Porosity in Spar Cap Laminate: TOFD Detection and Limits

Author: RAS Thermal Co., Ltd. · Updated 2026-10-07

TL;DR Acceptance limit: porosity cluster 3 mm diameter and under 3 % area per Table G.1. Method: TOFD reads volumetric clusters regardless of orientation, PAUT runs alongside. Distinction: the 3 mm porosity cap is tighter than the 6 mm delamination floor. Sizing: the diameter is the deciding number, not the presence of scatter.

A cap reads quiet on a planar scan, but the volumetric channel shows scattered reflections spread through the thickness. Porosity is a volumetric defect, so it does not sit in a plane and a planar scan alone can miss it. TOFD reads scattered energy from voids whatever their orientation, which is why it runs beside PAUT rather than after it. A porosity cluster capped at 3 mm sits well below the 6 mm volumetric floor, so clearing the table is not the whole job. The cluster also changes stiffness, and stiffness changes how the cap answers a fatigue load.

Common root causes

Root cause	What you see	Where it bites
Infusion voids	scattered reflections through the thickness with no single planar reflector	thick unidirectional cap sections
Out-of-window resin	small voids distributed where the resin front stalled	large laminates and slow wet-out
Entrained air at ply stacks	a concentration near a ply boundary across part of the cap	stacked plies and thick buildups
Vacuum-bag leaks	porosity spread across a whole panel rather than a point	bagged pre-preg and infusion alike

What the numbers allow

The table gives the ceiling. IEC 61400-5 Table G.1 caps a porosity cluster at 3 mm diameter and under 3 % area, which is tighter than the 6 mm floor that governs planar delamination. A cluster at 3 mm still passes, and a cluster at 3.2 mm does not, so the scan has to resolve the diameter rather than merely report a noise floor. TOFD does the resolving, and PAUT alongside it keeps the planar defects from being read as porosity.

Acceptance limit: porosity cluster 3 mm diameter and under 3 % area per Table G.1
Method: TOFD reads volumetric clusters regardless of orientation, PAUT runs alongside
Distinction: the 3 mm porosity cap is tighter than the 6 mm delamination floor
Sizing: the diameter is the deciding number, not the presence of scatter

How to confirm it

Work down this list in order. The first check that changes the result has found the cause.

Separate planar from volumetric. Confirm the planar channel has already cleared delamination, so a volumetric reading here is not a planar defect wearing the wrong label.
Set the gain to a known reference. Calibrate on the reference block before the run, because a 2 dB gain error is the difference between sizing a cluster and missing it.
Run TOFD through the thickness. Sweep the TOFD pair across the cap so scattered energy from voids is collected whatever orientation the voids hold.
Sizing the cluster diameter. Measure the cluster across its widest span and compare with 3 mm, and check the affected area against the 3 % ceiling at the same time.
Flag clusters against the serial. Record every cluster above the recording threshold with its coordinates so trend data can show whether an infusion process is drifting.

Prevention

Hold the infusion window and the vacuum integrity so the resin front never stalls inside a thick cap. Then let the scan measure what got through, because porosity that is close to the 3 mm ceiling is a process warning, not a pass. Control the process, and read the scan as the process meter it is.

These are the working numbers from our own machine and campaign records, restated rather than re-pitched.

Frequently Asked Questions

What porosity is acceptable in a spar cap?

IEC 61400-5 Table G.1 caps a porosity cluster at 3 mm diameter and under 3 % area. That is tighter than the 6 mm floor for delamination, so the cluster size is what the accept decision turns on.

Why does porosity need TOFD when PAUT is running?

Because porosity is volumetric and does not sit in a plane. TOFD reads scattered energy from voids regardless of orientation, while PAUT images planar separations, so the two methods answer different questions in the same pass.

How does a gain error affect porosity sizing?

Directly. A 2 dB gain error can mean the difference between sizing a cluster and missing it, which is why the run is calibrated on a reference block before the scan and not left to a factory default.

Can porosity in one blade warn about the process?

It can. Clusters sitting close to the 3 mm ceiling point at the infusion rather than at the blade, so logging each one against the serial turns a single inspection into a process trend.

Bottom line: Porosity is a volumetric defect read by TOFD, not a plane read by PAUT, and IEC 61400-5 Table G.1 holds the cluster to 3 mm and under 3 % area. Calibrate before the run, size the cluster across its widest span, and treat a result near the ceiling as a message about the infusion rather than a quiet pass.