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插图：1.1 THE TASK OF MECHANICS OF MATERIALSIn engineering practice, we are always dealing with structures. There are many examples of structures such as buildings, bridges, airplanes, boats, vehicles, trains and all kinds of machines. Structures are quite different in type, but they have a common character that all structures bear loads. For example, buildings bear wind toad, seismic load, and load from residents. Bridges carry wind load, earthquake load and the load from vehicles. Vehicles carry the load from tourists, etc. Structures consist of several main portions called members such as rods, beams, columns, plates, shells, shafts, bolts, wheels etc. In civil engineering, these members are called structural components, while in mechanical engineering, they are called component parts.MeChanics of materials studies members. Let us see what kind of requirements they should meet when members are working. First of all, they can not be broken otherwise the structures including these members will collapse. Bridges can not be damaged when vehicles are running on them. When a lathe is processing parts, its shafts can not be broken. If such things happen, disastrous consequences will occur. In other words, members have to have enough strength to bear the loads safely. Second, deformation or deflection of members should be limited within some allowable ranges. For example, the deflection of a crane beam should not be too large otherwise the card on the beam will be driven with difficulty. If the principal shaft of a lathe has large deformation, the accuracy of the parts being processed will not be guaranteed. That is, the members should have enough stiffness to resist deformation.The third, members have to have stability. Right now, we are not able to give the definition of stability, but we can give an example. We take a slender column with a compressive force exerted on top of it. If the force is sufficiently large, the column will bend. That is, the equilibrium state of the column with a straight-line shape becomes unstable, or the column loses its stability. Stability is also an important requirement for members. If a column of a building loses its stability, the building will collapse, the consequence is also catastrophic.

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