Chapter 2A - Sheet Metal Structures

Three methods of corrosion prevention are:

- cladding - oxide film - primer/paint

A torsional stress is made of what two types of stress acting perpendicular to each other?

- compression - tension

The five basic stresses encountered by an aircraft structure are:

- compression - tension - torsion - bending - shear

A protective film may be formed on the surface of a piece of aluminum alloy by what two methods?

- electrolytically - chemically

Aluminum alloy-faced honeycomb panels are useful as a structural material because of their:

- high strength - light weight - stiffness

The two basic types of sheet metal structure used for aircraft are:

- monocoque - semi-monocoque

Three requirements for the formation of corrosion on aluminum alloy are:

- potential difference - conductive path - electrolyte

A properly designed rivet joint is subjected to what types of stresses or loads?

- shear - tensile

A repair to an aircraft structure must restore its original:

- strength - stiffness - shape

Three drawbacks to the use of magnesium as a structural material are:

- susceptible to corrosion - tends to crack - burns easily

Commercially pure aluminum is identified by what number?

1xxx or 1100?

Most of the aluminum alloy used for aircraft structure is what alloy number?

2024

Describe the markings found on the heads of A, B, D, AD, and DD rivets.

A rivets are plain, B rivets have a cross, D rivets have a raised dot, AD rivets have a dimple, and DD rivets have a double dash.

Describe the process for determining the total length of a solid rivet for a particular installation.

Add the grip length ( thickness of the materials being joined) plus 1 1/2 times the rivet diameter.

2A—4 D05A When fabricating parts from Alclad 2024-T3 aluminum sheet stock, A — bends should be made with a small radius to develop maximum strength. B — all bends must be 90* to the grain. C — all scratches, kinks, tool marks, nicks, etc., must be held to a minimum.

Alcad 2024-T3 aluminum has a pure aluminum coating over the aluminum alloy core. If the coating is damaged, the aluminum alloy will be subject to outside corrosive elements.`

2A-11 D07A You can distinguish between aluminum and aluminum alloy by A — testing with a 10 percent solution of caustic soda. B — filing the metal. C — testing with an acetic acid solution.

Answer A. (AC43.13-1 B AC65-9A) Aluminum alloy sheets are marked with the specification number on approximately every square foot of material. If for any reason this identification is not on the material, it is possible to separate the heat-treatable alloys from the non-heat-treatable alloys by immersing a sample of the material in a 10% solution of caustic soda. The heat-treatable alloys will turn black due to the copper content.

2A-10 D06A What type loads cause the most rivet failures? A — Shear. B — Head. C — Bearing.

Answer A. JSAT 2-5, JSAT 2-51 (AC65-15A) Shear failure is perhaps the most common of rivet failures. It is simply a breakdown of the rivet shank by forces acting along the plane of two adjacent sheets, causing a slipping action which may be severe enough to cut the rivet shank in two. If the shank becomes loaded beyond the yield point of the material and remains overloaded, a permanent shift is established in the sheets and the rivet shank may become joggled.

2A-1 D07A The aluminum alloys used in aircraft construction are usually hardened by which method? A — Heat treatment. B — Aging. C — Cold working.

Answer A. JSAT 2-8 (AC65-9A) Heat treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for a definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact.

2A-2 DO5A When repairing a small hole on a metal stressed skin, the major consideration in the design of the patch should be A — that the bond between the patch and the skin is sufficient to prevent dissimilar metal corrosion. B — the shear strength of the riveted joint. C — to use rivet spacing similar to a seam in the skin.

Answer B. JSAT 2-5, JSAT 2-50 When repairing a small hole on a metal stressed skin, the riveted joint needs to be designed so that the rivets take the stress, and not the skin, so if there is a failure, it will be the rivets that shear and not the skin that tears.

2A-3 D05A Clad aluminum alloys are used in aircraft because they A — are stronger than unclad aluminum alloys. B — are less subject to corrosion than uncoated aluminum alloys. C — can be heat treated much easier than the other forms of aluminum.

Answer B. JSAT 2-8 (AC65-9A) Clad aluminum alloys consist of an aluminum alloy core coated with a layer of pure aluminum, to a depth of approximately 5-1/2% on each side. The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching, or from other abrasions.

2A-8 D06A When riveting dissimilar metals together, what precautions must be taken to prevent an electrolytic action? A— Avoid the use of dissimilar metals by redesigning the unit according to the recommendations outlined in AC 43.13-1 A. B— Treat the surfaces to be riveted together with a process called anodic treatment. C— Place a protective separator between areas of potential electrical difference.

Answer C. JSAT 2-13, JSGT 12-30 (AC65-9A) To prevent corrosion between dissimilar metal joints in which magnesium alloy is involved, two coats of zinc chromate are applied to each surface. Then a layer of pressure-sensitive vinyl tape is applied smoothly and firmly enough to prevent air bubbles and wrinkles. The separating material may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metals involved.

2A-9 D06A Joggles in removed rivet shanks would indicate partial A — bearing failure. B — torsion failure. C — shear failure.

Answer C. JSAT 2-5 (AC65-15A) When rivets are to be inspected which have been critically loaded, but show no visible signs of distortion, some of the rivet heads should be drilled off. If, upon examination, the shank appears joggled and the holes in the sheet are misaligned, the rivet has failed in shear.

2A-6 D06A Which part of the 2017-T36 aluminum alloy designation indicates the primary alloying agent used in its manufacture? A — 20. B — 17. C — 2.

Answer C. JSAT 2-7 (AC65-9A) In the 2xxx through 8xxx groups, the first digit indicates the major alloying agent used in the formation of the alloy. A head is the only way to tell what alloy the rivet is actually made of.

2A-5 D06A The primary alloying agent of 2024-T36 is indicated by the number A — 20. B — 24. C — 2.

Answer C. JSAT 2-7 (AC65-9A) The first digit in this aluminum alloy 2024 number indicates what the primary alloying agent is. 1000 series would indicate almost pure aluminum. 2000 series would indicate copper as the primary alloying agent. 3000 series would indicate manganese as the primary alloying agent, and so on.

2A-12 E05A Annealing of aluminum A — increases the tensile strength. B — makes the material brittle. C — removes stresses caused by forming.

Answer C. JSAT 2-9 (AC65-9A) When aluminum alloy is being formed, it will become strain hardened, and often before the forming can be completed, the metal must be annealed to relieve the stress.

2A-7 D06A Mild steel rivets are used for riveting A — nickel steel parts. B — magnesium parts. C — steel parts.

Answer C. JSGT 12-9 (AC65-9A) Mild steel rivets are used for riveting steel parts. Metals of the same galvanic grouping show little tendency to corrode.

What procedures should be followed to properly remove a solid-shank rivet?

Center punch the rivet and then drill just to the base of the rivet head with the same size or one size smaller drill. Once drilled, use a pin punch to tip off the rivet head and drive the remaining shank out of the hole while supporting the surrounding metal.

What is the difference between the tools required to pull a CherryLock rivet and a CherryMax rivet?

CherryLock rivets require a tool for each different size and head shape, while one pulling tool will set any size Cherry Max rivet.

6 Name at least three types of self-plugging mechanical lock rivets.

CherryMax, CherryLock, Olympic-Locs, and Huck.

What is the major type of damage to aluminum structures that is caused by exposure to the weather?

Corrosion

Give the temper designation for this condition: a. As fabricated

F

If a 2024 rivet must be replaced with a 2117 rivet, how do you determine the size to be used?

For 5/32" or smaller diameter, use the next larger size 2117 rivet, assuming that the edge distance and spacing meet the minimum requirements.

Give the temper designation for this condition: f. Strain hardened to quarter-hard temper

H12

Give the temper designation for this condition: g. Strain hardened to full-hard temper

H18

What action is taken to prevent cracks from forming while dimpling magnesium or some hard sheet metals?

Hot dimpling equipment is used to preheat and soften the metal before the dimple is formed.

Why is it important to use the proper size and weight bucking bar when performing sheet metal riveting?

If a bucking bar is too large or heavy it may be difficult to control and may cause damage to the surrounding structure, whereas a bucking bar that is too light will not properly upset the rivet before work hardening occurs.

What type of damage can occur when using a rivet set that does not properly fit the rivet?

If the radius of the set is too small, the rivet head may be damaged, whereas a set with an over-sized radius may cause damage to the sheet metal.

What minimum edge distance and spacing should be used for a single row of protruding head rivets?

Not less than two rivet diameters from the edge and not less than three rivet diameters apart.

Give the temper designation for this condition: b. Annealed

O

Give the temper designation for this condition: d. Solution heat treated and cold worked

T3

Give the temper designation for this condition: c. Solution heat treated

T4

Give the temper designation for this condition: e. Solution heat treated and artificially aged

T6

What are the dimensions of a properly formed rivet head?

The head should be 1 1/2 times the shank diameter in width and 1/2 the shank diameter in height.

5 What happens to the stem of a self-plugging (friction lock) rivet when the rivet is installed?

The stem is pulled until it snaps off and the remaining projecting part is trimmed flush with the head.

The type of metal most often used in the construction of civilian aircraft is heat-treated _____________________ alloy.

aluminum

The top of a wing spar, in flight, is subjected to what type of stress?

compressive

Aircraft structures are more likely to fail at points where stresses _____________________ due to abrupt changes in ____________________________.

concentrate; cross-sectional area

Name the primary alloying element in each in this aluminum alloy: a. 2117

copper

Name the primary alloying element in each in this aluminum alloy: b. 2024

copper

Because of it's strength and resistance to corrosion, stainless steal is well suited for ___________ temperature applications such as: -? -? -?

high - firewalls - junction boxes - exhaust systems

Magnesium is lighter than or heavier than aluminum?

lighter than

Name the primary alloying element in each in this aluminum alloy: c. 5056

magnesium

Will pure aluminum corrode?

no

Artificial aging is another name for what type of heat-treatment?

precipitation

Alclad is _________________ that is ________________ onto the surface of heat-treated aluminum alloys.

pure aluminum; rolled

In order to protect the alloy within an Alclad sheet and to prevent creation of potential stress points, the surface of aircraft sheet metal should be protected form what?

scratches and abrasions

If a properly designed rivet joint fails, the rivet will first do what?

shear

Which of the five stresses is the most common cause of rivet failure?

shear

Aluminum alloys may be ________________ by annealing.

softened

When an aluminum alloy is heated to its critical temperature and quenched, it is ____________ heat treated.

solution

To prevent a small crack from extending, you may ________________ its end.

stop-drill

Titanium is used in structural areas where a high _________ to ___________ ratio is required

strength; weight

The bottom of a wing spar, in flight, is subjected to what type of flight?

tensile

Are aluminum alloys susceptible to corrosion?

yes

Name the primary alloying element in each in this aluminum alloy: d. 7075

zinc

Name several methods for forming sheet metal?

Bending or folding, stretching, shrinking, bumping, and crimping.

How can a mechanic determine whether the countersink for a flush rivet should be dimpled or drilled?

By the thickness of the top sheet; thin sheets are dimpled while thick sheets may be countersunk.