CN104568605A - Method for testing fracture toughness of rattan canes based on three-point bending mode - Google Patents

Method for testing fracture toughness of rattan canes based on three-point bending mode Download PDF

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CN104568605A
CN104568605A CN201410855005.2A CN201410855005A CN104568605A CN 104568605 A CN104568605 A CN 104568605A CN 201410855005 A CN201410855005 A CN 201410855005A CN 104568605 A CN104568605 A CN 104568605A
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sample
load
rattan
curve
crack
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刘杏娥
杨淑敏
田根林
尚莉莉
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International Center for Bamboo and Rattan
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Abstract

The invention provides a method for testing the fracture toughness of rattan canes based on a three-point bending mode. The method comprises the following steps: preparing a sample, and regulating a water content rate so as to obtain a sample of which the water content rate is 10-12%; recording a load-crack opening displacement (PV) curve, measuring actual crack depths, and calculating critical loads PQ and critical stress intensity factors KQ. According to the method provided by the invention, the crack opening displacement is measured by a C.O.D Gauge, and the test precision is improved. Aiming at the characteristics of the rattan materials, the sampling method for testing the fracture toughness of rattan canes is ensured, and the measured results having high accuracy and good reproducibility are obtained. The calculation method for testing the fracture toughness of rattan cane, provided by the invention, provides qualitative and quantitative bases for the utilization of the rattan canes.

Description

Based on the method for three-point bending pattern test rattan material fracture toughness
Technical field
The invention belongs to fields of measurement, be specifically related to a kind of method that mechanical stress surveys rattan material strength characteristics.
Background technology
Fracturing mechanics simulates a kind of rational method having stress and concentrate timber compoment to destroy.The principle of fracturing mechanics is applied to the progress that wood materials is existing deep abroad, when the ultimate principle of fracturing mechanics is widely used in knot in timber structure and cementing port size Control, strength of wood are estimated, lumber grading, the drying of wood cause cracking, wood cutting by foreign scholar in the research such as crack propagation law, and there are many respective experimental techniques to determine the fracture toughness K of timber iC.According to the force way of component, its Crack Extension is divided into opening mode, sliding mode and tearing mode, and opening mode (I type) Crack Extension mode is the most common in loading, is also the most dangerous destruction form causing fracture, therefore, in wooden bamboo fracture mechanics to I mode-Ⅲ crack research maximum.Therefore patent provides emphatically is the fracture toughness K of windmill palm vine material I mode-Ⅲ crack iC.
There is the testing standard measuring material at present both at home and abroad.Such as: the ASTM-E399 of ASTM (American society for testing materials), ASTM-E1826, the BS5447-1977 of material standard association of Britain, CNS GB4161-2007, the ISO12737-2005 of International Standardization Association (ISA), their measurement ultimate principle is all consistent.But nature material category is thousands of, although some material is widely used, because of himself complex structure, performance test can not be carried out comprehensively, the development of materials application of the parameter influence of processor's the unknown.
Windmill palm vine material is important non-timber forest product, belongs to wood materials together, its profile and textural with bamboo wood relatively, difference to be bamboo wood be hollow have every, and windmill palm vine material is solid, identical with timber, but much less than timber in stem level.Windmill palm vine material is a kind of natural material, has obvious orthotropy, and xsect shows again the feature of functionally gradient material (FGM).The intensity of windmill palm vine material is not only by the impact of its structural attitude, chemical composition, and defect such as knot, crackle that rattan material is formed in growth course and in process also have significant impact to it.The existence of crackle can have influence on quality and the safe design of material, and also never someone relates to the fracture toughness of windmill palm vine material, therefore, inquires into and measure the K of windmill palm vine material iCvalue has important practice significance.
Summary of the invention
For the deficiency that prior art exists, the object of the invention is to propose a kind of method based on three-point bending pattern test rattan material fracture toughness.
The technical scheme realizing the object of the invention is:
Based on a method for three-point bending pattern test rattan material fracture toughness, comprise step:
1) sample preparation: rattan material to be measured is processed as rectangular block shape sample, and specimen width W is 1 ~ 2 times of the minimum diameter value of rattan material to be measured, and length L is 4 ~ 5W, and thickness is 0.4 ~ 0.7W; Crackle is processed in the middle of rectangular specimen;
2) water percentage is adjusted: temperature 20 DEG C put into by sample, humidity is climatic chamber adjustment under 65% condition, obtains the sample that water percentage is 10-12%;
3) load-crack opening mouth displacement (PV) curve is recorded: adopt three-point bending pattern to load, fulcrum span S is 4W.Sample is vertically put in the middle of backing roll, COD displacement meter is placed in crack opening mouth place.At the uniform velocity load with the speed of 0.5mm/min, record load-crack opening mouth displacement (PV) curve;
4) actual crack depth survey: sample, after destroying by curved loading, is gone bail for and stayed the sample of fracture section, measure the length a of crackle under the microscope n, because cut sharp crack processed with thin blade, section can clearly see straight line impression, due to bottom surface can not be parallel to completely when cutting crackle processed, thus measure the degree of depth a of sample crackle at sample different-thickness place 1, a 2... .a n, finally get its mean value (a 1+ a 2+ ... .a n)/n is the effective length of crackle; N is the positive integer between 2 ~ 10;
5) critical load P is calculated qwith critical stress intensity factors K q: according to step 3) load-crack opening mouth displacement (PV) curve of recording tries to achieve load p qvalue and K q;
6) validity of test result is assessed.
Wherein, described step 2) adjustment water percentage be sample is put into temperature 20 DEG C, humidity is climatic chamber under 65% condition, through 26-35 days, obtains the sample that water percentage is 10-12%.Normally through one month.
Further, the width of described sample is 10 ~ 50mm.The diameter of rattan material common are 10 ~ 50mm, can processing sample width 10mm; Crack depth a=0.4 ~ 0.6W.
Or, described crack depth a=0.45 ~ 0.55W.
Wherein, described step 3) middle load transducer and COD Gauge carry out automatic record to the bending load in process of the test and displacement.
Preferably, described step 4) in, the length a of crackle is measured at 1/4,1/2,3/4 thickness place of sample n, n=3.
Wherein, described step 5) in the computing formula of critical stress intensity factors be:
K Q = P Q S BW 3 / 2 · f ( a w ) - - - ( 1 )
f ( a w ) = 3 a w · 1.99 - ( a w ) ( 1 - a w ) [ 2.15 - 3.93 a w + 2.7 ( a w ) 2 ] 2 ( 1 + 2 a w ) ( 1 - a w ) 3 / 2 - - - ( 2 )
In formula, S is fulcrum span,
P qthe determination of value: for the PV curve of test gained, first do the matched curve of stretch section (OA), this matched curve and X-axis and crack opening displacement axle meet at O point, cross O point and be secant OP5, and make the slope of OP5 be 95% of initial matched curve slope, obtain the load p 5 corresponding to intersection point of secant and this curve, measure the maximum load Pmax that test specimen can bear simultaneously, calculate Pmax/P5, if both ratios are less than or equal to 1.1, then P5 is P qvalue, P qsubstitution formula (1) tries to achieve K qvalue.
Wherein, described step 6) method of assessment test result validity is: calculate 2.5 (K q/ σ s) 2value, and compare with sample thickness, if 2.5 (K q/ σ s) 2be less than or equal to sample thickness, then critical stress intensity factors is valid data; If 2.5 (K q/ σ s) 2be greater than sample thickness, then critical stress intensity factors data invalid; Wherein, σ sfor the yield strength of material.
The yield strength σ of material sthe method of this area routine can be adopted to measure, such as " windmill palm vine material tensile strength method of testing " [Lv Wenhua, Liu Xinge, Wang Youhong. wood processing machinery, 2010, (1): 20-23].
Beneficial effect of the present invention is:
(1) use C.O.D Gauge to measure the displacement of crack opening mouth, improve measuring accuracy.
(2) for the characteristic of rattan material, determine the fracture toughness test sampling method of windmill palm vine material, obtain that accuracy is high, the measurement result of favorable reproducibility.
(3) propose the fracture toughness measuring and calculation method of windmill palm vine material, provide quantitative and qualitative analysis foundation for rattan material utilizes.
Accompanying drawing explanation
Fig. 1 is three point bending specimen dimensional drawing.
Fig. 2 is rattan core SEB sample and end face (A face), and Fig. 2 (a) is full face, and Fig. 2 (b) is end face photo.
Fig. 3 is actual crack linear measure longimetry schematic diagram (observation place is the I cross section of Fig. 1).
Fig. 4 is the outdoor scene photo that crack opening displacement is measured.
Fig. 5 is the PV curve of hook leaf rattan material rattan core.
Embodiment
Following examples for illustration of the present invention, but are not used in restriction scope of the present invention.
Experimental apparatus: omnipotent mechanics machine, COD Gauge (U.S. instron Instron, 2670-116).
Embodiment 1:
1. experiment material and specimen size
Hook leaf rattan material, diameter 30-40mm, is collected in Dehong prefecture Yingjiang County, Yunnan Province.The size of SEB test specimen according to " ASTM E399 metal material is linear-elastic linear strain fracture toughness K ictest method " in requirement obtained (as shown in Figure 1, S=4W, B=W/2, a/W=-0.45 ~ 0.55).Specimen thickness is 10mm, and width is 20mm, and crack depth is 10mm, and test specimen number is no less than 20.Temperature 20 DEG C put into by the test specimen processed, humidity is climatic chamber adjustment under 65% condition, and sample water percentage is 10-12%.
2. crackle is prefabricated
Vascular bundle skewness in rattan diametric(al), vascular bundle distribution in rattan peel place is close, and vascular bundle distribution in rattan core place is dredged, and meets personally Fig. 2 (a) and Fig. 2 (b) from the crack tip that rattan core is prefabricated.In the centre position of piece lengths, be 0.45 ~ 0.55 precrack by a/w.First on sample, crack position and the degree of depth is marked with pencil before precrack, then on miniature band saw (saw blade thickness is about 0.7mm), saw out the saw kerf of 3/4ths of total crack length, then cut out remaining crack length with thin blade (thickness about 0.3mm) along saw kerf, make the sharp crack that total length is a.Measure B and W of each sample before test respectively at crackle section part, average at least in triplicate, precision is at 0.01mm.
3. actual crack depth survey
Sample, after destroying by curved loading, is gone bail for and is stayed the sample of fracture section, measure the length a of crackle under the microscope.Because cut sharp crack processed with thin blade, section can clearly see straight line impression, due to bottom surface can not be parallel to completely when cutting crackle processed, thus measure the degree of depth a of sample crackle at 1/4,1/2,3/4 3 places of sample thickness 1, a 2, a 3, finally get its mean value (a 1+ a 2+ a 3)/3 are the effective length of crackle, as shown in Figure 3.
4. loading equipemtn is the omnipotent test machine for mechanism of American I nstron 5582, and adopt three-point bending load mode, the diameter of loaded roll and backing roll is 5mm, and span is 4W.C.O.DGauge is adopted to measure the displacement of crack opening, as shown in Figure 4.At the uniform velocity load with the speed of 0.5mm/min, by load transducer and C.O.D Gauge, automatic record is carried out to the tensile load in process of the test and displacement, obtain PV curve.
5. computing formula and data processing
Computing formula is formula (1) ~ (3), K in formula qunit be p qunit be the unit of KN, B, a, W, S be all cm.Adopt EXCEL and SPSS software statistics measurement result.
6 results and analysis
1) the PV curve in hook leaf rattan material destructive process and P qdetermination
Rattan material is viscoelastic material, and in the bending starting stage, elastic deformation occurs, when stress reaches proportional limit, rattan material enters the viscoelasticity stage, until reach maximal destruction load and destroy.As shown in Figure 5, as can be seen from the figure, the starting stage that hook leaf rattan material is bending is elastic stage (OA section) to PV curve in hook leaf rattan material loading procedure, and bending load becomes positive correlation with bending displacement, and loading curve is straight line.Along with the increase of load, rattan material starts surrender, and plastic yield occurs, and bending load slowly increases with the increase of displacement.When load increases to failing load, different from hard brittle material, hook leaf rattan material partial destruction before this, then destroys gradually completely, and when load displacement reaches maximum, indentation destroys, similar with the failure mode of bamboo wood.PV curve with the hook leaf rattan material rattan core rupture failure of precrack meets the feature of I class curve.For the PV curve of test gained, first be stretch section (OA, line the thickest in Fig. 5) best matched curve, this matched curve and X-axis and crack opening displacement axle meet at O point, cross O point and are secant OP5, and make the slope of OP5 be 95% of initial matched curve slope, obtain the load p 5 corresponding to intersection point of secant and this curve, measure the maximum load Pmax that test specimen can bear simultaneously, calculate Pmax/P5, if both are less than or equal to 1.1 by ratio, then P5 is P qvalue.
2) fracture toughness of rattan core
The K of hook leaf rattan material rattan core qvalue is by P qvalue and obtained by formula the results are shown in table 1, carry out validity check: calculate 2.5 (K q/ σ s) 2value, compares with sample thickness; In the present embodiment, the yield strength σ of material sfor 9.8MPa, the 2.5 (K tried to achieve q/ σ s) 2all be less than or equal to the sample thickness of 10mm.
Pass through K qthe detection that value is carried out, can meet the condition of plane strain when showing that thickness is 10mm, therefore calculated K qvalue is the fracture toughness K of hook leaf rattan material rattan core iCvalue.
The nominal fracture toughness mean value of the rattan core measured by SEB test specimen is 0.476, much smaller than bamboo wood and the timber such as China fir, masson pine, show that Fracture Toughness is relevant with constituent to the structure of material on the one hand, on the other hand may be relevant with precision with the method calculated.
From the statistics of this test, the coefficient of variation of hook leaf rattan material rattan core name Fracture Toughness is 12.41% be less than 20%, and accurate index is 1.39% be less than 5%, shows that experimental data is effective.
The size of table 1 rattan core SEB test specimen and K qvalue
Above embodiment is only be described the preferred embodiment of the present invention; not scope of the present invention is limited; under not departing from the present invention and designing the prerequisite of spirit; the various modification that the common engineering technical personnel in this area make technical scheme of the present invention and improvement, all should fall in protection domain that claims of the present invention determine.

Claims (8)

1., based on a method for three-point bending pattern test rattan material fracture toughness, it is characterized in that, comprise step:
1) sample preparation: rattan material to be measured is processed as rectangular block shape sample, and specimen width W is 1 ~ 2 times of the minimum diameter value of rattan material to be measured, and length L is 4 ~ 5W, and thickness is 0.4 ~ 0.7W; Crackle is processed in the middle of rectangular specimen;
2) water percentage is adjusted: temperature 20 DEG C put into by sample, humidity is climatic chamber adjustment under 65% condition, obtains the sample that water percentage is 10-12%;
3) load-crack opening mouth displacement (PV) curve is recorded: adopt three-point bending pattern to load, fulcrum span S is 4W, sample is vertically put in the middle of backing roll, COD displacement meter is placed in crack opening mouth place, at the uniform velocity load with the speed of 0.5mm/min, record load-crack opening mouth displacement (PV) curve;
4) actual crack depth survey: sample, after destroying by curved loading, is gone bail for and stayed the sample of fracture section, measure the length a of crackle under the microscope n, the degree of depth a of sample crackle is measured at sample different-thickness place 1, a 2... .a n, finally get its mean value (a 1+ a 2+ ... .a n)/n is the effective length of crackle; N is the positive integer between 2 ~ 10;
5) critical load P is calculated qwith critical stress intensity factors K q: according to step 3) load-crack opening mouth displacement (PV) curve of recording tries to achieve load p qvalue and K q;
6) validity of test result is assessed.
2. method according to claim 1, is characterized in that, the width of described sample is 10 ~ 50mm; Crack depth a=0.4 ~ 0.6W.
3. method according to claim 2, is characterized in that, described crack depth a=0.45 ~ 0.55W.
4. method according to claim 1, is characterized in that, described step 2) adjustment water percentage be sample is put into temperature 20 DEG C, humidity is climatic chamber under 65% condition, through 26-35 days, obtains the sample that water percentage is 10-12%.
5. method according to claim 1, is characterized in that, described step 3) middle load transducer and COD Gauge carry out automatic record to the bending load in process of the test and displacement.
6. method according to claim 1, is characterized in that, described step 4) in, the length a of crackle is measured at 1/4,1/2,3/4 thickness place of sample n, n=3.
7., according to the arbitrary described method of claim 1 ~ 6, it is characterized in that, described step 5) in the computing formula of critical stress intensity factors be:
K Q = P Q S BW 3 / 2 · f ( a w ) - - - ( 1 )
f ( a w ) = 3 a w · 1.99 - ( a w ) ( 1 - a w ) [ 2.15 - 3.93 a w + 2.7 ( a w ) 2 ] 2 ( 1 + 2 a w ) ( 1 - a w ) 3 / 2 - - - ( 2 )
In formula, S is fulcrum span,
Wherein, P qbeing defined as of value: for the PV curve of test gained, do matched curve, this matched curve and X-axis and crack opening displacement axle meet at O point, cross O point and be secant OP5, and make the slope of OP5 be 95% of initial matched curve slope, obtain the load p 5 corresponding to intersection point of secant and this curve, measure the maximum load P that test specimen can bear simultaneously max, calculate P max/ P 5if both are less than or equal to 1.1 by ratio, then P5 is P qvalue, P qsubstitution formula (1) tries to achieve K qvalue.
8., according to the arbitrary described method of claim 1 ~ 6, it is characterized in that, described step 6) method of assessment test result validity is: calculate 2.5 (K q/ σ s) 2value, and compare with sample thickness, if 2.5 (K q/ σ s) 2be less than or equal to sample thickness, then critical stress intensity factors is valid data; If 2.5 (K q/ σ s) 2be greater than sample thickness, then critical stress intensity factors data invalid; Wherein, σ sfor the yield strength of material.
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CN113453818B (en) * 2019-02-26 2023-11-28 杰富意钢铁株式会社 Method for evaluating bending crack, system therefor, and method for manufacturing press-formed part
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