CN1847810A - Film pre-tension measuring method - Google Patents

Abstract

The film pre-tension measuring method includes the following steps: 1. applying two different loads to the measured point under the same condition and measuring two corresponding displacement values; 2. calculating the film tensions under different loads; and 3.solving simultaneous equations to find out elastic modulus, pre-tension and short direction prestress. During applying load, one vacuum pump is used to apply some local pressure to the stretched film and displacement sensor is used in measuring the out-of-plane displacement of the central point in the local area. One closed space is formed in the film for vacuum pump to pump so as to form one pressure difference across the film as the pressure acted to the film. The present invention has the advantages of low measurement cost, convenient operation and high precision.

Description

A kind of the pretension of film material is carried out to method for measuring

Technical field

The present invention relates to building membrane structure industry, particularly the measurement of the pretension of relevant membrane structure, film material.

Background technology

Pretension is exactly the pulling force of introducing when building membrane structure building, and this is a very important factor of membrane structure.The shape on film surface determines by the pretension ratio of two principal directions of curvature, and in design process, this part can be realized by the shape program of looking for of computing machine, and the absolute value of pretension can guarantee the shape of the each several part of membrane structure under any operating mode.The pretension level that will apply in film is also relevant with the load that needs in use procedure to bear, pretension is too little, under load action, film there will be lax, if pretension is inadequate simultaneously, certain load can cause that face sink, but if pretension is too large, the edge member stress of film material can increase, and this can increase the difficulty of installation.So consider the factors such as the result of force analysis and installation process, in the design phase, determine rational pretension level, and in work progress and in use procedure, the tension variation of Accurate Determining membrane structure reality is the key of successfully building membrane structure to can control and hold the tension level of designing requirement.

The mensuration that membrane structure after stretch-draw is installed is carried out film material pretension value belongs to Non-Destructive Testing category.How to measure exactly film material pretension value is that industry is devoted to the problem solving always.By principle of work, divide, mainly contain Raman spectrum method of testing, vibration-testing method and displacement calibrating method of testing three classes.

Film material is subject to after stretching force, and in contact bed, any two atoms leave equilibrium position and in new position, balance occur, and interatomic acting force changes thereupon.Interatomic acting force is determining the spacing of atom, and interatomic spacing is relevant with atomic vibration frequency.Along with the increase of additional tension, atomic distance increases, and atomic vibration frequency declines; Otherwise under compressive stress, atomic distance reduces, atomic vibration frequency increases.The available laser Raman spectroscopy of variation of this vibration frequency is measured out, just distortion (interatomic change) can be measured accordingly.The method of flash ranging can utilize laser Raman spectroscopy technical calibration to go out the relation between the suffered strain of fiber and Raman spectrum frequency shifting, then with laser Raman spectrometer, measure again the Raman spectrum frequency shifting of film material interlayer adjacent fiber, thereby converse the strain of fiber, according to the elastic modulus of fiber, obtain the stress of film material fiber.This is the ultimate principle of Raman spectrum method of testing.It is synthetic that building film material is actually fiber basic unit and coating, coating is still very large on the impact of the material character of film material and mechanical characteristic, this method has been ignored the impact of coating, obviously degree of accuracy is discounted, and adopt the method instrument used more expensive, it is also difficult in Practical Project, promoting on a large scale.

Vibration-testing is to test by the fundamental frequency in face region, the anti-face pretension that pushes away.Vibration-testing is more effective for the structural internal force test of ideal boundary condition, but in Practical Project, the face border of regional area is very complicated, sometimes be difficult to the fundamental frequency of identification face vibration, or the fundamental frequency precision of identifying is very low, this just greatly reduces the precision of vibration-testing method.

Displacement calibrating test is to apply vacuum load by the face to given area, makes face produce distortion, measures deformation values, the anti-pretension value that pushes away face.What employing this method relatively had representative is the film tension measuring device of Japanese Kajima Corporation exploitation, Japan Kajima Corporation utilizes and in advance the film material of appointment is carried out to a series of test, then according to test figure, return the funtcional relationship that obtains film tension force and plane outer displacement, the relation of the prestress of certain films material and the plane outer displacement under appointment load is just demarcated like this.

During in-site measurement, by vacuum pump, load, measure film material plane outer displacement, then corresponding relation of having demarcated is found the now pretension of film material.The nonlinear relationship between pretension that film material due to different, exists different loads---distortion---, vacuum test method need to be demarcated in advance to the film material of different materials.The shortcoming of the method is exactly the restriction that the scope of application is subject to proven film assortment class, the film assortment class that Kajima Corporation is only used Japan is demarcated, other film material that this instrument uses China need to be demarcated separately, use the method simultaneously, to the warp-wise of variety classes, different-thickness and one species film material and broadwise, also all must first demarcate displacement and pretension relation curve, workload is huge.Often there is very large difference in the condition during due to the actual conditions of on-the-spot film material and calibration experiment, test result also depends on elasticity modulus of materials, so the precision of this method is also limited.

In sum, the degree of accuracy of existing measuring method and equipment is all lower, measures cost compare high, applies also extensive not.In Practical Project, in a lot of situations, still by more original method, the method for striking by experienced workman's hand is estimated the degree that applies of film material pretension value.

Summary of the invention

The object of the present invention is to provide and a kind of the pretension of film material is carried out to method for measuring, overcome now methodical above-mentioned shortcoming.

For achieving the above object, solution of the present invention is: a kind of the pretension of film material is carried out to method for measuring, intend rope method, according to the inventive method, measuring process adopts following steps:

(1) on membrane structure face, in a little rectangular region, (claim that this region is measured zone, the central point in region is tested point, and the short side direction in region is direction to be measured, and bond length is a) to apply the air pressure q being more or less the same on twice numerical value ₁, q ₂as two evenly loads on region, survey to obtain central point corresponding plane outer displacement w under two loads ₁, w ₂, record two groups of data q ₁, w ₁and q ₂, w ₂;

1. use $H=\frac{q{a}^2}{8w}$ Obtain the tension force H of the measured zone film material in direction to be measured under each load ₁, H ₂;

2. with two groups of data q of above gained ₁, w ₁, H ₁and q ₂, w ₂, H ₂, Simultaneous Equations:

$\left\{\begin{array}{c}\frac{H_1-H_0}{E^{\′}A}=\frac{8}{3}\frac{{w_1}^2}{a^2}-\frac{32}{5}\frac{{w_1}^4}{a^4}\\ \frac{H_2-H_0}{E^{\′}A}=\frac{8}{3}\frac{{w_2}^2}{a^2}-\frac{32}{5}\frac{{w_2}^4}{a^4}\end{array}\right.$

Can obtain equivalent elastic modulus E ' and pretension H in film material measured zone direction to be measured ₀, the prestress in direction to be measured is σ ₀=H ₀/ (t * 1), wherein t is film material thickness.

Further, the air pressure load that applies described in step 1 refers to the film material setting measurement region good to stretch-draw, utilize vacuum pump to extract certain vacuum tightness from a face in this region, make to produce draught head on two faces of face, this draught head is the air pressure that puts in measured zone (in the situation that distortion is little, this air pressure can be regarded evenly load as), finally by the plane outer displacement of displacement sensor measured zone central point.

Realize in the space that setting measurement region can rely on certain equipment to construct a relative closure in a side of face, and face is a face in this space.Vacuum pump can be bled from this space, and inner and outer air pressure is poor is the air pressure acting on face.

The space of this relative closure can be with surveying instrument with the measuring box of a specific opening be buckled on face and form, the opening of measuring box is rectangular, is buckled in the region forming on face and is exactly said measured zone above.

Conventionally, this rectangle length-width-ratio requires to be greater than 3: 1.

The optimal length-width-ratio of this rectangle is to be greater than 5: 1.

Direction to be measured is the short side direction of rectangle measured zone, the central point of rectangular measuring box opening is overlapped with tested point and make the minor face of measuring box consistent with the direction that needs to measure, and the result of measurement is exactly the pretension of direction to be measured, and prestress is counted σ _x; Vertical with it direction measure again once can this direction pretension, prestress is counted σ _y; Then in the angle that angle is α with it, survey the pretension of this direction, prestress is counted σ _α, the shear stress of tested point is: τ _xy=[σ _α-σ _xcos ²α-σ _ycos ²(90-α)]/(2cos α sin α).

For the more coarse film material of face, during measurement, can be coated with lubricant in the surrounding in the region of measuring, do not affect measurement result, process like this, can make face tightr with contacting of measuring box, when increasing sealing, also make measuring box relative freely with the slip between face.

Method of the present invention has significant superiority at aspects such as saving measurement cost, operation facility and high precision.

Accompanying drawing explanation

Fig. 1 is the basic structure schematic diagram according to a kind of measuring instrument of the inventive method;

Fig. 2 is enough undersized rectangular areas schematic diagram of a membrane structure;

Fig. 3 is a load schematic diagram.

Embodiment

Building film material is flexible material, when in plane, pretension is zero, its ability of bearing plane external load is very weak, but while being applied with prestress in plane, membrane structure just becomes the structure with certain rigidity, not only can bear well the deadweight of self, can also effectively resist wind, rain, snow etc. from outside plane external load simultaneously.The principal feature of membrane structure that Here it is.The function of the measuring instrument of succeeding in developing according to this measuring method is exactly for measuring the pretension of film material each point in membrane structure, and making can their size of manual control.According to the basic comprising principle of a kind of measuring instrument of the inventive method as shown in Figure 1, it mainly comprises two parts: control box 1 and measuring box 4,4 of measuring boxes mainly by an organic glass box and attached to it, can from bleed in box with the pipe 3 vacuum pump (not shown)s that are connected, and the non-contact laser displacement transducer 2 that is attached to box top, the 5 measured zone central point displacements of energy measuring box opening part film material; Control box 1 mainly can control and coordinated measurement box on the operation of several parts, object is vacuum tightness when reading seal cavity when measuring box and face and reaching certain vacuum tightness and the displacement 6 of corresponding face central point, and according to this computing face pretension 7 and show.

With regard to a membrane structure, the area of film is all often very large, the plane that face neither be regular simultaneously, but complicated curved surface, but enough undersized rectangular area axb (being the size of the measuring box opening of measuring instrument) is as shown in Figure 2 basic or plane.When measuring, the measuring box of measuring instrument is buckled on face, just forms like this space of a relative sealing between measuring box and face, by extracting vacuum, formation inner and outer air pressure is poor, thereby in this scope, applies evenly load outside a plane, as shown in Figure 2.Under load action, film material deforms, there is plane outer displacement, now between each factor such as load, plane outer displacement and film material pretension, there is certain mutual relationship, by this relation, the in the situation that of known payload values and deflection, can instead push away the pretension of film material.

(1) planar dimension (axb as shown in Figure 2) of amount box determines; Determining of measuring box and face boundary condition.Measuring box is of a size of rectangle, and long edge lengths is greater than 3: 1 with the ratio of bond length; Membrane structure often area is very large, from two faces measure simultaneously operation very difficult, single-sided operation be only reality.Single-sided operation also makes surveying work become simpler and easy, during measurement, between measuring box and face, generally need not adopt other special processing, in the region of measuring, be coated with lubricant at most, between measuring box and face, can be free to slide, such boundary condition is easy to realize at engineering site.

(2) the local zonule film material pretension of given above-mentioned size and boundary condition and external load nonlinear theory relation in large-area face.

In certain area, there is pre-tension stress σ _x0and σ _y0film material under imposed load q effect, produce displacement w.The pre-tension stress σ of load q---displacement w--- _x0(σ _y0) between there is certain nonlinear relationship.

Such nonlinear relationship depends on that film material is through the elastic modulus of broadwise.But, film material elastic modulus and its stress state are also nonlinear relationship, also there is very large discreteness, not only for different film materials, even for the different pretension state of same film material, also all there is difference in its elastic modulus,, before mensuration obtains film material pretension state, is therefore in fact to get hold of the definite elastic modulus of film material.If test philosophy is based upon on the supposition basis that film material has constant elasticity modulus, can make measurement result have very large error.This is also the reason that more existing testing apparatuss and instrument produce error.

Through research, find, the research that is greater than the rectangle size film material of certain numerical value for length breadth ratio can be converted into the research to the film bar of the unit width of center to mould material length, and the film bar mechanical property of this unit width and having and the same area of film bar, the elastic modulus again mechanical property of the rope the same with film material short elastic modulus is consistent, like this, solving film material pretension measurement problem also just can discuss by theory revised of relevant single rope.We can measured zone length to the centre film bar that cuts out a unit width discuss, film material is can be along (when namely reality is measured of film material planar slide in the Contact Boundary of the sidewall with measuring box, measuring box and face do not need to process with regard to attainable boundary condition especially), Fig. 3 is the calculation diagram of measuring process.A in figure is that the span in film bar loading region is the bond length of measuring box.Under evenly load q effect, the displacement of regional center point is w, and the tension force of short also becomes T from original To.

Film bar under vertical uniform load effect, film bar can be assumed to be constant in the horizontal component of the tension force of measured zone,

H=constant (1)

In the situation that deformation comparison is little, the deformation curve of the film bar short of measured zone can be similar to regards a para-curve as.In the constant situation in border, two ends, according to the displacement w of central point under load, can determine this parabolical equation

$z=\frac{4\mathrm{wx}(a-x)}{a^2}...\left(2\right)$

The relational expression of the horizontal component of tension force and load and central point displacement

$H=\frac{\mathrm{<figure-callout\; id="2"\; label="q\; a"\; filenames="A20051002513900111.png"\; state="\{\{state\}\}">q</figure-callout>}{a}^{}{}^2}{8w}...\left(3\right)$

When displacement w is smaller, the tension force T ≈ H of film material.

In the process of measuring, film bar is stretched, and change has occurred length.Length at measured zone film bar was its span a originally, and the length of load application caudacoria bar can be obtained by integration type below

$s={\&Integral;}_0^{\mathrm{<figure-callout\; id="1"\; label="a"\; filenames="A20051002513900131.png"\; state="\{\{state\}\}">a</figure-callout>}}\sqrt{1+{\left(\frac{\mathrm{dz}}{\mathrm{dx}}\right)}^2}\mathrm{dx}...\left(4\right)$

Integration type is by series expansion and get its first three items, above formula can be reduced to

$s={\&Integral;}_0^a[1+\frac{1}{2}{\left(\frac{\mathrm{dz}}{\mathrm{dx}}\right)}^2-\frac{1}{8}{\left(\frac{\mathrm{dz}}{\mathrm{dx}}\right)}^4]\mathrm{dx}...\left(5\right)$

(2) formula substitution can be obtained

$s=a(1+\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4})...\left(6\right)$

The elongation of film bar is like this

$\mathrm{\&Delta;s}=s-a=a(\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4})...\left(7\right)$

By physics aspect, investigated, the elongation of film bar is mainly to increase because load application makes film bar internal force, and internal force increment has produced stretcher strain, can obtain following formula like this

$\mathrm{\&Delta;s}=\frac{\mathrm{\&Delta;T}}{\mathrm{EA}}{l}^{\&prime;}...\left(8\right)$

In little sag problem,

$\mathrm{\&Delta;s}=\frac{\mathrm{\&Delta;H}}{\mathrm{EA}}{l}^{\&prime;}=\frac{H-H_0}{\mathrm{EA}}{l}^{\&prime;}...\left(9\right)$

L ' in formula is the length that affects of load, as l '=l in Fig. 3 ₁+ a+l ₂l ' is relevant with the pretension of residing position, measured point and this point, because boundary condition is slidably, therefore evenly load not only causes the variation of measured zone inner membrance material internal force, outside measured zone, in certain scope, also caused the change of internal force simultaneously, the change of internal force all can make in region and extra-regional this scope produces stretching, and the inside and outside two parts of measured zone are because the change of internal force produces deflection amount of tension and that just equal measured zone inner membrance bar.The concept of coverage l ' used herein is that the STRESS VARIATION equivalence of handle in actual coverage is in a scope, think that stress is even with the same distribution in measured zone in this scope, and think beyond this scope it is the impact that is not subject to measured zone institute load application, a so l ' virtual concept just.Same point applies different loads that difference is not too large, and can to think that load affects length constant, needed two air pressure load q while that is to say for above-mentioned measurement ₁, q ₂, the length that affects of load is the same.By (7) formula and (9) Shi Ke get

$\frac{H-H_0}{\mathrm{EA}}{l}^{\&prime;}=a(\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4})...\left(10\right)$

Load in formula affects length and span can think there is following relation

l′＝ξ·a (11)

(10) formula becomes

$\frac{H-H_0}{\mathrm{EA}}\mathrm{\&xi;a}=a(\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4})...\left(12\right)$

Further become

$\frac{H-H_0}{\left(\frac{E}{\mathrm{\&xi;}}\right)A}=\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4}...\left(13\right)$

(E/ ξ) in formula can be described as the equivalent elastic modulus of tested point place film material measured zone short side direction while being measurement, with E ', represents.The same with l ', E ' is relevant with the pretension of residing position, measured point and this point, and same point applies different loads also can think that it is constant.By what introduce load, affect length l ' and equivalent elastic modulus E ' like this, we have just solved the processing of Measured Boundary condition and the problem of film material material discreteness considerable influence measurement result.Although the elastic modulus of film material can be different because of the difference of the factors such as the difference of material and the inner Two-way tension force of film material, we are only concerned about that it is measuring value constantly, and while measuring because add the external disturbance q being more or less the same on two numerical value ₁, q ₂can not cause the supposition of the significant change of elastic modulus is acceptable, and this is also an important prerequisite of the inventive method.

Like this, formula (13) becomes following formula

$\frac{H-H_0}{E^{\&prime;\&prime;}A}=\frac{8}{3}\frac{w^2}{a^2}-\frac{32}{5}\frac{w^4}{a^4}...\left(14\right)$

In actual measurement, we can adopt following step:

1. under similarity condition, in measurement point, apply two stage atmospheric pressure q ₁, q ₂, survey to obtain corresponding central point displacement w under load ₁, w ₂, record two groups of data (q ₁, w ₁) and (q ₂, w ₂);

2. with the formula of being derived $H=\frac{q{a}^2}{8w}$ Obtain the film tension force H under each load ₁, H ₂;

3. with above two groups of data (q ₁, w ₁, H ₁) and (q ₂, w ₂, H ₂), by containing pretension H ₀(equivalent elastic modulus of film short side direction, the boundary condition of the elastic modulus making progress the party with film material and this direction of film material is relevant with equivalent elastic modulus E '.This equivalent elastic modulus, only as intermediate quantity, there is no practical significance) system of equations of two unknown numbers:

$\left\{\begin{array}{c}\frac{H_1-H_0}{E^{\&prime;}A}=\frac{8}{3}\frac{{{\mathrm{<figure-callout\; id="1"\; label="w"\; filenames="A20051002513900131.png"\; state="\{\{state\}\}">w</figure-callout>}}_1}^2}{a^2}-\frac{32}{5}\frac{{{\mathrm{<figure-callout\; id="1"\; label="w"\; filenames="A20051002513900131.png"\; state="\{\{state\}\}">w</figure-callout>}}_1}^4}{a^4}\\ \frac{H_2-H_0}{E^{\&prime;}A}=\frac{8}{3}\frac{{{\mathrm{<figure-callout\; id="2"\; label="w"\; filenames="A20051002513900111.png"\; state="\{\{state\}\}">w</figure-callout>}}_2}^2}{a^2}-\frac{32}{5}\frac{{{\mathrm{<figure-callout\; id="2"\; label="w"\; filenames="A20051002513900111.png"\; state="\{\{state\}\}">w</figure-callout>}}_2}^4}{a^4}\end{array}\right.$

Equivalent elastic modulus E ' and pretension H like this ₀just can obtain, short prestress is σ ₀=H ₀/ (t * 1), t is film material thickness.

Film material is the flexible composite of orthotropic, material principal direction is the direction of fiber, two-way elastic modulus is different, simultaneously because film material material is discrete larger, different film materials and same film material are under different stress, each to elastic modulus all different, therefore based on the resulting measurement result of measuring method fixing, that known elastic modulus is prerequisite, be all inaccurate, feature of this method is under the condition of each material parameter of not knowing film material, to obtain measurement result (being verified) very accurately in experiment.

Processing to boundary condition

Press the measuring principle figure shown in Fig. 1, in the process of measuring, the situation that between measuring box and face, most probable is realized is to be exactly to allow film material along surface level (face), be free to slide on border, so must consider this factor in the derivation of formula.Fig. 3 is the calculation diagram of measuring process, and a in figure is that the span in film bar loading region is the bond length of vacuum box, l ₁, l ₂film material measured zone length is affected scope to both sides, the actual conditions that the boundary treatment that simultaneously can be free to slide in figure relatively contacts with film material close to the side of vacuum box.

L ' in figure is the length that affects of load, l '=l ₁+ a+l ₂=ξ a.We solve this influence factor by introducing the method for above-described equivalent elastic modulus.

To the regulation of measuring box opening size (regulation of the local loading area size on face)

For corresponding with our the derive conversion formula that obtains, we are of a size of rectangle by specifying measurement box opening, and long limit should be greater than 3: 1 with the ratio of minor face, and General Requirements is 5: 1.Direction to be measured is measuring box opening short side direction, the central point of the opening of measuring box is overlapped with tested point and make it minor face and need the direction of measurement consistent, and measurement result is exactly the pretension of direction to be measured, and prestress is counted σ _x; Vertical with it direction measure again once can this direction pretension, prestress is counted σ _y; Then in the angle that angle is α with it, survey the pretension of this direction, prestress is counted σ _α.According to Elasticity stress rotation axis formula: τ _xy=[σ _α-σ _xcos ²α-σ _ycos ²(90-α)]/(2cos α sin α)

The shear stress of tested point also can be obtained like this.

Claims (7)

1, the pretension of film material is carried out to a method for measuring, it is characterized in that adopting following steps:

(1) under similarity condition, in measurement point, apply two-stage load q ₁, q ₂, survey to obtain corresponding displacement w under load ₁, w ₂, record two groups of data q ₁, w ₁and q ₂, w ₂;

(2) use $H=\frac{{\mathrm{qa}}^2}{8w}$ Obtain the film tension force H under each load ₁, H ₂;

(3) with above two groups of data q ₁, w ₁, H ₁and q ₂, w ₂, H ₂, Simultaneous Equations:

$\left\{\begin{array}{c}\frac{H_1-H_0}{E^{\&prime;}A}=\frac{8}{3}\frac{{w_1}^2}{a^2}-\frac{32}{5}\frac{{w_1}^4}{a^4}\\ \frac{H_2-H_0}{E^{\&prime;}A}=\frac{8}{3}\frac{{w_2}^2}{a^2}-\frac{32}{5}\frac{{w_2}^4}{a^4}\end{array}\right.$

Obtain elastic modulus E ' and pretension H ₀, short prestress is σ ₀=H ₀/ (t * 1), wherein t is film material thickness.

2, according to claim 1 the pretension of film material is carried out to method for measuring, it is characterized in that the load application described in step 1 refers to utilizes vacuum pump to apply an air pressure to the good film material part of stretch-draw, then by the plane outer displacement of this regional area central point of displacement sensor.

3, according to claim 2 the pretension of film material is carried out to method for measuring, it is characterized in that forming the space of a relative closure on face, vacuum pump can be bled from this space, and inner and outer air pressure is poor is the air pressure acting on face.

4, according to claim 3 the pretension of film material is carried out to method for measuring, it is characterized in that the crossing region of the space of this relative closure and film material is rectangle.

5, according to claim 4 the pretension of film material is carried out to method for measuring, it is characterized in that this rectangular region length-width-ratio is greater than 3: 1.

6, according to arbitrary in claim 4-6, described the pretension of film material is carried out to method for measuring, it is characterized in that direction to be measured is for this rectangular short side direction, this rectangular central point is overlapped with tested point and make its minor face consistent with the direction that needs to measure, measurement result is exactly the pretension of direction to be measured, and prestress is counted σ _x; Vertical with it direction measure again once can this direction pretension, prestress is counted σ _y; Then in the angle that angle is α with it, survey the pretension of this direction, prestress is counted σ _α, the shear stress of tested point is: τ _xy=[σ _α-σ _xcos ²α-σ _ycos ²(90-α)]/(2cos α sin α).

7, according to arbitrary in claim 1-5, described the pretension of film material is carried out to method for measuring, it is characterized in that the more coarse film material of face, during measurement, can be coated with lubricant in the region of measuring, when increasing measuring box and contact sealing with face, make slip between measuring box and face more freely.

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