed method.

[0110] The specific implementation of the present invention has been described in detail above with reference to preferred embodiments. However, this is not intended to limit the present invention to the specific forms disclosed. Those skilled in the art should understand that, based on the understanding of the core concept of the present invention, various obvious modifications or variations can be made to some of the technical features. Therefore, any solution that can be obtained through logical deduction based on the design idea and principle of the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A rock stability failure testing method based on stress-strain hybrid control, characterized in that, Includes the following steps: S1: Determine the target stress state for the test and set the maximum allowable strain rate threshold. The target stress state is determined by the target stress level. And the target stress Lode angle Common characterization, in which the target stress Lode angle The value range is -30° to 30°, and the target stress level is... Basic mechanical parameters of the rock were obtained through preliminary experiments, and calculations were performed based on strength criteria to ensure the expected minimum principal stress at rock failure. ; S2: Perform hydrostatic pressure loading, simultaneously applying principal stresses in the three principal directions to the rock at the same stress loading and unloading rate until the target stress level is reached. The corresponding hydrostatic pressure state, at which point the stress state in the three principal directions satisfies ; S3: In stress control mode, to maintain and A constant proportional rate is used to load and unload the principal stresses in the three directions until the strain rate increase in any principal direction reaches a threshold. ; S4: Switch to stress-strain hybrid control mode to achieve strain control and stress control targets, and continue loading until the rock sample fails and is then unloaded; The strain control objective is to ensure that the strain rate increase in all three directions does not exceed the threshold. The method of implementation is to use the maximum principal stress The direction switching is strain feedback control, which adjusts the input energy of the hydraulic servo system in that direction in real time to constrain the strain growth rate in all three directions to not exceed a threshold. ; The stress control objective is to maintain the stress state of the specimen throughout the failure process. and Constant, achieved through intermediate principal stress. With minimum principal stress The direction remains in stress control mode, and a closed-loop predictive correction strategy based on discrete time steps is used for control.

2. The rock stability failure testing method based on stress-strain hybrid control according to claim 1, characterized in that: The target stress state described in step S1 uses stress invariants in the stress space of the Haigh-Westergaard coordinate system. , , ) indicates that, As the first stress invariant, The square root of the second deviatoric stress invariant. The Lode angle is the stress angle. The stress invariants and stress tensors The transformation relationship for (i = 1, 2, 3) is as follows: ， ， ； And stress tensor Expressed in terms of stress invariants: 。 3. The rock stability failure testing method based on stress-strain hybrid control according to claim 2, characterized in that, The constant proportional rate described in step S3 is achieved by controlling the minimum principal stress. Directional loading and unloading rates To achieve a constant value; The loading / unloading rate is defined as: ； in, For different measurement time points during the loading process, For fixed measurement time intervals; Given the minimum principal stress Directional loading and unloading rates Under the condition of maintaining and Constant constraints, maximum principal stress With intermediate principal stress Directional loading and unloading rates and They are respectively: ， 。 4. The rock stability failure testing method based on stress-strain hybrid control according to claim 1, characterized in that, The control logic of the strain feedback control satisfies: ； At this point, the maximum principal stress The rate of stress change in the direction is a dynamic feedback variable characterizing the real-time mechanical state of the specimen.

5. The rock stability failure testing method based on stress-strain hybrid control according to claim 1, characterized in that, The specific steps to achieve the stress control objective include: S41, Stress State Sensing: In At any given time, obtain the current stress values ​​in three directions ( , , ); S42, Target prediction: based on the current rate of stress change As the rate of stress change at the next moment Forecast Stress value at time : ； S43, and Target prediction: based on and Constant constraints, according to Solve Target stress value at time and : ， ； S44, Control Output: Calculate intermediate principal stress With minimum principal stress The next stress change rate in the direction and : ， ； S45, Closed-loop verification: In Measure the actual stress state at all times. , , The actual measured value is used as the initial state of step S41 and the above process is repeated to achieve closed-loop tracking and dynamic correction of the stress path.

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