CN115112495A - A dynamic beam method for testing the Poisson's ratio of wood - Google Patents

Abstract

The invention discloses a dynamic beam method for testing the Poisson's ratio of wood according to the beam theory, the lengths of a cantilever wood beam with a square section and a cantilever wood composite beam test piece are l, a 0-degree strain gauge and a 90-degree strain gauge are respectively stuck on the central lines of two opposite surfaces of the cantilever wood beam test piece (b/h is 1) with the square section and the cantilever wood composite beam test piece (b/h is less than or equal to 1 and less than 3), the 0-degree strain gauges on the two surfaces are symmetrically arranged and the 90-degree strain gauges are symmetrically arranged, the 0-degree strain gauge is closely attached to the 90-degree strain gauge, and the distance between the center point of the 90-degree strain gauge and the fixed supporting edge of the cantilever wood beam with the square section and the cantilever wood composite beam test piece is l/3; testing strain frequency spectrums of 0 degree and 90 degrees of a cantilever wood beam with a square cross section and a cantilever wood composite material beam test piece or vibration wave curves of first-order bending vibration; the poisson ratio is calculated through a frequency spectrum or a vibration wave curve. The invention has simple operation and high test precision, and one square section cantilever wood beam test piece can test two principal directions Poisson's ratio of wood.

Description

A dynamic beam method for testing the Poisson's ratio of wood

technical field

The invention relates to a method for testing the Poisson's ratio of wood, in particular to a dynamic beam method for testing the Poisson's ratio of wood.

Background technique

The Poisson's ratio of wood is statically tested by the axial tensile method and the four-point bending beam method. The Poisson's ratio tested by the axial tensile method is based on the definition of Poisson's ratio, that is, the absolute value of the ratio of the inner transverse strain to the longitudinal strain of the specimen under the action of axial tension is defined as the material Poisson's ratio. The axial tensile method is a classic method to test the Poisson's ratio of materials from the definition of Poisson's ratio. It is suitable for both isotropic materials and anisotropic materials. The location of the patch should be avoided near the chuck of the testing machine. There are no other special requirements. It should be clearly stated that when the specimen is axially stretched, only the longitudinal tensile stress exists in the specimen, and the transverse stress is zero.

The four-point bending beam method for the static test of the Poisson's ratio of wood uses the absolute value of the ratio of the transverse strain to the longitudinal strain of the point near the center of the pure bending section of the beam when the beam is loaded with symmetrical four-point bending to test the Poisson's ratio of the material. Since the upper and lower surfaces of the beam specimen in the pure bending section are subjected to equivalent reverse stress (tensile stress or compressive stress) during four-point bending loading, this method can more comprehensively and truly display the mechanics of wood compared with the axial tensile method. behavior, which is an advantage of the four-point bending beam method. However, according to the static analysis of the four-point bending beam, there is no point where the transverse stress is equal to zero in the pure bending section of the four-point bending beam. Although the transverse stress is small, due to the anisotropy of wood, its longitudinal elastic modulus is higher than The transverse elastic modulus is an order of magnitude larger, which leads to a certain error between the absolute value of the ratio of the transverse strain at the center point of the beam surface to the longitudinal strain test value -ε _y /ε _x and the reference value of Poisson's ratio. The test results show that in order to reduce the Poisson's ratio dispersion of wood tested by the four-point bending beam method, two sets of half-bridge measurements are required.

In earlier studies, the Poisson's ratio of wood and OSB was tested with tensile tests. In recent years, based on the board theory, a patch method with the transverse stress of Poisson's ratio equal to zero for dynamic and static testing of wood is proposed, which is suitable for testing wood and OSB boards with cantilever panels with an aspect ratio of 4-6 and a width-thickness ratio of 6-13.67. Poisson's ratio. It has been verified by experiments that the absolute value of the ratio of the test value of the transverse strain gauge to the test value of the longitudinal strain gauge pasted at the position of transverse stress σ _y =0 can be used as the test value of Poisson's ratio. The test also verifies that the patch position determined by the dynamic and static transverse stress σ _y =0 patch method is different, but it can test the same Poisson's ratio result. According to the plate theory, the transverse stress of wood and OSB plates is tested dynamically and statically with the Poisson's ratio equal to zero (referred to as σ _y = 0 patch method). OSB sheet has a principal Poisson's ratio. In addition, the disadvantage of the σ _y =0 patch method is that the patch position for testing Poisson's ratio is related to the main direction of the wood and the size of the specimen, which is cumbersome and inconvenient to implement. It is necessary to seek a Poisson's ratio method for dynamic testing of wood and wood composite panels that is simple and accurate.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention based on the beam theory is to provide a dynamic beam method for testing the Poisson's ratio of a plate in view of the deficiencies of the prior art. The square section cantilever beam specimen can test the Poisson's ratio of the two principal directions of the wood.

In order to realize the above-mentioned technical purpose, the technical scheme adopted in the present invention is:

A dynamic beam method for testing the Poisson’s ratio of wood. The wood specimens are square-section cantilevered wood beams and cantilevered wood composite beams, and the lengths of the square-section cantilevered wood beams and cantilevered wood composite beams are equal is l, which is characterized by:

Paste 0° strain gauge and 90° strain gauge on the center line of the two opposite surfaces along the length direction of the wood specimen, wherein the positions of the 0° strain gauge and 90° strain gauge on the same surface are: 0° The angle between the strain gauge and the center line of the wood specimen is 0°, and the angle between the 90° strain gauge and the center line of the wood specimen is 90°. The center point of the 90° strain gauge and the center point of the 0° strain gauge are The connection line coincides with the center line of the wood specimen, the 0° strain gauge is symmetrically placed close to the 90° strain gauge, and the distance between the center point of the 90° strain gauge and the fixed edge of the wood specimen is 1/3;

Tap the center line of the wood specimen at a distance of 0.5l-0.6l from the fixed edge to excite the free vibration of the wood specimen;

Two of the 0° strain gages and the two 90° strain gages adopt the half-bridge method, and the dual-channel signal acquisition and analysis display the 0° strain and 90° strain spectrum or the vibration curve of the first-order bending vibration;

Read the 0° strain linear spectrum amplitude and 90° strain linear spectrum amplitude at the first-order bending frequency from the frequency spectrum; or, read the 0° strain peak-to-peak value and 90° strain waveform from the vibration curve of the first-order bending vibration The test value of Poisson's ratio is obtained from the peak-to-peak value of the strain waveform;

The frequency domain Poisson's ratio test value of the wood specimen is:

The time-domain Poisson's ratio test value of the wood specimen is:

As a further improved technical solution of the present invention, when the upper and lower surfaces along the length direction of the square-section cantilevered wooden beam are LT chord sections, then along the length direction of the square-section cantilever beams and perpendicular to the LT chord section The plane is the LR radial section; when the upper and lower surfaces along the length direction of the square section cantilever beam are the RT transverse section, then the vertical section along the length direction of the square section cantilever beam is perpendicular to the RT transverse section. The plane is the RL radial section; when the upper and lower surfaces along the length direction of the square section cantilever beam are the TR transverse section, then the plane along the length direction of the square section cantilever beam and perpendicular to the TR transverse section It is a TL chord section; that is, if three types of specimens containing wood principal directions LT, RT and TR are produced, six principal Poisson's ratios of LT, LR, RT, RL, TR and TL can be dynamically tested.

As a further improved technical solution of the present invention, the length-thickness ratio of the square-section cantilever wooden beam test piece is: l/h=8-20, the width-thickness ratio is: b/h=1-1.5, and the width h=15-20mm ; The length-thickness ratio of the cantilevered wood composite beam specimen is: l/h=8-20, and the width-thickness ratio is: 1≤b/h<3.

As a further improved technical solution of the present invention, the distance between the center point of the 90° strain gauge and the fixed edge of the wood specimen is 1/3, that is, the 1/3 span patch method, 0° strain gauge and 90° strain gauge The location of the patch does not depend on the tree species and the principal direction of the wood and the longitudinal or transverse orientation of the wood composite panel for beam specimens.

The beneficial effects of the present invention are:

The dynamic beam method for testing the Poisson's ratio of wood proposed by the present invention: adopt a square section cantilever wood beam specimen with a length-to-thickness ratio of 8-20; paste 0° and 90° from the center line of the cantilever beam surface at 1/3 of the fixed edge. ° strain gauge (square section cantilever beam 1/3 span patch method, referred to as 1/3 span patch method); through the 0° and 90° strain spectrum or vibration wave curve of the first-order bending vibration of the beam specimen to test the main direction of the wood Poisson's ratio. The advantage of the 1/3-span patch method for dynamic testing of the Poisson's ratio of wood, in addition to the simple test operation and high test accuracy, is that a square-section cantilever beam specimen can test the two principal Poisson's ratios of wood, namely If three types of specimens containing wood main directions LT, RT and TR are produced, six main direction Poisson's ratios such as LT, LR, RT, RL, TR and TL can be dynamically tested. In this regard, the dynamic beam method for testing wood Poisson's ratio based on beam theory is superior to the test method based on slab theory.

The invention has simple operation and high test accuracy, and a square section cantilever wooden beam test piece can test the Poisson's ratio of two principal directions of wood. That is to say, if three types of specimens containing wood main direction LT, RT and TR are produced, six main direction Poisson's ratios such as LT, LR, RT, RL, TR and TL can be dynamically tested; for wood composite materials, such as OSB and MDF, the width-thickness ratio of beam specimens applicable to the 1/3 span patch method of dynamic test Poisson's ratio can be relaxed to 1≤b/h<3. This relaxation condition has practical significance for testing the Poisson's ratio of wood composite materials, because The thickness of the commonly used wood composite panels is 15mm-20mm, which is rare, and it is not easy to realize the cantilever beam specimen with square section.

Description of drawings

Figure 1 shows the cantilever beam coordinate system.

Figure 2 shows the -ε _y /ε _x -x/l curves of radial cantilever beams such as spruce, red pine and beech.

Figure 3 shows the three main direction cantilever beams -ε _y /ε _x -x/l curves of pine tree in chord, radial and transverse directions.

Figure 4 is the chordwise cantilever beam -ε _y /ε _x -x/l curve of European red pine.

Fig. 5 is the horizontal cantilever beam -ε _y /ε _x -x/l curve of European red pine.

Figure 6(a) is a schematic diagram of the main direction specimen of the square section wooden beam LT.

Figure 6(b) is a schematic diagram of the RT main direction specimen of a square section wooden beam.

Figure 6(c) is a schematic diagram of the main direction specimen of the square section wooden beam TR.

Figure 7 is a test block diagram for testing the spectrum and waveform of the cantilever beam.

Figure 8 is the chord-direction (LT) spectrum of larch No. 1 specimen.

Figure 9 shows the chordwise (LT) waveform of larch No. 1 specimen.

Figure 10 shows the radial (LR) spectrum of larch No. 1 specimen.

Figure 11 is the TR-direction spectrogram of Xijia Spruce No. 1 specimen.

Figure 12 is a waveform diagram of the TR direction of the specimen of Xijia spruce No. 1.

Fig. 13 is the TL-direction spectrogram of Xijia Spruce No. 3 specimen.

Figure 14 is a plot of OSB longitudinal and transverse cantilever beams -ε _y /ε _x -x/l.

Figure 15 is a graph of MDF cantilever beam -ε _y /ε _x -x/l.

Detailed ways

The specific embodiments of the present invention will be further described below according to the accompanying drawings:

In order to simplify the method for testing the Poisson's ratio of wood and improve its testing accuracy, this embodiment proposes a new method for dynamically testing the Poisson's ratio of wood based on beam theory, namely the dynamic beam method. First, for the three main directions (LT, LR, and RT) square-section cantilever beam specimens of tree species such as spruce, red pine and beech, whose length-to-thickness ratios are 8, 10, 12, 16, and 20, the application The ANSYS modal block calculates their first-order bending modal stresses and strains, and determines the absolute value of the ratio of transverse to longitudinal strain -ε _y /ε _x along the centerline of the cantilever beam surface x/l, i.e. -ε _y /ε _x -x/l curve; according to the -ε _y /ε _x -x/l curve of the square section cantilever beam specimen, the characteristics of the change in a flat line in the range of x/l = 0.2-0.7 and the above The value of _-εy /εx _is equal to the corresponding reference value of the Poisson's ratio in the principal direction of the wood, and the 1/3 span patch method for testing the Poisson's ratio of the wood is proposed (strain gauges are pasted at 1/3 of the cantilever end of the specimen). The dynamic beam method for testing the Poisson’s ratio of wood proposed in this example: using a cantilever wood beam specimen with a square section with a length-to-thickness ratio of 8-20; paste 0° and 90° strain gauge (square section cantilever beam 1/3 span patch method, referred to as 1/3 span patch method); test the main wood through the 0° and 90° strain spectrum or vibration wave curve of the first-order bending vibration of the beam specimen. to Poisson's ratio. Poisson's ratios of larch LT and LR, TR and TL, and LVL longitudinal and transverse directions of larch were tested by the 1/3-span patch method, and the four-point bending beam method and the axial tensile method were also used. The validity of the 1/3 span patch method for testing the Poisson's ratio of wood is verified. The advantage of the 1/3-span patch method for dynamic testing of the Poisson's ratio of wood, in addition to the simple test operation and high test accuracy, is that a square-section cantilever beam specimen can test the two principal Poisson's ratios of wood, namely If three types of specimens containing wood main directions LT, RT and TR are produced, six main direction Poisson's ratios such as LT, LR, RT, RL, TR and TL can be dynamically tested. In this regard, the dynamic beam method for testing wood Poisson's ratio based on beam theory is superior to the test method based on slab theory.

A square section beam specimen with a width-to-thickness ratio of b/h=1, eg a thickness of 20 mm, has been used to statically test E, μ and G of wood by the four-point bending beam method. In this example, the 1/3 span patch method of a cantilever beam specimen with a square section is proposed to dynamically test the principal direction Poisson's ratio of wood (dynamic beam method).

First, ANSYS calculates the first-order bending modal stress and modal strain of cantilever beam specimens with square cross-sections in three principal directions of tree species with different length-to-thickness ratios, such as spruce, pine and beech, to determine the center line of the beam specimen surface. The distribution law of -ε _y /ε _x , namely the -ε _y /ε _x -x/l curve; according to the -ε _y /ε _x -x/l curve of the square section cantilever beam specimen, at x/l=0.2 In the range of -0.7, there is a flat line change and the -ε _y /ε _x value is equal to the reference value of the Poisson's ratio of the wood. A 1/3 span patch method for testing the Poisson's ratio of the main direction of the wood is proposed, that is, the strain gauge is pasted on the cantilever beam. On the central line of the specimen surface and 1/3 from the cantilever end, l is the outrigger length of the cantilever beam.

For wood beam specimens with square cross-section (b/h=1), if one face along the length of the wood beam is a chord face, the face perpendicular to it is the radial face, or one face is RT face, then the face perpendicular to it It is the RL-oriented face, or a face is the TR-oriented face, then the face perpendicular to it is the TL-oriented face. Therefore, the 1/3 span patch method for dynamically testing the Poisson's ratio of wood based on the beam theory proposed in this embodiment can test the Poisson's ratio of two principal directions of wood from a beam specimen. That is to say, if three types of specimens including main direction LT, RT and TR are produced, six main direction Poissons such as LT, LR, RT, RL, TR and TL can be dynamically measured. In this regard, the 1/3 span patch method for testing the Poisson's ratio of wood proposed in this example is superior to the σ _y =0 patch method for testing the Poisson's ratio of wood based on the board theory.

The main points of this embodiment are as follows:

The dynamic beam method for dynamic testing of the Poisson’s ratio in the principal direction of wood is based on the following: the curve of -ε _y /ε _x -x/l on the central line of the surface of a cantilever beam with square section is flat in the range of x/l=0.2-0.7 during the first-order bending vibration. The characteristics of straight line change; the value of -ε _y /ε _x on the flat line is equal to the corresponding reference value of the Poisson's ratio in the principal direction of the wood; the 0° and 90° strain gauges are pasted on the central line of the surface of the cantilever beam with a square section, and the distance from the cantilever 1/3 at the end (ie 1/3 span patch method); suitable for cantilever beams with square section, its aspect ratio is 8-20, the width (thickness) of square section is 15mm-20mm; the width-thickness ratio can be relaxed to 1.5; A cantilever beam with a square section can test the Poisson's ratio of two principal directions of wood; it is suitable for testing the Poisson's ratio of six principal directions of wood, that is, to make LT, RT, TR specimens, and to test the six principal directions of wood Poisson's ratio ratio _μLT , _μLR , _μRT , _μRL , _μTR and _μTL .

For wood composite panels, i.e. cantilevered wood composite beam specimens, such as OSB and MDF cantilever beam specimens, the first-order bending vibration stress-strain analysis shows that the dynamic test Poisson's ratio 1/3 span patch method is suitable for beam test The width-thickness ratio of the piece can be relaxed to 1≤b/h<3. This relaxed condition has practical significance for testing the Poisson’s ratio of wood composite panels, because the thickness of commonly used wood composite panels is 15mm-20mm, which is rare, and it is not easy to realize a square section cantilever beam Specimen.

The dynamic beam method for testing Poisson's ratio has the advantages of simple test operation and high test accuracy.

1. Method:

1.1. The first-order bending modal stress and strain of the cantilever beam:

The cantilever beam coordinate system is shown in Figure 1, the beam length l, the beam width b, and the beam thickness h.

The ANSYS 19 modal program block solid-shell 3D Finite strain 190 element is used to calculate the first-order bending modal stress and strain of the cantilever beam specimen. The mesh is divided into 40×8×4, and the input material parameters come from wood science.

1.1.1. The first-order bending modal stress and strain of a square-section cantilevered wooden beam:

For spruce, red pine and beech radial square-section cantilever beam specimens (200mm×20mm×20mm), ANSYS was used to calculate the position of transverse stress σ _y = 0 and the absolute ratio of transverse strain to longitudinal strain during the first-order bending vibration the distribution of values along the center line of the beam surface (Fig. 2);

It can be seen from Fig. 2 that -ε _y /ε _x varies with x/l on the center line of the radial square-section cantilever beam of beech, pine and spruce, and it varies in a straight line in the range of x/l = 0.2-0.7 , and the -ε _y /ε _x values on the flat line are equal to the respective radial Poisson's ratios of 0.45, 0.42 and 0.37; the x/l = 0.156, 0.237 and 0.413 marked in Fig. 2 are beech, pine and cloud, respectively The transverse stress of a square-section cantilever beam specimen in the radial direction of fir is equal to zero, and the -ε _y /ε _x values at these positions are also very approximately equal to the respective Poisson's ratio values.

Figure 3 shows the first-order bending mode -ε _y /ε _x -x/l curve of the three main direction square-section cantilever beam specimens (200mm×20mm×20mm) of pine pine in chord, radial and transverse directions. It can be seen from Figure 3 that for the cantilever beam specimens with equal square cross-sections in the radial and transverse directions of pine, in the range of x/l=0.2-0.8, -ε _y /ε _x changes in a straight line, and their values are respectively equal to the diameter of pine. Poisson’s ratio of 0.42 in the longitudinal direction and 0.68 in the transverse direction of the pine; for the cantilever beam with a square section in the chordwise direction of the pine, in the range of x/l=0.3-0.7, -ε _y /ε _x also changes in a straight line, Its value is equal to the chordwise Poisson's ratio of 0.57 for European red pine.

1.1.2. The first-order bending modal stress and strain of a cantilevered wooden beam when the width-thickness ratio is equal to 1, 2, and 3:

For European red pine chord and transverse cantilever beam specimens (specimen length l=180mm, specimen thickness h=10mm, specimen width-thickness ratio b/h equal to 1, 2 and 3, respectively, the first-order bending modulus calculated by ANSYS 19 The state-εy/ _{εx-x /} l curves are shown in Fig. 4 and Fig. 5, respectively.

As shown in Figure 4: For the European red pine chordwise cantilever beam specimen of 180mm×10mm×10mm (b/h=1), the transverse stress σ _y =0 position x/l=0.265, -ε _y /ε _x at x /l=0.2-0.7 in the range of a flat line, and its value is equal to the Poisson's ratio of the European red pine chordwise 0.57; for the European red pine chordwise cantilever beam specimen 180mm×20mm×10mm (b/h=2), the transverse stress σ At the position of _y = 0, x/l = 0.385, -ε _y /ε _x shows a slight upward trend in the range of x/l = 0.3-0.5, and its value is slightly equal to the chordwise Poisson's ratio of European red pine of 0.57; for European red pine The chordwise cantilever beam specimen is 180mm×30mm×10mm (b/h=3), the position of transverse stress σ _y =0 is x/l=0.467, -ε _y /ε _x is in the range of x/l=0.4-0.6 The upward trend changes rapidly, and its value deviates from the Poisson's ratio of 0.57 in the string direction of European red pine. Therefore, the variation law of -ε _y /ε _x with x/l of the first-order bending mode of the European red pine chordwise cantilever beam is related to the width-to-thickness ratio of the cantilever beam. For the chordwise cantilever beam specimen of European red pine with a width-to-thickness ratio of b/h=1, -ε _y /ε _x changes in a straight line in the range of x/l=0.2-0.7, and its value is equal to the chordwise Poisson's ratio of European red pine. Reference.

The same analysis is used for the European red pine radial cantilever beam specimen. When its width-thickness ratio b/h=1, 2, 3, the variation law of -ε _y /ε _x with x/l is the same as that of the chordwise cantilever beam specimen.

For the transverse cantilever beam specimen of European red pine, when its width-thickness ratio b/h=1, 2, 3, in the range of x/l=0.2-0.6, the value of -ε _y /ε _x is equal to the transverse Poisson's ratio of European red pine , which is 0.68. Therefore, when the width-to-thickness ratio b/h of the cantilever beam specimen changes from 1 to 3, the variation law of -ε _y /ε _x -x/l in the transverse direction of the wood is different from the -ε _y /ε _x - in the chordwise and radial directions of the wood. Variation of x/l.

To sum up, the -ε _y /ε _x value of the cantilever beam specimen in the three main directions of the wood with the width-to-thickness ratio of b/h=1 changes in a straight line in the range of x/l=0.2-0.7. , whose value is equal to the reference value of the principal direction Poisson's ratio of the wood. Therefore, for the Poisson's ratio test of wood, if a square-section beam specimen is used, the error caused by the inaccurate position of the patch will be reduced, thereby improving the accuracy of the Poisson's ratio test.

Considering the value of -ε _y /ε _x on the central line of the surface of the square section timber cantilever beam specimen, it varies with the x/l flat line in the range of x=0.2l-0.7l, and its value is equal to the main direction of the timber. The reference value of Poisson's ratio (Figure 2-Figure 4), so to test the Poisson's ratio of wood, the strain gauge can also be pasted at 1/3 of the distance from the cantilever end, that is, the 1/3-span patch method. The 1/3-span patch method is suitable for dynamic testing of the three principal Poisson's ratios of wood in chord, radial and transverse directions.

The square-section main-direction beam specimens shown in Figure 6 (a), 6 (b), and 6 (c) were made, because the planes perpendicular to the main planes of LT, RT, and TR wood correspond to LR, RT, and TR, respectively. RL and TL wood principal planes, so the three types of square-section principal beam specimens shown in Figure 6 (a), 6 (b), and 6 (c) can dynamically test the six principal directions of wood. Songbi μLT, μLR, μRT, μRL, μTR and μTL.

1.3, Poisson's ratio definition:

Definition of frequency domain Poisson's ratio:

The ratio of the 90° strain linear spectrum amplitude to the 0° strain linear spectrum amplitude at the first-order bending frequency of the spectrum of the cantilever beam specimen, namely:

The definition of Poisson's ratio in time domain:

The absolute value of the ratio of the peak-to-peak value of the 90° strain waveform to the peak-to-peak value of the 0° strain waveform of the first-order bending vibration of the cantilever beam specimen, namely:

2. Test:

2.1. Specimen:

Three beam specimens were cut from the chord direction of larch to the large slab, their dimensions and main directions: larch LT direction (chord direction), size: 320mm×20mm×20mm; larch LR direction (radial), size: 320mm ×20mm×20mm.

Three beam specimens were cut down from the cross-section of spruce spruce, and their dimensions and main directions: TR direction of spruce spruce, size: 300mm×30mm×20mm; direction of spruce TL, size: 300mm×20mm× 30mm.

Three beam specimens were cut longitudinally and transversely from the whole LVL plate, and their dimensions were: 380mm×15.7mm×15.7mm.

2.1. Patch scheme:

On the chord plane (LT plane) of the larch beam, the 0° strain gauge and the 90° strain gauge are attached to the center of the upper and lower surfaces (chord plane) of the beam, with a total of 4 strain gauges;

On the radial surface of the larch beam, the 0° strain gauge and the 90° strain gauge are pasted at the center of the upper and lower surfaces (radial surface) of the beam, with a total of 4 strain gauges;

On the TR surface of the Xijia spruce beam, the 0° strain gauge and the 90° strain gauge sticker are pasted at the center of the upper and lower surfaces (TR surface) of the beam, a total of 4 strain gauges;

On the TL surface of the Xica spruce beam, the 0° strain gauge and the 90° strain are attached to the center of the upper and lower surfaces (TL surface) of the beam, with a total of 4 strain gauges.

LVL longitudinal and transverse, the upper and lower surfaces of the beam specimen and the center of the adjacent sides are respectively pasted with 0° strain gauges and 90° strain gauges, a total of 8 strain gauges;

For the four-point bending beam method and the axial tensile method to test the Poisson's ratio and the dynamic beam method to test the Poisson's ratio in this paper.

A total of 72 strain gauges. The specification of the strain gauge is BX-120, the strain grid is 5mm×3mm, and the strain gauge is pasted with 502 glue.

The specific positions of the 0° strain gauge and the 90° strain gauge on a certain surface of the above wood are: where the angle between the 0° strain gauge and the center line of the square-section cantilever beam is 0°, and the 90° strain gauge and the square-section cantilever beam are at an angle of 0°. The included angle of the center line of the wooden beam is 90°, the line connecting the center point of the 90° strain gauge and the center point of the 0° strain gauge is parallel to the center line of the cantilever beam with square section, and the 0° strain gauge is close to the 90° For the setting of the strain gauge, the distance between the center point of the 90° strain gauge and the fixed edge of the cantilever beam with a square section is 1/3.

2.3. Experimental design:

First, the four-point bending beam method is used to statically test the Poisson's ratio, and the upper and lower surfaces of the beam are half-bridged with strain gauges. Then, the 1/3-span patch method is used to test the Poisson's ratio. Finally, the axial tension method is used to statically test the Poisson's ratio.

The four-point bending beam method and the axial tensile method are used as experimental verifications for the validity of the 1/3 span patch method proposed in this example to test the Poisson's ratio of wood.

For the specimens that are statically tested for Poisson's ratio by the four-point bending beam method, the strain gauges pasted by the four-point bending beam method can meet the requirements of the 1/3-span patch method by adjusting the depth of the clamping specimen, and the Poisson's ratio of the wood is dynamically tested.

2.4. Test block diagram:

The 0° strain gauge and the 90° strain gauge on the upper and lower surfaces of the beam are collected by the half-bridge method, respectively, and the frequency spectrum and vibration wave curve of the first-order bending vibration of the cantilever beam are tested. The test block diagram is shown in Figure 7.

2.5. Spectrum and waveform:

Read out from Figure 8: The first-order bending frequency of larch No. 1 specimen in the chord direction is 265 Hz, the linear spectrum amplitude of the 0° strain gauge is 6.15 με, and the linear spectrum amplitude of the 90° strain gauge is 2.43 με. That is, formula (4)) obtains the test value of the larch chordwise Poisson's ratio of 0.395.

Read from Figure 9: 21.29ms-23.24ms, the peak-to-peak value of the 0° strain gauge waveform is 42.2με, the negative peak-to-peak value of the 90° strain gauge waveform is -17.05με, and the Poisson's ratio test value calculated according to formula (5) is 0.404 ;

36.72ms-38.48ms, the peak-to-peak value of the 0° strain gauge waveform is 35.11με, the negative peak-to-peak value of the 90° strain gauge waveform is -14.01με, the Poisson's ratio calculated according to formula (5) is 0.399; Corresponds to the negative peak of the 0° slice.

Read from Figure 10: The radial first-order bending frequency of larch No. 1 specimen is 320Hz, the linear spectrum amplitude of the 0° strain gauge is 8.18με, and the 90° strain gauge linear spectrum amplitude is 2.49με. Formula (4)) obtains the larch chordwise Poisson's ratio test value of this specimen is 0.304;

Read from Figure 11: the first-order bending frequency is 80 Hz, the linear spectrum amplitude of the 0° strain gauge is 42.69 με, the linear spectrum amplitude of the 90° strain gauge is 15.19 με, and the measured value of the Poisson’s ratio in the TR direction of spruce spruce is 0.356 ;

Read from Figure 12: 38.67ms-45.31ms, the peak-to-peak value of the 0° strain gauge waveform is 172.68με, the negative peak-to-peak value of the 90° strain gauge waveform is -63.57με, and the Poisson's ratio calculated according to formula (5) The test value is 0.368 ;76.56ms-82.81ms, the peak-to-peak value of the 0° strain gauge waveform is 142.76με, the negative peak-to-peak value of the 90° strain gauge waveform is -51.72με, the Poisson's ratio calculated according to formula (5) is 0.362; The peak corresponds to the negative peak of the 0° slice.

Read out from Figure 13: the first-order bending frequency is 67.5Hz, the linear spectrum amplitude of the 0° strain gauge is 6.91με, the linear spectrum amplitude of the 90° strain gauge is 0.17με, and the measured value of the Poisson’s ratio in the TL direction of Sica spruce is 0.025.

From Figure 8 to Figure 13: Poisson's ratio with approximately the same value can be determined whether from the frequency domain or from the time domain; the vibration wave curve in the time domain is attenuated, and the positive peak value of the 0° sheet strain corresponds to the 90° sheet strain The negative peak value of , which indicates that the phase difference between the transverse strain and the longitudinal strain is 180°, that is, the opposite direction, which has obvious physical meaning; in addition, the damping ratio of the material can be determined from the damped vibration curve.

2.5 Confirmatory test:

2.5.1 Symmetrically loaded four-point bending beam method test:

The four-point bending test bench is designed by itself, loaded with weights, and can measure E, μ.

Larch specimen: span is 300mm, symmetrical four-point bending loading: initial load is 24.99N (2.55kg weight), ΔP=24.99N, secondary loading.

Sika spruce specimen: span is 240mm, symmetrical four-point bending loading: initial load is 4.165N (0.425kg weight), ΔP=4.165N, secondary loading.

In the four-point bending test, two sets of two-channel half bridges were used to measure 0° strain gauges and 90° strain gauges, that is, after one set of three consecutive measurements, the specimen was turned 180° around the central axis of the beam for another set of three consecutive measurements. The mean of the two groups of E,μ test values was taken as the E,μ test value of the specimen.

2.5.2 Axial tensile test:

Four-channel measurement, one test piece simultaneously measures two main directions E and μ, 0° strain gauge and 90° strain gauge each occupy one channel. In order to eliminate the bending strain caused by the misalignment of the load during stretching, the 0° strain gauge and the 90° strain gauge on the two surfaces were connected in series, respectively, and the 1/4 bridge was used to test the strain in the 0° and 90° directions.

Microcomputer-controlled electronic universal testing machine, model UTM 4304, produced by Shenzhen Sansi, with a maximum test force of 30kN.

The loading scheme is:

Larch specimens: 500N-2500N-4500N; Western spruce specimens: 100N-200N-300N. The axial tensile test of each specimen is carried out three times, and the average value of E and μ of the last two tests is taken as the parameter test value of the specimen.

3. Results and Analysis:

Poisson's ratio testing for larch tangential (LT) and radial (LR) and TR and TL directions for larch tested by dynamic beam method (this example), four-point bending beam method and axial tensile method The values are shown in Table 1 (the percentage in brackets is the coefficient of variation, the same below).

Table 1 shows the Poisson's ratios for larch and spruce 1/3 span patch method, four-point bending beam method and axial tensile method.

As seen from Table 1: Poisson's ratios of larch tangential (LT) and radial (LR) and TR and TL directions of larch tested by dynamic beam method, four-point bending beam method and axial tensile method quite consistent.

Specimens used for the 1/3-span patch method can test both the Poisson's ratio in the principal direction of the wood and the modulus of elasticity in the principal direction of the wood. As long as the square-section wooden beam specimen is freely suspended with a rubber rope (stiffness coefficient less than 1N/cm), the first-order bending frequency is measured, and the elastic modulus is calculated according to the relationship between the elastic modulus of the Euler beam and the first-order bending frequency. After the elastic modulus E, the main direction Poisson's ratio of the wood can be dynamically tested by the 1/3 span patch method.

For the LVL beam of 380mm×20mm×20mm, the elastic modulus and Poisson’s ratio of LVL tested by dynamic beam method, four-point bending beam method and axial tensile method are shown in Table 2.

Table 2 shows the LVL elastic modulus and Poisson's ratio tested by the dynamic beam method, the four-point bending beam method and the axial tensile method.

x-LVL whole board longitudinal direction (along the length); y-LVL whole board transverse direction (along the width); z-LVL whole board thickness direction.

4 Conclusion:

4.1. During the first-order bending vibration of a cantilevered wooden beam with a square section, the absolute value of the ratio of transverse strain to longitudinal strain -ε _y /ε _x changes in a straight line in the range of x/l=0.2-0.7 on the center line of the beam surface, and its -ε The _y /ε _x value is equal to the Poisson's ratio reference value;

4.2. The dynamic beam method is suitable for the dynamic test of 6 timber main direction Poisson's ratios including LT, LR, RT, RL, TR and TL directions of wood beams with a length-to-thickness ratio of 8-20 or 8-12;

4.3 Validity of LT and LR directions of larch, TR and TL directions of spruce and LVL longitudinal and transverse Poisson's ratio of 1/3-span patch method dynamic tests obtained by four-point bending beam method and axial tension Test verification of extension method;

4.4. Dynamic beam method Since the patch positions for testing the Poisson's ratio of wood are all on the 1/3 span of a cantilever beam with a square section, this method is applied to test the Poisson's ratio of the main direction of wood, which has the advantages of simple operation and high test accuracy.

Appendix: First-order bending modal strain analysis of wood composite cantilever beam specimens (ie, cantilever wood composite beam specimens).

For OSB longitudinal, OSB transverse and MDF cantilever beam specimens (specimen length l=180mm, specimen thickness h=10mm specimen width-thickness ratio b/h are 1, 2 and 3 respectively) the first-order bending mode calculated by ANSYS 19 The -ε _y /ε _x -x/l curves are shown in Figure 14 and Figure 15, respectively. As shown in Figure 14, the OSB longitudinal Poisson's ratio=0.34, and the OSB transverse Poisson's ratio=0.13. As shown in Figure 15, MDF Poisson's ratio = 0.25.

It can be seen from Fig. 14 and Fig. 15 that the -ε _y /ε _x -x/l curves have the same change for the cantilever beam specimens with OSB longitudinal, OSB transverse and MDF equal aspect ratio b/h = 1, 2 and 3 temperament. For b/h=1, in the range of x/l=0.1-0.7, -ε _y /ε _x is a flat line transformation ratio, and the -ε _y /ε _x value in this range is equal to the reference value of the respective Poisson's ratio; for b/h=2, in the range of x/l=0.2-0.6 -ε _y /ε _x is a flat line transformation ratio, and the -ε _y /ε _x value in this range is equal to the reference value of the respective Poisson's ratio; for x /l=3, in the range of x/l=0.3-0.5, -ε _y /ε _x is a flat line transformation ratio, and -ε _y /ε _x in this range is equal to the reference value of the respective Poisson's ratio.

OSB and MDF cantilever beam specimens at the width-to-thickness ratio b/h=1, 2 and 3 -ε _y /ε _x vary with x/l although the variation rule is different from that of wood chordwise or radial (Fig. 2), But the change of -ε _y /ε _x with x/l in the transverse direction of wood is the same.

In this example, the 1/3 patch method of Poisson's ratio is tested, and the position of the patch is a beam specimen that does not depend on the tree species and the main direction of the wood and the longitudinal or transverse orientation of the wood composite board.

The protection scope of the present invention includes but is not limited to the above embodiments, the protection scope of the present invention is subject to the claims, and any replacement, deformation, and improvement that are easily thought of by those skilled in the art made by the present technology all fall within the scope of the present invention. protected range.

Claims (4)

1. A dynamic beam method for testing the Poisson’s ratio of wood, the wood specimens are square-section cantilevered wood beam specimens and cantilevered wood composite beam specimens, and square-section cantilevered wood beam specimens and cantilevered wood composite material beam specimens The length is all l, and it is characterized in that: Paste 0° strain gauge and 90° strain gauge on the center line of the two opposite surfaces along the length direction of the wood specimen, wherein the positions of the 0° strain gauge and 90° strain gauge on the same surface are: 0° The angle between the strain gauge and the center line of the wood specimen is 0°, and the angle between the 90° strain gauge and the center line of the wood specimen is 90°. The center point of the 90° strain gauge and the center point of the 0° strain gauge are The connection line coincides with the center line of the wood specimen, the 0° strain gauge is symmetrically placed close to the 90° strain gauge, and the distance between the center point of the 90° strain gauge and the fixed edge of the wood specimen is 1/3; Tap the center line of the wood specimen at a distance of 0.5l-0.6l from the fixed edge to excite the free vibration of the wood specimen; Two of the 0° strain gages and the two 90° strain gages adopt the half-bridge method, and the dual-channel signal acquisition and analysis display the 0° strain and 90° strain spectrum or the vibration curve of the first-order bending vibration; Read the 0° strain linear spectrum amplitude and 90° strain linear spectrum amplitude at the first-order bending frequency from the frequency spectrum; or, read the 0° strain peak-to-peak value and 90° strain waveform from the vibration curve of the first-order bending vibration The peak-to-peak value of the strain waveform is obtained to obtain the test value of Poisson's ratio. 2. The dynamic beam method for testing the Poisson's ratio of wood according to claim 1, characterized in that: when the upper and lower surfaces along the length direction of the cantilever beam with the square section are LT tangential planes, then along the square The plane in the length direction of the cross-section cantilever beam that is perpendicular to the LT chord section is the LR radial section; when the upper and lower surfaces along the length direction of the square section cantilever beam are the RT cross section, then along the square section The plane in the length direction of the cantilevered wooden beam and perpendicular to the RT transverse section is the RL radial section; when the upper and lower surfaces along the length direction of the square section cantilever beam are the TR transverse section, then the cantilever along the square section cantilever The plane in the length direction of the wooden beam and perpendicular to the TR transverse section is the TL chord section; that is, if the three types of specimens containing the main timber direction LT, RT and TR are produced, LT, LR, RT, RL, TR can be dynamically tested. and TL with six principal Poisson's ratios. 3. The dynamic beam method for testing the Poisson's ratio of wood according to claim 1 or 2, characterized in that: the length-to-thickness ratio of the square-section cantilevered wooden beam specimen is: 1/h=8-20, and the width-to-thickness ratio is: b/h=1-1.5, width h=15-20mm; the length-thickness ratio of the cantilever wood composite beam specimen is: l/h=8-20, and the width-thickness ratio is: 1≤b/h< 3. 4. The dynamic beam method for testing the Poisson's ratio of wood according to claim 1, wherein the distance between the center point of the 90° strain gauge and the retaining edge of the wood specimen is 1/3, that is, 1/3 For the cross-patch method, the patch positions of 0° strain gauges and 90° strain gauges are independent of the tree species and the principal direction of the wood and the longitudinal or transverse orientation of the wood composite panels for beam specimens.

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