CN105547861A - Method for enhancing capability of testing modulus of elasticity and precision of Poisson's ratio of wood by four-point bent beam - Google Patents

Abstract

The invention provides a method for enhancing the capability of testing modulus of elasticity and precision of Poisson's ratio of wood by a four-point bent beam. According to the method, the bent beam is cuboid with the width of b and thickness of h; the length of load span is l; a horizontal strain gage is pasted to the central point part in the middle of the bent beam; a longitudinal strain gage is in contact with the horizontal strain gage; the length to thickness ratio l/h is 16-20; the width to thickness ratio b/h is 1-2.

Description

Improve the method for four_point bending beam test modulus of elasticity of wood and Poisson ratio precision

Technical field

The present invention relates to the method improving four_point bending beam test modulus of elasticity of wood and Poisson ratio precision.

Background technology

The assay method of GB/T1936.2-2009 wood modulus of elasticity, extrapolates the Deflection Modulus of Elasticity of timber by measuring four_point bending beam mid-span deflection.The reckoning elastic modulus formula provided in standard does not count shearing to affect mid-span deflection.

Due to a modulus of elasticity parellel to grain generally order of magnitude larger than corresponding modulus of shearing of timber, therefore the impact of shearing on four-point bending wooden frame mid-span deflection is not allowed to ignore.

Measure timber Poisson ratio and commonly use axial compression or tension test, compression specimens is of a size of 20mm × 20mm × 30mm or 30mm × 30mm × 60mm.Axial compression need load by testing machine and load require centering, surface of test piece is smooth and contact with pressure head of testing machine and will reduce friction, therefore axial compression test condition is harsher, realize difficulty, so that test data dispersiveness is bigger than normal.

When four_point bending beam is used for testing elastic modulus and Poisson ratio, though can be similar to for the upper and lower surperficial each point of bent beam and be considered as uniaxial stressed state, but transverse strain and longitudinal strain ratio change with the position of putting, according to semi-girder or the four_point bending beam test specimen as static measurement Poisson ratio, then there is cross strain rosette and should be attached to where going up of beam surface, could the problem of material Poisson ratio be obtained by the transverse strain measured and longitudinal strain ratio? in fact, the upper and lower surface of four_point bending beam is apart from meridional stress σ _x, also there is transverse stress σ _y, only σ _y<< σ _x, therefore being called that the upper and lower surface of beam is in uniaxial stressed state approx, this approximation can be ignored when testing Poisson ratio isotropy, but such as, for orthotropic timber, dragon spruce, even if σ _yjust σ _x6 ‰, due to a modulus of elasticity parellel to grain order of magnitude larger than tangential elastic module, thus test Poisson ratio time also can cause flagrant relative error.

Summary of the invention

The object of this invention is to provide a kind of method that can improve four_point bending beam test modulus of elasticity of wood and Poisson ratio precision.

The method of raising four_point bending beam test modulus of elasticity of wood of the present invention and Poisson ratio precision, described bent beam is the rectangular parallelepiped of wide b, thick h, load the long l of span, transverse strain sheet is attached to the centre of bent beam across center position, and longitudinal strain sheet contacts with transverse strain sheet; Slenderness ratio l/h is 16 ~ 20; Flakiness ratio b/h is 1 ~ 2.

Above-mentioned raising four_point bending beam test modulus of elasticity of wood and the method for Poisson ratio precision, described loading Position is pressed l/3-l/3-l/3, l/4-l/2-l/4 or l/5-3l/5-l/5 four-point bending and is loaded.

Above-mentioned raising four_point bending beam test modulus of elasticity of wood and the method for Poisson ratio precision, when surveying timber tangential section, radial longitudinal section Poisson ratio, its bent beam is of a size of 280mm × 20mm × 20mm, and load by l/3 – l/3 – l/3 four-point bending, loading span l is 240mm.

Above-mentioned raising four_point bending beam test modulus of elasticity of wood and the method for Poisson ratio precision, when surveying wood transverse section Poisson ratio, its bent beam size 220mm × 20mm × 20mm, load by l/4 – l/2 – l/4 four-point bending, loading span l is 240mm.

Beneficial effect of the present invention: applicant finds, on the pure bending section that four-point bending loads, longitudinal strain is not substantially with change in location; But transverse strain increases and increase (x-axis is with the strong point of the side of bent beam for initial point, and the length direction along bent beam extends) with x/l by absolute value.Longitudinally, this variation characteristic of transverse strain causes: can calculate E with the longitudinal strain of arbitrfary point on pure bending section, and measure-the ε that Poisson ratio must use central point _y/ ε _xvalue is estimated, otherwise can cause comparatively big error.According to transverse strain and the longitudinal strain Changing Pattern at pure bending section, be the measuring accuracy ensureing Poisson ratio, the transverse strain sheet of strain rosette is pasted on the central point on the upper and lower surface of beam, and longitudinal strain sheet is critical pastes by transverse strain sheet.In addition, illustrate that sample dimensions and four-point bending loading Position are also two key factors affecting timber Poisson ratio measuring accuracy, slenderness ratio l/h be 16 ~ 20, flakiness ratio b/h be 1 ~ 2 when, test modulus of elasticity of wood and Poisson ratio, it is more accurate especially to test Poisson ratio.

Accompanying drawing explanation

Fig. 1 is that four_point bending beam loads schematic diagram;

Fig. 2 is P/2 effect l/3 mid-span deflection when counting shearing;

Fig. 3 is that dragon spruce test specimen is at pure bending section-ε _y/ ε _x-x/l changes schematic diagram;

Fig. 4 is that dragon spruce test specimen is at pure bending section-ε _z/ ε _x-x/l changes schematic diagram;

Fig. 5 is that dragon spruce four_point bending beam is at pure bending areal strain distribution plan;

Fig. 6 is strain rosette paste position schematic diagram.

Embodiment

Below the present invention is elaborated.

1 four_point bending beam is for measuring wood modulus of elasticity

In GB/T1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood, the beam adopting l/3, l/3, l/3 four-point bending to load is test specimen, calculates wood modulus of elasticity, as shown in Figure 1 by measuring beam mid-span deflection.

L/3 in the middle of beam is across being in pure bending, and namely beam is on all cross sections of this section, and shearing is zero, and moment of flexure is constant, and namely moment of flexure does not change with sectional position; And left and right l/3 is across being in shear bending, namely not only there is moment of flexure in cross section, also there is shearing.Being calculated in timber bullet flexing resistance tangent elastic modulus by beam mid-span deflection of providing in GB/T1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood only considers moment of flexure, do not count left and right l/3 span centre shearing to the impact of beam midway deflection, analysis shows timber, due to a timber modulus of elasticity parellel to grain order of magnitude larger than its modulus of shearing, therefore ignoring much larger than isotropic material of the impact of wood modulus of elasticity test value of shearing, cause wood modulus of elasticity test value to produce sizable error.

GB/T1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood specifies: four-point bending l/3-l/3-l/3 loads (Fig. 1), sample dimensions 300mm × 20mm × 20mm (loading span 240mm), namely on testing machine, realize four-point bending l/3-l/3-l/3 by 80mm-80mm-80mm to load, regulation lower limit load p _under=300N, upper limit load p _on=700N, therefore △ P=400N.

Elastic modulus is pressed mid-span deflection test value and is calculated:

$E=\frac{23{\mathrm{\&Delta;Pl}}^3}{108{\mathrm{\&Delta;ybh}}^3}---\left(1\right)$

In formula: △ P-load increment N; △ y-beam mid-span deflection increment; L-test specimen span mm; B-specimen width mm; H-test specimen thick (height) degree mm.E derived unit MPa.

Formula (1) not to count in beam shearing to the impact of its mid-span deflection, but what measure is amount of deflection under bending and shearing acting in conjunction, and so, the Deflection Modulus of Elasticity that applying equation (1) calculates is less than normal.

Count the impact of the shearing in four_point bending beam on its mid-span deflection below, with this, formula (1) is revised.

When beam l/3 place used load P/2 (Fig. 2), counting the amount of deflection y (l/2) that shearing produces at span centre l/2 place can be expressed as:

$EIy(l/2)=\frac{23}{2592}{\mathrm{Pl}}^3+\frac{E}{G}\frac{i^2}{12}Pl/k$

In formula: I-beam section moment of inertia; I-beam section turning radius; K-Section factor, to square-section k=0.913.

According to superposition principle, consider symmetry, during beam l/3,2l/3 place effect P/2 (as shown in Figure 1, l/3-l/3-l/3 four-point bending loads stand under load), the amount of deflection produced at span centre l/2 place is

$EIy(l/2)=\frac{23{\mathrm{Pl}}^3}{1296}+\frac{2E}{G}\frac{i^2}{12}Pl/k$

$E=\frac{23{\mathrm{Pl}}^3}{1296Iy(l/2)}(1+9.3913\frac{E}{G}\frac{i^2}{l^2}/k)$

For the rectangular cross section beam of wide b, high h, k=0.913, have

$E=\frac{23{\mathrm{Pl}}^3}{108y(l/2){\mathrm{bh}}^3}(1+0.8572\frac{E}{G}\frac{h^2}{l^2})---\left(2\right)$

Comparison expression (1) and formula (2), the Section 2 in the bracket of formula (2) the right is that shearing produces the percentage of mid-span deflection relative to moment of flexure.This percentage has relation with beam length to height ratio quadratic sum elastic modulus with modulus of shearing ratio.

Sample dimensions in GB GB/T1936.2-2009 meets l=12h, is substituted into formula (2),

$E=\frac{23{\mathrm{\&Delta;Pl}}^3}{108\mathrm{\&Delta;}y(l/2){\mathrm{bh}}^3}(1+0.00595\frac{E}{G})---\left(3\right)$

Formula (2) or formula (3) count the amendment type to formula (1) after shearing, the size of correction and E/G ratio and beam relevant across thickness rate.

For isotropic material E/G=2 (1+ μ), as μ=0.2-0.4, E/G=2.4-2.8, for the four_point bending beam of l/h=12, under l/3-l/3-l/3 load mode, in beam, shearing is on the impact of mid-span deflection within the scope of 1.4%-1.7%, therefore can ignore, but just different for timber, such as dragon spruce string surface elastic modulus E _l=11.6GPa and rift grain-string face shear modulus G _lT=0.72GPa, so E _l/ G _lT=16.1, then shearing produces the percentage 0.00595E/G ≈ 0.096 of mid-span deflection relative to moment of flexure, and namely 9.6%.Just larger for cork wood, because of cork wood E _l/ G _lT=31.5, that 0.00595E/G ≈ 0.187, namely 18.7%.

Formula comprises E and G in (3), and after measuring G, ability formula (3) calculates E, therefore formula (3) inconvenience is used for timber test E, unless across thickness rate l/h=30, at this moment, for cork wood, the percentage that its shearing produces mid-span deflection relative to moment of flexure just drops to 2.1%.

If the parameter measured is without mid-span deflection, use span centre strain instead, situation is just different.Because l/3 in the middle of beam is across being pure bending region, strain is the concept (local concept) of point unlike amount of deflection, though the left and right l/3 on four_point bending beam is across there is shearing, but span centre strain can not be affected, therefore elastic modulus can make its measuring accuracy improve to select strain parameter measured value to calculate.Calculate that the formula of elastic modulus is by the longitudinal strain of pure bending l/3 section

$\begin{array}{cc}E=\frac{\left(\mathrm{\&Delta;}P\right)l}{\left(\mathrm{\&Delta;}\mathrm{\&epsiv;}\right){\mathrm{bh}}^2}& \left(MPa\right)\end{array}---\left(4\right)$

In formula: △ P-load increment N; △ ε-strain increment; L-test specimen span mm; B-specimen width mm; H-specimen thickness mm.

The analysis of 2 impact test Poisson ratio measuring accuracy

The strain-stress relation of 2.1 timber

Timber is forward anisotropic material, is ignoring σ _ztime, strain-stress relation can show be:

${\mathrm{\&epsiv;}}_x=\frac{{\mathrm{\&sigma;}}_x}{E_x}-{\mathrm{\&mu;}}_{yx}\frac{{\mathrm{\&sigma;}}_y}{E_y}$

${\mathrm{\&epsiv;}}_y=\frac{{\mathrm{\&sigma;}}_y}{E_y}-{\mathrm{\&mu;}}_{xy}\frac{{\mathrm{\&sigma;}}_x}{E_x}$

$\&RightArrow;-\frac{{\mathrm{\&epsiv;}}_y}{{\mathrm{\&epsiv;}}_x}=\frac{{\mathrm{\&mu;}}_{xy}-E_x{\mathrm{\&sigma;}}_y/E_y{\mathrm{\&sigma;}}_x}{1-{\mathrm{\&mu;}}_{yx}{E}_x{\mathrm{\&sigma;}}_y/E_y{\mathrm{\&sigma;}}_x}---\left(5\right)$

So, work as σ _ywhen=0, just there is-ε _y/ ε _x=μ _xy, for cantilever slab (beam), it is dynamic for paying no attention to, or static, and the center line on the upper and lower surface of plate (beam) must exist σ _ythe point of=0, just the position of point is different.

To four_point bending beam, the ANSYS result of calculation of Shell63 unit is adopted to show to there is not σ on the upper and lower centre of surface line of plate (beam) _ythe point of=0.Therefore, four_point bending beam is approximate for testing timber Poisson ratio.Its approximation depends on test specimen flakiness ratio, slenderness ratio and four-point bending load mode (loading Position).

2.2 test specimen flakiness ratio impacts

For dragon spruce, consider that test specimen span 240mm, thickness 20mm are constant, change the p-ε of specimen width _y/ ε _x,-ε _z/ ε _ximpact.Specimen width gets 20mm respectively, 40mm, 60mm, 80mm and 100mm, carries out ANSYS static strain, Stress calculation, and ANSYS calculates and adopts sheel63 unit, stress and strain model 30 × 6, sends into string surface elastic constant (table 1) of dragon spruce.When ANSYS calculates, by load p/2 point on each node of x=l/3, x=2l/3.

Table 1 dragon spruce tangential section is main to elastic constant

According to the static strain ε that ANSYS calculates _x, ε _y, ε _z,-the ε of each node on the lower surface center line on pure bending section as shown in Figure 3,4 can be drawn _y/ ε _xwith-ε _z/ ε _xwith the change curve of x/l.

See from Fig. 3,4: when specimen width strengthens ,-ε _y/ ε _xwith μ _lTnormal value 0.47 ,-ε _z/ ε _xwith μ _lR0.37 difference becomes large.

Fig. 3,4 is the output strain stress sending into elastic constant calculating according to table 1 _x, ε _y, ε _zdraw, therefore, if to stop face or diametric plane sawing when making test specimen, can by string face being pasted foil gauge and same x place subsides foil gauge just can measure μ on perpendicular diametric plane _lTand μ _lR, but need to do twice test to four-point bending, this point is not so good as axial tension single test can measure μ simultaneously _lTand μ _lR.

For more clear explanation specimen width is on the impact of test timber Poisson ratio precision, get dragon spruce, l/3 – l/3 – l/3 four-point bending loads, load p=400N, four-point bending loads span l=240mm, h=20mm, with the ANSYS result of calculation of two of b=60mm and 20mm kinds of specimen widths, σ is described _ywhen ≠ 0, on the impact of timber Poisson ratio measuring accuracy.

Consider the point of span centre

As b=60mm,

σ is calculated according to ANSYS _x=3.965MPa, σ _y=0.0227MPa, σ _y/ σ _x=0.0057

Due to dragon spruce E _x=11.6GPa, E _y=0.5GPa, μ _xy=0.47, μ _yx=0.02,

Therefore-ε _y/ ε _x=0.339 (ANSYS calculated value 0.338);

As b=20mm,

σ is calculated according to ANSYS _x=11.994MPa, σ _y=0.0037532MPa, σ _y/ σ _x=0.00031357

Due to dragon spruce E _x=11.6GPa, E _y=0.5GPa, μ _xy=0.47, μ _yx=0.02,

Therefore-ε _y/ ε _x=0.463 (ANSYS calculated value 0.463);

2.3 test specimen slenderness ratio impacts

Width and the thickness of dragon spruce test specimen are constant, and get 20mm, and change piece lengths, slenderness ratio gets 12,15,16,18 and 20 respectively.-the ε that span centre calculates _y/ ε _x,-ε _z/ ε _xbe worth as shown in table 2.

Table 2 slenderness ratio impact (width 20mm is constant)

Dragon spruce normal value μ _lT=0.47, μ _lR=0.37.

In a word, for dragon spruce, when employing 240mm × 20mm × 20mm test specimen, l/3-l/3-l/3 the load mode ,-ε that span centre calculates _y/ ε _x,-ε _z/ ε _xthe difference (see table 2 second row) 1.5% is remained with normal value; Also know from table 2, work as l/h=18, this is equivalent to pure bending section slenderness ratio and equals 6 ,-ε _y/ ε _x=0.469 ,-ε _z/ ε _x=0.370, almost equal with dragon spruce normal value, this slenderness ratio showing to increase pure bending section can improve the precision of test Poisson ratio.

2.4 four-point bending loading Position impacts

Sample dimensions 300mm × 20mm × 20mm constant (loading span 240mm), under changing the condition of three kinds of four-point bendings loadings such as l/3-l/3-l/3, l/4-l/2-l/4 and l/5-3l/5-l/5, the tangential-ε of timber are calculated to six seeds such as dragon spruce, beech, Lapland pine, cork wood, mahogany and Bai Lamu _y/ ε _xwith-the ε of radial longitudinal section _z/ ε _x.Result of calculation is as shown in table 3.See from table 3 data, under l/4-l/2-l/4 four-point bending loads, the dragon spruce-ε calculated by span centre _y/ ε _x,-ε _z/ ε _xdiffer with normal value and reduce to 0.6%.

The impact of table 3. four-point bending loading Position

From table 3, under l/3-l/3-l/3 loads, except dragon spruce ,-the ε of other seeds _y/ ε _xwith-ε _z/ ε _xcalculated value differs with its normal value and is all less than 1%, in this sense, l/3-l/3-l/3 four-point bending load mode also can be adopted to test timber Poisson ratio, but measures in order to the four-point bending load mode improving measuring accuracy recommendation l/4-l/2-l/4.

In fact, loading Position on the impact of Poisson ratio measuring accuracy is and σ _y/ σ _xthe size of ratio is relevant.For dragon spruce, according to ANSYS result of calculation:

When l/3-l/3-l/3 loads σ _y/ σ _x=3.13 × 10 ^-4,

When l/4-l/2-l/4 loads σ _y/ σ _x=1.09 × 10 ^-4;

When l/5-3l/5-l/5 loads σ _y/ σ _x=6.21 × 10 ^-5.

I.e. σ _y/ σ _xratio is less ,-the ε of span centre _y/ ε _xcalculated value is more close to Poisson ratio.

3 four_point bending beam test wood modulus of elasticity and Poisson ratio patch location

3.1 four_point bending beam vertical and horizontal Strain Distribution

Dragon spruce test specimen 300mm × 20mm × 20mm (span 240mm), l/3-l/3-l/3 four-point bending loads, P=400N.The transverse strain of the pure bending section that ANSYS calculates and longitudinal strain (it is as shown in table 1 that ANSYS calculates feeding value) as shown in table 4.

The E estimated value of table 4 dragon spruce on diverse location and-ε y/ ε x calculated value

Known from table 4 data: on the pure bending section that four-point bending loads, longitudinal strain is not substantially with change in location; But transverse strain increases and increase with x/l by absolute value.Longitudinally, this variation characteristic of transverse strain causes: can calculate E with the longitudinal strain of arbitrfary point on pure bending section, and measure-the ε that Poisson ratio must use central point _y/ ε _xvalue is estimated, otherwise can cause comparatively big error.

Dragon spruce beam is when l/3-l/3-l/3 four-point bending loads, and pure bending section is the longitudinal strain ε of middle l/3 section _xwith transverse strain ε _yrelative to the ε of span centre _xand ε _yratio with x/l distribution as shown in Figure 5.ε as seen from Figure 5 _x(x/l)/ε _x(0.5)-x/l does not change substantially, and its numerical value changes to 1 from 0.999, and ε _y(x/l)/ε _y(0.5)-x/l but changes to 1 from 0.903.

3.2 test Deflection Modulus of Elasticity prediction equation and patch location

Survey elastic modulus prediction equation

L/3-l/3-l/3 loads: $E=\frac{\left(\mathrm{\&Delta;}P\right)l}{\left({\mathrm{\&Delta;\&epsiv;}}_x\right){\mathrm{bh}}^2}---\left(6a\right)$

L/4-l/2-l/4 loads: $E=\frac{3\left(\mathrm{\&Delta;}P\right)l}{4\left({\mathrm{\&Delta;\&epsiv;}}_x\right){\mathrm{bh}}^2}---\left(6b\right)$

L/5-3l/5-l/5 loads: $E=\frac{3\left(\mathrm{\&Delta;}P\right)l}{5\left({\mathrm{\&Delta;\&epsiv;}}_x\right){\mathrm{bh}}^2}---\left(6c\right)$

In formula: △ P-loads increment; △ ε _x-span centre longitudinal slice strain increment.

See from Fig. 5, the longitudinal strain sheet surveying E does not have particular/special requirement in the patch location of pure bending section, as long as at pure bending section, recommends to be affixed near in girder span.

3.3 the strain rosette paste position of test timber Poisson ratio

See from Fig. 5, ε _y/ ε _xratio changes with x/l at pure bending section, changes to 0.463 (dragon spruce μ from 0.419 _lTnormal value be 0.47), therefore measure the stickup strain rosette position of Poisson ratio μ and should be arranged in girder span.So, estimate Poisson ratio with the transverse strain of span centre and longitudinal strain increment ratio:

According to transverse strain and the longitudinal strain Changing Pattern at pure bending section, be the measuring accuracy ensureing Poisson ratio, the transverse strain sheet of strain rosette is pasted on the central point on the upper and lower surface of beam, and longitudinal strain sheet is critical pastes by transverse strain sheet, as shown in Figure 6.

Test timber Poisson ratio, four_point bending beam thereon, the central point of lower surface respectively pastes one piece of strain rosette, and its longitudinal strain sheet and transverse strain sheet press half-bridge connection respectively.

3.4 recommend four_point bending beam size and loading Position thereof

According to above analysis, the sample dimensions of test string face or diametric plane elastic modulus and Poisson ratio is recommended to be that (it is 240mm that four-point bending loads span to 280mm × 20mm × 20mm, and four-point bending load mode is l/3-l/3-l/3 or l/4-l/2-l/4.

And the sample dimensions of testing plane of structure elastic modulus and Poisson ratio is 220mm × 20mm × 20mm (it is 180mm that four-point bending loads span), four-point bending load mode is l/4-l/2-l/4.

The main test specimen to elastic constant can reduce to 180mm × 20mm × 20mm (test specimen is long is 220mm).

4 moduluss of elasticity of wood and Poisson ratio static test

Silver spruce, Chinese pine and poplar elastic modulus and Poisson ratio is measured with four_point bending beam, also by axial tension test, elastic modulus and Poisson ratio are measured for silver spruce, to contrast with four-point bending measurement result, to verify the correctness of four-point bending test modulus of elasticity of wood and the Poisson ratio method provided herein.

4.1 four-point bending

Test specimen: silver spruce radial longitudinal section test specimen nominal size 300mm × 12.2mm × 12.2mm, quantity 5; Chinese pine tangential section test specimen nominal size 300mm × 12.2mm × 12.2mm, quantity 5.The two span is all 240mm, l/3-l/3-l/3 four-point bending load mode; Silver spruce square section test specimen nominal size 220mm × 12.2mm × 12.2mm quantity 4, span 200mm, l/4-l/4-l/4 four-point bending load mode; Load: counterweight.

Radial longitudinal section and radial longitudinal section test specimen: lower limit load 4.165N, upper limit load 16.66N;

Square section test specimen lower limit load 1.019N, upper limit load 2.783N.

Each test specimen carries out three tests, gets the mean value of second and third trial value as this test specimen elastic modulus and Poisson ratio measured value.

Poisson ratio and elastic modulus are calculated as follows:

In formula: if b-mm, h-mm, △ be P-N, ε-μ ε, then the unit of E is MPa.

4.2 axial tension

Test specimen: silver spruce radial longitudinal section test specimen nominal size 300mm × 40mm × 12.2mm, quantity 3, be and the tensile test specimen of four-point bending test specimen from the large plate of same silver spruce (large board size 625mm × 107mm × 12.2mm) loading and unloading, therefore get the test specimen identical with four-point bending test specimen and number.

Glue one piece of strain rosette respectively on the surface at two of tensile test specimen, the transversal flaps on two sides, longitudinal slice are connected respectively, and the longitudinal slice of series connection meets a passage bridge box A, B, and the transversal flaps of series connection meets two-way bridge box A, B; Another test specimen of compensating plate, the longitudinal slice of series connection meets a passage bridge box B, C, and the transversal flaps of series connection meets two-way bridge box B, C.This connection is the bending strain misaligning generation in order to eliminate pulling force, ensures that monitor strain is the strain that axial tension produces.Foil gauge output meets YD-25A and moves statical strain indicator, and the output of strainmeter output transversal flaps connects a passage and two passages of vasculum respectively.

Load: testing machine loads continuously, foil gauge exports and connects dynamic strain indicator, and strainmeter exports and connects vasculum and by special software and computer recording longitudinal strain and transverse strain data.

Data processing: tensile loads is from lower limit load 2kN to upper limit load 3.5kN, record the corresponding time, the transverse strain from the civilian Ordering-the file reading lower limit load of image data to upper limit load and longitudinal strain value, get some groups of data, after verifying they linear, Poisson ratio can be determined from slope.Also Poisson ratio and elastic modulus can be calculated as follows simply:

In formula: if b-mm, h-mm, △ be P-N, ε-μ ε, then the unit of E is MPa.

4.3 silver spruces and Chinese pine elastic modulus and Poisson ratio test

The static test value of silver spruce and Chinese pine elastic modulus and Poisson ratio four-point bending and tension test is as shown in table 5.

Table 5 silver spruce and Chinese pine elastic modulus and Poisson ratio static test value (four-point bending beam span 240mm, 180mm)

See from table 5 data, four-point bending is very consistent with Poisson ratio with the elastic modulus of the silver spruce radial longitudinal section that axial tension test is tested, although experiment quantity is less, but the method that also show four-point bending testing elastic modulus and the Poisson ratio provided herein is reliable, and has sufficiently precision.

5. conclusion

5.1 according to the static stress of ANSYSshell6.3 unit, strain analysis, and four_point bending beam does not exist σ _ythe point of=0, for the anisotropic timber of forward ,-the ε on 4 bent beams loaded _y/ ε _xvalue perseverance is less than Poisson ratio, and adopts suitable sample dimensions or four-point bending load mode, with the central point-ε of upper or lower surface in girder span _y/ ε _xestimate that Poisson ratio has sufficiently precision;

5.2 survey timber tangential section, radial longitudinal section Poisson ratio recommends specimen size 280mm × 20mm × 20mm, load by l/3 – l/3 – l/3 four-point bending, realize four-point bending loading and are of a size of 240mm × 20mm × 20mm; Survey square section Poisson ratio and recommend specimen size 220mm × 20mm × 20mm, load by l/4 – l/2 – l/4 four-point bending, realize four-point bending loading and be of a size of 200mm × 20mm × 20mm;

5.3 four_point bending beam test Poisson ratios, foil gauge is attached to the central point of middle span, with transversal flaps centering adjustment point position;

5.4 four_point bending beam are practically applicable to test timber E _l, E _t, E _r, μ _lT, μ _lR, μ _rT, μ _tL, μ _rL, μ _tRmain to elastic constant Deng 9;

5.5 four_point bending beam measure timber Poisson ratio, adopt testing machine to load or counterweight loading.Testing machine loads upper recommended limit load 300N, upper limit load 700N; Counterweight loads, and is not less than 100 μ ε designs upper and lower limit load value with longitudinal slice strain difference.

In sum, due to a modulus of elasticity parellel to grain generally order of magnitude larger than corresponding modulus of shearing of timber, therefore the impact of shearing on four-point bending wooden frame mid-span deflection is not allowed to ignore, strain parameter measured value can extrapolate the elastic modulus of timber more accurately than mid-span deflection measured value.According to ANSYS static stress, strain calculation, though there is not transverse stress σ in four_point bending beam _ythe point of=0, but adopt suitable sample dimensions and four-point bending loading Position, with-the ε of centre of surface point upper and lower in girder span _y/ ε _xmeasured value estimates that timber Poisson ratio has sufficiently precision; Four_point bending beam test Poisson ratio, cross strain rosette is attached on the central point of simple bending tune upper and lower surface, and make transverse strain sheet centering adjustment point position, longitudinal strain sheet is pasted near transverse strain sheet; It is main to elastic constant that four_point bending beam is applicable to test modulus of elasticity of wood and Poisson ratio etc. 9.In addition, for four_point bending beam, particularly timber beam can load with testing machine, also can load with counterweight, for 300mm × 20mm × 20mm toothed oak wood test specimen, the four-point bending of l/3-l/3-l/3 span l=240mm loads, according to our test, namely testing machine is consistent with the Poisson ratio of two load mode tests of counterweight, in this sense, for timber, the four_point bending beam test Poisson ratio that can load with counterweight, counterweight loads simple, is easy to realize, and has its superiority.

Claims (4)

1. improve the method for four_point bending beam test modulus of elasticity of wood and Poisson ratio precision, described bent beam is the rectangular parallelepiped of wide b, thick h, load the long l of span, it is characterized in that: transverse strain sheet is attached to the centre of bent beam across center position, and longitudinal strain sheet contacts with transverse strain sheet; Slenderness ratio l/h is 16 ~ 20; Flakiness ratio b/h is 1 ~ 2.

2. the method improving four_point bending beam test modulus of elasticity of wood and Poisson ratio precision as claimed in claim 1, is characterized in that: described loading Position loads by l/3-l/3-l/3, l/4-l/2-l/4 or l/5-3l/5-l/5 four-point bending.

3. the method improving four_point bending beam test modulus of elasticity of wood and Poisson ratio precision as claimed in claim 2, it is characterized in that: when surveying timber tangential section, radial longitudinal section Poisson ratio, bent beam size 280mm × 20mm × 20mm, load by l/3 – l/3 – l/3 four-point bending, loading span l is 240mm.

4. the method improving four_point bending beam test modulus of elasticity of wood and Poisson ratio precision as claimed in claim 2, it is characterized in that: when surveying wood transverse section Poisson ratio, bent beam size 220mm × 20mm × 20mm, load by l/4 – l/2 – l/4 four-point bending, loading span l is 240mm.