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These tests are useful because they more 2. Common examples of such operations are forging turbine disks, extruding various components of aluminum ladders, drawing wire for making nails, and rolling metal to make sheets for car bodies.
Forming operations may be carried out at room temperature or at elevated temperatures, and at a low or a high rate of deformation. These operations are also used in forming and shaping nonmetallic materials such as plastics and ceramics, as described throughout this book. As indicated in Fig. This chapter covers those aspects of mechanical properties and behavior of metals that are relevant to the design and manufacturing of products and includes commonly used test methods employed in assessing various properties.
Metals are in Their Alloy Form. The tension test first requires the preparation of a test specimen, as shown in Fig. Although most tension-test specimens are solid and round, they can also be flat or tubular. Ultimate tensile strength UTS Typically, the specimen has an original gage length, lo, generally 50 mm 2 in. Uniform Necking It is mounted in the jaws of a tension-testing machine Fracture elongation Fig. If the load is removed, the spec- tension test, showing various features.
As the load is increased, the specimen begins to undergo nonlinear elastic deformation at a stress called the proportional limit. At that point, the stress and strain are no longer proportional, as they were in the linear elastic region, but Stress when unloaded, the specimen still returns to its original shape. Permanent Unload plastic deformation occurs when the yield stress, Y, of the material is reached. The yield stress and other properties of various metallic and nonmetallic materi- als are given in Table 2.
For soft and ductile materials, it may not be easy to determine the exact lo- Load cation on the stress—strain curve at which yielding occurs, because the slope of the curve begins to decrease slowly above the proportional limit. Therefore, Y is Strain usually defined by drawing a line with the same slope as the linear elastic curve, Elastic recovery but that is offset by a strain of 0. The yield stress is then defined as the stress where this offset line intersects the stress—strain curve.
This Permanent deformation simple procedure is shown on the left side in Fig. As the specimen begins to elongate under a continuously increasing load, its cross-sectional area decreases permanently and uniformly throughout its gage FIGURE 2.
If the specimen is unloaded from a stress level higher than the yield stress, the unloading of a tensile-test the curve follows a straight line downward and parallel to the original slope of specimen.
Note that, during the curve Fig. As the load is increased further, the engineering stress eventu- unloading, the curve follows a ally reaches a maximum and then begins to decrease Fig. The maximum en- path parallel to the original gineering stress is called the tensile strength, or ultimate tensile strength UTS , of elastic slope. Values for UTS for various materials are given in Table 2. Multiply gigapascals GPa by , to obtain pounds per square in.
If the specimen is loaded beyond its ultimate tensile strength, it begins to neck, or neck down. The cross-sectional area of the specimen is no longer uniform along the gage length and is smaller in the necked region. As the test progresses, the engi- neering stress drops further and the specimen finally fractures at the necked region Fig.
Hooke, — Note in Eq. The modulus of elasticity is the slope of the elastic portion of the curve and hence the stiffness of the material.
The higher the E value, the higher is the load required to stretch the specimen to the same extent, and thus the stiffer is the material. Compare, for example, the stiffness of metal wire with that of a rubber band or plastic sheet when they are loaded. The elongation of the specimen under tension is accompanied by lateral con- traction; this effect can easily be observed by stretching a rubber band.
Poisson, — and is denoted by the Stainless steels, annealed symbol n. There are two common measures of 10 ductility. Note that the elongation is based on the original gage length of the specimen and that it is calculated as a percentage. Reduction of area and elongation are generally interrelat- ed, as shown in Fig. Thus, the ductility of a piece of chalk is zero, because it does not stretch at all or reduce in cross section; by contrast, a ductile specimen, such as putty or chewing gum, stretches and necks considerably before it fails.
However, the instantaneous cross-sectional area of the specimen becomes smaller as it elongates, just as the area of a rubber band does; thus, engineering stress does not represent the actual stress to which the specimen is subjected.
However, they diverge rapidly as the strain increases. For example, when e " 0. Unlike engineering strains, true strains are consistent with actual physical phe- nomena in the deformation of materials. According to their definitions, the engineering strain that the specimen undergoes is 0! Note that the answer will be the same regardless of the original height of the specimen. Clearly, then, true strain describes the extent of deformation correctly, since the deformation is indeed infinite.
Because Ao and lo are constants, the engineering stress—strain curve obtained shown in Fig. In this example, Ao " 0.
Note that this curve has a positive slope, indicating that the material is becoming stronger as it is strained. The correction is due to the triaxial state of stress that exists in the necked region of the specimen.
The result is shown in Fig. This state gives higher stress values than the actual true stress; hence, to compensate, the curve must be corrected downward. The true stress—true strain curve in Fig. Typical values for K and n for several metals are given in Table 2. When the curve shown in Fig. The slope of the curve is equal to the exponent n. Thus, the higher the slope, the greater is the strain-hardening capacity of the material—that is, the stronger and harder it becomes as it is strained.
True stress—true strain curves for a variety of metals are given in Fig. When they are reviewed in detail, some differences between Table 2. Note that the elastic regions have been deleted, be- cause the slope in this region is very high. As a result, the point of intersection of each curve with the vertical axis in this figure can be considered to be the yield stress, Y, of the material.
The area under the true stress—true strain curve at a particular strain is the energy per unit volume specific energy of the material deformed and indicates the work required to plastically deform a unit volume of the material to that strain. Note that toughness involves both the height and width of the stress—strain curve of the material, whereas strength is related only to the height of the curve and ductility is related only to the width of the curve.
The curves start at a finite level of stress: The elastic regions have too steep a slope to be shown in this figure; thus, each curve starts at the yield stress, Y, of the material. Note that the slope of the load—elongation curve at this point is zero, and it is there that the specimen begins to neck. The speci- men cannot support the load because the cross-sectional area of the neck is becoming smaller at a rate that is higher than the rate at which the material becomes stronger strain-hardens.
The true strain at the onset of necking is numerically equal to the strain- hardening exponent, n, of the material. Thus, the higher the value of n, the higher the strain that a piece of material can experience before it begins to neck. This observation is important, particularly in regard to sheet-metal-forming operations that involve the stretching of the workpiece material Chapter It can be seen in Table 2. Ao where s is the true ultimate tensile strength.
Most materials display similar temperature sensitivity for elastic modulus, yield strength, ultimate strength, and ductility. The ductility and toughness incease, and b. The yield stress and the modulus of elasticity decrease. Temperature also affects the strain-hardening exponent of most metals, in that n decreases with increasing temperature.
The influence of temperature is, however, best described in conjunction with the rate of deformation. Some machines, such as hydraulic presses, form materials at low speeds; others, such as mechanical presses, form materials at high speeds. To incorporate such effects, it is common practice to strain a specimen at a rate corresponding to that which will be experienced in the actual manufacturing process. A short specimen elongates proportionately more during the same period than does a long specimen.
Thus, the strain rates are 0. Deformation rates typically employed in various testing and metalworking processes, and the true strains involved, are given in Table 2. The slope of these curves is called the strain-rate sensitivity 10 exponent, m. Note that C has the units of stress and is similar to, but not to be confused with, the strength coef- 1!
Note that, as the Also, the slope is relatively flat at room temperature; temperature increases, the slopes of the curves increase; that is, m is very low. This condition is true for most thus, strength becomes more and more sensitive to strain metals, but not for those that recrystallize at room rate as temperature increases.
Source: J. Typical ranges of m for metals are up to 0.
In this chapter, we present the current research program of dynamic mechanical behaviour of polymer materials. Aspects of Polyurethanes. Exploring the potentials of advanced materials and structures for serving in extreme conditions has been a hot research topic for decades [ 1 — 5 ]. Materials and material technologies can be tailored nowadays for the applications in structural engineering. They require the dedicated design concepts and optimized material properties. In many cases, these increases are unacceptable in structure design. For solving this problem, the trend in materials is towards weight reduction and strength increase at reasonable costs.
Save extra with 2 Offers. About The Book Mechanical Behaviour And Testing Of Materials Book Summary: This comprehensive book provides an insight into the mechanical behaviour and testing of metals, polymers, ceramics and composites, which are widely employed for structural applications under varying loads, temperatures and environments. Organized in 13 chapters, this book begins with explaining the fundamentals of materials, their basic building units, atomic bonding and crystal structure, further describing the role of imperfections on the behaviour of metals and alloys. The book then explains dislocation theory in a simplified yet analytical manner. The destructive and non-destructive testing methods are discussed, and the interpreted test data are then examined critically. Specifically designed for the undergraduate and postgraduate students of Metallurgical and Materials Engineering, the book will be equally beneficial for the undergraduate and postgraduate students of Mechanical engineering and related disciplines.
Figure 2. Stress-Strain Curve Figure 2. Nonmetallic materials Ceramics 0 Diamond Glass and porcelain Rubbers 0.
Hoteit, N. Numerous tests were performed in order to determine the thermo-hydro- mechanical behaviour of this rock. First, physical properties like density, porosity, water content, permeability and mineralogy were determined on all the stratum on core samples research laboratories and in boreholes in-situ. Some uniaxial and triaxial compression tests were performed on samples coming from different depths.
Скорее бы просигналил ее терминал. Но тот молчал. Конец лета. Солнце уже зашло. Над головой автоматически зажглись лампы дневного света. Сьюзан нервничала: прошло уже слишком много времени.
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