Why is prepreg composite material stored in a freezer?

Prepreg already has resin and its catalyst impregnated into the fabric, so the cure reaction starts slowly at room temperature. Freezing the roll, typically near zero degrees Fahrenheit, stalls that reaction and preserves the material's remaining out-time and shelf life. Freezing is not about keeping moisture out; the sealed bag does that, which is why you thaw the roll still sealed so condensation forms on the bag rather than on the fabric.

What is the difference between a tap test and an ultrasonic inspection?

A tap test uses a coin or a tap hammer and your ear. A sharp ring means the skin is bonded, a dull thud means it is not. It works only on thin skins and near-surface defects, and it is subjective. Ultrasonic inspection sends sound into the laminate and reads the returned signal, so it finds delamination and disbond deeper in the part and can indicate how deep the flaw lies.

Why do composite repairs need heat and vacuum?

Heat from a lamp or a hot bonder drives the resin through its full cure cycle. A repair cured at room temperature generally reaches lower strength and a lower service temperature than the elevated-temperature cure the repair data calls for. Vacuum bagging does the other half of the job. It compacts the plies, presses the patch against the parent structure, and pulls trapped air and volatiles out of the layup.

How is aramid (Kevlar) different from carbon fiber?

Aramid is tough. It resists impact and abrasion and tends to hold together after a strike rather than shatter, which is why it appears where debris is a threat. Its weaknesses are compressive strength and moisture absorption, and it is difficult to cut and drill cleanly without special tooling. Carbon is stiffer and much stronger in compression, but it is more brittle and it conducts electricity.

What damage should I look for in a honeycomb sandwich panel?

Look for crushed or dented core, skin-to-core disbond, delamination inside a face sheet, and water in the cells. Water is the one that gets missed. It adds weight, corrodes aluminum core, and expands when it freezes, which can drive the skin off the core. Tap testing finds many near-surface disbonds, while radiography is the usual method for confirming trapped water before you cut into a panel.

Non-Metallic Structures — FAA A&P Test Questions (ACS AM.II.B)

Non-metallic structures is the ACS area covering composites, honeycomb sandwich panels, plastics, and the bonded assemblies and transparencies that have replaced sheet metal across much of the modern airframe. The airframe written test treats it as a materials-and-process subject: what a given fiber does, why a core is there, how resin cures, and which inspection method finds damage you cannot see from outside. Every AMTprep question published against AM.II.B carries a worked rationale and a handbook reference, so a wrong answer tells you which idea to go back and fix.

What ACS AM.II.B covers

AM.II.B pulls from the composite and non-metallic chapters of FAA-H-8083-31, and the questions stay close to material behavior and repair process. Expect fiber identification first: carbon and graphite are stiff and electrically conductive, aramid (Kevlar) is tough and impact resistant but weak in compression, and fiberglass is the lower-cost, non-conductive option. Because carbon conducts, questions ask what happens where it contacts aluminum, which forms a galvanic couple that corrodes the aluminum unless the two are isolated. Matrix and process questions follow. You are asked why resin and hardener must be mixed at the ratio the manufacturer publishes, why pre-impregnated (prepreg) fabric lives in a freezer, and why a heat lamp or hot bonder is part of most repairs. Those answers all come back to cure chemistry: an off-ratio mix or a cold cure leaves resin that never reaches full strength. Sandwich construction is its own cluster. The core of a honeycomb panel separates and stabilizes the thin face sheets and carries shear, which is where the panel gets its stiffness for the weight; damage that crushes the core or admits water destroys that. Ply orientation questions test whether you understand that a laminate is directional, so repair plies have to match the original angles, not just the original count. Finally, inspection. Composite damage hides under an intact surface, so the test favors tap testing for near-surface disbond in thin skins and ultrasonic inspection for delamination deeper in a laminate, with radiography used to confirm water trapped in core cells.

Where this sits on the test

ACS AM.II.B is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-31

Three traps candidates fall into

Students count plies and ignore angles. A four-ply patch with every fiber running the same way is not a substitute for a four-ply laminate laid up at zero, forty-five and ninety degrees. A laminate carries load along its fibers, so wrong orientation leaves the repair weak in the direction that matters.
The idea that extra hardener makes resin cure faster or harder is common and wrong. Resin and hardener react in a fixed proportion, so an excess of either leaves unreacted material in the matrix and the finished part cures softer and weaker, not stronger.
Candidates treat all fibers as insulators because fiberglass is one. Carbon and graphite conduct, so a carbon laminate fastened directly against aluminum structure forms a galvanic couple and the aluminum, as the anode, corrodes. A fiberglass barrier ply or sealant isolates the two materials.

7 free sample questions from ACS AM.II.B

AM.II.BTap an answer
When using a wet layup repair, what does the term 'ply orientation' refer to?
AM.II.BTap an answer
A nondestructive method that uses sound to detect delamination or disbond in a composite panel is the:
AM.II.BTap an answer
When storing pre-impregnated (prepreg) composite material, why is it kept in a freezer?
AM.II.BTap an answer
What is the main reason a heat lamp or hot bonder is used during many composite repairs?
AM.II.BTap an answer
In a honeycomb sandwich structure, what is the primary purpose of the core?
AM.II.BTap an answer
Which fiber type is electrically conductive and therefore can cause galvanic corrosion when in contact with aluminum?
AM.II.BTap an answer
Aramid (Kevlar) fiber is best known for its high resistance to:

When using a wet layup repair, what does the term 'ply orientation' refer to?

ACS code: AM.II.B

Correct answer: The direction of the reinforcing fibers in each layer

Rationale: Ply orientation describes the direction in which the reinforcing fibers run in each individual layer (for example 0, 45, or 90 degrees). Maintaining the original orientation is critical because composite strength is directional. The count of layers is the ply count, and cure temperature is a separate process parameter.

A nondestructive method that uses sound to detect delamination or disbond in a composite panel is the:

ACS code: AM.II.B

Correct answer: Coin tap test

Rationale: The coin-tap (ring) test relies on the difference in sound between a solid bonded area (sharp ring) and a delaminated or disbonded area (dull thud). Dye penetrant is used for surface cracks in nonporous metals, and magnetic particle inspection works only on ferromagnetic materials, so neither is suitable for composites.

When storing pre-impregnated (prepreg) composite material, why is it kept in a freezer?

ACS code: AM.II.B

Correct answer: Low temperature slows curing and extends shelf life

Rationale: Prepreg already contains the resin in a partially uncured state, so the chemical cure reaction is continuously progressing. Freezer storage slows that reaction, preserving the material's working life and tack. The freezer does not stiffen the fibers for cutting or dry out the resin; in fact, frozen prepreg must be thawed sealed to avoid moisture condensation.

What is the main reason a heat lamp or hot bonder is used during many composite repairs?

ACS code: AM.II.B

Correct answer: To cure the resin properly and develop full strength

Rationale: Most repair resins require elevated temperature to complete the cross-linking (cure) reaction and reach full design strength; a hot bonder or heat lamp supplies and controls that heat. Heat is not applied to soften the parent skin for shaping, nor to burn off contaminants, which are removed by cleaning before layup.

In a honeycomb sandwich structure, what is the primary purpose of the core?

ACS code: AM.II.B

Correct answer: To separate the face sheets and resist shear loads

Rationale: In a sandwich structure the thin, stiff face sheets carry the bending tension and compression loads, while the lightweight core separates them (increasing stiffness like an I-beam web) and carries the shear between them. The core does not carry the in-plane tension or form the outer aerodynamic surface.

Which fiber type is electrically conductive and therefore can cause galvanic corrosion when in contact with aluminum?

ACS code: AM.II.B

Correct answer: Carbon (graphite) fiber

Rationale: Carbon/graphite fiber is electrically conductive and cathodic relative to aluminum, so direct contact in the presence of moisture sets up a galvanic cell that corrodes the aluminum; a barrier ply of fiberglass or a sealant is used to isolate them. Fiberglass and aramid fibers are nonconductive and do not create this galvanic problem.

Aramid (Kevlar) fiber is best known for its high resistance to:

ACS code: AM.II.B

Correct answer: Impact and tensile loads

Rationale: Aramid fiber has excellent tensile strength and outstanding impact and damage tolerance, which is why it is used where toughness matters. It performs comparatively poorly in compression, and it is not noted for high-temperature resistance, so those choices are incorrect.

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Frequently asked questions

Why is prepreg composite material stored in a freezer?: Prepreg already has resin and its catalyst impregnated into the fabric, so the cure reaction starts slowly at room temperature. Freezing the roll, typically near zero degrees Fahrenheit, stalls that reaction and preserves the material's remaining out-time and shelf life. Freezing is not about keeping moisture out; the sealed bag does that, which is why you thaw the roll still sealed so condensation forms on the bag rather than on the fabric.
What is the difference between a tap test and an ultrasonic inspection?: A tap test uses a coin or a tap hammer and your ear. A sharp ring means the skin is bonded, a dull thud means it is not. It works only on thin skins and near-surface defects, and it is subjective. Ultrasonic inspection sends sound into the laminate and reads the returned signal, so it finds delamination and disbond deeper in the part and can indicate how deep the flaw lies.
Why do composite repairs need heat and vacuum?: Heat from a lamp or a hot bonder drives the resin through its full cure cycle. A repair cured at room temperature generally reaches lower strength and a lower service temperature than the elevated-temperature cure the repair data calls for. Vacuum bagging does the other half of the job. It compacts the plies, presses the patch against the parent structure, and pulls trapped air and volatiles out of the layup.
How is aramid (Kevlar) different from carbon fiber?: Aramid is tough. It resists impact and abrasion and tends to hold together after a strike rather than shatter, which is why it appears where debris is a threat. Its weaknesses are compressive strength and moisture absorption, and it is difficult to cut and drill cleanly without special tooling. Carbon is stiffer and much stronger in compression, but it is more brittle and it conducts electricity.
What damage should I look for in a honeycomb sandwich panel?: Look for crushed or dented core, skin-to-core disbond, delamination inside a face sheet, and water in the cells. Water is the one that gets missed. It adds weight, corrodes aluminum core, and expands when it freezes, which can drive the skin off the core. Tap testing finds many near-surface disbonds, while radiography is the usual method for confirming trapped water before you cut into a panel.

All 40 ACS areas

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Airframe

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Primary sources

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