Contraction and expansion are reactions produced in metals as the result of heating or cooling. A high degree of heat applied to a metal will cause it to expand or become larger. Cooling hot metal will shrink or contract it. Contraction and expansion affect the design of welding jigs, castings, and tolerances necessary for hot-rolled material.

The selection of proper materials is a primary consideration in the development of an airframe and in the proper maintenance and repair of aircraft. Keeping in mind the general properties of metals, it is now possible to consider the specific requirements that metals must meet to be suitable for aircraft purposes. Strength, weight, and reliability determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and as light in weight as possible. There are very definite limits to which increases in strength can be accompanied by increase in weight. An aircraft so heavy that it could not support more than a few hundred pounds of additional weight would be of little use. All metals, in addition to having a good strength/weight ratio, must be thoroughly reliable, thus minimizing the possibility of dangerous and unexpected failures. In addition to these general properties, the material selected for definite application must possess specific qualities suitable for the purpose. These specific qualities are discussed in the following text.

The material must possess the strength required by the demands of dimensions, weight, and use. There are five basic stresses that metals may be required to withstand. These are tension, compression, shear, bending, and torsion. Each was discussed previously in this chapter.

The relationship between the strength of a material and its weight per cubic inch, expressed as a ratio, is known as the strength/weight ratio. This ratio forms the basis of comparing the desirability of various materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison. In some applications, such as the skin of monocoque structures, thickness is more important than strength; and in this instance, the material with the lightest weight for a given thickness or gauge is best. Thickness or bulk is necessary to prevent buckling or damage caused by careless handling.

Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections and the safety factors used in aircraft design and construction, it would be dangerous to select a material subject to severe corrosion if it were not possible to reduce or eliminate the hazard. Corrosion can be reduced or prevented by using better grades of base metals; by coating the surfaces with a thin coating of paint, tin, chromium, or cadmium; or by an electrochemical process called "anodizing." Corrosion control is discussed at length in Ratings Fundamentals, this TRAMAN.

Another significant factor to consider in the selection of metals for aircraft maintenance and repair is the ability of material to be formed, bent, or machined to required shapes. The hardening of metals by cold-working or forming is called work hardening. If a piece of metal is formed (shaped or bent) while cold, it is said to be cold-worked. Practically all the work you do on metal is cold-work. While this is convenient, it causes the metal to become harder and more brittle. If the metal is cold-worked too much (that is, if it is bent back and forth or hammered at the same place too often), it will crack or break. Usually, the more malleable and ductile a metal is, the more cold-working it can withstand.

Joining metals structuraly by welding, brazing, or soldering, or by such mechanical means as riveting or bolting, is a tremendous help in design and fabrication. When all other properties are equal, material that can be welded has the advantage.

Aircraft metals are subject to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials that are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending will ultimately cause a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until complete failure of the part occurs. This is termed "shock and fatigue failure. " Resistance to this condition is known as shock and fatigue resistance. It is essential that materials used for critical parts be resistant to these stresses.

The preceding discussion of the properties and qualities of metals is intended to show why you must know which traits in metals are desirable and which are undesirable to do certain jobs. The more you know about a given material, the better you can handle airframe repairs.