The Mechanical Behaviour of Metals
area = مساحة
a brittle material = a material that breaks under the applied load with very little or no plastic deformation
creep (n.) = التغيير في ابعاد معدن بسبب الضغط أو الحرارة العالية
cross-section(n.) = a cutting through something especially at right angles to the axis.
defect (n.) = something missing or not right,
deform (v.) = change form because of load,
deformation (n.)
dimensions = measurements (of length, width, height, thickness etc.)
exceed (v.) =become greater than
extent (n.) =degree
fatigue (n.) = loss of strength because of repeated application of different stresses.
indentation = غرز
in terms of = with regard to
permanent(adj.) = continuing without change
property (n.) =a very important quality
provided that =if
proportionality constant = ثابت التناسب
ratio = نسبة
rigid = that does not change its form under load.
rivet = a short piece of metal which is used to fasten two plates when it is pushed through holes in the plates
and then hammered flat.
scratch = cut a mark on surface
shear = القوة المبذولة في القص
structure = a body which resists loads
thereby = in that way
Word Study
a- Tension, tensile, tense
When a load is suspended from a rope, the rope is in tension.It is under tensile stress. The rope is not loose; it is tense.
b- Compress, compressible, compressive, compression
Foundations are compressed by the loads.They are in compression Gases are easily compressed. They are compressible.
Some objects are considered as rigid bodies, but in fact, rigid bodies do not exist because all known materials are deformed to some extent by the application of forces. To avoid structural failures engineers, therefore, must know the properties of the materials they use, and they must be able to calculate the magnitudes of any deformations.
Elasticity
One of the most important properties of a material is its degree of . elasticity. Whenever a body is acted upon by an external force, a change in shape of the body results. Generally, if a certain limit (called the elastic limit) is not exceeded, many materials will return almost exactly to their original length or shape.
It was found that the elongation of an object is directly proportional to the applied load, provided that the elastic limit of that object is not exceeded. This fact, called Hooke s law, may be written:
F = ks,
Where F is the applied force, s the resulting deformation, and K a constant whose value depends on the nature, dimensions and the cross-sectional area of the object. The proportionality constant is expressed in units of N/m (newton per meter) in the S. I. system.
The property of elasticity is used to increase our com fan safety. The various springs in cars, trains and pieces of furniture the tyres on automobiles are just a few examples where this prop»| elasticity is of great importance.
Plasticity
If force is so great that the body is deformed beyond mi • limit and does not return to its original shape after the is removed then we say that the body has plasticity. A plastic material will retain its deformed shape after the removal of the applied force. On other hand, elastic deformation, which happens as soon as any 14 applied, is not permanent i.e., after the load is removed, the body returns to its original shape.
Hardness and Ductility
Engineers, in selecting suitable materials for a structure, must consider many other properties in addition to elasticity and plasticity. Among them are hardness and ductility. The hardness of a material is described in terms of its ability to resist indenting and scratching by other materials. Most hardness tests consist of applying a standard load to force a standard object into the material. In the Brinell hardness test a 10 - mm chromium - steel ball is pressed into the metal to be tested, with a force of 3000 kg; the area of the resulting indention a related to the hardness of that material. Hardness, however, must not be the mixed with brittleness. Brittle substances such as glass and cast iron break at or near their elastic limits. Most metals, however, have the ability to withstand plastic deformation before fracturing this property known as ductility, permits the material to be drawn into a wire and a plate into a cup.
Stress and Strain
It is possible to deform solid bodies by applying stresses. Deformation body under stress is called strain. Strain, therefore, is a direct result of strain. There are three basic kinds of stress and strain: tension, and shear. When equal and opposite forces are applied along the line of action so that they tend to elongate or stretch the body, the body is said to be in state of tensile stress. Tension, for example, happens when a cable is tied to a rigid support and carries a weight. The foundation of a house, on the other, resists external forces which tend to push the part together, thereby shorting it. The foundation is said to be under compression. Another form of stress is that of shear. A shearing stress is appliad to a body when two parallel, equal and opposite forces are applied along different lines of action, thereby changing the shape of the body without changing its volume. An example of this kind of stress is that produced in rivets of a steel structure. A rivet has to keep one plate from sliding over another, as well as to hold the plates together.
The amount of deformation depends upon the material of the body itself, and on the magnitude, direction, and area of the application of stress. Stress can expressed as the ratio of the applied external force causing the deformation to the cross sectional area over which the force acts.
Stress = external applied force / cross sectional area over which the force acts.
S =f / k
In the S.I. system the units of stress are N/M2 (newton per square meter),
Fatigue and Creep
When a material is subjected to repeated applications of varying stresses over a long period, it gradually loses its strength; this is known as fatigue, and it happens sooner if the material has a defect. Fatigue is a common cause of failure in machines parts.
If a metal is subjected to a constant stress for a long period while it is kept at relatively high temperature, a kind of deformation called high temperature creep will happen in the metal. High temperature must be considered in devices such as boilers and blades of turbines, where metals are stressed at relatively temperatures. In some materials such as lead and glass, creep deformation occurs even at normal temperatures.
References
1- Bette,J. Physics for Technology, Reston Publishing Co. Inc., New York
1976
2- Gosling, P.E. Engineering Science for the General Course, George G. Harrap & Co., London 1963.
3- Suh, N, P. , and Turner, A. P. Elements of The Mechanical Behaviour of
Solids, 1975. Exercises
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