ASTM D2734 - 23

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Designation: D2734 −23
Standard Test Methods for
Void Content of Reinforced Plastics
1
This standard is issued under the ﬁxed designation D2734; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 These test methods cover the void content of reinforced
plastics or “composites.” The test methods are applicable to
composites for which the effects of ignition on the materials are
known. Most plastics, glass, and reinforcements fall into this
class. These test methods are not applicable to composites for
which the effects of ignition on the plastics, the reinforcement,
and any ﬁllers are unknown. This class may include silicone
resins, which do not burn off completely, reinforcements
consisting of metals, organic materials, or inorganic materials
which may gain or lose weight, and ﬁllers consisting of oxides,
carbonates, etc., which may gain or lose weight. Note that
separate weight loss tests of individual materials will usually,
but not necessarily, give the same result as when all the
materials are combined.
NOTE 1—There is no known ISO equivalent to these test methods.
1.2 The values stated in SI units are to be regarded as
standard.
1.3 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 appro-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
2
D618 Practice for Conditioning Plastics for Testing
D792 Test Methods for Density and Speciﬁc Gravity (Rela-
tive Density) of Plastics by Displacement
D883 Terminology Relating to Plastics
D1505 Test Method for Density of Plastics by the Density-
Gradient Technique
D2584 Test Method for Ignition Loss of Cured Reinforced
Resins
E456 Terminology Relating to Quality and Statistics
3. Terminology
3.1 Deﬁnitions—Terms used in this standard are deﬁned in
accordance with Terminology D883, unless otherwise speci-
ﬁed. For terms relating to precision and bias and associated
issues, the terms used in this standard are deﬁned in accordance
with Terminology E456.
4. Summary of Test Methods
4.1 The densities of the resin, the reinforcement, and the
composites are measured separately. Then the resin content is
measured and a theoretical composite density calculated. This
is compared to the measured composite density. The difference
in densities indicates the void content. A good composite may
have 1 % voids or less, while a poorly made composite can
have a much higher void content. Finite values under 1 %
should be recognized as representing a laminate density
quality, but true void content level must be established by
complementary tests or background experience, or both.
5. Signiﬁcance and Use
5.1 The void content of a composite may signiﬁcantly affect
some of its mechanical properties. Higher void contents
usually mean lower fatigue resistance, greater susceptibility to
water penetration and weathering, and increased variation or
scatter in strength properties. The knowledge of void content is
desirable for estimation of quality of composites.
6. Interferences
6.1 The density of the resin, in these test methods, is
assumed to be the same in the composite as it is in a large cast
mass. Although there is no realistic way to avoid this
assumption, it is nevertheless not strictly correct. Differences in
curing, heat and pressure, and molecular forces from the
1
These test methods are under the jurisdiction of ASTM Committee D20 on
Plastics and are the direct responsibility of Subcommittee D20.18 on Reinforced
Thermosetting Plastics.
Current edition approved Oct. 1, 2023. Published November 2023. Originally
approved in 1968. Last previous edition approved in 2016 as D2734 – 16. DOI:
10.1520/D2734-23.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
1

reinforcement surface all change the composite resin density
from the bulk resin density. The usual change is that bulk
density is lower, making void content seem lower than it really
is.
6.2 For composites with high void contents, this error will
lower the true value an insigniﬁcant amount, from a true 7 %
down to a calculated 6.7 %, for example. For composites with
low and void contents, the value may be lowered from a true
0.2 % to a calculated − 0.1 %. This would indicate an obvious
error, and illustrates that as the void content gets lower the
constant error in resin density gets progressively more impor-
tant. Note that these values are for example only, that different
resin systems can give different errors, and that it is left to the
individual tester to determine the accuracy of the calculated
result in his particular measurement.
6.3 For the special case of semi-crystalline plastics, such as
polyphenylene sulﬁde (PPS) and polyetheretherketone
(PEEK), an interference due to the level of crystallinity present
in the composite can cause signiﬁcant variation in the mea-
surement of void content by this test method. The level of
crystallinity can be affected by a variety of circumstances,
including the molding conditions. For these polymers, the
density used in the calculation must be the actual density of the
resin in the composite.
NOTE 2—The actual degree of crystallinity of the composite can be
measured by techniques such as differential scanning calorimetry (DSC)
or by X-ray diffraction.
7. Conditioning
7.1 Conditioning—Condition the test specimens at 23 6
2°C (73.4 63.6°F) and 50 610 % relative humidity for not
less than 40 h prior to test in accordance with Procedure A of
Practice D618, for those tests where conditioning is required.
In cases of disagreement, the tolerances shall be 1°C (1.8°F)
and 62 % relative humidity.
7.2 Test Conditions—Conduct tests in the standard labora-
tory atmosphere of 23 62°C (73.4 63.6°F) and 50 65 %
relative humidity, unless otherwise speciﬁed in the test meth-
ods. In cases of disagreement, the tolerances shall be 1°C
(1.8°F) and 65 % relative humidity.
8. Procedure
8.1 Density of the Resin and the Composite—Three test
methods are presented for these measurements. Measure the
density on pieces of resin that are bubble-free and that were
cured under heat, time, and pressure conditions that are as close
as practicable to the conditions under which the composite was
cured. Density measurements supplied by the resin manufac-
turer are acceptable if they are certiﬁed for each batch.
8.1.1 Test Method A—Measure densities using Test Methods
D792. Paragraph 1.1 of that test method requires the specimen
to have smooth edges and surfaces. For composites, this
requirement may necessitate hand sanding the specimen with
400-grit emery paper to remove fuzzy edges caused by cutting.
The specimen should be as free as possible from geometric
irregularities which tend to trap air bubbles. Remove any
bubbles by a wire, or other mechanical means. Do not use a
vacuum to remove bubbles, because cut surfaces of composites
may be porous and exposure to a vacuum will force water into
the pores, causing an error in the density measurement. In some
cases of extreme porosity, just immersing the composite in
water, without using a vacuum, will allow an unacceptably
large amount of water take-up by the composite. In these cases,
seal the porous surfaces by coating them with a known amount
of sealer of known density. This will require weighing the
specimen, then spraying-on or wiping-on the sealer, then
reweighing. A correction for this added material will then be
required in the calculations.
8.1.2 Test Method B—Measure densities in accordance with
Test Method D1505. The comments in 8.1.1 concerning fuzzy
edges and removal of bubbles apply here also.
8.1.3 Test Method C:
8.1.3.1 Densities calculated from weight and volume mea-
surements are acceptable if the specimens are smooth, uniform,
and of such shape that the volume can be calculated accurately
from the dimensions.
8.1.3.2 Procedure—The volume of each specimen shall not
be less than 2 cm
3
(0.125 in.
3
). Make dimensional measure-
ments with a micrometer at all edges (12 in all for a 6-sided
rectangular block). Use the averages for each dimension to
calculate the volume.
8.1.3.3 The tolerance on the accuracy of the micrometer
measurements shall be 60.0013 cm (60.0005 in.). With
maximum tolerance buildup on a small sample, this could
result in an error in the calculated volume of 0.6 %. For larger
samples, and with some measurements being in error on the
plus side and some on the minus side, the error in the
calculated volume should not exceed 0.2 %.
8.1.3.4 Calculate the density by dividing the weight by the
volume; express as grams per cubic centimeter.
8.2 Density of the Glass or Other Reinforcement—Most
glass reinforcement is E glass, which typically has a density
between 2.54 and 2.59 g/cm
3
; S glass density is 2.46 to 2.49
g/cm
3
. However, if a density determination is necessary, use
Test Methods D792. Pay particular attention to Note 11 of that
test method, which discusses removal of trapped air by
exposure of the sample to a vacuum. This step can be assumed
to be necessary in every determination of glass density. Use a
vacuum of 3 mm Hg or better. Several cycles of atmospheric
pressure-to-vacuum may be required before the trapped air is
completely removed.
NOTE 3—It is suggested that the density of the glass supplied be veriﬁed
with the glass ﬁber producer.
8.3 Resin Content of Composite—Determine in accordance
with Test Method D2584. The ignition loss in that test method
is the resin content of the sample and is to be recorded as the
weight percent as indicated.
9. Theoretical Density
9.1 Calculation—Using the values determined in 8.1,8.2,
and 8.3, calculate theoretical density of a composite as follows:
T
d
5100/
~
R/D1r/d
!
(1)
where:
T
d
= theoretical composite density,
D2734 − 23
2

R= resin in composite, weight %,

D= density of resin,

r= reinforcement in composite, weight %, and

d= density of reinforcement.

9.2 Examples:

From 8.1:

D51.230 g/cm

(2)

From 8.2:

d52.540 g/cm

(3)

From 8.3:

R528.55 weight %, (4)

r571.45 weight % (5)

5100/

28.55/1.230171.45/2.540

51.949 g/cm (6)

10. Void Content

10.1 Test Method A:

10.1.1 Calculation:

V5100

(7)

where:

V= void content, volume %,

= theoretical composite density, and

= measured composite density.

Note that dividing two densities gives an unlabeled ratio,

which in this case is the fraction of material missing. It is

equally correct to interpret this as a weight fraction or volume

fraction, but in expressing voids it is always considered to be

a volume fraction.

10.1.2 Example: From 9:

51.949 g/cm

(8)

From 8.1:

51.903 g/cm

(9)

V5100 ×

1.949 21.903

/1.949 52.36 % (10)

Note that four signiﬁcant ﬁgures have been used in these

calculations. This represents a level of accuracy achieved only

by careful work with optimum samples. For many, or perhaps

the majority of, determinations, such accuracy is not attained

and only three signiﬁcant ﬁgures are warranted.

10.2 Test Method B—Users may ﬁnd this test method more

convenient when only the void content value is wanted and the

theoretical density value is of no interest:

10.2.1 Calculation:

V5100 2M

D1r

(11)

where:

V= void content, volume %,

= measured composite density,

R= resin in composite, weight %,

r= reinforcement in composite, weight %,

D= density of resin, and

d= density of reinforcement.

10.2.2 Example—Using the same number as in 9.2 and

10.1.2 but to only three signiﬁcant ﬁgures:

V5100 21.90

28.6

1.23171.4

2.54

52.4 % (12)

11. Report

11.1 Report the following information:

11.1.1 Complete identiﬁcation of the materials tested,

11.1.2 Density of all specimens,

11.1.3 Weight fraction of resin and reinforcement, theoreti-

cal density (if calculated), and void content of all composite

specimens,

11.1.4 Method of test, and

11.1.5 Date of test.

12. Precision and Bias

12.1 This test method does not yet contain a numerical

precision and bias statement and it shall not be used as a referee

method in case of dispute. The precision and bias of this test

method are under investigation by a task group of Subcommit-

tee D20.18. Anyone wishing to participate in this may contact

the Chairman, Subcommittee D20.18, ASTM, 100 Barr Harbor

Drive, West Conshohocken, PA 19428.

13. Keywords

13.1 composites; ignition loss; plastics; reinforced plastics;

void content; weight fractions

D2734 − 23

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摘要：

ASTM D2734 - 23 是美国材料与试验协会发布的最新标准，专门用于测定增强塑料中空隙含量的试验方法。该标准通过密度测量法，计算复合材料中树脂、纤维与实测密度之间的差异，从而准确评估材料内部气泡、空洞等缺陷的比例。适用于玻璃纤维、碳纤维等增强塑料的质量控制与工艺优化，帮助工程人员判断成型工艺的完整性及材料密实度。标准涵盖试样制备、密度测试步骤、计算公式以及结果报告要求，广泛应用于航空航天、汽车制造、风电叶片及结构复合材料领域。遵循ASTM D2734-23能够提升产品可靠性与一致性，降低

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