Test to Determine the Void Content of Reinforced Plastics

What Is Void Content in Reinforced Plastics?

Void content refers to the volume fraction of air pockets, gas inclusions, or porosity present within the matrix of a fibre-reinforced plastic (FRP) composite. Voids form during the manufacturing process — typically autoclave cure, resin transfer moulding (RTM), or wet lay-up — when entrapped air or volatile reaction gases are not fully removed from the laminate before the resin gels.

Even small quantities of voids significantly degrade the mechanical properties of composite laminates, particularly interlaminar shear strength, compressive strength, and fatigue performance. Void content measurement is therefore a fundamental quality control parameter in composite manufacturing for the aerospace, automotive, marine, and wind energy industries.

Why Void Content Matters

Research has established clear quantitative relationships between void content and composite mechanical property degradation:

• Each 1% increase in void content reduces interlaminar shear strength (ILSS) by approximately 7–10%
• Compressive strength is similarly sensitive to void content, with significant reductions above 1% voids
• Fatigue life and delamination resistance decrease progressively with increasing void content
• Void content above 2–3% is generally considered the upper limit for aerospace-grade structural composites

Understanding and controlling void content allows manufacturers to verify laminate quality, identify process problems early, and ensure that delivered components meet design allowables.

Test Methods for Void Content

ASTM D2734 — Void Content of Reinforced Plastics

ASTM D2734 is the primary ASTM standard for void content determination in reinforced plastics. It calculates void content from the measured composite density and the theoretical resin and reinforcement densities:

Void Content (%) = 100 × (Theoretical Density − Measured Density) / Theoretical Density

The theoretical composite density is calculated from the known resin density, fibre density, and their respective weight fractions (from resin burn-off per ASTM D2584 or acid digestion per ASTM D3171). The measured density is determined by the Archimedes displacement method or by direct measurement of specimen mass and volume.

ASTM D3171 — Constituent Content of Composite Materials

ASTM D3171 determines fibre volume fraction, resin volume fraction, and void content by matrix digestion (acid dissolution or combustion) of the composite specimen. After matrix removal, the remaining fibres are weighed to determine fibre mass fraction, from which volume fractions and void content are calculated.

Optical Microscopy (Metallographic Image Analysis)

Cross-sections of composite specimens are prepared metallographically and examined by optical microscopy. Digital image analysis software measures the area fraction of voids in the polished cross-section as a direct, spatially resolved measurement of void distribution. This method provides information on void size, morphology, and distribution that density-based methods cannot reveal.

Ultrasonic C-Scan (Non-Destructive)

Ultrasonic C-scanning provides a non-destructive, spatially resolved void distribution map of the entire composite panel. Areas of high void content appear as regions of reduced ultrasonic signal transmission or increased attenuation. While C-scan does not provide absolute void volume fractions directly, it identifies void-rich regions requiring further characterisation.

Industrial Applications

In aerospace composite certification, void content measurement per ASTM D2734 or D3171 is mandatory for every production laminate qualification. Aerospace OEM specifications (Boeing BMS standards, Airbus AIMS) typically require void content ≤2% for structural laminates. In wind turbine blade manufacturing, void content monitoring ensures that large spar cap and shear web laminates achieve design mechanical properties. In automotive structural composites, void content data validates cure cycle parameters for high-rate RTM and press moulding processes.

Conclusion

Void content is one of the most critical quality parameters in fibre-reinforced plastic (FRP) composites because even small levels of porosity can significantly reduce structural performance. Excess voids adversely affect interlaminar shear strength, compressive strength, fatigue life, and delamination resistance, making accurate measurement essential for high-performance composite applications.

Standards such as ASTM D2734 and ASTM D3171 provide reliable methods to quantify void content and verify manufacturing quality. By controlling void levels during processes such as autoclave curing, RTM, and wet lay-up, manufacturers can ensure that composite components meet stringent mechanical and certification requirements for aerospace, automotive, marine, and wind energy applications.

Why Choose Infinita Lab for Void Content Testing?

Infinita Lab provides ASTM D2734 and D3171 void content testing, including density measurement, matrix digestion, and optical image analysis, through our nationwide accredited composite materials testing laboratory network.

Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.

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