1、INTERNATIONAL STANDARD INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXQYHAPOjJHAR OPrAHM3AuMR n0 CTAHAAPTkl3AMM 3 j P ,.I ” .fi- ,) I; ,q. ( ,.- r.,;, ,“. :.: ,., , “I i f. I.$ yJ&;j,- T% Q1 .;1, “ I I2:l” (3) V . . . 1 4.2 Load, loading pieces, dead-w
2、eight In equation (3) m is expressed in grams; I, I2 and AI in millimetres; V in cubic millimetres; At in seconds; and q in decipascal seconds (dPas). NOTE - In calculating the viscosity, corrections sometimes have to be applied (see 9.2). 5.2 Preliminary estimation of time interval and elongation T
3、he load consists of all parts of the apparatus on which gravity acts to produce a force on the lower end of the fibre - i.e. the loading pieces (variable), and the device for suspending the loading pieces below the specimen (a given value for the appar- tus) - plus the weight of the lower ball (to b
4、e estimated after preparing the test specimen). The load produces the force Fc acting on the test specimen in the axial direction. The dead-weight stems from the part of the fibre below the cross-section under consideration (see 9.2). The force due to gravity varies linearly from zero at the lower e
5、nd to g.e.1.S at the upper end of the fibre (g and Q being the acceleration of free fail, and the glass density, respectively). 4.3 Effect of surface tension The effect of surface tension is to decrease the surface area of the specimen, i.e. to shorten the fibre. Therefore, it is a force which acts
6、upon the fibre in the opposite sense to that of the gravitational force of the load and of the dead-weight. The table shows the expected values of the time interval At, for given loads in relation to the viscosity. The values are calculated for a fibre with a diameter d, = 0,737 mm at the beginning
7、of the measurement, and an elongation of 2 % dur- ing the time interval At, (Al/l, = 0,021. Between the two heavy stepped lines in the table the sensitivity of the measuring device for the elongation Al should be increased by the factor 10 (the elongation should be decreased by the factor 1 /IO) for
8、 shorten- ing the time interval At, by the factor I! 10. Below the lower line the factor is 100 instead of 10 (l/100 instead of l/IO). 5 Principle 5.1 Basic relation Table - Time interval At, for an extension Al/l1 = 0,02 under a load of mass m (as corrected for dead-weight and surface tension), for
9、 a fibre of initial diameter d, = 0,737 mm Consider the force Fe of the load acting on a specimen prepared from a glass with Newtonian behaviour and showing no defects. When the effects of dead-weight and surface ten- sion are negligible and all elastic deformations by the loading become constant, t
10、here exists to a good approximation an ex- tensional flow at constant volume as described by equation (1) : Time interval At, in seconds for load of mass 1 g,6i phi 1 ; i21gl dl 1 Fol* -=-xx dt 3rj V . . . where 100 2560 640 160 40 - 10” 25 600 6400 1600 400 100 102 256000 64000 16000 4 000 I 1 000
11、103 - I I- 104 ( I ( 1600 / 40: E( q is the dynamic viscosity; V = I x S is the volume of the fibre (the other symbols have the meaning as introduced above). For other diameters of the fibre the time interval At3 for an elongation of 2 % can be estimated from equation (4): During the time interval A
12、t of the measurement, the fibre elongates from the initial length I, to the final length 12 Then the viscosity is calculated using equation (2) : At; = Ats.ft . . . (4) The value of the dimensionless conversion factor ft as a func- tion of the diameter is found from the diagram in figure 1. g At 12
13、x 11 q=3XmxTX- * . . (2) 12 - 4 where g is the average acceleration of free fall; m is the mass of the load that produces the force Fo. The estimated values At, and At: derived from the table, figure 1 and equation (4) give only a survey of the expected time required for the measurement and an estim
14、ation of the whole elongation of the fibre during a sequence of measurements on the same fibre. For the quantitative evalua- tion of the elongation experiment, however, the starting point is equation (31, using the actual values (see clause 8). 2 IS0 7884-3 : 1987 (E) 0.6 I.0 I4 Conversion factor ft
15、 1.8 Figure 1 - Conversion factor ft as a function of the fibre diameter dl 6 Apparatus 6.1 Viscometer furnace Electrically heated furnace for temperatures at least up to 900 OC. The furnace shall be capable of accepting ther- mocouples for measuring the temperature and its distribution along the sp
16、ecimen. The temperature gradient at the locus of the specimen shall not exceed 0,2 C/cm. The furnace heaters shall be controlled by a device which en- sures that the temperature remains constant with respect to time, within the working space of the furnace, to f 1 OC or bet- ter. Linearly increasing
17、 time-temperature programmes with a maximum rate of 6 Y/min shall be achievable by the same device. The furnace and its control device shall have characteristics such that the desired time-temperature programme is attained at the latest 5 min after starting from a constant initial temperature. The v
18、alue of the rate shall be held within + 10 % throughout the determination. 6.2 Temperature measuring and indicating instruments 6.2.1 The alumina-insulated platinum-10 % rhodium/plati- num (type S according to IEC 584-l 1 thermocouples, or nickel- chromium/nickel (type K according to IEC 584-I) ther
19、mo- couples shall exhibit low thermal inertia (the diameter of the wires should not be greater than 0,5 mm). The wires shall have a sufficient length within the furnace (with respect to heat conduction along the wires). 6.2.2 Control thermocouples should be located as close as possible to the furnac
20、e windings for fast response. The hot junctions of the measurement thermocouple, however, shall be placed in the immediate vicinity of the specimen (see ther- mocouple A in figure 2). The temperature distribution along the fibre shall be monitored by suitable devices such as - a high-mass block of n
21、ickel or silver around the fibre, with a number of holes for fixed thermocouples; or - a mobile thermocouple (such as thermocouple B in figure 2). In this case care should be taken that the thermocouple does not affect the temperature of the fibre (see IS0 7884-l : 1987, sub-clause 5.3). In accordan
22、ce with IS0 7884-l the measurement thermo- couples shall be calibrated and the calibration checked regularly. 6.2.3 The electrical output of the thermocouples shall be determined at zero current by means of potentiometers, or high-resistance electronic amplifiers having a sensitivity of 1 pV for typ
23、e S (according to IEC 584-l 1, or 4 uV for type K (accord- ing to IEC 584-I) thermocouples. Precautions shall be taken that the ice-bath for the cold junction is maintained at 0 OC throughout the test. If the temperature measuring equipment is fitted with automatic cold junction compensation, the ic
24、e-bath can be omitted. 6.3 Hanging device for test specimen and load 6.3.1 The construction of the support for the test specimen depends on the type of furnace used. When the top of the working space of a chamber furnace is closed (as shown in figure 2). then a tube of fused silica is used as a supp
25、ort stand. The tube projects into the working space of the furnace from below. The centred support plate on top of the stand is made from ceramic or from stainless steel. The test specimen is suspended by its upper ball which locates in a central recess in the support plate. Because of possible adhe
26、sion of the glass, a centrally bored ceramic bead of appropriate dimensions (only to be used once) should be placed between the support plate and the upper ball of the test specimen. 6.3.2 When using a vertically mounted muffle furnace (both ends open), the fibre is connected to an upper rod of fuse
27、d silica. Examples of devices for connecting the test specimen to the rod (by means of balls) are shown in figures 3 and 4. It is essential that the rod and the fibre hang vertically, coaxially and centrally in the tube of the furnace. Aside from the support, the top of the furnace tube should be co
28、vered. 6.3.3 The loading linkage consists of a rod (made of fused silica, about 1 mm in diameter) below the test specimen, with fused balls on both ends and connectors as shown in figure 3 or 4. Below the lower connection (outside the furnace) the attach- ment of the load carries a suitable indicato
29、r for the elongation measurement (marker or transducer core). For the medium range of viscosity the total mass of all parts of the loading linkage device should amount to approximately 4 g. In the range of higher viscosities and heavier loads the diameter of the silica rod should be greater. 3 ISO 7
30、884-3 :1987 (El Hot junction o thermocouple I F-.-*- . . . . . . . . . 31 / 2 1 z?i! 1 Connector for the lower ball of the test specimen and the loading device (for connectors offering different magnitudes of load, see figures 3 and 4) 2 Test specimen 3 Support plate 4 Upper ball of the test specime
31、n Figure 2 - Fibre elongation method example of a testing device (schematic) - 8, Y 9 Hot junction of thermocouple B 5 Loading device (only the rod is shown) 6 Support stand : tube of fused silica 7 Screening between working space and heaters 8 Chamber furnace with heaters 9 Cover ring below the fur
32、nace (fixed at the silica tube) Figure 3 - Connector block (metal) bored to act as hanging device Figure 4 - Connector hooks (wire) to act as hanging devices 4 IS0 7884-3 : 1997 (I3 6.3.4 When measuring within the range from IOsto 10s dPas, the loading linkage device consists of a connector and lowe
33、r rod, with a diameter of about 0,5 mm, without further devices below the end of the rod (this end is below the furnace and acts as indicator). The total mass of this device should amount to approximately 1 g. NOTE - The measurement of the elongation by means of a travelling microscope is facilitate
34、d when the lower end of the rod is gently guided with minimum friction. 6.4 Loading pieces A set of loading pieces made from brass, with masses deter- mined to the nearest 0,Ol g. The pieces shall be fitted by suitable attachment devices (e.g. hooks). It is convenient to choose pieces of masses such
35、 that the total load corresponds nearly to the values given in the table. When the mass of the loading linkage device (in accordance with 6.3.3) is 4 g, then the appropriate masses of the pieces are 12 g, 60 g and 252 g, respectively. 6.5 Extensometer arrangements 6.51 Moving marker below the furnac
36、e as an indicator of the elongation of the fibre. 6.5.2 Device for determining the fibre elongation Al during the time interval At. A minimum sensitivity of 0,02 mm is needed. The instrument for observing the downwards moving marker shall be locked in position during any one single determination of
37、Al. It shall be possible to position it in advance so that the whole elongation length AI in a determination is within the range of observation. Errors arising from incorrect graduation of the scale, or deviations from linearity of the display, shall not exceed 0,Ol mm. NOTE - Suitable devices are f
38、or example travelling microscopes with micrometer scales, or linear differential transformers (core and coil separated from each other). The use of different ranges of sensitivity is an advantage. 6.5.3 Device for determining the length 1, over the whole amount of elongation (I, - /c) from the origi
39、nal length lc after preparing the specimen at room temperature (18 to 20 “Cl to the initial length 1, before starting the time interval At, see equation (2). The length lc shall be determined to the nearest 0,2 mm. The last value of I, - lo at the end of a sequence of measurements should not exceed
40、0,l x lo NOTE - The device might consist of, for example, a cathetometer fit- ted with scale and vernier, carrying the microscope or the coil of the differential transformer. 6.5.4 Timer for determining the interval At see equations (2) and (311, ranging from 20 to 2 000 s, with a least count of 0,l
41、 s. The timer shall be corrected for systematic errors greater than 0,2 %. 6.6 Equipment for preparation of test specimens 6.6.1 Blast flame burner or electrically heated furnace for melting the sample and drawing fibres. 6.6.2 Two rods made from platinum metal alloys, ceramics or hard glass for fla
42、me-working a sample portion. 6.6.3 Platinum crucible and rods made from platinum metal alloys or fused silica, when an electrically heated furnace for the preparation is used. 6.6.4 Slide-gauge with vernier graduation 1 /lO for the deter- mination of the fibre length. 6.6.5 Micrometer caliper, with
43、a least measurement of 0,005 mm, for measuring the fibre diameters. The surface of the fibre shall not be damaged by the advancement of the spin- dle head. Instead of the caliper, a microscope with micrometer scale may be employed by experienced users. 7 Sample and test specimen The sample shall be
44、uniform, bubble-free and homogeneous. 7.1 Preparation of a test specimen by flame- working Melt a test portion of about 2 to 3 cm3 of the sample in the flame. By turning the test portion between the rods, form a ball. Remove the test portion from the flame and draw out the ball to a long fibre. Proc
45、eed quickly because of possible evapor- ation of the more volatile components of the glass, NOTE - To produce sufficient regularity of the diameter along the fibre, a certain experience of the preparation technique is required, especially with respect to the - temperature at the beginning of drawing
46、; - temperature-viscosity relationship of the glass; - force and speed of drawing; - uniformity of drawing; and - conclusion of drawing at the correct time. 7.2 Preparation of a test specimen in a furnace The method applies to glasses with a tendency to crystallize. A test portion of about 100 g of
47、the sample is melted in a platinum crucible in the electrically heated furnace. The glass may be delivered in any form, but the diameter for grains should not be less than 3 mm. A cylindrical rod of platinum, ceramics or fused silica is dipped into the almost bubble-free molten test portion of the glass. After the rod tip is wetted by the glass, it is drawn upwards (see note to 7.1). The rod may be drawn by hand or any suitable device. 5
