CN105547861B - The method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision - Google Patents
The method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision Download PDFInfo
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- 238000012360 testing method Methods 0.000 title claims abstract description 92
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- 241000351396 Picea asperata Species 0.000 description 22
- 238000010008 shearing Methods 0.000 description 14
- 230000003068 static effect Effects 0.000 description 12
- 235000004507 Abies alba Nutrition 0.000 description 10
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Abstract
The present invention provides a kind of method that can improve four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision, in this method, the bent beam is the cuboid of width b, thickness h, load the long l of span, transverse strain piece is attached to the centre of bent beam across center position, and longitudinal strain piece is contacted with transverse strain piece；Slenderness ratio l/h is 16~20；Width-thickness ratio b/h is 1~2.
Description
Technical field
The present invention relates to the methods for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision.
Background technology
The assay method of GB/T 1936.2-2009 wood modulus of elasticity, by measuring four_point bending beam mid-span deflection
Extrapolate the Deflection Modulus of Elasticity of timber.The reckoning elasticity modulus formula provided in standard is not to be included in shearing to mid-span deflection
It influences.
Since the modulus of elasticity parellel to grain of timber is generally more order of magnitude greater than corresponding modulus of shearing, therefore shear to 4 points
The influence of bending-wood beam mid-span deflection is non-negligible.
Measure that timber Poisson's ratio is common to be compressed axially or tension test, the size of compression specimens is 20mm × 20mm × 30mm
Or 30mm × 30mm × 60mm.Be compressed axially need to by testing machine load and load require centering, surface of test piece smooth and with examination
Frictional force will be reduced by testing compressing head contact, therefore axial compression test condition is harsher, realizes difficulty, so that test data point
It is bigger than normal to dissipate property.
When four_point bending beam is used for testing elastic modulus and Poisson's ratio, though it can be approximate for the upper and lower surface each point of bent beam
Be considered as uniaxial stressed state, but transverse strain changes with longitudinal strain ratio with the position of point, according to cantilever beam or
Test specimen of the four_point bending beam as static measurement Poisson's ratio, then exist cross strain rosette should be attached to beam surface where on,
The problem of material Poisson's ratio could be worth to the transverse strain of measurement and longitudinal strain ratioIn fact, four_point bending beam is upper and lower
Surface, which is removed, longitudinal stress σx, there is also lateral stress σy, only σy<<σx, therefore referred to as the upper and lower surface of beam is in approximatively single
To stress state, this approximation can ignore isotropism when testing Poisson's ratio, but for orthotropic wood
Material, such as dragon spruce, even if σyOnly σx6 ‰, since modulus of elasticity parellel to grain is more order of magnitude greater than tangential elastic module, therefore
Flagrant relative error is will also result in when testing Poisson's ratio.
Invention content
The object of the present invention is to provide one kind capable of improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision
Method.
The method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision of the present invention, the bent beam are
The cuboid of wide b, thickness h, load the long l of span, and transverse strain piece is attached to the centre of bent beam across center position, longitudinal strain piece
It is contacted with transverse strain piece；Slenderness ratio l/h is 16~20；Width-thickness ratio b/h is 1~2.
The above-mentioned method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision, the loading Position
It is loaded by l/3-l/3-l/3, l/4-l/2-l/4 or l/5-3l/5-l/5 four-point bending.
The above-mentioned method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision, survey timber tangential section,
When radial longitudinal section Poisson's ratio, bent beam size is 280mm × 20mm × 20mm, is loaded by l/3-l/3-l/3 four-point bendings, load
Span l is 240mm.
The above-mentioned method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision, surveys wood transverse section pool
Pine than when, bent beam size 220mm × 20mm × 20mm, by l/4-l/2-l/4 four-point bendings load, load span l be
240mm。
Beneficial effects of the present invention：It has been found that longitudinal strain is basic on the pure bending section of four-point bending load
On not with change in location；But transverse strain is by being to increase with x/l and increased (x-axis is the side with bent beam for absolute value
Supporting point be origin, along bent beam length direction extend).This variation characteristic longitudinally, laterally strained causes:It can be with
E is calculated with the longitudinal strain of arbitrary point on pure bending section, and-the ε of central point must be used by measuring Poisson's ratioy/εxValue estimation, it is no
It will cause large errors.According to transverse strain and longitudinal strain pure bending section changing rule, to ensure Poisson's ratio
The transverse strain piece of measuring accuracy, strain rosette is pasted on beam central point upper and lower surfaces of, and longitudinal strain piece is critical is answered by transverse direction
Become piece to paste.In addition, illustrating that sample dimensions and four-point bending loading Position are also influence timber Poisson's ratio measuring accuracy two
A key factor tests modulus of elasticity of wood and Poisson in the case of slenderness ratio l/h is 16~20, width-thickness ratio b/h is 1~2
Than it is more accurate especially to test Poisson's ratio.
Description of the drawings
Fig. 1 is four_point bending beam load schematic diagram；
Fig. 2 is mid-span deflection when P/2 effects l/3 is included in shearing；
Fig. 3 is dragon spruce test specimen in pure bending section-εy/εx- x/l change schematic diagrams；
Fig. 4 is dragon spruce test specimen in pure bending section-εz/εx- x/l change schematic diagrams；
Fig. 5 is dragon spruce four_point bending beam in pure bending areal strain distribution map；
Fig. 6 is strain rosette paste position schematic diagram.
Specific implementation mode
It elaborates below to the present invention.
1 four_point bending beam is for measuring wood modulus of elasticity
In GB/T 1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood, using l/3, l/3, l/3 four-point bendings add
The beam of load is test specimen, calculates wood modulus of elasticity by measuring beam mid-span deflection, as shown in Figure 1.
L/3 among beam is across in pure bending, i.e., beam is on all sections of this section, and shearing zero, moment of flexure is constant, i.e.,
Moment of flexure does not change with sectional position；And left and right l/3, across in shear bending, i.e., not only there is moment of flexure on section, there is also cut
Power.What is provided in GB/T 1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood calculates timber bullet bending resistance by beam mid-span deflection
Property tangent elastic modulus in only take into account moment of flexure, be not included in left and right l/3 span centres and shear influence to beam midway deflection, analytical table
It is bright for timber, since timber modulus of elasticity parellel to grain is more order of magnitude greater than its modulus of shearing, thus shear ignore to wood
The influence of material Deflection Modulus of Elasticity test value is bigger than isotropic material more, and wood modulus of elasticity test value is caused to produce
Raw sizable error.
GB/T 1936.2-2009 Method for determination of the modulus of elasticity in static bending of wood provides:Four-point bending l/3-l/3-l/3 loads
(Fig. 1), sample dimensions 300mm × 20mm × 20mm (load span 240mm) are real i.e. on testing machine by 80mm-80mm-80mm
Existing four-point bending l/3-l/3-l/3 loads, provide lower limit load pUnder=300N, upper limit load pOn=700N, therefore △ P=400N.
Elasticity modulus is calculated by mid-span deflection test value:
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) spends mm.E derived units MPa.
Formula (1) is not included in influence of the shearing to its mid-span deflection in beam, but what is measured is that moment of flexure and shearing are common
Amount of deflection under effect, so, the Deflection Modulus of Elasticity that applying equation (1) calculates are less than normal.
It is included in influence of the shearing in four_point bending beam to its mid-span deflection below, formula (1) is modified with this.
At beam l/3 when used load P/2 (Fig. 2), being included in the amount of deflection y (l/2) that shearing generates at span centre l/2 can indicate
For:
In formula:I-beam section the moment of inertia；I-beam section the radius of gyration；K-Section factor, to rectangular section k=
0.913。
According to principle of stacking, symmetry is considered, (stand under load is as shown in Figure 1, l/3-l/3- when acting on P/2 at beam l/3,2l/3
L/3 four-point bendings load), the amount of deflection generated at span centre l/2 is
For the rectangular cross section beam of wide b, high h,K=0.913,Have
Comparison expression (1) and formula (2), the Section 2 on the right of formula (2) in bracket are that shearing generates mid-span deflection relative to moment of flexure
Percentage.The percentage has relationship with beam length to height ratio quadratic sum elasticity modulus with modulus of shearing ratio.
Sample dimensions in national standard GB/T 1936.2-2009 meet l=12h, are substituted into formula (2), obtained
Formula (2) or formula (3) are after being included in shearing to the amendment type of formula (1), modified size and E/G ratios and beam across thickness
Than related.
For isotropic material E/G=2 (1+ μ), as μ=0.2-0.4, E/G=2.4-2.8, for l/h=12's
Four_point bending beam, under l/3-l/3-l/3 load modes, influence of the beam internal shear force to mid-span deflection is in 1.4%-1.7% ranges
It is interior, therefore can ignore, but it is just different for timber, such as dragon spruce chord plane elastic modulus EL=11.6GPa and rift grain-chord plane
Shear modulus GLT=0.72GPa, then EL/GLT=16.1, then shear the percentage that mid-span deflection is generated relative to moment of flexure
0.00595E/G ≈ 0.096, i.e., 9.6%.For cork wood with regard to bigger, because of cork wood EL/GLT=31.5, that 0.00595E/G ≈
0.187, i.e., 18.7%.
Include E and G in formula (3), ability formula (3) calculates E after measuring G, therefore formula (3) is inconvenient for timber test E, removes
At this moment non-across thickness rate l/h=30 for cork wood, is sheared and is just dropped to relative to the percentage of moment of flexure generation mid-span deflection
2.1%.
If the parameter measured does not have to mid-span deflection, span centre strain is used instead, situation is with regard to different.Due among beam l/3 across
Be pure bending region, strain unlike amount of deflection is the concept (local concept) of point, though left and right l/3 on four_point bending beam across in the presence of
Shearing, but not span centre strain is influenced, therefore strain parameter measured value is selected to calculate that elasticity modulus can be such that its measurement accuracy is changed
It is kind.Calculate that the formula of elasticity modulus is by the longitudinal strain of pure bending l/3 sections
In formula:△ P- load increments N；△ ε-strain increment；L-test specimen span mm；B-specimen width mm；H-test specimen is thick
Spend mm.
2 influence the analysis of test Poisson's ratio measurement accuracy
The strain-stress relation of 2.1 timber
Timber is positive anisotropic material, is ignoring σzWhen, strain-stress relation can table be:
Then, work as σyWhen=0, just there is-εy/εx=μxy, for cantilever slab (beam), it is dynamic to pay no attention to, or static, in plate
There must be σ on the center line upper and lower surfaces of of (beam)y=0 point, the position only put are different.
To four_point bending beam, shown in plate (beam) upper and lower centre of surface using the ANSYS result of calculations of Shell63 units
σ is not present on liney=0 point.Therefore, four_point bending beam is approximate for testing timber Poisson's ratio.Its approximation depends on
Test specimen width-thickness ratio, slenderness ratio and four-point bending load mode (loading Position).
2.2 test specimen width-thickness ratios influence
By taking dragon spruce as an example, considers that test specimen span 240mm, thickness 20mm are constant, change the p- ε of specimen widthy/εx、-εz/εx
Influence.Specimen width takes 20mm, 40mm, 60mm, 80mm and 100mm respectively, carries out ANSYS static strains, Stress calculation,
ANSYS, which is calculated, uses sheel63 units, mesh generation 30 × 6 to be sent into the chord plane elastic constant (table 1) of dragon spruce.ANSYS is calculated
When, by load p/2 point on x=l/3, each node of x=2l/3.
1 dragon spruce tangential section of table is main to elastic constant
The static strain ε calculated according to ANSYSx,εy,εz, the following table on pure bending section as shown in Figure 3,4 can be drawn
- the ε of each node on the center line of facey/εxWith-εz/εxWith the change curve of x/l.
See from Fig. 3,4:When specimen width increases ,-εy/εxWith μLTNormal value 0.47 ,-εz/εxWith μLR0.37 difference becomes
Greatly.
Fig. 3,4 be to be sent into the output strain stress that elastic constant calculates according to table 1x,εy,εzAnd draw, therefore, make test specimen
Shi Ruo presses chord plane or diametric plane sawing, can be answered by pasting foil gauge in chord plane and being pasted at same x on perpendicular diametric plane
μ can be measured by becoming pieceLTAnd μLR, but four-point bending needs are done and are tested twice, this point can not as good as the primary experiment of axial tension
Measure μ simultaneouslyLTAnd μLR。
For the apparent influence for illustrating specimen width to test timber Poisson's ratio precision, dragon spruce, 4 points of l/3-l/3-l/3 are taken
Bending load, load p=400N, four-point bending load span l=240mm, h=20mm, with two kinds of examinations of b=60mm and 20mm
The ANSYS result of calculations of part width illustrate σyWhen ≠ 0, the influence to timber Poisson's ratio measuring accuracy.
Consider the point of span centre
As b=60mm,
σ is calculated according to ANSYSx=3.965MPa, σy=0.0227MPa, σy/σx=0.0057
Due to dragon spruce Ex=11.6GPa, Ey=0.5GPa, μxy=0.47, μyx=0.02,
Therefore-εy/εx=0.339 (ANSYS calculated values 0.338)；
As b=20mm,
σ is calculated according to ANSYSx=11.994MPa, σy=0.0037532MPa, σy/σx=0.00031357
Due to dragon spruce Ex=11.6GPa, Ey=0.5GPa, μxy=0.47, μyx=0.02,
Therefore-εy/εx=0.463 (ANSYS calculated values 0.463)；
2.3 test specimen slenderness ratios influence
The width and thickness of dragon spruce test specimen is constant, and takes 20mm, changes piece lengths, slenderness ratio takes 12 respectively, 15,16,
18 and 20.Calculated-the ε of span centrey/εx,-εz/εxValue is as shown in table 2.
2 slenderness ratio of table influences (width 20mm is constant)
Dragon spruce normal value μLT=0.47, μLR=0.37.
In a word, for dragon spruce, when using 240mm × 20mm × 20mm test specimens, l/3-l/3-l/3 load modes, span centre meter
- the ε calculatedy/εx、-εz/εx1.5% difference (being shown in Table 2 second rows) is still had with normal value；Also know from table 2, work as l/h=18,
This is equivalent to pure bending section slenderness ratio and is equal to 6 ,-εy/εx=0.469 ,-εz/εx=0.370, almost with dragon spruce normal value phase
Deng, this show increase pure bending section slenderness ratio can improve test Poisson's ratio precision.
2.4 four-point bending loading Position influences
Sample dimensions 300mm × 20mm × 20mm constant (load span 240mm), changes l/3-l/3-l/3, l/4-l/2-
Under conditions of three kinds of four-point bending loads such as l/4 and l/5-3l/5-l/5, to dragon spruce, beech, Lapland pine, cork wood, peach blossom
Six seeds such as heart wood and Bai Lamu calculate the tangential-ε of timbery/εxWith-the ε of radial longitudinal sectionz/εx.Result of calculation such as 3 institute of table
Show.See from 3 data of table, under the load of l/4-l/2-l/4 four-point bendings, passes through the dragon spruce-ε of span centre calculatingy/εx、-εz/εxWith
Normal value difference is reduced to 0.6%.
The influence of 3. four-point bending loading Position of table
From table 3, under l/3-l/3-l/3 loads, in addition to dragon spruce ,-the ε of other seedsy/εxWith-εz/εxCalculated value and its
Normal value difference is respectively less than 1%, in this sense, l/3-l/3-l/3 four-point bending load modes test wood can also be used
Material Poisson's ratio, but measured with the four-point bending load mode of l/4-l/2-l/4 to improve measuring accuracy recommendation.
In fact, loading Position is and σ on the influence of Poisson's ratio measuring accuracyy/σxThe size of ratio is related.For dragon spruce, root
According to ANSYS result of calculations:
When l/3-l/3-l/3 loads σy/σx=3.13 × 10-4,
When l/4-l/2-l/4 loads σy/σx=1.09 × 10-4；
When l/5-3l/5-l/5 loads σy/σx=6.21 × 10-5。
That is σy/σxRatio is smaller ,-the ε of span centrey/εxCalculated value is closer in Poisson ratio.
3 four_point bending beams test wood modulus of elasticity and Poisson's ratio patch location
3.1 four_point bending beam vertical and horizontal Strain Distributions
Dragon spruce test specimen 300mm × 20mm × 20mm (span 240mm), the load of l/3-l/3-l/3 four-point bendings, P=400N.
The transverse strain for the pure bending section that ANSYS is calculated and longitudinal strain are as shown in table 4 (it is as shown in table 1 that ANSYS calculates feeding value).
E estimated value of 4 dragon spruce of table on different location and-ε y/ ε x calculated values
It was found from 4 data of table:Longitudinal strain is not substantially with change in location on the pure bending section of four-point bending load；
But transverse strain by be for absolute value with x/l increase and it is increased.This variation characteristic longitudinally, laterally strained causes:It can
To calculate E with the longitudinal strain of arbitrary point on pure bending section, and-the ε of central point must be used by measuring Poisson's ratioy/εxValue estimation,
It is no that it will cause large errors.
For dragon spruce beam when l/3-l/3-l/3 four-point bendings load, pure bending section is the longitudinal strain ε of intermediate l/3 sectionsx
With transverse strain εyε relative to span centrexAnd εyRatio it is as shown in Figure 5 with the distribution of x/l.ε as seen from Figure 5x(x/l)/εx
(0.5) being basically unchanged of-x/l, numerical value changes to 1 from 0.999, and εy(x/l)/εy(0.5)-x/l is but changed to from 0.903
1。
3.2 test Deflection Modulus of Elasticity prediction equations and patch location
Survey elasticity modulus prediction equation
In formula:△ P- load increment；△εxSpan centre longitudinal slice strain increment.
See from Fig. 5, surveying the longitudinal strain piece of E, there is no particular/special requirements in the patch location of pure bending section, as long as
Pure bending section, recommendation be affixed in girder span near.
The strain rosette paste position of 3.3 test timber Poisson's ratios
See from Fig. 5, εy/εxRatio changes in pure bending section with x/l, and 0.463 (dragon spruce μ is changed to from 0.419LTRule
Typical value is that 0.47), therefore the stickup strain rosette position for measuring Poisson's ratio μ should be located in girder span.Then, with the transverse strain of span centre with
Longitudinal strain increment ratio estimates Poisson's ratio:
According to transverse strain and longitudinal strain pure bending section changing rule, to ensure the measuring accuracy of Poisson's ratio,
The transverse strain piece of strain rosette is pasted on beam central point upper and lower surfaces of, and longitudinal strain piece is critical is pasted by transverse strain piece,
As shown in Figure 6.
Timber Poisson's ratio is tested, four_point bending beam is on it, the central point of lower surface is each pastes one piece of strain rosette, and longitudinal direction is answered
Become piece and transverse strain piece and presses half-bridge connection respectively.
3.4 recommend four_point bending beam size and its loading Position
According to the above analysis, recommend test chord plane or the sample dimensions of diametric plane elasticity modulus and Poisson's ratio be 280mm ×
(it is 240mm that four-point bending loads span to 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 for testing plane of structure elasticity modulus and Poisson's ratio are that (four-point bending loads 220mm × 20mm × 20mm
Span is 180mm), four-point bending load mode is l/4-l/2-l/4.
It is main to may decrease to 180mm × 20mm × 20mm (a length of 220mm of test specimen) to the test specimen of elastic constant.
4 moduluss of elasticity of wood and Poisson's ratio static test
Silver spruce, Chinese pine and poplar elasticity modulus and Poisson's ratio are measured with four_point bending beam, silver spruce is also used
Axial tension test measures elasticity modulus and Poisson's ratio, to be compared with four-point bending measurement result, is given herein with verification
The correctness of the four-point bending test modulus of elasticity of wood and Poisson ratio method that go out.
4.1 four-point bending
Test specimen:Silver spruce radial longitudinal section test specimen nominal dimension 300mm × 12.2mm × 12.2mm, quantity 5；Chinese pine tangential section
Test specimen nominal dimension 300mm × 12.2mm × 12.2mm, quantity 5.The two span is all 240mm, l/3-l/3-l/3 four-point bendings
Load mode；Silver spruce cross section test specimen nominal dimension 220mm × 12.2mm × 12.2mm quantity 4, span 200mm, l/4-
L/4-l/4 four-point bending load modes；Load:Counterweight.
Radial longitudinal section and radial longitudinal section test specimen:Lower limit load 4.165N, upper limit load 16.66N；
Cross section test specimen lower limit load 1.019N, upper limit load 2.783N.
Each test specimen is tested three times, takes the average value of second and third secondary test value as the test specimen elasticity modulus and pool
Pine compares measured value.
Poisson's ratio and elasticity modulus are calculated as follows:
In formula:If b-mm, h-mm, △ 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 dimension 300mm × 40mm × 12.2mm, quantity 3 are and four-point bending
Test specimen takes and four from the tensile test specimens of the big plate of same silver spruce (big board size 625mm × 107mm × 12.2mm) loading and unloading
The identical test specimen number of point bending specimen.
Glue one piece of strain rosette respectively on two surfaces of tensile test specimen, the transversal flaps, longitudinal slice on two sides are connected respectively, are gone here and there
The longitudinal slice of connection meets channel bridge box A, a B, and concatenated transversal flaps meet two-way bridge box A, B；Another test specimen of compensating plate, it is concatenated
Longitudinal slice meets channel bridge box B, a C, and concatenated transversal flaps meet two-way bridge box B, C.This connection is not right in order to eliminate pulling force
The bending strain of middle generation ensures that measuring strain is the strain for being axially stretched and generating.Foil gauge output connects YD-25A sound states
The output of deformeter, deformeter output transversal flaps connects the channel and two channels of vasculum respectively.
Load:Testing machine continuously loads, and foil gauge output connects dynamic strain indicator, and deformeter output connects vasculum and by special
With software and computer record longitudinal strain and transverse strain data.
Data processing:Tensile loads record the corresponding time from lower limit load 2kN to upper limit load 3.5kN, from acquisition number
According to literary Ordering-the files read lower limit load to the transverse strain and longitudinal strain value of upper limit load, take several groups data, verify it
It is linear after, can determine Poisson's ratio from slope.Also Poisson's ratio and elasticity modulus can simply be calculated as follows:
In formula:If b-mm, h-mm, △ P-N, ε-μ ε, then the unit of E is MPa.
4.3 silver spruces and Chinese pine elasticity modulus and Poisson's ratio test
Silver spruce and Chinese pine elasticity modulus and Poisson's ratio four-point bending and the static test value of tension test such as 5 institute of table
Show.
5 silver spruce of table and Chinese pine elasticity modulus and Poisson's ratio static test value (four-point bending beam span 240mm,
180mm)
See from 5 data of table, four-point bending and axial tension test test silver spruce radial longitudinal section elasticity modulus and
Poisson's ratio is extremely consistent, although experiment quantity is less, also shows four-point bending testing elastic modulus given herein and pool
The method of loose ratio is reliable, and with sufficiently precision.
5. conclusion
σ is not present in 5.1 static stress according to ANSYS shell6.3 units, strain analysis, four_point bending beamy=0 point,
For positive anisotropic timber ,-the ε on the bent beams of 4 points of loadsy/εxValue is permanent to be less than Poisson ratio, and uses appropriate
Sample dimensions or four-point bending load mode, with the central point-ε of upper or lower surface in girder spany/εxIt is enough to estimate that Poisson's ratio has
Ground precision；
5.2 survey timber tangential section, radial longitudinal section Poisson's ratio recommends specimen size 280mm × 20mm × 20mm, by l/3-l/3-
L/3 four-point bendings load, and realize that four-point bending load size is 240mm × 20mm × 20mm；It surveys cross section Poisson's ratio and recommends examination
Sample ruler cun 220mm × 20mm × 20mm, is loaded by l/4-l/2-l/4 four-point bendings, realizes that four-point bending load size is 200mm
×20mm×20mm；
5.3 four_point bending beams test Poisson's ratio, and foil gauge is attached to the central point of middle span, with transversal flaps centring point
It sets；
5.4 four_point bending beams are practically applicable to test timber EL,ET,ER,μLT,μLR,μRT,μTL,μRL,μTRIt is main to elastic normal Deng 9
Number；
5.5 four_point bending beams measure timber Poisson's ratio, are loaded using testing machine load or counterweight.Under testing machine load is recommended
Limit for tonnage lotus 300N, upper limit load 700N；Counterweight loads, and not be less than 100 μ ε with longitudinal slice strain difference designs upper and lower limit load
Value.
In conclusion the modulus of elasticity parellel to grain due to timber is generally more order of magnitude greater than corresponding modulus of shearing, therefore
The influence sheared to four-point bending wooden frame mid-span deflection is non-negligible, and strain parameter measured value is than mid-span deflection measured value energy
More accurately extrapolate the elasticity modulus of timber.According to ANSYS static stress, strain calculation, though there is no cross in four_point bending beam
To stress σy=0 point, but sample dimensions appropriate and four-point bending loading Position are used, in upper and lower surface in girder span
- the ε of heart pointy/εxMeasured value estimates that timber Poisson's ratio has sufficiently precision；Four_point bending beam tests Poisson's ratio, cross strain rosette
It is attached on the central point of simple bending tune upper and lower surface, makes transverse strain piece centring point position, longitudinal strain piece is against laterally
Strain gauge adhesion；It is main to elastic constant that four_point bending beam is suitable for test modulus of elasticity of wood and Poisson's ratio etc. 9.In addition, right
Can be loaded, can also be loaded with counterweight with testing machine in four_point bending beam, especially timber beam, for 300mm × 20mm ×
20mm toothed oak wood test specimens, the four-point bending load of l/3-l/3-l/3 spans l=240mm, according to our experiment, i.e., testing machine and
The Poisson's ratio of the two load modes test of counterweight is consistent, and in this sense, for timber, can be loaded with counterweight
Four_point bending beam tests Poisson's ratio, and counterweight load is simple, it is easy to accomplish, there is its superiority.
Claims (4)
1. the method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision, the bent beam is width b, thickness h
Cuboid loads the long l of span, it is characterized in that：Transverse strain piece is attached to bent beam centre upper and lower surfaces of across center position,
Longitudinal strain piece is contacted with transverse strain piece；Slenderness ratio l/h is 16~20；Width-thickness ratio b/h is 1~2.
2. the method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision as described in claim 1, special
Sign is：The loading Position is loaded 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 for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision as claimed in claim 2, special
Sign is：When surveying timber tangential section, radial longitudinal section Poisson's ratio, bent beam size 300mm × 12.2mm × 12.2mm, by l/3-l/3
- l/3 four-point bendings load, and load span l is 240mm.
4. the method for improving four_point bending beam test modulus of elasticity of wood and Poisson's ratio precision as claimed in claim 2, special
Sign is：When surveying wood transverse section Poisson's ratio, bent beam size 220mm × 12.2mm × 12.2mm, by l/4-l/2-l/4
Four-point bending loads, and load span l is 200mm.
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