Concrete asymmetric fatigue test system and method based on ultrasonic vibration loading

Abstract

The application discloses a kind of concrete asymmetric fatigue test systems based on ultrasonic vibration loading, including universal testing machine, ultrasonic frequency generator, ultrasonic transducer, upper displacement amplifier and lower displacement amplifier and for reducing system loading eccentric moment ball hinge mechanism, the ball hinge mechanism is connected with lower displacement amplifier bottom by clamping connecting disc.This application also provides a kind of concrete asymmetric fatigue test method based on ultrasonic vibration loading, fix the concrete test piece of equal cross section cylinder between upper displacement amplifier and lower displacement amplifier, then open universal testing machine, ultrasonic frequency generator and ultrasonic transducer, realize the ultrasonic vibration asymmetric compression-compression fatigue cycle loading of concrete test piece.The application can realize the asymmetric compression-compression fatigue loading of concrete test piece by ultrasonic fatigue test technology, solve the problem of high time cost in the super-long life fatigue test of traditional concrete fatigue test system.

Description

Technical Field

[0001] This invention relates to ultra-high circumference (10) concrete materials 7 In the field of fatigue life testing technology (more than 100 cycles), the specific invention relates to a concrete asymmetric fatigue testing system and method based on ultrasonic vibration loading. Background Technology

[0002] Concrete is one of the most common building materials, widely used in major projects such as large bridges, high-speed railways, highways, and urban subways. During their service life, these structures often need to withstand 10... 7 Up to 10 9 Secondary fatigue alternating load, while current concrete structure design assumes that concrete material is subjected to 2×10 6 If no fatigue failure occurs within a cycle, the fatigue failure of concrete need not be considered. This leads to the actual service life of concrete components and structures often being lower than their design life, causing performance degradation or even failure of the concrete components in the aforementioned structures during their service life.

[0003] Fatigue life is measured by the number of cycles of loading a material can withstand before fatigue failure. Based on the concept of fatigue life, fatigue can be subdivided into three main categories: low-cycle fatigue, with a fatigue life of less than 10 cycles. 4 In high-cycle fatigue, the applied stress is generally greater than the macroscopic yield strength; the fatigue life is 10 cycles. 4 -10 7 Within the cycle range, the applied stress is generally lower than the macroscopic yield strength; for ultra-high cycle fatigue, the fatigue life is greater than 10. 7 The number of cycles is lower, and the loading stress is also lower. Traditional concrete fatigue testing systems, based on their structural design, are mainly suitable for 10 cycles. 7 Fatigue tests and studies conducted within a few weeks are not applicable to 10 weeks. 7 Fatigue studies involving more than 10 cycles, i.e., ultra-long life fatigue studies. This is because the operating frequency of traditional concrete fatigue testing systems is generally less than 200Hz, and to achieve 10 cycles... 9 The average time cost for testing fatigue life per cycle is 57.87 days, which is too high.

[0004] Ultrasonic vibration accelerated fatigue testing technology utilizes the resonance of the specimen, with a loading frequency reaching 20kHz, which can significantly shorten the time required for ultra-long life fatigue testing of concrete. Therefore, it has become a popular method for conducting 10... 7Ultrasonic fatigue testing is the most effective method for fatigue testing and research involving more than one cycle. A typical ultrasonic fatigue testing system mainly consists of an ultrasonic frequency generator, an ultrasonic transducer, a displacement amplifier, and a fatigue specimen. The ultrasonic frequency generator provides a high-frequency excitation electrical signal. The ultrasonic transducer, based on the inverse piezoelectric effect of piezoelectric ceramics, converts the high-frequency excitation electrical signal into a mechanical longitudinal wave, and the displacement amplifier amplifies the amplitude of the mechanical wave. The lower end of the specimen is free, while the upper end is fixed to the end of the displacement amplifier. Through the geometric design of the displacement amplifier and the specimen, the specimen, displacement amplifier, and ultrasonic transducer can form a resonant system, achieving ultra-high frequency cyclic loading.

[0005] In addition, there is an ultrasonic fatigue testing system in which the lower end of the specimen is clamped by a fixture, and the specimen is provided with superimposed load by a combination of ultrasonic vibration loading and static load loading, such as the ultrasonic fatigue testing machine disclosed in patent publication number CN110411872A.

[0006] However, concrete is typically used in building structures to withstand compressive loads, and fatigue tests on it are mostly conducted using asymmetric compressive-compressive cyclic loading. The aforementioned ultrasonic fatigue testing system, however, can only achieve symmetric tensile-compressive cyclic loading and is primarily designed for ultra-long fatigue life testing of metallic materials (such as aero-engine parts). It cannot be directly applied to asymmetric compressive-compressive cyclic loading and ultra-long fatigue life testing of concrete. Furthermore, the specimens used in uniaxial tensile-compressive ultrasonic fatigue technology are mostly cylindrical specimens with transition arcs. However, because the molding precision of concrete is lower than that of metallic materials tested by traditional ultrasonic fatigue testing, the molding precision of the transition arcs is difficult to control. Therefore, using traditional cylindrical shapes with transition arcs for testing is challenging. Summary of the Invention

[0007] To address the shortcomings of the existing technology, this invention provides a system and method for asymmetric fatigue testing of concrete based on ultrasonic vibration loading, which can effectively achieve ultra-long service life (1000-1000 km) of concrete materials. 7 Fatigue testing (more than 10 weeks).

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The asymmetric fatigue testing system for concrete based on ultrasonic vibration loading includes a universal testing machine, an ultrasonic frequency generator, an ultrasonic transducer, an upper displacement amplifier, and a lower displacement amplifier. The upper displacement amplifier and the lower displacement amplifier work together to fix the concrete specimen. The system also includes a ball joint mechanism connected to the universal testing machine to reduce the eccentric moment of the system load and thus obtain an ideal axial compressive load. The ball joint mechanism is connected to the bottom of the lower displacement amplifier through a clamping connecting plate.

[0010] Furthermore, the present invention also includes a cooling airflow application device for applying a strong convective airflow to a concrete specimen when the system applies a load to the concrete specimen.

[0011] Specifically, the ball joint mechanism includes a rolling element connected to the clamping connecting plate, thereby having the same sliding angle as the lower displacement amplifier, and a ball seat located below the rolling element; the ball seat has an I-shaped cross-section, longer at the top and shorter at the bottom, with the upper part of the I-shape connected to the rolling element and the lower part connected to the universal testing machine base, and the maximum radial compressive stress p on the upper and lower contact surfaces of the ball joint mechanism satisfies the following relationship with the vertical bearing capacity F:

[0012]

[0013] In the formula, R is the spherical radius of the contact surface of the ball joint mechanism; R1 is the projected radius of the ball seat groove; the vertical bearing capacity F is calculated according to the following formula:

[0014] F = 1.05σS

[0015] In the formula, σ is the strength of the concrete specimen; S is the cross-sectional area of ​​the concrete specimen.

[0016] Furthermore, the rolling element and the ball seat are connected by a first screw. The two ends of the first screw are held in position by flange nuts to limit the relative sliding position of the two. A spring is also sleeved in the first screw, which is located between the rolling element and the ball seat and is used to automatically return the position of the ball joint mechanism after unloading.

[0017] Preferably, the ball seat is made of 304 stainless steel.

[0018] Preferably, the surface roughness Ra of the ball joint mechanism is less than 1.6 μm.

[0019] This invention also provides a method for asymmetric fatigue testing of concrete based on ultrasonic vibration loading. The hardware adopts the aforementioned system, and the testing process is as follows: a cylindrical concrete specimen with a uniform cross-section is fixed between an upper displacement amplifier and a lower displacement amplifier. Then, a universal testing machine, an ultrasonic frequency generator, and an ultrasonic transducer are turned on. By adjusting the output amplitude of the upper and lower displacement amplifiers and the magnitude of the static pressure load applied by the universal testing machine, asymmetric compression-compression fatigue cyclic loading of ultrasonic vibration on the concrete specimen is achieved. The ultrasonic vibration is intermittent, which allows the heat inside the concrete specimen to be conducted to its surface. Cooling is achieved by applying enhanced convection through a cooling airflow application device, thereby controlling the temperature of the concrete specimen.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) This invention achieves ultrasonic vibration compression-compression fatigue loading on the specimen by adjusting the output amplitude of the upper and lower displacement amplifiers and the magnitude of the static pressure load applied by the universal testing machine. Since compression loading cannot achieve self-adjustment of the load through small-amplitude slippage of the specimen clamping position, the ideal axial compression load usually becomes the eccentric compression load in actual loading. The eccentric moment will cause the actual stress on one side of the specimen to be too large, reducing the fatigue life measured in the test. Based on this, this invention designs a ball joint mechanism at the bottom of the loading sequence. This ball joint mechanism consists of an upper rolling element and a lower ball seat. By designing the shape and structure of the rolling element and the ball seat and their connection relationship with the lower displacement amplifier, combined with the superposition of ultrasonic vibration loading and static load loading, the asymmetric compression-compression fatigue test of concrete specimens by ultrasonic fatigue testing technology is realized. Therefore, this invention effectively replaces the traditional concrete fatigue test system and solves the problem of excessive time cost in ultra-long life fatigue testing of the traditional concrete fatigue test system.

[0022] (2) This invention connects the lower displacement amplifier, rolling elements, and ball joint through the cooperation of a screw, flange nut, and clamping connecting plate. This ensures that the rolling elements and ball joint do not separate when subjected to large eccentric torques, and also makes the lower displacement amplifier and rolling elements form a geometrically invariant whole with the same sliding angle. Simultaneously, the spring design allows for automatic return of the ball joint mechanism to its original position after unloading. This not only ensures the safety of the system but also provides the lower displacement amplifier with the same sliding angle, giving it a rotational degree of freedom. When subjected to eccentric torque, the specimen and lower displacement amplifier will rotate and stop when the eccentric torque reaches zero. Therefore, this invention, through its ingenious connection design, utilizes the ball joint mechanism to reduce eccentric torque during fatigue testing.

[0023] (3) Considering the poor molding precision of concrete, this invention uses cylindrical specimens with uniform cross-sections for ultrasonic fatigue testing. Compared to cylindrical specimens with transition arcs, the testing operation of cylindrical specimens with uniform cross-sections is more convenient, and the test results are more accurate. Simultaneously, considering that concrete itself is a material with poor thermal conductivity, this invention employs externally applied airflow for strong convection cooling and utilizes intermittent ultrasonic vibration to conduct internal heat to the surface. This combination of intermittent ultrasonic vibration and the application of cooling airflow effectively controls the temperature of the specimen. Through the design of the aforementioned ultrasonic vibration method and cooling structure, the fatigue testing system can be better adapted to the testing of concrete specimens, further ensuring the accuracy and reliability of ultra-long life fatigue testing of concrete materials.

[0024] (4) The present invention is ingeniously designed and easy to use, providing a reliable foundation for the ultrasonic fatigue testing system in the ultra-long fatigue life testing of concrete. Therefore, the present invention is suitable for widespread application. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the system structure in an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the ball joint structure in an embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram of the structure of the rolling element in an embodiment of the present invention.

[0028] Figure 4 This is a schematic diagram of the ball seat in an embodiment of the present invention.

[0029] Figure 5 This is a simplified schematic diagram illustrating the use of a ball joint mechanism to reduce eccentric torque in an embodiment of the present invention.

[0030] The component names corresponding to the reference numerals in the attached drawings are as follows:

[0031] 1-Universal testing machine, 2-Ultrasonic frequency generator, 3-Ultrasonic transducer, 4-Upper displacement amplifier, 5-Concrete specimen, 6-Cooling airflow application device, 7-Lower displacement amplifier, 8-Clamping connecting plate, 9-Rolling element, 10-First screw, 11-Spring, 12-Ball seat, 13-Second screw. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0033] Example

[0034] This embodiment provides an asymmetric fatigue testing system for concrete based on ultrasonic vibration loading, used to replace traditional concrete fatigue testing systems for ultra-long life (10) tests on concrete materials. 7 Fatigue testing (more than 10 weeks). The structure of the system is as follows: Figure 1 As shown, similar to existing ultrasonic fatigue testing systems, this embodiment integrates the ultrasonic loading sequence (including ultrasonic frequency generator 2, ultrasonic transducer 3, and upper displacement amplifier 4) with the universal testing machine 1. The upper displacement amplifier 4 is connected to the upper end of the concrete specimen 5. The superposition of static load and cyclic load is achieved by clamping the ultrasonic loading sequence on the loading path of the universal testing machine.

[0035] Specifically, since ultrasonic transducers cannot withstand pressure loads, force transmission through the ultrasonic transducer must be avoided when integrating the ultrasonic loading sequence into the loading path of the universal testing machine. To address this, this embodiment designs a shoulder protrusion on the upper displacement amplifier, using a clamping tool to hold and connect the upper displacement amplifier to the clamping plate connected to the universal testing machine sensor. This provides sufficient space for the ultrasonic transducer to be connected to the top of the upper displacement amplifier, enabling ultrasonic cyclic loading. Ultrasonic vibration compression-compression fatigue loading of the specimen is achieved by adjusting the output amplitude of the upper displacement amplifier and the magnitude of the static pressure load applied by the universal testing machine. The output amplitude of the upper displacement amplifier is mainly affected by the power of the input electrical signal of the ultrasonic transducer and the amplifier's amplification factor.

[0036] One difference from existing ultrasonic fatigue testing systems is that in this embodiment, a lower displacement amplifier 7 is connected to the lower end of the concrete specimen 5. Since the input power of the ultrasonic transducer's electrical signal can be adjusted by a computer system, and the amplification factor of the upper displacement amplifier 4 and the lower displacement amplifier 7 is mainly affected by the size of the amplifier, the ideal displacement amplification factor can be obtained by reasonably designing the displacement amplifier according to the longitudinal wave transmission principle.

[0037] Another difference from existing ultrasonic fatigue testing systems is that, because compressive loading cannot achieve self-adjustment of the load through small-amplitude slippage of the specimen clamping position, the ideal axial compressive load usually becomes an eccentric compressive load in actual loading. The eccentric moment causes the actual stress on one side of the specimen to be larger, thus reducing the fatigue life measured in the test. To address this, this embodiment incorporates a ball joint mechanism at the bottom of the loading sequence to reduce the eccentric moment.

[0038] like Figure 1 , 2 As shown, in this embodiment, the ball joint mechanism consists of a lower ball seat 12 and an upper rolling element 9, which are connected by springs and bolts. Molybdenum disulfide grease is applied to the contact surfaces to reduce friction. The bottom of the ball seat 12 is connected to the base of the universal testing machine via M10 bolts. The ball seat 12 has an I-shaped cross-section, longer at the top and shorter at the bottom. The upper part of this I-shape has three through holes, each with a radius of 7.5mm. To prevent the two parts of the ball joint mechanism from separating under large eccentric torques, thus posing a safety risk during use, a first screw 10 (M10 specification) passes through the ball seat 12 and connects to the upper rolling element 9. Flange nuts are used at both ends of the first screw 10 to maintain its position and limit relative sliding. Simultaneously, a spring 11 is fitted over the first screw 10 and positioned between the rolling element 9 and the ball seat 12 to automatically return the ball joint to its original position after unloading. Six through holes are evenly arranged on the upper part of the rolling element 9. Figure 3 As shown ( Figure 3The top view of the rolling element and its projected front view are shown. Three through holes correspond to three through holes on the ball seat 12 and are connected by the first screw 10 to limit the relative sliding position. The other three through holes are connected and fixed to the clamping connecting plate 8 of the upper lower displacement amplifier 7 by the second screw 13, so that the two form a geometrically invariant whole with the same sliding angle.

[0039] The process of conducting fatigue tests on concrete specimens in this embodiment is as follows:

[0040] A cylindrical concrete specimen with a uniform cross-section is fixed between an upper displacement amplifier and a lower displacement amplifier. Then, the universal testing machine, ultrasonic frequency generator, and ultrasonic transducer are turned on. By adjusting the output amplitude of the upper and lower displacement amplifiers and the magnitude of the static pressure load applied by the universal testing machine, asymmetric compression-compression fatigue cyclic loading of ultrasonic vibration on the concrete specimen can be achieved. The use of a cylindrical concrete specimen with a uniform cross-section in this embodiment is due to the relatively poor molding accuracy of concrete. Compared to cylindrical specimens with transition arcs, the testing operation of a cylindrical specimen with a uniform cross-section is more convenient, and the test results are more accurate.

[0041] The following section introduces the principle of reducing eccentric torque in ball joint mechanisms during fatigue testing.

[0042] First, the ball joint mechanism needs to provide sufficient load-bearing capacity and ensure omnidirectional rotation. Therefore, the design of the ball joint mechanism must consider whether the vertical load-bearing capacity and surface roughness meet the requirements. To this end, the cross-sectional shape of the ball seat 12 is designed as an "I" shape with a longer upper section and a shorter lower section. The maximum radial compressive stress p on the upper and lower contact surfaces of the ball joint mechanism satisfies the following relationship with the vertical load-bearing capacity F:

[0043]

[0044] In the formula, R is the spherical radius of the contact surface of the ball joint mechanism; R1 is the projected radius of the ball seat groove plane, such as... Figure 4 As shown; the vertical bearing capacity F is calculated according to the following formula:

[0045] F = 1.05σS

[0046] In the formula, σ is the strength of the concrete specimen; S is the cross-sectional area of ​​the concrete specimen.

[0047] Based on the above relationships, it can be seen that to meet the load-bearing capacity requirements, the radial compressive stress should not exceed the elastic limit of the ball joint material. Since the compressive strength of concrete is typically 150 MPa and the maximum radial compressive stress is 4.48 MPa, and the elastic limit of most metallic materials meets this strength requirement, considering the availability of materials, this embodiment preferably uses 304 stainless steel to make the ball joint seat. Furthermore, to ensure the omnidirectional rotation of the ball joint mechanism, in this embodiment, the surface roughness Ra of the ball joint mechanism is <1.6 μm.

[0048] Secondly, during the fatigue test, the specimen and the upper and lower displacement amplifier sections were cut out and simplified to... Figure 5 As shown in the two-dimensional schematic diagram (a), when the two contact surfaces of the specimen and the upper and lower displacement amplifiers are not parallel, the contact point of the upper contact surface deviates from the axis of the specimen. The load is transmitted to the specimen through the eccentric contact point. The stress state of the specimen at this time is as follows: Figure 5 As shown in (b). The force effect in 5(b) can be equivalent to the superposition of a concentrated axial force F and an eccentric moment M, as follows: Figure 5 As shown in (c).

[0049] In the two-dimensional case, the ball joint mechanism provides a rotational degree of freedom for the lower displacement amplifier. Therefore, under the action of the eccentric torque M, the specimen and the lower displacement amplifier rotate and stop when the eccentric torque M = 0. Figure 5 As shown in (d). At this time, the contact area between the specimen and the upper displacement amplifier is the area of ​​the upper bottom of the specimen. The contact surface of the specimen is uniformly stressed, and the magnitude is a uniformly distributed load p. At this time, the eccentric moment M = 0. In this way, the purpose of reducing the eccentric moment can be achieved.

[0050] Furthermore, considering that concrete itself is a material with poor thermal conductivity, this embodiment employs intermittent ultrasonic vibration, which allows heat from the interior of the concrete specimen to be conducted to the surface during the test. Simultaneously, this embodiment also utilizes externally applied airflow for strong convection cooling of the concrete specimen. Specifically, two cooling airflow application devices 6 (preferably cooling airflow nozzles) are symmetrically designed on both sides of the concrete specimen 5. During the test, the two cooling airflow application devices simultaneously spray cooling gas onto the specimen, creating strong convection to cool the concrete specimen 5. Thus, by combining intermittent ultrasonic vibration and the application of cooling airflow, effective temperature control of the specimen can be achieved.

[0051] The fatigue testing system designed in this invention, while seemingly simple, is actually quite sophisticated. Only through a deep understanding of the properties of concrete materials, combined with the details and functional characteristics of the ultrasonic fatigue testing system, can a reasonable system structure and testing procedure be designed as a whole, thereby effectively achieving ultra-long service life (1000-2000 km) for concrete materials. 7This invention addresses the problem of excessively high time costs in traditional concrete fatigue testing systems for ultra-long-life fatigue testing (more than 100 cycles). Each step of this invention is interconnected and mutually reinforcing, providing a reliable foundation for ultrasonic fatigue testing systems in ultra-long-life concrete fatigue testing. Therefore, compared with existing technologies, this invention has outstanding substantive features and significant progress.

[0052] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A concrete asymmetric fatigue testing system based on ultrasonic vibration loading, comprising a universal testing machine (1), an ultrasonic frequency generator (2), an ultrasonic transducer (3), an upper displacement amplifier (4), and a lower displacement amplifier (7); wherein the upper displacement amplifier (4) and the lower displacement amplifier (7) work together to fix the concrete specimen, characterized in that, It also includes a ball joint mechanism connected to the universal testing machine (1) for reducing the system loading eccentric moment to obtain the ideal axial compression load, the ball joint mechanism being connected to the bottom of the lower displacement amplifier (7) via a clamping connecting plate (8); The ball joint mechanism includes a rolling element (9) connected to the clamping connecting plate (8) and thus having the same sliding angle as the lower displacement amplifier (7), and a ball seat (12) located below the rolling element (9); the ball seat (12) has a cross-sectional shape that is longer at the top and shorter at the bottom, with the upper part of the "I" shape connected to the rolling element (9) and the lower part connected to the base of the universal testing machine (1), and the maximum radial compressive stress on the upper and lower contact surfaces of the ball joint mechanism is... With vertical bearing capacity The following relationship exists between them: In the formula, Let be the radius of the sphere at the contact surface of the ball joint mechanism; Radius of the ball seat groove in plan view; vertical bearing capacity Calculate using the following formula: In the formula, For the strength of concrete specimens; The cross-sectional area of ​​the concrete specimen; The rolling element (9) and the ball seat (12) are connected by a first screw (10). The two ends of the first screw are held in place by flange nuts to limit the relative sliding position of the two. A spring (11) is also sleeved in the first screw. The spring (11) is located between the rolling element (9) and the ball seat (12) and is used to automatically return the position of the ball joint mechanism after unloading.

2. The concrete asymmetric fatigue testing system based on ultrasonic vibration loading according to claim 1, characterized in that, It also includes a cooling airflow application device (6) for applying a strong convective airflow to the concrete specimen when the system applies a load to the concrete specimen.

3. The asymmetric fatigue testing system for concrete based on ultrasonic vibration loading according to claim 2, characterized in that, The ball seat (12) is made of 304 stainless steel.

4. The asymmetric fatigue testing system for concrete based on ultrasonic vibration loading according to claim 2 or 3, characterized in that, The surface roughness Ra of the ball joint mechanism is less than 1.6 μm.

5. A method for asymmetric fatigue testing of concrete based on ultrasonic vibration loading, using the system described in any one of claims 1 to 4, characterized in that, The experimental procedure is as follows: a cylindrical concrete specimen with a uniform cross-section is fixed between an upper displacement amplifier and a lower displacement amplifier. Then, the universal testing machine, ultrasonic frequency generator, and ultrasonic transducer are turned on. By adjusting the output amplitude of the upper and lower displacement amplifiers and the magnitude of the static pressure load applied by the universal testing machine, the ultrasonic vibration asymmetric compression-compression fatigue cyclic loading on the concrete specimen is achieved.

6. The method for asymmetric fatigue testing of concrete based on ultrasonic vibration loading according to claim 5, characterized in that, Ultrasonic vibration is an intermittent vibration that conducts heat from inside the concrete specimen to its surface. Cooling is achieved by applying enhanced convection through a cooling airflow device, thus controlling the temperature of the concrete specimen.

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