A multi-axis electromagnetic impact fatigue test system and test method

Abstract

This invention discloses a multi-axis electromagnetic impact fatigue testing system and method. The system includes a stress wave generation module, a capacitor bank, a charging circuit, high-voltage thyristors, and a charge / discharge control center. The stress wave generation module contains at least four spatially opposed electromagnetic stress wave generating devices, each containing a discharge coil, an induction coil, and a waveform amplifier. The length of the waveguide rod is determined based on one-fiftieth of the product of the loaded wave pulse width and the material wave velocity, while also satisfying engineering constraints. The charge / discharge control center includes a stress wave acquisition unit, a fatigue-timing controller, a voltage detection module, and a switch controller. The fatigue-timing controller performs independent microsecond-level delay trigger control on the conduction time of each high-voltage thyristor and voltage closed-loop compensation after discharge, realizing synchronous and repeated loading of multi-axis impact loads and long-term constant amplitude output. This significantly reduces the system's spatial size and can be used for multi-axis impact fatigue life testing of materials and structures.

Description

Technical Field

[0001] This invention belongs to the technical field of fatigue testing equipment, specifically relating to a multi-axis electromagnetic shock fatigue testing device and testing method. Background Technology

[0002] Impact fatigue refers to the type of load caused by repeated impact loading. In structures such as aircraft landing gear, carrier-based aircraft arresting gear, and automatic firearm mechanisms, fatigue problems caused by repeated impacts must be considered. Impact fatigue testing is commonly used to test the life and failure behavior of materials and structures under cyclic loading. In practical engineering, structural components subjected to impact fatigue are often in a complex triaxial stress state, with strain rate effects deeply coupled with multiaxial stress states, necessitating the development of methods and equipment capable of testing multiaxial impact fatigue performance.

[0003] Existing impact fatigue testing equipment includes pendulum-type, falling-weight type, and split Hopkinson bar (SHB) methods, all of which are uniaxial impact methods. Because impact testing equipment generates short pulses with pulse widths ranging from microseconds to milliseconds, it is difficult to achieve multiaxial synchronous loading, and repeated impact loading places higher demands on multiaxial synchronization. Currently, the testing method that is advantageous for achieving multiaxial synchronous dynamic loading is electromagnetic Hopkinson bar technology. Examples include the electromagnetic force-based Hopkinson tension / compression bar stress wave generator and experimental method disclosed in Chinese Patent No. 201410173843.1, and an electromagnetically driven Hopkinson bar multiaxial tension / compression composite system disclosed in Chinese Patent No. 202310297713.8. This type of method utilizes pulse power conversion technology to directly convert electromagnetic force pulses into stress wave pulses via stress wave amplifiers, achieving multiaxial dynamic loading. This technology eliminates the need for intermediate mechanical transmission, significantly improving the synchronization of multiaxial loading and showing certain advantages in single-cycle multiaxial synchronous loading.

[0004] However, the above-mentioned electromagnetic multi-axis Hopkinson bar loading technology still has the following defects: (1) Its design goal is only a single high strain rate loading, without involving repeated impact fatigue conditions, and lacks systematic consideration for long-term cyclic loading; (2) It does not provide a solution to the load amplitude attenuation during multiple charging and discharging processes. The electromagnetic loading system uses capacitor banks for energy storage. During multiple charging and discharging processes, voltage attenuation will occur due to internal resistance loss, switching characteristic changes, etc., resulting in a gradual decrease in the impact load amplitude, which cannot meet the constant amplitude loading conditions required by the fatigue test; (3) It still follows the traditional long rod design concept of Hopkinson bars. In order to meet the one-dimensional stress wave propagation conditions, the length of the rod is usually required to be tens or even hundreds of times the diameter. If the rod system is extended to a multi-axis system, multiple long rods will form a three-dimensional cross structure in space, which puts forward harsh requirements on the experimental space and makes it difficult to align the rods coaxially, install and debug them, resulting in poor engineering practicality; (4) It lacks the means to control the timing of multi-channel discharge devices, making it difficult to compensate for the synchronization error caused by the difference in line impedance. Summary of the Invention

[0005] To address the comprehensive technical challenges of existing uniaxial impact fatigue testing equipment being unable to simulate multiaxial stress states, existing electromagnetic multiaxial Hopkinson bar technology only supporting single loading and lacking synchronous holding and constant amplitude control mechanisms under long-term fatigue conditions, and the excessive size of traditional waveguide rods making multiaxial systems difficult to implement in engineering, this paper proposes a multiaxial electromagnetic impact fatigue testing system and method that can simultaneously meet the objectives of multiaxial synchronous loading, repeated constant amplitude impact, and compact and usable structure.

[0006] To achieve the above objectives, the technical solution provided by this invention is:

[0007] A multi-axis electromagnetic impact fatigue testing system includes a stress wave generation module, a capacitor bank, a charging circuit, a high-voltage thyristor, and a charge / discharge control center. The stress wave generation module comprises at least four spatially opposed electromagnetic stress wave generating devices. Each electromagnetic stress wave generating device is used to transfer the generated pulse load to the specimen and includes a discharge coil, an induction coil, a push rod, a variable cross-section waveform amplifier, and a miniaturized waveguide rod.

[0008] The discharge coil is used to receive discharge pulses and generate a transient magnetic field. The induction coil is coupled to the discharge coil to generate eddy current repulsion. One end of the push rod is fixedly connected to the induction coil.

[0009] The large end of the variable cross-section waveform amplifier is connected to the other end of the push rod, and its small end is connected to one end of the miniaturized waveguide rod. The other end of the miniaturized waveguide rod is used to contact the sample; the length of the miniaturized waveguide rod... satisfy And simultaneously satisfy The actual value is the maximum of the two; where To load the pulse width, The wave velocity is the wave velocity of the waveguide rod material. The diameter of the waveguide rod;

[0010] Each electromagnetic stress wave generating device is independently equipped with a high-voltage thyristor. Each high-voltage thyristor is connected in series between the corresponding capacitor bank and the discharge coil to control the discharge on and off of the discharge coil. The capacitor bank is connected to each discharge coil to store the discharge energy required by the discharge coil. The input terminal of the charging circuit is used to connect to an external power source, and its output terminal is connected to the capacitor bank to charge the capacitor bank.

[0011] The charge and discharge control center is configured to: control the electromagnetic stress wave generator to apply constant amplitude loading impact to the sample; acquire the stress wave waveform generated by the electromagnetic stress wave generator in real time; monitor the voltage of the capacitor bank; independently control the triggering time of each high-voltage thyristor; and control the charging of the capacitor bank.

[0012] Furthermore, the discharge coil and the induction coil are arranged in a close-fitting coaxial manner, with an axial gap of 0.5mm to 2mm between them.

[0013] Furthermore, the charge / discharge control center includes a stress wave acquisition unit, a fatigue-timing controller, a voltage detection module, and a switch controller; among which:

[0014] The capacitor bank includes multiple capacitors connected in parallel; the positive terminal of the capacitor bank is connected to one end of the charging circuit via a switch controller, and the negative terminal is connected to the other end of the charging circuit.

[0015] The voltage detection module is connected to the positive terminal of the capacitor bank and is used to monitor the voltage of each capacitor in the capacitor bank in real time.

[0016] The input terminal of the fatigue-timing controller is connected to the output terminal of the voltage detection module to receive the voltage feedback signal output by the voltage detection module; its output terminal is connected to the gate of each high-voltage thyristor to independently control the triggering time of each high-voltage thyristor; its output is also connected to the feedback terminal of the switch controller to control the charging circuit to recharge the capacitor bank after each discharge according to the voltage feedback signal, so as to keep the voltage constant before each impact.

[0017] The stress wave acquisition unit is used to acquire the stress wave waveform generated by each electromagnetic stress wave generator in real time during the loading and impact process.

[0018] Furthermore, the stress wave acquisition unit includes an impact load data acquisition and waveform display and multiple high-frequency strain gauges; the multiple high-frequency strain gauges are respectively attached to the middle of the corresponding miniaturized waveguide rods;

[0019] The signal output terminal of the high-frequency strain gauge is connected to the impact load acquisition and waveform display for real-time recording of the stress wave waveform of each channel.

[0020] Furthermore, it also includes a high-speed camera for acquiring speckle images of the sample surface in real time during the impact fatigue test; the high-speed camera has a frame rate of 50,000 to 200,000 fps.

[0021] Furthermore, the electromagnetic stress wave generating device is arranged in four groups, horizontally opposed in a biaxial four-way configuration; or in six groups, orthogonally opposed in a triaxial six-way configuration.

[0022] This invention also provides a multiaxial electromagnetic impact fatigue testing method, implemented based on the aforementioned designed multiaxial electromagnetic impact fatigue testing system. The multiaxial electromagnetic impact fatigue testing method includes the following steps:

[0023] Step 1, Sample clamping and parameter setting:

[0024] The sample is clamped between multiple miniaturized waveguide rods; test parameters including target impact force, pulse frequency and total number of cycles are set.

[0025] Step 2, No-load pre-trigger calibration:

[0026] A single low-level trigger is performed using a fatigue-timing controller. The arrival time of each stress wave is observed, and the conduction delay time of each high-voltage thyristor is obtained and set so that each stress wave pulse arrives at the sample synchronously.

[0027] Step 3, Automated Cyclic Impact and Voltage Constant Amplitude Control:

[0028] The following process is automatically repeated according to the preset loading frequency: Perform discharge impact and collect the actual loading amplitude of this time; after the discharge ends, perform closed-loop compensation of the charging voltage to the preset target voltage based on the deviation between the actual loading amplitude and the target impact force, so that the voltage before each impact remains constant; repeat the above process until the preset number of cycles is reached or the sample fails.

[0029] Step 4, fatigue life determination:

[0030] The impact fatigue life of the specimen is determined based on the measured stress wave waveform distortion or the total number of loading cycles.

[0031] Furthermore, the specific process of obtaining and setting the conduction delay time of the high-voltage thyristor in step 1 to ensure that each stress wave pulse arrives at the sample synchronously is as follows:

[0032] Using the wavefront rise point or the first characteristic peak point of the stress wave in each channel as a time marker, the arrival time of each waveform is recorded, and the maximum time deviation is calculated. If the maximum time deviation If the error exceeds the preset microsecond-level allowable error threshold, mechanical backlash compensation or electrical timing compensation will be performed, and no-load pre-trigger calibration will be performed again until the arrival time difference of each waveform is less than or equal to the preset microsecond-level allowable error threshold.

[0033] Furthermore, in step 3, the actual loading amplitude of this impact is collected by a high-frequency strain gauge attached to a miniaturized waveguide rod, the deviation between the actual loading amplitude and the preset target impact force is calculated, and the charging voltage adjustment required for the next impact is calculated by the adaptive compensation algorithm built into the fatigue-timing controller.

[0034] Adjust the charging voltage threshold according to the fine-tuning amount of the charging voltage, and recharge the capacitor bank after the current impact ends, so that the voltage error before each impact is controlled within ±0.5%.

[0035] Furthermore, in step 4, if the impact fatigue life of the specimen is defined by the specimen fracture, then the total number of loading cycles at the time of fracture is taken as the specimen fracture life.

[0036] If crack initiation is used to define the impact fatigue life of a specimen, it is determined based on the measured stress wave waveform distortion. The specific method is as follows: when there are no abnormalities in the incident wave peak value, incident wave pulse width, and loading energy, the number of loading cycles that first meet the following combination of conditions is taken as the crack initiation life of the specimen:

[0037] Condition 1: If the measured amplitude of the reflected wave or the transmitted wave exceeds the preset corresponding amplitude threshold range, it indicates that the stress wave waveform is distorted.

[0038] Condition 2: The transmitted wave amplitude decreases or the reflected wave amplitude increases at least 3 times consecutively, and cannot recover to the preset corresponding amplitude threshold range in subsequent loading cycles.

[0039] The advantages of this invention are:

[0040] 1. The four or six sets of electromagnetic stress wave generating devices used in this invention are spatially opposed. The conduction time of each high-voltage thyristor is independently delayed and triggered by a programmable fatigue-timing controller. Combined with pre-trigger calibration and high-frequency strain gauge feedback compensation, multi-axis stress wave pulses can act on the sample simultaneously. This invention can also set the phase difference as needed to simulate complex dynamic load sequences, realizing the synchronous loading of multi-axis impact loads. This overcomes the problem that existing impact equipment is difficult to achieve microsecond-level multi-axis synchronization due to extremely short pulse width and large mechanical transmission inertia.

[0041] 2. This invention integrates a voltage detection module, a fatigue-timing controller, and a switch controller in the charge / discharge control center, forming a closed-loop feedback control circuit. After each discharge, the voltage detection module reads the residual voltage of the capacitor bank in real time. The charge / discharge control center triggers a recharging command, and the switch controller immediately triggers recharging to compensate the voltage of each capacitor bank to the target value. This ensures that the initial voltage error before each impact is controlled within ±0.5%, guaranteeing the consistency of the impact load amplitude from the energy source. This achieves a constant output of load amplitude during long-term cyclic impacts, overcoming the problem that existing electromagnetic drive devices suffer from load attenuation due to internal resistance loss and changes in switching characteristics during multiple charge / discharge cycles, thus failing to meet the constant amplitude requirements of fatigue tests.

[0042] 3. Combining waveguide rod design principles and considering the requirements of Saint-Venant's principle for establishing a one-dimensional strain field, this invention employs a miniaturized waveguide rod design. Its length is determined based on one-fiftieth of the product of the loading wave pulse width and the material wave velocity, while simultaneously satisfying the engineering constraint that the rod length must be ≥ 4 to 5 times the waveguide rod diameter. Taking a loading pulse width of 200 microseconds and a steel waveguide rod as an example, the waveguide rod length is only 100mm (compared to 0.5m to 2m for traditional Hopkinson waveguide rods), significantly reducing the overall device size. This overcomes the limitation that traditional Hopkinson rods must meet the aspect ratio requirement for one-dimensional stress wave propagation, and overcomes the problem of difficult arrangement of multi-axis systems in limited experimental space due to excessively long rods.

[0043] 4. This invention attaches high-frequency strain gauges to each waveguide rod to capture and record the loading waveform acting on the sample in real time, which facilitates online identification of reflected waves and extraction of the local strain evolution law before and after fatigue crack initiation; at the same time, it is equipped with a high-speed camera and DIC technology to obtain the full-field deformation of the sample surface, revealing the damage evolution law of multiaxial impact fatigue, realizing global monitoring and failure determination of the multiaxial impact fatigue process, and overcoming the shortcomings of the information limitation of single monitoring methods. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the overall structure of the triaxial six-axis electromagnetic impact fatigue testing device in an embodiment of the present invention;

[0045] Figure 2 This is a wiring diagram of the control circuit of the triaxial six-directional electromagnetic impact fatigue testing device in an embodiment of the present invention.

[0046] Figure 3 This is a cross-sectional view of the electromagnetic stress wave generating device of the present invention. The connecting flange has been removed from the view to clearly show the positional relationship of the discharge coil, induction coil, push rod, variable cross-section waveform amplifier and miniaturized waveguide rod.

[0047] In the figure: 1-Six-way rigid frame, 2-Flange, 3-High-speed camera, 4-Sample, 5-High-frequency strain gauge, 6-Variable cross-section waveform amplifier, 7-Miniaturized waveguide rod, 8-Electromagnetic stress wave generator, 9-Impact load data acquisition and waveform display, 10-Fatigue-timing controller, 11-Charging and discharging control center, 12-Voltage detection module, 13-Switch controller, 14-Capacitor bank, 15-Discharge coil, 16-Induction coil, 17-Push rod, 18-High voltage thyristor, 19-Charging circuit. Detailed Implementation

[0048] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0049] Reference Figure 1 This embodiment uses a triaxial six-axis electromagnetic impact fatigue testing system as an example. The system includes a multi-channel electromagnetic stress wave generator 8, a capacitor bank 14, a charging circuit 19, high-voltage thyristors 18, and a charge / discharge control center 11. The electromagnetic stress wave generator 8, used to transmit the generated pulse load to the sample, includes a discharge coil 15, an induction coil 16, a push rod 17, a variable cross-section waveform amplifier 6, and a miniaturized waveguide rod 7. The charge / discharge control center 11 is configured to: control the electromagnetic stress wave generator to apply constant amplitude impact to the sample; acquire the stress wave waveform generated by the electromagnetic stress wave generator in real time; monitor the voltage of the capacitor bank; independently control the triggering time of each high-voltage thyristor; and control the charging of the capacitor bank. The input terminal of the charging circuit 19 is connected to an external power source, and its output terminal is connected to the capacitor bank 14 for charging the capacitor bank 14.

[0050] Specifically, in this embodiment, the core support is a spatially orthogonal six-directional rigid frame 1. Six sets of electromagnetic stress wave generating devices 8 are arranged opposite each other in the horizontal direction (positive X-axis, negative X-axis, positive Y-axis, negative Y-axis) and vertical direction (positive Z-axis, negative Z-axis). Each set of electromagnetic stress wave generating devices is fixed to the six-directional rigid frame 1 via a flange 2, ensuring that the loading axes in the six directions precisely intersect at the center of the sample mounting position. The sample 4 is clamped at the intersection center of the six electromagnetic stress wave generating devices by a self-aligning clamp, ensuring that the multiaxial load can be precisely applied to the sample along the preset axis.

[0051] Reference Figure 2 and Figure 3Each electromagnetic stress wave generating device independently corresponds to a high-voltage thyristor 18, used to control the discharge on / off state of the discharge coil 15. Each high-voltage thyristor is connected in series between the corresponding capacitor bank 14 and the discharge coil 15. The anode A (main current inflow terminal) is connected to the anode of the capacitor bank, the cathode K (main current output terminal) of the high-voltage thyristor is connected to the discharge coil 15, and the gate G is connected to the fatigue-timing controller. Specifically, the discharge coil 15 and the induction coil 16 are coils encapsulated in epoxy resin. The discharge coil 15 is connected to the output terminal of the high-voltage thyristor. The discharge coil 15 is used to receive discharge pulses and generate a transient magnetic field. The induction coil 16 is coupled to the discharge coil to generate eddy current repulsion. One end of the push rod 17 is fixedly connected to the induction coil 16. In this embodiment, the discharge coil and the induction coil are arranged coaxially in close contact, with a gap between them maintained at 0.5mm to 2mm to obtain the maximum transient repulsion. In this embodiment, a gap of 1mm is preferred.

[0052] The large end of the variable cross-section waveform amplifier 6 is connected to the other end of the push rod 17, and its small end is coaxially connected to one end of the miniaturized waveguide rod 7. The other end of the miniaturized waveguide rod is used to contact the sample. In this embodiment, the variable cross-section waveform amplifier 6 is a frustum-shaped amplifier. Through its variable cross-section amplification structure, the variable cross-section waveform amplifier 6 converts the instantaneous kinetic energy obtained by the induction coil 16 into a high-amplitude stress wave pulse, thereby concentrating the energy density.

[0053] In this embodiment, the loading pulse width is set to 200 microseconds, and the miniaturized waveguide rod is made of high-strength steel with a material wave velocity of 5100 m / s. Based on design criteria, the theoretical length of the waveguide rod is calculated to be approximately 20 mm. Specifically, based on preliminary experiments and simulation results, the designed length of the waveguide rod only needs to be approximately one-fiftieth of the product of the loading pulse width and the material wave velocity, which satisfies the requirements. Therefore, the calculated result is 20.4 mm. We round this to 20 mm as the preliminary design result, which represents the theoretical limit for the waveguide rod to achieve an extremely short reflection time. However, considering the requirements of Saint-Venant's principle for establishing a one-dimensional strain field, in order to eliminate the local three-dimensional stress concentration at the loading end and allow the stress wave within the waveguide rod to evolve into a uniform one-dimensional plane stress wave, the length of the waveguide rod must be at least 4 to 5 times its diameter. That is, it needs to meet the following requirements. , The waveguide rod diameter. If the waveguide rod diameter... If the value is 20mm, then the waveguide rod length is 100mm. Therefore, the maximum value of 100mm is selected as the actual design length of the miniaturized waveguide rod in this invention between the two design results (the preliminary design result and the design result considering Saint-Venant's principle).

[0054] Taking a 100mm rod as an example, the stress wave takes approximately 40 microseconds to travel back and forth once. Within a pulse width of 200 microseconds, it can reflect 5 times, which is sufficient to establish quasi-static equilibrium at the end of the sample. This short rod design of the present invention allows the stress wave to complete multiple back-and-forth reflections in a very short time, enabling the sample to quickly reach a quasi-static stress equilibrium state within the pulse duration, thus solving the problem of space constraints in traditional long rod systems.

[0055] Reference Figure 2 The charge / discharge control center 11 mainly includes a stress wave acquisition unit, a fatigue-timing controller 10, a voltage detection module 12, and a switch controller 13. The stress wave acquisition unit is used to acquire the stress wave waveform generated by each electromagnetic stress wave generator in real time during the loading impact process. In this embodiment, the stress wave acquisition unit includes an impact load data acquisition and waveform display 9 and several high-frequency strain gauges 5 attached to miniaturized waveguide rods. The signal output terminals of the high-frequency strain gauges are connected to the impact load data acquisition and waveform display 9, which acquires and displays each stress wave waveform acquired by the high-frequency strain gauges 5 in real time. Specifically, multiple high-frequency strain gauges are respectively attached to the middle of the corresponding miniaturized waveguide rods 7 to eliminate the interference of local stress concentration at the loading end on the stress wave waveform.

[0056] The capacitor bank 14 includes multiple capacitors connected in parallel. The positive terminal of the capacitor bank is connected to one end of the charging circuit 19 via the switch controller 13, and the negative terminal is connected to the other end of the charging circuit. The voltage detection module 12 is connected to the positive terminal of the capacitor bank 14 and is used to monitor the voltage of each capacitor in the capacitor bank in real time. The input terminal of the fatigue-timing controller is connected to the output terminal of the voltage detection module and is used to receive the voltage feedback signal output by the voltage detection module. The output terminals of the fatigue-timing controller 10 are respectively connected to the gate G of each high-voltage thyristor and are used to independently control the triggering time of each high-voltage thyristor. Specifically, the fatigue-timing controller 10 adopts a programmable PLC controller, and its output terminals are respectively connected to the gate G of the corresponding high-voltage thyristor to independently control the triggering time of each high-voltage thyristor, thereby realizing microsecond-level independent triggering control of the discharge circuit of each electromagnetic stress wave generating device. This fatigue-timing controller can achieve absolute synchronous impact of multiple loads, and can also set a specific phase difference according to actual needs, so that each load arrives at the specimen according to its own preset delay time, in order to simulate complex dynamic load sequences and realize asynchronous impact tests of each part.

[0057] Due to slight differences in the impedance and coil gap of each circuit, the electromagnetic stress wave generating device of each circuit needs to be pre-triggered and calibrated by the fatigue-timing controller before the test. That is, observe the arrival time of the waveform of the high-frequency strain gauge 5 on the six miniaturized waveguide rods, and set the conduction delay time of each high-voltage thyristor through programmable logic. The delay time is on the order of microseconds to ensure that the six stress wave pulses act on the sample at the same time.

[0058] The fatigue-sequence controller output is also connected to the feedback terminal of the switch controller 13. Based on the voltage feedback signal output by the voltage detection module 12, it controls the charging circuit to replenish the capacitor bank after each discharge, ensuring that the voltage of each capacitor remains constant before each impact. Specifically, the fatigue-sequence controller can control the switch controller 13 to open and close based on the voltage feedback signal from the voltage detection module 12, thereby controlling the charging circuit 19 to replenish the capacitor bank 14 after each discharge, ensuring that the voltage remains constant before each impact. During prolonged repeated impacts, the voltage detection module 12 and the switch controller 13 cooperate to achieve replenishment charging. Specifically, after each discharge, the voltage detection module 12 reads the residual voltage of the capacitor bank 14 and sends it to the fatigue-sequence controller. The switch controller 13 immediately triggers replenishment charging (switch closed, charging circuit begins charging) according to the control command of the fatigue-sequence controller, ensuring that the initial voltage of each capacitor bank before the next impact is controlled near the preset target voltage, with an error not exceeding ±0.5%, thus guaranteeing the consistency of the repeated impact load amplitude.

[0059] The device of this invention is also externally equipped with a high-speed camera 3 and an illumination source, the illumination source being used to provide a good imaging environment. In this embodiment, a high-speed camera 3 capable of synchronous triggering is arranged in front of and to the side of the sample, and a high-temperature anti-stripping speckle pattern is sprayed onto the surface of the sample 4. During impact, the high-speed camera 3 records the deformation image of the sample, and uses digital image correlation technology (i.e., DIC technology) to capture and analyze the speckle image on the sample surface in real time, obtaining the non-uniform deformation field and dynamic strain evolution law of the sample during multi-axis impact. In this embodiment, in order to capture a sufficient number of deformation images within a pulse width of 200 microseconds to fully observe the local deformation process, a high-speed camera with a frame rate of 50,000-200,000 fps is required. In this embodiment, a high-speed camera with a frame rate of 100,000 fps is preferably used, that is, one frame is captured every 10 microseconds, synchronously triggered by the rising edge of the fatigue-timing controller 10 or the high-frequency strain gauge 5. Furthermore, the high-speed camera is equipped with a high-power, high-frequency, flicker-free LED array to ensure the amount of light entering during the extremely short exposure time. Before conducting the formal material impact test, the theoretical error of the stress waves reaching the sample under various loading conditions is calibrated through an unloaded test. The total delay time of the high-speed camera is adjusted by measuring the stress wave arrival time, the camera pre-start time, and the camera hardware response delay time to ensure that the high-speed camera starts in advance before the stress wave actually acts on the sample.

[0060] This embodiment also provides a biaxial four-directional electromagnetic impact fatigue testing device, suitable for impact fatigue testing under plane stress. The difference from the aforementioned triaxial six-directional electromagnetic impact fatigue testing device is that only four sets of electromagnetic stress wave generating devices 8 are set, arranged along the positive X-axis, negative X-axis, positive Y-axis, and negative Y-axis respectively, forming orthogonal opposing loading in the horizontal plane; no loading head is set in the Z-axis direction. The six-directional rigid frame 1 is correspondingly simplified to a four-directional planar frame. The structure and working principle of the remaining components, such as coil assemblies, waveform amplifiers, miniaturized waveguide rods, and charge / discharge control centers, are the same as those of the aforementioned triaxial six-directional electromagnetic impact fatigue testing device, and will not be repeated here. The biaxial four-directional electromagnetic impact fatigue testing device provided in this embodiment is suitable for biaxial impact fatigue testing of thin plates, planar cross-shaped specimens, etc.

[0061] This embodiment takes multi-path load synchronous impact as an example to specifically describe the multi-axis electromagnetic impact fatigue test method based on a triaxial six-axis or biaxial four-axis electromagnetic impact fatigue testing device, including the following steps:

[0062] Step 1: Sample clamping and parameter setting

[0063] The dummy sample (used only for calibration in step 2, and replaced by the test sample during actual testing) is clamped between each miniaturized waveguide rod through the central clamp, and the miniaturized waveguide rod 7 is adjusted to make it in close contact with the end face of the dummy sample.

[0064] Input the target impact force, pulse frequency, and total number of cycles for the fatigue test into the system's control interface. The target impact force can be set according to the corresponding charging voltage.

[0065] Step 2: No-load pre-trigger calibration

[0066] A single low-energy-level trigger is performed using a fatigue-timing controller. The arrival time of each stress wave is observed, and the conduction delay time of each high-voltage thyristor is acquired and set to ensure that each stress wave pulse arrives at the sample synchronously. The specific process is as follows:

[0067] A single low-energy trigger pulse is set in the system's control interface to limit the stress wave amplitude generated by the ultra-high-speed dynamic strain gauge within the system's purely elastic deformation range. This ensures that the maximum stress of the rod after the superposition of the incident and reflected waves during the test is less than 0.5 times the yield strength of the small waveguide rod, preventing plastic deformation and guaranteeing the repeatability of the calibration results. Before executing step 2, the present invention adjusts the low voltage value of the charging voltage step by step through the charging and discharging control center 11 and obtains the relationship between the charging voltage and the strain of the small waveguide rod to determine the magnitude of the low-energy trigger pulse. In this embodiment, the single low-energy trigger uses a capacitor discharge pulse lower than the formal test energy level. The calibration charging voltage is preferably set to 10% to 30% of the formal loading voltage to keep the peak value of the strain wave measured on the waveguide rod within a certain range. Within the specified range; the gate of the high-voltage thyristor uses 5–15V, 0.1–0.5A, and continuous voltage. The isolation trigger pulse.

[0068] The ultra-high-speed dynamic strain gauge is activated and placed in a ready-to-trigger state; a single low-energy discharge trigger is executed via the fatigue-timing controller 10. At the moment of discharge, the ultra-high-speed dynamic strain gauge synchronously records the voltage signals collected by the high-frequency strain gauges attached to the middle of the six miniaturized waveguide rods, forming a six-channel incident waveform.

[0069] The six-channel incident waveforms recorded by the ultra-high-speed dynamic strain gauge were extracted. The arrival time of each of the six waveforms was recorded using the wavefront rise point (i.e., the moment the waveform begins to rise from the baseline) or the first characteristic peak point (i.e., the moment corresponding to the first peak) as time markers. , , , , , Then, the difference between the maximum and minimum arrival times of the six routes is calculated to obtain the maximum time deviation. If the maximum time deviation If the error is less than or equal to the preset microsecond-level allowable error threshold (e.g., ±5 microseconds as set in this embodiment), it indicates that the synchronization of each channel during electromagnetic conversion or waveform transmission is qualified. Save the calibration parameters and proceed directly to step 4; if the maximum time deviation ... If the error exceeds the microsecond-level allowable threshold, it indicates that there is physical asynchrony among the channels during electromagnetic conversion or waveform transmission. In this case, mechanical backlash compensation or electrical timing compensation is required based on the channels with leading waveforms (shorter arrival times) and lagging waveforms (longer arrival times). Specifically:

[0070] (1) Mechanical backlash compensation

[0071] Based on the time deviation of stress waves (including reflected and transmitted waves) in each channel, the axial position of the corresponding push rod, miniaturized waveguide rod end face, or electromagnetic stress wave generating device 8 is slightly adjusted. Specifically, for channels with advanced waveforms, the force transmission path gap or axial distance is appropriately increased, while maintaining the contact or pre-tightening state between the miniaturized waveguide rod and the sample. The corresponding electromagnetic stress wave generating device 8 or push rod is then moved axially away from the sample, increasing the initial distance between the electromagnetic stress wave generating device and the input end of the miniaturized waveguide rod, thus delaying the contact loading time. For channels with lagging waveforms, the corresponding electromagnetic stress wave generating device 8 or push rod is moved axially towards the sample, decreasing the initial distance between the electromagnetic stress wave generating device and the input end of the miniaturized waveguide rod, thus advancing the contact loading time. After each adjustment, a synchronous impact test is performed again, acquiring six waveforms and calculating the maximum time deviation. The synchronization error continues until it meets the preset microsecond-level allowable error threshold, and then all mechanical connection components are locked.

[0072] During mechanical backlash compensation, the single adjustment amount is 0.01 to 0.05 mm, and the cumulative adjustment amount is controlled within 0.1 mm to 0.2 mm.

[0073] (2) Electrical timing compensation

[0074] Based on the arrival time deviation of the stress waves in each channel, the trigger delay of the corresponding electromagnetic drive channel is corrected. Specifically, the trigger delay is increased for channels with advanced waveforms and decreased for channels with lagging waveforms. Similarly, after each compensation, a synchronous impact test is performed again, acquiring six waveforms and calculating the maximum time deviation. The synchronization error continues until it meets the preset microsecond-level allowable error threshold, and then the timing parameters of each channel are saved.

[0075] After the compensation adjustment is completed, the single low-level trigger test is repeated until the six characteristic waveforms highly overlap on the time axis, that is, the maximum time deviation of the six waveforms. Microseconds. After the synchronization is qualified, the compensation parameters of each channel are saved to the fatigue-timing controller as the trigger timing reference for subsequent automated fatigue implementation.

[0076] Step 3: Automated Cyclic Impact and Voltage Constant Amplitude Control

[0077] The automated constant amplitude loading test is initiated. The system automatically performs charge-discharge impact and constant amplitude control according to the preset loading frequency, which includes the following sub-steps:

[0078] Step 3.1: Perform a single discharge impulse.

[0079] When the preset loading frequency trigger cycle point is reached, the fatigue-timing controller 10 simultaneously sends microsecond-level TTL trigger commands to each high-voltage thyristor. Upon receiving the microsecond-level TTL trigger command, the high-voltage thyristors instantly turn on, allowing the electrical energy stored in the capacitor bank 14 to be released instantaneously. Each capacitor bank 14 synchronously releases peak current to the corresponding discharge coil 15 through the high-voltage thyristors. The discharge coil 15 generates a transient strong magnetic field, which generates eddy current repulsion with the induction coil 16. This force drives the miniaturized waveguide rod 7 via the push rod 17 and the variable cross-section waveform amplifier 6, applying an impact load to the sample 4. While the stress wave propagates back and forth within the miniaturized waveguide rod, the high-speed camera 3 triggers the recording of sample deformation images.

[0080] Step 3.2: Collect the actual load amplitude of this impact.

[0081] After a single impact, the charge-discharge control center extracts the dynamic electrical signal transmitted back by the high-frequency strain gauge 5 attached to the waveguide rod, and uses the stress peak value of this incident wave as the actual loading amplitude of this cycle, denoted as . ,in This represents the current loop count.

[0082] Step 3.3: Calculate the amplitude deviation

[0083] The charge / discharge control center will extract the actual load amplitude. Impact force against the preset target Perform the difference operation to calculate the stress amplitude deviation value for this cycle. This amplitude deviation value reflects the amplitude decay or overshoot of the current cycle. This indicates the current cycle number. It should be noted that the actual loading amplitude mentioned here refers to the actual stress, and the target impact force refers to the target stress.

[0084] Step 3.4: Calculate the compensation voltage for the next cycle and set the target charging voltage for the next cycle.

[0085] The fatigue-sequence controller has a built-in adaptive compensation algorithm that calculates the compensation energy required for the next cycle based on the current and historical deviation values, and converts it into a charging voltage adjustment.

[0086] Before the next cycle begins, the charge and discharge control center calculates the adjustment amount of the charging voltage based on the end of the previous cycle, and automatically fine-tunes the charging voltage threshold of the external high-voltage charger according to the adjustment amount of the charging voltage to achieve constant amplitude control.

[0087] Step 3.5: Six independent charging channels and voltage regulation

[0088] Six independent charging branches charge their respective capacitor banks 14. The voltage detection module 12 is responsible for monitoring the voltage status of each capacitor in the capacitor bank 14 in real time. The fatigue-timing controller 10 receives the voltage feedback signal monitored by the voltage detection module until all six voltages reach their respective voltage setting thresholds and remain stable. In this embodiment, the voltage error is controlled within ±0.5%.

[0089] Step 3.6: Waiting for the trigger cycle and loop execution

[0090] After charging is complete, discharging and loading impact are performed, and the circuit enters a standby state, waiting for the next charging trigger cycle. The charging time of the charging circuit is designed to be much shorter than the impact interval corresponding to the preset loading frequency. For example, when the loading frequency is 1Hz, the impact interval is 1000ms, and the charging time is usually tens of milliseconds, ensuring that the charging operation can be completed before the next impact.

[0091] Return to step 3.1 and repeat steps 3.1 to 3.6 to automatically perform cyclic impact and constant amplitude control according to the preset loading frequency until the preset total number of cycles is reached or the sample fails.

[0092] Step 4: Fatigue life determination

[0093] When the specimen fails and fractures, the system automatically shuts down and saves the data. Fatigue life is determined based on the measured stress wave waveform distortion or the total number of loading cycles. If crack initiation is used to define the impact fatigue life of the specimen, it is determined based on the stress wave waveform distortion measured on the high-frequency strain gauge 5. If the specimen fracture is used to define the impact fatigue life of the specimen, the total number of loading cycles at fracture is taken as the specimen fracture life. If crack initiation is used to define the impact fatigue life of the specimen, it is determined based on the measured stress wave waveform distortion. The specific method is as follows: when there are no abnormalities in the incident wave peak value, incident wave pulse width, and loading energy, the number of loading cycles that first meet the following combination of conditions is taken as the crack initiation life of the specimen:

[0094] Condition 1: If the measured amplitude of the reflected wave or the amplitude of the transmitted wave exceeds the preset corresponding amplitude threshold range, it indicates that the stress wave waveform is distorted.

[0095] Condition 2: The transmitted wave amplitude decreases or the reflected wave amplitude increases at least 3 times consecutively, and cannot recover to the preset corresponding amplitude threshold range in subsequent loading cycles.

[0096] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the scope of the technology disclosed in the present invention, and such modifications or substitutions should all be covered within the scope of protection of the present invention.

Claims

1. A multi-axis electromagnetic impact fatigue testing system, characterized in that, It includes a stress wave generation module, capacitor bank, charging circuit, high-voltage thyristor, and charging / discharging control center; The stress wave generation module comprises at least four sets of spatially opposed electromagnetic stress wave generating devices. Each set of electromagnetic stress wave generating devices is used to transfer the generated pulse load to the sample, and includes a discharge coil, an induction coil, a push rod, a variable cross-section waveform amplifier, and a miniaturized waveguide rod; wherein: The discharge coil is used to receive discharge pulses and generate transient magnetic fields; the induction coil is coupled to the discharge coil to generate eddy current repulsion; one end of the push rod is fixedly connected to the induction coil. The large end of the variable cross-section waveform amplifier is connected to the other end of the push rod, and its small end is connected to one end of the miniaturized waveguide rod. The other end of the miniaturized waveguide rod is used to contact the sample; the length of the miniaturized waveguide rod... satisfy And simultaneously satisfy The actual value is the maximum of the two; where To load the pulse width, The wave velocity is the wave velocity of the waveguide rod material. The diameter of the waveguide rod; Each electromagnetic stress wave generating device is independently equipped with a high-voltage thyristor. Each high-voltage thyristor is connected in series between the corresponding capacitor bank and the discharge coil to control the discharge on / off of the discharge coil. The capacitor bank is connected to each of the discharge coils and is used to store the discharge energy required by the discharge coils; the input terminal of the charging circuit is used to connect to an external power source, and its output terminal is connected to the capacitor bank for charging the capacitor bank. The charge / discharge control center is configured to: control the electromagnetic stress wave generating device to apply constant amplitude loading impact to the sample; acquire the stress wave waveform generated by the electromagnetic stress wave generating device in real time; monitor the voltage of the capacitor bank; independently control the triggering time of each high-voltage thyristor; and control the charging of the capacitor bank.

2. The multi-axis electromagnetic impact fatigue testing system according to claim 1, characterized in that, The discharge coil and the induction coil are arranged coaxially in close contact, with an axial gap of 0.5mm to 2mm between them.

3. The multi-axis electromagnetic impact fatigue testing system according to claim 1, characterized in that, The charge / discharge control center includes a stress wave acquisition unit, a fatigue-timing controller, a voltage detection module, and a switch controller; wherein: The capacitor bank includes multiple capacitors connected in parallel; the positive terminal of the capacitor bank is connected to one end of the charging circuit through the switch controller, and the negative terminal is connected to the other end of the charging circuit. The voltage detection module is connected to the positive terminal of the capacitor bank, and is used to monitor the voltage of each capacitor in the capacitor bank in real time. The input terminal of the fatigue-timing controller is connected to the output terminal of the voltage detection module to receive the voltage feedback signal output by the voltage detection module; its output terminal is connected to the gate of each high-voltage thyristor to independently control the triggering time of each high-voltage thyristor; its output is also connected to the feedback terminal of the switch controller to control the charging circuit to recharge the capacitor bank after each discharge according to the voltage feedback signal, so as to keep the voltage constant before each impact. The stress wave acquisition unit is used to acquire the stress wave waveform generated by each electromagnetic stress wave generator in real time during the loading impact process.

4. The multi-axis electromagnetic impact fatigue testing system according to claim 3, characterized in that, The stress wave acquisition unit includes an impact load data acquisition and waveform display and multiple high-frequency strain gauges; the multiple high-frequency strain gauges are respectively attached to the middle of the corresponding miniaturized waveguide rod; The signal output terminal of the high-frequency strain gauge is connected to the impact load acquisition and waveform display for real-time recording of the stress wave waveform of each channel.

5. The multi-axis electromagnetic impact fatigue testing system according to claim 1, characterized in that, It also includes a high-speed camera for acquiring speckle images of the sample surface in real time during the impact fatigue test; the high-speed camera has a frame rate of 50,000 to 200,000 fps.

6. The multi-axis electromagnetic impact fatigue testing system according to claim 1, characterized in that, The electromagnetic stress wave generating device consists of four groups, arranged in a biaxial, four-directional horizontally opposed configuration; or six groups, arranged in a triaxial, six-directional spatially orthogonal opposed configuration.

7. A multiaxial electromagnetic impact fatigue testing method, characterized in that, Based on the multiaxial electromagnetic impact fatigue testing system according to any one of claims 1 to 6, the multiaxial electromagnetic impact fatigue testing method includes the following steps: Step 1, Sample clamping and parameter setting: The sample is clamped between the miniaturized waveguide rods described above; test parameters including target impact force, pulse frequency, and total number of cycles are set. Step 2, No-load pre-trigger calibration: A single low-level trigger is performed using a fatigue-timing controller. The arrival time of each stress wave is observed, and the conduction delay time of each high-voltage thyristor is obtained and set so that each stress wave pulse arrives at the sample synchronously. Step 3, Automated Cyclic Impact and Voltage Constant Amplitude Control: The following process is automatically repeated according to the preset loading frequency: Perform discharge impact and collect the actual loading amplitude of this time; after the discharge ends, perform closed-loop compensation of the charging voltage to the preset target voltage based on the deviation between the actual loading amplitude and the target impact force, so that the voltage before each impact remains constant; repeat the above process until the preset number of cycles is reached or the sample fails. Step 4, fatigue life determination: The impact fatigue life of the specimen is determined based on the measured stress wave waveform distortion or the total number of loading cycles.

8. The multiaxial electromagnetic impact fatigue testing method according to claim 7, characterized in that, The specific process of obtaining and setting the conduction delay time of the high-voltage thyristor in step 1 to ensure that each stress wave pulse arrives at the sample synchronously is as follows: Using the wavefront rise point or the first characteristic peak point of the stress wave in each channel as a time marker, the arrival time of each waveform is recorded, and the maximum time deviation is calculated. ; If the maximum time deviation If the error exceeds the preset microsecond-level allowable error threshold, mechanical backlash compensation or electrical timing compensation will be performed, and no-load pre-trigger calibration will be performed again until the arrival time difference of each waveform is less than or equal to the preset microsecond-level allowable error threshold.

9. The multiaxial electromagnetic impact fatigue testing method according to claim 7, characterized in that, In step 3, the actual loading amplitude of the impact is collected by a high-frequency strain gauge attached to a miniaturized waveguide rod, the deviation between the actual loading amplitude and the preset target impact force is calculated, and the charging voltage adjustment required for the next impact is calculated by the adaptive compensation algorithm built into the fatigue-time controller. The charging voltage threshold is adjusted according to the fine-tuning amount of the charging voltage, and the capacitor bank is recharged after the current impact ends, so that the voltage error before each impact is controlled within ±0.5%.

10. The multiaxial electromagnetic impact fatigue testing method according to claim 7, characterized in that, In step 4, if the impact fatigue life of the specimen is defined by the specimen fracture, then the total number of loading cycles at the time of fracture is taken as the specimen fracture life. If crack initiation is used to define the impact fatigue life of a specimen, it is determined based on the measured stress wave waveform distortion. The specific method is as follows: when there are no abnormalities in the incident wave peak value, incident wave pulse width, and loading energy, the number of loading cycles that first meet the following combination of conditions is taken as the crack initiation life of the specimen: Condition 1: If the measured amplitude of the reflected wave or the amplitude of the transmitted wave exceeds the preset corresponding amplitude threshold range, it indicates that the stress wave waveform is distorted. Condition 2: The transmitted wave amplitude decreases or the reflected wave amplitude increases at least 3 times consecutively, and cannot recover to the preset corresponding amplitude threshold range in subsequent loading cycles.

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