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    • 专利摘要:
    Device for flexural stress relaxation tests on curved wooden sheets with constant radius. The present invention refers to a device that allows the realization of tests of materials and structures. The device comprises a support structure (102); a pair or more of clamping and curving elements (104) of sheets; a pair or more of tie rods and load cells (106); a pair or more of regulation elements (108). (Machine-translation by Google Translate, not legally binding)
    公开号:ES2649543A1
    申请号:ES201730481
    申请日:2017-03-30
    公开日:2018-01-12
    发明作者:Antonio José LARA BOCANEGRA；María Almudena MAJANO MAJANO；Jorge CRESPO OUTES；Manuel GUAITA FERNÁNDEZ
    申请人:Universidade de Santiago de Compostela；Universidad Politecnica de Madrid；
    IPC主号:
    专利说明:

    DEVICE FOR RELAXATION TENSION TESTS TO FLEX IN CURVED WOODEN SHEETS WITH CONSTANT RADIO TECHNICAL SECTOR OF THE INVENTION
    The present invention falls within the field of materials science and structural engineering; more specifically, the invention relates to a device that allows the realization of tests of materials and structures related to the building sector and structural engineering in wood. STATE OF THE TECHNIQUE
    Wood, like other polymeric materials, exhibits a time-dependent viscoelastic rheological behavior that manifests itself in two phenomena of great importance in materials engineering and structural calculation: 1) stress relaxation; 2) creep. The relaxation of tensions (in English, stress relaxation) implies the decrease of the tension level that occurs over time in a material subjected to constant deformation. On the contrary, creep (in English, creep) consists in increasing the deformation that occurs over time in a material under constant load.
    The knowledge of the evolution of the relaxation of tensions over time is a key aspect in the design and analysis of various structural types that use initially straight but subsequently curved wood sheets, as is the case with reticular and ribbed laminar structures (in English gridshell and ribbed shell, respectively). During the construction process of this type of structures, the wood sheets reach a high level of tension produced by their bending, being able to easily acquire values close to the maximum permissible. However, due to the aforementioned rheological behavior of wood and the associated plasticizing process produced in its microstructure, the initial bending stresses are partially reduced over time (stress relaxation).
    These laminar structures of curved wood on site are becoming increasingly important in recent years due to their high structural efficiency, sustainable nature and attractive aesthetics. However, and despite the interest of knowing the phenomenon of stress relaxation for the design and dimensioning of this type of structures, there is little, incomplete and uncertain information about speed and speed in literature.


    intensity with which tensions are reduced in the sections of wood sheets subjected to constant deformation. To shed light on these issues, it is necessary to conduct extensive experimental campaigns with equipment that enable appropriate test configurations and minimizes uncertainties in measurement procedures.
    The relaxation of stresses in wood has been analyzed primarily under a material characterization approach by conducting unidirectional tests, mainly of tension and compression, in universal testing machines (eg Kitazawa, 1947; Bach & Rovner, 1967; Echenique-Manrique, 1969 ).
    However, in gridshells-like structures where the shape is achieved by curving the sheets, the analysis of stress relaxation under bending stresses is of greater interest, for which the existing scientific literature is very scarce.
    In general, the relaxation tests of flexural stresses in wood included in scientific literature have consisted in restricting the deformation of the piece by one or two point forces applied externally in its length (analogously to the configurations of static flexural tests a three and four points) and record the variation in the value of these forces, necessary to maintain constant deformation over time (eg Grossman, 1954; Urakami, 1971; Perkitny & Hoffmann, 1976; Kubát et al., 1989; Ebrahimzadeh & Kubat, 1993; Hunt et al., 2015).
    These procedures have the advantage of relatively easy execution. However, the drawback is that they generate combined bending and shear stresses and of variable value along the length of the piece. This implies that the radius of curvature and the tension level along the part are also variable and therefore the relationship between the stress relaxation value and the curvature cannot be easily and directly analyzed.
    In addition, the mentioned test procedures have been focused on the characterization of the material using small-sized specimens and generally too short test periods in relation to those necessary for structural calculation.
    At the regulatory level there are no guidelines for the execution of stress relaxation tests on wood, although for other materials. In this sense, the American standard ASTM E328-13 "Standard Test Methods for Stress Relaxation Tests for Materials and Structures" defines the general test methods to determine tension relaxation


    over time of materials and structures subjected to traction, compression, flexion and torsion. In the case of relaxation of bending stresses, it provides recommendations on general test methods including a four-point static bending configuration and a cantilever specimen, which have the same drawbacks as the existing scientific research in this regard in wood mentioned above ( combined efforts and variable value throughout the piece).
    In the Spanish regulatory environment, the UNE-CEN / TS 14689: 2007 EX standard defines the physical and mechanical tests necessary for the determination of plastic deformation, creep and relaxation in leather following a flexural test procedure with a centered load similar to previous. However, there is no standardized stress relaxation test procedure specific to other types of materials.
    As for patents of existing stress relaxation test equipment, there are also some contributions focused on compression, tensile, torsion and bending tests for different materials, but none specific for determining the relaxation of bending stresses in curved sheets of wood or in curved pieces with constant radius. The machine designed by Díaz Calleja et al. (Pat. ES-8801862 A1) to study interchangeably the creep and relaxation of stresses in tensile or flexural mode in polymers, fibers, films, composites and biological materials. Although the description included in said patent focuses on the tensile process, it follows that the configuration for the flexural test corresponds to a lateral compression flexural test as a controlled buckling. With this test configuration, once again, the tensions produced have variable value throughout the piece and from combined stresses (axial and flexion).
    There are other equipment patents whose objective is to determine the relaxation of flexural stresses, such as the "Method of testing of bending stress relaxation" (Pat. RU2485475 C1), and "Bending stress relaxation test rig for plane samples" (Pat. RU2349894 C1). In these, the test scheme is again similar to that of static four-point bending with two point forces applied externally on the length of the piece. With this test configuration constant tensions of pure bending are achieved in the area between the points of application of the loads. However, in the areas between the loads and the supports the bending stresses are not constant and also appear combined with tensions from shear stresses.
    In the scientific field, the inventors only have knowledge of a test method to determine the relaxation of bending stresses in curved sheets of wood


    for application to gridshells using large specimens. It consists of sheet bending by means of cable tensioning as an arc (Happold, E. & Liddell,
    W.I. (1975) Timber Lattice Roof for the Mannhein Bundesgartenschau. Struct. Eng., 53 (3): 99-135.). This method, although very simple to execute, has similar drawbacks to those discussed above since the pieces tested are subjected to combined flexural and axial forces and do not have constant curvature along the sheet. Consequently, the measurement of relaxation cannot be associated with a tension level of section, but corresponds to the entire piece. This forces us to test a multitude of possible combinations of sheet lengths and bending levels in order to draw useful conclusions.
    As mentioned above, we can conclude that the different equipment currently available for measuring the relaxation of stresses in parts subjected to bending stresses present a series of common drawbacks, which are summarized below:
    - The test configurations used (three or four point flexion, point-load overhang, lateral compression flexion as a controlled buckling or arc tension flexion) produce in all cases stresses combined with flexion (shear or axial) ) so that the measured stress relaxation does not correspond exclusively to pure flexion.
    - Said test configurations produce variable value stresses along the beam. This fact implies that the measured relaxation value corresponds to the piece and therefore cannot be directly associated with a specific curvature or bending stress value.
    - The size of the pieces they use is small and not structural (except the one proposed by Happold, E. & Liddell, W.I. in 1975).
    References:
    Kitazawa, G. (1947) Stress relaxation of wood under constant strain: a study of the viscoelastic property of wood. New York State College of Forestry Tech. Bull. No. 67
    Bach, L. and Rovner, B. (1967) Stress relaxation in wood at different grain angles, Forest Products Laboratory Vancouver, British Columbia. Information Report VP-X-14, 22pp.
    Echenique-Manrique, E. (1969) Stress relaxation of wood at several levels of strain. Wood Science and Technology, 3: 49-73.


    Grossman, P.U.A. (1954) Stress relaxation in wood. Nature, 173: 42-43.
    Urakami, H. (1971) Stress relaxation of wood treated with the formaldehyde in bending and in torsion during adsorption of water vapor. Sci. Rep. Kyoto Pref. Univ, Agr, 23: 88-99.
    Perkitny, T. and Hoffmann, Z. (1976) Zur Relaxation von Holz nach verschieden langer, 5 konstanter Durchbiegung (in german). Holz als Roh- und Werkstoff, 34: 167-170.
    Kubát, D.G., Samuelsson, S. and Klason, C. (1989) Stress relaxation in wood (Scots pine veneer). Journal of Materials Science, 24: 3541-3548.
    Ebrahimzadeh, P.R. and Kubát, D.G. (1993) Effects of humidity changes on damping and stress relaxation in wood. Journal of Materials Science, 28: 5668-5674.
    10 Hunt, J.F., Zhang, H. and Huang, Y. (2015) Analysis of cantilever-beam bending stress relaxation properties of thin wood composites. Bioresources, 10 (2): 3131-3145. DESCRIPTION OF THE INVENTION
    Thus, it is interesting to develop devices that solve the exposed problems presented by existing devices in the state of the art.
    The present invention relates to a laboratory test equipment capable of bending wood sheets of structural size at a constant radius and in the long term to evaluate the relaxation of stresses due to the bending caused by said bending.
    Since the curved blades with constant radius are subjected exclusively to constant value bending stresses throughout their length, the measured relaxation value
    20 in the sheet can be directly associated with both the radius of curvature of the sheet and the bending stress since both are constant throughout the piece. In this way the equipment eliminates the inconveniences described in the previous section of the equipment used to date.
    In addition, the proposed invention incorporates another series of advantages:
    25 - The equipment allows to bend the sheets to multiple radii of curvature by having easily configurable moving parts.
    - The equipment allows several sheets to be tested simultaneously, considerably reducing the time needed to obtain results.


    - The equipment does not need an electrical supply for the application of the force necessary to maintain the curved position of the sheets. The proposed tensioner system reacts against the support structure without the need for any motor to operate, as is the case with universal testing machines. This is a great advantage especially in conducting long-term trials (more than one year).
    The present invention supposes a novel contribution in terms of equipment available for testing the rheological behavior of wood, allowing to analyze with relative ease the relaxation of tensions in curved sheets with constant radius.
    The device for conducting stress relaxation tests on wood sheets (100) curved with constant radius object of the present invention comprises:
    a) a support structure (102);
    b) a pair or more of fastening and bending elements (104) of sheets;
    c) a pair or more of tension bars and load cells (106); Y
    d) a couple or more of regulatory elements (108).
    The support structure comprises a rigid frame open on its upper side and incorporates in its lower base a pair or more of longitudinal profiles (110) that allow the free placement of the fastening and bending elements (104). In addition, the support structure incorporates in its vertical elements longitudinal grooves (114) that allow the free positioning of the tension bars (106) and the regulating elements (108).
    The fastening and curving elements (104) of sheets fix the ends of the sheets
    (100) to the support structure (102) producing the bending of said sheets. Each of the clamping and bending elements (104) of sheets comprises two pieces, one lower
    (112) and a superior one (116), between which the end of the sheet is housed, and four or more bolts that produce the tightening of the assembly preventing the sliding between the sheet and the clamping parts.
    In a particular embodiment, the lower clamping part (112) of the clamping and curving elements of sheets is fixed to the support structure (102) by a hinge-like system that prevents the sheet from moving but allows its rotation.
    In a particular embodiment, the upper clamping part (116) of the clamping and curving elements incorporates a lever which, by tilting


    Simultaneously, two of them located at each of the ends of the sheet make it possible to bend it.
    The tension bars and load cells (106) measure the evolution in time of the force necessary to keep the levers of the fasteners and curved in their fixed position.
    In a particular embodiment each regulating element (108) comprises a nut and a washer that allow adjusting the length of the tension bar.
    The device also comprises means for viewing and / or reading the measurements made by the load cells. BRIEF DESCRIPTION OF THE FIGURES
    The modalities detailed in the figures are illustrated by way of example and not by way of limitation
    Figure 1 shows the perspective image of the device object of the present invention including as an example the representation of curved sheets with three different radii of curvature.
    Figure 2 shows an exploded perspective image of the device object of the present invention in which the main components are shown.
    Figure 3 shows the elevation image of the device object of the present invention including as an example the representation of three pairs of curved sheets with three different radii of curvature.
    Figure 4 shows the device object of the present invention in plan in which three pairs of curved sheets with three different radii of curvature are identified.
    Figure 5 shows the profile of the device object of the present invention with the regulating elements of each of the six sheets.
    Figure 6 shows a detail of the device object of the present invention in which the fastening and curving elements of the sheets are presented. EXAMPLE OF REALIZATION
    The description of a possible preferred embodiment of the invention will now be made.


    A more complete idea of the test equipment referred to in the invention is provided by the following description together with the accompanying figures in which only the assemblies and preferred details for the invention referred to are represented by way of non-limiting example to a possible case of practical realization.
    The equipment consists of a support structure (102), six pairs of fasteners and sheet bending (104), six pairs of tension bars and load cells (106), and six pairs of regulating elements (108). With this specific configuration, the equipment allows simultaneous bending and testing up to a maximum of six curved sheets of wood arranged in parallel. The equipment also has four swivel castors (118) that make it possible to move comfortably in a laboratory environment.
    The support structure (102) functions as a rigid self-stable open frame on its upper side. At its base, which in a particular embodiment has a length of 3.25 meters, the fastening and bending elements of the wood sheets are fixed. On the sides, which in a particular embodiment have a height of 0.65 meters, the tension bar adjustment systems are fixed. Said structure incorporates in its lower base a series of longitudinal profiles (110) that allow the fastening and curving elements to be placed in different positions in order to be able to study sheets of different lengths. In a particular embodiment, the length of the sheets on which tests can be made is between 2 meters and 3 meters, and it is possible to work with radii of curvature in the range 1.5 meters and 10 meters.
    The fastening and bending elements of the sheets (104) are responsible for fixing the sheets to the support structure and producing its bending. Each sheet has one of these elements at each of its ends.
    In a particular embodiment, the clamping and curving elements of sheets are formed by two pieces (112 and 116) that embrace the sheets at their ends by their upper and lower face, both of which are pressed against the wooden sheet by means of threaded bolts that prevent sliding from the same. The system allows to use sheets of different thickness, depending on the length of the bolts used.
    The lower part of the clamping element (112) is composed of a flat piece that is fixed to the support structure with a "hinge" type system that prevents translation but allows the end of the sheet to be rotated for bending.


    The upper part of the clamping element (116) is a flat piece that incorporates a lever, which in a particular embodiment has a length of 55 cm, which has at its end a connecting element (120) for the tension bars.
    The bending of each sheet is produced by the simultaneous inclination of the levers of the 5 fasteners arranged at both ends.
    To keep the levers in a constant position, tension bars are used. Tension bars and load cells (106) that join the ends of them with the support structure. Each tension bar incorporates a load cell that allows to measure in time the evolution of force necessary to keep the lever in its fixed position. The evolution in
    The time of the quotient between the force measured at the initial time t0 and the force measured in an instant tn describes the relaxation of flexural stresses produced in the curved sheet in the period of time n.
    As a display and / or acquisition element of the measurements made by the load cells, any system existing in the market can be used for this purpose.
    15 At the end anchored to the support structure of each of the tension bars there is a system of regulating elements (108). In a particular embodiment, a regulating element is formed by a nut and washer that allow a precise adjustment of the useful length of the tension bar and, consequently, of the angle of the corresponding lever which, being tangent to the arc, establishes the radius of objective curvature of
    20 essay.
    Relationship of position between the different elements. Since the equipment has moving parts and allows infinite possible combinations, it is necessary to establish a procedure to determine the exact position of the different elements once the length and radius of the sheet to be tested are defined. This procedure, however, is
    25 based on the rules of geometry and therefore is not the subject of this patent.

    权利要求:
    Claims (1)
    [1]
    one- Device for conducting stress relaxation tests on sheets of
    wood (100) curved with constant radius, comprising:
    5 a) a support structure (102);
    b) a pair or more of fastening and bending elements (104) of sheets;
    c) a pair or more of tension bars and load cells (106);
    d) a couple or more of regulatory elements (108);
    characterized in that the support structure comprises a rigid frame open in its
    10 upper side and incorporates in its lower base a pair or more of profiles
    lengths that allow free placement of the fasteners and
    curved (104), wherein said clamping and curving elements comprise two
    pieces, one lower (112) and one upper (116), between which the end of the
    blade, and four or more bolts that produce the tightening of the assembly preventing the
    fifteen sliding between the sheet and the clamping parts; further said structure
    support incorporates in its vertical elements longitudinal grooves (114) that
    allow the free placement of tension bars (106) and the elements of
    regulation (108).
    2- The device according to claim 1, characterized in that the clamping piece
    twenty bottom (112) of the fasteners and curved sheets is fixed to the
    support structure (102) by a hinge type system that prevents the translation of
    the blade but allows its rotation.
    3- The device according to claim 1, characterized in that the upper part of
    clamping (116) of the clamping and sheet curling elements incorporate a
    25 lever that, by simultaneously tilting two of them located in each
    one of the ends of the sheet allows it to be curved.
    4- The device according to claim 1, characterized in that the tension bars and
    the load cells (106) keep fixed the position of the end of the levers of
    the fasteners and curved ..

    The device according to claim 1, characterized in that each regulating element (108) comprises a nut and a washer that allow adjusting the length of the tension bar.
    6- The device according to claim 1, characterized in that it further comprises means for viewing and / or reading the measurements made by the load cells.

    FIGURE 1

    FIGURE 2

    FIGURE 4

    FIGURE 5

    FIGURE 6
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    同族专利:
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    ES2649543B2|2018-05-17|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    SU565228A1|1974-12-27|1977-07-15|Предприятие П/Я М-5671|Method for testing bending stress relaxation in band material|
    JP2007132759A|2005-11-09|2007-05-31|Toyota Central Res & Dev Lab Inc|Relaxation test method, testing device and testing tool|
    CN103743635A|2013-06-20|2014-04-23|华北电力大学|Creep deformation test method and platform of full-dimension pipeline bend|
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