Free from today. A composites handbook is free on Kindle, with three other engineering books, until Tue 10 Nov 23:59 PT. Time remaining: ends Tue 10 Nov, 11:59 pm PT.
This laminate explainer was published during a free Kindle week for four engineering books in November 2026.
A sheet of aluminium arrives with its properties already set. A carbon fibre laminate doesn't. Engineers design the material at the same time as the part, one thin layer at a time.
The ply is the building block
Each ply holds carbon fibres in a resin. Along the fibres it's very strong and stiff, and across them it depends mostly on the much weaker resin. A single ply on its own would split easily, so plies are stacked at different angles.
The standard angles are 0, plus and minus 45, and 90 degrees. Plies at 0 degrees carry axial load, the 45s handle shear, and the 90s support the transverse direction and help around holes. The proportions depend on the job. A skin dominated by bending may hold many 0-degree plies, while a shear web leans on the 45s.
Balance and symmetry
Stacking order matters as much as the angles. A laminate is usually kept symmetric, so the plies above the mid-plane mirror those below. Without that, the part can warp as it cools after curing.
It's also usually balanced, with every plus-45 ply matched by a minus-45. An unbalanced laminate can twist when it's pulled. Designers also avoid grouping too many identical plies together, because cracks can run more easily between them.
From stack to prediction
Classical laminate theory combines the properties and angles of each ply to predict how the whole stack stretches, bends and twists. Failure criteria then estimate when the plies will start to fail. These are models, so test data sets the allowable values used in design.
Making the part
Aerospace laminates are commonly laid up from pre-impregnated tape, then cured under heat and pressure, often in an autoclave. Small errors such as wrinkles, gaps or trapped air can reduce strength, which is why process control is part of the design.
For the wider differences from metal, see why carbon fibre isn't just lighter aluminium.
The four books
Practical Airframe Stress Analysis, Volume 2: Composite Structure
GD&T Professional Exam Guide
Practical Airframe Stress Analysis, Volume 1: Metallic Structure
Python for Aerospace Stress Engineers (Volume 3)
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Related gear
- Practical Airframe Stress Analysis, Volume 2: Composite Structure (Sandeep Bandyopadhyay) on Amazon — Related gear
- Aero Structures and Composite Materials with Artificial Intelligence (book) — Sandeep's book
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