ASTM D2734-23

Standard Test Methods for Void Content of Reinforced Plastics

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.
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 appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 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.

General Information

• Status: Published
• Publication Date: 30-Sep-2023
• ICS: 83.120 - Reinforced plastics
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.18 - Reinforced Thermosetting Plastics

Relations

• Replaces: ASTM D2734-16 - Standard Test Methods for Void Content of Reinforced Plastics
• Effective Date: 01-Oct-2023
• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM D883-22 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2022
• Refers: ASTM E456-13a(2022) - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Refers: ASTM E456-13a(2022)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Referred By: ASTM D5448/D5448M-22 - Standard Test Method for Inplane Shear Properties of Hoop Wound Polymer Matrix Composite Cylinders
• Effective Date: 01-Oct-2023
• Referred By: ASTM D7248/D7248M-23 - Standard Test Method for High Bearing - Low Bypass Interaction Response of Polymer Matrix Composite Laminates Using 2-Fastener Specimens
• Effective Date: 01-Oct-2023
• Referred By: ASTM C273/C273M-20 - Standard Test Method for Shear Properties of Sandwich Core Materials
• Effective Date: 01-Oct-2023
• Referred By: ASTM D5467/D5467M-97(2017) - Standard Test Method for Compressive Properties of Unidirectional Polymer Matrix Composite Materials Using a Sandwich Beam
• Effective Date: 01-Oct-2023
• Referred By: ASTM D6671/D6671M-22 - Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites
• Effective Date: 01-Oct-2023
• Referred By: ASTM D8387/D8387M-23 - Standard Test Method for High Bypass – Low Bearing Interaction Response of Polymer Matrix Composite Laminates
• Effective Date: 01-Oct-2023
• Referred By: ASTM D7136/D7136M-20 - Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event
• Effective Date: 01-Oct-2023
• Referred By: ASTM D3410/D3410M-16e1 - Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading
• Effective Date: 01-Oct-2023
• Referred By: ASTM D7905/D7905M-19e1 - Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites
• Effective Date: 01-Oct-2023

Overview

ASTM D2734-23 – Standard Test Methods for Void Content of Reinforced Plastics establishes procedures to determine the void content in reinforced plastics, also known as composites. Developed by ASTM International, this standard is essential for assessing the quality and mechanical performance of composites by quantifying the amount of voids present. High void content can negatively impact fatigue resistance, increase water and weathering susceptibility, and introduce larger variability in mechanical strength. Understanding void content is a critical step in the quality assurance process for manufacturers and users of composite materials.

Key Topics

• Applicability:
  The test methods are designed for reinforced plastics and composites where the effects of ignition on all constituent materials are known. Most standard plastics, glass, and common reinforcements are within scope. Composites with unknown ignition effects, such as those containing silicone resins, certain fillers, and some metallic reinforcements, are excluded.

• Measurement Procedures:

  • Determination and comparison of densities of resin, reinforcement, and composite samples.
  • Calculation and reporting of theoretical and measured composite densities.
  • Estimation of void content as a percentage by comparing theoretical density (based on resin and reinforcement content) with the measured density.

• Influence of Voids:
  Voids in composites may reduce fatigue strength, increase water ingress, and cause variation in measured mechanical properties, making low void content desirable for high-quality composites.

• Interferences and Limitations:
  The standard acknowledges assumptions in resin density, possible errors due to porosity, and special considerations for semi-crystalline plastics such as PPS and PEEK, where crystallinity can affect density measurement and, consequently, void content calculation.

Applications

ASTM D2734-23 is broadly applied across industries utilizing reinforced plastics and composite materials, including:

• Aerospace and Automotive:
  Ensuring structural composite components meet safety, durability, and performance standards by minimizing void content.

• Construction:
  Evaluating reinforced polymer components such as panels and beams for suitability in moisture-sensitive and load-bearing applications.

• Manufacturing and Quality Control:

  • Used in process validation and routine quality assessments.
  • Assists in diagnosing issues with composite production and establishing benchmarks for product consistency.

• Research and Development:

  • Aids materials scientists in evaluating new composite formulations.
  • Provides a method for comparing the effectiveness of different manufacturing techniques in reducing void content.

Related Standards

The following ASTM standards are commonly referenced alongside ASTM D2734-23:

• ASTM D618 – Practice for Conditioning Plastics for Testing: Ensures testing under controlled environmental conditions.
• ASTM D792 – Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement: Used for density measurements of both resins and composites.
• ASTM D883 – Terminology Relating to Plastics: Supplies standardized definitions for terms used in plastics testing.
• ASTM D1505 – Test Method for Density of Plastics by the Density-Gradient Technique: An alternative density measurement method.
• ASTM D2584 – Test Method for Ignition Loss of Cured Reinforced Resins: Used to determine resin content via ignition loss.
• ASTM E456 – Terminology Relating to Quality and Statistics: Ensures clarity in reporting and statistical evaluation.

Note: There is currently no ISO equivalent to ASTM D2734-23. All users must observe appropriate safety, health, and environmental requirements while applying this standard and reference relevant regulatory constraints.

---

By following ASTM D2734-23, organizations can ensure reliable, consistent measurement of void content in reinforced plastics, supporting high standards of quality and performance in composite applications. For up-to-date and detailed procedures, always consult the official ASTM documentation.

Standards Content (Sample)

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.
Designation: D2734 − 23
Standard Test Methods for
Void Content of Reinforced Plastics
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* D792 Test Methods for Density and Specific Gravity (Rela-
tive Density) of Plastics by Displacement
1.1 These test methods cover the void content of reinforced
D883 Terminology Relating to Plastics
plastics or “composites.” The test methods are applicable to
D1505 Test Method for Density of Plastics by the Density-
composites for which the effects of ignition on the materials are
Gradient Technique
known. Most plastics, glass, and reinforcements fall into this
D2584 Test Method for Ignition Loss of Cured Reinforced
class. These test methods are not applicable to composites for
Resins
which the effects of ignition on the plastics, the reinforcement,
E456 Terminology Relating to Quality and Statistics
and any fillers are unknown. This class may include silicone
resins, which do not burn off completely, reinforcements
3. Terminology
consisting of metals, organic materials, or inorganic materials
which may gain or lose weight, and fillers consisting of oxides, 3.1 Definitions—Terms used in this standard are defined in
accordance with Terminology D883, unless otherwise speci-
carbonates, etc., which may gain or lose weight. Note that
separate weight loss tests of individual materials will usually, fied. For terms relating to precision and bias and associated
issues, the terms used in this standard are defined in accordance
but not necessarily, give the same result as when all the
materials are combined. with Terminology E456.
NOTE 1—There is no known ISO equivalent to these test methods.
4. Summary of Test Methods
1.2 The values stated in SI units are to be regarded as
4.1 The densities of the resin, the reinforcement, and the
standard.
composites are measured separately. Then the resin content is
1.3 This standard does not purport to address all of the
measured and a theoretical composite density calculated. This
safety concerns, if any, associated with its use. It is the
is compared to the measured composite density. The difference
responsibility of the user of this standard to establish appro-
in densities indicates the void content. A good composite may
priate safety, health, and environmental practices and deter-
have 1 % voids or less, while a poorly made composite can
mine the applicability of regulatory limitations prior to use.
have a much higher void content. Finite values under 1 %
1.4 This international standard was developed in accor-
should be recognized as representing a laminate density
dance with internationally recognized principles on standard-
quality, but true void content level must be established by
ization established in the Decision on Principles for the
complementary tests or background experience, or both.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
5. Significance and Use
Barriers to Trade (TBT) Committee.
5.1 The void content of a composite may significantly affect
2. Referenced Documents
some of its mechanical properties. Higher void contents
usually mean lower fatigue resistance, greater susceptibility to
2.1 ASTM Standards:
water penetration and weathering, and increased variation or
D618 Practice for Conditioning Plastics for Testing
scatter in strength properties. The knowledge of void content is
1 desirable for estimation of quality of composites.
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.
6. Interferences
Current edition approved Oct. 1, 2023. Published November 2023. Originally
6.1 The density of the resin, in these test methods, is
approved in 1968. Last previous edition approved in 2016 as D2734 – 16. DOI:
10.1520/D2734-23.
assumed to be the same in the composite as it is in a large cast
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
mass. Although there is no realistic way to avoid this
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
assumption, it is nevertheless not strictly correct. Differences in
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. curing, heat and pressure, and molecular forces from the
*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
D2734 − 23
reinforcement surface all change the composite resin density may be porous and exposure to a vacuum will force water into
from the bulk resin density. The usual change is that bulk the pores, causing an error in the density measurement. In some
density is lower, making void content seem lower than it really cases of extreme porosity, just immersing the composite in
is. water, without using a vacuum, will allow an unacceptably
large amount of water take-up by the composite. In these cases,
6.2 For composites with high void contents, this error will
seal the porous surfaces by coating them with a known amount
lower the true value an insignificant amount, from a true 7 %
of sealer of known density. This will require weighing the
down to a calculated 6.7 %, for example. For composites with
specimen, then spraying-on or wiping-on the sealer, then
low and void contents, the value may be lowered from a true
reweighing. A correction for this added material will then be
0.2 % to a calculated − 0.1 %. This would indicate an obvious
required in the calculations.
error, and illustrates that as the void content gets lower the
8.1.2 Test Method B—Measure densities in accordance with
constant error in resin density gets progressively more impor-
Test Method D1505. The comments in 8.1.1 concerning fuzzy
tant. Note that these values are for example only, that different
edges and removal of bubbles apply here also.
resin systems can give different errors, and that it is left to the
8.1.3 Test Method C:
individual tester to determine the accuracy of the calculated
8.1.3.1 Densities calculated from weight and volume mea-
result in his particular measurement.
surements are acceptable if the specimens are smooth, uniform,
6.3 For the special case of semi-crystalline plastics, such as
and of such shape that the volume can be calculated accurately
polyphenylene sulfide (PPS) and polyetheretherketone
from the dimensions.
(PEEK), an interference due to the level of crystallinity present
8.1.3.2 Procedure—The volume of each specimen shall not
in the composite can cause significant variation in the mea- 3 3
be less than 2 cm (0.125 in. ). Make dimensional measure-
surement of void content by this test method. The level of
ments with a micrometer at all edges (12 in all for a 6-sided
crystallinity can be affected by a variety of circumstances,
rectangular block). Use the averages for each dimension to
including the molding conditions. For these polymers, the
calculate the volume.
density used in the calculation must be the actual density of the
8.1.3.3 The tolerance on the accuracy of the micrometer
resin in the composite.
measurements shall be 60.0013 cm (60.0005 in.). With
maximum tolerance buildup on a small sample, th
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D2734 − 16 D2734 − 23
Standard Test Methods for
Void Content of Reinforced Plastics
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 appropriate safety and healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
D792 Test Methods for Density and Specific Gravity (Relative 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
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 Sept. 1, 2016Oct. 1, 2023. Published September 2016November 2023. Originally approved in 1968. Last previous edition approved in 20092016
as D2734 – 09.D2734 – 16. DOI: 10.1520/D2734-16.10.1520/D2734-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. 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
D2734 − 23
3. Terminology
3.1 Definitions—Terms used in this standard are defined in accordance with Terminology D883, unless otherwise specified. 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 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 important. 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 measurement 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 6 3.6°F) and 50 6 10 % 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 laboratory atmosphere of 23 6 2°C (73.4 6 3.6°F) and 50 6 5 % relative
humidity, unless otherwise specified in the test methods. 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 manufacturer are acceptable
if they are certified for each batch.
D2734 − 23
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 pa
...