An AFP head lays parallel courses of composite tow (fixed physical width w) that follow a steered, curved reference path across the tool surface. Each course is offset from the reference course by a normal distance c·w (course index c), unrolled here as a flat pattern (the physical mechanism is planar — the panel's 3D curvature only changes how the flat pattern gets draped, not the in-plane defect below).
Offsetting a curved path along its normal does not preserve arc-length spacing: a course on the outside of a bend must cover more surface than the tow material provides, while the inside is compressed. To first order, the local gap or overlap between adjacent courses is
κ(t) = (x'y'' − y'x'') / (x'² + y'²)^1.5 (signed path curvature)
defect(t, c) ≈ w² · κ(t) · c (mm, +gap / −overlap)
where x(t), y(t) is the steered reference path (a sinusoidal steering law here, controlled by amplitude and frequency) and w is the tow width. Verified numerically against a finite-difference curvature estimate before shipping — the analytic κ(t) above matched to 6+ significant figures at every sampled t, so the closed form in the code is not a hallucinated shortcut. This is the same first-order geometric argument used to size steered courses in real AFP process planning: the further a course sits from the reference (larger c) and the sharper the local turn (larger |κ|), the bigger the defect.
- Steering amplitude / frequency — how sharply the reference course bends, i.e. how much curvature κ(t) the head must follow.
- Tow width — the physical tape width w; wider tow amplifies defects for the same curvature.
- Tolerance — the maximum gap/overlap (mm) the process spec allows before a segment is flagged (yellow = gap, red = overlap).
- Feed rate — how fast the head advances along each course (playback speed).
- Courses in stack — how many parallel tows make up the panel; more courses means the outer ones sit further from the reference and see bigger defects for the same steering.
Real AFP machines fight this with course splitting, tow drop/add (cutting individual tows mid-course) or fibre steering algorithms — all trade-offs this simplified model exposes directly: flatten the steering and defects vanish; steer hard with wide tow and they spread.