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Cold bend radius that avoids crazing in acrylic

Aviation Mechanic Airframe (AMA) practice question 17 of 30 on Non-Metallic Structures. ACS element AM.II.B.K23 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONAn acrylic sheet 0.100 inch thick is to be cold-bent into a single curvature. What is the smallest bending radius that will not produce crazing?

  1. A1.8 inches, or 18 times thickness
  2. B18 inches
  3. C180 inches, the multiplier itself
Show the answer and explanation

Correct answer: B. 18 inches

  • A — incorrect The published multiplier is 180 times the thickness, not 18 times, so this radius would overstress the surface.
  • B — correct The bending radius must be at least 180 times the sheet thickness, and 180 multiplied by 0.100 inch is 18 inches.
  • C — incorrect The figure 180 is a multiplier applied to the sheet thickness, not a radius expressed directly in inches.

BACKGROUND

Acrylic plastic is a thermoplastic that softens with heat and can be formed to almost any shape, retaining that shape when it cools. It may also be cold-bent into a single curvature if the material is thin and the bending radius is at least 180 times the thickness of the sheet. Cold bending beyond these limits imposes so much stress on the surface of the plastic that tiny fissures or cracks, called crazing, form. Crazing is also produced by solvents such as gasoline, alcohol, acetone, and lacquer thinner, which is why only water and mild soap are used for cleaning. Scratches and other damage that obstruct the pilots' vision are not acceptable.

STUDY TIP

Convert the multiplier before you compare answers: multiply sheet thickness by 180 and read the result in inches. Remember that crazing is the defect the limit is written to prevent.

KEY POINTS

  • Cold bending requires a radius of at least 180 times the sheet thickness.
  • Exceeding that limit produces tiny fissures called crazing.
  • Solvents and harsh cleaners also craze acrylic surfaces.

SOURCES

  • ama-airframe/ch07.txt "Forming Procedures & Techniques"
  • ama-airframe/ch07.txt "Cleaning"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.