ASTM E471-96
(Test Method)Standard Test Method for Obtaining Char Density Profile of Ablative Materials by Machining and Weighing
Standard Test Method for Obtaining Char Density Profile of Ablative Materials by Machining and Weighing
SCOPE
1.1 This test method covers the determination of the char density profile of a charred ablator that can be used with the following limitations:
1.1.1 The local surface imperfections must be removed, and the char must be able to be machined off in a plane parallel to the char-virgin material interface before the density profiles can be determined.
1.1.2 The char must be strong enough to withstand the machining and handling techniques employed.
1.1.3 The material should have orderly density variations. The total thickness of the char and degradation zone must be larger than the machining thicknesses required.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 471 – 96
Standard Test Method for
Obtaining Char Density Profile of Ablative Materials by
Machining and Weighing
This standard is issued under the fixed designation E 471; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The ablation characteristics of charring materials must be well known in order to design the material
for a specific set of environmental conditions. The char density profile and the environmental
conditions 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 in the
following 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 profile
measurement described in this method is applicable if the char is strong enough to be machined
without breaking.
1. Scope g, and a machining technique capable of removing material in
increments as small as 0.025 mm (0.001 in.). For example, flat
1.1 This test method covers the determination of the char
specimens can be machined with a surface grinder using a
density profile of a charred ablator that can be used with the
medium fine grit ceramic grinding wheel of a soft grade
following limitations:
dressed to the proper contour. Cylindrical specimens can be
1.1.1 The local surface imperfections must be removed, and
mounted in a lathe and the char can be removed with a sharp
the char must be able to be machined off in a plane parallel to
carbide or diamond tip tool.
the char-virgin material interface before the density profiles
can be determined.
4. Sampling
1.1.2 The char must be strong enough to withstand the
4.1 The charred sample selected for machining and weigh-
machining and handling techniques employed.
ing should be taken from a representative section of the ablated
1.1.3 The material should have orderly density variations.
specimen where the environmental conditions are well known,
The total thickness of the char and degradation zone must be
and where the surface is parallel to the char-virgin material
larger than the machining thicknesses required.
interface. Where large sections are available, this condition is
1.2 This standard does not purport to address all of the
usually met. For small samples which have been exposed to
safety concerns, if any, associated with its use. It is the
varying environmental conditions along the length of the
responsibility of the user of this standard to establish appro-
sample, the sample size will be smaller.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
5. Test Specimens
2. Summary of Test Method 5.1 A typical specimen size obtained from a channel, pipe
flow, or rocket motor section may be 12.7 by 12.7 mm (0.5 by
2.1 Density variation throughout a charred ablator material
0.5 in.), or 12.7 mm (0.5 in.) in diameter. The sample thickness
is determined by successively measuring, machining, and
is determined by the available thickness of material. Smaller or
weighing a sample of known size to obtain the density of the
larger samples can be used depending upon the accuracy,
material removed by machining.
weighing apparatus, and specimen size. Larger samples reduce
3. Apparatus edge effects. Excess virgin material should be eliminated, if
possible.
3.1 The apparatus required for this method includes a
5.2 The specimen is rough-cut out of the ablated section,
laboratory balance capable of measuring to the nearest 0.0001
and then machined so as to make the sides perpendicular and
the front surface parallel to the char-virgin interface as shown
This test method is under the jurisdiction of ASTM Committee E-21 on Space
in Fig. 1.
Simulation and Applications of Space Technology and is the direct responsibility of
5.3 For simplicity and ease of handling, the base of the
Subcommittee E21.08 on Thermal Protection.
Current edition approved Oct. 10, 1996. Published December 1996. Originally
sample (the backface or side opposite the charred surface) may
published as E 471 – 73. Last previous edition E 471 – 81 (1992).
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 471
FIG. 1 Typical Specimen for Measuring Char Density Profile
be cemented to a thin piece of steel. This permits the sample to 6.4 Weight Measurements:
be handled with tweezers and facilitates the mounting of the 6.4.1 Maintain the specimen at constant humidity condi-
sample on a magnetic chuck or other mounting jig for tions (ideally dry) during the machining, measuring, and
machining the surface from a fixed reference plane. weighing operation. A freshly charred ablator has a chemically
active surface like that of activated charcoal. Previous experi-
6. Procedure
ence indicates equilibrating the specimen to 50 % relative
6.1 Sample Preparation:
humidity and 20 C (70 F) dry should yield good results. Make
6.1.1 Take the dimensions of the specimen with microme-
a separate measurement on the char to determine how much
ters or other suitable measuring devices to the nearest 0.025
moisture is contained at this relative humidity.
mm (0.001 in.), and record. Weigh the specimen on a balance
6.4.2 Some chars have chemically active species which are
to the nearest 0
...
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