1、Designation: E 471 96 (Reapproved 2002)Standard Test Method forObtaining Char Density Profile of Ablative Materials byMachining and Weighing1This standard is issued under the fixed designation E 471; the number immediately following the designation indicates the year oforiginal adoption or, in the c
2、ase of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe ablation characteristics of charring materials must be well known in order to design
3、 the materialfor a specific set of environmental conditions. The char density profile and the environmentalconditions under which it was formed can provide useful information about the ablation performance.A method of obtaining the char density profile from a charred sample of material is described
4、in thefollowing sections. Some chars are very friable and are easily broken before they can be measured.Other chars are relatively strong and can be handled with ease. The type of char density profilemeasurement described in this method is applicable if the char is strong enough to be machinedwithou
5、t breaking.1. Scope1.1 This test method covers the determination of the chardensity profile of a charred ablator that can be used with thefollowing limitations:1.1.1 The local surface imperfections must be removed, andthe char must be able to be machined off in a plane parallel tothe char-virgin mat
6、erial interface before the density profilescan be determined.1.1.2 The char must be strong enough to withstand themachining and handling techniques employed.1.1.3 The material should have orderly density variations.The total thickness of the char and degradation zone must belarger than the machining
7、 thicknesses required.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior
8、to use.2. Summary of Test Method2.1 Density variation throughout a charred ablator materialis determined by successively measuring, machining, andweighing a sample of known size to obtain the density of thematerial removed by machining.3. Apparatus3.1 The apparatus required for this method includes
9、alaboratory balance capable of measuring to the nearest 0.0001g, and a machining technique capable of removing material inincrements as small as 0.025 mm (0.001 in.). For example, flatspecimens can be machined with a surface grinder using amedium fine grit ceramic grinding wheel of a soft gradedress
10、ed to the proper contour. Cylindrical specimens can bemounted in a lathe and the char can be removed with a sharpcarbide or diamond tip tool.4. Sampling4.1 The charred sample selected for machining and weigh-ing should be taken from a representative section of the ablatedspecimen where the environme
11、ntal conditions are well known,and where the surface is parallel to the char-virgin materialinterface. Where large sections are available, this condition isusually met. For small samples which have been exposed tovarying environmental conditions along the length of thesample, the sample size will be
12、 smaller.5. Test Specimens5.1 A typical specimen size obtained from a channel, pipeflow, or rocket motor section may be 12.7 by 12.7 mm (0.5 by0.5 in.), or 12.7 mm (0.5 in.) in diameter. The sample thicknessis determined by the available thickness of material. Smaller or1This test method is under th
13、e jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved Oct. 10, 1996. Published December 1996. Originallypublished as E 471 73. Last previous edition E 471 81 (
14、1992).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.larger samples can be used depending upon the accuracy,weighing apparatus, and specimen size. Larger samples reduceedge effects. Excess virgin material should be eliminated, ifpos
15、sible.5.2 The specimen is rough-cut out of the ablated section,and then machined so as to make the sides perpendicular andthe front surface parallel to the char-virgin interface as shownin Fig. 1.5.3 For simplicity and ease of handling, the base of thesample (the backface or side opposite the charre
16、d surface) maybe cemented to a thin piece of steel. This permits the sample tobe handled with tweezers and facilitates the mounting of thesample on a magnetic chuck or other mounting jig formachining the surface from a fixed reference plane.6. Procedure6.1 Sample Preparation:6.1.1 Take the dimension
17、s of the specimen with microme-ters or other suitable measuring devices to the nearest 0.025mm (0.001 in.), and record. Weigh the specimen on a balanceto the nearest 0.0001 g and record the weight. If the sample hasnot been prepared with parallel sides, take additional measure-ments to determine the
18、 surface area being removed. Section 6.4has additional comments on moisture content and chemicalreactions of some chars with atmospheric constituents.6.1.2 Take care to prevent breaking off any of the charduring the measuring or weighing operations.6.2 Machining:6.2.1 If the char-virgin material int
19、erface is not parallel tothe front surface, then machine the latter parallel to theinterface.6.2.2 It is preferable to have a grinding machine, or lathe setup specifically for this machining operation so as to leave thesettings untouched between weighings. For a surface grindingmachine, advance the
20、grinding wheel dressed flat to 60.0127mm (0.0005 in.) in 0.025-mm (0.001-in.) increments until asuitable amount has been removed. This is usually about 0.254mm (0.010 in.). After the surface has been cleaned up (whenthe machining process has resulted in a continuous surface allover the specimen) all
21、ow the specimen to stabilize at theselected relative humidity and temperature conditions beforemeasuring and weighing. For consistency, remove the machingdust after each machining operation with about 34 kPa (5 psi)air through a nozzle held about 25 mm (1 in.) from themachined surface.6.3 Dimensiona
22、l Measurements:6.3.1 The increment of material removed between weigh-ings is usually 0.254 mm (0.010 in.) depending upon the totalchar thickness and the uniformity of the charred specimen. Ifthe char is relatively uniform in density down to the char-virginmaterial interface, increase the machining i
23、ncrements. If thechar-virgin material interface is sharply defined, take smallermachining increments. Exercise extreme care when making thedimensional measurements with a micrometer to avoid crush-ing the char.6.4 Weight Measurements:6.4.1 Maintain the specimen at constant humidity condi-tions (idea
24、lly dry) during the machining, measuring, andweighing operation. A freshly charred ablator has a chemicallyactive surface like that of activated charcoal. Previous experi-ence indicates equilibrating the specimen to 50 % relativehumidity and 20C (70F) dry should yield good results.2Make a separate m
25、easurement on the char to determine howmuch moisture is contained at this relative humidity.6.4.2 Some chars have chemically active species which areunstable in atmospheric conditions. Special handling tech-niques would be required for these chars and this is beyond thescope of this test method.6.4.
26、3 Repeatedly machine, measure, and weigh the speci-men until the char and the char-virgin material interface hasbeen removed, and the density approaches the virgin materialdensity.7. Calculation7.1 Determine the average density of the material removedusing the weight and dimensional changes as follo
27、ws:r5W12 W2!/AH12 H2! (1)where:r = average density, kg/m3,W1= weight before machining, kg,W2= weight after machining, kg,A = surface area of specimen face, m2,H1= height of specimen before machining, m, andH2= height of specimen after machining, m.2Liston, E. M., “Experimental and Analytical Studies
28、 of Radiation-OnlyPyrolysis of Model Char-Forming Polymers”, Contract NAS-7-341, SRI Report No.19, Final Report 1968.FIG. 1 Typical Specimen for Measuring Char Density ProfileE 471 96 (2002)28. Report8.1 Report the density calculated for each machining step atthe point mid-way between the height mea
29、surement for eachincrement removed. Report the density on a moisture-free basisusing the measured relative humidity and temperature condi-tions taken when weighing the specimen. Report the averagerelative humidity and temperature conditions. If the moisturecontent of the char has not been determined
30、 as a function ofrelative humidity and temperature then report these conditionsand the time of equilibration at these conditions beforeweighing.9. Precision and Bias9.1 The precision resulting from the machining and weigh-ing technique described here is inversely proportional to theheight removed be
31、tween weighings and the accuracy of themeasuring device used to determine the dimensions betweenweighings.An error of less than 3 % can be achieved where theheight removed is on the order of 0.254 mm (0.010 in.) and thesample is not smaller than 12.7 mm (0.5 in.) in length andwidth, or diameter. The
32、 precision is also dependent upon themoisture content of the char and the variation in relativehumidity and temperature during machining and weighing.39.2 The bias of this method is dependent upon the friabilityof the sample, the density gradient, the size of the specimen,the uniformity of the char,
33、 the moisture content of the char, andthe variation in relative humidity.10. Keywords10.1 ablation; char density; density profileASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard ar
34、e expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not
35、revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which yo
36、u may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United
37、 States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).3Hiester, N. K., and Clark, C. F., “Comparative Evaluation of Ablating Materialsin Arc Plasma Jets,” NASA CR-1207, 1968.E 471 96 (2002)3
