PRODUCT DESIGN

PLASTICITY - STRESS–STRAIN RELATIONSHIP - ELASTIC LIMIT

PLASTICITY is that state of matter where permanent deformations or strains may occur without fracture. A material is plastic if the smallest load increment produces a permanent

deformation. A perfectly plastic material is nonelastic and has no ultimate strength in the ordinary meaning of that term. Lead is a plastic material. A prism tested in compression will deform permanently under a small load and will continue to deform as the load is increased, until it flattens to a thin sheet. Wrought iron and steel are plastic when stressed beyond the elastic limit in compression. When stressed beyond the elastic limit in tension,

they are partly elastic and partly plastic, the degree of plasticity increasing as the ultimate strength is approached.

STRESS–STRAIN RELATIONSHIP gives the relation between unit stress and unit strain when plotted on a stress–strain diagram in which the ordinate represents unit stress and the abscissa represents unit strain. Figure 5 shows a typical tension stress–strain curve for medium steel. The form of the curve obtained will vary according to the material, and the curve for compression will be different from the one for tension. For some materials like

cast iron, concrete, and timber, no part of the curve is a straight line.

PROPORTIONAL LIMIT is that unit stress at which unit strain begins to increase at a faster rate than unit stress. It can also be thought of as the greatest stress that a material can stand without deviating from Hooke’s law. It is determined by noting on a stress–strain diagram the unit stress at which the curve departs from a straight line.

ELASTIC LIMIT is the least stress that will cause permanent strain, that is, the maximum unit stress to which a material may be subjected and still be able to return to its original form upon removal of the stress.

JOHNSON’S APPARENT ELASTIC LIMIT. In view of the difficulty of determining precisely for some materials the proportional limit, J. B. Johnson proposed as the ‘‘apparent elastic limit’’ the point on the stress–strain diagram at which the rate of strain is 50% greater than at the origin. It is determined by drawing OA (Fig. 5) with a slope with respect to the vertical axis 50% greater than the straight-line part of the curve; the unit stress at which the line O A which is parallel to OA is tangent to the curve (point B, Fig. 5) is

the apparent elastic limit.

YIELD POINT is the lowest stress at which strain increases without increase in stress. Only a few materials exhibit a true yield point. For other materials the term is sometimes used as synonymous with yield strength.

YIELD STRENGTH is the unit stress at which a material exhibits a specified permanent deformation

or state. It is a measure of the useful limit of materials, particularly of those whose stress–strain curve in the region of yield is smooth and gradually curved.

ULTIMATE STRENGTH is the highest unit stress a material can sustain in tension, compression, or shear before rupturing.

RUPTURE STRENGTH, OR BREAKING STRENGTH, is the unit stress at which a material breaks

or ruptures. It is observed in tests on steel to be slightly less than the ultimate strength because of a large reduction in area before rupture.

MODULUS OF ELASTICITY (Young’s modulus) in tension and compression is the rate of change of unit stress with respect to unit strain for the condition of uniaxial stress within the proportional limit. For most materials the modulus of elasticity is the same for tension and compression.

MODULUS OF RIGIDITY (modulus of elasticity in shear) is the rate of change of unit shear stress with respect to unit shear strain for the condition of pure shear within the proportional limit. For metals it is equal to approximately 0.4 of the modulus of elasticity.

MECHANICAL DESIGN

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

The casting design

In the casting design, factors to consider are:

1. The function of the casting,

2. The ability of the casting,

3. Strength casting,

4. Ease of production,

5. Considerations for safety

6. Economies in production.

In order to meet this requirement, we must have a thorough knowledge of production methods including pattern making, molding, core making, melting and flow, etc.

The best design will be achieved only when one is able to make the right choice from a variety of methods available. However, some rules for designing castings are given below to serve as a guide:

1. Sharp corners and often use the fillet should be avoided to avoid stress concentrations.

2. All parts must be designed in a casting of uniform thickness, as far as possible. If, however, the variation is unavoidable, it should be done gradually.

3. A sudden change from the very thick to very thin sections should always be avoided.

4. Casting should be designed as simple as possible, but with a good appearance.

5. Large flat surface on the casting should be avoided because it is difficult to obtain the correct surface on large castings.

6. In designing the casting, various allowances should be provided in making the pattern.

7. The ability to withstand pressure of casting contraction of some members can be enhanced by providing for example a curved shape, arms, pulleys and wheels.

8. The rigid members such as webs and ribs are used in the casting must be at least possible amount, because it can cause various defects such as hot water and shrinkage, etc.

9. Casting should be designed in such a way that would require a simple pattern and the mold easier.

10. In order to design cores for casting, consideration should be given to provide them adequate support in the mold.

11. Deep and narrow pockets in the casting should always be avoided to reduce cleanup costs.

12. The use of metal inserts in the casting must be kept at a minimum.

13. Signs such as names or numbers, etc., should not be given on vertical surfaces because they provide a barrier in the withdrawal pattern.

14. A tolerance of ± 1.6 mm in the casting of small (under 300 mm) must be provided. In terms of accuracy over the desired dimensions, tolerance ± 0.8 mm can be given.

Classes and Characteristics of Composite Materials

There is no universally accepted definition of a composite material. For the purpose of this work, we consider a composite to be a material consisting of two or more distinct phases, bonded together.

Solid materials can be divided into four categories: polymers, metals, ceramics, and carbon, which we consider as a separate class because of its unique characteristics. We find both reinforcements and matrix materials in all four categories. This gives us the ability to create a limitless number of new material systems with unique properties that cannot be obtained with any single monolithic material. Table 1 shows the types of material combinations now in use.

Composites are usually classified by the type of material used for the matrix. The four primary categories of composites are polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon/carbon composites (CCCs). At this time, PMCs are the most widely used class of composites. However, there are important applications of the other types, which are indicative of their great potential in mechanical engineering applications.

Figure 1 shows the main types of reinforcements used in composite materials: aligned continuous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numerous forms of fibrous architectures produced by textile technology, such as fabrics and braids. Increasingly, designers are using hybrid composites that combine different types of reinforcements to achieve more efficiency and to reduce cost.

A common way to represent fiber-reinforced composites is to show the fiber and matrix separated by a slash. For example, carbon fiber-reinforced epoxy is typically written ‘‘carbon/ epoxy,’’ or, ‘‘C/Ep.’’ We represent particle reinforcements by enclosing them in parentheses followed by ‘‘p’’; thus, silicon carbide (SiC) particle-reinforced aluminum appears as ‘‘(SiC)p/ Al.’’

Composites are strongly heterogeneous materials; that is, the properties of a composite vary considerably from point to point in the material, depending on which material phase the point is located in. Monolithic ceramics and metallic alloys are usually considered to be homogeneous materials, to a first approximation.

Many artificial composites, especially those reinforced with fibers, are anisotropic, which means their properties vary with direction (the properties of isotropic materials are the same in every direction). This is a characteristic they share with a widely used natural fibrous composite, wood. As for wood, when structures made from artificial fibrous composites are required to carry load in more than one direction, they are used in laminated form.

Many fiber-reinforced composites, especially PMCs, MMCs, and CCCs, do not display plastic behavior as metals do, which makes them more sensitive to stress concentrations. However, the absence of plastic deformation does not mean that composites are brittle materials like monolithic ceramics. The heterogeneous nature of composites results in complex failure mechanisms that impart toughness. Fiber-reinforced materials have been found to produce durable, reliable structural components in countless applications. The unique characteristics of composite materials, especially anisotropy, require the use of special design methods.

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Metal Casting Processes

Casting is making components in a way pour the melted material into the mold. Material herein may the form of metal and non-metal. To melt the ingredients necessary furnace (cupola kitchen). Furnace is a kitchen or a place equipped with a heater (heating). The solid material melted until temperature melting point and can be added to the mixture of materials such as chrome, silicon, titanium, aluminum and other materials in order to become more good. Materials that are liquid can be poured into molds. Molds for casting can be made with sand or

metal. For components that are complex and numerous usually use sand mold, while the components that form simple and can use any mass-produced metal molds. In making molds that need to be considered is the porosity and tolerance for sringkage (depreciation) after casting. porosity the higher the better mold to release the gases trapped inside the mold.

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