What void content means in materials testing and why it matters

The short answer

Void content is the volume percentage of unfilled space inside a material or finished part. In materials testing, the term is most often used for fiber-reinforced composites, reinforced plastics, cast materials, ceramics, foams and additively manufactured components. A low value often points to better consolidation or densification, but the meaning depends on the material system and the part’s intended function. Unintended voids in a carbon fiber laminate, for example, can reduce fatigue resistance and create pathways for moisture. In a filter, biomedical scaffold or foam, designed pores may be essential to performance.

For that reason, the useful question is not only how much void content is present. Laboratories and manufacturers also need to know where the voids are, whether they are open or closed, how large they are, and whether the selected measurement method is appropriate for the material.

For related materials evaluation topics, visit the Testing section.

What void content means in different material systems

In its simplest form, void content is the ratio of void volume to total volume, usually reported as a percentage. If a specimen has 2% void content, the reported interpretation is that about 2% of its bulk volume is not occupied by the intended solid constituents. In composite testing, this percentage is commonly calculated by comparing measured density with theoretical density. In other applications, it may be estimated by microscopy, image analysis, X-ray computed tomography, gas pycnometry or liquid displacement methods.

Void content is closely related to porosity, but the terms are not always used in the same way. Porosity is a broader materials science term for empty space within a material. Void content is often used in manufacturing quality control, especially when the voids are defects created during processing. Void ratio is different again: it compares void volume with the volume of solids rather than with total volume. Mixing up these terms can lead to serious reporting errors, particularly when data are compared across composites, concrete, ceramics, powders and geotechnical materials.

In fiber-reinforced polymer composites, voids may occur between plies, within fiber bundles, around fibers, near resin-rich regions or close to tool surfaces. In powder metallurgy and additive manufacturing, voids may appear as lack-of-fusion pores, gas pores, keyhole pores or shrinkage-related defects. In concrete and cementitious materials, air voids may be intentional, harmful or neutral depending on size, spacing and exposure condition. In engineered porous materials, the objective may be controlled porosity rather than minimum void content.

Why void content affects performance

Standards and technical literature generally treat unintended voids as a quality concern because they interrupt load paths and change how moisture, heat, gases or fluids move through a material. ASTM D2734-23, which covers void content test methods for reinforced plastics, states that higher void contents usually correspond to lower fatigue resistance, greater susceptibility to water penetration and weathering, and greater scatter in strength properties. ASTM D3171-22, which addresses constituent content of composite materials, makes a similar point about the effect of void volume on composite quality and mechanical behavior.

The mechanism is easy to understand. A void is a local absence of load-bearing material. Under tension, compression, bending, shear or fatigue, the surrounding matrix or interface must carry stress around that empty region. This can create stress concentration, encourage crack initiation or accelerate delamination. The effect can be more severe when voids are elongated, clustered, located near ply interfaces or connected into moisture pathways.

Void content alone, however, is not a complete performance predictor. A 1% void content made up of many tiny, isolated pores may behave differently from a 1% void content made up of a few long interlaminar voids. A review published in the Journal of Composite Materials on voids in fiber-reinforced polymer composites emphasized that void formation, morphology and mechanical effects depend on reinforcement architecture, matrix chemistry, process route and target property. A useful test report should therefore include the method, specimen location, density inputs, imaging resolution where applicable, and any assumptions used in the calculation.

How laboratories measure void content

No single method fits every material. The right choice depends on whether the specimen can be destroyed, whether its constituents are known, whether the pores are open or closed, and how much spatial information is needed. A density-based calculation can be efficient for production checks. Computed tomography can show where voids are located. Microscopy can reveal morphology, but unless the sampling plan is carefully designed, it may represent only a cut plane.

Density and theoretical density methods

For reinforced plastics and many composite coupons, the classic approach is to measure the actual density of the specimen, calculate the theoretical void-free density from constituent weight fractions and constituent densities, and compare the two values. ASTM D2734-23 is commonly associated with this approach for reinforced plastics when the effects of ignition on the materials are known. ASTM D792-20 is often used for density and specific gravity measurement of solid plastics by displacement, while constituent content may be determined through ignition, digestion or related procedures depending on the material system.

The basic relationship is often expressed as: void content (%) = 100 × (theoretical density − measured density) ÷ theoretical density. When weight percentages are used, theoretical density for a two-constituent composite can be calculated from the resin and reinforcement weight percentages and their individual densities. The calculation becomes more complex when fillers, hybrid fibers, coatings, high-temperature residues or moisture-sensitive constituents are present.

Ignition, digestion and constituent content

ASTM D3171-22 covers test methods for determining constituent content of composite materials. Its first method can physically remove the matrix by digestion, ignition or carbonization under specified conditions, leaving the reinforcement for calculation of reinforcement content, matrix content and, where applicable, void volume. The method is mainly intended for two-part composite systems, although special provisions may be needed for filled systems. The standard also notes that a calculation-only approach based on fiber areal weight does not measure void volume.

That limitation matters in practice. If a matrix does not burn off completely, if reinforcement changes mass during heating, or if fillers gain or lose weight, an ignition-based method can misstate resin content and therefore void content. Silicone resins, metal reinforcements, some organic reinforcements and reactive fillers require extra care. In these cases, laboratories should verify whether the procedure is valid for the actual material rather than simply applying a familiar composite test method.

Microscopy and image analysis

Polished cross-sections can reveal void size, shape, clustering and location. This is especially useful when a density value is not enough to explain a failure mode. For example, a laminate may show acceptable average void content but still contain void-rich layers caused by poor resin flow, entrapped air or inadequate compaction. Optical microscopy and scanning electron microscopy can support root-cause analysis, but they are destructive and local. Results depend heavily on section location, polishing quality, contrast, thresholding and statistical sampling.

X-ray computed tomography

X-ray computed tomography, often called CT or micro-CT at smaller scales, is useful when internal void distribution must be examined without cutting the part. ASTM E1570-19(2026) provides practice guidance for fan-beam CT examination, while ASTM E1441-19(2026) gives broader guidance on CT fundamentals and performance parameters. CT can provide three-dimensional information about pore volume, shape and location, but detection depends on resolution, contrast between phases, artifact control and the density or thickness of the part. A CT result should therefore state voxel size, thresholding approach and the minimum feature size that can be reliably detected. See also: Application.

Method comparison for practical testing

What the numbers can and cannot tell you

A reported void content value is meaningful only when the test method and assumptions are visible. A single number without method details can be misleading. Density-based methods, for example, provide a bulk estimate, but they do not show whether voids are clustered at a bond line. Microscopy can show morphology, but a small number of sections may miss a defect-rich region. CT can provide volume mapping, but pores below the detection limit may not be counted.

There is no universal acceptable void content for all materials. Aerospace laminate specifications, marine composite guidelines, automotive parts, wind blade laminates, printed polymer composites and structural repair coupons may each use different acceptance criteria. Some criteria are based on historical process capability; others are linked to mechanical qualification. A specification should define the material, process, specimen location, test method, reporting units and acceptance limits. Without that context, a void content value is a data point, not a pass-or-fail conclusion.

Another common mistake is comparing values from different methods as if they were interchangeable. CT porosity, polished-section area fraction and density-derived void volume can differ because each method observes the material in a different way. A practical program may use one method for routine screening and another for root-cause investigation. Consistency usually matters more than using the most sophisticated method for every sample.

Common causes of excessive void content

Void formation is usually linked to processing, material handling or design details. In liquid composite molding, resin flow imbalance, dry spots, air entrapment, fiber preform compaction and vacuum quality can all affect porosity. In prepreg laminates, trapped air, inadequate debulking, poor cure pressure, resin viscosity and volatile removal can matter. In compression molding and thermoplastic consolidation, temperature, pressure, dwell time and cooling rate influence whether voids close or remain trapped.

For additively manufactured polymers and composites, voids may result from bead geometry, insufficient interlayer bonding, poor filament impregnation, moisture, print temperature, raster strategy or inadequate post-consolidation. For metal additive manufacturing, porosity mechanisms are different and may include trapped gas, incomplete melting, keyhole instability or powder contamination. Although the word void is shared, the root cause and test method must match the process.

Material storage is another overlooked factor. Moisture uptake in fibers, powders or resins can create volatiles during heating. Incorrect mixing, poor degassing and expired or contaminated material can also contribute to voids. The best control strategy is preventive: stable materials, controlled environment, validated process windows and regular verification testing.

How to build a reliable void content test plan

A useful test plan starts with the decision that needs to be made. If the goal is incoming material comparison, a repeatable density-based method may be enough. If the goal is failure analysis, microscopy or CT may be necessary. If the goal is production release, the method should be tied to a specification and sampling plan rather than chosen after results are known.

• Define the material system. List matrix, reinforcement, fillers, coatings and expected density values.
• Select the method before testing. Choose a standard or validated internal procedure that fits the material.
• Control specimen location. Record whether samples come from edges, center, bond lines, thick sections or known risk areas.
• Report assumptions. Include density inputs, weight fractions, temperature conditions, image thresholding and detection limits where relevant.
• Separate screening from diagnosis. Use routine methods for process monitoring and detailed methods for root-cause analysis.
• Avoid overgeneralization. Do not treat a single coupon result as proof of full-part uniformity unless the sampling plan supports it.

For new materials, hybrid composites and complex printed parts, method validation is especially important. A standard developed for reinforced plastics may not apply cleanly to a filled thermoplastic, ceramic matrix composite or multi-material lattice. When the chemistry or architecture is unusual, laboratories should verify that the test method measures the intended feature rather than an artifact of preparation, heating, fluid absorption or image segmentation.

Frequently asked questions

Is lower void content always better?

Not always. For structural composites and dense engineering parts, unintended voids are usually undesirable. For foams, filters, acoustic materials, scaffolds and some thermal management structures, controlled porosity can be part of the design. The key question is whether the voids are intended, controlled and compatible with the performance requirement.

Can density testing locate where voids are inside a part?

No. Density testing can estimate bulk void content, but it does not show location, shape or clustering. If spatial information is needed, microscopy, CT or another imaging method should be considered.

Why can two laboratories report different void content values?

Differences can come from specimen location, drying condition, constituent density inputs, balance precision, immersion procedure, ignition behavior, image resolution or thresholding. A good comparison requires the same method, similar sampling and transparent reporting.

Which standards are most relevant to composite void content?

For reinforced plastics, ASTM D2734-23 is a key void content reference. ASTM D3171-22 is important for constituent content of composite materials, and ASTM D792-20 is widely used for density and specific gravity of solid plastics by displacement. ISO 7822:1990 addresses void content determination for textile glass reinforced plastics. CT-related references such as ASTM E1570-19(2026) and ASTM E1441-19(2026) are relevant when non-destructive internal imaging is required.

What should a void content report include?

At minimum, it should include the material description, specimen location, test method, measured density or imaging parameters, theoretical density inputs if used, calculation method, number of specimens and any deviations or limitations. Without these details, the result is difficult to reproduce or compare.