Додаткові тексти для читання

STRENGTH OF MATERIALS

When a load is applied to a rod or structural member, the latter is deformed, stretched or compressed. If the member is elastic, it returns to its normal shape when the load is removed; if not, it is either broken down or remains deformed. Analyzing loads, calculating stresses and strains, and proportioning members for the loads they are to withstand is a fundamental operation in engineering design.

The ancients had little concept of mechanics, either of nature of stresses and strains which acted upon the body or of the strength of materials to resist such stresses and strains. They advanced by trial and error, and were guided in their work by the experience of their predecessors . Hence, the sombre arches and massive columns of Norman architecture; these columns have strength and too spare, and they say so quite frankly. The Gothic architects, on the other hand, had almost uncanny instinct for stresses and strains and the strength of materials. They delighted in demonstrating on how a roof could be supported by a slender stone cluster.

There were also tragedies, when machines broke down and bridges failed. At one time it became a matter of professional ethics for the designer of a bridge to stand under during the first run of heavy loaded waggons.

The problems of strength of materials were hidden deep in the mysteries of atomic and molecular structure and their solution had to wait for the progress made by man in all branches of knowledge.

Since the 19th century both scientific research and practical application of its results have escalated. New materials and various testing machines for compressive and tensile, and bending tests have been developed. To these were later added photoelectric and optical devices for the investigation of stresses in solid materials. The engineer of today has the mathematical abilities to calculate stresses and strains developed in the structures and predict the behavior of material under loading conditions.

IRON

Specimens of iron have been found in Egypt as far back as 2, 500 BC, though its extensive use did not begin until a much later times.

Iron was smelted in a primitive blast-furnace. It consisted of a hole, in which the ore and charcoal were placed; to blow the fire there was a hand bellows at the bottom. By these means two men could produce ten pounds of pig iron per day; a modem blast furnace will produce from 500 to 1,500 tons per day.

Pig iron is the iron as it comes from the blast-furnace. It contains so much carbon and other inclusions, that it has a very little strength. This excess carbon is removed by heating the pig iron with iron ore or by blowing air through the molten iron. The process, known as Bessemer process, was developed only in the 18th century. It is often said that it was the innovation that ushered an Era of Steels. The removal of impurities from pig iron made it malleable at room temperature. Forgings could be made. Sheets could be rolled. Steel as we know it today was finally here. Although the Bessemer process is no longer used, it was a start of the family of steel production techniques that we use today.

Iron alone is very weak and soft. Mixing iron with other elements varies its quality and strength. Carbon is by far the most important element that is mixed with iron; in fact, the distinctive properties of the different types of steel are due more to the variation in the carbon content than to all other alloying elements together. Carbon steels are specified for general engineering applications. Alloy steels contain definite concentration of various elements: cobalt, nickel, chromium, molybdenum, titanium, etc. Alloying elements influence hardness, strength, machinability, corrosion resistance and wearing qualities. These steels are used us structural components to resist wear.

In addition to carbon steels and alloy steels, there are tool steels, steels for special purposes, etc. All these steels can be useful in engineering but the most important are the carbon low-alloy steels and tool steels.

Many of the finest properties of steel can be obtained by heat treatment. If a bar of iron is heated to the melting point (1,500°C-or 2,730°F), the crystalline structure of a metal undergoes a series of changes at various points. If the steel is allowed to cool slowly, the process is reversed, but if it cools suddenly, any desired crystalline structure can be obtained. Whereas this kind of heat-treatment is comparatively new, the tempering process by quenching in water or oil was already known to the ancients. That the Greeks of the time of the writing of the Odyssey were acquainted with steel and with its heat-treatment is demonstrated by these words:

"As when the smith plunges the hissing blade deep in cold water"

(Hence the strength of steel).

Tempering makes steel less brittle and enables it to be used in the production of parts that require very hard steel. High-carbon steels are much harder than tempered ones, but they are difficult to work. To use them or not, is often a matter of a designer. He should consider the use of steels from the point of view of their property, advantages and economics. If the application under design calls for a wear-resistant surface, the carbon and alloy steel is the best choice. If cost is a primary selection factor, carbon steels are cheaper than any others.

Non-ferrous metals

Though iron is by far the most widely used metal today, it certainly was not the earliest. Copper and copper-tin alloys known as bronzes were known and used centuries before iron was discovered.

We do not know how primitive people came to discover these metals; probably by accident when they built their homes of certain hard rocks, which happened to be copper or iron ore.

The early bronzes were mixtures of metals obtained by early man which later were improved by the admixture of other metals, especially tin.

Bronze was the first metal to be used widely for implements. The stage of civilization in which bronze tools and weapons superseded stone and proceeded iron has been called the "Bronze Age".

Copper, the chief constituent of bronze, was obtained in relatively pure form at an early date, lead and tin were likewise known and used in ancient times.

Copper ranks next to iron in usefulness. As it is the second-next conductor of electricity (silver coming first), it is widely used in electric equipment. But it is also important in account of the many useful alloys of which it forms a part. Coins and jewelry made of gold and silver are usually alloyed with copper. A small amount of nickel can hide its reddish colour; this has led to many uses where copper acts as a cheap substitute for silver. Brass contains about one part of zinc to two parts of copper. Bronze is the name given to a number of copper alloys. Especially noteworthy is a combination of about 90% of copper and the rest of aluminium, known as aluminium bronze, which resembles gold in colour and is cheap substitute of gold.

Other non-ferrous metals whose uses have advanced in recent years are tin, zinc, lead, magnesium, chromium, aluminium and tungsten. Tin keeps bright and resists corrosion so well that it is widely used as a coating to protect iron from rusting. It is malleable and commonly used as tin foil for wrapping articles. Zink is used as negative plates and as a protective coating of iron. Lead finds more uses in the form of its compounds than pure lead, its greatest use is in lead plates for storage batteries.

Within the recent years, another non-ferrous metal, aluminium, has come to the fore. Aluminium was first obtained in the laboratory in 1825. However, wide application of aluminium did not occur until the World War II. Aluminium is the most abundant metal. Some 8% by weight of the earth crust aluminium. As aluminium is difficult to extract it took almost 60 years of research to find an economical way of making aluminium from ore.

Aluminium is a good conductor of electricity. It is ductile and can be readily machined and cast. There are several properties that set aluminium apart from other engineering materials.

About 25% is used for containers and packaging where lightness, and corrosion resistance is important. About 20 % of aluminium is used for architectural applications. 10 % of aluminium products is used for electrical conductors, and the rest is used for goods in industry, consumer products, and in vehicles.

As engineering material aluminium is widely used in machine design for all sorts of components in electrical wiring it has replaced more expensive and heavier copper. The useful properties and reasonable cost of aluminium have resulted in such increased use that in 1985 it was the second most widely used metal (second only to steel).

RUBBERS

Rubbers are a class of materials which serve an enormous number of engineering needs in the fields dealing with noise, vibration or corrosion protection, friction protection, electrical or thermal insulation, etc.

Among some 2,000 plants which contain rubber, only a few have ever produced it for commercial use. Two of these - the "rubber tree" growing in Africa and Southeast Asia ( Hevea Brasilliensis) and the "rubber shrub" (Parthenium Argentatus) have been the only sources of commercial rubber prior to the World War II.

Originally, the term "rubber" meant the material obtained from Hevea Brasilliensis. Today it means any material capable of extreme deformability. Synthetic materials like neoprene, butadiene are grouped together with natural rubbers. Rubber chemists have even developed a "synthetic" natural rubber which duplicates the chemistry and properties of the product of nature.

It was automotive industry with its needs for large quantities of tyres and other products which made the researchers to look for a synthetic rubber with roughly the same properties of natural rubber. The first significant progress was made by German scientists who developed a synthetic material called "methyl rubber". It was a start but not a successful one. Methyl rubber was expensive.

Prior to the World War II three events marked the evolution of synthetic rubber material. Two of these-polysulfide in the late 1920s and neoprene in the early 1930s are still being used in many products. The third event was the development of Buna rubbers in Germany. Although the quality of these materials was poor, the technologies - much improved and modified - formed the basis for major synthetic rubber production in the early 1940s. Since that time a lot of new improved synthetic rubbers have been developed. The demand for this low-cost material was so great that some experts predicted a sharp fall-off in demands for natural rubber. But it has not happened. In fact, current demand exceeds production. Whereas the most synthetic rubbers are used in passenger-car tyres, natural rubber is specified for more demanding areas, such as bus, aircraft, truck, etc.

It is against natural rubber that all other rubbers should be measured. No synthetic material has the properties equal to overall engineering characteristics of natural rubber and wide range of applications available with natural rubber. Natural rubber is the best choice for sealing devices, tyres, isolators, springs, bearings, etc.

Natural rubber has some disadvantages for example, the useful service temperature ranges from 60 to (in special cases) + 250°F. Other drawbacks of natural rubber can be minimized by specific processing technologies.

In selecting a rubber for an application a number of things must be considered mechanical and physical requirements, a reasonable service life, manufacturability of the part and its cost.

Plastics

We are more closely dependent on plastics than most of us realize, and yet the use of plastics has only just begun.

Plastics are a large group of materials consisting of carbon, oxygen, hydrogen, nitrogen and other organic and inorganic elements. A plastic is a solid in its finished form, but at some stages of manufacture it is a liquid which can be formed into various shapes, forming is done by heating and pressure. More than 50 families of plastics are used today, and each may have dozens of subtypes and variations.

One of the most widely used kinds of plastics is "bakelite". Bakelite is a hard strong, but rather brittle material which is made by heating a mixture of phenol and formaldehyde. Most bakelite articles are molded into their finished form; but after it has once hardened, it cannot be softened, or reshaped.

Cellulose is the base for another popular kind of plastics. It differs from bakelite by getting soft, when heated, so it can be easily reshaped. Properties of the celluloses which started the plastic industry as early as 1868 vary and greatly depend on plasticizer. Cellulosic parts can be used in service over broad temperature ranges. They are tough, have low specific heat and low thermal conductivity. Their application is wide-ranging from toys and optical frames to gears and tool handles.

There are a great number of other popular kinds of plastics, but only one more can be mentioned here "nylon". Although nylon is somewhat similar to silk and wool, it is different chemically from any known natural product. Nylon fibres are elastic and have great tensile strength.

Automotive industry is the largest user of nylon. Good mechanical properties and good resistance to heat make this material suitable for mechanical and electrical items speedometer, gears, cooling fans, connectors, etc.

Bearings are another major use of nylon parts. Low friction, good abrasion resistance qualifies nylon for such applications. Nylon bearings do not require lubrication and serve well for a long period of time.

Good design of plastic structures, however, requires an understanding of basic engineering principles and appreciation of the differences between metals and plastics. To achieve the best results requires sometimes a considerable investigation.

CORROSION

Corrosion attacks all engineering materials, especially me­tals. Corrosion is any chemical action which harms "the properties of a material. It reduces the life of a material and increases th cost of a structure. For example, a steel bridge must be repain­ted regularly to protect it from rust. Various metals have there­fore been developed to resist corrosion. Among them are the stain less steels. These metals contain from 12 to 35Й chromium which forms a very thin layer or film of chromium oxide on the surface of the metal. This film protects the metal from corrosion. Alloys made from copper and nickel are also corrosion-resistant. For example Monel metal, which containa roughly 60% nickel and ЗОЙ copper, is resistant to both fresfc and salt water corrosion. It is therefore used for marine engine parts, and for other surfaces like ships' propellers which are in contact with sea water. Supro nickels, which contain a smaller proportion of nickel, have a si­milar resistance to fresh and sea water. They are mainly used to make tubes.

'.Then two different metals touch each other in the presence of moisture, corrosion occurs. This type of corrosion is known as galvanic or electrolytic corrosion because it has an electrical cause. The metals and the moisture act like a weak battery and the chemical action which results corrodes one of the metals. If, for example, aluminium sheets are riveted with copper rivets, tialuminium near the rivets will corrode in damp conditions.

What is corrosion? To many people corrosion means rust. Bust is a by-product of corrosion, but a simple definition of corrosion is deterioration, of a material or its properties because of the reaction with environment. Sometimes it is a weight garn sometimes it is a weight reduction sometimes mechanical properties are affected. Most corrosions are electrochemical in nature; some are not. Why is corrosion prevention important? If a designer is concerned with an assembly machine that will not be subjected to corrosion, he may ignore all corrosion considerations in design. This can be unwise because many iron containing materials rust at a normal room temperature. If a service machine will be dripping with oil, corrosion may not occur, but if the parts are to be lubricated in service, it may be wise to use corrosion prevention measures on every part.

Despite the many new materials being used today, steel remains the principal material for the construction of transportation and industrial equipment. In the automotive industry, for example, steel maintains about 60 % share of the weight of a passenger car despite the increased use of plastics and aluminium.

Although steels meet most of design requirements they are vulnerable to deterioration from aggressive chemical environment or even from simple atmospheric oxidation. Various degrees of protection have been developed to prevent steel members from corrosion.

Coatings may range from electroplated metals to polymeric paints. Those common methods add a protective layer between the steel and the environment.

Selecting the best coating material, the designer should evaluate all effects of corrosive environment including temperatures and mechanical conditions.

METALS AND ALLOTS

Everybody knows that metals and alloys play important part ia any branch of technique as well as in our everyday life. But when did man come to know them? How early were metals and alloys used by man?

From the earliest times man has made ' hings of materials obtained from the earth. For thousands of years of the Stone Age for making tools he used mostly stone. Then came the discovery that metal can be produced from certain types of stones when they were treated with fire. By heating stones to very high tem­peratures man made the metal the stones contained melt and run out of them. Sometimes, as we mow it now, man had to add some carbon (C) to produce compounds of metals. We know also copper (Cu) and tin (Sn) to be two of the earliest metals obtained this way. Then man noticed that the two metals, when melted together, produced a new material that was much harder and stronger than either of.them had been.Scientists think this discovery opened a new period in man's development. Since the discovery the period to continue for about seven hundred years is considered to be the age of bronze. That's why the scientists gave it the name of the Bronze Age. It had lasted for about 700 years when came the time when man learned to produce iron, which became one of the most

important metals for him. It marked the beginning of a new age - the Iron Age.

Since then man has learned how to produce a lot of other nie- t.jj r. .mad how to obtain thousands of alloyp from th=ftn. Jo producean alloy man melted together two or more'metals. As the time pas­sed many different types of alloys were discovered. How we know some alloys to contain not only metals but also non-metals, such as crbon, sulphur (S), phosphorus (Eh), etc. Generally speaking, it is in the form of alloys that one considered metals to be the most useful. A lot of metals are converted into alloys of much importance and scientists and metallurgists want much more new alloys to serve man's needs. We want you to know that only about as little as 30 metallic element . serve modern needs of man, but there are over five thousand alloys, hundreds of which are in com­mon use. So many different alloys have, been elaborated because modern industry requires metals to be used for different purposes.

We know scientists to classify all the alloys into some ty­pes or classes according to their chemical composition and physi­cal properties. According to composition, all the alloys are to be classified as ferrous alloys (those containing iron) and non-fer- rous ones (those containing no iron or only a small quantity of it). The classes of alloys based upon their physical properties inclu­de light-weight alloys, low melting-point alloys and others.

An alloy to answer the purpose it has been elaborated for is to have certain properties or certain combination of properties which no metal in nature has. Many industries need alloys having certain chemical and physical properties. For example, new kinds of stainless steel are being designed to rer.ist the action of acids and corrosion due to the atmospheric sgento under high tem­peratures. Some of the alloys to be used in the automobile engines are to withstand very high temperatures. Future improvements in gas turbines depend in large measure on the development of new improved heat-resisting alloys too. In the nearest future we ex­pect metallurgists to elaborate new ways of controlling both chemical and physical properties of alloys and elaborating many new ways of important alloys for our technique and economy.

SOME IMPORTANT PROPERTIES OP METALS

Why are metals of such importance for man's life? The answer to this is to be found in their characteristic proper­ties. The most important of these is strength, which means that metals can withstand weight without bending or breaking, and are also corrosion-resistant and may be formed into different shapes, which distinguishes them from many other classes of materials. Some metals also have other special properties, two of which are conductivity and the property to be magnetized.

Strength is of great importance for most industrial purpo­ses. That is why steel is widely used in modern industry. We know steel constructions to be used not only for building ships, planes and engines but in building industry too. Steel is also used for producing tubes, pipes, etc. For this purpose, engineers often use special kinds of steel.

As iron and steel are used more often than any other metal, we usually divided metallurgical materials into two classes: fer­rous metals and non-ferrous ones. Where the requirement for strength are combined with the requirements for resistance to rusting, such non-ferrous metals as aluminium, bronze or brass may be used.

We know engineers to use copper and aluminium for conducting electric current since these metals offer less resistance to it than ferrous metals do. A copper wire offers less resistance to current than an aluminium wire of the same size does- But due toits lighter weight aluminium offers less resistance per unit of weight.

When current passes through a conductor, resistance results i^- giving off heat; the greater the resistance, the greater is the heat for a given current. That's why metals with high elec­tric resistance are used for electric heating. This property is found in the alloys; among the best for the purpose are alloys of nickel and chromium.

IRON OR E

Iron orea are mined by open-pit or underground methods and are crushed, screened and shipped to pig iron blast furnace plants. Advances in technology have led to the development of the pelle- tising or balling process in which fine grained iron ore concen­trates, that is iron ore from which some of the valueless materi­al has been removed, are made into pellets in a special plant.

The pellets so produced have a somewhat higher iron content than the ore as mined, and are in a very suitabJ 3 physical form for smelting.

Another method used to agglomerate iron ores is to heat them in a special furnace to enable the fine material to Join together.

Metallurgical coke, lump and sintered ore and pellets, plus limestone .and other slag-forming materials are the main feed ma­terials for blast furnaces. A hot blast of air blown through the furnace helps the coke to burn and to produce carbon monoxide, the gas mainly responsible for reducing the iron oxide to iron. In addition, the burning of the coke produces the high temperature necessary to melt the reduced iron and remove the waste materials as a molten slag.

Today this reaction takes place in large blast furnaces which operate continuously and can produce more than two thousand tons of iron in twenty-four hours. The blast furnaces are the central feature of complex large plants which must also include coke ovens, hot blast stoves, dustcollecting and gas-cleaning equipment.

The molten iron is run off into moulds to solidify into in­gots known aa 'pigs', or it ia conveyed directly, while still mol­ten, to a steel works situated close to the blast furnace plant for r.-fining into steel.