DEVICE FOR PERFORMING BIAXIAL TENSILE-COMPRESSION TESTS ON A UNIAXIAL MACHINE
Patent Information
- Application Number
- ES2025032647U
- Authority / Receiving Office
- ES · ES
- Patent Type
- Utility models
- Current Assignee / Owner
- Filing Date
- 2025-12-30
- Publication Date
- 2026-09-21
- Estimated Expiration
- 2035-12-30
Abstract
Description
DESCRIPTION DEVICE FOR PERFORMING BIAXIAL TENSILE TESTS- COMPRESSION IN A UNIAXIAL MACHINE Field of invention This invention relates to a device for performing biaxial tensile-compression tests on a uniaxial testing machine and falls within the field of devices that enable biaxial testing of cruciform specimens using uniaxial testing machines. This device transforms the uniaxial displacement into movement in two perpendicular directions, intended for analyzing the mechanical response of solid materials subjected to simultaneous loads in two perpendicular (biaxial) directions. More specifically, it relates to devices that allow biaxial testing of cruciform specimens for the precise characterization of the shear properties of carbon fiber-reinforced polymer matrix composites. Background of the invention Uniaxial tests are often insufficient to describe the actual behavior of structures, where loads are applied simultaneously in multiple directions. To address this complexity, biaxial tests allow for the analysis of the material's mechanical behavior under loads in two perpendicular directions. This is especially relevant in the design of composite materials with marked anisotropy (different mechanical properties depending on the loading direction). Fiber-reinforced polymer matrix (FRP) composites are increasingly used in sectors such as aerospace, automotive, wind energy, rail, and sporting goods, thanks to their high stiffness-to-weight and strength-to-weight ratios, making their accurate characterization crucial. Characterizing the mechanical response to shear is of particular interest, given that this type of stress is common in most practical applications of FRPs. Currently, several standardized methods are available for this purpose, notably the ASTM D3518, ASTM D5379, and ASTM D7078 tests, which are widely used to evaluate the properties associated with the material's shear response. However, these test methodologies have limitations in accurately estimating shear strength, thus necessitating new procedures for determining these properties. The UNE 0074:2023 Specification has recently been developed, establishing the basis for a new test method that induces a pure shear state at the center of a cruciform specimen, even under high levels of angular deformation. To perform the test described in UNE 0074:2023, a biaxial test is required using a cruciform specimen subjected simultaneously to tension in one direction and compression in the orthogonal direction, applying forces of equal magnitude. This configuration induces a pure shear state in the central region of the specimen, on planes oriented at ±45° with respect to the loading directions, allowing for the analysis of the shear response of interest. Although this test is designed for use on biaxial testing machines, the UNE 0074:2023 specification also allows for its implementation on uniaxial machines by incorporating an auxiliary device. This option is justified by the greater availability and lower cost of uniaxial machines in laboratories, which increases the accessibility and feasibility of the method. Consequently, the test can be carried out on a universal testing machine, provided that a device is available that can transform its uniaxial displacement into movement in two perpendicular directions, thus simulating the conditions of a biaxial machine. Several devices have been proposed in the literature for performing biaxial tests on cruciform specimens using universal testing machines. These systems are usually based on bar mechanisms that change their orientation during the test, generating variations in the ratio of the loads applied in each direction, as well as in the rate of load application to the cruciform specimen. Among the most representative designs are those developed by Ferron & Makinde (Design and development of a biaxial strength testing device, J. Test. Eval. 16 (3) (1988) 253-256), Barroso et al. (Biaxial testing of composites in uniaxial machines: Manufacturing of a device, analysis of the specimen geometry and preliminary experimental results, Proceedings of the 15th European Conference on Composite Materials, 2012), and Merklein & Biasuti (Development of a biaxial tensile machine for characterization of sheet metals, J. Mater. Process. Technol.213 (6) (2013) 939-946), Habanusa (US Patent 8671771) and Nagayasu et al. (Modification of Compact Biaxial Tensile Testing Apparatus Using Conventional Material Testing Machine and Evaluation of the Test Results, Proceedings of the International Conference on Technology and Science 29 (12) (2020) 238-243). In this context, the device proposed by Nagayasu et al., incorporated in the ISO 16842:2021 standard, stands out, which is aimed at the mechanical characterization of metallic materials under biaxial tensile-tension loads. Likewise, the devices designed by Bhatnagar et al. (Development of a biaxial tensile test fixture for reinforced thermoplastic composites, Polym. Test.26 (2) (2007) 154-161), Jianjun & Zhiyong (China Patent 101561376), Andrusca et al. (Design and development of a device used in biaxial testing of materials, Buletinul Institutului Politehnic Din Iasi 62 (2016) 9-14) and Puente-Medellín & Diosdado de la Peña (Design of a biaxial test module for uniaxial testing machine, Materials Today: Proceedings 4 (8) (2017) 7911-7929) employ components with relative angles of 45° that allow maintaining a constant loading rate and forces of equal magnitude in both loading directions during the test. This characteristic makes them particularly stable and precise solutions for performing controlled biaxial tests. However, the only existing device capable of performing the biaxial tensile-compression test (TC test) is the bar mechanism developed by Barroso et al. However, it has a significant limitation, since it does not manage to maintain equal loads applied in the two perpendicular directions and a constant load transfer during the test due to the progressive reorientation of the bars, which according to Specification UNE 0074:2023 are necessary conditions to reliably analyze a pure shear state. In this context, no device currently exists that satisfies the main requirements, justifying the development of a new, specific system, the concept of which gives rise to the present invention. Furthermore, the device must allow instrumentation of the specimen in the biaxially loaded zone to acquire strain measurements using sensors such as strain gauges or fiber optics, and even provide visual access to the area of interest for better control and monitoring of the test's progress, thus opening the possibility of using strain and displacement measurement techniques such as video strain measurement or Digital Image Correlation (DIC). Likewise, the device is sought to be versatile enough to be used on any uniaxial testing machine. The present invention offers all these advantages over currently known biaxial devices. Brief description of the invention The invention relates to a device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen comprising a first trapezoid attached to a movable shaft of the uniaxial machine, a second trapezoid facing the first trapezoid and fixed in place, a third trapezoid and a fourth trapezoid facing the third trapezoid, sliding skids fixed by means of skid fixing screws on the third and fourth trapezoids; guide rails mounted on the first and second trapezoids and fixed by means of rail fixing screws. In the device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen of the invention, the sliding skids are configured to allow controlled movement on the guide rails, the displacement of the sliding skids on the guide rails being limited by positioning stops that are fixed by means of stop fixing screws. In the device for performing biaxial tensile-compressive tests on a uniaxial machine on a cruciform specimen of the invention, the first trapezoid and the second trapezoid comprise a hollow intended to accommodate the cruciform specimen, the third trapezoid and the fourth trapezoid comprise a hollow intended to accommodate the cruciform specimen, and the four trapezoids are configured to join together and accommodate the cruciform specimen between them. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen, which is the subject of the invention, comprises gripping plates that also include clamping wedges with the cruciform specimen between them. In the device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen of the invention, the gripping plates comprise a zone for fixing to the trapezoid, which is housed inside the hollow of each trapezoid, and an anti-buckling zone, of lesser thickness, which is outside the trapezoid and has a triangular shape with the vertex chamfered. In the device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen of the invention, the clamping wedges are located in the hollow of each trapezoid, parallel to the gripping plates, and reproduce the shape of the gripping plates with a uniform thickness. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen, which is the subject of the invention, comprises screws for securing the wedges and plates in the area of attachment to the trapezoid, configured to fix the cruciform specimen. In the device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen of the invention, the clamping wedges are knurled to increase friction. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen, the object of the invention, comprises individual wedge clamping screws configured to secure the position of each clamping wedge. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen of the invention comprises anti-buckling reinforcement clamping screws configured to fix the anti-buckling zone and the corresponding zone of the clamping wedge. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen, which is the subject of the invention, comprises protective blocks on each face of the arms of the cruciform specimen, between said arms and the clamping wedges, and between said arms and the gripping plates. Brief description of the figures To complement the description that follows and to help in a better understanding of the characteristics of the invention, this descriptive report is accompanied by a set of drawings based on which the innovations and advantages of the object of the invention will be more easily understood. Figure 1 represents an exploded view of the device that is the subject of the invention, in which the different components that make it up are distinguished and listed. Figure 2 presents a schematic perspective overview of the device for performing biaxial tensile-compression tests on uniaxial machines, the subject of the invention. Figure 3 provides a schematic perspective overview of the first and second trapezoids, which are identical. Figure 4 shows an exploded view of the first and second trapezoid assembly, in which the plates and wedges associated with each trapezoid, as well as their respective fixing screws, are shown separately. Figure 5a shows an exploded view of the first and second trapezoid assembly, in which the separation of the head from the body of the trapezoid is represented. Figure 5b shows an exploded view of the first and second trapezoid assembly, in which the head is replaced by a cup, representing its separation from the body of the trapezoid and allowing the device to be adapted to an alternative uniaxial machine model. Figure 6 presents an exploded view of the first and second trapezoid assembly, showing the separation of the rails from the trapezoid body along with their respective screws. Figure 7 provides a schematic perspective overview of the third and fourth trapezoids, which are identical. Figure 8 shows an exploded view of the third and fourth trapezoid assembly, in which the separation of the skates from the trapezoid body is shown along with their respective screws. Figure 9 presents a perspective view of a plate, highlighting the detail of the contact surface between the plate and the test specimen. Figure 10 presents a perspective view of a wedge, highlighting the detail of the contact surface between the wedge and the test specimen. Figure 11a presents the initial setup of the test, showing the arrangement of the assembly before the application of the load. Figure 11b shows the limit configuration of the test, in which the first trapezoid has descended applying compression in the vertical direction, while the third and fourth trapezoids have separated, generating tension in the horizontal direction. The various numerical references shown in the figures correspond to the following elements: 1A. first trapezoid 1B. second trapezoid, 2. grip plate, 3. positioning stop, 4. guide rail, 5. coupling head, 6. clamping wedge, 7. Rail fixing screw, 8. Stop fixing screw, 9. Wedge and plate clamping screw, 10. Anti-buckling reinforcement fastening screw, 11. Individual wedge fastening screw, 12. fixing nut of the head or cup to the trapezoid, 13. sliding skate, 14A. Third trapezoid, 14B. fourth trapezoid, 15. Skate fixing screw, 16. cruciform test tube with three median planes of symmetry, 17. protective block, 18. hole of first and second trapezoid, 19. hole of the third and fourth trapezoid, 20. Fixing zone to the trapezoid of the grip plate, 21. Anti-buckling zone of the grip plate, and 22. knurled. Preferred embodiment of the invention In view of the above statement and referring to the numbering adopted in the figures, the object of the invention is a device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16) that allows the precise determination of the shear properties even at high levels of deformation on a cruciform specimen (16) and with three median planes of symmetry. The device that is the subject of the invention comprises a first trapezoid (1A), a second trapezoid (1B), facing the first trapezoid (1A), a third trapezoid (14A) and a fourth trapezoid (14B), facing the third trapezoid (14A), wherein the four trapezoids (1A, 1B, 14A, 14B) are configured to join together and accommodate between them the cruciform test specimen (16) on which the biaxial test is to be developed. The first trapezoid (1A) and the second trapezoid (1B) are identical components whose main function is to compress the cruciform specimen (16) in the vertical direction. The first upper trapezoid (1A) is attached to the moving shaft of the testing machine, while the second trapezoid (1B) remains fixed. The first trapezoid (1A) and the second trapezoid (1B) comprise a recess (18) intended to house the cruciform test specimen (16), such that said recess (18) has been dimensioned with a width slightly greater than the width of the cruciform test specimen (16), thus ensuring a proper fit within the permitted tolerances. The first trapezoid (1A) and the second trapezoid (1B) have heads (5) for coupling to the uniaxial testing machine, ensuring the correct anchoring of the biaxial testing device of the invention to said testing machine, and are secured by means of fixing nuts (12) of the head (5) to the trapezoid (1A, 1B). To accommodate the head (5) properly, the first trapezoid (1A) and the second trapezoid (1B) have a thicker section where the head (5) is located. However, to avoid an unnecessary increase in weight, the thickness transitions towards the head (5) area. Furthermore, the area designed to support the nut includes reinforcement to ensure a stable and secure fastening, thus contributing to the rigidity of the anchoring system. The third trapezoid (14A) and the fourth trapezoid (14B) are identical components, with the function of pulling the cruciform specimen (16) in the horizontal direction. The third trapezoid (14A) and the fourth trapezoid (14B) comprise a cavity (19) intended to house the cruciform specimen (16), which has been dimensioned with a width slightly greater than that provided for the cruciform specimen (16) and with dimensions identical to those of the first and second trapezoids (1A, 1B). All trapezoids (1A, 1B, 14A, 14B) are made of steel to provide the necessary rigidity for the test, thus increasing the integrity of the apparatus. To reduce the device's weight, thickness reductions are implemented wherever possible in non-critical areas. The device for performing biaxial tensile-compressive tests on a uniaxial machine on a cruciform specimen (16) that is the subject of the invention comprises: - some sliding skates (13) fixed by means of some skate fixing screws (15) on the third and fourth trapezoid (14A, 14B); - guide rails (4) mounted on the first and second trapezoid (1A, 1B) secured by rail fixing screws (7). In the device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen (16) of the invention, the sliding skids (13) are configured to allow controlled movement on the guide rails (4), the displacement of the sliding skids on the guide rails (4) being limited by positioning stops (3) which are fixed by fixing screws of the stops (8). With the above configuration, in the device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16) object of the invention, the different trapezoids (1A, 1B, 14A, 14B) are joined together. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen (16) of the invention includes gripping plates (2) which also comprise clamping wedges (6) with the cruciform specimen (16) between them, so that the position is fixed and the slippage of the cruciform specimen (16) to be tested is prevented. The gripping plates (2) serve a dual purpose, acting simultaneously as a plate and a clamping wedge. These plates have two distinct zones: a zone for attaching to the trapezoid (20), which is housed inside the recess (18, 19) of each trapezoid, and a thinner anti-buckling zone (21) that extends outside the trapezoid (1A, 1B, 14A, 14B) and has a triangular shape with a chamfered vertex. The zone for attaching to the trapezoid (20) contains screws (9) for securing the wedges and plates. Their function is to firmly fix the cruciform specimen (16) by applying the appropriate tightening torque. The anti-buckling zone (21) acts as a clamping wedge and features knurling (22) in the area in contact with the cruciform specimen (16) to improve friction and prevent slippage. The clamping wedges (6) hold the cruciform specimen (16) during the test and are knurled to increase friction. They are secured by wedge and plate clamping screws (9), and their position is further ensured by individual wedge clamping screws (11). They are positioned in the recess (18, 19) of each trapezoid (1A, 1B, 14A, 14B), parallel to the gripping plates (2). The clamping wedges (6) replicate the shape of the gripping plates (2) with uniform thickness. The anti-buckling zone (21) and the corresponding zone of the clamping wedge (6) are designed to prevent buckling of the arms of the cruciform specimen (16), fixing said zone by means of clamping screws of the anti-buckling reinforcement (10). All clamping wedges (6) are made of high-strength steel that has high tensile strength and hardness, requirements for carrying out the test. The cruciform specimen (16) tested has a cruciform geometry, with three median planes of symmetry, and is made of the material under study. The clamping wedges (6) firmly hold the cruciform specimen (16) up to the central area to prevent buckling of its arms. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen (16) that is the subject of the invention comprises protective blocks (17) for the specimens, which are placed on each face of the arms of the cruciform specimen (16), between the said arms and the clamping wedges (6) and between the said arms and the gripping plates (2), avoiding stress concentrations in the bite zone. The central area of the cruciform test specimen (16) remains free to allow analysis using strain gauges or digital image correlation (DIC) techniques. The device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen (16) does not have nuts for the different fixing elements used such as screws, but rather threaded holes into which the different screws used are screwed, with the aim of reducing weight, but also to facilitate the assembly and disassembly of the device. During the performance of a test with the device for carrying out biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16) object of the invention, when the first trapezoid (1A) descends compressing the cruciform specimen (16) in the vertical direction the third and fourth trapezoids (14A, 14B) separate pulling the cruciform specimen (16) in the horizontal direction carrying out a biaxial tensile-compression test in any uniaxial machine.
Claims
1. Device for performing biaxial tensile-compression tests on a uniaxial machine on a cruciform specimen (16) characterized in that it comprises: - a first trapezoid (1A) attached to a movable shaft of the uniaxial machine, - a second trapezoid (1B), facing the first trapezoid (1A), remains fixedly anchored, - a third trapezoid (14A) and - a fourth trapezoid (14B), facing the third trapezoid (14A), - sliding pads (13) fixed by means of sliding pad fixing screws (15) on the third and fourth trapezoids (14A, 14B); - guide rails (4) mounted on the first and second trapezoids (1A, 1B) fixed by means of rail fixing screws (7) wherein the sliding pads (13) are configured to allow controlled movement on the guide rails (4),Having limited the displacement of the sliding pads on the guide rails (4) by positioning stops (3) that are fixed by means of stop fixing screws (8), and wherein the first trapezoid (1A) and the second trapezoid (1B) comprise a recess (18) intended to house the cruciform specimen (16), wherein the third trapezoid (14A) and the fourth trapezoid (14B) comprise a recess (19) intended to house the cruciform specimen (16), and wherein the four trapezoids (1A, 1B, 14A, 14B) are configured to join together and house the cruciform specimen (16) between them.
2. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to claim 1 characterized in that it comprises gripping plates (2) that also comprise,3. Device for performing biaxial tensile-compressive tests in a uniaxial machine on a cruciform test specimen (16), according to claim 2, characterized in that the gripping plates (2) comprise: - a zone for fixing to the trapezoid (20), which is housed inside the hollow (18, 19) of each trapezoid (1A, 1B, 14A, 14B), - an anti-buckling zone (21), of lesser thickness, which is outside the trapezoid (1A, 1B, 14A, 14B) and has a triangular shape with a chamfered vertex.
4. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to claim 2, characterized in that the clamping wedges (6) are located in the hollow (18, 19) of each trapezoid (1A, 1B, 14A, 14B), parallel to the gripping plates (2),and reproduce the shape of the gripping plates (2) with a uniform thickness.
5. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to claim 3, characterized in that it comprises clamping screws for the wedges and plates (9) in the area of attachment to the trapezoid (20) configured to fix the cruciform specimen (16).
6. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to claim 3, characterized in that the clamping wedges (6) are knurled to increase friction.
7. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to claim 3,characterized in that it comprises individual wedge clamping screws (11) configured to secure the position of each clamping wedge (11).
8. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to any of claims 1 to 7, characterized in that it comprises anti-buckling reinforcement clamping screws (10) configured to fix the anti-buckling zone (21) and the corresponding zone of the clamping wedge (6).
9. Device for performing biaxial tensile-compression tests in a uniaxial machine on a cruciform specimen (16), according to any of claims 2 to 8, characterized in that it comprises protective plugs (17) on each face of the arms of the cruciform specimen (16), between said arms and the clamping wedges (6) and between said arms and the gripping plates (2).