𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞 : 𝐏𝐨𝐫𝐨𝐬𝐢𝐭𝐲 𝐢𝐧 𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐞𝐬: 𝐖𝐡𝐲 𝐯𝐨𝐢𝐝 𝐜𝐨𝐧𝐭𝐞𝐧𝐭 𝐢𝐬 𝐧𝐨𝐭 𝐨𝐧𝐞 𝐧𝐮𝐦𝐛𝐞𝐫.

𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞

𝐏𝐨𝐫𝐨𝐬𝐢𝐭𝐲 𝐢𝐧 𝐜𝐨𝐦𝐩𝐨𝐬𝐢𝐭𝐞𝐬: 

𝐖𝐡𝐲 𝐯𝐨𝐢𝐝 𝐜𝐨𝐧𝐭𝐞𝐧𝐭 𝐢𝐬 𝐧𝐨𝐭 𝐨𝐧𝐞 𝐧𝐮𝐦𝐛𝐞𝐫.

A few percent porosity can turn a composite part from structurally acceptable to unusable. Not because the void volume is large, but because pores are effectively built-in crack starters.

In fiber-reinforced polymers, porosity is usually trapped air in the matrix or at fiber/matrix interfaces. The penalty appears first in 𝗺𝗮𝘁𝗿𝗶𝘅-𝗱𝗼𝗺𝗶𝗻𝗮𝘁𝗲𝗱 and 𝗶𝗻𝘁𝗲𝗿𝗳𝗮𝗰𝗲-𝗱𝗼𝗺𝗶𝗻𝗮𝘁𝗲𝗱 failure modes, not necessarily in UD tension along the fibers.

𝗪𝗵𝗲𝗿𝗲 𝗽𝗼𝗿𝗼𝘀𝗶𝘁𝘆 𝗵𝘂𝗿𝘁𝘀 𝗳𝗶𝗿𝘀𝘁

- interlaminar shear strength (ILSS)

- transverse tension and compression

- compression (microbuckling and kink-band sensitivity)

- fracture toughness and delamination

- fatigue

A recent CF/epoxy study reported approximate ILSS knockdowns of:

- ~15% at 1.0% porosity

- ~30% at 1.5%

- ~40% at 2.0%

- ~45% at 2.5%

Fatigue is especially sensitive because voids act as an initial damage population: local notches, earlier matrix cracking, interface delamination, and increased scatter.

𝗪𝗵𝘆 "<𝟭% 𝗽𝗼𝗿𝗼𝘀𝗶𝘁𝘆" 𝗯𝗲𝗰𝗮𝗺𝗲 𝗮𝗲𝗿𝗼𝘀𝗽𝗮𝗰𝗲 𝗱𝗼𝗴𝗺𝗮

The rule persists because autoclave prepreg can achieve it repeatably, many matrix-dominated knockdowns remain manageable, and certification prefers simple acceptance criteria.

But the real target depends on:

- the critical property (compression, ILSS, CAI, fatigue)

- void morphology

- what can be measured reliably

𝗧𝗵𝗲𝗿𝗺𝗼𝘀𝗲𝘁𝘀 𝘃𝘀 𝘁𝗵𝗲𝗿𝗺𝗼𝗽𝗹𝗮𝘀𝘁𝗶𝗰𝘀

Thermosets can lock in voids during gelation and outgassing. Thermoplastics shift the problem toward incomplete melt wetting and consolidation, especially in in-situ AFP and welding-type processes.

That is why in-situ thermoplastic AFP often lands around 2–3% porosity, while VBO or autoclave post-consolidation remains the common route to <1% when specifications require it.

Prof. Vasiliev once described a filament-wound structure delivered with roughly 2–3% porosity. The customer initially called it unacceptable. The structure still met performance requirements.

His point was simple: Porosity is only bad in how it moves your property distribution. If you can control it, measure it, and demonstrate acceptable margins, void content becomes a quality variable rather than a moral failure.

Do you define acceptable porosity by the number itself or by the properties the structure still has to achieve?

source : Fedor Antonov