1、INTERNATIONAL STANDARD IS0 7884-7 First edition 1987-12-15 INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXAYHAPOAHAR OPI-AHM3Al;:,: ? ” b.,., :p;:;, .,_, ! (I. i/ . . ,. ,i, !K$ j- m is the mass of the centrally applied load, in grams; Z, is the cross-
2、sectional moment of inertia of the test beam, in millimetres to the fourth power (see annex A). NOTE - The deflection rate cJf/dt which defines the annealing point by equation (I ), corresponds to a viscosity of approximately 10i3,2 dPa.s. 4.3 strain point, 19 : The temperature at which internal str
3、esses in a glass are substantially relieved in a matter of hours. The strain point is determined by extrapolation of the annealing point data and is the temperature at which the viscous deflec- tion rate is 0,031 6 times that observed at the annealing point. NOTE - This extrapolated deflection rate
4、corresponds to a viscosity of approximately 1014.7 dPa.s. 5 Principle The annealing point is determined by measuring the rate of midpoint viscous bending of a simply loaded glass beam (see annex D). The strain point is subsequently determined by an extrapolation method. The annealing and strain poin
5、ts shall be obtained following a specified procedure after direct calibration of the apparatus us- ing beams of reference glassest) having known annealing and strain points. 6 Apparatus 6.1 Furnace The furnace shall be electrically heated by resistance-wire wind- ings of suitable alloys capable of m
6、aintaining the appropriate temperature. Dimensions and details of the furnace construction are not critical. Examples are given in IS0 7884-4 and in annex B. The temperature distribution shall be such that differences in temperature greater than 2 “C do not result over the length of the specimen bea
7、m and along the axis of the furnace from the undeflected beam plane to a point 13 mm below. 6.2.2 Control thermocouples should be located as close as possible to the furnace windings for fast response. The hot junction of the measurement thermocouple, however, shall be placed within 5 mm of the test
8、 specimen near the axis of the furnace. In accordance with IS0 7884-l. the measurement thermocouple 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 electroni
9、c amplifiers having a sensitivity of 1 pV for type S (according to IEC 584-11, or 4 pV for type K (accor- ding to IEC 584-l) 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 w
10、ith automatic cold junction compensation, the ice-bath can be omitted. 6.3 Furnace control Suitable means shall be provided for idling the furnace, con- trolling the heating rate and, in the case of very hard glasses, limiting the cooling rate to not more than 5 Y/min. Although commercially availabl
11、e programming equipment can be used, a continuously variable transformer with manual control may also be used. 6.4 Specimen support stand and loading rod A ceramic support stand and a ceramic loading rod shall be pro- vided for supporting the test specimen and applying the load to the test specimen,
12、 respectively. The thermal expansion characteristics of both stand and rod materials shall be very similar so as to minimize motion of the loading rod on cooling due to expansion differences (see annex Cl. A rectangular alumina muffle makes a suitable support stand (see note). The side walls of this
13、 muffle can be notched to define the test specimen position. The supporting surfaces of these notches shall be flat and lie in a plane perpendicular to the axis of the furnace. The inside edges of these supporting surfaces define the support span once the test specimen beam starts to deflect. A supp
14、ort span of about 50 mm is recommended. A suitable loading rod can be provided by a single-crystal sapphire rod2) flame-bent at one end in the form of a shepherds crook. The arrangement is shown in annex 8. I) See, for example IS0 7884-l : 1987, annex B, “Examples of certified reference glasses for
15、viscometric calibration”. 2) Sapphire rods according to 6.4 (after ASTM designation C 598-72) may be obtained from lnsaco Inc., P.O. Box 422, Quakertown, Pa., USA. This information is given for the convenience of users of this part of IS0 7884 and does not constitute an endorsement by IS0 of this pr
16、oduct. 2 IS0 7884-7 : 1987 1 El NOTE - Vitreous silica is a suitable material for both support stand and loading rod. It is not recommended for temperatures above 900 T, however. 6.5 Extensometer for measuring midpoint deflection The means of observing the rate of midpoint deflection of the beam sho
17、uld be such as to indicate reliably over a range of at least 2,5 mm. The graduated scale of the extensometer shall permit direct reading to 0,025 mm and estimates of 0,002 5 mm. Its accuracy shall be such that the error of indica- tion will not exceed + 0,005 mm for any length change. To en- sure th
18、is accuracy, the extensometer shall be precalibrated. A linearly variable differential transformer (LVDT) is suitable for this purpose but any device (optical, capacitative, or other) may be used, provided that the length changes are reliably measured as specified. The arrangement with the LVDT is s
19、hown in annex B. The core of the LVDT is attached to the end of the loading rod, whereas the coils are attached to the leg of the furnace platform. A screw arrangement is provided in the coil attachment assembly tb move the coils vertically for zero- ing purposes. 6.6 Micrometer calipers with an acc
20、uracy of at least 0,Ol mm for measuring specimen dimensions. 7 Preparations that the total mass of the loading device - consisting of the loading rod, LVDT core, hooks, fixtures and the weight piece - is close to the optimum load. This loading mass m shall be used throughout, both for calibra- tion
21、and for test measurements. o-a- 2 4 b tl 10 Cross-sectional moment of inertia of test beam, mm4 Figure 1 - Optimum load versus cross-sectional moment of inertia for test beams 7.1 Preparation of the specimens 8 Procedure 7.1.1 Specimens from reference glass Choose a reference glass whose annealing p
22、oint lies close to the expected annealing point of the glass under test. Specimens may either be flame-drawn or centreless ground into cylindrical form, or diamond-saw cut and mill ground into rectangular form. Non-uniformity of any dimension along the length of the specimen shall not exceed 2 %. Fo
23、r a support span of 50 mm, the cross-sectional moment of inertia shall be between 2 and IO mm4. Corresponding ranges for other values of the span may be derived from the relations given in IS0 7884-4. Prepare a number of specimens (at least two) with different cross-sectional moments of inertia (to
24、be calculated according to annex A), but all within the limits given above. 7.1.2 Test specimens Prepare the test specimens from the glass under test in the same way as in 7.1.1, second paragraph. Take care that the cross-sectional moments of inertia of the reference glass beams bracket the respecti
25、ve values of the beams from the glass under test. 7.2 Adjustment of the loading device From the mean of the cross-sectional moments of inertia of all the beams which will be measured, determine an optimum load by means of the graph in figure 1. Choose a weight piece such 8.1 Preparation of a run All
26、 runs, both for calibration (specimens from reference glass) and for determining the annealing and strain point (test specimens), shall be performed in the same manner. 8.1.1 With the furnace at least 25 OC below the estimated annealing point, remove the top plug and place the specimen beam across t
27、he support stand at the notch points. Carefully engage the loading rod with the test specimen and centre it using long calipers. Replace the top plug. 8.1.2 Apply the weight piece, chosen according to 7.2, to the hook on the end of the LVDT core as shown in figure 8. 8.1.3 Adjust the position of the
28、 extensometer to the lower end of its measuring range. Then start heating the furnace at a convenient rate, preferably at about 5 OC/min. Stop heating and establish a cooling rate of (4 f 1) YYmin when the specimen midpoint deflection rate, in millimetres per second, reaches 7 x lo-10 x i$m . . . zc
29、 (2) where the symbols used are defined below equation (I 1. Reset the extensometer to the lower end of its range. NOTE - This deflection rate, corresponding to a viscosity of 1012 dPa.s, guarantees erasure of previous thermal history. 3 IS0 7884-7 : 1987 (E) 8.1.4 Immediately after cooling has been
30、 established, take readings of both the extensometer and potentiometer alter- nately at 30 s intervals so that each will be read at 1 min inter- vals. Continue the readings until the temperature is 10 OC below the annealing point. Such a temperature will generally be reached when the extensometer in
31、dicates a deflection rate three times less than that expected at the annealing point. If the extensometer goes off range during the test, reset it to the lower end of the range by means of the vertical zeroing screw. Total beam deflections greater than 10 mm are excessive. 8.1.5 Take the change in e
32、xtensometer readings during each 1 min interval as the rate of midpoint deflection at the temperature recorded for the middle of that minute. Plot it logarithmically against its corresponding temperature, using suitable co-ordinated paper with linear abscissa (about 400 mm) against logarithmic ordin
33、ate with three decades (about 250 to 300 mm). The relation should be substantially linear; draw a straight line to represent the plotted points as shown in figure 2. Temperature (linear scale), C Annealing point F temperature flf3 Figure 2 - Graphical method of analysing deflection rate temperature
34、data 8.2 Calibration Carry out the measurements according to 8.1 .I to 8.1.4 on each reference glass beam prepared according to 7.1 .I, and plot the data according to 8.15 and figure 2. From the known annealing point of the reference glass, the related midpoint deflection rate (dfldt), is derived fr
35、om the graph as shown in figure 2 for each beam of that reference glass. Make a linear diagram as shown in figure 3, plotting the values (dfldt), (as found above) against the values of l/Z, (having calculated I, according to annex A) for each beam of that reference glass. This is the calibration cur
36、ve to be used for the test measurements. It is recommended that the apparatus be recalibrated periodically, depending on the incidence of usage. Reciprocal of cross-sectional moment of inertia, 1 /I, (linear scale), mm -4 Figure 3 - Graphical calibration plot of deflection rate versus reciprocal of
37、moment of inertia of reference glass test beams 8.3 Test measurement Carry out the measurements according to 8.1 .I to 8.1.4 on a beam of the glass under test, prepared according to 7.1.2, and plot the data according to 8.1.5 and figure 2. 9 Expression of results 9.1 Evaluation of annealing point Fr
38、om the known dimensions of the test beam, calculate the cross-sectional moment of inertia Z, according to annex A. From the values 1 lZ, find on the calibration curve, as in figure 3 plotted according to 8.2, the related midpoint deflection rate at the annealing point (dfldt), for the beam under tes
39、t. Then, from the Ig(dfldt), versus temperature plot for that beam, drawn according to 8.3 as shown in figure 2, find the related temperature value on the abscissa. This is the annealing point Lpf3 of the glass under test. 9.2 Evaluation of strain point Calculate the midpoint rate of deflection at the strain point (dfldt), for the beam under test by means of equation (3) : (dfldt), s = 31,6 . . . (3) Extrapolate the straight line on the data plot (as shown in figure 2) for that beam towards lower temperatures. 4
